Improvement of Copper Metal Leaching in Sulfuric Acid Solution by Simultaneous Use of Oxygen and Cupric Ions

Abstract

A new concept for copper (Cu) metal leaching by the simultaneous use of cupric ions (Cu²⁺) and oxygen (O₂) was proposed to improve Cu metal leaching in sulfuric acid. According to this concept, Cu²⁺ oxidizes Cu metal into cuprous ion (Cu⁺), and O₂ oxidizes Cu⁺ into Cu²⁺. The improvement in Cu leaching efficiency from Cu metal was investigated experimentally in the sulfuric acid solution using Cu²⁺ and O₂ simultaneously. Furthermore, the result was compared with that for the sulfuric acid solution containing neither Cu²⁺ nor O₂ and with the sulfuric acid solution without Cu²⁺ and O₂. When both Cu²⁺ and O₂ were used in the leaching solution, the leaching rate of Cu from Cu metal powder was higher than at other leaching conditions, and the leaching efficiency of Cu increased to more than 99.9% in 1 mol/L sulfuric acid solution at 400 rpm and 50 °C with ≤75 µm Cu metal powder, 1% pulp density, 10,000 mg/L initial Cu²⁺ concentration, and 100 cc/min O₂ introduction. These results indicated that the leaching of Cu from Cu metal could be accelerated by adding Cu²⁺ and O₂ in the sulfuric acid solution.

1. Introduction

The International Copper Study Group (ICSG) reported that 33% of global copper use came from recycled copper in 2017 [1]. Although more than 70% of refined copper is produced by smelter [1,2], there have been many studies to recover copper from secondary sources such as printed circuit boards (PCBs) using hydrometallurgical ways, which have some advantages such as less gas emission and lower capital cost than pyrometallurgical processes [3,4,5,6,7,8,9,10,11,12]. Leaching is the first step of hydrometallurgical process for copper recovery from secondary sources, where copper metal must be oxidized into copper ion such as cuprous ion (Cu⁺) or cupric ion (Cu²⁺) because copper in the secondary sources is present as elemental copper [3,4].

The standard reduction potential of copper is +0.34 V (Cu²⁺ + 2e = Cu) [13], which is higher than that of hydrogen (0 V); therefore, copper cannot be oxidized by sulfuric acid. Hence, strong oxidants such as nitric acid, hydrogen peroxide, and ferric ion have been investigated to oxidize copper for the leaching of copper [3,14,15,16,17,18]. These oxidants have the following disadvantages: NO_x gas could be generated from nitric acid leaching [3]; hydrogen peroxide has been found to be unstable and expensive [3,14,15,16]; and when Fe³⁺ ion is used as an oxidant, an additional process after leaching is required to separate the Cu ion from Fe ions [17,18].

Cupric ions (Cu²⁺) have been recognized as an alternative to the strong oxidants [4,19,20,21]. However, it was reported that the cupric ions can act as oxidants only in hydrochloric acid or ammonia solutions. The oxidation reaction can be summarized as follows [19,20]:

$${\mathrm{Cu}}^{2+}+\mathrm{Cu}=2{\mathrm{Cu}}^{+}\left(\mathrm{in} \mathrm{hydrochloric} \mathrm{acid} \mathrm{or} \mathrm{ammonia} \mathrm{solution}\right),$$

(1)

where the cuprous ion (Cu⁺) plays a key role in the reaction because the reaction cannot proceed without Cu⁺. Generally, Cu⁺ ions can exist as complex ions in hydrochloric acid or ammonia solutions. These ions are unstable in a sulfuric acid solution; hence, it is understood that the reaction represented by Equation (1) is unlikely to occur in a sulfuric acid solution.

In conventional studies [22,23,24], the instability of cuprous ions in a sulfuric acid solution has been reported by examining the leaching behavior of Cu from cuprous oxide (Cu₂O). When Cu₂O was leached in a sulfuric acid solution under anoxic conditions, the Cu⁺ ions dissolved out from Cu₂O, but they were immediately transformed into Cu²⁺ ions and Cu due to their instability. The transformation reaction can be represented as follows [22,23,24]:

$${\mathrm{Cu}}_2\mathrm{O}+2{\mathrm{H}}^{+}={\mathrm{Cu}}^{2+}+\mathrm{Cu}+{\mathrm{H}}_2\mathrm{O}.$$

(2)

As shown in Figure 1a, under anoxic conditions, when one of the two Cu⁺ ions, which are dissolved out from Cu₂O, oxidizes to cupric ion (Cu²⁺) by losing an electron, the other Cu⁺ ion gains the electron and is transformed into elemental copper (Cu⁰). Thus, a balanced chemical reaction occurs. Figure 2 shows the XRD data of Cu₂O reagents (before leaching) and leaching residue (after leaching), respectively, where Cu metal peaks were observed after leaching.

