The invention has particular application to recovery of gold, such as by cyanidation. However, the invention also is applicable to recovery of values of other metals by processes necessitating liquid treatment of mineralised substances, such as by a leaching operation. Also, it is to be appreciated that the invention can be used to treat a wide variety of other particulate materials capable of processing as a pulp or slurry, such as foodstuffs, fine wood or paper pulps and particulate coals.

In one aspect, the invention provides apparatus for treatment of a pulp of a liquid and particulate material, such as a mineralised substance, the apparatus including a housing defining chamber, a partition dividing the chamber into an agitation zone and a recovery zone, the recovery zone being of lesser depth than the agitation zone and being laterally adjacent an uppermost region of the agitation zone, the zones being in communication through at least one slot extending adjacent the lowermost edge of the partition, the apparatus further including agitation means for agitating the pulp and generating vertical circulation of the pulp, the arrangement being such that, during operation of the agitation means and with addition of feed to the agitation zone, the pulp is caused to circulate vertically in the agitation zone to provide flow of pulp downwardly past the at least one slot so that a flow of the liquid occurs between the agitation and recovery zones, with particulate material being transported from the agitation zone to the recovery zone and settling in the recovery zone to be subsequently returned to the agitation zone.

The apparatus may consist of a single cell, or a bank of cells, which in the mineral processing industry would be

classed as an all slime processing cell.

A process according to the invention comprises charging particulate material, such as mineralised substance, and a liquid to the agitation zone of such apparatus and, while adding further feed thereto, operating the agitation means to agitate a pulp of the material and cause the pulp to circulate vertically to provide a flow thereof downwardly past the at least one slot so that a flow of the medium and transported particulate material from the agitation zone to the recovery zone occurs through the at least one slot, and allowing the liquid to overflow from the recovery zone with settling of the transported material in the recovery zone for eventual return to the agitation zone. In the process, the liquid overflowing from the recovery zone in the treatment of mineralised substances has recovered values in solution. The process thus is clearly distinguished from flotation processes in which selected mineral particles are attached to bubble surfaces and overflow a flotation cell as a froth. In the case of recovery of gold by cyanidation or, for that matter in leaching of a metal value from a particulate mineralised substance, the metal ^" value leached is recovered from the medium flowing to the recovery zone. The particulate substance transported to the recovery zone thus is the resultant leached solids residue, the "recovery" of that zone thus being i relation to the liquid. However,- it will be appreciated that where a mineralised substance is given a leach to remove an impurity metal, "recovery" will be relative to the resultant upgraded leached solids. The apparatus most conveniently has a launder into whi medium overflowing the recovery zone is received. However, in some forms of the invention, the medium may simply overflow a weir to be received in a secondary treatment zone.

The agitating means may comprise means for agitating the pulp in the agitation zone and for causing in that zone vertical circulation of the pulp such that there is generated a flow of the pulp downwardly past the slot(s) . In some uses of the invention, it is required that the liquid or particulate material of the pulp be treated with a gas. In such case, the

agitating means may be adapted to agitate and circulate the pulp by charging such gas to the agitation zone. Thus, in the case of recovery of gold by cyanidation, the agitation means may charge pressurized air or oxygen to the agitation zone to additionally provide the necessary aeration for recovery of the gold. However, depending on the liquid or particulate mat ^¬ erial, other reactive gases may be used, examples of these being S0 ₂ , S0 ₃ , CO and/or C0 ₂ as used in dilute acid leaching of for example uranium or vanadium ores or concentrates. However, it is to be appreciated that the gas charged for agitation and circulation of the pulp may be one that is inert to both the liquid and particulate material.

Where the agitation means is to provide the required agitation and circulation by a charged gas, the flow of gas released into the agitation zone is to be such as to cause circulation of the pulp. One particularly suitable arrangement for this is agitation means in the form of an air lift.

It is to be appreciated that other forms of agitation means can be used. Thus, where for example it is required to avoid aeration or treatment with a gas, the agitation means may be of a mechanical or electro-mechanical form, such as,a motor driven impeller or pulsed plunger, or a sonic agitator. However, it is to be understood that the agitation means also can be of a combination of such types, with for example, an impeller or plunger and sonic agitator being used together or any being used in conjunction with an air lift.

