Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02917497 2016-01-13
METHODS AND APPARATUS FOR CRYSTALLIZATION OF SALTS
Technical Field
[0001] Some embodiments of the present invention pertain to methods for
crystallizing salts from a solution. Some embodiments pertain to apparatus for
crystallizing salts from a solution. Some embodiments pertain to apparatus or
methods for increasing the rate of salt crystal formation in a crystallization
pond.
Some embodiments pertain to apparatus or methods for enhancing the quality of
salt crystals formed in a crystallization pond.
Background
[0002] The crystallization of salts from solution is important in a
number
of processes. Crystallization from solution can be used in the production of
many
important inorganic crystals, for example, potassium chloride (KC1), sodium
chloride (NaC1), sodium sulphate (Na2SO4), potassium sulphate (K2504),
magnesium sulphate (Mg504.7H20), zinc sulphate (ZnSO4), copper sulfate
(CuSO4), urea, thiourea, potassium di-hydrogen phosphate (KH2PO4), ammonium
di-hydrogen phosphate (NH4PO4), or the like.
[0003] One example of a commercially important crystallization process
is
the crystallization of KC1 (potassium chloride). Crystallized KC1 is widely
used in
various fertilizers (for example, as applied to agricultural land to increase
crop
yields). One source of KC1 is mined potash. Examples of patents pertaining to
the
production of KC1 using crystallization include US 4,386,936 to Geesen; US
4,276,117 to Geesen; US 3,966,541 to Abraham; and US 3,384,459 to Carter, each
of which is incorporated by reference herein.
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[0004] Two techniques that can be used to produce crystalline salt
products, including crystallized KC1products, include pond crystallization
(typically used for solution mining operations and in naturally occurring salt
lakes)
and flotation bubble separation of mined potash crystals (typically used for
mechanically mined small particles from potash mines).
[0005] Solution mining is achieved by pumping heated water or weak
brine solutions down distributed and directionally drilled injection wells and
extracting a nearly equal volume of saturated or nearly saturated salt
solutions
from nearby production wells. The produced saturated or nearly saturated
solution
is often warm, e.g. at a temperature between 35 to 40 C (which is, for
example,
the mine salt layer rock temperature in example operations in Saskatchewan,
Canada). In some deposits, the ore body contains 50 to 55% wt/wt NaC1 and 40
to
45% wt/wt KC1 plus other salts and other impurities. Solution mining at the
ore
body temperatures common in Saskatchewan, Canada typically results in high
concentrations of K + Cl- ions in the solution, low concentrations of Na'
ions, and
trace mass fractions of other materials.
[0006] During mining, as the KC1 salt deposit becomes depleted in one
underground area, the KC1 concentration in the pumped-out solution will
decrease
and the relative concentration of NaC1 will increase slowly. At some time
duration
after the start of production in this production area, which has been
producing
saturated solution at a selected rate, it will be advantageous to move the
supply and
production wells to new positions. This decision is mostly based on a cost-
benefit
analysis for any given operation. Using horizontal drilling in the pay-zone or
production¨zone of the geological formation, the total production from each
particular set of well pipes can be large before new wells are required;
however the
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rate of production from a set of wells may have to be limited to get the
maximum
total mass production.
[0007] Differential crystallization is used to produce the desired KC1
crystal products from the solution and separate them from the Na ions in
solution.
This can be done by cooling and evaporating the solution water such that the
solution state remains within the metastable saturation state of KC1 on a
phase
diagram. Crystalline KC1 products can be produced from this solution in large
outdoor ponds or indoors in evaporated, circulated and cooled crystallizers
that
have large energy input rates to control the state of the solution. Outdoor
crystallization ponds have been operated in batch modes of production and have
little or no control of conditions ¨ so there is a much greater variation in
the crystal
product sizes, quality and rate of crystal production as compared with indoor
crystallizers. Since weather conditions over a typical year are so variable,
often for
any given operation both indoor plant and outdoor means of salt crystal
production
are used. The cost of producing crystallized product using an outdoor
crystallization pond is typically low compared with the cost of production in
an
indoor processing plant.
[0008] In one example installation in Saskatchewan, Canada, the outdoor
salt ponds are open ponds which are each approximately 3.5 m deep, covering
about 0.5 km2. KC1 crystals are produced as the solution cools, primarily by
natural pond cooling (i.e. heat convection to the air). After KC1 crystals
grow to a
certain size, they settle to the bottom of the pond from the open surface as
the pond
temperature drops. Over time a thick layer of KC1 crystals accumulates on the
bottom and the solution pond becomes stratified in average pond liquid
density,
with density increasing toward the bottom. The temperature increases with
depth
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because almost all the cooling takes place at the open surface of the pond.
This
pond stratification restricts any natural circulation of the liquid solution
(for
example due to wind and destabilizing temperature gradients) in the pond to a
fraction of the pond depth. Only a negligible fraction of the heat loss occurs
at the
bottom of the pond compared to that lost on the top surface of the pond.
Energy
transfer by evaporation from the top surface of the pond is not very
significant
compared to convective heat transfer because, in cold or cool weather, the
moisture
carrying capacity of air is low, and so convective heat transfer dominates
over the
entire surface area of the ponds.
[0009] This situation is illustrated schematically for a hypothetical
example in Figure 1, showing different regions of temperature and salt
concentration vertically within a typical prior art crystallization pond. The
variations of these properties in the horizontal direction are likely to be
negligible
except for in the pond perimeter region and near the pond surface. Upper
region
18 of the illustrated crystallization pond is at a cooler temperature than the
middle
region 19 or bottom region 21. Upper region 18 also has a lower concentration
of
salt in solution, e.g. KH and a ions. The concentration of KC1 increases with
increasing depth through middle region 19 and bottom region 21. Salt crystals
accumulate at the bottom of bottom region 21. During variable wind, ambient
air
temperature and solar irradiation conditions, the solution in upper pond
region 18
may move and mix. Solution temperatures in this region will vary spatially and
temporally. The portion of the crystallization pond with conditions within the
metastable zone width region (MSZW) where salt crystals may be produced is
illustrated as region 23. Region 23, although shown as an exact known
location, is
in fact uncertain and variable in location and size. As well, each of the
interfaces
shown in Figure 1 will vary with the time duration of exposure.
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[00101 When a large fraction of the dissolved KC1 has formed crystals
that
have been deposited on the bottom of the pond, the pond is drained and the bed
of
crystals is mechanically harvested. This is a batch production process. In the
example installation described above, this batch process may be staged at
three
levels of crystal production, with varying factions of the crystal volume
taken out
at each stage. That is, when about 1/2 of the crystals are deposited in the
first pond,
the brine solution is pumped into another pond and crystals are harvested from
the
first pond, when about the next 1/3 of crystals are deposited in the second
pond, the
second pond is drained, mechanically harvested and the solution is pumped into
the
third pond where the last 1/6 of the crystals are produced. In this cascade of
ponds
the average temperature of each of these ponds decreases by about the same
absolute amount over time as the solution cools before being pumped to the
next
pond (e.g. 10 to 15 C in winter and 5 to 10 C in summer). The duration of
time
the salt solution spends in each pond in the cascade is approximately the
same, but
pond production of KC1 crystals is several times slower in summer than in
winter
when the temperature differences between the solution and the atmospheric air
are
large.
[0011] Although the above-described example process using outdoor
crystallization ponds produces crystalline KC1, it is not optimal because, at
any
time, only a small fraction of the pond volumes are producing crystals and of
the
produced crystals only a small fraction are considered to be of good quality
(i.e. to
be generally transparent and clear with cubic morphology). The duration the
salt
solution remains in each pond varies from a minimum of about 6 days for cold
weather periods in winter to about 18 days for warm weather in summer. The
size
and quality of crystals produced increases from the first to the last pond in
the
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sequence of three ponds; the first pond produces crystals with a size range
from 0.2
to 1.2 mm, the second produces crystals with a size range from 0.9 to 2.0 mm,
and
the third pond produces crystals with a size range from 1.6 to 3.0 mm. The
first
pond crystals are slightly colored and opaque due to impurities and particles
comprised of agglomerated small crystals, while the last pond produces a high
fraction of crystals that are mostly transparent and clear. Pure large KC1
crystals,
which have the best market price, have a cubic morphology, with three nearly
equal characteristic lengths, or they have a truncated square morphology with
only
two sides equal. These high quality crystals are clear and transparent.
[0012] In the above-described example installation, the rate of
production
is limited primarily by the total surface area of the crystallization ponds
and the
temperature differences between the supply feed inlet of solution and the
ambient
air.
[0013] It is desirable to provide new methods and apparatus for
increasing
the production rate of crystalline salt products, particularly methods and
apparatus
applicable to an outdoor crystallization pond.
[0014] It is desirable to provide new methods and apparatus for
increasing
the crystal size and/or quality of crystalline salt products produced in a
crystallization pond, including in an outdoor crystallization pond. For
example,
the typical crop seeds used in many agricultural applications may have a size
in the
range of, for example, 1.5 to 5 mm. In agricultural applications, it may be
desirable
to distribute fertilizer and crop seeds concurrently and of nearly the same
size
and/or to reduce any dust caused by handling fertilizers of very small
particle size
(i.e. less than 0.1 mm). As well, pure large salt crystals have a lower risk
of caking
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during storage, and, because they are mechanically stronger, are not as easily
damaged by typical mechanical bulk handling. Thus, larger salt crystals resist
mechanical damage and the formation of dust, and also have a slightly longer
shelf
life than smaller salt crystals.
[0015] It is desirable to provide new methods and apparatus for
improving
the quality of salt crystals produced in a crystallization pond, including in
an
outdoor crystallization pond. Higher quality crystals include crystals having
a
particular desired morphology, having an absence of inclusions, and/or not
comprising small agglomerated particles.
[0016] The foregoing examples of the related art and limitations related
thereto are intended to be illustrative and not exclusive. Other limitations
of the
related art will become apparent to those of skill in the art upon a reading
of the
specification and a study of the drawings.
Brief Description of the Drawings
[0017] Exemplary embodiments are illustrated in referenced figures of
the
drawings. It is intended that the embodiments and figures disclosed herein are
to
be considered illustrative rather than restrictive.
[0018] Figure 1 shows schematically a hypothetical cross-sectional
elevation view of the different regions of temperature and salt concentration
within
a typical prior art crystallization pond. This figure shows regions in the
pond
where the temperature and salt concentration may vary versus the pond depth in
a
typical prior art crystallization pond.
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[0019] Figure 2A shows the saturation line for the equilibrium phase
diagram of a solution of KC1 and water.
[0020] Figure 2B shows the preferred metastable region of
crystallization
for a KC1 + H20 system in the super-saturation region adjacent to the
equilibrium
saturation line for phase change from solution-to-(KC1 crystals + solution)
for
quasi-equilibrium crystal production superimposed on part of the thermodynamic
phase diagram for KC1 solutions with and without the addition of a chelating
agent.
[0021] Figure 3 shows schematically an example embodiment of a
crystallization pond having an apparatus for actively cooling the bottom of
the
crystallization pond.
[0022] Figure 4A shows schematically an exemplary configuration for the
cooling tubes provided at the bottom of a crystallization pond. Figure 4B
shows
schematically a second exemplary configuration for the cooling tubes that
allows
for position-dependent control of the rate of cooling using the distribution
of the
cooling tubes and a plurality of valves.
[0023] Figure 5 shows schematically an example embodiment of a
crystallization pond having an auxiliary cooling system.
[0024] Figure 6 shows schematically an example embodiment of an
apparatus for sub-cooling the groundwater used as a heat sink for the
auxiliary
cooling system.
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[0025] Figure 7 shows schematically an example embodiment of a
crystallization pond having a bubble injection apparatus for enhancing
circulation
within the crystallization pond and/or for enhancing the water vapor
evaporation
rate of the pond.
[0026] Figures 8A and 8B show schematically a plan view of bubble
injection tubes on the bottom of a crystallization pond having bubble
injection
apparatus for generating hexagonal circulation cells (Figure 8A) or parallel
circulation cells (Figure 8B).
[0027] Figures 9A and 9B show a schematic diagram of a parallel
circulation cell showing one streamline or particle pathline for each of the
counterclockwise and clockwise helical circulation cells in a plan isometric
view
(Figure 9A, viewed along line A-A indicated in Figure 9B) and elevation view
(Figure 9B, viewed along line B-B indicated in Figure 9A).
[0028] Figure 10 shows a partial top view of a portion of an example
embodiment of a bubble injection tube having two parallel bubble injection
tubes
each adapted to provide a different size of bubble to the solution in a
crystallization
pond.
[0029] Figures 11A and 11B show schematically a plan view of two
example embodiments of a warm solution supply system.
[0030] Figures 12A, 12B and 12C show a schematic elevation view,
elevation view and plan view, respectively, of an example embodiment of a warm
solution supply system.
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[0031] Figure 13 shows schematically a cross-sectional view illustrating
the different regions of temperature and salt concentration within an example
embodiment of a crystallization pond.
[0032] Figure 14 shows a schematic view of a crystal vacuum harvesting
device according to an exemplary embodiment.
[0033] Figure 15 shows schematically a method for controlling features
of
an example embodiment of a crystallization pond based on various exemplary
inputs.
Description
[0034] Throughout the following description specific details are set
forth
in order to provide a more thorough understanding to persons skilled in the
art.
However, well known elements may not have been shown or described in detail to
avoid unnecessarily obscuring the disclosure. Accordingly, the description and
drawings are to be regarded in an illustrative, rather than a restrictive,
sense.
[0035] Some embodiments of the present invention provide methods or
apparatus for increasing the production rate of salt crystals produced from
salt-
containing solutions in crystallization ponds. In some embodiments, the
increased
production rate of salt crystals does not result in a significant decrease in
the size
range of salt crystals produced in the crystallization ponds. In some
embodiments,
the methods or apparatus provide an increase in the size range of salt
crystals
produced in the crystallization ponds. In some embodiments, the quality of
salt
crystals produced in the crystallization ponds is improved. In some
embodiments,
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the crystallization ponds are outdoor crystallization ponds.
[0036] In some embodiments, apparatus is provided to actively cool sub-
surface regions of the crystallization pond. In some embodiments, apparatus is
provided to actively cool layers of solution within the pond that are below a
surface layer of the pond.
[0037] In some embodiments, apparatus is provided to increase or sustain
circulation of the salt-containing solution in the crystallization pond. In
some
embodiments, apparatus is provided to increase the evaporation rate of water
from
the salt-containing solution and/or to increase circulation of the salt-
containing
solution in the pond by the injection of air at selected pond locations. In
some
embodiments, air is injected in the form of bubbles. In some embodiments, the
bubbles are injected as clusters of small bubbles. In some embodiments, the
distribution pattern of the injected air is selected to produce or sustain a
predetermined circulation cell pattern within the crystallization pond. In
some
embodiments, injection of air to provide a predetermined circulation cell
pattern
within the crystallization pond minimizes the energy input required to sustain
a
desired circulation rate within the pond.
[0038] In some embodiments, apparatus is provided to inject warm
solution into the cooling pond in a manner that increases circulation of the
salt-
containing solution in the cooling pond. In some embodiments, the warm
solution
is injected generally upwardly into the cooling pond. In some embodiments,
apparatus is provided to inject warm solution generally horizontally at an
inlet end
of a pond. In some such embodiments, the injection apparatus can be rotated to
adjust the direction in which the warm solution is injected generally
horizontally in
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the pond. In some embodiments, the direction of injection of warm solution
into
the pond is selected to assist in the formation and maintenance of
predetermined
circulation cells within the pond. In some embodiments, formation and
maintenance of predetellnined circulation cells within the pond minimizes the
amount of energy required to sustain a desired circulation rate within the
pond.
