Note: Descriptions are shown in the official language in which they were submitted.
1 163782
It is necessary in many chemical process industries to separate
small diameter partlculate solids from their mother liquor. In the Bayer
process, for example, red muds residues are produced in the treatment of
bauxite with hot caustic sodium aluminate solution, and it is necessary to
separate these solids from liquor, clean the solids, and recover chemical
values from the solids. The separation and cleaning of the solids, which
includes iron oxides, sodium aluminum silicate, titanium oxide, and various
other oxides, however, is not a simple matter for the particulate solids
range from sand to collold size. Commonly, 90 percent of the solids particles
are less than 100 microns average diameter, and many are less than one micron.
Three methods are possible for the separation and cleaning of solids, and
recovery of chemicals from Bayer process liquors, viz. hydrocyclone centri-
fugal force separatlon, fllter and counter current decantation. Whereas
centrifugal force separatlon of a relatlvely coarse solids residue in hydro-
cyclones is possible, with washing of the solids particles in a mechanical
classifier (e.g. an Aklns spiral classifier) this method is ineffective
because of the very fine solids particles that comprise the bulk of the
residue. Filtering, with subsequent washing of the collected residue (e.g.
in pressure type leaf filters), is likewise unfeasible because of the high
labor and equipment costs. Counter current decantation is commercially
practical today in most alumina plants. This method too has many drawbacks.
In counter current decantation the red mud residues from the Bayer
process, after sand separation, are treated at temperatures near their
atmospheric boiling points in a large number of settling tanks, or vessels,
equipped with mechanical rakes to transport the settled mud along the
bottoms of the vessels to an outlet, or outlets. Chemical flocculating
agents are added to cause the solids particles to flocculate, or coalesce.
The mud solids usually constitute from about 5 to 7 wt.~ of the slurry feed
1 163782
to a settling vessel, the flocculated residues being retained in the bottoms
of the vessels until settling has resulted in solids densification in the
bottoms of the vessels. Commonly, the bottoms of the vessels are conical
and a single mud slurry outlet is located at the apex of the cone so that
the raking is accompanied by gravity assistance. The red mud residues are
washed by successive dllutions and concentrations of the mud residues.
Capital costs are quite considerable. Dilution washing of the red
mud solids in this manner too is very cumbersome, particularly since the
solids particles must be washed and settled with minimum dilution in order
to avoid impermissable disturbance of the water balance in the total process.
Mud densification, or mud solids concentration, require inordinately large
vessel volumes and long slurry residence periods in order to obtain acceptable
concentrations of the solids in the bottoms of the vessels. And all too
: often, a large number of washlngs are required to recover the valuable
chemicals from the solids. And too, the solids particles because of their
small size are difficult to clean.
It is, accordingly, the primary ob~ective of the present invention
to provide new and novel apparatus and process for the continuous hydraulic
elutriation, and washing of particulate solids.
A specific objective is to provide apparatus and process for the
treatment and recovery of particulate solids from muds, or slurries, cleaning
the solids, and recovering chemical values from the solids.
A further object is to provide apparatus and process of such
character which is particularly efficient in recovering and cleaning such
solids particles, and economical in terms of capital investment, and operat-
ing costs.
Yet another, and more specific object is to provide apparatus and
process, as characterized, which is particularly suitable for the washing,
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1 ~ 63782
elutriation, cleaning and recovery of solids from Bayer process muds, supra,
without any requirement for the use of apparatus for desanding such muds prior
to ~:reatment, or the use of mechanical raking apparatus, or the use of multi-
ple vessels, mud slurry pumps, flow measurement and flow control devices or
the like; and wherein better counter current washing of the solids can be
accomplished in a given vessel volume, because there is less loss of dissolved
alumina by recrystallization, because shorter residence times are required
for separation of solids from the liquor, and the expenditures for chemical
flocculants are appreciably reduced because of the elimination of the con-
ventionally used mud pumps and flow control valves.
