Leaching of Copper from Cuprous Oxide in Aerated Sulfuric Acid
2017 Volume 58 Issue 10 Pages 1500-1504
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2017 Volume 58 Issue 10 Pages 1500-1504
The leaching behavior of copper from Cu2O in H2SO4 solution was investigated to establish the leaching process for cathode powders produced by the recycling of waste printed circuit boards. When air was not introduced in sulfuric acid solution, the dissolution of copper from Cu2O was inhibited by the formation of elemental copper (Cu0). The dissociated cuprous ions (Cu+) transformed into elemental copper (Cu0) or cupric ions (Cu2+) owing to the instability of Cu+ in H2SO4. Cu+ can be reduced to elemental copper (Cu0) by accepting an electron generated from the oxidation of another Cu+ to Cu2+, which is known as a “disproportionation reaction.” The introduction of air enhanced the leaching efficiency of copper due to the role of oxygen in the air as oxidant by accepting the electron generated from the oxidation of Cu+ to Cu2+. In the leaching test using Cu2O reagent, the leaching efficiencies of copper increased with increasing air flow rate, temperature and agitation speed, but decreased with increasing pulp density. Copper leaching efficiency increased to up to 99% within 60 min in the aerated sulfuric acid solution at 30℃, 400 rpm, and pulp density of 2%.
The rapid growth in the rate of global electronic waste (e-waste) generation is a growing concern. E-wastes, such as spent printed circuit boards (PCBs), may contain more valuable metals than their typical ores1–3). As shown in Table 14), even though PCBs contain rather small amounts of precious metals, recovery of precious metals with a high recovery ratio is important because they account for 93% of the total economic value of PCBs4). Conventional studies and processes have adopted the approach of extensive comminution followed by leaching for the recovery of precious metals from waste PCBs3,5). However, loss of precious metals has been reported in the process, especially during grinding4).
| Mass% | Fe | Al | Cu | Plastics | Ag/ppm | Au/ppm | Pd/ppm |
| TV board | 28 | 10 | 10 | 28 | 280 | 20 | 10 |
| PC board | 7 | 5 | 20 | 23 | 1000 | 250 | 110 |
| Cellphone | 5 | 1 | 13 | 56 | 1380 | 350 | 210 |
| DVD player | 62 | 2 | 5 | 24 | 115 | 15 | 4 |
| Value Share | Fe | Al | Cu | Sum PM | Ag | Au | Pd |
| TV board | 4% | 11% | 42% | 43% | 8% | 27% | 8% |
| PC board | 0% | 1% | 14% | 85% | 5% | 65% | 15% |
| Cellphone | 0% | 0% | 7% | 93% | 5% | 67% | 21% |
| DVD player | 13% | 4% | 36% | 47% | 5% | 37% | 5% |
A recycling process comprising of smelting and subsequent electrorefining, as depicted in Fig. 1, was established by the Korea Institute of Geoscience and Mineral Resources (KIGAM)6). In this process, precious metals are concentrated as anode slimes after smelting and electrorefining, which alleviated the loss of these metals. Other metals such as copper, nickel, and iron are recovered as cathode powder. The copper was found to be cuprous oxide (Cu2O) and elemental copper (Cu0) in the preliminary study.
Flow sheet of recycling process developed by KIGAM.
Only a few studies examined the leaching of cuprite (Cu2O)7–10). In these studies, leaching tests were carried out using various solutions, such as sulfuric acid with ferric sulfate7), sulfuric acid with perchloric acid8,9), and sulfuric acid with sulfur dioxide10). The leaching efficiency of copper was reported to be about 50% in the case of leaching in sulfuric acid solution without other oxidants7–9). Although Sullivan and Oldright7) and Majima et al.9) observed that the introduction of oxygen or air enhanced the leaching efficiency of copper by employing single experimental set without further discussion, they did not investigate the effects of gas type or gas flow rate as their work focused on the effects of ferric sulfate and perchloric acid as oxidants. Temperature and agitation speed can also be considered as indirect parameters because increasing the temperature decreases dissolved oxygen concentration, and increasing the agitation speed increases the air inflow into solution by the formation of swirling motions.