[0039] In some embodiments, the crystallization process is actively
controlled by controlling some or all of: active cooling of sub-surface
regions of
the crystallization pond; injecting clusters of small bubbles at selected pond
locations; injecting walin salt-containing solution into the crystallization
pond;
controlling a concentration gradient of the salt within the salt-containing
solution
so that a large portion of the pond solution remains within a metastable
region for
enhanced crystal growth by adjusting the pond circulation rate; or, adding a
chemical chelating agent to the circulated region of the pond.
[0040] In some embodiments, the process of producing and/or harvesting
salt crystals from the cooling pond is carried out continuously. In some
embodiments, a vacuum harvester is provided to harvest salt crystals. In some
embodiments, the vacuum harvester operates continuously.
[0041] In some embodiments, conditions within the crystallization pond
are controlled so that at least 60%, at least 70%, at least 80%, at least 90%,
at least
91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at
least
97%, at least 98%, or at least 99% of the volume of the crystallization pond
is at
conditions of temperature and salt concentration that fall within the
metastable
zone width (MSZW). In some such embodiments, the crystallization pond is
exposed to ambient weather conditions (e.g. ambient temperature, humidity,
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precipitation and/or wind conditions) while the conditions within the
crystallization
pond are controlled.
[0042] In some embodiments, the salt crystals are KC1 crystals and the
salt-containing solution is obtained as a product of solution mining of a
potash
mine.
[0043] In some embodiments, the overall size of crystallization ponds
required to process a given outflow (e.g. to handle production from a single
solution mine) is reduced as compared with crystallization ponds operated
according to previously known protocols.
[0044] As used herein, enhancing the formation of salt crystals means
any
or all of: increasing the purity of the salt crystals produced, increasing the
size of
the salt crystals produced, producing relatively more salt crystals having a
desired
morphology, producing salt crystals with fewer inclusions, producing salt
crystals
with less agglomeration, or producing relatively more salt crystals having a
desired
size range.
[0045] For the creation of crystal nuclei leading to macroscopic crystal
growth, an important prerequisite is the degree of super-saturation of the
solution,
which is the driving chemical potential and is defined to be the super-
saturated
concentration (SSC) divided by the saturation line concentration (SLC) minus
1.0
at the solution temperature and pressure (i.e. S-1.0, where S is the super-
saturation
ratio,¨ssrc). The concentration of salt in solution is measured to yield the
value of
S. Controlling the degree of super-saturation over time will usually result in
a
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particular growth rate and size distribution of the crystals. At a constant
degree-of-
saturation, the nucleation rate will also increase with increasing solubility
of the
surrounding solution. Solubility is defined to be the saturation concentration
and it
is a function of the solution temperature. Solubility affects the probability
that one
ion in solution will encounter another of dissimilar charge, and therefore
affects the
rate of inter-molecular and inter-ion collisions. When changes in solution
composition lead to increases in solubility, the interfacial or Gibbs energy
decreases since the affinity between crystallizing medium and crystal
increases [1].
Consequently, the degree of super-saturation required for spontaneous
nucleation
decreases with increasing solubility [2].
[0046] In view of the above thermodynamics, the metastable zone width
(MSZW) is an important parameter for the growth of large size crystals from a
solution, since it is the direct measure of stability of the solution in its
supersaturated region. The metastable zone width is the difference (i.e.
region)
between the saturation temperature at a given salt concentration and the
temperature at which crystals are first detected under a constant cooling rate
for
that salt concentration. Larger metastable zone-widths imply more stability
for
crystal growth [3, 4].
[0047] Stability for crystal growth implies the formation of a high
fraction
of perfect or high quality, clear and strong crystals having a desirable
morphology
(e.g. cubic or truncated-cubic for KC1 crystals), generally without
inclusions,
impurities, or complex and fragile morphology. Stability for crystal growth
generally means the solution is super-saturated and local diffusional
processes
occur around each nucleus or crystal in the solution. Since these diffusional
physical processes are not equilibrium processes, the crystallization process
is
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described as a quasi-equilibrium process when it is carried out slowly (i.e.,
slow
cooling is conducted to match the crystallization rate), so a greater
proportion of
the crystals formed are perfect. When this process is carried out too quickly,
as
occurs during rapid cooling of a super-saturated solution, the crystals formed
tend
to be very small and the resultant particles are often weak agglomerates with
many
inclusions.
[0048] An assessment of crystal quality can involve many different
attributes, for example, the size, size distribution, morphology,
agglomeration,
clarity or transparency, and/or percent inclusion of impurities. For different
applications, the desirable ranges of these properties may differ. As an
example, in
embodiments where suitability of a product for long-term storage is desirable,
clearer crystals may be desirable because substantially clear crystals can be
less
subject to mechanical damage when bulk materials are handled or shipped, and
will be less subject to caking when exposed to high humidity during shipping
or
storage. As an example, substantially clear crystals, when protected from high
or
super-deliquescence humidity (e.g. 85% relative humidity for KC1) during
storage,
may have very long shelf lives (e.g. one year or more). As well, crystal
particles
with a narrow range of sizes (i.e. a generally uniform size distribution) can
result in
more uniform distributions when distributed as a soil fertilizer.
[0049] The metastable zone width depends on a number of parameters
such as temperature and thermal diffusivity of the solution, diffusivity of
ions in
solution, rate of generating super-saturation crystal generation conditions
(e.g. if
the solution is cooled too quickly, poor quality crystals will be produced
because
the salt concentration may go well above the saturation limit), any fluid
dynamic
macroscopic motions, and the presence of foreign chemical particles or
impurities
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[5-9]. There are several reports in the literature on the effect of some
specific
impurities on the nucleation kinetics of crystal growth processes [10-15].
[0050] The process of crystal growth consists of several stages through
which growth units (e.g. ions) pass. These include (a) transport from the bulk
solution to a site at the crystal surface, (b) adsorption of the growth unit
onto the
impingement site, or (c) diffusion from the impingement region to a growth
site,
and (d) incorporation of charge pairs of ions into the crystal lattice.
Dissolution can
take place in steps b-d; however, the solvent may possibly be adsorbed into
the
crystal structure, and avoiding this requires careful control of these
parameters.
Any one of the above steps may be rate-limiting depending on the growth
conditions, such as the degree of super-saturation, temperature, presence of
additives or solvent, and transport or diffusional controlled properties of
the
system. Consequently, crystal growth mechanisms fall into two main categories
[16]: volume diffusion controlled and surface or interface integration
controlled
reactions. The goals of crystal growth theories are to determine the best
conditions
to achieve the most desirable crystal growth.
[0051] At equilibrium conditions the state of a salt solution is
determined
only by its temperature, concentration and phase of each chemical component.
Figure 2A shows the saturation line for the equilibrium phase diagram of a
KC1+
H20 solution.
[0052] Crystallization occurs during non-equilibrium conditions. When
the
solution is close to thermodynamic equilibrium, crystals of high quality and
market
value can be grown; but, when the solution is supersaturated and far from
equilibrium, the phase changes will result in phase transitions with a large
fraction
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of included H20 molecules and random small crystal orientations in the
resulting
solid structure, and a weak chemical crystal structure with agglomerated
particles.
A weak chemical crystal structure can arise from the presence of crystal
inclusions
such as water and/or impurities, and/or a complex morphology of interconnected
bonded small crystals forming an agglomeration mass. Such particles will not
have
a good market value. In some embodiments, such crystals with low market value
are recycled by putting them into the warm supply solution inlet. For example,
harvested crystals can be screened to separate out crystals having lower than
a
desired size, and such crystals can be partially or wholly redissolved and
recycled
back into the crystallization pond.
[0053] Figure 2B shows the preferred metastable region of
crystallization
15 for a KC1 + H20 system adjacent to the equilibrium saturation line 14 for
phase
change from solution-to-(KC1 + solution) for quasi-equilibrium crystal
production
superimposed on part of the theimodynamic phase diagram for KC1 solutions,
with
(16) and without (17, shaded region) the addition of a chelating agent. Region
15A
forms part of the metastable zone width for solutions including a chemical
chelant.
Outside this region and into the region above the metastable region for the
solid
crystal formation (i.e. above the supersaturation limit for nucleation of
crystals,
indicated by dashed lines 16 with chelating agent and 17 without chelating
agent)
the solution will produce mostly low quality crystals. Within the metastable
region,
nucleation of crystals will occur and these can grow beyond a critical size as
high
quality KC1 crystals so that subsequent crystal growth can be achieved by
cooling
the solution at a controlled rate to maximize the high quality crystal
production
throughout the crystallization pond.
[0054] The metastable zone width (or region) is the region of transition
on
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a plot of salt concentration versus temperature between the saturation line
and the
supersaturation line. The metastable zone width (or region) is the region
where
high quality crystals will grow. On the saturation line (i.e. line 14 in
Figure 2B),
crystals will just start to grow slowly, and may dissolve if they fall into
non-
saturated solution (i.e. below line 14 in Figure 2B). On and above the
supersaturation line (i.e. dashed line 16 with chelating agent and dashed line
17
without chelating agent in Figure 2B), nucleation will be rapid, which can
lead to
clusters of poor-quality agglomerated crystals. The metastable zone width
(MSZW) is the zone between the saturation curve and the labile region where
well-
controlled and spontaneous nucleation takes place. It is the most stable zone
of a
critical supersaturation level where nucleation begins.
[0055] The entire metastable zone width (MSZW) area shown in Figure
2B (i.e. region 15 in the case without chelating agent, and both regions 15
and 15A
in the case with chelating agent, i.e. the region between the equilibrium
saturation
line 14 and saturation limit for nucleation of crystals, 17 without chelating
agent or
16 with chelating agent, on a plot of salt concentration versus temperature)
can be
used to produce good quality KC1 crystals. Around each nucleation site or
small
crystal undergoing growth (e.g. crystal particle diameter, dõ about 0.01 to
2.0 mm
in one example) there will be a small liquid zone of mass diffusion
interaction (4)),
diameter equal to about 2 to 6 mm. This particle-surrounding salt solution is
a
nearly isolated diffusion cell with a crystal particle growing near its
center. The
number of these cells in a pond could be as large as the volume of the pond
divided
by the average volume of 4). Within one typical cell, or mass diffusion zone,
4),
with a crystal growing near its center, radial concentration gradients in the
supersaturated solution will be such that, at any time, t, the concentration
in the
solution (c*) will decrease slightly the closer one gets to the growing
crystal [i.e.
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gradient (c*) will be positive with respect to the radial distance from the
center of
each crystal 6].
[0056] The total mass of KC1 within the diffusion cell, 4), will tend to
be
invariant or nearly constant over time until such time that the particle
buoyancy or
gravity forces cause the particle to descend rapidly with respect to the
surrounding
solution (e.g. at relative speeds greater than 0.1 mm/s with respect to the
cell
liquid) even though the bulk mass of pond contents may also be in motion with
speeds greater than 1.0 mm/s. Although each diffusion cell is somewhat
isolated
for mass diffusion over this crystal growth duration, these cells are not
isolated for
heat conduction and convection. Temperature variations and gradients will
exist
throughout the pond. As well, some diffusion of water or salt concentration
may
take place between cells when there are significant gradients of water or salt
concentration between different areas of the pond. In general, measurement of
the
concentration and temperature variations within a diffusion cell, 4), is
impractical
but property variations throughout a large pond are readily measured.
[0057] At any time, t, there will exist an average temperature, T, of
solution and average concentration of KC1 solution ions in 4) of (C) such
that, as t
and dc increases, C decreases. In general, both T and C are functions of the
independent spatial coordinates (x, y, z) in the pond and time (t) (i.e., vary
any one
of these independent variables in a pond and the value of C and T may well be
different in a pond). In some embodiments, the local pond temperature T(x, y,
z, t)
is controlled such that C(x, y, z, t) is always or as much as possible within
the
MSZW as shown in Figure 2B for good quality crystallization.
[0058] For a typical prior art outdoor crystallization pond operation,
19
CA 02917497 2016-01-13
temperature distribution T is controlled by its initial inlet conditions of
the solution
and the surrounding ground temperature distribution, ambient weather,
evaporation
rate of water vapor, and solar irradiation history. For a passive pond with no
controls, this means that T is not controlled anywhere in the pond such that C
will
be within the MSZW. All that is known is that, if a large pond is permitted to
cool
from say, 40 C inlet conditions to 5 C above ambient temperature, there is a
good
probability that each region of the pond will spend a short fraction of the
total
period of time of pond batch production within the MSZW. During this fraction
of
the total time, crystals will grow; but, they are unlikely to be good quality
large
crystals. Most of the time, each region of the passive pond solution will be
either
producing small-size agglomerated crystals with many impurities or there will
be
no significant crystal growth. In contrast, in some embodiments of the present
invention, the local pond temperature T(x, y, z, t) is controlled such that
C(x, y, z, t)
is always or as much as possible within the MSZW as shown in Figure 2B for
good
quality crystallization. In some embodiments, conditions within the
crystallization
pond are controlled so that at least 60%, at least 70%, at least 80%, at least
90%, at
least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least
96%, at
least 97%, at least 98%, or at least 99% of the volume of the crystallization
pond is
at conditions of temperature and salt concentration that fall within the
metastable
zone width (MSZW).
[0059] The
state of salt-containing solution within the pond and of crystals
within the pond at any point are determined by the thermodynamic, chemical and
mechanical states. By assessing conditions that affect the crystallization
pond such
as pond temperature, salt concentration, crystal properties (when particles
exist as
crystals), atmospheric pressure, solar irradiation, pond fluid velocity (e.g.
three
components), turbulence (if turbulence exists), pond depth and other geometric
CA 02917497 2016-01-13
factors such as pond shape and circulation cell size, and bubble size
distribution
and concentration where bubbles float to the surface of the pond; conditions
that
affect the top surface of the pond such as ambient air temperature, humidity
ratio,
wind speed and direction, together with the effect of any control measures
taken as
described in this specification to control the flux of heat or moisture
transfer on the
top surface of the pond; and conditions that subsurface regions of the pond
such as
heat flux induced by various methods of control as disclosed in this
specification.
In some embodiments, only a few of the foregoing properties are monitored, and
other properties are simulated, for example based on the results of
experimental
tests or simulations. Certain properties, such as transport properties (e.g.
properties
for heat and mass convection) cannot be directly measured but can be
determined
indirectly.
[0060] In some embodiments, a chemical chelating agent is used to
enhance the growth of salt crystals. There are several chelating agents that
may be
used and each has somewhat different effects. Chelating agents generally act
to
increase the size range of the metastable region for crystallization without
changing the solution composition significantly (i.e. only very low
concentrations,
e.g. mole or mass fractions much less than 10-6, of the chelating agent are
typically
used). Exemplary chelating agents used to enhance salt crystal formation
include,
for example, EDTA (ethylene-diamine-tetra-acetic acid) (C10H16N208), DTPA
(diethylene-triamine-penta-acetic acid) (C14H23N3 01 0), DMSO (dimethyl
sulfoxide)
((CH3)2S0), DMSA (dimercapto-succinic acid) (C4H604S2), NTA (nitrile-triacetic
acid) (C6H9N06), citric acid (C6H807), oxalic acid (C2H204), acetic acid
(CH3COOH), and the like. The chelating agent enlarges the metastable region by
preventing heavy metals from being included within the formed crystals. The
enlarged metastable region provided by addition of a chelating agent can allow
for
21
CA 02917497 2016-01-13
more or better control of the salt crystal production rate and resulting
crystal sizes,
and/or slower circulation rates in each circulation cell. Faster cooling rates
depend
on the metastable zone width (MSZW) to ensure large-sized crystals are grown.
The addition of chelating agents can not only enhance the metastable zone
width,
but can also suppress the contamination of produced salt crystals by trace
amounts
of heavy metals in the solution (i.e. the fraction of perfect crystals can be
increased).
[0061] According to some embodiments of the present invention, provision
is made to increase and/or control the cooling rate of a crystallization pond
by
actively cooling sub-surface regions of the pond. In some embodiments, the sub-
surface regions of the pond that are cooled are the bottom and/or sides of the
pond.