These and other ob~ects are achieved in accordance with the present
invention, embodying apparatus and process, the apparatus embodiment compris-
lng an elongate vessel formed by relatlvely long enclosing side walls and
relatively short enclosing end walls, the cross-section of the vessel, taken
in a direction perpendicular to the ma~or axis of the vessel, (i.e. across
the width of the vessel) being preferably of substantially V-shape. The
upper slde, or sides of the ves~el are provided with liquid overflow weirs,
or launders. Suitably, the overflow weirs, or launders are compartmented and
aligned in series one ad~acent and contiguous with another. The bottom of
the vessel can be formed by physically bringing the two alternately disposed
relatively long slde walls together in true V-shaped cross-section, or a
substantially horizontally oriented floor can be provided. The bottom apex
of the vessel or its horizontal floor, includes an aperture, or apertures,
along its entire length communicating with a bottom liquid inlet enclosure,
which enclosure may be compartmented. In other words, the two side walls
can be brought together as a "V", or spaced apart and provided with a horizon-
tal floor and, in either event, provided with a bottom liquid inlet enclosure,
which may be compartmented.
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~ 1637~2
In all embodiments, a liquid and solids inlet, either a separate
liquid inlet, or inlets, or a common slurry inlet a~ provided at one end of
the vessel for introduction of the liquid and solids, or slurry. This inlet
or inlets is located at a position below the launders and above the bottom of
the vessel; suitably above the vessel bottom. An outlet, or outlets, for the
cleaned solids is provided at the opposite end of the vessel. This outlet
may be in an extension of the vessel which is not equipped with a bottom inlet
compartment or launders. Liquid overflowing into a launder, launders, or
series of launders, is withdrawn therefrom, suitably by the use of pumps, and
directly in~ected into the bottom of the vessel. Suitably, staging is pro-
vided by withdrawlng liquid from a launder, or from the last launder of a
series located relatively close to the clean solids outlet, and the liquid is
in~ected into a portion of the vessel bottom more distantly removed from said
clean solids outlet. Preferably, where the bottom liquid inlet enclosure is
compartmented, liquid is withdrawn from a launder relatively close to the
clean solids outlet, and in~ected into a bottom liquid inlet compartment more-
distantly removed from said clean solids outlet, or liquid inlet compartment
closer to the liquids and solids, or slurry, inlet at the opposite end of the
vessel. In its preferred form, the bottom liquid inlet enclosure is compart-
mented, launders are located on the two side walls of the vessel, and the
launders are equal in number to the vessel bottom compartments. In effect,
the net movement of liquid is in one horizontal direction, and solids flow in
the opposite horizontal direction. Some liquid, in net effect, is also flowed
continuously upwardly prior to removal from the vessel.
In terms of process, particulate solids, or a particulate contamin-
ated solids slurry, is introduced into an end of the vessel and contacted from
below with a vigorously agitated, ascending stream of liquid, the solids also
being swept upwardly but in net effect at a slower rate than that of the
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1 163782
ascending stream of cleaning liquid. The particulate solids, in a state of
hindered settling, are countercurrently washed and cleaned of contaminants by
the cleaning liquid, contaminants being displaced by the cleansing liquid.
In total effect, the cleaning liquid with dissolved contaminants and very
finely divided flnes solids particles flow upwardly into the launders, with
little or no carryover of particulate solids of significant size. In the
very bottom of the liquid pool the particulate solids undergo a vigorous
scrubbing action, the scrubbing action continuing as the solids rise to a more
quiescent zone intermediate the bottom and top surface of the pool of liquid.
This solid cleaning action continues as the solids are moved, in net effect,
horizontally throughout the length of the vessel, the solids being depleted
of contaminants as they progress from the end of the vessel wherein the
slurry is introduced, to the clean solids dlscharge end of the vessel. The
contaminated particulate solids are continuously counter currently contacted
by relatively clean portions of liquid to obtain maximum scrubbing effective-
ness as the solids are ved from the input end to the clean solids discharge-
end of the vessel. The compartment of the bottom liquid inlet enclosure
nearest the cleaned solids discharge end of the vessel is supplied with wash
liquid composed in part of fresh water from an outside source and in part of
liquid separated from the cleaned mud in a final mud settling basin, or
storage pond. In net effect, the suspended solids flow substantially hori-
zontally at a level intermediate the top level and bottom of the liquid pool
after introduction of the slurry, and clean solids are removed from the
opposite, or clean solids discharge end of the vessel.
