Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
This invention rela~es to a process and appara~us
particularly applicable to the mining industry by which
pa~ticles are separated according to density.
BACKGROUND OF THE INVENTION
Typically gold ore is composed of up to a few ounces
of gold per tone of host medium. Gold has a dQnsity oE about
17 gmJcm3, and silica, a typical host medium, has a density
of about 3 gm/cm3. Traditionally gold ore has been crushed
into fine particles in a mill, then a cyanide leaching process
is used to chemically remove the gold contained in the crushed
ore. Once the gold is remo~ed the crushed ore or tailings (as
they are now called) are disposed of. Invaria~ly ~his method
is inefficient in that not all the gold is removed. The
primary reason for this inefficiency lies in the fact that not
all the gold is exposed to the leaching ac~ion due to
inadequate crushing. This inadequate crushing is due to
interference caused by the smaller particles protecting the
larger particles which require further crushing.
Forms of an elutriation techni~ue have been in use in
laboratories for decades where particles are sorted according
to diameter, as in a ~Cyclo-sizer~ (Trademark of Warman
International)O
The other major form of density separation previously
used employed a stationary high densi~y slurry into which
- 1 -
"
material to be separated was simply dropped then vibrated.
This batch method of density separa~ion has proven to be too
slow and expensive to be used on a large scale.
Two Canadian patent~ of interest relating to the use
5 o~ fluids to separate mixed material6 aLe Canadian Patent No.
102,673 of Trottier i6sued DecembQr 18, 1906 and Canadian
Patent No. 373,878 o Remick issued May 17, 1938. The Trottier
reference describes and illustrates a single column apparatus
for separating at differe~t levels in the column mate~ial of
constant size but of different density. The column contai~s a
plurality of vertically spaced 60rting tables and collection
vanes which act to sort materials according to density and
permit higher density material6 to fall while lighter density
mater;als are drawn off at appropriate heights within the
column do~n-shoots. The Remick patent describes an apparatus
and proce~s for separating low density materials such as coal
from a higher density host material such as slate. The mixed
materials are int~oduced to a tank provided wi~h an u~wardly
flowing current of water which carries particles of lesser
s~e~ific gravity, such as coal, upwardly and out of the upper
end o~ the tank onto an overflow wier, where they are collec~ed
on a screen. ~gain the Remic~ apparatu~ is inef~icient over a
large range of particle sizes.
The object of the invention i~ to apply a unique,
controlled, combined elutria~ion screening technique to density
separation~, pArticularly applicable to gold and diamond
tailings in order to economically recover previo~ly
unavailable valuable ma~erials.
SUMM~RY OF THE INVENTION
According to the present invention, there i8 pro~ided
a counter-flow sedimentation separator and process designed to
separate mixed aggregate materials by den6ity frac~ioning.
The invention embodie~ a process which utilizes both
the classifying effect of a column o~ fluid, such as wa~er,
flowing vertically and standard size screening techniques used
in an alternating sequence. This process begins by introducing
the particulate material, to be separated, into an upward
flowing column of water known as an ~lutriation Column. The
rate of flow of the water will depend on ~he size and density
characteristics of the particles to ba separated, bu~ a
velocity of about 20 cm~s would be typical. The high density
underflow is retained, and the low density ~or smaller size)
overflow is introduced into the next lower velscity classifying
flow which allows smaller high densi~y particles to settle
out. The underflow is screened, such that the material which
is underflow due only to its relatively large size ~ecomes the
di~carded oversize, the undersize being of high densi~y is
retained. The overflow from this second classifying column is
introduced into the next Elutriation column which again has a
lower flow rate~ and the underflow is sub3ec~ed to a screening
-- 3 --
~ ~6~
process with a smaller mesh SiZ2. Again it is the undersized
underflow which is retained and the overflow continues to be
introduced to lower velocity flows and finer mesh screening
proces~es as described until the overflow is so fine that it is
believed to contain very little gold.
The invention also embodies an apparatus consisting of
a series of elutriation columns with decreasing flow rates and
increasing column diameters such that the overflow from each
column is introduced into the next column o~ the series. This
apparatus employs a constant feed and can effect separations at
a high rate with little expense. In a preferred embodiment,
the apparatus is further desi~ned to introduce the material to
be separated into a cyclon~ mixing chamber in order to break up
any large lumps of tailings which might erroneously become
under10w if not broken in~o particulate componen~s.
This apparatus according to the present in~ntion
allows large quantities of tailings ~o be processed and high-
graded. The highgraded tails can then be returned ~o ~he
extraction plant where more gold can be extracted using ~radi-
tional techniques. In order to effect the se2aration across
~he spectrum ~f particle sizes. multiple elutriation columns
are coupled with screening techniques such tha~ the retained
material is both under~low in the elu~ria~ion column and under-
size with respect to the screening process.
