Fundamental Application of Basket Electrolysis Method for Black-Copper Anode

Abstract

Crude copper with a high content of impurities derived from secondary raw materials, known as black-copper, cannot be used in the conventional electrorefining process because of passivation. The current industrial process for black-copper, which involves a combination of high-temperature acid dissolution under pressure, and subsequent electrowinning, results in high power consumption. In this study, the basket electrolysis method was investigated as an alternative process for black-copper. Basket electrolysis experiments were performed using black-copper alloy samples shaped into shot of diameter 2.5 mm. The anodic dissolution behaviors with respect to the connection between the anode and the supporting conductor, anode shape, number of anodes, and anode composition, i.e., with and without Ag, were investigated. Relatively high anodic dissolution ratios, i.e., greater than 70%, were obtained in all cases when multiple-shot anodes with 10 or more shots were used. The anode film formed on the black-copper surface is thought to contribute to the maintenance of electrolysis by providing ion-conducting paths via electrolyte-filled pores, and electron-conducting paths via metal particles. This enabled application of the basket electrolysis method.

1. Introduction

Waste electronic circuit boards from PCs, smartphones, and other devices, which are referred to as E-scrap, are highly valuable secondary raw materials because of their high contents of Cu and precious metals. Copper smelting using secondary raw materials has been attracting attention in recent years from the perspective of sustainable development goals. The impurity contents of crude copper anodes are expected to increase as the proportion of secondary raw materials in the feedstock for copper smelting rises.^1,2) In particular, the crude copper produced mainly from secondary raw materials, which is called black-copper, has high contents of impurities. When black-copper anodes are used in electrorefining, electrolysis is easily stopped because an insoluble film is formed on the anode surface, and this inhibits the elution of Cu ions into the electrolyte. This passivation makes treatment of black-copper anodes via conventional processes difficult.^3,4) Among impurity elements, Ag significantly promotes anode passivation. The Ag contents of crude copper is carefully controlled at 10–750 ppm,⁵⁾ and it has been reported that crude copper anodes containing Ag at 2600 ppm hardly work because of passivation.^6–8) The development of suitable methods for processing black-copper anodes is important for enabling secondary raw materials to be used as the main raw material.

Many studies of the use of black-copper anodes in processes based on conventional electrorefining methods have been performed, e.g., by spraying fresh electrolyte on the anode surface^9–11) and stirring the electrolyte with a rotating-rod impeller to suppress passivation.^12–14) Studies have been conducted in which electrolyte composition adjustment and additives were used to promote the detachment and sedimentation of anode slime.^15,16) The industrial process that is currently used for black-copper processing, which is designed for treating crude copper with a wide range of compositions, involves two steps: black-copper powder is produced by gas atomizing and dissolved in acid under pressure at high temperatures, and then electrolytic copper is obtained by electrowinning.^17,18) However, this process has the drawback of complex process and high operating costs because of large power consumption in both steps.⁵⁾

In this study, we focused on use of the basket electrolysis method for the electrorefining of black-copper anodes. Use of black-copper anodes in the form of small-diameter shot in insoluble conducting baskets was expected to give a high anodic dissolution ratio because of the large specific surface area, even when the anodes were passivated during electrolysis. Based on a preliminary calculation using a black-copper anodic dissolution depth of 0.5 mm, as reported by Tokushige et al.,¹⁹⁾ an anodic dissolution ratio of over 70% was expected to be achieved with a shot anode of diameter 2.5 mm (Fig. 1); this is much higher than the ratio achieved with a conventional plate anode. Owais et al.^20,21) and Gana²²⁾ investigated basket electrolysis with relatively high-grade copper scrap shots and cut copper wires as anodes and insoluble titanium baskets. Iizuka et al. reported that basket electrolysis with Cu–Pt alloy anodes with a Pt content of 1 mass%, which are easily passivated during electrolysis, gave a dissolution ratio that was significantly higher for small shot anodes than for plate anodes.²³⁾

Fig. 1

Schematic diagrams of dissolution of black copper anodes in shape of (a) the conventional plate type and (b) the shot type with small diameter, and their calculated dissolution ratios based of the anode sizes and the dissolution depths of 0.5 mm.