It is expected that the economic efficiency will be improved if the leaching efficiency of copper is enhanced by introducing air without adding any oxidizing agent, such as HClO4 and Fe3+ used in the conventional studies7–9). Therefore, in the present study, leaching tests using cuprous oxide were conducted in sulfuric acid solution and the effects of leaching parameters, such as air flow rate, gas type, temperature, agitation speed, sulfuric acid concentration, and pulp density, on the leaching efficiency of copper were discussed with the aim of improving the dissolution of copper from the cathode powder obtained from the recycling process shown in Fig. 1.
The cathode powder sample used in the experiments was acquired from KIGAM. The sample was sieved using a 75 µm screen. Figure 2(a) shows the SEM image (with JSM-6490, JEOL Ltd.) of the cathode powder, where agglomerated submicron particles were observed. The X-ray diffraction (XRD) pattern (see Fig. 3) of the cathode powder sample revealed that copper existed in the form of cuprous oxide and elemental copper (Cu0). The sample was digested with HNO3 and HF to measure the metal content. Acid digestion was performed by Yucheon Tech Co. (Daejeon, Korea). The chemical composition of the sample is presented in Table 2. It contained 89.4% copper, 2.32% iron, 0.9% tin, 0.77% nickel, and 0.51% zinc; other metals were not detected. Cuprous oxide and sulfuric acid used in the experiments were of reagent grade. As shown in Fig. 2(b), the reagent-grade cuprous oxide consists of 1–3 µm particles.
SEM images of cathode powder and Cu2O reagent.
XRD pattern of cathode powder sample used in this study.
| Element | Cu | Fe | Sn | Ni | Zn |
|---|---|---|---|---|---|
| Mass% | 89.4 | 2.32 | 0.9 | 0.77 | 0.51 |
Experiments on the dissolution of the cathode powder sample and Cu2O reagent in H2SO4 solution were conducted in a 500-mL three-necked Pyrex reactor set in a heating mantle. The reactor was fitted with an agitator and reflux condenser. The reflux condenser was inserted in one port to prevent solution losses at high temperatures. In a typical run, 200 mL of acid solution (1–5 M H2SO4) was poured into the reactor and permitted to attain thermal equilibrium (30–90℃). Predetermined amount of the cathode powder sample or Cu2O reagent was added to the reactor, and the agitator was set to operate at speeds of 200–1000 rpm. Nitrogen gas, air, or oxygen gas was introduced into the reactor at a flow rate of 0–1000 cc/min. During the experiment, 3 mL of the sample solution was withdrawn periodically at a desired time interval by means of a syringe. The sample solution was filtered using a 0.45-µm membrane filter, and then, the filtrate was diluted with 5% HNO3 for copper, iron, nickel, lead, and zinc analyses or 15% HCl solution for tin analysis.
The sample solutions were analyzed by atomic absorption spectrometry (AA7000, Shimadzu Co., Ltd., Japan) and inductively coupled plasma-atomic emission spectrometry (ICP-AES, Optima 8300DV, PerkinElmer Co., Ltd., USA). The leaching residue was also characterized by XRD (D/Max-2500PC, Rigaku Co., Japan).
As shown in Fig. 3, copper was present in the form of Cu2O and elemental copper (Cu0) in the cathode powder sample. Therefore, the copper leaching tests were performed in sulfuric acid with a Cu2O reagent to investigate the optimum conditions for the dissolution of the cathode powder sample, aiming to improve the economic efficiency of the recycling process shown in Fig. 1.
The effects of temperature on the leaching efficiency of copper were investigated under the following conditions: 1 M H2SO4, 400 rpm, pulp density of 2%, and without the introduction of air. As shown in Fig. 4, the leaching efficiencies first increased rapidly and then gradually afterwards. Generally, an increase in temperature can enhance metal dissolution in an exothermic reaction; however, in the current study, results showed the opposite. The leaching efficiency for copper at 120 min was found to be 82.3% at 30℃ but it decreased to 69.4% at 90℃. The leaching residues were examined by XRD and were compared to the reagent-grade Cu2O, to find what caused the opposite effect of temperature on leaching. The XRD patterns are shown in Fig. 5. As can be seen in this figure, peaks of Cu metal are present in all leaching residues. This observation has been reported in previous studies9,10), and it can be represented by the reaction as expressed in eq. (1):
| \[ Cu_{2}O + 2H^{+} = Cu^{2+} + Cu + H_{2}O \] | (1) |
Effect of temperature on leaching efficiency of copper in 1 M H2SO4 solution at 400 rpm with pulp density of 2% and without the introduction of air.