-- 4
BRIEF D~SCRIPTION O~ TH~ DRAWINGS
In drawings which illustrate example embodiments of
the invention:
FIGURE 1 is a graph illustrating typical theoretical
design considerations used to set flow rates, decide how man~
columns should be employed, and decide where screening pro-
cedures should be employed in the apparatus and process of the
present invention;
FIGURE 2 is a cross-sectional schematic view of a
typical elutriation column of an example apparatus according to
the present invention; and
FIGURE 3 is a schematic representation of an example
four-stage elutriation process according to the present
lnvention.
While the invention will be described in conjunction
with an example embodiment, it will be understood that it is
not intended to limit ~he invention to such embodiment. On the
contrary, it is intended to cover all alternatives, modifi-
cations and equivalents as may be included within ~he spirit
and scope of the invention as defined by the appended claims.
DETA~LED DESCRIPTION OF~THE IMVENTION
The elutriation process described here refers to a
type of dynamic counter-flow sedimentation in which the sep-
~~` ~æ~
aration medium (liquid) is moved rather than the solid matterto be separated. Aggregate materials and tailings materials of
restricted si~e ranges, generally less than 20 mesh, are
ideally su;ted for elutriator sep~ration. The purpose of the
design is to fab~icate a heavy density concentrate (underflow
material) from a mixed aggregate material; the lighter density
waste material is swept away by the upward flow of a separation
medium such as water or air while the heavy density valuable
material i~ allowed to fall into an entrapmen chamber for
collection.
Theoretically, a particle of diameter "d~ and density
" Ps", will fall at a velocity "v" through any liquid o~
density "~Q " and viscosity "~", as indicated by the equation:
d 9 (PS - P~)
lg~
where "g" is the acceleration due to gravity. In an
elutriation process, the variable "v" is used to set ~he flow
rate of the liquid column such that particles of a required
density "Ps" and diameter "d" will be at terminal velo~ity
within the liquid and thus su~pended. Particles of greater
diameter or density will drop to the bot~om as underflow and
particles of a lesser size or density will be washed away by
the fluid column, as overflow. Thus this system can be used to
separate particles varying in size or density.
one probable use of the present invention would be to
% ~-
separate industrial grade diamonds from their host material as
present separation techniquas often miss many of the smaller
diamonds which are still of value in industrial applications.
The apearatus and process discussed here with respect to gold
tailings would most likely require somewhat higher flow rate
(up to several meters per second~ and slightly larger mesh
screening ope~ations based on the di~ferent size and density
characteristics of diamond tailings.
Turning to FIGURE 1, ideally line "X" represents the
optimum separation cutoff line. This line is set just higher
than the true solids density oP the tailsO thus only host
material containing other higher density elements will settle
out. In practice, however. separations a6 along line "X" must
be approximated as indicated by the shaded area ahove lines 1
~hrough 4 where the areas above lines 1 through 4 depict the
product placement characteristics of the underflow for each
column in a series of four. The dashed lines, "A", "Bl', and
"C" indicate where screening processes are employed such ~hat
the oversize is discarded, and the undersize which is to the
righ~ of ~hese lines i5 retained for reprocessing. Thus ~he
entire shaded area in FI~URE 1 represen~s the size density
characteristics of the particles which will be re~ained. The
curved lines, 1 through 4, are calculated as based on the pre-
viously stated equation such that flow rate "v" is set and the25 density of particles of a given size is calcula~ed. Decreasing
~he flow ra~e has the effect of broadening the range of the
~%~
curve. All the curves asymptotically approach the density of
the separation medium itself. Thus as can be seen in FIGURE 1,
~he curves can be broadened to the right by decreasing the flow
such that at zero flow tha parabola i5 flattened into a
~trai~ht line along the density of the separation medium.
Similarly the vert;cal lines representing the screening proc2ss
can be adjusted in order to op~imize efficiency.
The number of stages ~o be used is also an important
consideration. The greater the number of stages the greater
the efficiency of the apparatus as a whole, howe~er, as the
overflow from each column is introduced to ~he subsequent
column, the inflow volume increases rapidly from one column to
the next. Very quickly siæe considerations limit the number of
stages to be used.
In the actual portion of the apparatus where the sep-
arations are being made, the ideal flow pattern is ~hat o~ plug
flow, (ie. the exact design velocîty across the entire cross-
section o~ the separation column) however in ~ractice this must
be app~oximated due ~o the finite viscosity of the separation
medium. Thus the higher the Reynold ' 8 number describing the
flow in the aolumn the more accurate the separation, and thus
the greater e~ficiency of the appara~u~ as a whole. Maximizing
the Reynold's number must however be weighed against restric-
tions of physical size. This problem becomes especially25 cLitical when dealing with the lo~er ~low rates. However as it
is improbable that the tradi~ional leaching process would
erroneously miss much of the gold associa~ed with minus 270
mesh particles, separa~ions this low in the spectrum need not
be carried out. Further, this lack of gold associated with the
veLy fine particles allows desliming of the slurry before it is
introduced to the first mixing chamber without adversely
affecting column e~ficiency. Th;s desliming process is useful
in that it removes the Eines and thus inhibits larger
conglomerate particles from corming as these particles would be
mistakenly retained.