In some embodiments, the surface area for heat transfer from a crystallization
pond
is increased by actively cooling the bottom and/or sides of the pond. This
provides
an additional dimension for cooling the pond (i.e. in addition to the surface
of the
pond).
[0062] In some embodiments, to maintain production of high quality salt
crystals, the temperature of a solution in a crystallization pond is reduced
by active
controlled cooling at a rate that is directly related to the rate of
production of high
quality crystals. In some embodiments, as the concentration of the desired
salt in
solution decreases, the temperature of the solution is decreased by an amount
sufficient to keep the solution in the metastable zone width. In some
embodiments,
both salt concentration and temperature are measured at one or more locations
to
determine whether the solution is within the metastable zone width for
crystallization of a desired salt product. In some embodiments, the rate of
cooling
of the solution in the crystallization pond is maintained at a rate that is
directly
22
CA 02917497 2016-01-13
related to the rate of production of high quality crystals at all or nearly
all positions
in the crystallization pond so that most or all regions of the crystallization
pond
remain within the metastable region.
[0063] In some embodiments, to maintain production of high quality salt
crystals, the concentration of salt in a solution in a crystallization pond is
controlled. In some embodiments, the salt concentration is controlled by
suppressing water vapor evaporation rate when it is larger than a desired
level. In
some embodiments, both the pond evaporation rate (i.e. moisture flux) and
cooling
rate (i.e. heat flux) are controlled. In some embodiments, the solution
temperature
and concentration of a desired salt in solution is controlled throughout
substantially
the entire pond volume so that, collectively, the majority of locations within
the
crystallization pond lie within the metastable zone width region (i.e. within
regions
15/15A in the example embodiment illustrated in Figure 2B). In some
embodiments, the amount of auxiliary energy required to achieve control of
temperature and salt concentration is minimized. In some embodiments, the
amount of auxiliary energy required to achieve a desired circulation rate
within the
crystallization pond is minimized by encouraging fluid flow in a manner that
produces circulation cells within the crystallization pond.
[0064] In conventional outdoor cooling ponds, cooling is provided only
at
the top surface of the pond at the air-liquid interface by wind and natural
convection. Thus, the cooling rate of such ponds is highly dependent on
factors
such as environmental air temperature, relative humidity or moisture content
of the
air, wind speed and direction, and solar heat gains. Surface convective heat
loss
and evaporation rates are determined by the surface water to air convective
heat
transfer coefficient times the air to surface temperature difference, and,
similarly,
23
CA 02917497 2016-01-13
vapor mass transfer convection coefficient times the moisture content
difference
between the air and the surface of the liquid in the cooling pond.
[0065] For the case of the ambient air moisture content greater than the
pond surface equivalent moisture content, water vapor will condense from the
air
onto the pond surface which will dilute the solution contained in the pond, so
the
surface salt content will decline near the pond surface. In some embodiments,
if
this situation takes the solution out of the metastable region, the active
cooling rate
for the pond is reduced to return the solution back to the metastable region.
In
some embodiments, if the temperature and salt concentration measured in the
pond
at one or more regions indicate that precipitation has caused the solution to
move
outside the metastable zone width, active cooling, e.g. using apparatus 50
described below, is reduced or stopped. In some circumstances, if the moisture
content difference cannot be corrected as aforesaid (e.g., during rain), the
active
cooling rate for the pond is reduced, or active cooling is stopped altogether,
prior
to the forecasted event (e.g. the start of precipitation), and active cooling
is
increased or resumed only after the precipitation has stopped and the air
humidity
has dropped. In some embodiments, active cooling is increased or resumed only
after the dew-point temperature of the air has decreased below the surface
temperature of the crystallization pond.
[0066] In some embodiments, a salt solution feed is supplied to a
crystallization pond at a concentration and temperature such that it has a
salt
concentration close to saturation conditions for its particular supply
temperature.
A supply jet momentum is provided at the solution inlet supply in such a
manner
that a circulating motion in the liquid solution is induced through
substantially the
entire volume of the solution pond in such a manner that the viscous or
frictional
24
CA 02917497 2016-01-13
energy dissipation rate for the pond is reduced or minimized for a selected
number
of liquid rotations. In some embodiments, this is achieved by inducing fluid
flow
in circulation cells, as described in greater detail below. In some
embodiments, the
resulting circulatory liquid motion is used to control the temperature
variations for
the entire pond such that the solution at substantially each and every point
within
the crystallization pond is within or very close to the metastable zone width
(MSZW). In some embodiments, the metastable zone width (MSZW) region is
enhanced by adding a chelating agent. In some embodiments, the crystallization
pond is subjected to a wide range of environmental conditions for the ambient
air,
precipitation, solar irradiance and surrounding soil. In some embodiments, an
apparatus for collecting or harvesting and selecting high quality salt
crystals from
the bottom of the crystallization pond is provided. In some embodiments as a
result of one or more of the foregoing, the rate of production of the most
valuable
or high quality salt crystals is high per unit mass through-put within the
crystallization pond.
[0067] With reference to Figure 3, an exemplary embodiment of a cooling
pond 20 has a bottom 22 and sides 24. Pond 20 has a liquid-impermeable pond
liner 26. In some embodiments including the illustrated embodiment, pond 20
includes a layer of clay 28 outside of liner 26, although depending on
external site
conditions, layer of clay 28 may be omitted in some embodiments.
[0068] In the illustrated embodiment, cooling pond 20 further has a
layer
of coarse gravel 30 interposing the layer of clay 28 and impermeable liner 26
on
the bottom 22 of pond 20. Impermeable liner 26 contains the liquid contents of
cooling pond 20 and prevents environmental contamination to and from the
adjacent soil. In some embodiments, coarse gravel layer 30 is comprised of
CA 02917497 2016-01-13
coarse-sized gravel with generally uniform particle size. In some embodiments,
ambient air is circulated through coarse gravel layer 30, for example using
one or
more fans, shown schematically as 31. Circulation of ambient air through
coarse
gravel layer 30 may act to isolate heat gain in the surrounding soil from
cooling
tubes 60 described below, and/or may carry away any small amounts of
accumulated water collecting in gravel layer 30 from ground sources or from
leaks
in liner 26. In some embodiments, the coarse gravel layer 30 is omitted.
[0069] In the illustrated embodiment, the bottom 22 of pond 20 is
provided
with a gravel layer 34 above impermeable liner 26, and with a bottom screen 36
above gravel layer 34. Crystals 38 of the desired salt form and accumulate
above
bottom screen 36. In the illustrated embodiment, gravel layer 34 is a layer of
approximately pea-sized gravel. Bottom screen 36 has a pore size smaller than
the
particles comprising gravel layer 34 to contain gravel layer 34. Bottom screen
36
may be made from any suitable inert material, for example plastic.
[0070] A salt-containing solution 32 from which a desired salt is to be
crystallized is introduced into and contained within pond 20. Salt-containing
solution 32 is typically a warm salt-containing solution. In some embodiments,
warm salt-containing solution 32 is at least 10 C, at least 20 C, at least 30
C, or at
least 40 C warmer than the ambient air temperature in the geographic location
of
pond 20. In some embodiments, warm salt-containing solution 32 has a
temperature difference between the supply inlet and the pond surface dew-point
temperature in the range of 10 C to 80 C, or any value therebetween, e.g. 15
C,
20 C, 25 C, 30 C, 35 C, 40 C, 45 C, 50 C, 55 C, 60 C, 65 C, 70 C, or 75 C. In
some embodiments, warm salt-containing solution 32 is injected at conditions
of
temperature so that salt-containing solution 32 is near the saturation point
given the
26
CA 02917497 2016-01-13
salt concentration of solution 32.
[0071] In some embodiments, pond 20 has a depth in the range of 0.5 m to
m, or any value therebetween, for example, 1.0 m, 1.5 m, 2.0 m, 2.5 m, 3.0 m,
3.5 m, 4.0 m, or 4.5 m. Control of operating conditions in some embodiments of
the invention may allow for the use of deeper crystallization ponds than could
be
used in situations where operating conditions are not controlled. In some
embodiments, the depth of pond 20 could be variable across its length and/or
width. In some embodiments, the depth of pond 20 is maintained at
approximately
a constant level for a given set of inlet flow temperature and salt
concentration
conditions, and/or for given ambient air conditions. In some embodiments, the
depth of pond 20 is varied based on changing operating conditions, for
example, in
winter versus summer conditions.
[0072] In some embodiments, pond 20 has a surface area in the range of
0.1 km2 to 1.0 kin2, or any value therebetween, for example, 0.2 km2, 0.3 km2,
0.4
km2, 0.5 km2, 0.6 km2, 0.7 km2, 0.8 km2, or 0.9 km2. In some embodiments, pond
20 has a length in the range of 10 m to 100 m, or any value therebetween, e.g.
20
m, 30 m, 40 m, 50 m, 60 m, 70 m, 80 m, or 90 m. In some embodiments, pond 20
has a width in the range of 10 m to 100 m, or any value therebetween, e.g. 20
m,
30 m, 40 m, 50 m, 60 m, 70 m, 80 m, or 90 m. Pond 20 may have any suitable
shape. In some embodiments, pond 20 is rectangular, square, elliptical or
circular
in shape. In some embodiments, pond 20 has an asymmetrical shape.
[0073] In some embodiments having parallel circulation cells (as
described
below), pond 20 has a generally rectangular planar shape with a generally
constant
pond depth (with the exception of locations adjacent sloping interfaces
between
CA 02917497 2016-01-13
pond liner 26 and the bottom of pond 20). In some such embodiments, the inlet
flow of solution enters on one side of the pond width (indicated as inlet end
44),
providing a net or circulation flow average velocity normal to the inlet end
44 (i.e.
in the direction of the length of pond 20, towards an outlet end 46).
[0074] In some embodiments, the geometric size of the crystallization
pond is characterized by two dimensionless ratios: the pond width (w) divided
by
the pond depth (d) or the pond width ratio (w/d) and the length ratio (//d)
where (1)
is the pond length. In some embodiments, these ratios have the ranges 2 < w/d
<
50 and 5 < l/d < 50. For example, for a pond depth of 3 m the pond width may
range from 6 to 150 m and the pond length range will be from 15 to 150 m.
[0075] In some embodiments having hexagonal circulation cells, the pond
20 may be rectangular or square. In some embodiments having hexagonal
circulation cells, the pond 20 may be somewhat circular or elliptical. In some
embodiments having hexagonal circulation cells, the ratio for pond width (w)
or
length (/) to depth (d) ranges from about 2 < w/d< 80 and 2 < l/d < 50.
[0076] With reference to Figure 3, according to one exemplary
embodiment, an apparatus 50 for actively cooling the bottom and/or sides of a
cooling pond using a coolant 52 is provided. The apparatus 50 has a heat
exchanger in pond 20. In the illustrated embodiment, the cooling apparatus is
provided with a tubing network 54 for moving the coolant 52 along the bottom
22
of the cooling pond 20 to produce a warm coolant. In some embodiments,
including the illustrated embodiment, the warm coolant is directed to one or
more
surface heat exchangers 56. In some such embodiments, the cooled coolant is
then
recycled back to tubing network 54. While the exemplary embodiment described
28
CA 02917497 2016-01-13
herein uses tubing network 54 to exchange heat between solution 32 and coolant
52, other suitable heat exchangers could be used, for example, cooling plates.
[0077] Coolant 52 is circulated through tubing network 54 by one or more
pumps 58. In one example embodiment, coolant 52 is an aqueous solution of
glycol. Any suitable substance may be used for coolant 52, for example, saline
water solutions using a suitable salt (e.g. NaCl, CaCl2, LiBr, or the like) or
any
suitable refrigerant used in the refrigeration or HVAC industries, for example
those
listed in the ASHRAE Handbook of Fundamentals (2013) published by ASHRAE
ISBN 9781936504459), which is incorporated by reference herein. In some
embodiments, two or more pumps 58 are provided, and pumps 58 are operated in
parallel to circulate coolant 52 through tubing network 54.
[0078] To cool salt-containing solution 32, tubing network 54 extends
through the bottom 22 and/or sides 24 of pond 20. In one example embodiment
shown in Figure 4A, tubing network 54 includes a plurality of approximately
evenly spaced parallel tubes 60 extending over the bottom 22 of pond 20, above
impermeable liner 26 and below pond bottom screen 36. In the illustrated
embodiment, tubes 60 are buried within gravel layer 34. While tubes 60 have
been
shown as extending over the bottom 22 of the pond, it will be appreciated that
tubes 60 could be positioned at any vertical location within salt-containing
solution
32 and still be used to cool pond 20. However, placement of tubes 60 beneath
bottom screen 36 facilitates harvesting of salt crystals unimpeded by tubes
60.
[0079] In a second example embodiment illustrated in Figure 4B, a
configuration of tubes 60 and valves 62 that allows for position-dependent
control
of the rate of cooling of the bottom of pond 20 is shown. A plurality of
valves
29
CA 02917497 2016-01-13
62A, 62B, 62C and 62D are provided to regulate the rate of flow of coolant 52
into
a plurality of regions 60A, 60B, 60C and 60D of tubes 60. The rate of flow of
coolant 52 through each of valves 62A, 62B, 62C and 62D can be controlled, for
example in response to a signal provided by a controller, shown schematically
as
64, to independently regulate the rate of cooling in each of regions 60A, 60B,
60C
and 60D. In the illustrated embodiment, region 60A is closest to the inlet end
44
of the pond, and region 60D is closest to the outlet end 46.
[0080] In some embodiments, one or more temperature sensors 66 are
provided to measure the temperature at various locations within pond 20. In
the
illustrated embodiment, one temperature sensor 66 is provided in each of
regions
60A, 60B, 60C and 60D. In some embodiments, temperature sensors 66 provide
feedback to controller 64 to allow controller 64 to regulate the rate of
cooling in
each of regions 60A, 60B, 60C and 60D by independently controlling the rate of
flow of coolant 52 through each of valves 62A, 62B, 62C and 62D, respectively
to
provide a desired rate of cooling in each region.
[0081] While in the embodiments illustrated in Figures 4A and 4B, tubes
60 have been shown as being parallel and evenly spaced, any desired
configuration
could be used for tubes 60 that allows the coolant 52 carried inside tubes 60
to
absorb heat from salt-containing solution 32. For example, in some
embodiments,
for example those having parallel circulation cells as described below, the
pond 20
has a unidirectional flow from inlet end 44 to outlet end 46, and the
resulting pond
mean bulk flow temperature will decrease from inlet end 44 to outlet end 46
(i.e.
pond 20 will have a negative bulk mean temperature gradient along the axial
direction of the circulation vortices). This, along with surface cooling and
evaporation, may make it desirable to provide at least some control over the
pond
CA 02917497 2016-01-13
bottom cooling as a function of distance from the pond solution supply inlet
or
inlet end 44 in order to maintain the bulk mean pond temperature at every
position
at its most desired value for its particular bulk mean concentration of salt
ions in
solution.
[0082] For example, in the embodiment illustrated in Figure 4B,
controller
64 may control the flow of coolant through valve 62A to be higher than the
flow of
coolant through valve 62B, which in turn is controlled to be higher than the
flow of
coolant through valve 62C and so on, so that the rate of cooling is higher in
region
60A than in region 60B, which is higher than in region 60C, and so on, to
counteract the naturally occurring pond mean bulk flow temperature that tends
to
decrease from inlet end 44 to outlet end 46.
[0083] In some embodiments, coolant 52 may be passed through tubes 60
in a direction that allows for control over the pond bottom cooling as a
function of
distance from the inlet end 44. For example, in one embodiment coolant 52
flows
through tubes 60 in a direction from inlet end 44 to outlet end 46 to provide
a
warm coolant with a higher temperature at outlet end 46 as compared with inlet
end 44.
