THE PRODUCTION OF COPPER

THIS INVENTION relates to the production of copper. In particular, the invention relates to a method of producing copper and to a method of recovering copper from a copper-containing substrate.

Copper is usually recovered from copper-containing ores either pyrometallurgically, by electrolytic reduction of a copper-containing solution or by reduction of a copper-containing solution with a suitable reducing agent. The reduction processes are generally carried out on solutions of copper (II). However, a problem associated with the reduction of copper (II) solutions is that the metal which plates out from such solutions during reduction traps impurities. In consequence, the copper which is produced by electrolytic or chemical reduction usually requires further purification in order to produce a metal having a purity of greater than 99%.

According to one aspect of the invention, there is provided a method of producing copper from a solution containing a copper (II) salt, the method including the steps of reducing at least some of the copper (II) salt to a copper (I) salt in a first reduction step; solubilising the copper (I) salt to produce a soluble copper (I) complex; and reducing the soluble copper (I) complex to copper in a second reduction step.

Most copper (I) salts are insoluble and the solubilising step is necessary to obtain a dissolved copper (I) complex for the second reduction step. The copper (II) salt may be essentially any water-soluble copper salt excluding copper nitrate or any other oxidative salts. Typically, the copper (II) salt will be copper sulphate or copper chloride.

The solubilising step may be achieved by conducting the first reduction step in the presence of a soluble halide, e.g. a chloride as a solubilising agent, or by adding a soluble halide, e.g. a chloride, subsequent to or during the first reduction step. Instead, solubilising the copper (I) salt may be achieved by using copper (II) chloride as the copper (II) salt. The reduction of the copper (II) salt and the solubilising of the copper (I) salt may thus take place essentially simultaneously.

Preferably the soluble halide will be used in excess. If sodium chloride is used as the solubilising agent, the soluble copper (I) complex will be a sodium chloride- copper chloride complex. Any water-soluble chloride salt of which the metal cation doesn't have an oxidative reaction with copper (I) e.g. ferric cations can, in principle, be used as the solubilising agent. Examples include AICI3, CaCI ₂ , FeCI ₂ or the like (but not FeCb). Without being bound thereby, the Applicant believes that the soluble copper (I) complex when a soluble chloride is used is probably a CuCI ₄ ^3" species, e.g. with an Al ³⁺ or 3Na ⁺ to balance the charge.

When a reducing agent is used in the first reduction step for the reduction of copper (II) to copper (I), the reducing agent will preferably be a mild reducing agent such as sulphur dioxide or copper metal. Sodium sulphite or sodium thiosulphate may also be used. Preferably, only a catalytic amount, i.e. less than a stoichiometric amount

of a reducing agent will be used in the first reduction step to initiate reduction of the copper (II) to copper (I) so that further reduction of the copper (II) to copper (I) will result from the copper metal produced in the second reduction step. For example, if copper powder is used in the first reduction step, it may be used in an amount of about 5%-25% of the stoichiometric amount required to reduce all the copper (II).

In this way, for example, the addition of a small amount of copper metal powder to the copper (II) solution in the presence of an excess of sodium chloride will initially result in the reduction of part of the copper (II) to produce some insoluble CuCI. The insoluble CuCI will then be solubilised by the sodium chloride and the solubilised CuCI will be reduced to copper in the second reduction step, e.g. by using a reducing agent. The copper produced in the second reduction step will, in turn, reduce more copper (II) to copper (I) whilst itself being oxidised to copper (I). This process will then continue until all of the copper (II) has been consumed. The remaining copper (I) will then be reduced by the reducing agent to copper metal.

In one embodiment of the invention, a reducing agent is thus used in the second reduction step. Preferably, the reducing agent used in the second reduction step is a reducing metal.

The reducing metal may be iron, aluminium or zinc. Instead, the reducing metal may be nickel, cobalt, manganese or magnesium.

The reducing agent used in the second reduction step may be iron if copper crystals are sought after. The reducing agent used in the second reduction step

may be aluminium if copper powder is sought after. It was found that reduction with aluminium was highly exothermic and that the addition of aluminium to the reaction mixture needed to be controlled to prevent overheating. Zinc was found to be less exothermic than aluminium and iron less exothermic than zinc. The Applicant believes that the mild reaction of iron with copper (I) resulted in copper crystal formation while an exothermic reaction like aluminium with copper (I) resulted in copper powder formation. It is therefore the Applicant's conclusion that nickel and cobalt as reducing agents with copper (I) will also result in the formation of copper crystals while other reducing agents like manganese and magnesium with copper (I) will result in the formation of copper powder.

