WO1989005866A1

WO1989005866A1 - Metal recovery process

Info

Publication number: WO1989005866A1
Application number: PCT/AU1988/000479
Authority: WO
Inventors: Gerald Joseph Ward Cairns
Original assignee: Systratec Services Pty. Ltd.
Priority date: 1987-12-15
Filing date: 1988-12-15
Publication date: 1989-06-29

Abstract

A metal (e.g. Ag) may be removed from solution (e.g. photographic fixer) by passing the solution through a packing containing one or more zones (40) of metal, preferably cast iron chips, the metal being lower/more active in the emf series than the metal to be removed. The solution flows from inlet (35) to outlet (36) in a flow path which extends adjacent to zones (40) and preferably extends through an open cell foam (39). A minor portion of the solution is able to permeate from foam (39) into the permeable zone (40).

Description

e o e nven on

This invention relates to a method of and apparatus for removing useful or valuable metal from solutions containing the dissolved metal to enable subsequent recovery.

5 Background to the Invention

It is well known to remove metals such as silver from black and white photographic fixer solutions, copper from copper sulphate leachates and gold from _ι gold leachates by passing the solution containing the metal through a bed lOpacked with a metal lower in the electromotive series than metal to be removed.

The metal most commonly used for the bed packing is iron. Silver ions in the solution react with the iron metal packing, the iron forming ferrous iron in solution and being

15replaced by a deposit of silver. A similar reaction occurs when considering removal of copper and gold from solution.

Gold is removed from gold leachates such as where the gold is in solution as aurocyanide ions by passing the solution containing the dissolved metal through a bed packed 0with zinc metal powder or by electroplating onto iron wool anodes. Whilst possible, it is not efficient to deposit gold onto iron as a layer of iron hydroxide forms rapidly in neutral to alkaline cyanide solutions thereby preventing further reaction and deposition. A further disadvantage of

SUB^STITUTE© using metals such as zinc and electroplating methods is that the resultant gold product is of low concentration.

The art of depositing copper from acidic copper sulphate solutions is well known but the resultant copper product is often very low in copper content, and the efficiency of using the reducing capacity of metals such as iron is low, because of the generation of hydrogen gas.

The iron in the metal packing has usually been in the form of iron wool, iron filings, iron wire mesh or other porous packing. Iron wool has been preferred because of its large potential reactive surface to weight ratio. In the case of silver the silver fixer solution percolates directly through a bed of the packing arranged for maximum surface contact between the iron and the solution. A packing of that type is described, for example, in Australia Patent 428,478. Such a packing shows low efficiency, undesirable side relations with thiosulphate and a risk of exothermic reactions. The alternative art of electrowinning is expensive and in practice does not sustain high recovery of silver.

Conventional recovery packings also suffer from a number of other disadvantages. Ideally, the solution leaving a recovery packing should contain no silver. However in practice, the packings (usually 20 Litres) packed with iron wool often operate with only 50% efficiency and consequently it is common practice to run two or more such packings in sequence to maximise silver recovery.

Further when the packing is exhausted it contains a large quantity of corrosive solution and is susceptible to rupture in transit. It is not usual for these packings to be drained before dispatch because of the danger of exothermic reactions and the risk of loss of valuable metal.

Many undesirable side reactions also occur in conventional iron wool packed packings. For example, one side reaction tends to consume thiosulphate, which can cause the production of sulphuric acid and similar sulphur containing compounds in photographic fixer. Under these conditions the iron wool medium may dissolve making it difficult if not impossible to accurately determine whether or not the cell is exhausted. Alkaline side reactions can also occur leading to formation of ammonia from degradation of the residues, from the photographic process. In addition, ferric ions may reach sufficient concentrations to eventually precipitate as the oxide, causing blockages, loss of metal and spillage.

Another serious problem with conventional packings is the evolution of a large volume of gases from the side reactions in the packing. These gases include hydrogen sulphide and sulphur dioxide which have an unpleasant odour and are toxic. Their evolution also creates a considerable health hazard. Further in conventional packings the precipitated metal plug has been known to block the outlet port. The cell rapidly pressurizes and two results can occur. Firstly, the precious metal sludge is forced into the inlet blocking it and the fixer solution caused to bypass the packing and go directly to drain causing loss of precious metal. Secondly, if the outlet is also blocked, the body of the packing may split under the pressure with consequent spillage of precious metal.

It is an object of the present invention to provide an improved method and apparatus for removing valuable or useful metal from solutions containing the dissolved metal.

