Copper(II) reduction

Abstract

A method for reducing copper from Cu(II) to Cu(I) comprises the step of reacting an Fe(II) reducing agent with Cu(II) in basic conditions. The method may be carried out in basic solution, for example ammoniacal solution. The method has particularly utility in the context of solutions which have been produced by the selective leaching of copper over other metals from elemental copper-containing mixtures into basic lixiviants and, in particular, into ammoniacal lixiviants.

Description

[0001] COPPER(II) REDUCTION

[0002] Field of the invention

[0003] The present invention relates to a process for the reduction of copper ions in basic solutions.

[0004] Background to the invention

[0005] Copper is a valuable metal used in a wide range of applications, particularly in electronic applications. Demand for copper is projected to increase significantly over the next 20 years. To meet this demand, and to reduce the environmental impact of mining operations, it is important that copper is recycled from secondary sources. This is often achieved through hydrometallurgical techniques, which involve oxidative leaching of scrap metallic copper to obtain copper ions in solution, separation of impurities from the leach solution and a subsequent electrowinning step to generate pure metallic copper.

[0006] It is known to leach copper using both basic and acidic lixiviant solutions. The use of basic lixiviant solutions such as ammoniacal lixiviant solutions has become increasingly popular owing to the potentially superior attributes of basic leaching agents over acidic leaching agents such as strong acids e.g. H2SO4 utilised with oxidizing agents such as H2O2. These attributes include a higher metal selectivity and less generation of harmful byproducts. Examples of leaching copper using basic lixiviants such as ammoniacal lixiviants are disclosed in the following documents: Koyama, K., Tanaka, M. and Lee, J.C., 2006. Copper leaching behaviour from waste printed circuit board in ammoniacal alkaline solution. Materials Transactions, 47(7), pp.1788-1792; Lin, P. and Werner, J.M., Kinetic Study of Cu Leaching Mechanism from Waste RAM Chips Using Cu (ll)-NH3-SO4 Solution. Available at SSRN 4200137; Oishi, T., Koyama, K., Alam, S., Tanaka, M. and Lee, J.C., 2007. Recovery of high purity copper cathode from printed circuit boards using ammoniacal sulfate or chloride solutions. Hydrometallurgy, 89(1-2), pp.82-88.

[0007] The lixiviant typically contains an oxidizing agent, for example, a Cu(ll) species which may oxidise the scrap elemental copper into Cu(l) species, the Cu(ll) species itself being simultaneously reduced into Cu(l) species. Both Cu(ll) and Cu(l) ions can form stable complexes with ammonia ligands when an aqueous ammonia lixiviant is used. The initial Cu(ll) species which acts as an oxidant may be added in the form of a salt such as copper sulfate (CuSC ), along with additives such as ammonium sulfate ((NH^SC ) to maintain electroneutrality.

[0008] There is a higher energy net potential associated with reducing Cu(ll) species to elemental copper in acidic solutions (Cu2+ / Cu system in acidic solution Enet = 0.89 V) compared the energy net potential associated with reducing Cu(l) species to elemental copper in ammoniacal solutions (Cu7Cu system in ammoniacal solution Enet = 0.20 V) (Koyama, Tanaka and Lee, 2006). of the invention

[0009] In accordance with an aspect of the invention, there is provided a method for reducing copper from Cu(ll) to Cu(l), the method comprising the step of reacting an Fe(ll) reducing agent with Cu(ll) in basic conditions.

[0010] The basic conditions may be ammoniacal conditions. The reaction may be carried out in basic solution, for example in ammoniacal solution. As will be understood by a person skilled in the art, “basic solution” means that the solution is basic as a whole, i.e. alkaline, i.e. has a pH of greater than 7. This is in contrast to solutions which may have had a base component added but which are not basic overall. Where the basic conditions are ammoniacal, optionally the concentration of ammonia may be between approximately 2 M and 6 M, or between approximately 3 M and 5 M, for example approximately 4 M.

[0011] Optionally, the order of addition may be such that the Fe(ll) reducing agent is added to the Cu(ll) species. For example, the Fe(ll) reducing agent may be added to the Cu(ll) species in basic solution (e.g. in ammoniacal solution).

[0012] The Fe(ll) reducing agent is consumed during the method of the present invention.

[0013] Therefore, it acts as a reagent, not a catalyst. This method has utility in the context of solutions which have been produced by the selective leaching of copper over other metals from elemental copper-containing mixtures into basic lixiviants and, in particular, into ammoniacal lixiviants.