The preferential reduction of copper (I) to copper in the presence of copper (II) in the second reduction step is a reflection of the difference in the redox potentials for the reduction of copper (I) and copper (II) to copper as can be seen below

Cu(I) +e → Cu (0,52V) Cu (ll) + 2e → Cu (0,34V)

Since the redox potential for the reduction of copper (II) to copper (I)

Cu (II) +e → Cu (I) (0,153V)

is lower than that for the reduction of copper (II) to copper, this can explain why copper

(II) is directly reduced to copper in the presence of a reducing agent. If a piece of iron plate is dipped into a solution of copper sulphate only, it is immediately poisoned by a

strong plated layer of copper. No further plating of copper occurs. In this case, due to the lack of a soluble copper (I) intermediate, copper plating and subsequent poisoning of the reducing agent thus occurs. However, it appears that the prior reduction using a mild reducing agent of part of the copper (II) to soluble copper (I) before exposure to the stronger reducing agent allows for the selective reduction of the copper (I) in the presence of the copper (II). In this case, if a piece of iron plate is dipped into a solution containing copper (I) and copper (II), all the copper (II) via the soluble copper (I) intermediate is reduced to copper crystals and a clean active iron surface remains afterwards. The piece of iron can thus be used again.

A further advantages of the reduction mechanism via the copper (I) intermediate is it selectivity over other metal impurities except gold and silver that will plate before copper (I). The high nobility of copper (I) ensures that only copper will be reduced in preference to any other soluble metals.

In another embodiment of the invention, the second reduction step is an electrolytic reduction. In this case a membrane may be used to separate the chlorides from the anode because of the formation of chlorine gas during the electrolysis process. However, chlorine is a corrosive and usually undesirable oxidising agent and the Applicant has found that there is a drop in the chloride concentration of the reaction mixture as chlorine gas is produced. This embodiment would therefore probably not be a preferred embodiment.

The solution containing the copper (II) salt may be produced from a copper-containing ore, preferably a basic copper-containing ore, such as malachite,

C11CO3. Cu(OH) ₂ , azurite, 2C11CO3. Cu(OH) ₂ or cuprite, Cu ₂ O. Sulphide ores such as CuFeS ₂ , CUsFeS ₄ , Cu ₂ S, CuS, may also be used after roasting or oxidation.

Thus, in an embodiment of the invention, an excess of sodium chloride is added to a copper (II) sulphate solution followed by a catalytic amount of copper powder as a reducing agent. As the copper (I) forms, it precipitates as a white insoluble precipitate. The high chloride concentration then causes dissolution of the copper (I) chloride and the solution turns from blue to green. The reactions which take place are:

CuSO ₄ (aq) + Cu(powder) + NaCI(aq) →2CuCI(white precipitate) + Na ₂ SO ₄ (aq)

Excess NaCI(aq) + CuCI(white precipitate) → NaCI-CuCI(aq, green solution)

In an experiment in which excess sodium chloride was added to a copper sulphate solution without the prior addition of copper powder or another mild reducing agent and in which an aluminium rod was used as the reducing agent, copper plated onto the aluminium and eventually poisoned the aluminium. No copper powder was formed during this experiment.

It was also found that the catalytic amount of the mild reducing agent needed to initiate the reaction depended on the surface area of the strong reducing agent used. Generally, 0.2 mole of copper powder or Na ₂ Sθ ₃ per 1 mole of copper (II) produced a satisfactory result.

According to another aspect of the invention, there is provided a method of recovering copper from a copper-containing substrate, the method including the steps of converting the copper-containing substrate to a copper (II) containing solution; reducing at least some of the copper (II) in the copper (II) containing solution to produce an insoluble form of copper (I) in a first reduction step using a less than stoichiometric amount of a first reducing agent; solubilising the insoluble form of copper (I) with a solubilising agent to produce a soluble form of copper (I) by adding the solubilising agent prior to, during or subsequent to the first reduction step; and reducing the solubilised form of copper (I) to copper in a second reduction step using a second reducing agent.

The copper-containing substrate may be a copper-containing ore, e.g. malachite or copper oxides. The copper-containing ore may thus be as hereinbefore described.

The first reducing agent may be selected from sodium sulphite and copper powder.

The second reducing agent may be selected from iron, zinc and aluminium.

The invention extends to a method of producing copper, the method including the step of reducing a soluble form of Cu (I) in solution with a reducing metal.

The reducing metal may be iron, aluminium or zinc.

Typically, the dissolved Cu(I) is reduced in the presence of dissolved Cu(II).

The invention is now described, by way of example, with reference to the following example.

Example 1

Finely milled malachite (30Og; -250μm) with a copper content of 50 % (2,36 mol Cu) was added to water (1 ,2t) in a 21 polypropylene beaker. The suspension was stirred vigorously and sulphuric acid (20Og; 98%; approximately 2 mol) was slowly added over a period of 20 minutes to control foaming resulting from this production of carbon dioxide. The reaction was exothermic and the temperature rose from 20 ⁰ C to AA ⁰ C. The resulting suspension was filtered producing a filter cake which was washed with water (2x1 OOmt).

Excess malachite was recovered from the filter cake. An excess of malachite was used to ensure that no free sulphuric acid was present in the filtrate. The resulting copper sulphate solution (1 ,Ai), at a pH of between 3 and 4, was transferred to a 21 polypropylene beaker and copper powder (25g; -75μm; 0,39 mol) and sodium chloride (24Og; 4,1 mol) were added and the mixture stirred for 5 minutes to produce pale insoluble copper (I) chloride. As the salt dissolved in the presence of the excess

chloride, the solution turned green. Aluminium buttons (4Og; 1 ,48 mol) in a perforated plastic container were then added to the green solution with vigorous stirring over a period of two hours whilst maintaining the temperature at 70 ⁰ C. It is important that the aluminium is static relative to the solution during agitation. The green colour disappeared and successively became grey, pink, maroon and then reddish when the reaction was complete.