The wash liquid with which the solids are cleaned is introduced at
the compartment of the bottom liquid inlet enclosure at the cleaned solids
exit end of the vessel, is overflowed into the launder compartment above this
inlet compartment, and pumped from this launder compartment into the next
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1 ~63782
bottom inlet enclosure compartment toward the slurry inlet end of the vessel.
This wash liquid, with the dissolved chemicals displaced fro~ the suspended
solids, is successively taken from one launder section and pumped into the
next bottom inlet section toward the slurry inlet end, until the launder
section at the slurry inlet end is reached. The liquid overflowing into this
end launder compartment contains all of the dissolved chemicals displaced from
the solids, and is pumped back to the main process stream for reuse of the
chemicals.
The invention, in its several aspects, will be better understood by
reference to the following detailed description and to the drawings to which
reference is made in the description. Corresponding parts of the apparatus
are designated by similar whole numbers in the different figures, and sub-
scrlpts are used to designate portions of a given component. Letters are also
used to lndicate simllar portions, or plural similar components.
In the drawings:
Figure 1 tepicts an elevation view, in partial section, of a pre- -
ferred particulate solid elutriation and washing apparatus; and
Figure 2 depicts a cross-section of the apparatus taken along
line 2-2 of the preceding figure.
Referring to Figures 1 and 2 there is shown an elongate vessel 10
formed by enclosing, relatively long, parallel side walls 111, 112 and rela-
tively short, parallel aligned end wall 121, 122. The vessel 10 is of length
several times its width, and of substantially V-shaped cross-section.
The upper edges of side walls 111, 112 are level and may be notched
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~ ~63782
to form a continuous weir, and each side wall is arrayed with a series of
laullders 13 ~ 131B' 131C' 131D and 132A' 132B' 2C 2D
Each launder 131, 32 of a series is formed by an outer side wall 141, 142
which runs parallel with the top of side walls 111, 112, respectively, and
includes a bottom floor 151, 152, respectively. Each launder in a series is
separated one from another of a series by end walls 161A, 161B, 161C and 162A,
162B, 162C, respectively. The outer side wall 141, 142 is higher than the
top edge of the side walls 111, 112 of the vessel, respectively, the latter
forming a lip over which liquid can flow into a launder. The last launder
of a series, i.e. 131D, 132D , is provided with an overflow 191, 192, for
the withdrawal of liquid from the launders.
The bottom of vessel 10 can be arrayed with a series of upwardly
opening compartments 171, 172, 173, 174. The compartments 171, 172, 173,
174 are formed by the presence of parallel aligned side walls lllA, 112A,
end walls 121A, 122A, the bottom wall or floor 18 and intermediate walls
171A, 172A, 173A which separate compartments 171, 172, 173, 174. In its
preferred form, the compartments 17 are equal in number to the number of
launders in a series. Preferably, the number of compartments 17 and launders
in a series, respectively, range from about 2 to about lS, more preferably
from about 4 to about 10. The first of the series of compartments 171 is
provided with an inlet 20 for the injection of a wash liquid, and each of
compartments 172, 173, 174 are provided with pumps 211A, 211B, 211C for the
withdrawal of liquid from launders 131A, 131B, 131C, and 132A, 132B, 132C,
respectively, for pumping into compartments 172, 173, 174, respectively.
