WO1982000478A1

WO1982000478A1 - Recovery of metal values

Info

Publication number: WO1982000478A1
Application number: PCT/AU1981/000096
Authority: WO
Inventors: Pty Ltd Anumin
Original assignee: Parker A
Priority date: 1980-08-06
Filing date: 1981-07-24
Publication date: 1982-02-18
Also published as: EP0057684A1

Abstract

Gold and silver, and their sulfides, selenides and tellurides are oxidized with water miscible dipolar aprotic solvents containing bromides and/or chlorides and as oxidant cupric and/or ferric ions. This oxidation reaction is used to recover gold and silver; to produce gold mirrors and to etch copper and gold.

Description

RECOVERY OF METAL VALUES

THIS INVENTION relates to the reccvery of metal values and particularly gold and silver.

Australian Patent Application No. 39230/78 is directed to a process for the recovery or refining of silver compris ing the steps of, if required, converting the silver to silver chloride, dissolving the silver chloride in dipolar aprotic solvent such as dimethylsulfoxide in the presence of additional chloride salts such that the silver chloride dissolves as the dichloroargenate salt, separat ing any insoluble solids, precipitating purified silver chloride from solution by the addition of water or methanol and separating the silver chloride precipitate.

Throughout the following description and claims the term halide refers only to chloride or bromide. It has now been established that copper (II) halides, or ferric halides dissolved in dimethylsulfoxide, dimethylformamide or acetonitrile or other water miscible dipolar aprotic solvents or mixtures of such dipolar aprotic solvents containing halide salts of cations such as sodium, copper, potassium, lithium, tetraalkylammonium, calcium and magnesium are very good oxidising agents for oxidising metals which form halo-complexes, such as copper, silver and gold, and that such an oxidising agent is very useful in the recovery of metal values from materials which can be oxidised to form halo-complexes. Other oxidants, such as chlorene or bromine in acetonitrile containing halide ions, can oxidise gold or silver.

Thus in one form the invention resides in an oxidising agent comprising a water miscible dipolar aprotic solvent or mixtures of such solvents containing halide ions, copper (II) and/or ferric ions and cations (including cupric and ferric) which forms soluble salts with the halo complex anions in the appropriate solvent. In another form the invention resides in a method of oxidising gold, silver and their sulfides selenides or tellurides and dissolving them as their gold or silver halo-complex salts leaving insoluble elemental sulfur, selenium or tellurium as appropriate, comprising leaching the material containing gold and/or silver with a water miscible dipolar aprotic solvent containing halide salts and an oxidant which does not react extensively with the solvent but does oxidise gold, silver and their sulfides selenides and tellurides in solution in the solvent.

The various aspects of the invention and the application of the oxidising agent in the recovery of metal values will be readily apparent from the following description in which dimethylsulfoxide (DMSO) is taken as a typical water miscible dipolar aprotic solvent and cupric chloride is the oxidant. Dimethylformamide and acetonitrile are also preferred water miscible dipolar aprotic solvents, and cupric bromide or ferric halides are other preferred oxidants.

The invention may be applied to a method of dissolving copper, silver, and gold such as are found in anode slimes and of recovering silver halide and gold from the resulting solutions. It also may be applied to a method of refining silver and gold from oxidisable materials such as bullion or dental alloys containing those elements. Refining is achieved by leaching said materials with copper (II) halide in a dipolar aprotic such as dimethylsulfoxide (DMSO) containing halide salts of inert cations such as sodium, potassium, lithium, calcium, tetraalkyl ammonium, magnesium, copper and iron, separating the resulting solution, containing halo-complexes of silver, gold and copper, from insoluble material then adding a hydroxylic solvent, such as water or methanol. On addition of the hydroxylic solvent, silver halide precipitates and is separated. If copper (I) halide and gold halo-complex are present, gold is reduced and precipitated and is separated also. Any remaining copper (I) halide may be oxidised to copper (II) halide for recycling, using for example oxygen and HCl or chlorine or bromine as oxidants.

The invention is also applicable as a method of recovering gold from solutions containing halo-complexes of gold in dipolar aprotic solvents, whereby water or methanol are added to such solutions to destabilize the gold halo complex. In the presence of copper (I) halide or other reductant, gold is reduced and precipitates.