[0084] Several flow configurations could potentially be used to provide
the
desired control over the cooling rate within pond 20, including the example
configuration shown in Figures 4A and 4B. In other embodiments, for example
those having hexagonal circulation cells as described below, there may be no
appreciable mean bulk flow temperature across the length of pond 20, and the
operation of cooling apparatus 50 is configured and regulated to provide a
relatively consistent or homogeneous rate of cooling on the bottom of pond 20.
It
31
CA 02917497 2016-01-13
is within the expected ability of one of ordinary skill in the art to
determine a
desirable configuration for the placement of tubes 60, for example by
conducting
appropriate pond temperature simulation studies.
[0085] In some embodiments, the size, spacing, and positioning of tubes
60 is determined based on the need to provide a selected range of cooling
rates for
the salt-containing solution 32 in pond 20. In some embodiments, the size,
spacing, and positioning of tubes 60 is determined to provide a small pressure
drop
to allow for high rates of flow of coolant 52 through tubes 60. In some
embodiments, the size, spacing and positioning of tubes 60 is determined based
on
the provision of a selected range of circulation cell size (as described
below) to
pond depth. In some embodiments, the ratio of circulation cell diameter to
pond
depth is close to or approximately 2.0, e.g. in the range of 1.5 to 2.5 or any
value
therebetween. In some such embodiments, the circulation cells are hexagonal
circulation cells or parallel circulation cells.
[0086] In some embodiments having parallel circulation cells as
described
below, different cooling rates are used for different parts of the bottom of
pond 20.
In some such embodiments, the bottom surface cooling rate is varied with
distance
from the inlet end 44. In some embodiments, the flow of coolant 52 through
tubing network 54 is controlled by using pumps or valves to regulate the rate
of
cooling along the bottom of pond 20, for example using the configuration shown
in
Figure 4B, for example by stopping the flow of coolant 52 through some tubes
in
tubing network 54 in regions where it is desired to decrease the rate of
cooling by
shutting off appropriate valves 62. In some embodiments, the degree of cooling
is
increased with increasing distance from the inlet end 44. In some embodiments,
the degree of cooling is decreased with increasing distance from the inlet end
44.
32
CA 02917497 2016-01-13
In some embodiments, the degree of cooling in a particular region of pond 20
is
decreased by putting fewer tubes of tubing network 54 in that particular
region of
pond 20 where it is desired to provide a lower rate of cooling.
[0087] Tubing network 54, including tubes 60, can be made from any
suitable inert material. In an example embodiment, tubing network 54 is made
from plastic tubing.
[0088] In the illustrated embodiment, surface heat exchanger 56 is a
plurality of heat-pipe heat exchangers. The heat-pipe heat exchangers release
heat
captured from the salt-containing solution 32 by a circulating coolant 52 as
sensible energy or heat to the atmospheric air. It is within the expected
ability of
one skilled in the art to design and build a heat-pipe heat exchanger suitable
for a
given application. In some embodiments, a fluid-to-fluid heat exchanger could
be
used to cool warm coolant, for example warm coolant could be cooled by a fluid-
to-fluid heat exchanger cooled by a natural coolant such as a flowing water
source
or groundwater.
[0089] In the illustrated embodiment, the only auxiliary energy input
for
cooling apparatus 50 is the electrical energy required to operate pumps 58. In
such
embodiments, surface heat exchangers 56 are entirely passive devices that
transport sensible energy within them by gravity-induced internal flows
coupled
with two-phase energy transfer using a suitable available refrigerant within
each
heat pipe.
[0090] In operation, the flow through pumps 58 is controlled to meet the
cooling requirement needs for pond 20 at any given time. In some embodiments,
33
CA 02917497 2016-01-13
the rate of flow of coolant through pumps 58 is controlled to provide a
desired
cooling rate for salt-containing solution 32 in pond 20. In some embodiments,
physical modeling, numerical modeling, simulation studies and/or laboratory
scale
studies are, for a given set of typical hourly or time averaged ambient air
properties, solar irradiance, and soil conditions, used to select the
parameters for
pond depth, length, width, supply solution feed rate and solution feed
recirculation
rate, vortex circulation rate, and/or pond bottom cooling rate for the salt-
containing
solution.
[0091] In some embodiments, sub-surface regions of pond 20 could be
cooled by circulating groundwater directly through coarse gravel layer 30.
[0092] With reference to the embodiment illustrated schematically in
Figure 5, in another example embodiment, an auxiliary cooling system 80, using
groundwater as a heat sink, is provided to assist in cooling pond 20. In the
embodiment of Figure 5, groundwater 82 is used as an auxiliary heat sink. Use
of
groundwater 82 as an auxiliary heat sink may be particularly desirable during
periods of high external air temperatures (e.g. during summer) to further cool
coolant 52. In the embodiment of Figure 5, moderately deep well pumping by a
pump (shown schematically as 83) is used to supply groundwater to the surface
42,
where additional heat exchangers (shown schematically as 84) are used to cool
coolant 52 after it exits heat exchanger 56 and before coolant 52 is returned
to
tubes 60 below pond 20. Slightly heated groundwater 86 produced by the
exchange of heat from coolant 52 is disposed of in any suitable manner, for
example by injection into wells some distance away from pond 20 and the source
of groundwater 82. Injection into wells some distance from pond 20 and the
source of groundwater 82 can avoid returning heat removed from salt-containing
34
CA 02917497 2016-01-13
solution 32 back to either pond 20 or the source of groundwater 82.
[0093] Without being bound by theory, it is anticipated that the
temperature of coolant 52 during summer periods could be reduced by up to
about
C to 15 C beyond what could be achieved by the use of apparatus 50 in the
absence of auxiliary cooling system 80 because the source of groundwater 82 is
anticipated to have a maximum temperature ranging from about 5 C to 15 C in
late
summer. Thus, auxiliary cooling system 80 may facilitate enhanced cooling of
pond 20 cooling rates during warmer periods (e.g. during summer), and allow
for
improved year-round production rates of salt crystals 38.
[0094] With reference to Figure 6, in a further example embodiment
illustrated schematically, groundwater 82 is sub-cooled to less than 5 C, or
even
cooler depending on the salt content of groundwater 82 which may allow for
even
cooler temperatures, during periods of low prevailing atmospheric temperature
(e.g. during winter) using a sub-cooling apparatus 90. Sub-cooling apparatus
90
has one or more heat-pipe heat exchangers (shown schematically as 92) embedded
in the ground and extending down to a low level of groundwater 82. For
example,
where groundwater 82 is at a depth of 100 to 200 feet, a fluid conduit 93
extends
approximately 100 to 200 feet to heat exchanger 92. Below-ground heat-pipe
exchangers 92 are coupled in fluid communication with above-ground air-cooled
heat-pipe exchangers 94 via fluid conduit 93. A suitable coolant, for example
an
aqueous solution of glycol, saline water solutions using a suitable salt (e.g.
NaCl,
CaCl2, LiBr, or the like), or any suitable refrigerant used in the
refrigeration or
HVAC industries, for example those listed in the ASHRAE Handbook of
Fundamentals (2013) published by ASHRAE ISBN 9781936504459), is circulated
between heat-pipe exchangers 92 and 94. Coolant is circulated between the heat-
CA 02917497 2016-01-13
pipe exchangers using a pump (shown schematically as 96) or set of pumps in
parallel. Thus, heat is transferred from the source of groundwater 82 to the
coolant
by below-ground heat-pipe exchangers 92, and then released to atmospheric air
by
air-cooled heat-pipe exchangers 94 to sub-cool groundwater 82 below the native
temperature of the source of groundwater 82.
[0095] Operation of heat-pipe exchangers 92, 94 during periods of
prevailing low temperatures, e.g. during winter, will result in the production
of a
sub-cooled heat sink that can be advantageously used as a source of cold
groundwater 82 for auxiliary cooling system 84 during periods of prevailing
high
temperatures (e.g. during summer). Stopping the flow of coolant between heat-
pipe exchangers 92, 94 during periods of prevailing high temperatures (e.g.
during
summer) will help to avoid additional warming of the source of groundwater 82
during periods of prevailing high temperatures. In some embodiments, summer
months are June, July, August and September. In some embodiments, during the
summer months, ground water temperatures are below the air temperatures most
of
the time (for example, in some embodiments, the groundwater temperature is
expected to vary between about 5 C and 10 C, or between just over 0 C and 5 C
in
embodiments in which sub-cooling apparatus 90 is used to sub-cool the source
of
groundwater 82 during the winter months).
[0096] In some embodiments, heat-pipe heat exchangers 92, 94 are
operated during periods of prevailing low atmospheric temperatures during the
winter months, for example, from November to March or any interval
therebetween, including December, January and February. During this period,
coolant is circulated between heat-pipe heat exchangers 92, 94 to sub-cool the
source of groundwater 82. The flow of coolant between heat-pipe heat
exchangers
36
CA 02917497 2016-01-13
92, 94 is then stopped. Groundwater 82 can continue to be used as an auxiliary
heat-sink to cool coolant 52.
[0097] Determination of whether to operate auxiliary cooling system 80
and/or sub-cooling apparatus 90 can be made by assessing the trade-off between
cooling coolant 52 below the ambient air wet-bulb temperature conditions on
one
hand, and the rate of evaporation from the top surface of pond 20 (which will
change with changing weather conditions) and circulation rate within pond 20,
on
the other hand. In some embodiments, the operation of auxiliary cooling system
80 and sub-cooling apparatus 90 is controlled to approximately match the
cooling
rate of pond 20 with the crystallization rate, so that most or all of the
solution 32
throughout pond 20 remains within the metastable zone width (MSZW). Thus, as
crystallization proceeds, the concentration of salt in solution 32 will
decline, and
the temperature of solution 32 should be cooled to keep the solution within
the
MSZW. Cooling of solution 32 can then continue until the pond air-liquid
surface
temperature declines to the dew-point temperature of the ambient air to avoid
excessive air to liquid condensation of airborne water vapor on the surface of
the
pond. In some embodiments, during periods of prevailing low atmospheric
temperatures (e.g. winter), the ambient air dew-point temperature may be well
below 0 C. In some embodiments, during periods of prevailing high atmospheric
temperatures (e.g. summer), the dew-point temperature may be well above 0 C,
e.g. 5 C to 10 C.
[0098] In some embodiments, the cooling of solution 32 is controlled so
that the surface temperature of pond 20 is maintained slightly above the dew-
point
for the adjacent atmosphere at the lowest expected temperature of the day. In
one
example embodiment, the lowest expected temperature of the day is the
37
CA 02917497 2016-01-13
temperature expected at approximately 4:00 a.m. in the absence of changes in
weather.
[0099] In some embodiments, operation of auxiliary cooling system 80
and/or cooling apparatus 50 is controlled based on changes in precipitation.
Precipitation such as rain falling on pond 20 will decrease the concentration
of salt
in solution 32 near the pond surface 40. This may remove solution 32 from the
metastable zone width region, and slow the rate of crystal formation. However,
decreasing the concentration of salt in solution 32 will not generally
decrease the
quality of salt crystals 38 formed. In some embodiments, operation of
auxiliary
cooling system 80 and/or cooling apparatus 50 is stopped or reduced shortly
before
the forecasted arrival of precipitation, e.g. one to two days before the
forecasted
arrival of precipitation. In some embodiments, the expected amount of
precipitation is factored in to the determination of when to stop or reduce
operation
of auxiliary cooling system 80 and/or cooling apparatus 50. In some
embodiments,
operation of auxiliary cooling system 80 and/or cooling apparatus 50 is
resumed
when the dewpoint temperature of the atmospheric air decreases to below the
surface temperature of the pond 20.
[0100] In some embodiments, operation of cooling apparatus 50 and/or
auxiliary cooling system 80 is controlled based on the time of day and/or the
level
of sunlight prevailing on a particular day. Sunlight will result in solar
gain, i.e. the
temperature of solution 32 in pond 20 will increase as a result of sunlight
shining
on the surface 40 of the pond 20. In some embodiments, cooling apparatus 50 is
operated during the day to provide a higher degree of cooling of pond 20 than
at
night to counteract solar gain (e.g. by increasing the rate of flow of coolant
52
through tubing network 60). In some embodiments, auxiliary cooling system 80
is
38
CA 02917497 2016-01-13
operated during the day but not at night to counteract solar gain.
[0101] In some embodiments, a removable cover, shown schematically as
65 in Figure 3, is provided to shelter pond 20 from precipitation, wind and/or
sun.
In some embodiments, the cover is removable, so that the cover can be put in
place
over the pond when the expected precipitation, wind or sun is being
experienced,
and so that the cover can be removed from pond 20 when the precipitation, wind
and/or sun are no longer affecting pond 20. In some embodiments, pond 20 can
be
sheltered by a permanent structure or enclosure, with ventilation air provided
through large apertures in the side of the structure or enclosure, or through
air
pumped into the structure or enclosure by auxiliary power means.
[0102] In some embodiments, air is injected into pond 20 to enhance
circulation and/or to increase the evaporation rate from pond 20. In some
embodiments, as illustrated in Figure 7 and Figures 8A and 8B, a bubble
injection
apparatus 110 for enhancing circulation in pond 20 by promoting formation or
sustenance of natural liquid circulation cells is provided.
[0103] Two types of natural liquid circulation cells may be exploited
for
the low rate of input energy required to sustain their circulation rate in a
crystallization pond: (a) hexagonal cylindrical circulation cells and (b)
parallel
circulation cells, each comprised of two counter-rotating circulation vortices
within
a square or rectangular cross-section and a length equal to the length of the
pond.
In some embodiments, each of these circulation cells extends through
substantially
the entire depth of the pond, top to bottom, and together the circulation
cells cover
substantially the entire volume of the pond.
39
CA 02917497 2016-01-13
[0104] For natural convection, induced by liquid body-force-viscous-
force
instabilities, cell (a) is more common than cell (b) for natural convection
arising
from bottom heated pond heat transfer. Nonetheless, for some crystallization
ponds
there may be an advantage for configuration (b) because it favors a
unidirectional
net flow along the axis of each parallel circulation cell for a solution
supply feed
inlet to the pond while the outlet is distributed along the floor of the pond
where
crystals are deposited and where crystal harvesting occurs, for example as
described below with reference to crystal vacuum harvesting device 150. In
contrast, in embodiments using hexagonal circulation cells, incoming salt-
containing solution 32 is supplied relatively evenly across the bottom of pond
20
and in some embodiments is removed by a crystal vacuum harvesting device 150
as outlined below, and in such embodiments there is no net flow of solution
across
the pond. Hence, the temis "inlet end" and "outlet end" as used with respect
to
ponds having parallel circulation cells are not applicable to such embodiments
having hexagonal circulation cells.
[0105] In some embodiments having parallel or hexagonal circulation
cells, the amount of input energy required to sustain a given circulation rate
is
approximately equal to the minimum rate of viscous energy dissipation in the
solution 32 in pond 20.
[0106] Release of bubbles continuously or at regular intervals at a
controlled rate and allowing the bubbles to rise to the top surface of pond 20
in a
manner that entrains the surrounding solution provides a flow that forms
circulation cells that are stable at low Reynolds number. In some embodiments,
the release of bubbles is not continuous. In some embodiments, the release of
bubbles is approximately steady. In some embodiments, the rate of flow of
CA 02917497 2016-01-13
circulation-enhancing bubbles is chosen to sustain the required bulk flow
rotational
rate of the circulation cells. These cells can cover the entire pond volume
and can
cause the temperature differences from the top to the bottom of pond 20 to be
controlled to a small variance at any distance from the solution feed inlet
end 44.
The circulation cells can also enhance the bottom and top surface cooling and
evaporation.
[0107] With reference to Figures 8A and 8B, respectively, bubble
injection
apparatus 110A for injecting bubbles to produce hexagonal circulation cells
and
bubble injection apparatus 110B for injecting bubbles to produce parallel
circulation cells are illustrated. Each parallel circulation cell has a pair
of counter-
rotating vortices with rectangular cross-section, as illustrated in Figures 9A
and
9B. Elements that perfoini the same function in apparatus 110A and 110B are
referred to with identical reference numerals.