[0014] As discussed above, conventionally, copper leaching processes in ammoniacal solution have been conducted to produce Cu(l) containing solutions. This is because conventional subsequent electrowinning techniques for copper recovery have been carried out when copper is present as Cu(l) and not as Cu(ll). This requires the leaching conditions to be controlled so that oxygen is excluded, preventing overoxidation of Cu(l) to Cu(ll).

[0015] Surprisingly, we have found an alternative way forward, where oxidation of Cu(l) to Cu(ll) is allowed to take place during or after leaching. This has the advantage of not having to exclude air from the leaching stage, enabling faster and more efficient leaching of elemental copper-containing mixtures such as scrap copper, resulting in Cu(ll) solutions.

[0016] Without wishing to be bound by theory, conventionally, leaching Cu(0) using Cu(ll) in ammoniacal lixiviants operates in accordance with the following equation:

[0017] Cu + [Cu(NH3)4]2+2[Cu(NH3)2]+(1)

[0018] Equation (1) indicates how a Cu(ll) oxidizing agent present in the lixiviant can be used to oxidise Cu(0) species to Cu(l) species through the reduction of Cu(ll) to Cu(l), in a process known as comproportionation.

[0019] In accordance with the present invention method, there is no need to exclude oxygen in the leaching stage. This allows for the following reaction to occur in ammoniacal solution as shown in equation (2) below:

[0020] 2[CU(NH3)2]++ 0.5O2+ 3NH3+ NH4+2[Cu(NH3)4]2++ OH' (2)

[0021] Equation (2) indicates that the presence of oxygen allows for the Cu(l) species to be oxidised to Cu(ll) in ammoniacal solution. The present invention thus allows the leaching process to be carried out in a more cost-effective manner and under less stringent conditions because there is no need for the leaching to be carried out under inert gas conditions.

[0022] Conveniently, the method of the present invention allows the preparation of copper (I) in solution rather than as a solid or precipitate.

[0023] In some embodiments, the reducing agent is an Fe(ll) ion-containing salt. This may, for example, be FeSC . Fe(ll) salts are preferred as Fe(ll) ion-containing compounds are particularly effective at reducing Cu(ll) to Cu(l) in solution. Moreover, Fe(ll) ioncontaining compounds are cheap and readily available. The reducing agent may be provided in any form. In some embodiments, the reducing agent is a solid. In some embodiments, the reducing agent is provided in solution.

[0024] In some embodiments, the method further includes the step of separating the Cu(l) ion-containing solution from the by-products of the reduction process. In this way, a substantially pure Cu(l) ion-containing species can be obtained which can be used directly for electrowinning to recover high purity elemental copper therefrom. In some embodiments, the step of separating the Cu(l) ion-containing solution from the reaction mixture comprises filtering off a precipitate formed following the addition of the reducing agent. In this way, the Cu(l) ion-containing solution is isolated from the precipitate formed and it can be used directly in a further step for copper recovery e.g. in an electrowinning step. The step of separating the resulting Cu(l) ion-containing solution may be performed in the absence of oxygen. In this way, the Cu(l) ions are stabilised and conversion to Cu(ll) ions via oxidation is substantially prevented. This ensures a high efficiency when copper is subsequently recovered, for example, by electrowinning procedures.

[0025] In some embodiments, the method further includes the step of adding Cu(0) to the Cu(l) ion-containing mixture. The Cu(0) may be in the form of copper granules. This additional step may act to reduce any remaining Cu(ll) ions in solution such that a high reduction conversion from Cu(ll) ions to Cu(l) ions can be achieved.

[0026] The lixiviant solution may be an ammoniacal lixiviant solution. The use of an ammoniacal lixiviant solution provides high stabilisation of copper ions in solution and the leaching process can proceed at room temperature. Ammoniacal lixiviants are typically readily available materials and cheap to obtain. Moreover, by using an ammoniacal solution, when an Fe(ll) reducing agent is added and oxidised to Fe(lll), this Fe(lll) species can precipitate out of ammoniacal solution thus allowing for easy separation from the Cu(l) solution. Ammonia is also considered a green recovery reagent due to its evaporative recyclability and low toxicity (Wang, J., Chen, S., Zeng, X., Huang, J., Liang, Q., Shu, J., Chen, M., Xiao, Z., Zhao, H. and Sun, Z., 2021 , Recovery of high purity copper from waste printed circuit boards of mobile phones by slurry electrolysis with ammonia-ammonium system., Separation and Purification Technology, 275, p.119180.). By combining the advantageous properties of the ammoniacal lixiviant and the fast reaction times for leaching copper in air, the leaching stage of the reaction is therefore particularly effective.