AuCl-₂ + CuCl-₂ → CuCl₂ Au + 2Cl- -(1)

The gold is believed to be in solution as Au(Eal₂)-. Other reductants besides Cu(I) salts include ferrous salts, oxalic acid, SO₂, hydrazine as are familiar to those skil led in the art of reducing gold (I) and gold (III). The invention is also applicable as a method of leaching copper, silver or gold from suitably masked materials to produce circuit boards for electronic devices. The reactions in dipolar aprotic solvents containing chloride ions are believed to be as follows:-

Cu + CuCl₂ + 2Cl- → 2CuCl-₂ -(2)

Ag + CuCl₂ + 2Cl- → AgCl₂- + CuCl-₂ -(3)

Au + CuCl₂ + 2Cl- →

AuCl-₂ + CUCI₂- -(4)

The method of leaching halo-complexes of precious metals from materials containing the metals, their sulfides, their selenides their tellurides and their halides is especially well suited to processing anode slimes from copper refineries. The precious metals are recovered as metal, or halide by adding hydroxylic solvents, such as water or methanol, to the dipolar aprotic leach solution containing the metal halo-ccmplexes and ferrous or cuprous halides and collecting the precipitated solids. The halo-complexes are destabilized by adding hydroxylic solvents to the dipolar aprotic. Impurities such as silicates and sulfur selenium and tellurium remain in the leach residue.

The concept of oxidation and dissolution with cupric or ferric halides of oxidisable materials which form stable halo-complexes, in dipolar aprotics, separation from insolubles, precipitation of precious metal values either as halide or metal by adding a hydroxylic solvent to the dipolar aprotic solution, in the presence of a reductant if gold complexes are present and separation of the precipitated precious metal values, provides a new method of recovering and refining materials containing silver and gold, such as gold bullion, coins, anode slimes, gold plate, dental alloys, cemented silver, computer scrap, photographic residues and catalysts in inert supports.

The methods are based en the following chemistry. In dry dipolar aprotics such as DMSO, containing a source of halide ions, either as a suspension or in solution iron (III) or copper (II) halides are powerful oxidising agents of E° greater than 0.6V nhe. In dry acetonitrile cupric halides are even more powerful , even in the absence of other sources of halide ions, because of strong solvation of copper (I) by acetonitrile.

Thus many species which form halo-complexes can be generated in dipolar aprotic solvents containing halide ions by oxidising the appropriate compounds. The oxidant is useful for oxidising sulfides, selenides and tellurides of many metals, leaving a residue of sulfur, selenium or tellurium. Typical reactions are shown in equations 2 to 10 inclusive. Ag₂S + 2CuCl₂ + 4Cl- → 2AgCl₂- + 2CuCl₂- + S -(5

Ag₂Se + 2CuCl₂ + 4Cl- → 2AgCl₂- + 2CuCl₂- + Se -(6) Pb + 2CuCl₂ + 3Cl- → PbCl₃- + 2CuCl-₂ -(7)

Cu₂S + 2CuCl₂ + 4Cl- → 4CuCl₂- + S -(8)

ZnS + 2CuCl₂ + 4Cl- → ZnCl² ₄- + 2CuCl-₂ + S -(9)

CuFeS₂ + 4CuCl₂ + 4Cl- → FeCl⁴ ₂ + 5CuCl₂- + 2S-(10)

Of course, other halo-complexes, e.g. AgCl² ₃- than we have written may be present. In some cases (eg. with chlorine as oxidant) , the gold may be oxidised to the +3 oxidation state. It is possible to use other dry dipolar aprotic solvents than DMSO e.g. dimethylformamide, which are good cation solvators or to mix them with DMSO. It is of course essential that the halide salt be chosen so that its cations form soluble salts with halo-complexes of the metals being treated, even if the halide salt {eg. NaCl) itself is not appreciably soluble in the dipolar aprotic solvent (eg. DMF ) . Acetonitrile is a somewhat special case, since it solvates Cu⁺ so selectively, such that salts like Cu⁺AgCl-₂, Cu⁺AuCl₂- are soluble, whereas Na⁺ and Li⁺ salts are not. Thus CuCl₂ or CuBr₂ without other halide ions are especially good oxidants in acetonitrile.

The following examples illustrate some aspects of the invention as discussed above.