[0108] Bubble injection apparatus 110, including apparatus 110A/110B,
includes a plurality of bubble injection tubes 112 for releasing bubbles that
rise
through the salt-containing solution 32 to induce the development of large
natural
circulation cells and to sustain their motion within pond 20. The circulation
cells
so formed have an effective surface plane vortex diameter nearly equal to
about
two times the depth of the pond (i.e. each vortex is approximately equal to
the
pond depth), and in some embodiments, cover the entire pond. The presence of
natural circulation cells produced by apparatus 110AJ110B may reduce the power
required to operate the air compressors that feed bubble injection tubes 112,
for
example, as compared with the power that would be required to operate the air
compressors that feed bubble injection tubes 112 if the apparatus was not
configured to induce formation of natural circulation cells.
41
CA 02917497 2016-01-13
[0109] In the illustrated embodiment of Figure 8B, bubble injection
tubes
112 are generally equally spaced and extend generally parallel to one another.
Bubble injection tubes 112 are positioned on the bottom 22 of pond 20, below
bottom screen 36 (Figure 7). In some embodiments in which cooling apparatus 50
is used together with bubble injection apparatus 110, bubble injection tubes
112 are
positioned above tubes 60 so that tubes 60 do not interfere with the release
of
bubbles. In some embodiments, bubble injection tubes 112 are embedded within
pea gravel 34, but are positioned very close to the lower surface of bottom
screen
36 so that the bubbles do not have to travel a significant distance through
pea
gravel 34.
[0110] In the embodiment of a bubble injection apparatus 110A
illustrated
in Figure 8A, hexagonal circulation cells are produced by the positioning of
two
sets of parallel, spaced apart bubble injection tubes 112A, 112B near the
bottom of
pond 20. A first set of bubble injection tubes 112A extend in a generally
diagonal
direction across pond 20. Bubble injection tubes 112A are spaced apart at
approximately equal intervals and extend generally parallel to one another in
a
plane to form first and second opposite sides of a hexagon. A second set of
bubble
injection tubes 112B extend across pond 20 in the same plane as but at an
angle to
bubble injection tubes 112A to form third and fourth opposite sides of a
hexagon.
Bubble injection tubes 112B are spaced apart at approximately equal intervals
and
extend generally parallel to one another. Apertures 118 are provided at spaced
apart intervals on bubble injection tubes 112 so that the apertures on
adjacent
portions of bubble injection tubes 112A, 112B define four sides of a hexagon,
as
shown within dashed hexagonal outline 120 for illustrative purposes. In some
embodiments, the fifth and sixth sides of hexagon 120 are provided by the warm
42
CA 02917497 2016-01-13
solution supply system 130, described below. In some embodiments, the
hexagonal circulation cell can be produced by bubble injection tubes 112A,
112B
alone (i.e. with no additional flow component providing the fifth and sixth
sides of
hexagon 120). In some embodiments, apertures 118 provided on bubble injection
tubes 112 could be used to define all six sides of hexagon 120 by providing an
additional set of parallel bubble injection tubes (not shown) intersecting
tubes
112A and 112B.
[0111] In some embodiments having hexagonal circulation cells, the waini
salt-containing solution 32 is supplied directly by an inlet within each
hexagonal
cell. In some such embodiments, the wainl solution is supplied directly
through an
inlet located at approximately the center of each hexagonal cell. In some such
embodiments, waini solution is introduced only at or near the center of each
hexagonal cell.
[0112] In the embodiment of a bubble injection apparatus 110B
illustrated
in Figure 8B, parallel circulation cells, each comprised of two counter-
rotating
circulation vortices within a rectangular cross-section with a length equal to
the
length of pond 20, are produced by the positioning of one set of bubble
injection
tubes 112C that are spaced apart at approximately equal intervals and extend
generally parallel to one another in a direction parallel to the path of the
mean cell
flow or travel from the inlet end 44 to the outlet end 46 of pond 20.
[0113] A parallel circulation cell produced by the apparatus shown in
Figure 8B is shown in greater detail in Figures 9A and 9B, which illustrates
typical
streamlines or particle path lines in solution flow downstream of the inlet
flow.
Each parallel circulation cell has two rotating helical flows, each resulting
in the
43
CA 02917497 2016-01-13
net circulation of fluid in a helical path from the inlet end 44 to the outlet
end 46 of
pond 20. One helical flow vortex 100 rotates in a clockwise direction, and the
second helical flow vortex 102 rotates in a counterclockwise direction. Each
of
helical flow vortices 100 and 102 extends from substantially the bottom 22 to
the
top surface 40 of pond 20 (i.e. throughout the pond depth 104). When the
streamlines are viewed in helical axis elevation view (Figure 9B), in some
embodiments they appear elliptical or circular, depending on the depth 104 of
the
pond 20 relative to the width of the circulation cell. In some embodiments, at
the
interfaces between each vortex 100, 102, the sides, top and bottom of the
outside
surface of each vortex are nearly square, as shown in Figure 9B.
[0114] In some embodiments, a number of complete parallel
circulation
cells (N) are formed, each of which has one parallel circulation clockwise
vortex
100 and one counter-clockwise vortex 102. In some embodiments, a half
circulation cell (i.e. having only a clockwise vortex 100 or a counter-
clockwise
vortex 102) is provided at one or both sides of pond 20.
>
[0115] In some embodiments, the circulation rate is measured at one
or
more locations within pond 20. As illustrated in Figure 9A, one or more
sensors
106 can be provided at predetermined locations within pond 20 to measure the
flow rate to evaluate the circulation rate within pond 20. Examples of sensors
that
can be used to measure the pond circulation include velocity meters, flow
meters,
turbine flow meters, or particle position change sensors. In some embodiments,
flow rate sensors 106 provides feedback to controller 64 that is used to
control the
operation of some or all of the features of pond 20.
[0116] With reference to Figure 7, one or more air compressors 114
pumps
44
CA 02917497 2016-01-13
compressed air through bubble injection tubes 112. In some embodiments, a
dryer
116 is used to dry the supplied compressed air before it passes into bubble
injection tubes 112. In some embodiments, a controller is provided to control
the
amount of compressed air supplied by air compressors 114. In some embodiments,
the amount of air supplied by air compressors 114 is controlled based on the
need
to sustain a selected circulation rate in the pond. In embodiments utilizing
hexagonal circulation cells, it is anticipated that the feed rate of incoming
warm
salt-containing solution 32 will not provide as much inlet momentum and mixing
as for embodiments utilizing parallel circulation cells, for which the supply
feed
inlet flow will provide most or all of the vortex momentum needed to initiate
circulatory cell flow. For both parallel and hexagonal circulation cells, once
the
circulation cells have been generated, the amount of air supplied by air
compressors 114 can be reduced to the level required to sustain circulation in
each
circulation cell at a rate that will result in a good production rate of
crystals.
[0117] Circulation within a vortex or within the pond 20 generally can
be
measured in any suitable manner, for example using velocity meters, flow
meters,
turbine flow meters, or particle position change sensors.
[0118] The spacing between bubble injection tubes 112 is selected to
enhance the formation of and to sustain the natural convection cells that
circulate
the salt solution throughout pond 20. In some embodiments, the spacing between
bubble injection tubes 112 is selected to minimize the auxiliary energy input
required to create and sustain fluid flow of the convection cells. Apertures
118 of
a predetermined size, which in some embodiments is optimized empirically, are
positioned along the length of bubble injection tubes 112 to provide bubble
flows
that define the shared interfaces between adjacent circulation cells as the
bubbles
CA 02917497 2016-01-13
rise to the surface 40 of pond 20. The rising bubbles cause an upward buoyancy
shear stress on adjacent circulation cells, which results in a cell
circulation flow
with an axis of rotation that is parallel to the bottom 22 or top 40 surface
of pond
20 for any vertical cross section through solution 32. The number and length
of
circulation cells for any given pond 20 can be deteimined using simulation
studies,
model studies, laboratory scale studies, or a combination thereof.
[0119] In some embodiments, the size of bubbles released by bubble
injection tubes 112 is used to regulate the solution water vapor evaporation
rate.
Small bubbles tend to form rafts of small bubbles that can accumulate on the
surface 40 of pond 20 and reduce the surface heat loss rate when these rafts
are
sufficiently large relative to the pond surface area. Large bubbles tend to
rise to
the surface 40 of pond 20 more quickly, and cause greater momentum transfer to
the liquid and enhanced mixing of solution 42 and greater circulation rates
and
vertical shear forces in the circulation cells.
[0120] In some embodiments, the size of bubbles released by bubble
injection tubes 112 is regulated by regulating the size of apertures 118. With
reference to Figure 10, in some embodiments, a set of apertures 118 are
provided
to produce bubbles of a size sufficient to establish and maintain the flow of
solution 32 in circulation cells, and a second set of apertures 122 are
provided to
produce small bubbles that can float to and remain on the surface 40 of pond
20 to
provide surface rafts (or a surface foam). In the illustrated embodiment,
apertures
118 are provided on a first bubble injection tube 112D and apertures 122 are
provided on a second bubble injection tube 112E extending parallel and
adjacent to
tube 112D. In some embodiments, the flow of compressed air through bubble
injection tubes 112D and 112E can be independently controlled, to allow for
46
CA 02917497 2016-01-13
injection of bubbles having a desired size at appropriate times.
[0121] In some embodiments, bubble injection tubes 112 comprise a pair
of bubble injection tubes bound together in a side-by-side fashion. For
example, in
the embodiment illustrated in Figure 10, bubble injection tubes 112D and 112E
are
parallel to one another and are bound together by suitable fasteners 124. The
use
of two adjacent bubble injection tubes 112 can provide greater control over
momentum flux in solution 32 and/or the accumulation of surface bubbles on the
surface 40 of pond 20. In some embodiments, the size of apertures 118 and/or
122
provided on each of the two adjacent bubble tubes can be different or varied,
to
provide a broader final mixed bubble diameter range. In some embodiments, the
flow of air through each adjacent bubble tube can be independently controlled,
to
facilitate greater control over the circulation rate within each circulation
cell. In
some embodiments, the flow of air through each adjacent bubble tube is
regulated
by controller 64.
[0122] In some embodiments, these adjacent bubble injection tubes 112
can be used to produce either parallel circulation cells or hexagonal
circulation
cells by passing air only through the bubble injection tubes 112 having a
configuration that will result in production of the desired circulation cell.
In
embodiments having hexagonal circulation cells, apertures 118 are
intermittently
distributed along bubble injection tube 112. In some embodiments having
parallel
circulation cells, apertures 118 are substantially continuously spaced apart
along
bubble injection tube 112.
[0123] In some embodiments, bubble injection tubes 112 having large
apertures 118 and/or small apertures 122 are operated without producing
47
CA 02917497 2016-01-13
circulation cells to enhance the evaporation rate of pond 20. In some such
embodiments, one or more of the location of bubble injection tubes 112,
apertures
118 and/or apertures 122 and the timing of release of bubbles are randomly
selected.
[0124] The flow rate of compressed air supplied by air compressors 114
is
selected to meet the circulation requirements of pond 20 to enhance the rate
of
formation of salt crystals 38. In some embodiments, the rate of flow of
compressed air supplied by air compressors 114 is controlled to provide a
desired
flow rate of salt-containing solution 32 in pond 20. In some embodiments, the
flow rate of compressed air supplied by air compressors 114 is controlled by
controller 64. In some embodiments, physical modeling, numerical modeling,
simulation studies and/or laboratory scale studies are used to select the
desired
flow rate for the salt-containing solution.
[0125] Without being bound by theory, the circulation cells provided by
bubble injection apparatus 110 can provide an increase in the evaporation rate
as
the warm liquid salt-containing solution 32 is brought to the surface 40 by
the
bubble flow, which induces the adjacent liquid to flow up to the top surface
where
the saturated bubbles release water vapor directly into the air. Thus, the
induced-
flow warm-solution evaporates more quickly at the surface. Without being bound
by theory, it is believed that bubble flow rates are directly related to the
solution
water evaporation rates.
[0126] In some embodiments, small apertures such as apertures 122 are
used to produce small bubbles. In some embodiments, small bubbles produced
through small apertures 122 can be used to form rafts of small bubbles (e.g.
in the
48
CA 02917497 2016-01-13
nature of a surface foam) that can reduce surface heat loss rate when the
rafts are
sufficiently large to cover all or most of the surface 40 of pond 20. In
contrast, the
larger bubbles produced by apertures 118 will rise more rapidly to the
surface,
causing enhanced mixing of solution 32, greater circulation speeds within the
circulation cells, and vertical shear forces on the circulation cells. The
rate of
production of both small bubbles through small apertures 122 and large bubbles
through apertures 118 can be controlled in any suitable manner, for example by
regulating the volume flow rate of compressed air supplied by air compressor
114.
In some embodiments, desirable ranges of diameters for small bubbles and large
bubbles are determined empirically. In some embodiments, a non-toxic foaming
agent is added to pond 20 to stabilize the surface foam provided by rafts of
small
bubbles.
[0127] In some embodiments, an evaporation suppressing surface liquid is
added to pond 20 to decrease the rate of evaporation from pond 20. Examples of
evaporation suppressing surface liquids include oils, including organic oils.
In
some embodiments, the oil is a plant-based oil, such as canola oil, corn oil,
soybean oil, flax oil, or the like.
[0128] In some embodiments, the circulation requirements of pond 20 are
determined partly by the need to decrease at a selected rate the average pond
temperature, and/or by the need to decrease the temperature variations at any
particular distance from the solution supply inlet in each circulation cell,
and/or by
the need to cause surface evaporation with time and to maintain a
predetermined
average salt solution concentration within the metastable region adjacent the
equilibrium saturation line on the phase diagram for the salt solution
throughout a
large region of the pond.
49
CA 02917497 2016-01-13
[0129] In some embodiments, apparatus is provided to measure the
temperature and/or salt concentrations at one or more locations in pond 20. In
some embodiments, apparatus is provided to measure the temperature and/or salt
concentrations at one or more locations within one or more typical circulation
cells
within pond 20. Any suitable apparatus to measure temperature can be provided,
for example, a thermocouple, thermistor, resistance temperature detector, or
the
like. In some embodiments, any metal components of such apparatus are not in
direct contact with salt solution 32. Any suitable apparatus to measure salt
concentration can be provided, for example, a salt meter using an optical
refractometer or a conductivity meter or a liquid density meter.
[0130] As shown in Figure 4B, one or more temperature sensors 66 can be
provided at different locations around pond 20. In some embodiments, the
temperature sensors are provided at a plurality of different depths within
pond 20,
as an alternative or in addition to being provided at a plurality of different
positions
across the length and width of pond 20. In some embodiments, data collected by
temperature sensors 66 is returned to controller 64 and used to control the
operation of cooling apparatus 50, auxiliary cooling system 80, bubble
injection
apparatus 110, warm solution supply system 130, and/or crystal vacuum
harvesting
device 150. In some embodiments, a reading is taken from one or more
temperature sensors placed within pond 20 continuously or periodically.
[0131] As shown in Figure 7, one or more devices for measuring the salt
concentration in solution 32, shown schematically as 68, can be provided at a
plurality of different locations in pond 20, including at different depths
within pond
20. In some embodiments, the salt concentration is determined by continuously
or
CA 02917497 2016-01-13
periodically removing samples of solution 32 from pond 20 at one or more
different locations via a sample tube and then measuring the salt
concentration
using meters located outside pond 20. In some embodiments, the salt
concentration measured by devices 68 or measured in a sample taken from pond
20
is returned to controller 64 and used to control the operation of cooling
apparatus
50, auxiliary cooling system 80, bubble injection apparatus 110, warm solution
supply system 130, and/or crystal vacuum harvesting device 150. In some
embodiments, information regarding both temperature and salt concentration at
various locations within pond 20 is used by controller 64 to control the
operation
of cooling apparatus 50, auxiliary cooling system 80, bubble injection
apparatus
110, waini solution supply system 130, and/or crystal vacuum harvesting device
150. In some embodiments, information regarding temperature and/or salt
concentration at one or more locations within a single circulation cell is
used by
controller 64 to control the apparatus as aforesaid. In some embodiments,
infoimation regarding temperature and/or salt concentration at one or more
location within a single vortex of a parallel circulation cell is used by
controller 64
to control the apparatus as aforesaid.