In the basket electrolysis process, the characteristics of the anode film formed on the surface during electrolysis are especially important. In electrolysis with plate anodes, the effect of the anode film on passivation is mainly related to elution of Cu ions into the electrolyte.^24–27) In contrast, in basket electrolysis in which the anodes fill a conducting support, the anode film provides a conductive path between the support and the undissolved anode metals, i.e., passivation is affected by both ionic conduction and electronic conduction. For example, in copper-electroplating processes that use basket electrolysis, phosphorus-containing copper balls are used as anodes. This process is designed to form a suitable film on the anode surface, called a black film, which is conductive because of the contained copper phosphide or copper chloride, and contributes to the maintenance of electrolysis by providing a current path between the anode and the basket or other anodes.^28–31) In the basket electrolysis process using black-copper anodes, the properties of the anode film are considered to be closely related to the anodic dissolution behavior. However, use of the basket electrolysis method for black-copper anodes with similar compositions to those used industrially have not been reported, therefore the applicability of this method is still unclear. In this study, the use of basket electrolysis for black-copper processing was investigated in terms of the anode size, specific surface area, and composition, and the connection between the anode and the supporting conductor.³²⁾

2. Experimental

Cu alloy samples in shot form were used as simulated black-copper anodes. The sample diameter was 2.5 mm, the weight was 0.85 g, and they contained Ni, Sn, Pb, Sb, Ag, and Au (Rare Metallic Co., Ltd., Tokyo, Japan). Their average compositions (Table 1(a)) were similar to that of black-copper in industrial use.^3,33,34) For comparison, a Cu alloy block (10 mm × 10 mm × 5 mm, 4.00 g) with the same composition as the shot samples, and a Ag-free Cu alloy in the same shot form (Table 1(b)) were also used. The average compositions and variations were determined four times for three shot samples by chemical analysis, i.e., dissolution in aqua regia and then inductively coupled plasma atomic emission spectroscopy (Spectro Arcos, Spectro Analytical Instruments, Kleve, Germany) measurements. The electrolyte consisted of copper sulfate pentahydrate (99.5% purity, FUJIFILM Wako Pure Chemicals Corp., Osaka, Japan), sulfuric acid (95% purity, FUJIFILM Wako Pure Chemicals Corp.), and ultrapure water with an electrical resistivity greater than 18.4 MΩ cm, which was prepared with a Q-POD Element unit (Merck, KGaA, Darmstadt, Germany). Their concentrations were adjusted to be similar to those used in industrial processes: [Cu²⁺] = 40 g L⁻¹ and [H₂SO₄] = 185 g L⁻¹. The volume of the electrolyte was 80 mL and it was kept at 333 ± 2 K. The electrolyte was degassed by N₂ bubbling for 10 min before electrolysis.

Table 1 Compositions of Cu alloy samples (mass%; average ±2 standard deviations).

Electrolysis experiments were performed using the three-electrode method with a potentiostat (VersaSTAT 4, AMETEK Inc., Berwyn, Pennsylvania, USA) with an electrolysis cell with a rotating mechanism (RRDE-3A, BAS Inc., Tokyo, Japan), as shown in Fig. 2. Cu alloy anodes were used as the working electrode, a Cu disc (diameter 7 mm, 99.99% purity, BAS Inc.) or a cylindrical Cu foil cathode (diameter 46 mm, height 30 mm, thickness 0.2 mm) was used as the counter electrode, and a Ag | AgCl electrode immersed in 3.33 M KCl solution at 333 ± 2 K (+181 mV vs. SHE,³⁵⁾ BAS Inc.) was used as the reference electrode. The reference electrode was connected via a KCl–agar salt bridge in a Lugin tube and placed so that the distance from the tip to the anode was within 5 mm. In all experiments, the electrolyte was stirred at a rotational speed of 2000 rpm to achieve smooth deposition at the cathode. The anodes were placed at the bottom of the electrolysis cell and a current was applied through a Pt mesh (99.9% purity, thickness 0.08 mm, 80 mesh, Nilaco Corp., Tokyo, Japan); different installation methods were used according to the shape of the anode sample and the number of anodes used. For the experiments using a single anode shot or block, the anode was wrapped in Pt mesh and fixed by spot-welding at six points to ensure fixation with reproducible strength. Note that the welded connection disappeared during electrolysis because of the small thickness of the Pt wire. In the experiments using multiple anode shots, the anodes were placed inside a Pt mesh basket (12 mm × 12 mm × 3 mm) without fixation. The number of anodes was 18 or 28, and they were placed in a single layer or a double layer, respectively.