Example I

0.65g of particulate gold was stirred for one hour with 5ml of lMCaCl₂ in dry DMSO at 80°C, containing lg CuCl₂. Some of the gold dissolved, leaving 0.23g undissolved and a solution containing 84g/l gold. On addition of 10ml of water gold metal (0.42g) slowly precipitated. Example II

6.5g of finely divided silver selenide was stirred with 20g of CaCl₂.2H₂O in 100ml of dry DMSO containing 17g of CuCi₂.2H₂O at 80°C for one hour. The silver dissolved to give 41g/l of silver in solution. The residue contained selenium and a little unreaσted Ag₂Se. On addition of 100ml of water to the DMSO solution, silver chloride precipitated quantitatively. A similar result was obtained with silver sulfide, except that sulfur was found in the residue after leaching, rather than selenium.

Example III

5g of particulate silver was stirred with 20g of CaCl₂. 2H₂O in 100ml of dry DMSO containing 17g of CuCl₂.2H₂O at 80 °C. the silver dissolved to give 45g/l silver in solution. On addition of 150ml of water, silver chloride precipitated quantitatively. A similar result was obtained when a suspension of 15g NaCl or KCl replaced the CaCl₂.2H₂O and when 17g FeCl₃.6H₂O replaced the CuCl₂.

Oxygen was bubbled through the filtrate for 5 hours and the PH was maintained below 2 by the addition of HCl. The resulting CuCl₂, CaCl₂, DMSO, H₂O solution was distilled under reduced pressure to remove the water and the dry oxidant in DMSO dissolved a further 5g of particulate silver. On addition of 150ml of methanol, silver chloride precipitated. Caution: At temperatures aiove 130 °C, CuCl₂ reacts vigorously with DMSO, so such temperatures should be avoided, or DMF used in place of DMSO.

Example IV

Experiment III was repeated with a solution containing 90ml of DMSO and 10ml of DMF in place of the 100ml of DMSO. All the silver dissolved and precipitated as AgCl on addition of water.

Example V

1.5g of dry anode slimes from Copper Refineries Pty. Ltd. of Townsville, Australia, containing about 20% silver as well as copper lead, sulfur, selenium and other components including DMSO soluble organic material, was stirred for one hour with 4.0g of CaCl₂.2H₂O in 30 ml DMSO containing 4.2g CuCl₂.2H₂O from which water had been distilled. The slurry was centrifuged and the insoluble fraction weighed 1.13g. It contained no gold or silver. Water (15ml) was added to the DMSO filtrate to precipitate 0.1g of a white solid containing silver and a barely detectable trace of gold. The remainder of the silver (60%) and most of the gold (74%) remained in the filtrate possibly as colloidal gold because the leach solutions were less concentrated in gold and silver (<3g/1) than desirable (>10g/1).

Example VI

The following materials dissolved in 50ml of dry DMSO containing 5g CuCl₂.2H₂O, and 5g CaCl₂.2H₉O from which water had been distilled under reduced pressure. Solutions were stirred and heated at 80ºC for one hour, filtered then 50ml of water was added and the precipitate was weighed.

a) 25ml of solvent b) equal weights c) as gold Example VII

Some scrap material (1.07g) frcm a computer consisting of mild steel, copper and silver in layers, such as is used for preparing printed circuits, was first treated with

CuSO₄ in water at 80ºC to dissolve the mild steel and cement out copper. The solid was dried and dissolved over 10 hours at 80ºC in 50ml of dry DMSO containing 5g CaCl₂. 2H₂O and 5g CuCl₂.2H₂O. The residue weighed 0.02g. On addition of water (50ml) white silver chloride precipitated.

Example VIII

In another experiment with the material of Example VII, the mild steel face was masked as were portions of the silver face. After submersion in the DMSO oxidising solution for one hour, the unmasked silver faces were etched. Thus composition has application for etching copper and silver. In another form the invention resides in a method of recovering gold and silver from materials containing same which comprises leaching the material containing the gold and silver with a water miscible dipolar aprotic solvent containing halide salts and an oxidant which does not react extensively with the solvent but does oxidise gold and silver and their sulfides and selenides in the solvent, separating the leach solution from the material, adding water to the leach solution to precipitate silver halide and, in the presence of a reducing agent such as CuCl-₂ or FeCl₂ or SO₂ to precipitate gold, said reducing agent being sufficiently reactive to reduce chlorine, bromine and gold salts in aqueous dipolar aprotic solvents but insufficiently reactive to reduce copper halides to copper metal .