[0132] In some embodiments, the number and location of apparatus for
measuring temperature 66 and/or salt concentrations 68 is determined for a
given
pond through model pond measurements and/or simulation studies. In some
embodiments, each sensor and its sampling method is periodically calibrated.
[0133] In some embodiments, a controller 64 is provided that receives
feedback from the apparatus for measuring temperature 66 and/or salt
concentrations 68 at one or more locations throughout pond 20. In some
embodiments, the controller 64 controls the various operating parameters of
pond
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CA 02917497 2016-01-13
20 based on such feedback, for example by adjusting the rate of cooling
provided
by cooling apparatus 50 by adjusting the flow rate of coolant 52 therethrough,
by
adjusting (e.g. turning on or off) the operation of auxiliary cooling system
80 or
sub-cooling apparatus 90, by adjusting the rate of air supplied by air
compressor
114 to bubble injection apparatus 110, by adjusting the rate of air supplied
by air
compressor 114 to bubble injection tubes 112 to adjust the rate of production
by
small apertures 122, by adjusting the rate of inflow of wain' salt-containing
solution 32 through warm solution supply system 130 described below, and/or by
regulating the rate of harvesting of salt crystals 38 by crystal vacuum
harvesting
device 150 described below.
[0134] In one example embodiment, the temperature and salt concentration
are measured in at least one region of pond 20. The temperature and salt
concentration measured are returned to controller 64. Controller 64 then
determines based on the known metastable zone width whether the measured
conditions of temperature and salt concentration are within the metastable
zone
width. In some embodiments, controller 64 determines whether the measured
conditions of temperature and salt concentration are within a desired portion
of the
metastable zone width, for example, at least 2 C higher than the
supersaturation
limit and at least 2 C lower than the equilibrium saturation line. As used
herein, a
reference to determining whether conditions are within the metastable zone
width
includes determining whether conditions are within a selected portion of the
metastable zone width.
[0135] In one embodiment, if controller 64 determines that, given the
salt
concentration measured in the region of pond 20, the temperature is too high
relative to the metastable zone width or a desired portion thereof, for
example, the
52
CA 02917497 2016-01-13
temperature is above the metastable zone width or within about 2 C of the
equilibrium saturation line, active cooling via apparatus 50 is initiated
and/or
increased for that region of the pond by controller 64. In some embodiments,
the
amount of coolant 52 passed through tubing network 54 is adjusted based upon
the
difference between the measured temperature and the metastable zone width for
the measured salt concentration in the region of pond 20. For example, more
coolant will be passed through tubing network 54 in the region of the pond if
the
temperature is above the equilibrium saturation line than if the temperature
is near
or just below the equilibrium saturation line. In some embodiments, the rate
of
cooling of pond 20 is adjusted for any given region of the pond 20 or for pond
20
as a whole depending on how far the measured temperature is from the coolest
temperature that still falls within the metastable zone width for the measured
concentration of salt, so that the farther the temperature is above the
coolest
temperature within the metastable zone width, the greater the rate of active
cooling. In some embodiments, if controller 64 determines that the temperature
is
too high relative to the mestastable zone width, and if the temperature
measured at
a second location within pond 20 is too low relative to the metastable zone
width
or is within a desired portion of the metastable zone width, the rate of flow
of air
through bubble injection tubes 112 is increased to increase the circulation
rate
within pond 20. In some embodiments, controller 64 takes some or all of the
foregoing actions.
[0136] In one
embodiment, if controller 64 determines that, given the salt
concentration measured in the region of pond 20, the temperature is too low
relative to the metastable zone width, operation of active cooling via
apparatus 50
is decreased and/or stopped for that region of the pond by controller 64. In
another
embodiment, if controller 64 determines that, given the salt concentration
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CA 02917497 2016-01-13
measured in the region of pond 20, the temperature is too low relative to the
metastable zone width, the flow of warm salt-containing solution 32 through
warm
solution supply system 130 is increased. In another embodiment, if controller
64
determines that the temperature is too low relative to the mestastable zone
width
and the measured air atmospheric air temperature is lower than the temperature
of
solution 32, controller 64 activates the introduction of small bubbles via
small
apertures 122 into pond 20 to fat in a surface foam. In another embodiment,
if
controller 64 determines that the temperature is too low relative to the
metastable
zone width and a measurement of temperature at a second location within pond
20
is within a desired region of the metastable zone width, the rate of air flow
through
bubble injection tubes 112 is increased to increase the circulation rate in
the pond
20. In some embodiments, controller 64 takes some or all of the foregoing
actions.
[0137] In some embodiments, circulation rates of salt-containing
solution
32 within the circulation cells are chosen so that the conditions within the
crystal
growth region in the pond can be selected to be as optimal as practical for
each
operating and weather condition while maintaining production of high quality
crystals and producing a selected size range of crystals. Operating conditions
can
be optimized through mathematical and/or laboratory modeling and simulation of
the bubble induced and maintained circulation flows provided by bubble
injection
apparatus 110. Larger scale tests may be conducted using selected scaling
factors
to assist in the design and operation of full scale ponds 20.
[0138] In some embodiments, during periods of prevailing low
atmospheric temperature (e.g. during winter), the rate of cooling at the
surface of
pond 20 tends to be increased by both wind speed and surface to air
temperature
difference. In some embodiments, during periods of prevailing low atmospheric
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CA 02917497 2016-01-13
temperature, the rate of sub-surface cooling of pond 20 is decreased. In some
embodiments, the rate of cooling at the surface of pond 20 is decreased by
flooding
the surface of pond 20 with small bubbles to produce a foam. In some
embodiments, the foam so produced covers most of the surface of pond 20. In
some embodiments, the small bubbles used to produce the foam are produced by
bubbling air through small apertures 122 in bubble injection apparatus 110. In
some embodiments, the determination of whether a surface foam should be
produced on pond 20 is made by a person assessing prevailing conditions. In
some
embodiments, weather conditions with low ambient air temperatures and strong
winds are most likely to lead to higher-than-desired cooling rates for the top
surface of pond 20, and under such conditions deployment of an upstream wind
barrier (e.g. wind barrier 70) and/or formation of a surface foam are likely
to be
desirable.
[0139] In some embodiments, a non-toxic foaming agent is added to pond
20 to increase the surface tension for bubbles. In some embodiments, addition
of a
foaming agent increases the lifespan of the bubbles, helping to retain a
surface
foam on pond 20. Example foaming agents that may be used in some
embodiments include octadecylamine (ODA), dodecylamine (DDA), sodium
dodecyl sulphate (SDS), polyphenylsulfone (PPSF), carboxylated polysulfone
(CPSF), and the like.
[0140] In some embodiments, the rate of cooling at the surface of pond
20
is decreased by providing a wind suppression device, shown schematically in
Figure 7 as 70, such as a wind barrier along at least a windward side of the
surface
of pond 20. In some embodiments, the wind barrier is a wind suppression fence
or
a physical barrier (e.g. in the nature of a snow fence) that reduces the wind
speed
CA 02917497 2016-01-13
above and across the surface 40 of pond 20. In some embodiments, the wind
speed
adjacent to the pond surface 40 is reduced by deploying a wind suppression
device
along at least a windward side of pond 20.
[0141] In some embodiments, the wind barrier reduces the wind speed
over the pond surface by a factor of two or more, and reduces the convective
cooling rate at the surface of pond 20 by a similar ratio. In some
embodiments, a
wind barrier is used if design data for a particular crystallization pond 20
shows
that typical wind speeds in the region of the particular crystallization pond
20 are
likely to cause excessive cooling rates.
[0142] In some embodiments, the wind suppression device, e.g. a wind
barrier, is controllable, i.e. can be erected or taken down in an automated
fashion
either upon receiving a manual signal and/or a signal from a controller. In
some
embodiments, the wind barrier is controlled by controller 64. In some
embodiments, an apparatus for measuring prevailing wind speed and direction
provides feedback to controller 64, which raises or lowers wind barrier 70 on
the
basis of such feedback.
[0143] In some embodiments, the wind suppression device, e.g. a wind
barrier, is used with crystallization ponds that are not too large (i.e. do
not have a
length and/or width that is too large) relative to the height of the wind
barrier. In
some embodiments, the ratio of the pond length or width to the height of the
wind
barrier is less than 10, including any value between 1 and 10, e.g. 2, 3, 4,
5, 6, 7, 8
or 9. For example, in one exemplary embodiment, if the wind barrier has a
height
of 3 m, then the pond length and width should be 30 m or less.
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CA 02917497 2016-01-13
[0144] In some embodiments, pond 20 is operated in a batch process. In
some such embodiments, an active cooling apparatus 50, auxiliary cooling
system
80, sub-cooling apparatus 90, bubble injection apparatus 110, cover 65 and/or
wind
barrier 70 are provided, and are operated as described in this specification
to
control the conditions within pond 20. In some embodiments, pond 20 is
operated
in a continuous process. In some such embodiments, a system is provided for
supplying waini solution to pond 20 and a system is provided for removing
excess
solution and produced crystals from pond 20.
[0145] In some embodiments, as illustrated in Figures 11A and 11B, a
warm solution supply system 130 is provided to distribute warm incoming salt-
containing solution 32 (e.g. as obtained from a solution mining operation)
approximately equally to each pond circulation cell in pond 20. In some
embodiments, solution is supplied through warm solution supply system 130 at a
salt concentration such that the solution 32 is close to saturation conditions
for its
particular supply temperature.
[0146] In some embodiments, warm solution supply system 130 is used
together with parallel circulation cells in pond 20. In such embodiments, a
plurality of bottom inlet flow spreaders 132 are connected to a bottom inlet
solution supply pipe network 134. In some embodiments, bottom inlet flow
spreaders 132 are apertures foimed within pipe network 134. Bottom inlet
solution
supply pipe network 134 receives warm salt-containing solution 32 from a
supply
pipe 136, which receives input from the source of salt-containing solution 32
for
pond 20. In some embodiments, supply pipe 136 is fed from the output of a
potash
solution mine. In some embodiments, supply pipe 136 also receives chelant from
a
chelant supply source, to provide a desired concentration of chelant within
pond
57
CA 02917497 2016-01-13
20. In some embodiments, supply pipe 136 also receives recycled salt crystals
38
(for example, salt crystals 38 that are considered too small or low quality to
be sent
for further processing) and salt-containing solution 32.
[0147] In the illustrated embodiment of Figures 8A and 8B, bottom inlet
solution supply pipe network 134 (shown as 134A and 134B, respectively) is
positioned above bubble injection tubes 112. Bottom inlet flow spreaders 132
(shown as 132A and 132B, respectively) are oriented to inject salt-containing
solution 32 upwardly into pond 20 at a small angle with respect to the
vertical
direction, so that the resulting flow of solution will have both horizontal
and
vertical components of momentum along the length of each circulation cell. In
some embodiments, bottom inlet flow spreaders 132 are located on opposite
sides
of each pond circulation cell as compared with the apertures 118 of bubble
injection apparatus 110.
[0148] Bottom inlet flow spreaders 132 and bottom inlet solution supply
pipe network 134 may be made from any suitable material, for example, plastic.
In
some embodiments, bottom inlet solution supply pipe network 134 is clamped to
bubble injection tubes 112.
[0149] In one example embodiment illustrated in Figure 11A, the bottom
inlet flow spreaders 132C of warm solution supply system 130A are configured
to
supply incoming waini salt-containing solution 32 at approximately equally
spaced-apart intervals. In some embodiments having parallel rectangular
circulation cells, inlet flow spreaders 132 extend parallel to bubble
injection tubes
112. In some embodiments having hexagonal circulation cells, at least one
inlet
flow spreader 132 is positioned within each circulation cell 120. In some such
58
CA 02917497 2016-01-13
embodiments, one inlet flow spreader 132 is positioned at the center of each
circulation cell 120. In some such embodiments, air is bubbled in to the
circulation
cell at or near the center of the circulation cell, for example through bubble
injection tubes 112 (not shown in Figure 11A).
[0150] In an example embodiment having parallel rectangular circulation
cells within pond 20 illustrated in Figure 11B, the bottom inlet flow
spreaders
132D of warm solution supply system 130B are configured to supply incoming
salt-containing solution 32 at spaced-apart intervals along bubble injection
tubes
112 in the first approximately 10% to 15% of the length of pond 20 at the
inlet end
44 of pond 20. The remaining 85% to 90% of the length of pond 20 is dominated
by crystal deposition flux and, in some embodiments as described below, vacuum
harvesting of such crystals by a crystal vacuum harvesting device. Once
formed,
salt crystals 38 tend to sink toward the bottom 22 of pond 20, where they are
deposited in an accumulative layer on bottom screen 36.
[0151] In some embodiments, the size and number of inlet flow spreaders
132 is selected to provide exponentially decreasing flow rates at each inlet
flow
spreader 132 location farther from the inlet end 44 of pond 20. In some
embodiments, the flow of salt-containing solution 32 through inlet flow
spreaders
132 enhances the natural flow for adjacent circulation cells (i.e. carries the
incoming salt-containing solution 32 toward the surface 40 where it is cooled
by
ambient air cooled convection and evaporation), and has a net flow along each
circulation cell from the inlet end 44 of the pond 20 to the other end of the
circulation cell at the outlet end 46 of pond 20.
[0152] In some embodiments, warm solution supply system 130 is used in
59
CA 02917497 2016-01-13
the absence of a bubble injection apparatus 110.
[0153] In some embodiments, for example that shown in Figures 12A, 12B
and 12C, waim solution supply system 130 delivers wailil salt-containing
solution
32 to pond 20 via two different and potentially alternative mechanisms. As
described above, solution 32 is supplied through the bottom 22 of pond 20 by a
bottom inlet solution supply pipe network 134. Solution 32 is additionally or
alternatively supplied throughout the depth of pond 20 at inlet end 44 by one
or
more secondary distribution nozzles that extend through at least a portion of
the
depth of pond 20. In the illustrated embodiment, the secondary distribution
nozzle
is a rotary inlet 138. Additionally, a recycle supply pipe 137 feeds recycled
crystals and/or solution 32 (for example, as recovered from vacuum harvesting
device 150 described below) to supply pipe 136. In some embodiments, rotary
inlets 138 are used together with parallel circulation cells within pond 20.
[0154] With reference to Figure 12B, rotary inlet 138 has a solution
supply
tube 140, which has one or more apertures 142 for allowing solution 32 to flow
into pond 20 as a stream. Rotary inlet 140 is coupled through a rotatable
coupling
144 to supply pipe 136.
[0155] In use, as described above, bottom inlet flow spreaders 132 are
used
to initiate and mix warm salt-containing solution 32 into each circulation
vortex
within pond 20. In some embodiments having parallel circulation cells, rotary
inlet
138 is additionally or alternatively used to induce flow throughout each
circulation
cell. In such embodiments, solution supply tube 140 is rotated and solution is
pumped therethrough by pump 146 so that solution 32 exits apertures 142 and
induces flow.
CA 02917497 2016-01-13
[0156] In some embodiments having parallel circulation cells, the
initial
flow induced by warm solution supply system 130 persists for about one
rotation
of the circulation vortex. Without being bound by theory, beyond this region,
the
effects of friction on the bottom surface of the pond and viscous dissipation
within
the vortex due to the nearly square elevation shape of the vortex would
dissipate
the momentum imparted by warm solution supply system 130. In some
embodiments, such dissipation of the inlet rotational energy is partly or
fully
counterbalanced by mechanical energy supplied by bubbles from bubble injection
apparatus 110 rising through solution 32 on one of the vertical sides of each
vortex.