[0027] In some embodiments, the step of adding an Fe(ll) ion-containing reducing agent to the Cu(ll) ion-containing solution to obtain a Cu(l)-ion containing solution is conducted in the absence of oxygen. This allows for a high degree of Cu(l) ions to be formed from the Cu(ll) ions. The removal of oxygen from the solution may be achieved in various ways. In some embodiments, the removal of oxygen from the reaction mixture is achieved by purging the reaction mixture with an inert gas. For example, the reaction may be purged with nitrogen. Inert gases such as nitrogen are easily obtainable and easy to implement in large scale reactions for the removal of oxygen.

[0028] In some embodiments, a pH range of the reaction mixture is maintained at between 8 and 12, optionally between 9 and 11. For example, the pH may optionally be approximately 10. By keeping the pH of the reaction mixture within this range, the stabilisation of the Cu(l) ions in solution is enhanced.

[0029] In some embodiments, a concentration of Cu(ll) ions in the Cu(ll) ion-containing solution is between 0.0001 M and 1.2 M, optionally between 0.08 M and 0.8 M. For example, the concentration of Cu(ll) may optionally be approximately 0.4 M.

[0030] In some embodiments, the concentration of ammonia in the ammoniacal lixiviant solution is between approximately 2 M and 6 M, preferably between approximately 3 M and 5 M. For example, the concentration of ammonia in the lixiviant may be approximately 4 M. In some embodiments, the Cu(ll) ion-containing solution is obtained by leaching an elemental copper containing mixture under aerobic conditions in a basic solution, such as in an ammoniacal solution. The presence of oxygen allows for the Cu(l) species to be oxidised to Cu(ll) in ammoniacal solution, allowing the leaching process to be carried out in a more cost-effective manner and under less stringent conditions because there is no need for the leaching to be carried out under inert gas conditions.

[0031] In some embodiments, the lixiviant solution, such as the ammoniacal lixiviant solution is mixed with the elemental copper-containing mixture for from at least 1 hour up to 24 hours. In this way the leaching stage has enough time to result in a solution which has been well leached. In some embodiments, the mixing of the elemental copper- containing solution with the basic solution occurs for approximately 2 hours, approximately 3 hours or approximately 4 hours. The time required for leaching depends largely on the form of the elemental copper-containing mixture. For example, longer leaching times may be required for copper scrap which is provided in solid chunks compared to copper scrap provided in powder form. The mixing time is therefore conducted for a long enough time for efficient leaching to occur, however, does not require exceptionally long leaching times as typically required for known leaching procedures.

[0032] The Cu(l) ion-containing solution may be used in a subsequent step to recover copper. In some embodiments, the subsequent step of the method involves using the Cu(l) ioncontaining solution in an electrowinning stage. The Cu(l) ion-containing solution may, for example, be introduced into an electrodeposition reactor where the Cu(l) ions may be reduced to Cu in the form of, for example, a copper foil. The fact that the solution contains substantially all Cu(l) ions results in a less energy intensive electrowinning stage compared to electrowinning using Cu(ll) ions in solution which can significantly reduce the costs of copper recover from secondary sources such as copper scrap from electrical waste at industrial scales.

[0033] In the process of electrowinning, elemental copper is formed at the cathode and Cu(ll) may be regenerated at the anode. This Cu(ll) species can be recycled to regenerate the oxidising agent species and alternatively or additionally, this Cu(ll) species could be reduced according to the method of the present invention to form Cu(l) species. The present invention may therefore be used as a method of reducing a Cu(ll) containing solution to a Cu(l) containing solution, wherein the Cu(ll) solution has been obtained by leaching an elemental copper containing solution such as a copper scrap in an ammoniacal lixiviant. The Cu(l) solution resulting from the reduction may be used to obtain elemental copper, for example, by electrowinning.

[0034] In some embodiments, the method involves an additional step of obtaining a postelectrowinning solution following the step of electrowinning copper from the Cu(l) ioncontaining solution, the post-electrowinning solution comprising Cu(ll) ions, and using the post-electrowinning solution to generate a further Cu(l) ion-containing solution by adding an Fe(ll) ion-containing reducing agent to the post-electrowinning solution comprising Cu(ll) ions. In this way, the newly generated Cu(l) ion-containing solution obtained can be used in a further electrowinning stage. Accordingly, the postelectrowinning solution may still be a rich source of Cu(ll) ions that can be reduced by a suitable reducing agent as above. In this way, the recovery of copper can be extremely efficient as the materials may be recycled such that the waste from Cu(ll) ions is minimal.