FIGURE 2 i6 a ~chematic representation of a typical
appa~atus where the overflow from each column enters the mixing
chamber ~ of the next column 2. Each succe6sive column 2 also
has a lower flow rate produced by water pump 5 to allow finer
particles as underflow. Generally the system would be ~et to
concentrate the desired element by a factor of about 100, but
this can be altered.
In a further effort to break down large particles in
the mixing chamber 4 the slurry is introduced such that it will
cau~e a cyclone effect in the mixing chamber. The lower set of
baffles 8 prevent these currents from in~erfering with the
elu~riation action. The upper set of baffles 10 again reduce
the turbulence in the upper part of the mixing chamber 4. This
allows particles which should have become underflow, but were
caught up in the turbulence of the mixing chamber, to settle
back down to the elutriation column 12.
The apparatus repre6ented in FIGUR~ 3 comprises ~our
6~20
separate columns 2 like those in FIGURE 2 such that the over-
flow of each column is introduced to the mixing chamber of the
nex~ column in ~he series. The underElow to be screened is
removed from the undeLflow valve 14, at the bottom of settling
chamber 16 below each separation column 12.
As an alternative, the process may be carried out
essentially in reverse, ~hat is, using a series of screening
~rocedures, then making individual counter-flow sedimentation
density separations on the individual size fractions from each
screening procedure.
Typically ~he process of the invention as applied to
gold ore recovery employs 3 to 8 elutria~ion columns with flow
rates in the range of 100 to 0.3 cm/sec., as well as 2 to 7
screening operations usually between the 20 to 270 mesh sizes.
(For diamond tailings, the flow rates of the columns would be
in the range of 1000 to 1 cm/sec.~. Further, as the density
difference between diamonds and host material is often
relatively small, additional columns (perhaps up to ten) will
be required to achieve adequate separation efficiency.
Possibly the optimum approach would embody an initial screening
proee~s to separate the tailings into various size fractions,
which could ~hen be introduced in~o separate apparatus, with
the flow rates appropriate to that portion of the tailings. An
example apparatus of this type designed to process 100 tonnes
of gold tailings per hour would be comprised of four steel
columns standing side- by-side, each about 10 m in height and
~:~æ~
up to two metres in diameter. A system of these dimensions
would require a water supply with a capacity of about 2500
l/min and a h~ad of around 15 metres. This new appara-tus
allows the economical recovery of quantities of gold or other
a~propriate materials which until this time have been hidden ln
worthless tailinys.
The previously described apparatus could also be used
to highgrade material other than gold, given that tha required
material be of a different density than i~5 host ma~erial.
FIGURE 1 graphically represents a majori~y of the design
considerations pertinent to this apparatus. As stated earlier
the horizontal straight line labelled "X" represents the ideal
point at which the desi~ed separation is to be made. The
elu~riation highgrader could also be incorporated in a standard
extraction procedure on the ore itself ra~her than just the
tails. Finally in order to highgrade by a very large factor,
large quantities of underflow (after standard screening) could
be re-introduced to the apparatus. I~ association with this
system, a system for pumping and recycling the separation
medium, and a slurry feed system will also be required, however
the nature and design of these systems are obvious to a person
skilled in the art.
The single elutriation technique ha~ been employed in
the pa~t to effect classifications based on par~icle size with
a great deal o~ success as the terminal velocity of a particle
falling through a liquid varies directly with the square of the
o6~2~
particle diameter. Further, as most aggregate materials are of
very consis~ent densi~y, accurate size separations by
elutriation are ~uite simple. The apparatus and process
according to the prasent invention are unigue in that they
perorm separations based on particle density with little
regard to particle size. This system should prove to be
economically successful as the tailings which are to be
concentrated are abundant, pre-crushed, above-ground (and thus
far more accessible than ore), and can have considerable
concentrations of residual gold especially if the tailings were
processed years ago when the cyanide leaching process was less
refined. Furthermore, as water will probably be ~he most
common separation medium, the environmental and health problems
associated with the leaching proces6es used in gold e~traction
can be minimi2ed.
Thus it is apparent that ~here has been provided in
accordance with the invention coun~er-flow ~edimentation
separator that fully satisfies the objects, aim~ and advanta~es
set forth above. While the invention has been described in
conjunction wi~h a specific embodiment thereof, it is evident
that many alternatives, modi~ications and ~ariati~ns will ~e
apparent to those skilled in the art in light of the foregoing
description. Accordingly, it is in~ended to embrace all such
alternatives, modifications and variations as fall within the
spirit and broad scope of th~ appended claims.
