Various methods, which make use of different precipitation and cementation agents, have been proposed to recover silver from acidic brine solutions.

Sulfide ions can be used to precipitate silver sulfide". Disadvantages are that foreign ions are introduced into the system and the resulting silver sulfide product needs further processing.

Iodide precipitation is fairly effective to recover silves"', especially if selectivity over copper is required, but again foreign ions are introduced. Furthermore the iodide reagent is potentially expensive and the silver iodide product formed needs further processing.

Cementation of silver with various metals is possible and metals such as iron, zinc aluminium3) and copper'"'have been suggested for this purpose. The choice of metal depends largely on the cementation kinetics, the cost and availability of cementation agents, the brine solution pH and the consequences of foreign ions entering the system.

At low pH, high metal consumption may result. Depending on the silver product required, cementation could be favoured over precipitation since silver-metal is formed.

Contamination from the undissolved metallic cementation agent, however, could present a problem. Co-cementation of other elements in the brine solution can also cause

contamination.

It is possible to use copper (I) to cement silver from an ammoniacal brine solution at pH > 7 ('). For an acidic brine solution containing silver, however, this involves addition process steps of pH adjustment and ammonia removal and recycle. In a system containing iron the formation of ammonium jarosite on re-acidification and recycling of the brine solution could also be problematic.

SUMMARY OF INVENTION The invention provides a method of recovering silver from an acidic brine solution which includes the steps of: (a) adding copper (I) to the solution to cement silver (I) and to produce a slurry, and (b) recovering silver as silver cement by separation from the slurry.

The method may include the step of ensuring, when necessary, that the pH of the solution is at a value of between 0,1 and 5. Preferably the pH of the solution is between 1,5 and 3,0. Thus, where necessary, the method may include the step of: (c) adjusting the pH of the solution prior to, or during, step (a).

Step (a) may be carried out in one or in a plurality of stages.

The silver cement may be separated in any appropriate way but preferably use is made of a liquid/solid separation process.

At least part of a brine solution which is produced by step (b) may be used to provide at least part of the copper which is required for the cementation step (a).

The method thus preferably includes the steps of: (d) splitting the solution which is produced by step (b) into at least first and second streams, (e) removing copper from the first stream, (f) reducing copper (II) at least in the second stream to copper (I), (g) clarifying at least the second stream by removing metallic copper, and (h) using at least the clarified second stream to provide copper (I) for step (a).

Step (f) may be preceded by a step of : (i) removing impurities from the second stream.

In step (g) the copper solution may be clarified in any appropriate way but preferably use is made of a solid/liquid separation process.

The method of the invention may include the step of : (j) using metallic copper, which is separated in step (g), in step (f) by adding the metallic copper to the second stream.

Step (j) may be preceded by the step of: (k) combining at least part of the first stream with the second stream, obtained after step (i), and reducing the resulting combined stream in step (f).

Step (k) may be preceded by the step of: (I) removing impurities from the first stream.

Copper may be removed from the first stream, in step (e), in any appropriate way, for example by using sulfide to precipitate copper as copper sulfide.

The acidic brine solution may arise in different ways and, for example, may arise from the brine leaching of a silver and copper bearing bio-leach residue. In this particular case the copper sulfide may be recycled to the biological leaching stage.

Losses of copper and silver are avoided by recycling the copper sulfide, in the manner indicated, but this is not limiting for alternative methods such as solvent extraction, ion exchange and standard cementation techniques can be used to remove the copper.

It is also to be understood that the silver bearing material which is subjected to brine leaching can be produced in any appropriate way and need not necessarily be produced as a bio-leach residue. Such material may for example include concentrates, pressure- leach residues, roasted concentrates or anode slimes from electrowinning operations.

BRIEF DESCRIPTION OF THE DRAWING The invention is further described by way of example with reference to the accompanying drawing which is a flow chart representation of one way of carrying out the method of the invention.

DESCRIPTION OF PREFERRED EMBODIMENT The accompanying drawing illustrates in block diagram form one embodiment of the invention.

A silver bearing material 10 is subjected to a bio-leaching process 12 to produce a silver and copper bearing residue 14. The residue is subjected to a leaching step 16 which involves the addition of a brine solution 18. This is followed by a solid/liquid separation step 19 to produce a silver (I) containing brine solution 20 which contains copper and, possibly, other impurities. The copper may be present naturally or, as emerges hereinafter, may be added.

Normally the pH of the brine solution 20 is of the order of 2,0. Where necessary though the pH of the brine solution is adjusted in a step 22 so that it lies between 0,1 and 5,0 and, preferably, is between 1,5 and 3,0.

The solution 20 is then subjected to a cementation process 24 which involves the cementation of silver (I) in the solution by copper (I) (block 26). The following reaction applies: Ag + Cu"= Ag° + Cu'* During the cementation step 24 the solution should be maintained between ambient and 90°C and preferably is between ambient and 50°C. The cementation process may be carried out for up to two hours but normally from twenty to forty minutes is adequate.