[0035] In some embodiments, ammonia is added to the post-electrowinning solution prior to the step of adding the reducing agent. During leaching of copper in ammoniacal solution, ammonia is consumed as shown in Equation 2. This is converted to ammonium sulphate during the reduction step. Accordingly, adding a fresh supply of ammonia into the post-electrowinning solution enables recycling of the solution, allowing for a further leaching cycle following a first electrowinning stage.

[0036] In some embodiments, a portion of ammonium sulphate is removed from the postelectrowinning solution prior to the step of adding the reducing agent. Owing to increased production of ammonium sulphate after the reduction step, removal of a portion of excess ammonium sulphate from the post-electrowinning solution is beneficial to prevent a build-up of ammonium sulphate.

[0037] The method may be performed on a small scale, for example, from 1 g to 100 g of an elemental copper-containing mixture or on a larger scale, for example, from 500 g to 100 kg of a copper-containing mixture. In some embodiments, the method is performed on a scale of approximately 10 g of an elemental copper containing mixture. The copper-containing mixture may be provided as copper scrap from electrical or industrial waste, as a product of cementation from various metallic waste sources, or in any other form which contains copper. The copper-containing mixture may therefore contain other metals such as iron, lead and aluminium. All materials in the copper- containing mixture may vary in their percentage contributions to the overall copper- containing mixture. The term “ammoniacal solution” is understood to mean any aqueous solution comprising ammonia.

[0038] Figures

[0039] Non-limiting examples in accordance with the present invention will now be described in further detail with reference to the following Figures in which:

[0040] Figure 1 shows a schematic example of a process (100) according to a non-limiting embodiment of the invention.

[0041] Figure 2 shows a schematic example of a process (200) according to a non-limiting embodiment of the invention.

[0042] General Experimental Details

[0043] Reagents and equipment

[0044] Reagents were used as received from Sigma-Aldrich, Fisher Scientific, Fluorochem. Deionised water was sourced from a Milli-Q purification system.

[0045] Leaching experimental details

[0046] The copper (0) containing scrap (containing approx. 1 kg Cu) is stirred with an ammoniacal lixiviant (40 L) comprised of 4 M ammonia (NH3), 0.4 M Cu(ll) ions and 1 M ammonium sulfate ((NFDSCU) in the presence of air for at least 2 hours. The mixing occurs at 750 rpm at room temperature. Following leaching, a solution of approx. 0.8M Cu(ll) ions is obtained.

[0047] Reduction experimental details

[0048] 45 mL 0.08 M Cu(ll) in 4 M NH3 with 1 M (NH^SCU is purged with N2 for 10 minutes. 5 mL of 0.75 M FeSCU in 1 M H2SO4 is slowly added with stirring. The dark blue Cu(ll) solution changes to a colourless Cu(l) solution, along with the formation of an orange precipitate of Fe(lll). The solid Fe(lll) is separated from the Cu(l) solution through filtration under a N2 atmosphere.

[0049] Detailed Description of Example Embodiments

[0050] The following are non-limiting example embodiments of the invention.

[0051] As shown in Fig. 1 , the process (100) may involve a first leaching step (110) where copper scrap is stirred with a basic lixiviant (the basic lixiviant, for example, comprising ammonia, Cu(ll) ions and ammonium sulfate) in the presence of air for at least 2 hours. Following leaching (110), a Cu(ll) ion-containing solution is obtained. To this Cu(ll) ion containing solution, a reducing agent such as FeSC is added with stirring in a reduction step (120). The reduction step (120) results in a Cu(l) ion-containing solution and a precipitate, for example of Fe(OH)s. This precipitate may be separated from the Cu(l) containing solution by filtration (130). Optionally (not shown), residual Cu(ll) may be reduced to Cu(l) by using copper granules. The Cu(l) ion-containing solution may then be used in a subsequent electroplating step (140) to obtain elemental copper.

[0052] As shown in Fig. 2, in process (200) the copper-containing mixture may be provided as copper scrap such as electrical waste, or in any other form which contains elemental copper. This copper-containing mixture (containing approximately 1 kg Cu) is leached (210) into an ammoniacal lixiviant (40 L) comprised of 4 M ammonia (NH3), 0.4 M Cu(ll) ions and 1 M ammonium sulfate ((NH^SC ) in the presence of air for at least 2 hours. The mixing occurs at 750 rpm at room temperature. Following leaching (210), a solution of 0.8 M Cu(ll) ions is obtained.