The vertical extent or depth of the recovery zone most conveniently is substantially less than that of the agitation zone such that flow to the recovery zone occurs adjacen the lowermost level of the recovery zone but intermediate upper and lower levels of the agitation zone. However, it is found to be most practical to. have the volume of the recovery zone of the same order as the portion of the volume of the agitation zone above the level at which communication between the zones is provided.

Communication between the agitation and recovery zones may be by means of a single horizontally extending slot or two or more such slots in end-to-end spaced relation. Most

conveniently, the or. each such slot is provided at or adjacent the lower edge of. the partition between the zones which normally is of a vertical extent such as to prevent communication between the zones other than through the slσt(s) . The width of the slot(s) most conveniently is a minimum having regard to the throughput per hour for the apparatus or process, the particle size of the mineralised substance and, at least at relatively high pulp densities in the agitation zone, the actual pulp density. The recovery zone in a direction transversely of the communicatio slot most conveniently increases in width upwardly from the slot(s) . The zone may, for example, be of V-section in that direction and in such case it may be bounded by a wall of the housing and the partition being oppositely inclined at, for example, substantially 60° from the horizontal, substantially symmetrically with respect to the vertical.

Where such V-section recovery zone is provided, inclination of the housing wall and partition can be other than 60 . An import-ant factor in this regard is the desir¬ ability of preventing particulate substance transported from the agitation zone from simply building up on the wall and/or partition. That is, it is preferable to have an arrangement which results in transported substance settling in the recovery zone such that further liquid flowing to that zone percolates upwardly through the settled solids and particulate substance transported by the further liquid is trapped by the settled solids for eventual return to the agitation zone. Such operation can facilitate further treatment, such as leachin of the settled solids to optimise overall efficiency of treatmen

The extent of departure from an inclination of 60° is dependent on particle settling characteristics, such as particle size. A steeper inclination, up to about 80°, is possible for particles of substances which will settle more readily under gravity; while a flatter wall inclination, down to about 50°, is desirable to provide adequate settling area for particles of substances with slow settling characteristics. However, a furth factor is the volume of the recovery zone relative to the portion of the volume of the agitation zone above the level of

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communication therebetween.

The settling area requirement for a particulate substance of a given density and size range is an important factor of the apparatus design. However, this requirement can be determined by laboratory test work.

Particularly where the agitation means is an air lift, strong agitation occurs in the agitation zone. The pulp of particulate material and liquid is caused to circulate upwardly from he outlet of the air lift, downwardly past the slot(s) , and then upwardly again through the air lift. This circular flow, which is required for all forms of the agitation means, is an important factor in causing flow of liquid and transported particulate material into the recovery zone so as to achieve settling of the particulate material in that zone and maintenance of clear liquid above the settled solids for overflow from the recovery zone. A further important factor is the pulsing action resulting from the agitation, such as the action of the air lift where this is the means by which agitation is achieved in the agitation zone. This superimposes on the circular flow a vertical pulsing which assists in achieving efficient settling of solids transported to the recovery zone.

As implicit in the foregong, the agitation means may not be adapted for charging gas to the agitation zone and comprising an air lift. In one important variant, a gas, such as air, may be charged from a relatively low pressure source, such as to provide aeration. In such cases where an air lift is not used, the apparatus may include a mechanical agitation mechanism, such as a pulsed plunger or impeller, or an electro-mechanical or sonic agitator of suitable form.

While, in the foregoing, reference is made to solids settling in the recovery zone, it is found that an equilibrium is established and that, while particulate material is contin- ually transported from the agitation zone to the recovery zone, there also is return of particulate material. The recovery zone receives particles having a range of settling characteristics and there is achieved in that zone a flow path which is upward and circular, resulting in a return of particles in a denser state to the slot(s) and thus bacl

the agitation zone. Further, at the lower region in the agitati zone, t _h e larger or more dense particles are centrifugally thrown down onto the base plate, before being drawn upwardly again through the air lift with smaller or less dense particles. The larger or more dense particles thus can be drawn off at the base plate level, as required. However, it is to be appreciated that operation without the need for classification or concentration results in a range of particles being processed through the apparatus, to be finally discharged as residues. The return of particles to the agitation zone from the recovery zone can occur through the s-ame slot(s) as permit the prior transportation of particles from the agitation zone to the recovery zone. The above-mentioned upward and circular flow path results in settling of particles in the recovery zone and formation of a relatively dense layer thereof, within that zone, on the partition. That layer progressively moves down the partition toward the level of the slot(s) and either passes back through the slot(s) , or is guided through ducts associated with the slot(s) such as between successive portions of a single slot or between successive slots.