In acetonitrile and other nitriles the Cu⁺ ion is very strongly solvated (other cations are not). Thus in dry CH₃ CN the following useful reaction can occur without excess halide ions being present and cupric halides (but not ferric halides) are especially good oxidants.

Cu²⁺ + 2 Cl- + Au → Cu⁺ + Au Cl^-2 or (Ag, Ag₂S, Ag₂ Se) or Cu²⁺ + 2 Br- + Au → Cu⁺ + AuBr-₂

Thus in dry acetonitrile additional chloride or bromide salts are not necessary to form the desired complex. The source of chloride and bromide ions may be excess Cu Cl₂ or Cu Br₂ when using dipolar aprotic solvents other than acetonitrile. Cu²⁺ in the presence of bromide ions in DMF and DMSO is a more powerful oxidant for gold and silver than Cu²⁺ in the presence of chloride ions. Materials containing silver are oxidised faster and at lower potentials than are materials containing gold.

In another form the invention resides in a method of recovering gold and silver separately from materials containing same which comprises leaching the material containing the gold and silver with a water miscible dipolar aprotic solvent containing halide salts and an oxidant which does not react extensively with the solvent but does oxidise gold and silver in solution in the solvent, separating the leach solution from the material, if necessary oxidising the leach solution to remove any reducing agents present in the leach solution, adding water to the leach solution to precipitate silver halide, separating the precipitated silver halide, and adding a reducing agent to the separated solution to precipitate the gold, said reducing agent being sufficiently reactive to reduce chlorine, bromine and gold salts in aqueous dipolar aprotic solvents but insufficiently reactive to reduce copper halides to copper metal .

Some further aspects of the invention will be better understood by reference to the following specific examples.

Example IX

A 0.69g lump of a dental allow containing 90% gold was leached with 15ml 1M CuCl₂, 1M CaCl₂ in DMF at room temperature for 7 days. 35% of the alloy dissolved and 0.23g of gold entered the solution. After 1 further day, only 2% more gold dissolved. On addition of water, 95% of the leached gold precipitated. The remaining lump (0.442g) was leached with 0.2M CuCl₂, 0.5M LiBr in 15ml of DMSO. After 6 hours the weight had reduced to 0.420g.

Example X

0.5g of an anode slime supplied by the Electrolytic Refining and Smelting Co. of Pt. Kembla, was leached with 10ml of DMSO containing 0.15M CuCl₂ and 0.05M CuCl₂- in DMSO containing 0.2M CaCl₂. After 22.5 hours at 25° with stirring, 70% of the silver and 15% of the gold had dissolved. When 0.2M LiBr was added, a further 60% of the gold and the remainder of the silver dissolved in 2 hours.

Example XI

0.2475g of -200 mesh chalcopyrite was oxidised with 5ml of 0.7M CuCl₂, 0.7M CaCl₂ in DMSO at 80° for 1 hour. The residue weighed 0.115g and contained 0.08g sulfur. Thus CuFeS₂ is oxidised by this oxidant.

Example XII

60ml of DMSO containing 1M CaCl₂ and 1M CuCl₂ was dehydrated by distilling off the water and mixed with 40ml of acetone. It was stirred for one hour with 2.5g of -40 micron silver powder and 2.5g copper powder. The residue weighed 1.077g and was mainly silver. On addition of water, easily filterable AgCl was obtained.

Example XIII

A solution of anhydrous CuCl₂ (1.5g), LiCl (5.8g) in 50ml acetone was stirred with 0.51g of gold powder for 2 hours. The solution had the odour of mesityl oxide. After 2 hours, 0.21g gold had dissolved and was precipitated by adding water. Example XIV

1M NEt₄ Cl plus 0.5M anhydrous CuCl₂ in 15ml acetonitrile was stirred at 35°C for 3 hours with 0.503g gold powder. The residue weighed 0.222g. On dilution with 15ml of water, 0.247g of gold precipitated.

Example XV

10ml of 0.1M AuCl₃ in dry DMSO was stirred with 0.550g gold powder at 40 °C for 2 days. 0.526g of the gold dissolved. On addition of H₂O no gold precipitated but on addition of CuCl, gold precipitated. Thus AuCl₃ in dipolar aprotics containing chloride ions is also an oxidant for gold and a reducing agent like CuCl is needed if gold is to be precipitated from solutions of AuCl-₂ in such solvents by adding water.