Atmospheric pressure may prevail during the cementation step.

The cementation step 24 can be done in a single stage or in a number of stages with multiple additions of copper (I).

The cementation step produces a slurry 28 which is then subjected to a liquid/solid separation step 30 to produce silver cement 32 and a substantially silver-free brine solution 34. The silver cement 32 may be treated as required to recover silver products.

The silver cement may contain some copper contamination if conditions are such that some disproportionation of copper (I) to copper metal and copper (II) occurs.

The composition of the slurry 28 is substantially the same as of the solution 20, but with most of the silver removed and higher copper levels.

The solution 34 is, as indicated in the flow chart, split into a first stream 34A and a second stream 34B. The ratio of the split depends on the amount of copper (I) which is required for the cementation step 24 and the amount of copper (I) which is expected to be produced by oxidation of metallic copper by oxidants other than copper (If), eg. iron (III). The first stream 34A may include from 20 to 80% of the solution 34 but normally from 35 to 65% of the solution is adequate.

It should be borne in mind that sufficient excess copper (I) (biock 26) should be produced so that all of the silver (I) is cemented. From 0 to 100% excess copper (I) ensures complete silver cementation but the excess should be limited to prevent formation of metallic copper and copper (II), which would cause contamination of the silver product 32.

The use of a non-oxidising atmosphere in the cementation step 24 could be considered to prevent degradation of copper (I).

The first stream 34A undergoes a copper removal step 36 of any appropriate type. For example use may be made of sulfide 38 to precipitate copper as copper sulfide 40 which can then be recycled to a sulfide oxidation stage, for example the biological leaching step 12 (in the case in which use is previously made of the biological leaching step).

Any remaining silver that precipitates with the copper will hence be returned to the leaching, and consequently the cementation, circuit. A substantially copper-free brine solution 42 is produced. This solution may be subjected to a stage 44 for the removal of impurities 46. The solution 42 contains the impurities that were present in the original brine solution 20 and the step 44 is used to remove most impurities besides elements like the alkaline metals. Further impurity removal treatment such as jarosite precipitation or the introduction of a bleed stream 48 may be resorted to if such elements become problematic. The brine solution produced after removal of the impurities 46 carries the reference numeral 50.

The second stream 34B of the solution 34 undergoes an optional impurity removal stage 52 for the removal of impurities 54. The stage 52 may make use of precipitation, solvent extraction or ion exchange techniques depending on the nature of the impurities. Possibly problematic impurities are removed except copper and a limited amount of ferric iron. The resulting copper (II) solution 56 is then combined with the solution 50 and the copper (II) in the combined solution is reduced to copper (I) in a step 58 by the addition of metallic copper 60 to produce copper (I) (62).

Single or multiple stages can be used for the production of the copper (I). An alternative to the metallic copper 60, eg. brass or bronze, may be resorted to but with limitations.

Other strong oxidants such as ferric iron may also react with the metallic copper to produce copper (I).

The following reactions demonstrate the generation of copper (I): <BR> Cu+ Cu°==2Cu*<BR> Fe3++ Cu°= Cu + Fe2 The copper (I) solution 62 is clarified by subjecting the solution to a solid/liquid separation stage 64 wherein excess metallic copper 60 is separated from the solution 62. The resulting clarified copper (I) solution 26 is used in the cementation reaction 24.

The production of copper (I) (step 58) should be carried out at temperatures between ambient and 90°C, typically between ambient and 50°C. The pH of the solution should be maintained between 0,1 and 3,0 and preferably lies between 0,5 and 1,5. Up to four hours may be required for the reaction but typically less than one hour is adequate. The chloride content of the solution should be between 20 and 200 g/l depending on the chloride concentration in the brine feed stream 20.

The brine solution 20 should contain 20-200 g/l chloride ions and some silver (I), from 1 ppm to saturation levels. Other elements that may be tolerated in the brine solution are As, Ca, Cu, Fe, K and Na, at levels of the same order of magnitude as or higher than that of silver, as well as traces of Al, B, Ba, Be, Bi, Cd, Co, Cr, Li, Mg, Mn, Mo, Ni, Pb, Sb, Si, Sn, Sr, Ti, V, Zn and Zr. Non-oxidising anions and non-precipitating anions with respect to Cu* and Ag+, such as oxidised sulfur species (sulfates, thiosulfate, thionates, etc), acetates and short chain carboxylates can also be tolerated. Some of the more oxidising

anions (such as chromates and sulfites) are not expected to be present in any significant quantity due to their reduction during reduction of copper (II) and hence can be tolerated.

The list of impurities is not comprehensive and moderate levels of most impurities that do not affect the stability of copper (I) and do not cause precipitation of silver or copper are expected to be acceptable.

It is apparent that in the method of the invention copper (I) is used to cement silver at an acidic pH and does not result in unwanted contamination by foreign species in the copper system. The invention thus enables silver to be recovered directly from an acidic brine solution without resorting to an intervening step of rendering the solution ammoniacal.