Summary of the Invention

According to one aspect, the invention consists of apparatus for removal of a metal from a solution containing the metal, the apparatus comprising:- a container having an inlet and an outlet; a flow path or paths for conducting the solution from the inlet to the outlet; and at least one zone comprising a reactant metal which, in the emf series, is lower/more active than the metal in the solution; the zone being disposed adjacent to the flow path to permit a minor portion of the solution to permeate from the flow path into the zone. In another aspect of the invention a method is provided for removing a metal from a solution containing the metal comprising the steps of:-

(1) supplying the solution into a flow path having 5 at least one zone adjacent thereto, the zone comprising a reactant metal which, in the e.m.f. series, is lower/more active than the metal in the solution, and

(2) maintaining the supply of the solution into the 10 flow path at a rate sufficient to permit a minor portion of the solution to permeate from the flow path into the zone and to establish a concentration of the metal in the solution in the zone different to the concentration of the 15 metal in the solution in the flow path.

The initial reaction within the zone is

XMe₁ ⁺ + Me₂ -> X Me- .+ Me₂ ^X+

wherein Me- is a metal ion in solution

Me_ is a metal which, in the e.m.f. series,

20 is lower/more active than Me., and is comprised in the zone.

A region of low metal ion concentration thus develops in the zone causing a concentration cell to be established across the zone and flow path which helps to drive the deposition process at the interface between the flowing liquid and the relatively stationary liquid in the zone.

In the case of metals Me„ which have two common oxidation states a further reaction enhancing Me., recovery can occur in the flow path within the flowing solution: Me₂ ^x+ + y e^ -> yMe_χ + Me₂^^x+Y^⁺ where Me-., and Me« have the above stated meanings.

Me., crystallises from solution upon the surface of any available nucleation site in the flow path. This process further extends the working surface for Me., deposition by increasing cathode area in the dissimilar metal couple. By x+ this means and by the high concentration of Me„ in the slow flowing solution in the zone, the consumption of Me„ from the zone is limited to close to the theoretical stoichiometric values.

Description of Preferred Embodiments

In a preferred embodiment of the invention the zone is disposed as a bed or layer extending generally parallel to the flow path. Typically the zone may have a generally tubular or annular cross-section. Similarly the flow path may have a generally tubular or annular cross-section.

One particularly preferred form of the invention requires that the flow path and the zone is a spiral of a layer of material and a permeable bed of metal respectively. The permeable bed of metal is bound on each side by the layer of material, the material having a permeability to the solution greater than that of the permeable bed, and wherein the flow path extends through the material.

The flow path can be of any configuration provided that it permits the flow of the solution. In one example the flow path may be an open matrix. This open matrix may comprise electrically conductive connected zones disposed in a three dimensional array and providing a plurality of passageways between the zones. The passageways have a low resistance to flow of the solution in comparison with the resistance to flow of the solution through the zones.

Further it has been found that pulsating the supply of the solution into the flow path has advantageous mixing effects which enhances the precipitation of metal. Mixing can also be enhanced by introducing the solution into narrow and/or convoluted flow passages in the flow path. These may define an open cell matrix of the type referred to earlier.

The apparatus and method of the invention can be applied to the removal of a number of different types of metals from metal bearing solutions. The metals of most immediate application are silver, gold and copper. For example photographic fixer solutions containing silver sulphate solutions, containing copper and chloride, or cyanide solutions containing gold residues. The reactant metal used in the zone is typically copper, zinc, iron, and mixtures of these with carbon including both simple physical mixtures and intimate mixtures of iron and carbon.

Iron/carbon alloys other than cast iron may also be used in certain circumstances, as may be alloys chosen from zinc/copper, zinc/tin, copper/tin, aluminium/tin or combinations of these alloys. Metals such as iron, steel, copper, brass and bronze can also be used from suitable solutions.

By way of attempted explanation of the chemistry the following describes what is believed to be the operation of the cell.

Using as an example of the treatment a photographic fixer solution containing dissolved silver. The following reactions are believed to occur.

Fe + 2Ag + -> 2Ag + Fe2+ (in the zone)

Iron ions diffuse into the flow path and silver metal, in pure form, crystallises from solution upon the surface of or at any available nucleation site in the flow path. The consumption of iron from the metal bed has been limited to close to the theoretical one mole for each 2 moles of silver removed from solution. Corrosion reactions which normally occur upon contact of the solution with iron in the metal bed appear to be reduced or inhibited.