In some embodiments, the amount of rotational energy to be supplied by warm
solution supply system 130 and the bubble flow rate through bubble injection
apparatus 110 are determined by scale model experimental and/or simulation
studies.
[0157] As illustrated in Figure 12C, in embodiments having parallel
circulation cells, a pair of solution supply tubes 140 can be provided in each
circulation cell to induce flow along each outer side of the circulation cell
(illustrated by dashed outline 148). Solution 32 entering pond 20 through
rotary
inlet 138 is directed horizontally so that the stream of liquid output by each
rotary
inlet 138 is directed to a vertical supply jet impact baffle plate 149. The
momentum of the stream of liquid output by one rotary inlet 138 for one
circulation vortex is thus balanced by the stream of liquid output by the
adjacent
rotary inlets 138. In some embodiments, the length of baffle plate 149 is
equal to
the axial distance (i.e. the distance in the direction of arrow 45 in Figure
9A)
traveled across one rotation of the circulating vortex particles. The angular
rotational speed of each vortex is approximately equal for any circulation
vortex in
61
CA 02917497 2016-01-13
the pond 20 at the inlet end 44. Thus, the momentum of the stream of liquid
output
by rotary inlet 138 will increase from zero at the center of the vortex to a
maximum near the outer perimeter of each vortex. In some embodiments, this
result is achieved by varying the spacing or diameter of the stream of liquid
output
by rotary inlet 138, or a combination thereof, to provide a rotational
momentum
distribution for a viscous flow rotation with approximately the lowest rate of
energy dissipation for the stream of liquid output by rotary inlet 138. In one
embodiment having parallel circulation cells, the circulation vortices are
approximately square in cross section (i.e. the depth of the pond is
approximately
1/2 of the width of each parallel circulation cell).
[0158] In some embodiments, solution supply tube 140 extends across
substantially the entire depth of pond 20. In some embodiments, including the
illustrated embodiment, solution supply tube 140 extends across only
approximately the top half of the depth of pond 20, or across some other
portion
thereof, e.g. the top one third, top two thirds, top quarter, or the like.
[0159] With reference to Figure 13, through the use of cooling apparatus
50, auxiliary cooling system 80, bubble injection apparatus 110, and/or warm
solution supply system 130, conditions such as temperature and/or salt
concentration can be controlled throughout the volume of pond 20. Accordingly,
in contrast to the exemplary prior art crystallization pond illustrated in
Figure 1A,
pond 20 illustrated schematically in Figure 13 has only small regions 18 and
21
where conditions are outside a desired temperature and/or salt concentration
within
the metastable zone width region. In region 18, the concentration of salt may
be
slightly lower than in the rest of pond 20, and the temperature may be
somewhat
cooler than the rest of pond 20. In region 21, the salt concentration may be
62
CA 02917497 2016-01-13
somewhat higher than in the rest of pond 20, and the temperature may be
slightly
higher or slightly lower than the rest of pond 20, depending on whether
cooling
apparatus 50 is being operated. Most of the solution 32 in pond 20 has a
relatively
uniform temperature and/or salt concentration, illustrated as a relatively
large
middle region 19. Moreover, because the temperature, salt concentration and/or
rate of cooling can be controlled to remain within the metastable zone width,
most
of the solution 32 is within the metastable zone width region 23.
[0160] In some embodiments, the harvesting of salt crystals 38 and salt-
containing solution 32 from pond 20 is conducted on a continuous or
approximately continuous basis. In some such embodiments, the warm solution
supply system 130 supplies a volume of warm salt-containing solution 32 that
is
approximately equal to the volume removed by harvesting, such that the volume
flow rate of solution 32 into pond 20 is approximately equal to the volume
flow
rate of solution 32 out of pond 20.
[0161] In some embodiments, at least a portion of the warm salt-
containing
solution 32 harvested from pond 20 together with salt crystals 38 is re-
injected
back into pond 20 through warm solution supply system 32. In some
embodiments, at least a portion of the warm salt-containing solution 32
harvested
from pond 20 together with salt crystals 38 is re-injected back into an
underground
solution mining operation. The decision to return a portion of the harvested
solution 32 back to pond 20 or into a solution mine, or to direct the
harvested
solution to waste, can be made based on the remaining fraction of the desired
salt
still remaining in the harvested solution 32. In some embodiments, although a
portion of the harvested warm salt-containing solution 32 is re-injected back
into
pond 20 or back into a solution mine, the overall size of crystallization
ponds
63
CA 02917497 2016-01-13
required to handle the output of a given solution mining operation can still
be less
than would be required using previous crystallization apparatus and methods.
[0162] In some embodiments in which a waini solution supply system 130
is used together with bubble injection apparatus 110, the warm solution supply
system 130 is configured to distribute waiin salt-containing solution 32
approximately equally to each circulation cell within pond 20.
[0163] In some embodiments, the warm solution supply system 130 is
used to inject salt-containing solution on a substantially continuous basis so
that
cooling pond 20 can be operated in a continuous (rather than a batch) process.
[0164] In some embodiments, a crystal vacuum harvesting device 150 is
provided to harvest salt crystals 38 from the bottom of pond 20. Salt crystals
38
tend to be more dense than salt-containing solution 32. For example, crystals
of
KC1 have a density that is approximately 20% to 50% higher than the
surrounding
solution during crystallization. Thus, as salt crystals 38 grow in size, they
will
drop toward bottom screen 36 with a free fall rate that is predictable in a
stationary
pond (e.g. the free fall rate is very slow for small particles, and this free
fall rate
increases with increasing particle diameter). In embodiments in which
circulation
is provided in pond 20, the rate of free fall will be greater in regions of
pond 20
experiencing a down-flow, and less in regions of pond 20 experiencing an up-
flow.
[0165] With reference to Figure 14, crystal vacuum harvesting device 150
has a flow-driven sweeper reel 152 that helps feed deposited salt crystals 38
into a
vacuum intake 154 of vacuum harvesting device 150. Vacuum intake 154 feeds
material to vacuum tube 156, which is connected to a liquid and crystal vacuum
64
CA 02917497 2016-01-13
system pump 158 at the surface 42 adjacent pond 20.
[0166] Vacuum harvesting device 150 has set of wheels 160 that are
positioned to support part of the weight of vacuum harvesting device 150 on
bottom screen 36. Wheels 160 facilitate movement of vacuum harvesting device
150 around the bottom 22 of pond 20 on screen 36.
[0167] In the illustrated embodiment, a pond float 162 is coupled to
vacuum tube 156. Pond float 162 helps to at least partially support the weight
of
the vacuum tube 156 in the salt-containing solution 32.
[0168] In some embodiments, pond float 162 is used to guide the vacuum
intake 154. In one example embodiment, a corrosion-protected pond harvester
system is provided to guide vacuum harvesting device 150 around the bottom 22
of
pond 20 by moving pond float 162. For example, one or more plastic-coated
positioning guide cables can be provided above the surface 40 of pond 20 and
operated from the edges of the pond at the surface 42 to move device 150
around
pond 20.
[0169] In some embodiments, crystal vacuum harvesting device 150
moves along the bottom 22 of pond 20, on top of bottom screen 36, to harvest
produced crystals. In some embodiments, crystal vacuum harvesting device 150
moves in generally parallel paths. In some embodiments, crystal vacuum
harvesting device 150 moves in paths that generally cover the entire bottom 22
of
pond 20. In some embodiments, crystal vacuum harvesting device 150 is moved
over the bottom of the pond 20 so that over one traversing cycle, the entire
bottom
surface of the pond is traversed. Since the deposition depth of salt crystals
38 on
CA 02917497 2016-01-13
bottom screen 36 is expected to vary across the pond bottom (e.g. within each
circulation cell) and along the mean flow direction for supply feed flow,
variations
in the deposition depths may be significant. Thus, in some embodiments, the
feed
velocity of the vacuum harvesting device 150 is controlled by controlling the
operation of vacuum system pump 158. In some embodiments, the vacuum force
caused by vacuum system pump 158 is increased at locations within pond 20
where it is anticipated that salt crystals 38 will accumulate to a greater
depth, and
decreased at locations within pond 20 where it is anticipated that salt
crystals 38
will accumulate to a lesser degree.
[0170] In some embodiments, the position, direction of motion, and rate
of
advance of vacuum intake 154 are externally controlled by a controller. In
some
embodiments, the controller is controller 64 that receives feed back from
temperature sensors 66 and/or apparatus for measuring salt concentration 68
located at a plurality of different spatial locations within pond 20. In some
embodiments, the operation of vacuum harvesting device 150 are determined by
one or more of: the mean depth variation of crystal deposition at any given
time;
the maximum depth of crystal deposition at any time; the average rate of
crystal
deposition at any given time; the variation in rate of crystal deposition over
time;
the maximum rate of crystal deposition at any given time; expected diurnal
variations in crystal deposition; or expected weather-related variations in
any of the
foregoing factors. In some embodiments, the operation of vacuum harvesting
device 150 is regulated to minimize the energy expended per unit of crystal
production based on measured conditions such as salt concentration,
temperature,
or the rate of cooling at selected locations throughout pond 20.
[01711 Crystal vacuum harvesting device 150 harvests salt crystals 38,
66
CA 02917497 2016-01-13
together with salt-containing solution 32 that is sucked up by vacuum intake
154,
and transports this mixture to the surface 42. The salt crystals 38 are then
partially
separated from the liquid fraction, for example by gravity separation in
settling
vessel 166, to achieve a high concentration of crystals in the produced
crystal
solution 164. Excess liquid from the liquid fraction, as well as salt crystals
that are
smaller than a desired size, can be returned to pond 20 as part of the supply
solution (for example, through wailil solution supply system 130), or can be
pumped to a different pond for further crystallization. The produced crystal
solution 164 is conveyed for further processing, e.g. to a processing plant
(not
shown) through a suitable outlet pipe.
[0172] In some embodiments, excess liquid obtained from settling vessel
166 is used as a carrier for crystals in produced crystal solution 164, is
pumped
back to pond 20, is pumped to a different crystallization pond, or is pumped
underground (e.g. by being returned to a solution mine). The deteimination of
where excess liquid should be sent can be made based on the amount or fraction
of
solution needed for pumping and transporting the produced crystals in crystal
solution 164 over a known distance and elevation changes to a processing
plant,
and the concentration of salt ions remaining in the excess liquid. In some
embodiments, parameters such as the concentration of salt ions remaining in
the
excess liquid and the size of crystals in produced crystal solution 164 are
periodically measured to deteimine the most efficient use for the excess
liquid. In
some embodiments, the amount of excess liquid that is combined with produced
crystal solution 164 is periodically adjusted to maintain approximately a
fixed
mass fraction of liquid:crystal solids to facilitate transportation of the
produced
crystals to the processing plant.
67
CA 02917497 2016-01-13
101731 The crystal vacuum harvesting device 150 can be operated to
continually harvest salt crystals 38 from the bottom of pond 20. For example,
when crystal vacuum harvesting device 150 has completed one traverse of the
bottom of pond 20, it can initiate a new traverse of the bottom of pond 20 to
initiate another crystal harvesting cycle. The cycle rate and flow rate for
vacuum
harvesting device 150 can be controlled based on the need to control the depth
of
crystals 38 foiming on the bottom screen 36 of pond 20, and/or to reduce any
inter-
crystal bonding or caking among salt crystals 38 deposited on bottom screen
36. In
some embodiments, caking among perfect crystals is avoided by controlling the
solution conditions in the region of the deposited crystals 38 to be very
close to
saturation conditions for the salt-containing solution 32, and/or to be
slightly
cooler than the remainder of the pond. Caking is expected to be most
problematic
in situations where imperfect crystals are formed and deposited in contact
with one
another, and further are surrounded by a highly supersaturated solution.
[0174] In some embodiments, one crystal vacuum harvesting device 150 is
provided in a single pond 20. In some embodiments, two or more crystal vacuum
harvesting devices 150 are provided in a single pond 20.
[0175] At the processing plant, the two phases of the produced crystal
solution 164, liquid waste and salt crystal product particles, are
mechanically
separated. The salt crystal product particles are dried to yield the desired
crystalline product, while the liquid waste is disposed of in any suitable
manner,
for example delivery to another salt pond, pumping back underground for
deposition and mixing with underground solution flows to dissolve more of the
desired salt, or the like.
68
CA 02917497 2016-01-13
[0176] In some embodiments where the salt-containing solution 32 is
produced from a potash mine, the resulting liquid waste is primarily a weak
solution of NaC1 with a residual concentration of KC1.
[0177] In some embodiments, some or all of the rate of cooling, salt
concentration gradients, or circulation within pond 20 is controlled.
Controlling
the rate of cooling, salt concentration gradients, and/or circulation within
crystallization ponds can increase the production rate, quality and/or size of
salt
crystals produced by a particular crystallization pond. For example, it is
desirable
that KC1 crystals be transparent and have a cubic morphology, and crystals
that
have these characteristics would be considered to be of higher quality. Higher
quality crystals can have a higher market value than crystals of lower
quality.
[0178] The formation of salt crystals from a salt-containing solution
are
non-equilibrium, locally time-dependent processes. Although it is accepted
practice to show the metastable region of crystallization on a phase diagram,
these
processes occur only in quasi-equilibrium conditions because, for example,
crystallization of KC1 requires the simultaneous diffusion of I(' and Cr ions
toward each nucleation site for crystal growth, and the diffusion of H20 away
from
these sites. Also, the heat of the crystallization phase change to form KC1 is
diffused away from these sites. These diffusion processes are time dependent
and
coupled. They will differ from site-to-site and over regions of a
crystallization
pond, and especially along the length of the pond from supply inlet to the
outlet
end of the pond. Models can be developed to help understand the
crystallization
processes occurring within the crystallization pond, and to assist in
controlling the
conditions within the pond to enhance the foimation of salt crystals therein.
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CA 02917497 2016-01-13
[0179] In developing analytical/numerical models for crystallization
ponds, it is convenient to refer to the bulk mean properties [i.e. temperature
T(K),
solution salt concentration C, particle crystal size dp(mm), particle crystal
concentration Cc, solution velocity V(n-i/s), particle crystal velocity
Vp(m/s)] of a
circulation cell as the planar averaged values at any position, x, along the
parallel
bulk mean flow paths of each vortex from inlet to outlet. For steady supply
flows
and operating conditions these bulk mean properties are expected to be only a
function of x. Within the circulation cells there will be a cyclic variation
of
properties as the solution mixture rotates about an axis of rotation and the
solution
moves toward the top or bottom. This circulation rate Rc (rpm) is another
variable.
Modelling these processes will require complex numerical models for systems of
equations, which may be quasi-steady at any point in the pond. In order to
keep the
numerical analysis tractable, it will be necessary to make justifiable
assumptions
about the boundary conditions and how they vary with time. The relative size
of
each term is important when using a volume averaging method of modeling for
the
solution space for the set of governing physical balance equations (i.e.
continuity
of each chemical species, energy balance including phase change, solar gains
and
heat transfer, and momentum including gravitational and internal fluid shear
forces).
[0180] When several salts are present in the same solution, the
metastable
region for each salt is different. These are each somewhat complex
crystallization
processes particularly when they are coupled with more than one
crystallization
process and several salts in the solution. There will be some three-
dimensional
liquid and crystal movements that will change over time and space for a
crystallization pond. Laboratory models may be used to quantify the solution
processes. Expanded scale models may require theoretical/numerical models and
CA 02917497 2016-01-13
simulations to determine the optimum dimensionless parameters for each
operating
condition. Thus, it is anticipated that both physical scaled modeling and
model
analysis and simulations may be used to develop models that can be used to
better
control crystallization within a crystallization pond. Particularly when
chelating
agents are added to the solution to better control the size of the metastable
region
for enhanced crystal growth and size distribution and increased rate of
production
of KC1 crystals, these relationships are not linear and there is coupling
between
temperature and circulation flow controls.