Thus, wash liquid is in;ected via line 20 into compartment 171. Wash liquid
containing progressively greater amounts of recovered dissolved chemicals is
pumped from launders 131A, 132A by pump 211A and discharged into compartment
172, from launders 131B, 132B by pump 211B and discharged into compartment
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1163782
173, from launders 131C, 132C by pump 21C and discharged into comparement
174, and liquid is removed from launders 131D, 132D by lines 19l, 192,
respectively. Simultaneously, a slurry of particulate solids in liquid is
in~ected into the vessel 10 via line 22, and a mud is taken from the vessel
via line 23.
The slurry discharge end of the vessel 10 is optionally extended
to provide an additional V-trough 170 which aids in the removal, and discharge
of slurry from the vessel. Slurry is removed therefrom via line 23. No
fresh liquid is introduced via an inlet into V-trough 170, and no overhead
launder is associated therewithO
In net effect then, a solids mixture, mud, or slurry to be elutriat-
ed is introduced into vessel 10 vla line 22, and simultaneously with its
introduction the slurry, or the particulate solids are dispersed, the solids
falling while the liquid countercurrently ascends. The particulate solids
are in effect suspended in the vertical upflow of liquid in the V-shaped
vessel in hindered settling, the solids forming a strata above t~e vessel
bottom compartments 17, and below the launders 13l, 132. The geometric shape
of the vessel is such that the rate of flow of the liquid decreases as it
rises within the vessel so that essentially no particulate solids of larger
than a desired separation size are carried over into the launders 131, 132.
With the continued introduction of slurry into the vessel lO via inlet 22
the solids flow as a strata along the length of the vessel 10, and they are
discharged via line 23 at the opposite end of the vessel. The solids during
their travel are washed, elutriated, and cleaned of impurities, chemicals,
or undesired solids fines. Simultaneously, there is a net flow of liquid in
the opposite direction through the vessel 10, the liquid being introduced
via inlet 20 and discharged via lines 191, 192, the clean liquid introduced
into the vessel 10 displacing impurities, or chemicals, as it moves through
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1 163782
the vessel lO to its point of discharge at outlet l9l, 192.
In accordance herewith, hindered settling thus refers to the
hindrance of the normal settling rate of the solids caused by the upward
velocity of flGw of the liquid, without regard to solids particle contact;
which may also occur. In effect, the coarser particles are suspended near
the bottom of the V-shaped trough, or vessel, in a zone of relatively high
upward liquid velocity, and the slower settling finer particles are suspended
higher up in the trough, or vessel, where the liquid upward velocity is
slower. It thus follows that a more flexible and efficient operation can be
provlded by providing additional, optional means to pump some of the overflow
from a launder back into the same inlet section of the same compartment, if
desired. This will permit maintaining the necessary upward liquid velocity
to suspend the solids even if the wash liquid supply rate is temporarily
reduced, or stopped.
The pre~ent apparatus, and process, is particularly suitable for
the elutriation and washing of solids contaminated with caustic from alumina
processing, particularly the red mud residue produced in the Bayer process
wherein alumina is extracted from bauxites, or aluminum hydrate ores. Such
muds include iron oxides, sodium aluminum silicate, titanium oxide, and
various other metal oxide impurities. The muds contain finely divided parti-
culate solids, some particles ranging less than one micron diameter, for
which reason these muds are particularly difficult to wash clean. Water, or
aqueous medium capable of displacing dissolved caustic contaminant from the
red mud residue, constitutes the preferred wash liquid. The following exem-
plifies the production of a red mud slurry from bauxite, the separation of
the red mud from the slurry, and the washing of the recovered red mud residue
pursuant to the practice of the present inventionO
1 163782
Example 1
The present example is illustrative of a hindered settling dis-
placement washing operation applied to the processing of bauxite in a medium
size commercial operation, to wit:
Alumina production ~ ------- 2300 metric tons/day
Bauxite usage ---------------- 4300 metric tons/day
Bauxite quality --------------
Available trihydrate alumina --- 50%
Reactive silica ---------------- 2%
Bauxite is admixed with caustic sodium aluminate liquor in ratio
of 140 grams of dry bauxite per liter of liquor, the liquor used containing
210 grams per liter of caustic (expressed as sodium carbonate) and 73.5 grams
per liter of alumina (expressed as alumlnum oxide). This mixture is slurried
in a ball mill, and the partlculate solids reduced to slzes which will pass
through a 35 mesh screen (U.S. Standard). The slurry is then heated to 150C
under pressure, and digestion continued at such conditions for a time suffi-
cient for the soluble alumina and silica in the bauxite to dissolve, and for
essentially all of the dissolved silica to react to form solid sodium aluminum
silicate. After the digestion is completed, it is found that the alumina
content in the slurry of insoluble residues suspended in the strong sodium
aluminate liquor is increased to about 136.5 grams per liter. The mud slurry,
which contains red mud solids residue in the amount of about 6 wt.%, is cooled
to its atmospheric boiling point, i.e. about 105C (220F), by reducing
pressure and releasing steam in incremental steps.