Description of the Drawings

By way of example only the invention will now be described with reference to the accompanying drawings wherein:

Figure 1 shows a cross sectional representation of a packing constructed according to the invention,

Figure 2 shows, diagrammatically, a cross sectional representation of a packing with a flow path usable in the invention.

Figure 3, 4 and 5 are graphs showing performance characteristics of the packing of Figure 2.

Figure 6 is a flow profile of a packing according to the invention. Figure 7 shows, diagrammatically, a two dimensional representation, not a section, of three dimensional form of a packing according to the invention.

Description of Preferred Embodiments

A first embodiment of a packing according to the invention is shown in Figure 1 in which packing 1 is comprised of a leak-proof casing constructed from an inert plastic. The casing has a threaded cylindrical outer wall 2 on which is screwed a threaded lower end plate 3 and top end plate 4. These End plates 3, 4 are removable in order to facilitate removal of precipitated metal and replacement of packings. Top plate 4 is provided with an internally threaded downwardly-extending flange 5.

An inlet/outlet manifold 6 is mounted on and partially in flange 5. The metal containing solution A is admitted to the packing 1 through an inlet tube 7 extending through manifold 6 to communicate with a passageway 8 in packing 1. Inlet tube 7 communicates with passageway 8 via a sealing tube 9.

A lower baffle plate 11 is spaced above lower end plate 3 by a downwardly extending rim 12 and is providing with a central opening 13 with passageway 8. Baffle plate 11 is provided with openings 14 and 15. A first inert porous disk 16, overlies baffle plate 11.

Supported upon disk 16, and concentric with passageway 8 is an annular column 20. Column 20 is composed of a very porous matrix which extends between disk 16 and a second inert porous disk 27.

The annular space bounded by the radially inner wall of column 20 and the radially outer wall of passageway 8 is filled with a first permeable zone of densely-packed cast iron chips 21. The space between the outer wall of column 20 and the inner side of passageway 2 is also filled with a densely-packed bed or layer of cast iron chips 22 which constitutes a second permeable zone.

The disk 27 supported on column 20 serves to divide packing 1 into an upper and lower chamber and acts as a support for a layer of cast iron chips 23 constituting a third permeable zone disposed between layers of porous plastic foam matrix 24. The layers are wound in a roll disposed about the packing in axial direction. The space between the roll and the inner side of packing wall 2 is filled with cast iron chips 25. The space between the roll and passageway 8 and sealing tube 9 is likewise provided with a permeable zone of cast iron chips 26. However cast iron chips 26 do not completely fill this space and an empty chamber 28 is provided. Chamber 28 communicates with an outlet port 29, concentric with inlet tube 7.

In use, a solution A containing a valuable metal higher than iron in the emf series, for example a fixer solution containing silver, is admitted into the packing via inlet 7, passageway 8 and opening 13 of baffle plate 11 to lower chamber 10. From chamber 10, the liquid flows upward through openings 15 of baffle plate 11 and through first inert porous disk 16 to column 20.

The plastic foam matrix in column is highly porous and solution A flows through column 20, then through second porous disk 27 and via porous plastic matrix 24 of the upper compartment to packing 1. The solution A flows into empty chamber 28 and out through outlet port 29. There is thus a flow path from inlet 7 to outlet 29 which flows adjacent to beds 21, 22 of cast iron chips and adjacent to layers 23 of cast iron chips.

Beds of cast iron chips 21 and 22 are of low porosity in comparison with the porosity of column 20 and therefore the flow is predominantly if not entirely through the inert matrix. Substantially, no flow occurs through beds 21, 22, 23, 25 and 26 of cast iron.

Typically the chips having dimensions of approximately 4 - 5 mm x 2 - 3 mm x 1 mm for removing silver from fixer solution. However, the optimum size and shape varies with the type of solution being passed.

To further prevent flow of solution longitudinally through beds 21 and 22 of cast iron, lower seal rings 30, 31 and upper seal rings 32 and 33 are provided. The seal rings further ensure that solution flow is predominantly through column 20. However, diffusion of liquid into and out of beds 21, 22 occurs at the interface between beds 21, 22 and matrix column 20.

Likewise, in the upper compartment flow is predominantly through inert porous plastic matrix 24 in preference to flow through the layers of cast iron chips. Typically, the lower compartment defined between disk 16 and 27 occupies from 33% to the whole of the packing volume and 90% of the metal recovered is deposited in this compartment. The compartment above disk 27 constitutes a second stage in which the remaining dissolved metal is reduced and which removes approximately 95% of the metal not reduced in the first stage.