Example XVI

10 ml of solution containing 0.25 M CuCl₂ and 1 M LiCl in dry acetonitrile was stirred with 0.2 g gold at 17°C for 2.5 hours. The gold dissolved to give 0.1 M gold in solution. On addition of 10 ml of water all the gold precipitated. When the experiment was repeated with silver an insoluble salt, LiAgCl₂, was formed. This salt dissolved in DMSO containing 1 M LiCl .

Example XVII

10 ml of a solution containing 4% bromine and 1 M lithium chloride in dry acetonitrile rapidly dissolved 0.2 g of finely divided gold at 17°C. When hydrazine was added as a reductant, 99% of the gold precipitated. Example XVIII

10 ml of a solution containing 0.25 M CuBr₂ in dry ace tonitrile was stirred with 0.2 g silver at 17°C for three hours. The silver dissolved. On addition of 5 ml of water silver bromide precipitated.

The experiment was repeated with 0.1 g gold. On addition of water, gold precipitated.

The forms in which gold metal precipitates from solutions of gold (I) or gold (III) salts include powders, flakes, colloidal suspensions and mirrors. Gold flakes, powders and colloids have many practical applications which are described in the literature. Gold mirrors are especially desirable and so called electroless or chemical deposition of gold onto surfaces relies on in-situ reduction of a soluble gold compound with chemical reductants, such as hydrazine, and borohydrides or else via replacement or a less noble metallic surface (e.g. copper or nickel). Such methods have the disadvantage that the reaction products of the reductant build up in the solution in a continuous process. The soluble gold compounds are often cyano comp lexes which are undesirable for environmental reasons. Halo complexes of gold (I) are preferable as sources of soluble gold, for mirrors, colloids, flakes and powders.

The discovery that solutions of gold halo-complexes containing cuprous or ferrous halides and other halide salts in dimethylformamide, acetonitrile or dimethylsulfoxide give flakes, powders, colloidal suspensions and mirrors on surfaces such as glass, when water is added to the solutions, suggests a new method of making gold films, gold flakes, colloidal gold and gold powders. As disclosed herein solutions of a variety of concentrations up to 0.5M gold can be prepared by dissolving gold with cupric halide in dimethylformamide acetonitrile or dimethylsulfoxide containing salts of halide ions. On addition of water, the copper (I) halide acts as a reductant and a gold mirror forms on surfaces, such as glass, which are clean. The solution is colloidal but in time, gold powders and flakes form in addition to the gold mirror.

It should be appreciated that the method involves a reversible redox couple CuCl₂/CuCl₂- or CuBr₂/CuBr-₂ or FeCl₃/FeCl₂ or FeBr₃/FeBr₂ so that the solutions can be recycled, after removing water, to dissolve more gold and precipitate new films without further addition of oxidant or reductant. Thus impurities do not build up in the solutions on cycling. The method involves dissolving gold as the AuCl-₂ or AuBr-₂ complex as described herein, then reducing it with cuprous or ferrous halide as a film onto surfaces prepared by methods familiar to those skilled in the art, by adding water to the dry DMF acetonitrile or DMSO solution.

Thus in another form the invention resides in a method o producing gold mirrors which comprises applying a solution of gold as the AuCl-₂ or AuBr-₂ complex in a water miscible dipolar aprotic solvent as described herein as a film to a suitably prepared glass surface, reducing it by the addition of water in the presence of cuprous or ferrous halides to deposit a film of gold on the surface.

Claims

THE CLAIMS defining the invention are as follows :-

1. A method of oxidising gold, silver and their sul fides selenides or tellurides and dissolving them as their gold or silver halide complex salts leaving elemental sulfur selenium or tellurium as appropriate, comprising leaching the material containing gold and/or silver with a water-miscible dipolar aprotic solvent containing halide salts and an oxidant which does not react extens ively with the solvent but does oxidise gold, silver and their sulfides, selenides and tellurides in the solvent.

2. A method of recovering gold from solutions of gold halo-complexes in water miscible dipolar aprotic solvents wherein the gold complex is reduced to gold by the addition of water in the presence of a reducing agent or by the addition of water followed by the addition of a reducing agent, said reducing agent being sufficiently reactive to reduce chlorine, bromine and gold salts in aqueous dipolar aprotic solvents but insufficiently reactive to reduce copper halides to copper metal.