Fig. 2

Schematic diagram of (a) electrolysis experiment with fixed single shot anode and Cu disc cathode; and (b) basket electrolysis experiment with fixed block anode or in stacked basket anode surrounded by Cu foil cathode.

Electrolysis was performed under constant current conditions, and the current was set so that the initial anode current density was 200 A m⁻². The current value differed depending on whether the anode was a shot or block sample, and the number of anode shots. Two types of cathode with different areas were used to ensure that the cathode current density was sufficiently small to obtain a smooth deposit. The cathode current density was 136 A m⁻² for the setup in Fig. 2(a) and 22–30 A m⁻² for the setup in Fig. 2(b). The anode potential was monitored until anodic dissolution stopped because of anode passivation and/or electronic disconnection. The anodes were covered with a fragile film after electrolysis, therefore it was difficult to collect and weigh all the anodes. The anodic dissolution ratio was therefore calculated by using eq. (1), based on the assumption that the current was used only for dissolution of Cu to Cu²⁺.   

\begin{equation} D_{\text{Cu}} = \frac{C_{\text{total}}M_{\text{Cu}}}{zF W_{\text{ini}}P_{\text{Cu}}}\times 100 \end{equation} (1)

where D_Cu is the anodic dissolution ratio, C_total is the total charge applied until electrolysis stopped, M_Cu is the atomic weight of Cu, z (= 2) is the number of electrons involved in the reaction, F is the Faraday constant, W_ini is the anode weight, and P_Cu is the Cu content of the anode. At least four repeat tests were conducted under all experimental conditions, and the variations in the dissolution ratio of the anode were also examined.

After electrolysis, the anode was examined by scanning electron microscopy (SEM; SU-6600, Hitachi High-Technologies Corp., Tokyo, Japan); a cross section of the anode specimen embedded in resin was examined by energy-dispersive X-ray spectroscopy (EDX; Xact, Oxford Analytical Instruments, Abingdon, UK); and the anode film was examined by X-ray diffraction (XRD; D2PHASER, Bruker Corp., Billerica, MA, USA) with Cu Kα radiation (1.542 Å).

3. Results and Discussion

3.1 Anodic dissolution of single anode in shot form or block form

Figure 3 shows the changes in the anode potential during electrolysis for single black-copper anodes in shot form and block form (Fig. 3(a)), their dissolution ratios, and photographs of the anodes after electrolysis (Fig. 3(b)). The average dissolution ratios are marked with error bars indicating ± double the standard deviation for each run. Electrolysis proceeded at a stable anode potential for several hours in the early stage for both the shot and block anodes. After approximately 4 and 14 h for the shot anode and block anode, respectively, the potentials began to shift in the positive direction, and then increased sharply before electrolysis ended. The average dissolution ratio of the shot anode was approximately 65% and that of the block anode was approximately 48%. The variation for the shot anode was greater than that for the block anode. In both cases, the anode was covered with a hard, brittle, black film after electrolysis.

Fig. 3

(a) Changes in anode potentials during electrolysis with black-copper anode in shot form and block form; and (b) average dissolution ratios (error bars indicate ±2 standard deviations) with the photographs of anodes after electrolysis experiments.

In these setups, the electron-conducting path is considered to be Pt mesh–anode in the early stage of electrolysis, and Pt mesh–anode film–inner anode metal after formation of the anode film. It is thought that electrolysis stops because either or both of ion-conducting paths at the anode–electrolyte interface and the electron-conducting were no longer maintained after development of the anode film. Based on their average dissolution ratios, the dissolution depth from the anode surface was estimated to be 0.5 mm for the shot anode and 0.7 mm for the block anode. The dissolution depth for the shot anode was smaller than that for the block anode, although its dissolution ratio was higher. This is because the specific surface area of the shot anode was three times greater than that of the block anode. The variation in the anodic dissolution ratio was larger for the shot anode than for the block anode, probably because the smaller surface area of the shot anode provides fewer contact points with the Pt mesh, which results in lower reproducibility of the stability of the conduction path.