Of course, it will be understood by those skilled, in the art that, the first stage can occupy the whole of the packing and the second stage may be in a different packing in series with the first.

For preference, the porous matrix which provides the flow path and nucleation sites may comprise an open cell plastic foam, and/or a wire mesh wound to form a mesh cylinder of low flow resistance, or a concertina folded wire mesh preferably having the concertina fold lines extending in the axial direction. A preferred porous matrix is a polyester polyether copolymer open cell foam having in excess of 98% free space.

It has been found to be beneficial but not essential to dispense chips 35 of cast iron through plastic foam column 20 as isolated islands in the flow path. In use, the embodiment of Figure 1 is preferably provided with solution at the regulated flow rate. This may be achieved by coupling a suitable metering pump to deliver the solution to the packing. The embodiment of Figure 1 may be provided with an integral buffer tank (not illustrated) from which a pump draws the solution to be treated containing the dissolved metal. The buffer tank is desirable because the delivery of solution from a photographic processor is intermittent and variable.

The present invention allows the use of a simple low (12 volts) voltage power supply to drive the pump and the monitoring system and requires little or no operator supervision. Solutions delivered into the buffer tank may be monitored by simple level switches which switch ^'the power on and off to the pump and if necessary to an alarm should an overflow or other fault condition occur.

With reference to Figure 2 there is shown diagrammatically, in cross-sectional elevation a cylindrical packing 34 provided with an end inlet 35 and an outlet 36. The inlet and the outlet are situated on the cylinder axis.

Adjacent inlet 35 is a porous cylinder 37 of open cell foam matrix. A similar cylinder 38 of foam matrix is fitted in adjacent outlet .36. A formation 39 of open cell foam matrix extends between cylinder 37 and 38. The foam matrix has in excess of 98% free space. Formation 39 has a hollow cylindrical interior and reduces stepwise in outside diameter in the axial direction being of greatest diameter at the feed end. The formation may be integral with cylinder 39 or 38. The remainder of the space interior of packing 34 which is not occupied by the open cell foam is, in this embodiment, initially packed with granular cast iron chips 40. Cast iron chips 40 have the range -2.8mm to +2.0mm.

In use, solution from inlet 35 flows through the open cell foam 37, 39, 38 to outlet 36 in preference to flowing through the densely packed cast iron packing 40.

Nevertheless cast iron packing 40 interfaces with the open cell foam and is wet by the solution. Metal ions are able to diffuse across the foam/cast iron interface from the flow path into cast iron packing 40 and vice versa.

The silver deposits on the plastic foam, or any suitable nucleation surface in the flow path and progressively forms an extremely dense but porous mass which may be 2 - 3 cm thick. There is little penetration of the cast iron bed by the silver and, indeed, if silver-containing solution passes through the cast iron bed the efficiency of the system drops.

Figure 3 shows a graph of silver recovery against operating time from which the high efficiency of the unit is evident.

Figure 4 shows that the efficiency of the unit was not significantly effected by shut downs of 17 hours after 121 hours of operation or by 64 hours after 153 hours of operation. Figure 5 illustrates the effect on recovery of start-up after shut down as the concentration cell is re-established.

More particularly treatment of the daily production of effluent solution can be catered for. One of the benefits of the slow reduction of the silver according to the invention is that photographic fixing solutions do not appear to degrade appreciably and may be used again at least once for fixing and the reduced silver remains pure and crystalline. The solution leaving the cell typically is clear and provided the feed solution does not contain suspended particles and the cell is not physically disturbed, no solid particles should be present.

The effective life of packings according to the invention is extended in comparison with conventional metal replacement packings.

Packings according to the invention may be much smaller than prior art conventional packings. Packings as small as 1.6 litres in capacity and holding 3 - 4 Kgs of silver have been tested. This equals the most common retention from 20 litre packings of the iron wool replacement packing. Moreover, in packings according to the present invention, the silver is deposited in the inert matrix as pure white silver crystals which are not sulphided. This makes the refining of silver recovered a simple matter of initially flushing all residual solution from the packing, draining and melting.

Figure 6 shows the initial flow velocity profile of the solution through a packing having cast iron beds 41 and flow paths 42. Clearly the flow velocity of the solution through beds 41 is much smaller than that through flow path 42. This differentiation is believed to increase further upon continued use of the packing.

Figure 7 is a two dimensional representation, not a section, of a three dimensioned embodiment of the invention in which a network of interconnected chanels extends throughout a packed bed of cast iron chips. In the actual embodiment both the cast iron packed bed and the network of chanels are individually continuous.