XRD pattern of precipitates recovered from leaching test performed under conditions of 1 M H2SO4 solution, 400 rpm, pulp density of 2%, and without the introduction of air.
Generally, cuprous ions (Cu+) are unstable in sulfuric acid11). If there is no any other oxidant in the system, when one of two Cu+ oxidizes to cupric ions (Cu2+) by losing an electron, the other Cu+ must reduce to elemental copper (Cu0) by gaining the electron; this balances out the reaction. This kind of reaction is known as a “disproportionation reaction.” During agitation, a swirl is formed which introduces oxygen from air into the leach solution and acts as an oxidant. In this case, Cu metal is not formed and all copper can be dissolved as Cu2+ as follows:
| \[ Cu_{2}O + 4H^{+} + \frac{1}{2} O_{2} = 2Cu^{2+} + 2H_{2}O \] | (2) |
However, the increase in temperature reduced the concentration of dissolved oxygen, and higher leaching efficiency was obtained at the lower temperature as shown in Fig. 4.
As shown in Fig. 6, when air was introduced into the reactor at a flow rate of 1000 cc/min, the copper leaching efficiencies increased up to 99% within 30 min with an increase in temperature at the beginning of the leaching test. This result indicates that the introduction of air could enhance the dissolution of Cu2O in sulfuric acid.
Effect of temperature on leaching efficiency of copper in 1 M H2SO4 solution at 400 rpm with pulp density of 2% and air flow rate of 1000 cc/min.
The effects of agitation speed on the leaching efficiency of copper were investigated alongside the leaching test to examine the effects of liquid film boundary diffusion surrounding the solid particles on the leaching efficiency of the Cu2O reagent in 1 M H2SO4 solution at 30℃ with a pulp density of 5% and air flow rate of 1000 cc/min. The results of this leaching test are shown in Fig. 7. The leaching efficiencies increased rapidly within 10 min and then gradually with time. The leaching efficiencies also increased when the agitation speed was increased to 800 rpm. A higher agitation speed could possibly improve the distribution of dissolved oxygen. However, no improvement was observed when the agitation speed was increased from 800 rpm to 1000 rpm.
Effect of agitation speed on leaching efficiency of copper in 1 M H2SO4 solution at 30℃ with pulp density of 5% and air flow rate of 1000 cc/min.
Figure 8 shows the effects of the air flow rate on the leaching efficiency of copper in 1 M H2SO4 at 30℃ and 800 rpm with a pulp density of 2%. The leaching efficiency increased with increasing air flow rate. However, the improvement in the leaching efficiency was not significant after 60 min when the air flow rate was increased further from 100 cc/min to 1000 cc/min. At these air flow rates, leaching efficiencies higher than 99% were observed within 60 min.
Effect of air flow rate on leaching efficiency of copper in 1 M H2SO4 solution at 30℃ and 800 rpm with pulp density of 2%.
Figure 9 shows the effects of gas type on the leaching efficiency. The introduction of oxygen gas significantly enhanced the leaching efficiency compared to the introduction of air; the leaching efficiency of copper increased to almost 100% within 15 min with oxygen flow rate of 50 cc/min, whereas it took 120 min to dissolve all copper sample with air flow rate of 50 cc/min when temperature and pulp density were fixed at 30℃ and 2%, respectively. Furthermore, the introduction of nitrogen gas suppressed the dissolution of copper. These results are in good agreement with the reactions in eqs. (1) and (2).
Effect of gas type on leaching efficiency of copper in 1 M H2SO4 solution at 30℃ and 400 rpm with pulp density of 2% and gas flow rate of 50 cc/min.
Figure 10 shows the effects of sulfuric acid concentration on the leaching efficiency in sulfuric acid solution at 30℃ and 1000 rpm with a pulp density of 5% and air flow rate of 1000 cc/min. The difference in leaching efficiencies was negligible at sulfuric acid concentrations of 1–4 M after 30 minutes; however, at the concentration of 5 M, the leaching efficiency decreased to 53% at 120 min. The leaching residue was collected and investigated by XRD, and it was determined to be copper sulfate hydrate (CuSO4·5H2O), as shown in Fig. 11.