During agitation, liquid from the pulp is discharged from the recovery zone, while fresh liquid or pulp is added to maintain the volume within the chamber. The liquid discharge from the recovery zone may be pregnant solution where metal values or contaminants are extracted from the particulate material and, in such case, the added liquid may be to achieve a constant reactant strength. However, where the particulate material is to effect a treatment of the liquid, such as to remove suspended solids by filtration or to remove dissolved contaminants, the liquid discharged will be feed liquid clarified or purified by the treatment.

The partition between the agitation and recovery zones, is such that those zones are of specific volumes and configurations. It is in these respects that a principle of operation differing from that of flotation cells, such as the Southwestern air-lift flotation cell, is established and a liquid rather than a froth may thus be discharged from the recovery zone.

The apparatus may vary in configuration. However,

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two practical forms are rectangular and circular, such as illustrated in the accompanying drawings to which the description now is directed, and in which:

Figure 1 shows a transverse section through a cell of rectangular form;

Figure 2 shows an isometric view of the cell of Figure 1;

Figure 2A is a partial view, in section of a cell similar to that of Figure 2; Figure 2B is a sectional view on line IIB-IIB of Figure 2A;

Figure 3 shows a diametric transverse section through a cell of circular form;

Figures 4A and 4B show respective flowsheets for a cyanidation recovery with conventional practice and operation with cells as in Figure 1;

Figure 5 shows diagramatically a layout for operation with cells as in Figure 1 for recovery of gold from tailings by cyanidation; Figure 6 shows in longitudinal section a continuous leaching operation by application of counter-current decantation;

Figures 7 to 9 show further cell configurations- In Figures 1 and 2, the cell 10 is rectangular in plan view and is bounded by a rear wall 12, opposed side walls 14 and a front wall 16. As* shown, walls 12 and 14 are substantially vertical, while wall 16 extends upwardly and forwardly from a position adjacent the lower edge of wall 12. A rectangular base plate 18 closes the bottom of the cell around the lower edges of walls 12,14 and 16.

Within the chamber defined by walls 12,14 and 16, there is a partition 20 extending between walls 14 and upwardly and rearwardly from wall 16. The lower edge of partition 20 is slightly spaced from wall 16 to define an elongate slot 22. The upper edge of partition 20 has a vertical extension 23 which terminates at a common level with the upper edges of walls 12 and 14, although it will be noted that those edges are above the upper edge of wall 16. Partition 20 divides the chamber defined by walls

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12,14 and 16 to provide an agitation zone 24 and a recovery zone 26. The volume of .zone 26 is substantially the same as the portion of zone 24 above the level of slot 22. The latter provides communication between zones 24,26 for a liquid medium charged to the cell, and it will be appreciated that the liquid will overflow the upper edge of wall 16 to prevent communication between the zones other than via slot 22.

Parallel with wall 12 there is provided a panel 28 which defines with that wall a narrow duct 30 open at its upper and lower ends. Duct 30 extends upwardly from a position slightly above base plate 18 to about the level of slot 22. Gas lines 32 extend downwardly in duct 30 for supply of compressed gas to transverse manifold 34 located slightly above the lower end of duct 30; manifold 34 having a series of outlet nozzles for dissipation of compressed air from lines 32 into duct 30.

Counter-current liquid supply lines 36 pass down¬ wardly into agitation zone 24 and terminate adjacent the lower end of duct 30, exteriorily of the latter. In operation with cell 10, a pulp 38 of particulate mineralised material to be treated and a treating liquid is cha to the cell via line 39to a convenient level below that of the upper edge of wall 16. The material and liquid is charged to agitation zone 24 and agitated by compressed gas supplied via lines 32. Duct 30 operates as an air lift with the dissipated gas causing liquid and particulate material to circulate vertically in zone 24; the pulp moving upwardly through duct 30 along wall 12, horizontally to partition 20, and down the latter past slot 22 to the base of zone 24 where the pulp again is lifted through duct 30, as shown by arrows in Figure 1. During this agitation and circulation, and with increase in the volume of liquid supplied to zone 24, the liquid level rises, with flow of liquid and transported particulate material into recovery zone 26 via slot 22 and overflowing of wall 16.