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1 163782
The mud slurry analyzes as follows, to wit:
Density of liquor ~ ------- 1.4 grams/ml.
% Solids in slurry --~ ----- 6 wt.%
(ranger S wt.% to 7.5 wt.%)
Temperature, C -------------- 105
This mud slurry, produced by the digestion, without desanding is
separated into a relatively particulate free sodium aluminate liquor and
relatively soluble chemicals free mud by treatment in a vessel of V-shaped
cross section, wide at its top and narrow at the bottom, the side walls con-
verging and essentially meeting at the bottom of the vessel. Liquid inlet
apertures are provided in the bottom of the vessel for the introduction of
wash liquid, and a series of launders are provided at the top of the vessel
along each of lts two sides to collect overflow~ng wash liquor. An inlet is
provided at one end of the vessel for the introduction of the mud, or slurry,
and an outlet for the washed solids is provided at the opposite end of the
vessel. A fresh water inlet is provided in the bottom of the vessel at the
washed solids discharge outlet, and a liquids discharge outlet is provided in
the launders at the opposite end of the vessel.
To aid in the separation, a commercially available chemical floccu-
lent is first added to the mud slurry, and the mud slurry is then introduced
into the vessel via the slurry inlet at an end of the vessel. Sufficient of
the flocculent is added to produce flocs which settle at a velocity of 5 to
10 feet per hour, since the very finely divided of the solids mud particles
(<35 mesh size) will not settle out on their own in any practical time period.
Sufficient of the chemical flocculent causes the finely divided solids
particles to settle out leaving a clear supernatent liquor containing less
than 100 milligrams/liter, generally less than 50 milligrams/liter of suspend-
ed fine solids particles.
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I lS3782
On injection of the chemical flocculent treated mud slurry into the
vessel, the slurry is contacted with wash water consisting of the water de-
canted from the second to last launder of the series, or launder adjacent
that wherein the final wash water is withdrawn from the vessel. The wash
water is introduced into the bottom inlet at the slurry entry end of the
vessel. Under the conditions of operation essentially no solids particulates
are carried into the launders, and the velocity of flow of the ascending
liquid dlminishes upwardly to a minimum of on the order of two feet per hour,
and not more than 5 feet per hour.
Countercurrent displacement washing of the solids particles is con-
tinued throughout the length of the vessel by collecting overflow liquid from
each launder of the series and pumping this into the bottom of the vessel at
a location nearer the point where the mud, or slurry is introduced. At the
ent of the vessel where the clean mud is discharged, the solids are contacted
with relatively pure water, suitably a clear liquid decanted from the surface
of a mud lake to which the clean solids are fed, and fresh makeup water.
Calculations indicate that with 10% solids mud slurry in the washing
section, as expected as a minimum value, and allowing for only 85% effective
displacement, each compartment section will be as effective as one stage of
conventional dilution washing at 85% efficiency and with 25% solids achieved
in each stage.
The strong sodium aluminate liquor introduced with the mud at the
slurry inlet end of the trough is displaced by the wash liquor introduced at
the bottom of that section of the vessel, and is diluted due to the fresh
makeup water added at the opposite end of the vessel.