Effect of sulfuric acid concentration on leaching efficiency of copper in H2SO4 solution at 30℃ and 1000 rpm with pulp density of 5% and air flow rate of 1000 cc/min.
XRD pattern of leaching residue recovered from leaching test performed in 5 M H2SO4 solution at 30℃ and 1000 rpm with pulp density of 5% and air flow rate of 1000 cc/min.
Since pulp density is an essential factor to consider for system upgrades, the leaching behavior of copper was investigated in 1 M H2SO4 solution at 30℃ and 400 rpm with an air flow rate of 1000 cc/min. Figure 12 shows that the leaching efficiencies decreased with an increase in pulp density from 1% to 10%. A leaching efficiency over 99% was obtained at the pulp density of 5% at 60 min, while an efficiency of 68.3% was obtained at the pulp density of 10% at 120 min, in which case the pH was measured to be 3.5. Equation (2) indicates that hydrogen ions are consumed during the dissolution of copper in the case where oxygen is introduced in sulfuric acid solution. The increase in pH decreased the leaching efficiency of copper with higher pulp density.
Effect of pulp density on leaching efficiency of copper in 1 M H2SO4 solution at 30℃ and 400 rpm with air flow rate of 1000 cc/min.
To affirm the results of leaching tests using Cu2O reagent, a leaching test using the cathode powder obtained from the recycling process shown in Fig. 1 containing copper, nickel, iron, zinc, tin (Table 1). The leaching conditions for the test were 1 M H2SO4 solution, 50℃, 1000 rpm, pulp density of 5% and air flow rate of 1000 cc/min. As shown in Fig. 13, the leaching efficiencies of copper, nickel, iron, and zinc increased to values over 99% within 10 min. This leaching rate was found to be very fast compared to that of the leaching of reagent-grade Cu2O and can be attributed to the much smaller particle size of the cathode powder as shown in Fig. 2. Furthermore, iron dissolved from the cathode powder, could be changed into ferric ions by aeration, and the ferric ion has been known as a strong oxidant12,13) enhancing the leaching rate of copper.
Leaching behaviors of copper, nickel, iron, tin, and zinc in 1 M H2SO4 leaching test at 50℃ and 1000 rpm with pulp density of 5% and air flow rate of 1000 cc/min.
The leaching efficiency of Sn first increased and then decreased to almost 0%. Sn has been reported to have lower solubility in sulfuric acid compared to hydrochloric acid13), and generally, Sn4+ ions precipitate to SnO214–16). The leaching efficiencies of copper, nickel, iron, and zinc were relatively low owing to the formation of elemental copper (Cu0) in the sulfuric acid leaching test without the introduction of oxygen (or air) (data not shown); however, the introduction of air resulted to sufficiently enhanced leaching efficiencies. Therefore, dissolution of copper from cuprous oxide was achieved successfully by sulfuric acid leaching with the introduction of air.
The dissolution of Cu2O in sulfuric acid has been reported to be suppressed by a disproportionation reaction, wherein half of the cuprous ions are reduced to elemental copper (Cu0). In this study, effects of oxygen introduction on the leaching behavior of copper were evaluated via examination of the effects of leaching factors such as the air flow rate, sulfuric acid concentration, temperature, agitation speed, and pulp density on the leaching efficiency.
The leaching efficiencies of copper increased with increasing air flow rate, temperature, and agitation speed but decreased with increasing pulp density. Sulfuric acid concentrations of 1–4 M did not affect the leaching efficiency after 30 minutes, but at a concentration of 5 M, the efficiency decreased owing to the formation of cupric sulfate precipitate. Since oxygen in the air acts as an oxidant for cuprous ions, the introduction of oxygen or air could enhance the dissolution of Cu2O in sulfuric acid solution.
This research was supported by the Basic Research Project of the Korea Institute of Geoscience and Mineral Resources (KIGAM) funded by the Ministry of Trade, Industry & Energy (MOTIE) of Korea.