Due to the construction of cell 10, transported material passing to zone 26 settles therein adjacent slot 22 as shown at 40 to leave clear liquid 42 above the settled solids. As will be appreciated, it is the clear liquid 42 which overflow

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wall 16 and, as shown in Figure 1, that liquid is recovered in launder 44.

The principle of operation depends upon the estab¬ lishment of a state of settling in the upper part of the cell 10 comprising recovery zone 26 whilst a state of agitation and aeration (where the gas is air) is maintained in the adjoining upper part of agitation zone 24 and the pulp 38 within zone 24 is being subjected to other functions such as leaching. In particular, counter-current decantation can be provided for by supply of water via lines 36. Settling is aided by a pulsating counter-balancing effect created by the linked functions of agitation and aeration.

As indicated above, the settled solids 40 are not static but are pulsed (as is the pulp in zone 24) due to the pulsating action of the air lift. Under this action, an equilibrium is established in solids 40, with incoming particulate material moving up wall 16 from slot 22 and further material moving down partition 20 and back through slot 22 to zone 24. However, the flow through slot 22 is such that a proportion of the liquid percolates upwardly through solids 40 to overflow into launder 44.

The arrangement of Figures 1 and 2 can be modified so as to permit two such cells to operate in back-to-back relationship in an arrangement which, in the sectional view of Figure 1, is similar to the sectional view of Figure 3. In this, rear wall 12 can be omitted so that the one air lift provides agitation and aeration in zone 24 of each of the two cells.

In Figures 2A and 2B there is shown, on an enlarged scale, a sectional view through the recovery zone of a cell simi to that of Figures 1 and 2 and part of the agitation zone adjacent the slot providing communication between those zones. As indicated, the flow of the pulp down partition 20 past slot 22, resulting from circulation in zone 24, gives rise to circulation of pulp in the reverse direction in the vicinity of slot 22. It is this reverse circulation which results in the flow of liquid and transported particulate material from zone 24 to zone 26. The reverse flow extends throughout a substantial portion of the volume of chamber 26 and is facilitated by the

V-section of the latter; resulting in the transported material passing, up wall 16 and across zone 26, to settle on and pass down partition 20. .As a consequence, a region of clear liquid is established in the uppermost region of zone 26, above pulp/liquid interface 25, from this region that liquid overflows from that zone. Also, density of the pulp established in zone 26 decreases in the direction of flow up wall 16, with the reverse occuring in the direction of flow down partition 20. This increase in density along partition 20, and settling of the particles overall, results in a relatively dense build-up of particles on the lower extent of the partition; this build up moving slowly back to slot 22 and being drawn back into agitation zone 24 by the flow in that zone down past the slot. Slot 22 most conveniently is defined by the lower edge of partition 20 and the inner surface of wall 16, i.e. it extends horizontally and comprises the spacing between that edge and inner surface. Such simple arrangement facilitates both the transportation of particulate material into zone 26 and its return to zone 24. However, a modified arrangement is shown in Figures 2A and 2B.

In the modified arrangement, a return duct 22a_ extends down into zone 24 from the lower edge of partition 20. As shown, duct 22a extends part way across slot 22, but may extend across the full width of the latter; while the width of duct 22a in the direction of the longitudinal extent of slot 22 is relatively small.

Duct 22a_ serves to pass particulate material that has built up on partition 20 back into the mainstream of flow, in zone 24, past slot 22. In view of the limited extent of duct 22a along slot 22, ribs 22b are provided on partition 20 to collect and guide into duct 22a particulate material building up across the full width of partition 20. Ribs 22b stand up from partition 20, into zone 26, and diverge upwardly and outwardly from the inlet to duct 22a. However, to minimise their effect on circulation in zone 26, ribs 26b may taper to decrease in depth away from duct 22a. Also, the inlet to duct 22a_ may have a cover portion 22£ which projects from the side of the duct remote from the lower edge of partition 20, so as to

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extend into zone 26, such as parallel to partition 20.

The modified arrangement is such that the flow of liquid and transported particulate material from zone 24 into zone 26 occurs to the sides of duct 22a.. However, the return of particulate material occurs to a substantial extent through the duct and is aided by the flow in zone 24 down around the duct and beyond the lower end of the latter.

While duct 22a is shown centrally disposed in Figure 2B, this is not necessary. Thus duct 22a could, for example, be located against one side wall 14, with there then being only the one rib 22b extending therefrom, upwardly over partition 20 to* the other side wall 14. Also, there c-an be more than one duct 22a_ spaced longitudinally of slot 22.