[0053] Removal of oxygen from this solution then occurs by purging the reaction mixture with nitrogen for 10 minutes and the reduction (220) of the Cu(ll) ions to Cu(l) ions takes place by adding 1.8kg of an Fe(ll) ion-containing reducing agent. The Fe(ll) ioncontaining reducing agent is obtained via a cementation step (201), whereby Fe(0) containing scrap metal is added to a CuSC solution in dilute sulfuric acid while keeping the reaction mixture under a flow of nitrogen. The resulting solution is a 0.8M Cu(l) ioncontaining solution. This Cu(l) ion-containing solution is filtered (230) to remove an Fe(OH)s precipitate, a result of the oxidation of the Fe(l I) ion-containing reducing agent. An 0.8M Cu(l) ion-containing solution is then obtained and isolated, stored under nitrogen. This Cu(l) ion-containing solution may then be used in an electrowinning process (250) to recover elemental copper as copper plate. To the solution may be added Cu(0) granules (240) to reduce any remaining Cu(ll) species present in solution to Cu(l). From this Cu(l) ion-containing solution, 1 kg of copper plate may be recovered, in addition to 40 L of lixiviant containing 0.4 M Cu(ll) ions. The ammonia concentration will have decreased in the lixiviant and the ammonium sulfate concentration will have increased. A portion of the lixiviant solution may be removed to prevent the build-up of ammonium sulfate and fresh ammonia may be added to regenerate the lixiviant for subsequent use - i.e. recycling of the lixiviant may occur. Accordingly, the 0.4 M Cu(ll) solution may be regenerated (260) and used for a further round of leaching.

[0054] Throughout the description and claims of this specification, the words “comprise” and “contain” and variations of them mean “including but not limited to”, and they are not intended to and do not exclude other components, integers, or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

[0055] Features, integers, characteristics, or groups described in conjunction with a particular aspect, embodiment, or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. All the features disclosed in this specification (including any accompanying claims, abstract and drawings), and / or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and / or steps are mutually exclusive. The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

Claims

Claims1. A method for reducing copper from Cu(ll) to Cu(l), the method comprising the step of reacting an Fe(ll) reducing agent with Cu(ll) in basic conditions.

2. The method according to claim 1, wherein the method is carried out in basic solution and results in a Cu(l) containing solution.

3. The method according to claim 2, wherein the basic solution is an ammoniacal solution.

4. The method according to claim 2 or claim 3, further comprising the step of separating the Cu(l) containing solution from an Fe(lll) precipitate.

5. The method according to any of the preceding claims, wherein the Fe(ll) reducing agent is FeSC .

6. The method according to claim 4 or claim 5, wherein the step of separating the Cu(l) containing solution is conducted in the absence of oxygen, optionally by purging in nitrogen.

7. The method according to any of claims 3 to 6, wherein a pH range of the ammoniacal solution is maintained at between 8 and 12, preferably between 9 and 11.

8. The method according to any of claims 2 to 7, wherein the starting concentration of Cu(ll) is between 0.0001 M and 1.2 M.

9. The method according to any of claims 3 to 8, wherein the starting concentration of ammonia is between 2 M and 6 M.

10. The method according to any of claims 2 to 9, wherein the Cu(ll) is obtained by leaching an elemental copper containing mixture under aerobic conditions in ammoniacal solution.

11. The method according to any preceding claim, further comprising the step of electrowinning copper from the Cu(l).

12. The method according to claim 11, further comprising the step of: obtaining a post-electrowinning solution following the step of electrowinning copper from the Cu(l) containing solution, the post-electrowinning solution comprising Cu(ll) ions.

13. The method according to claim 12, further comprising the step of: adding an Fe(ll) ion-containing reducing agent to the post-electrowinning solution comprising Cu(ll) ions to obtain a further Cu(l) containing solution.

14. The method according to claim 13, further comprising the step of separating the further Cu(l) containing solution.

15. The method according to claim 14, further comprising the step of electrowinning the further Cu(l) containing solution.

16. The method according to any of the preceding claims, wherein the method is performed on a scale of 10 g to 100 kg of an elemental copper containing mixture.

17. The method according to any of the preceding claims, wherein the elemental copper-containing mixture is electrical waste.

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