More particularly cylinders 42, 44 of open cell foam are situated respectively adjacent the inlet and the outlet. However, a continuous bed of cast iron chips 45 is shown in cross hatching in Figure 7. Similarly the chanels 47 are also continuous providing flow paths extending from the inlet to the outlet and passing adjacent to the boundaries of the continuous bed 45.

While the above described embodiments are of preferred geometry, it will be apparent that the apparatus may be embodied in other forms and that modifications of the apparatus and method can be made which fall within the scope of the invention as it has been broadly described herein.

The following are illustrative examples of the invention

Example 1

A single packing of the structure described in figure 2 was used to recover silver from a silver-laden fixer solution from an operating X-ray film processor. The apparatus was operated at a steady flow rate of 1.2 litres per hour with silver concentrations in the feed of 3 to 5 grams per litre the packing. After a 20 hour start up period during which efficiency rose to between 98 and 99%, 98 to 99% of the silver passing through the inlet for a period of in excess of 400 operating hours was recovered. The efficiency of the unit was not significantly affected by shut down periods of 17 hours after 121 hours of operation and 64 hours after 153 hours of operation.

The packing was subsequently operated at variable flow rates which further showed that the flow rate only caused small variations in silver recovery efficiency and the packing was still operating at in excess of 97% efficiency after 600 hours when the run was discontinued.

The invention has also been applied to recover silver from a range of photographic colour fixers namely EP-2 and RA types. These solutions behave very similarly to one another regardless of the origin of the manufacturer.

Example 2

Solutions of EP-2 were transferred to a 2-packing in series prototype for processing.

The pump speed was set to deliver 25 litres of fixer per 24 hour day.

Samples of fixer were collected daily for analyses from the feed and effluent streams. The effluent stream was also monitored using a copper strip detector. The silver content of the effluent remained fairly constant for the best part of the test at 2 to 5 parts per million. At these levels silver remained undetectable by the copper strip. Eventually, as the first packing became exhausted of active elements, the silver level rose to 50 parts per million (pp ) . At this level the copper strip was effective in detecting the presence of silver. At this point the experiment was stopped.

In all, 375 litres of EF-2 solution had been processed containing 3.61 grams of silver per litre.

The packing was then cut open, dried weighed and analysed. The contents weighed 1530 grams and assayed 83.73%. Therefore, a silver content of 1281 grams recovered in the packing.

The content of the packing was then melted in a crucible using sodium carbonate, silica flour and borax, and poured into an ingot.

The silver ingot weighed 1215 grams and assayed 98.15%. Some silver having being lost during the melting through sublimation and as shot in the slag.

Example 3

Solutions of RA fixer were transferred to the prototype packing for processing. The additive tank of the prototype was filled with a 25% ammonium citrate solution and the pump was calibrated to deliver the additive at a rate of 20 ml of additive per litre of fixer to be processed (2%) . The pump was set to deliver 25 litres of fixer per 24 hour day. As in example 2, samples of fixer were collected from the feed and effluent streams and spot checks were made with the copper strip detector. Silver content in the effluent stream remained fairly constant for the best part of the test at under 5 parts per million. The test was stopped as the silver level peaked at 60 ppm. well over the detection threshold of the copper strip tester.

In all 475 litres of RA fixer containing 2.90 grams of silver were treated in this way. The packing was then cut open, dried, weighed and assayed. The contents weighed 1639 grams and assayed 78.59% silver. The silver content recovered in the packing being 1288 grams.

The contents of the packing was then melted as in example 2 and poured into an ingot.

The silver ingot weighed 1235 grams and assayed 98.00%. Some silver having being lost during the melting through sublimation and/or as shot in the slag.

The invention was also applied to recover copper from copper sulphate solutions.

Example 4

A copper sulphate solution was prepared by dissolving 6000 grams of copper sulphate into 100 litres of water. The pH of the solution was recorded as 3.6. This solution was gravity fed into a prototype packing at a rate of 16 litres per day. The test was stopped when blockage of the packing occurred.

In all 83 litres of the copper sulphate solution had gone through the packing.

The packing was then cut open, dried, weighed and analysed. The contents weighed 1533 grams and assayed 80.3% copper. The copper content recovered in the packing therefore would be 1230 grams.

The contents were not treated further. It was noted however that the resulting product was very fine and that upon drying the copper oxidized very quickly.

The invention was also applied to the recovery of gold from cyanide solutions.