In Figure 3, a modified form of cell is illustrated. This is of circular form, but otherwise corresponds to the arrangement of Figures 1 and 2 and similar parts therefore are identified by the same reference plus 100. It will be appreciated that wall 116 and partition 120 are conical; with there being no counterpart to walls 12,14 of Figures 1 and 2. Also, the air lift is defined by a cylindrical tube 128, corresponding to panel 28 of Figures 1 and 2, while launder 144 is annular.

Operation with the cell of Figure 3 is as for that of Figures 1 and 2. As shown, a valve 60 is provided below agitation zone 124 for discharge from the latter of solids, if required, via launder 62 for recovery of concentrate or removal of oversize particles for regrinding. For simplicity of illustration, such valve and launder is not shown in Figures 1 and 2, but can be provided.

As in the modification of Figures 1 and 2 shown in Figures 2A and 2B, the cell of Figure 3 can have one or more ducts similar to ducts 22a located in slot 122. In this case, ribs corresponding to ribs 22b would curve around the surface of conical partition 120. The volumes of the above-mentioned upper parts of the cells are carefully calculated so that the required corresponding densities will be established and maintained between the solids + solution + air mixture in agitation and the solids + solution mixture in settling. The net result is that cell 10

enables a change of solution or washwater to be passed through a pulp of very fine particle size in a relatively short space of t.ime whilst various other functions can be performed within the cell. The principle of operation may be incorporated in cell designs of varied configuration (e.g. rectangular, cylindrical and conical combinations) providing wall and partition components are featured in vertical or suitably inclined attitude to take advantage of the effect of gravity. Agitation for dissolution may be accomplished solely by the submerged air-lift as shown in Figures 1 and 2, or mechanical agitation may be incorporated to be used in conjunction with compressed air provided for aeration.

A single cell, or a battery of cells, as shown in Figures 1 to 3 can be operated to perform one or more of a variety of treatments on or with particulate material. Such treatments include filtration, agitation, conditioning, aeration, leaching, counter-current decantation, thickening, settling, washing, classification, concentration, flotation, amalgamation and neutralization. While as indicated above, and as implicit in this range of treatments, the mineralised material can vary widely, the cell has been found in experimental operation to be particularly well -≤uited to recovery of gold, such as from tailings, by cyanidation. An illustration of such recovery now will be described with reference to Figures 5 and 6, in relation to an installation able to treat 40 tonnes per day for recovery of gold from a tailings _^ dump.

The installation of Figures 5 and 6 has a battery of six cells (numbered Cl to C6) forming a rectangular construction. The cells may be portable, if required, and the battery illustrated has an overall length of nine metres, width of 1.5 metres, height of 3 metres. The individual cells thus are 1.5 by 1.5 metres in plan view and have a height of 3 metres. Each cell has a recovery zone and submerged air-lift as shown in Figures 1 and 2 and is connected to the adjoining cell by a 160 mm x 160 mm opening in the adjacent wall 14, below the inlet to the air-lift, at the base plate 18. Each air-lift is a rectangular slot which is positioned as shown in

Figure 2 and operates along almost the entire length of each cell. Compressed air is released into each air-lift slot through 0.5 mm diameter holes drilled into a 10 mm diameter steel air-supply manifold 34. A regulated flow of counter- current decantation water is supplied through 25 mm diameter lines 36 to the discharge opening at the base of each cell.

The six cell construction with a total weight of 8 tonnes sits on a level, 100 mm thick, concrete slab. The cells extend from a discharge end at Cl to a feed end at C6 and filter tanks are contained in the base of cells C3 to C6 inclusive. A single level launder 44 is welded along the dis¬ charge weirs defined by the respective walls 16 of the settling zones 26 to collect the overflow from each cell. This launder is provided with a partition between cells numbered 4 and 5 so that overflow from cells 5 and 6 may be collected separately from overflow from cells 1 to 4 inclusive.

In a typical operation, feed to the battery comprised auriferous dump tailings at the rate of about 1.71 tonnes per hour dry solids at 95% minus 200 mesh, about 0.05 to 0.06% NaCN equivalent treatment solution, and about 0.5 kg of lime per tonne of treatment solution for necessary alkalinity. The solids and treatment solution was fed as a pulp into one end of the battery (cell C6) and water for counter-current decantation is fed into, and leached pulp discharged from, the other end of each cell. The overflows into launder 44 comprised pregnant solution; that from cells 5 and 6 passing through filter tanks 50, for clarification, to zinc box 52 for stripping of recovered gold values therefrom by precipitation.