It was found that 2 moles of Cu (II) feed required 2x2/3 moles of aluminium (36g) to be fully reduced. However, because of the acidic conditions produced after the reduction (pH of approximately 2) more aluminium was consumed than needed if an access of aluminium was available. Consequently only about 10% excess aluminium was used for the reduction.

The copper suspension produced after the aluminium reduction step was filtered and the filtrate was washed with water. The resulting copper filter cake was passed through a 75μm screen with a small amount of water. Concentrated hydrochloric acid was added to the resulting powder slurry to reduce the pH to less than

1 to remove unreacted aluminium, co-precipitated aluminium salts and other impurities.

After filtering, washing and drying, the resulting product was a fine copper powder (151 ,5g) having a purity of greater than 99%. The copper recovery was therefore

126,5g (1 ,99 mol) taking into account the addition of the catalytic amount of copper powder (25g). Care must be taken to exclude air from the powder to prevent oxidation.

The powder can be stored under acidic water or when dried, under argon. The powder prepared this way typically has a d ⁵⁰ of 15μm.

The filtrate, which largely contained soluble aluminium and sodium salts, was optionally neutralized and the aluminium precipitated which is a waste or byproduct and sodium chloride reused.

Example 2

1.5 kg CuSO ₄ .5H ₂ O and 1 kg NaCI were dissolved in Al of water at 25°C. To the green solution 25 g Na ₂ SOs was added which turned the solution into a black colour. This solution (5.1 I in total) was poured into a plastic container with sheets of pickled mild steel. The surface area of the mild steel sheets was 214,600mm ² . A mild exothermic reaction took place while forming the copper crystals. The following Table 1 shows the temperature of the solution as a function of time.

Table 1

After 3 hours the reaction was complete. The copper crystals dropped to the bottom as the mild steel sheets were lifted out. After washing the crystals with dilute sulphuric acid 0.1 N, they were melted at 1 ,200 ⁰ C. This yielded a 99.99% copper ingot of 249.5 g which correlates with a 98% recovery of copper values from the solution.

The filtrate (approximately 51) mainly contained NaCI and FeSO ₄ . To separate the iron and sodium, crystallisation cannot be used because it results in a co- crystallisation of Na ₂ SO ₄ and FeSO ₄ . The iron (II) must first be electrolytically or chemically (H ₂ O ₂ ) oxidised to iron (III) while maintaining the pH between 4-6 via alkaline addition (slake lime) to precipitate FeSO ₄ OH. This specie filtrates with ease while the NaCI stays in solution which can be recycled for the next run.

Example 3

In remote places it would be advantageous to recover H ₂ SO ₄ from the process. This can be achieved by using FeCI ₂ instead of NaCI as the solubilisation agent for copper (I). 15Og FeCI ₂ .4H ₂ O and 100 g of CuSO ₄ .5H ₂ O were dissolved in 400 ml water at 30 ⁰ C. 2.5g of Na ₂ Sθ3 was added to the solution. This solution was poured into a plastic beaker with 2 vertical pickled mild steel plates. The surface area of the plates was 1 ,880mm ² . Together the plates weighed 193g. After 3 hours the plates were lifted and the copper crystals recovered as described in Example 2. 24g of copper was recovered which has an efficiency of 94.5%. The mass of the iron plates was reduced to 171 .7g. From the filtrate 636g, 520m{ was evaporated in a mild steel vessel at 100 ⁰ C until 31 OmE. It was then cooled to 15°C where crystallisation took place. 9Og moist FeSO ₄ .7H ₂ O was filtered from a remaining 32Og, 25OmE filtrate. The filtrate

contained mainly FeCI ₂ which can be used in a subsequent run while the FeSO ₄ .7H ₂ O can be converted via known oxidative pressure leaching techniques into Fe ₂ Os and H ₂ SO ₄ .

The Applicant has discovered that when soluble copper (I) is reduced with a reducing metal such as iron, copper crystals grow from the surface of the reducing iron and if the metal is aluminium or zinc, a soft copper agglomerate precipitates from the surface of the reducing metal, so that the surface of the reducing metal is left clean and active again. This agglomerate readily disperses into fine (<75μm) copper powder.

The copper crystals that form when iron is used as a reducing agent can be as long as 10mm, but typically the bulk of the crystals is between 0.25mm to 4mm. Both the copper crystals or copper powder that form are very selective and do not trap any significant amount of impurities.

The method of the invention thus produces a fine, pure copper powder or crystals from essentially any water soluble copper solution. The powder has a purity of over 99% and can be used either in powder metallurgy or powder coating applications. The purity of the copper crystals is greater than 99.9%, and the copper crystals are much less sensitive to air oxidation than the powder and can be melted into any desirable shape. Table 2 is a comparison with prior art electro-winning processes and the crystal growth process of the invention.

Table 2