The observed anodic dissolution ratios were high for a Cu anode containing 1.9 mass% (i.e., 19000 ppm) Ag and comparable to the industrial anode residual ratios of 15%–20% observed for the conventional process.⁵⁾ This suggests that the basket electrolysis method can be used for black-copper-containing anodes. The specific surface area, which is determined by the size and shape of the anode, has a significant effect on anodic dissolution, therefore anode sizes of a few millimeters are desirable. Basket electrolysis experiments were then performed using multiple-shot anodes.

3.2 Basket electrolysis with shot anodes

Figure 4 shows the changes in the anode potential during electrolysis with black-copper anodes consisting of 18 shots and 28 shots (Fig. 4(a)), their dissolution ratios, and photographs of the anodes after electrolysis (Fig. 4(b)). The average anodic dissolution ratio was 79.6% for 18 shots placed in a single layer and 72.7% for 28 shots stacked in a double layer. After electrolysis, all the anodes were covered with hard, brittle anode films similar to that observed for a single shot anode. The 28 shots remained stacked.

Fig. 4

(a) Changes in anode potentials during basket electrolysis with black-copper anodes consisting of 18 shots placed in a single layer and 28 shots stacked in a double layer; and (b) average dissolution ratios (error bars indicate ±2 standard deviations) with the photographs of anodes after electrolysis experiments.

During electrolysis, noise-like peaks often appeared in the anode potential from an early stage; these were not observed when single anodes were used. This is considered to be because the positions of the shot anodes in the Pt basket were not fixed. Their positions therefore fluctuated because of factors such as agitation of the electrolyte, and this resulted in unstable contact with the Pt basket. The anodic dissolution ratios for multiple anodes were higher than those for single anodes. This could be because multiple-shot anodes dissolve independently, therefore even when anodic dissolution of some shots did not continue, anodic dissolution was maintained. The installation of electronic conduction paths between not only the anode shot and the Pt mesh, but also between each anode shot, contributed to the continuation of electrolysis as well. The average anodic dissolution ratios for 28 shots stacked in a double layer were slightly smaller than those for 18 shots placed in a single layer. This is speculated to be because the shots placed in the second layer were not in contact with the Pt mesh and the current applied via the anodes and the anode film of the lower layer. This resulted in a less-stable electronic conduction path and therefore a lower dissolution ratio. The load of the upper layer of shots might give rise to reproducibility of the contact strength between the lower layer of shots and the Pt mesh, which would suppress variations in the anodic dissolution ratios for each run.

Figure 5 shows optical microscopy (Fig. 5(a)) and SEM (Fig. 5(b)) images, and Ag, Pb, Sb, and Cu EDX mappings (Fig. 5(c)) of a cross section of the anode covered with a black film after electrolysis. Similar trends were observed for each experiment, therefore representative results for one anode of the 18-shot anode are shown. The anode film had a skeletal structure containing many voids, as reported by Tokushige et al.,²¹⁾ and a multilayered structure. The outer layer had high Ag and Pb contents and the inner layer had high Sb and S contents. The XRD pattern of the anode film, which was separated from the undissolved metal part and ground into a powder (Fig. 5(d)), indicates the presence of metal-phase Ag in the film. The Ag content of the outer layer increased during electrolysis by repeated anodic dissolution of a Ag–Cu alloy phase and substitutional precipitation via a sementation reaction with Cu. This is similar to the reported behavior of Ag in an anode film formed on a Ag-rich anode.^36,37)

Fig. 5

(a) Optical micrograph, (b) SEM image, and (c) EDX mappings of cross section of a Ag-containing shot anode after electrolysis, and (d) XRD pattern of the passivation film separated from the undissolved metal part.

During electrolysis, the voids in the anode film were filled with electrolyte, through which Cu ions dissolved from the metal diffused into the bulk electrolyte. The anode film is thought to be electronic-conductive because it contains Ag and Cu particles, therefore the surface provides an electronic conduction path via the Pt mesh–shot anode and/or each shot anode during electrolysis. In basket electrolysis using black-copper anodes, the film formed on the anode surface is therefore considered to play an important role in providing the electron- and ion-conducting paths necessary for electrochemical dissolution (Fig. 6).