The cyanide solution typically decreased substantially uniformly from 0.06% NaCN equivalent in cell C6 to about 0.005% in cell Cl. The weaker pregnant solution from cells 1 to 4 was fed to solution tank 54 to be charged, with additional cyanide and lime from tank 55, to provide further pulp feed via feed tank 56. The pulp feed to the battery had about 30 per cent solids on a dry basis to achieve the required pulp density in the cells of about 1,23 S.G. (the dry solids used having a density of 2.65 and the pulp containing 14% by volume of solids) . Agitation in the agitation zone 24 of each cell preferably was

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achieved by using about 1.85 cubic metres of compressed air per minute for the battery of cells.

In the arrangement illustrated, the battery provided a settling area of approximately 0.25 square metres per tonne per 24 hours operation, while the total capacity of the agitation zones was approximately.15 cubic metres. Under these conditions, and with the above feed conditions, it was found tha 90% recovery of gold from the tailings feed was achieved in a le time of three hours for a throughput of 40 tonnes of tailings per 24 hour period. This was obtained, despite the short retention time of three hours, with a slot 22 in each cell of approximately 2 inches in width; the slots, as evident from Figures 1 and 2, extending the full width of the cells.

The arrangement of Figures 5 and 6 required the zinc box to be of 25 litre per minute capacity, with feed to this being by gravity from the filters. The solution tank was of 1000 litre capacity and was located so that barren solution overflow from the zinc tank flowed under gravity to the solution tank. In addition to the features descr.ibed, the arrangement necessitated low powered feed, fresh water and return water pumps, a small loader to charge tailings to be treated and an air compressor. As will be appreciated from a comparison of Figures 4A and 4B, the capital and operating costs are significantly less than with conventional plant, while the invention pe.rmits compact, enclosed continuous closed-circuit processing. Also, there need be "no moving parts within the battery.

If required, an upper edge portion of walls 16 of each cell and launder 44 can be adjustable in height to control overflow volume of pregnant solution.

Under ideal operating conditions it is desirable that the pulp density be maintained substantially uniform at corresponding levels over the full extent of the cell group. Control means, such as float controls, can be provided to facilitate this.

Figure 7 shows a further modified cell which may be of rectangular form as in Figures 1 and 2. Alternatively, it may be of circular form as in Figure 3 and has parts correspond¬ ing to those of the latter identified by the same reference

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numeral, plus 100.

The cell 210 of Figure 7 is somewhat schematic and the gas lines for agitation in ^" zone 224, and the lines for counter-current water have been omitted for simplicity of ₅ illustration. As shown, the air-lift defining member 228 diverges outwardly, toward its lower end and may be vertical, or slightly outwardly flared as shown, at its upper end. This form for member 228 can facilitate circulation of pulp in zone - 224. Also, wall 220 has an upward extension 221 which defines • _j _0 an exhaust duct for gas charged to zone 224 by the air lift.

Operation with cell 210 is essentially as described with reference to cell 10 of Figures 1 and 2.

Figure 8 shows an alternative form of the cell of Figure 7. In this, parts corresponding to those of Figure 7 _5 have the same reference numeral, plus 100.

The cell 310 of Figure 8 also is somewhat schematic, with gas lines for agitation in zone 324 and lines for counter- current water omitted for simplicity. Cell 310 is of circular form, as for the arrangement of Figure 3 and is designed for a 0 large capacity through-put. A reinforcing peripheral structure 307, either in the form of a continuous wall or angularly spaced uprights, supports, the upper periphery of wall 316 and carries launder 344. Also, wall 316 is of two part construction; having a lower conical portion 308, and an encircling portion 309 which 5 is cylindrical over its lower extent enclosing zone.324 and conic at its upper extent enclosing zone 326. The base of portion 309 rests on a support surface to increase the overall strength of the structure.

Operation of cell 310 is essentially as described with reference to cell 10 of Figures 1 and 2.

With reference to Figure 9, there is shown therein a further modified cell which is of rectangular form as in Figures 1 and 2, and which is suitable for use as either end cell of a battery of cells, such as cell Cl or C6 in the battery Figures 5 and 6. Parts corresponding to those of the Figures 1 and 2 have the s-ame reference numeral plus 400.