In a typical operation, the mud, or slurry exiting from the vessel
is flowed into a settling section from which the slurry, at about 10% solids,
is removed and pumped to a disposal mud lake. The fresh water added for
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1 ~37~2
dilution is added to the slurry introduced into the lake. This fresh water,
whic:h also make up for the liquor trapped in the mud left permanently in the
lake, will normally range to about twice the weight of the dry mud, but may
vary from 1.5 to 4 times the mud weight. The mud left in the lake eventually
settles to about 50% solids, and the liquor released is pumped back to the
bottom of the vessel. The concentration of soluble sodium aluminate and
caustic in this dilute liquor may be controlled by the design of the mud wash-
ing equipment and by selection of the amount of fresh makeup water to be used.
In accordance with this example, these factors are chosen to result in dilute
wash liquor containing about 10 grams per liter total alkali (expressed as
Na2C03) and about 5 grams per liter of dissolved alumina (expressed as A12O3).
The chemical values lost with the mud in the mud lake would then amount to 15
tons per day of alkali and 7.4 tons per day of alumlna.
In the examples lmmediately following, the efficiency and utility
of the apparatus and process of this invention is illustrated by the beneficia-
tion of of bauxite ore by washing to remove clay. In conventional operations
the fine clay particles are generally removed from the ore by washing with
water in revolving screens, or trommels, or by the use of mechanical vibrating
screens. Fine bauxite particles particularly those ranging about 20 mesh and
smaller, passing through such screens are then recovered, conventionally, by
use of a hydraulic cyclone, or gravity redimentation step, which is not very
efficient.
Example 2
Similar apparatus is employed in this example as in the preceding
example, coarsely crushed bauxite ore being introduced as feed into the solids,
or slurry inlet to the vessel, the solids particles being suspended in wash
water introduced into, and rising from the bottom of the vessel. The
relatively coarse bauxite particles are tumbled in the relatively high
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1 1 637~2
The mud slurry analyzes as follows, to wit:
Density of liquor ~ -- 1.4 grams/mlO
% Solids in slurry ----------- 6 wt.%
(ranger 5 wt.% to 7.5 wt.%)
Temperature, C -------------- 105
This mud slurry~ produced by the digestion, without desanding is
separated into a relatively particulate free sodium aluminate liquor and
relatively soluble chemicals free mud by treatment in a vessel of V-shaped
cross section, wide at its top and narrow at the bottom, the side walls con-
verging and essentially meeting at the bottom of the vessel. Liquid inlet
apertures are provided in the bottom of the vessel for the introduction of
wash liquid, and a series of launders are provided at the top of the vessel
along each of its two sides to collect overflowing wash liquor. An inlet is
provided at one end of the vessel for the introduction of the mud, or slurry,
and an outlet for the washed solids is provided at the opposite end of the
vessel. A fresh water inlet is provided in the bottom of the vessel at the
washed solids discharge outlet, and a liquids discharge outlet is provided in
the launders at the opposite end of the vessel.
To aid in the separation, a commercially available chemical floccu-
lent is first added to the mud slurry, and the mud slurry is then introduced
into the vessel via the slurry inlet at an end of the vessel. Sufficient of
the flocculent is added to produce flocs which settle at a velocity of 5 to
10 feet per hour, since the very finely divided of the solids mud particles
(<35 mesh size) will not settle out on their own in any practical time period.
Sufficient of the chemical flocculent causes the finely divided solids
particles to settle out leaving a clear supernatent liquor containing less
than 100 milligrams/liter, generally less than 50 milligrams/liter of suspend-
ed fine solids particles.
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~ ~63782
desired for recovery of the chemical values.
The number of stages of displacement washing can be determined by
the efficiency of a given stage. The number of stages required for dilution
washing and displacement washing, at about 85% efficiency per stage, are about
equal. The effectiveness of displacement washing can readily provide about
85% to 90% efficiency. Accordingly, the preferred number of vessel bot~om
compartments, and launders, are subject to some variation, and determinable
by the type, and degree of washing desired.
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