Example 5

In this test the packing dimension was changed but the overall structure was retained and had the same characteristics as the packings used for silver recovery.

The dimension of the packing being 3 cm inside diameter by 10 cm high. Only one packing was used for this application.

A solution of gold cyanide was prepared by dissolving 20 grams of gold in 2 litres of a 5% cyanide solution, made alkaline with caustic soda to a pH of 8.5 and with the addition of 100 ml of 35% hydrogen peroxide. The solution was diluted to 100 litres and gravity fed through the packing.

The effluent was conserved and treated with zinc dust to recover any gold still present. The residual zinc and precipitate was recovered, washed and digested in hydrochloric acid. The residue was collected and melted in a reducing flask of litharge. The resulting lead button was coupled and the gold recovered and weighed.

The total gold recovered in the effluent stream was 0.9152 grams at a loss of 4.58%. ^' On the recovery side this translates into a 95.424% recovery. Perhaps better recovery could be achieved with two packings in series. However, due to the costly nature of such tests the testing was stopped.

The invention is applicable for recovery of metals from solution for example of silver from photographic fixer solution as well as for recovery of valuable metals from minerals, ores and the like but is not limited to these uses.

Packings according to the invention have a number of surprising advantages over prior art. In particular dealing with silver recovery the silver obtained is generally of higher purity than obtained with conventional iron replacement packings. The process proceeds to an end point which is readily determinable. The system of the invention is much better able to operate on intermittent flows and indeed will tolerate long idle periods. The cast iron shows no tendency to precipitate ferric hydroxide when used for black and white acid fixer, on standing for periods up to six months even when the internally generated pH rises above 9. Surprisingly, also, the efficiency achieved is higher than obtainable with conventional metal replacement packings and electrolytic systems and permits a smaller packing volume to be used than has previously been practicable. For example, in the "Kodak" System using only iron or steel wool in a replacement packings, precipitation of hydroxides causes blockages which require the volume of the packing to be of the order of 20 litres at least. Embodiments of the invention permit packing volumes as low as 1.0 litres to provide equal removal.

Claims

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:-

1. An apparatus for removal of a metal from a solution containing the metal, the apparatus comprising:- a container having an inlet and an outlet; a flow path for conducting the solution from the inlet to the outlet; and at least one zone comprising a reactant metal which, in the emf series, is lower/more active than the metal in the solution; the zone being disposed adjacent to the flow path to permit a minor portion of the solution to permeate from the flow path into the zone.

2. Apparatus according to claim 1 wherein the zone is disposed as a bed or layer extending generally parallel to the flow path.

3. Apparatus according to claim 1 wherein the flow path is of generally tubular or annular cross-section.

4. Apparatus according to claim 1 wherein the zone is of generally tubular or annular cross-section.

5. Apparatus according to claim 1 wherein the flow path and the zone is a spiral of a layer of material and a permeable bed of metal in which the permeable bed is bounded on each side by the layer of material, the material having a permeability to the solution greater than that of the permeable bed, and wherein the flow path extends through the material.

6. Apparatus according to claim 1 wherein the flow path extends through an open matrix.

7. Apparatus according to claim 6 wherein the open matrix comprises a network of interconnected passageways extending throughout the zone, the zone being continuous, the passageways having a low resistance to flow of the solution in comparison with the resistance to flow of the solution through the zone.

8. Apparatus according to claim 1 wherein the flow path has a cross-section which is reduced in dimension progressively or stepwise as distance along the flow path from the inlet increases.

9. Apparatus according to Claim 1 further comprising at least one nucleation site for precipitation of the metal to be removed from the solution, in or at the border of the flow path and the zone.

10. A method for removing a metal from a solution containing the metal comprising the steps of:-

(1) supplying the solution into a flow path having at least one zone adjacent thereto, the zone comprising a reactant metal which, in the emf series, is lower/more active than the metal in the solution, and (2) maintaining the supply of the solution into the flow path at a rate sufficient to permit a minor portion of the solution to permeate from the flow path into the zone and to establish a concentration of the metal in the solution in the zone different to the concentration of the metal in the solution in the flow path.

11. A method according to claim 10 wherein the supply of the solution into the flow path is pulsated.

12. A method according to claims 10 further comprising the step of mixing the solution in the flow path by introduction the solution into narrow and/or convoluted flow passages.

13. A method according to claim 10 further comprising the step of establishing electrical conductive connection between the metal in the solution in two or more of the zones.

14. A method according to any one of claims 10 to 14 wherein the solution flows through one or more passageways defined by an open cell matrix in the flow path.