The cell 410 of Figure 9, as with that of Figure 8, has a peripheral support structure 407 which rests on base 408 and may be a continuous wall or spaced uprights. This structure

407 supports the upper edge of wall 416 and carries launder 444; while wall 416 is further s rengthened by spaced, upwardly extending ribs 417. Also, partition 420 separates zones 424 and 426 and, with wall 416, defies slot 422 and, at its upper periphery, partition 420 has an extension 421 which defines an exhaust duct for gas charged to zone 424.

Cell 410 is suitable for low head (height of about 1.5 metres) design, xequiring only a supply of low pressure gas which may be provided by blower 433 instead of a convention compressor. The gas is charged by blower 433 via a slot or pipes to be discharged into the air-lift just above the base of the latter at a pressure and volume sufficient to achieve the desired degree of agitation in zone 424. Operation of cell 410 is essentially as described in relation to the preceding embodiments. However, in this case, launder 444 is fitted with a weir overflow control vertically adjustable by means of mechanism 445 which is operated by float 445a in zone 426 and a suitable linkage 445b pivoted at 445£ to maintain an even overflow over the weir length of a battery of cells during periods of start-up or shut-down or when a malfunction occurs in the feed system causing a significant change in pulp density at one end of a battery of cells. Thus, raising or lowering of float 445a_ with pulp level in cell 410 causes closing or opening of gate 445d, pivoted at 445e_, to vary the overflow of liquid from cell 410 and resultant adjustment of pulp density along successive cells of a battery of which cell 410 is the end unit. As will be appreciated from the illustrated arrangements, the direction of movement of particulate material transferred with liquid from zone 24 (124, 224, 324, 424) to zone 26 (126, 226, 326, 426) via slot 22 (122, 222, 322, 422) is reversed from the downward direction of flow adjacent the slot in the agitation zone. This is of practical .importance as the particles undergoing that reversal are entrained or filte by previously settled particles and, as a consequence, liquid overflowing from the recovery zone is clarified by upward percolation. It is found that clear overflow from the recovery zone is achieved even with heavily slimed mineralised material

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which can be difficult .to treat by other procedures.

The apparatus of each of the arrangements illustrated achieves optimum efficiency if the volume of the recovery zone and that of the portion of the agitation zone above the slot is such that a substantial equality of density at given heights above the slot is achieved. For example, in the arrangement of Figures 5 and 6, if the density in the portion of the recovery zone occupied by settled solids is about 1.0, then desirably about 80% of the volume of the portion of the agitation zone at the level of the settled solids should (for .the above described pulp feed) be occupied by pulp with a density of about 1.23 to creat the desired balance; with the remaining 20% of that volume being occupied by air bubbles. In the arrangement of Figures 5 and 6 based on cells as in Figures 1 and 2, in whic the angle between wall 16 and partition 20 is about 60 , optimum results were obtained when provision was made for 35% of the pulp to occupy 81% of the agitation zone volume above the discharge «from duct 30.

In the description with reference to Figures 5 and 6, it is apparent that utilization of the battery of cells entailed agitation, aeration, leaching, counter-current decantation, thickening (of the barren solids relative to the feed pulp) , settling and washing. It readily will be appreciated that, depending on the composition of the liquid feed to the agitation zone, neutralization could be provided. Similar, it will be apparent that classification and/or concentration of a required metal value relative to gangue miner could be achieved for a suitable feed, such as by oversize or denser particles being drawn off from the bottom of the agitation zone. Also, amalgamation can be achieved by providing a facility for this purpose in the agitation zone of the cell; while flotation can be achieved with suitable additives to recover or remove selected minerals from the pulp by providing a suitable facility for the collection and removal of float. Additionally, while the apparatus can handle treatment of slurrie material without need for flocculents, it is to be appreciated that a flocculent could be added intermittently or continuously, for ex-a ple to the recovery zone to aid settling without such agent necessarily effecting the agitation zone. Moreover, while

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18. it will be appreciated that the apparatus has application in such treatment of particulate material by a liquid, the converse also is applicable. Thus, the particulate material can be used to clarify a liquid by filtering suspended solids from the latter. Also, the particulate material can be such as to enable removal of dissolved contaminants from a liquid, such as in removing dyestuff from a liquid using, for example, activated carbon as the particulate material. Finally, it is to be understood that various alterations, modifications and/or additions may be introduced into the constructions .and arrangements of parts previously described without departing from the spirit or ambit of the invention.