3. A method of recovering gold which comprises dissolving the gold as a gold halo complex in a water miscible dipolar aprotic solvent containing halide salts and an oxidant which does not extensively react with the solvent but does oxidise the gold in solution, adding water to the solution in the presence of a reducing agent, or adding water to the solution followed by the addition of a reducing agent, said reducing agent being sufficiently reactive to reduce chlorine, bromine and gold salts in aqueous dipolar aprotic solvents but insufficiently reactive to reduce copper halides to copper metal.

4. A method of recovering gold and silver from materials containing same which comprises leaching the material containing the gold and silver with a water miscible dipolar aprotic solvent containing halide salts and an oxidant which does not react extensively with the solvent but does oxidise gold and silver in solution in the solvent, separating the leach solution from the material , adding water to the leach solution to precipitate silver halide and, in the presence of a reducing agent gold, said reducing agent being sufficiently reactive to reduce chlorine bromine and gold salts in aqueous dipolar aprotic solvents but insufficiently reactive to reduce copper halides to copper metal.

5. A method as claimed in claim 4 wherein the reducing agent is present in the leach solution as a result of reactions occurring during the leaching step.

6. A method of recovering gold and silver separately from materials containing same which comprises leaching the material containing the gold and silver with a water miscible dipolar aprotic solvent containing halide salts and an oxidant which does not react extensively with the solvent but does oxidise gold and silver in solution in the solvent, separating the leach solution from the material , if necessary oxidising the leach solution to remove any reducing agents present in the leach solution,adding water to the leach solution to precipitate silver halide, separating the precipitated silver halide, and adding a reducing agent to the separated solution to precipitate the gold, said reducing agent being sufficiently reactive to reduce chlorine bromine and gold salts in aqueous dipolar aprotic solvents but insufficiently reactive to reduce copper halides to copper metal.

7. A method as claimed in any one of claims 1 to 6 wherein the solvent is selected from the group comprising dimethylsulfoxide, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, acetonitrile, acetone and mixtures thereof.

8. A method as claimed in any one of claims 1 to 7 wherein the halide salts are salts of cations selected from the group comprising sodium, potassium, lithium, calcium, magnesium, tetraalkyl ammonium, copper and iron.

9. A method as claimed in any one of claims 1 to 8 wherein the oxidant is selected from the group comprisinng cupric bromide, cupric chloride, ferric chloride, ferriic bromide, chlorine and bromine.

10. A method as claimed in any one of claims 1 to 6 wherein the solvent is dimethylsulfoxide or dimethylforraamide, or mixtures of dimethylsulfoxide and dimethylformamide and the oxidant is selected from the group comprising cupric chloride, cupric bromide, ferric chloride and ferric bromide.

11. A method as claimed in any one of claims 1 to 6 wherein the solvent is acetonitrile and the oxidant is cupric chloride or cupric bromide.

12. A method as claimed in claims 2, 3, 4 or 6 or any claim appended thereto wherein the reducing agent is selected from the group comprising ferrous chloride, ferrous bromide, ferrous sulfate, oxalic acid, hyαroxylamine, sulfur dioxide, hydrazine, cuprous chloride, cuprous bromide and copper sulfide.

13. An oxidising agent comprising a water miscible dipo lar aprotic solvent or mixtures of such solvents containing halide ions, copper II, and/or ferric ions and cations (including cupric and ferric) which form soluble salts with halo complex anions in the appropriate solvent.

14. An oxidising agent as claimed in claim 1 wherein the solvent is selected from the group comprising dimethylsulfoxide, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, acetonitrile, acetonitrile mixtures, dimethylsulfoxide-acetonitrile mixtures, mixtures of the foregoing and mixtures of the foregoing with acetone.

15. A method of producing gold mirrors which comprises applying a solution of gold as AuCl₂- or AuBr-₂ in a water miscible dipolar aprotic solvent as a film to a suitably prepared glass surface and reducing it to deposit a film of gold on the surface by the addition of water in the presence of CuCl-₂, CuBr-₂, FeCl₂ or FeBr₂.

16. A method of oxidising gold and silver substantially as herein described.

17. A method of recovering gold and silver substantially as herein described.