101811 High quality crystallization of salt from a salt-containing
solution in
a crystallization pond occurs only in a metastable crystallization region of
temperature and salt concentration. The variables that can be controlled to
maintain the salt-containing solution within the metastable zone and thereby
control the crystallization rate and crystal quality and size distribution
include the
cooling rate of the salt-containing solution, the circulation rate of the salt-
containing solution within the pond, and the concentration of any chelating
agent
added to the crystallization pond. In some embodiments, the cooling rate of
the
salt-containing solution is controlled by controlling the rate of coolant
supply to
cooling apparatus 50 by pumps 58. In some embodiments, the circulation rate of
the salt-containing solution within the pond is controlled by controlling
either or
both of the amount of air supplied to bubble injection apparatus 110 by air
compressor 114 or the rate of supply of solution 32 to watin solution supply
system 130. In some embodiments, the concentration of a chelating agent added
to
a crystallization pond 20 is controlled by adding a desired concentration of
the
chelating agent to incoming salt-containing solution.
[0182] In some embodiments, the concentration of chelating agent added
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to crystallization pond 20 is sufficient to give a final concentration in pond
20 in
the range of 300 and 3000 ppm, including any value therebetween, e.g. 500,
1000,
1500, 2000 or 2500 ppm. Lower concentrations of chelant can be used in
embodiments where the concentration of impurities in salt-containing solution
32
is lower.
[0183] Other factors that may influence the rate and quality of crystal
formation in a crystallization pond include the depth of the pond, the
evaporation
rate from the top surface of the pond, local pond temperature and salt
concentration, and the gradients of the temperature and salt concentration at
that
location, relative velocity of the pond liquid and the salt crystals, the feed
rate of
incoming salt-containing solution and the removal rate of salt crystal product
and
solution. Thus, in some embodiments, the average pond depth is controlled. In
some embodiments, the rate of removal of salt crystal product and solution by
crystal vacuum harvesting device 150 is controlled.
[0184] With reference to Figure 15, a schematic diagram of a method 200
for controlling an exemplary embodiment of pond 20 is shown. At 202, the
temperature of solution 32 in pond 20 can be measured at one or more
locations,
for example using temperature sensors 66A, 66B, 66C disposed at a plurality of
different locations within pond 20. At 204, the concentration of salt in
solution 32
in pond 20 can be measured at one or more locations, for example using
apparatus
for measuring solution salt concentration 68A, 68B, 68C disposed at a
plurality of
different locations within pond 20, or by assaying solution samples withdrawn
from different locations within pond 20. At 206, wind speed and/or direction
can
be measured. At 208, an assessment of the forecast can be made to determine if
precipitation (e.g. rain, snow or hail) is anticipated and if so, how much,
and/or
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current conditions can be assessed to determine if precipitation is presently
occurring. At 210, the dew point temperature of the external atmosphere can be
deteunined. At 212, atmospheric humidity can be determined. At 213, current
atmospheric temperature can be determined. At 211, the temperature of a source
of groundwater used by auxiliary cooling system 80 can be detelmined. At 209
atmospheric pressure can be measured. At 207, the level of solar irradiation
occurring or expected to be occurring can be assessed. At 205, the circulation
rate
within one or more circulation cells within the pond can be assessed, for
example
using appropriately placed velocity meters, flow meters, turbine flow meters,
or
particle position change sensors.
[0185] At 214, input from some or all of the foregoing steps can be used
to
determine how to operate the control features of pond 20. In some embodiments,
a
controller is provided at 214 to receive input from the foregoing steps and
control
some or all of the features of pond 20, including as described below. In some
embodiments, some or all of the features of pond 20 are controlled to maintain
substantially all of solution 32 in pond 20 within the metastable zone width
(MSZW) region. In some embodiments, some or all of the features of pond 20 are
controlled to minimize the auxiliary energy demand rate associated with the
operation of pond 20. In some embodiments, some or all of the features of pond
20 are controlled to both maintain substantially all of solution 32 in pond 20
within
the metastable zone width and minimize the auxiliary energy demand rate
associated with the operation of pond 20.
[0186] In some embodiments, some or all of the control features of pond
20 are adjusted in a slow manner and on a somewhat continuous basis (i.e.
abrupt
and/or significant changes in the operation of the control features of pond 20
are
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avoided). In some embodiments, for example where the conditions within pond 20
deviate significantly from ideal crystallization conditions within the
metastable
zone width region, some or all of the control features of pond 20 are
regulated
more aggressively, and more rapid and/or extreme changes in the operation of
some or all control features of pond 20 can be used to return conditions
within
pond 20 to the metastable zone width region throughout as much of the volume
of
pond 20 as possible.
[0187] At 216, the operation of cooling apparatus 50 is controlled to
provide a desired rate of cooling of solution 32 within pond 20, including at
specific regions within pond 20 if desired. For example, if the solution
temperature measured at 202 is above the metastable zone width for the salt
concentration measured at 204, or is above a desired region of the metastable
zone
width (e.g. is higher than 2 C less than the equilibrium saturation line for
the salt
concentration measured at 204), the rate of cooling provided by cooling
apparatus
50 can be increased, or cooling apparatus 50 can be activated, at least at the
region
where the temperature was measured.
[0188] In some embodiments, the rate of cooling provided by cooling
apparatus 50 is increased (or cooling apparatus 50 is activated) only if the
ambient
air temperature measured at 213 is warmer than a predetermined amount below
the
temperature of the solution measured at 202, e.g. warmer than about 30 C below
the temperature of the solution measured at 202. In some embodiments, cooling
apparatus 50 is activated only if the ambient air temperature is above about
10 C
(e.g. 15 C, 20 C, 25 C, 30 C, 35 C, 40 C or higher).
[0189] In some embodiments, the rate of cooling provided by cooling
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CA 02917497 2016-01-13
apparatus 50 is adjusted depending on how far the temperature measured at 202
is
from the metastable zone width for the salt concentration measured at 204. For
example, cooling apparatus 50 could be activated to 50% cooling capacity if
the
temperature measured at 202 is close to the equilibrium saturation line
temperature, but could be activated to 100% cooling capacity of the
temperature
measured at 202 is more than 5 C above the equilibrium saturation line
temperature.
[0190] In some embodiments, if the solution temperature measured at 202
is below the metastable zone width for the salt concentration measured at 204,
or is
below a certain predetermined portion of the metastable zone width, e.g. less
than
2 C above the supersaturation limit concentration for nucleation of crystals
for the
salt concentration measured at 204, the operation of cooling apparatus 50 is
reduced or stopped at 216.
[0191] At 218, the operation of auxiliary cooling system 80 can be
controlled to achieve a higher degree of cooling of coolant 52 within cooling
apparatus 50 if desired, for example based on the temperature difference
between
solution 32 and the external atmosphere, and/or the relative humidity of the
atmosphere.
[0192] At 220, the operation of sub-cooling apparatus 90 can be
controlled, for example based on the temperature difference between the
external
atmosphere measured at 213 and a source of groundwater used for auxiliary
cooling system 80 measured at 211. In some embodiments, sub-cooling apparatus
90 is operated only if the temperature difference between the source of
groundwater measured at 211 and the atmospheric air measured at 213 is at
least
CA 02917497 2016-01-13
15 C. In some embodiments, sub-cooling apparatus 90 is operated only if the
temperature of the source of groundwater used for auxiliary cooling system 80
rises close to or above the typical air dew-point temperature of the ambient
atmosphere.
[0193] At 222, operation of bubble injection apparatus is controlled. In
some embodiments, including the illustrated embodiment, the flow of air
through
large apertures 118 can be controlled at 224 independently of the flow through
small apertures 122, controlled at 226.
[0194] In some embodiments, the flow of air through large apertures 118
controlled at 224 is controlled based on the concentration of salt and/or
temperature measured at a plurality of different locations within pond 20 at
steps
202, 204, for example to provide a faster rate of flow when different regions
of
pond 20 are at appreciably different temperatures and/or salt concentrations.
For
example, if the temperature measured at a first location in pond 20 at step
202 is
more than 2 C or more than 5 C different than the temperature measured at a
second location in pond 20, the flow of air through large apertures 118 may be
increased at 224 to increase the circulation rate in pond 20 and thereby
reduce
temperature differences across the pond.
[0195] In some embodiments, if the temperature measured at a first
location in pond 20 at step 202 is either above the metastable zone width or
above
a predetermined region of the metastable zone width (e.g. higher than 2 C less
than
the saturation line temperature or below 2 C more than the supersaturation
limit for
nucleation of crystals) for the salt concentration measured at 204, but the
temperature measured at a second location in pond 20 at step 202 is within the
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CA 02917497 2016-01-13
metastable zone width (or the predetermined region of the metastable zone
width),
the flow of air through large apertures 118 may be increased at 224 to
increase the
circulation rate in pond 20.
[0196] In some embodiments, if the salt concentration measured at a
first
location in pond 20 at step 204 is appreciably different than the salt
concentration
measured at a second location in pond 20, the flow of air through large
apertures
118 may be increased at 224 to increase the circulation rate in pond 20.
[0197] In some embodiments, the flow of air through small apertures 122
is controlled at 226 to provide rafts of small bubbles that form a foam on the
top 40
of pond 20, for example to reduce the rate of cooling at the surface 40 of
pond 20
or to reduce the rate of evaporation from pond 20. In some embodiments, the
flow
of air through small apertures 122 is initiated or increased when the
atmospheric
temperature measured at 213 is more than 40 C below the temperature of
solution
32 measured at 202, or when the atmospheric humidity measured at 212 is less
than 70%. In some embodiments, the flow of air through small apertures 122 is
stopped or decreased when the atmospheric temperature measured at 213 returns
to
less than 40 C below the temperature of solution 32 measured at 202, or when
the
atmospheric humidity measured at 212 returns to greater than 70%.
[0198] At 228, operation of warm solution supply system is controlled.
In
some embodiments, the flow of warm salt-containing solution 32 through bottom
inlet solution supply pipe network is controlled independently of the flow of
warm
salt-containing solution 32 through rotatable solution supply tube 140. In
some
embodiments, the flow of warm salt-containing solution 32 is increased if
there are
significant differences in temperature and/or salt concentration measured at
77
CA 02917497 2016-01-13
different locations within pond 20 at 202, to increase the rate of circulation
within
pond 20. In some embodiments, if the temperature measured at a region of pond
20 at 202 is below the metastable zone width for the salt concentration
measured at
204, or below a desired region of the metastable zone width, e.g. 2 C above
the
supersaturation limit for crystal nucleation, the flow of warm salt containing
solution 32 thorough rotatable supply tube 140 and/or bottom inlet flow
spreaders
132 is increased to increase the temperature of pond 20. In some embodiments,
if
the temperature measured at a region of pond 20 at 202 is above the metastable
zone width for the salt concentration measured at 204, or above a desired
region of
the metastable zone width, e.g. higher than 2 C below the equilibrium
saturation
line for the salt concentration measured at 204, the flow of warm salt
containing
solution 32 through rotatable supply tube 140 and/or bottom inlet flow
spreaders
132 is decreased.
[0199] At 230, the operation of crystal vacuum harvesting device 150 is
controlled. In some embodiments, harvesting device 150 is controlled to remove
solution at approximately the same rate that solution 32 is introduced by warm
solution supply system 130. In some embodiments, the operation of crystal
vacuum harvesting device 150 is controlled to spend more time harvesting
crystals
at locations along the bottom of pond 20 where a higher salt concentration is
measured, and/or where a higher volume of deposited crystals 38 is expected to
form based on anticipated flow patterns within pond 20, and/or where a higher
depth of deposited crystals 38 occurs.
[0200] In some embodiments, when it is determined that the depth of pond
20 should be increased, the rate of solution supply by warm solution supply
system
130 is increased at 228 and/or the rate of removal of solution by harvesting
device
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CA 02917497 2016-01-13
150 is decreased at 230. In some embodiments, when it is determined that the
depth of pond 20 should be decreased, the rate of solution supply by warm
solution
supply system 130 is decreased at 228 and/or the rate of removal of solution
by
harvesting device 150 is increased at 230. In some embodiments, once the pond
20
has reached a desired depth, the rate of solution supply by system 130 and the
rate
of solution removal by harvesting device 150 are adjusted to be approximately
the
same.
[0201] At 232, operation of cover 65 is controlled. For example, where
precipitation is expected or occurring at step 208, cover 65 can be extended
to
cover some or all of the surface 40 of pond 20. Where precipitation has
stopped
and/or is forecast to stop, cover 65 can be retracted to expose the surface 40
of
pond 20 to the atmosphere.
[0202] At 234, operation of wind suppression device 70 is controlled.
For
example, where the wind speed measured at 206 indicates significant wind is
occurring (e.g. greater than about 20 km/h or greater than about 30 km/h),
wind
barrier 70 can be erected on the upwind side of pond 20. In some embodiments,
wind barrier 70 is erected manually. In some embodiments, wind barrier 70 is
erected automatically, for example by a controller activating a mechanism to
raise
wind barrier 70. In some embodiments, the terrain surrounding pond 20 (e.g.
the
presence of hills or other natural wind barriers) is taken into account in
determining when wind barrier 70 should be used. In some embodiments, wind
barrier 70 is erected only if the atmospheric temperature measured at 213 is
above
or below a predetermined value, e.g. below 0 C or above 30 C.
[0203] At 236, the depth 104 of pond 20 is controlled. In some
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CA 02917497 2016-01-13
embodiments, the depth 104 of pond 20 is increased by increasing the flow of
solution 32 through warm solution supply system 130 and/or decreasing the rate
of
flow of solution 32 and crystals 38 through vacuum harvester 150. In some
embodiments, the depth 104 of pond 20 is decreased by decreasing the flow of
solution 32 through warm solution supply system 130 and/or increasing the rate
of
flow of solution 32 and crystals 38 through vacuum harvester 150. In some
embodiments, the depth 104 of pond 20 is altered based on prevailing weather
or
climate conditions. In general, larger diameter vortex flows present in a
deeper
pond will respond more slowly to temperature changes on the top and bottom
surfaces than will smaller diameter vortices present in a shallower pond.
[0204] At 238, the flow of recycled crystals to wann solution supply
system 130 through recycle supply pipe 137 is controlled. In some embodiments,
crystals harvested by crystal vacuum harvester 150 are screened, and any
crystals
that pass through the screen together with the solution 32 sucked up by
crystal
vacuum harvester 150 are wholly or partially redissolved and returned to pond
20
via a pump or pumps used for recirculation via recycle supply pipe 137.
[0205] At 240, the concentration of chelant in the pond is controlled,
for
example by adding additional chelant to pond 20 and/or to inlet supply pipe
136.
In some embodiments, chelant is added to pond 20 from a chelant supply tank
129
(Figure 12A).
[0206] At 242, a non-toxic foaming agent is added to the pond, for
example in situations where it is desired to foini a surface foam as described
above.
CA 02917497 2016-01-13
[0207] At 244, an evaporation suppressing surface oil is added to the
pond,
for example to reduce the evaporation rate from the surface of the pond.
[0208] It is anticipated that in some embodiments, the expected
production
rate of quality crystalline salt (e.g. kg of KC1 per m2 of pond and time s
(kg.m-2.s-
1)) can be more than doubled for a given depth of pond, and that the size and
quality of the salt crystals obtained can be increased significantly over
previous
methods.
[0209] While a number of exemplary aspects and embodiments are
discussed herein, those of skill in the art will recognize certain
modifications,
permutations, additions and sub-combinations thereof. It is therefore intended
that
the following appended claims and claims hereafter introduced are interpreted
to
include all such modifications, permutations, additions and sub-combinations
as
are within their broadest interpretation consistent with the specification as
a whole.
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CA 02917497 2016-01-13
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