Fig. 6

Schematic diagram of possible ionic and electronic conduction paths during basket electrolysis for black-copper shot anodes, covered with anode film with high-Ag content in the outer part.

3.3 Anodic dissolution with Ag-free shot anodes

The presence of Ag is expected to affect the anode film properties. The effects of Ag were investigated by performing electrolysis experiments with black-copper anodes that did not contain Ag. Figure 7 shows the changes in the anode potential during electrolysis experiments using a single-shot anode and 28 shots stacked in a double layer (Fig. 7(a)), and their dissolution ratios (Fig. 7(b)). For comparison, the dissolution ratios for anodes containing Ag are included. Anodic dissolution of the Ag-free anodes proceeded with relatively stable anode potentials and the average anodic dissolution ratios were 57.9% for a single shot and 76.3% for 28 shots stacked in a double layer. The appearances of the anodes after electrolysis were similar to those of the anodes containing Ag. This is confirmed by the optical microscopy image, SEM image, and cross-section EDX mappings, and XRD patterns of the anode film shown in Fig. A1.

Fig. 7

(a) Changes in anode potentials during electrolysis with Ag-free anodes of a single shot and 28 shots stacked in a double layer; and (b) average dissolution ratios (error bars indicate ±2 standard deviations). Average dissolution ratios for Ag-containing anode are shown again for comparison.

The average anodic dissolution ratio with a single-shot Ag-free anode was lower than that for a Ag-containing anode. This may be because the lack of Ag metal particles in the anode film resulted in fewer stable electronic conduction paths on the surface and inside the anode film. When a Ag-free 28-shot anode was used, the variation was larger than that for a Ag-containing anode, although the average anodic dissolution ratios were similar. A dense, firm anode film is formed on a crude copper surface with a high Ag content.^6–8) This suggests that the strengths of the anode films formed on the Ag-free anodes were lower than those of the films formed on the Ag-containing anodes. The load of the stacked anodes probably caused breakdown of the anode film, in which case the fresh surface of the inner metal part would be exposed and could make contact with the electrolyte or Pt mesh. This would promote anodic dissolution. It can be inferred that the variation in the anodic dissolution ratio of the Ag-free multiple-shot anode was increased by both positive and negative effects on the conductivity and the mechanical strength of the anode film.

Basket electrolysis using small-shot black-copper anodes yielded an anodic dissolution ratio greater than 70% under all conditions. This suggests that this method could be used for electrorefining of black-copper anodes. Because Ag in the anode significantly affects the properties of the anode film, but does not affect the anodic dissolution ratio, the presence or absence of Ag in the raw material does not limit its use. For the development of more practical applications of basket electrolysis with black-copper anodes, issues such as the introduction of insoluble dummy anodes in the plating process to ensure stable electronic conduction paths, the recovery of precious metals from the anode film involving the separation and the removal of undissolved Cu, use of a cell with a suitable optimal shape and structure for producing a smooth Cu cathode, the electrolyte composition, and the use of additives need to be considered in the future.

4. Conclusion

The use of basket electrolysis with black-copper anodes was examined with respect to the size, specific surface area, and composition of the anodes, and their attachment with a conducting support. Relatively high anodic dissolution ratios were achieved under all conditions. In particular, the use of multiple-shot anodes in the basket electrolysis method resulted in average anodic dissolution ratios greater than 70%. The film formed on the surface of the black-copper anode is thought to facilitate basket electrolysis because of its porous structure filled with electrolyte and the contained Ag or Cu metal particles, which are mainly distributed in the outer part, providing both electronic and ionic conductive paths. Although the Ag content of the anode affects properties such as the conductivity and mechanical strength of the anode film, its effect on the anodic dissolution ratio is small and does not limit its use in the basket electrolysis method.

Acknowledgements

This work was supported by the Japan Oil, Gas, and Metals National Corporation. The SEM observations were performed with a SEM-EDX instrument (Hitachi/SU6600) at the Fundamental Technology Center, Research Institute of Electrical Communication, Tohoku University.

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Appendix

Fig. A1

(a) Optical micrograph, (b) SEM image and EDX mappings of cross section of a Ag-free shot anode after electrolysis.