In the previous study [23], the leaching tests with and without oxidants (1000 cc/min air) were performed to investigate the leaching behaviors of Cu from reagent Cu₂O in 1 mol/L sulfuric acid solution at 400 rpm and 30–90 °C with 2% pulp density. The leaching efficiency of Cu increased to 100% within 60 min with the aeration, while a lower leaching efficiency was observed without the introduction of air. The leaching with aeration was given by

$${\mathrm{Cu}}_2\mathrm{O}+4{\mathrm{H}}^{+}+\frac{1}{2}{\mathrm{O}}_2=2{\mathrm{Cu}}^{2+}+2{\mathrm{H}}_2\mathrm{O},$$

(3)

where oxygen in the air acts as an electron acceptor, as shown in Figure 1b. Thus, all the cuprous ions could be oxidized to cupric ions during Cu₂O leaching. Park et al. reported that the leaching efficiency of Cu from Cu₂O increased to 100% under aerated leaching conditions [23].

As discussed previously, due to the instability of the Cu⁺ ions, it is understood that the reaction represented by Equation (1) is unlikely to occur in sulfuric acid. However, the behavior has not been investigated in detail. We assumed that the oxidation of copper metal by cupric ions has taken place in the sulfuric acid solution without oxygen, and the new concept of copper leaching is shown in Figure 3b. Although copper metal could be oxidized by the cupric ion in sulfuric acid as shown in Equation (1), the Cu⁺ ions oxidize to Cu²⁺ ions or reduce to Cu⁰, as shown in Figure 3a. This is because the Cu⁺ ions of the reaction product are unstable in sulfuric acid. These reactions can be summarized by the following equation:

$${\mathrm{Cu}}^{2+}+\mathrm{Cu}\to 2{\mathrm{Cu}}^{+}\to {\mathrm{Cu}}^{2+}+\mathrm{Cu} \left(\mathrm{in} \mathrm{sulfuric} \mathrm{acid} \mathrm{solution}\right).$$

(4)

Equation (4) indicates that even if the Cu²⁺ ions could oxidize Cu metal in the sulfuric acid solution, it may appear that no reactions have occurred because the Cu⁺ ions transform immediately into Cu²⁺ ions or Cu. As shown in Figure 3b, when oxygen is introduced into the system, the oxygen oxidizes the Cu⁺ ions, which are generated by the oxidation of Cu metal by the Cu²⁺ ions. The reactions can be represented as follows:

$${\mathrm{Cu}}^{2+}+\mathrm{Cu}=2{\mathrm{Cu}}^{+}\left(\mathrm{in} \mathrm{sulfuric} \mathrm{acid} \mathrm{solution}\right),$$

(5)

$$2{\mathrm{Cu}}^{+}+2{\mathrm{H}}^{+}+\frac{1}{2}{\mathrm{O}}_2=2{\mathrm{Cu}}^{2+}+{\mathrm{H}}_2\mathrm{O},$$

(6)

where the Cu⁺ ions act as an intermediate. Hence, the overall reaction can be rearranged as follows:

$${\mathrm{Cu}}^{2+}+\mathrm{Cu}+2{\mathrm{H}}^{+}+\frac{1}{2}{\mathrm{O}}_2=2{\mathrm{Cu}}^{2+}+{\mathrm{H}}_2\mathrm{O},$$

(7)

Equation (7) indicates that the combination of Cu²⁺ ions and O₂ could leach the Cu²⁺ ions from copper metal. However, the effects of the addition of the Cu²⁺ ions and introduction of oxygen on the enhancement of copper leaching have not been investigated in a sulfuric acid solution. Since oxygen has a relatively high standard redox potential to oxidize copper metal (O₂ + 2H₂O + 4e⁻ = 4OH⁻, + 0.40 V) [13], only O₂ can oxidize copper and has been used in de-copperization from the anode slime. Therefore, in this study, four experimental conditions were compared to investigate the oxidation of copper metal powder in the sulfuric acid solution. These conditions were as follows: the addition of Cu²⁺ ions with the introduction of O₂, the addition of Cu²⁺ ions with the introduction of N₂, the introduction of O₂ without the addition of Cu²⁺ ions, and the introduction of N₂ without the addition of Cu²⁺ ions. Thus, the synergistic effect produced by using the Cu²⁺ ions and O₂ was investigated for copper metal leaching.

2. Materials and Methods

Two different sizes (≤ 75 µm (≥ 95%, Junsei Chemical Co., Ltd., Tokyo, Japan) and 150–420 µm (99.5%, Alfa Aesar Chemical Co., Ltd, Ward Hill, MA, USA)) of the copper metal powder were used in this study. For leaching tests, 1 mol/L sulfuric acid solutions with and without 10,000 mg/L Cu²⁺ were prepared by dissolving CuSO₄·5H₂O (99.0%, Junsei Chemical Co., Ltd., Tokyo, Japan) in sulfuric acid (≥ 95%, Junsei Chemical Co., Ltd., Tokyo, Japan) and distilled and deionized water. The solutions were transferred to a 500 cm³ four-necked Pyrex glass reactor (Daihan Scientific Co., Ltd, Wonju, Korea) and the temperature of the solutions was maintained at 50 °C using a heating mantle. The reactor was fitted with a stirrer, gas inlet/outlet tubes, and a reflux condenser. In addition, 100 cc/min of O₂, air and N₂ gas were introduced through the inlet tube during the leaching tests, and the purity of all gases is 99.99%. When the temperature of the solution was stabilized, 2 g of Cu metal powder was added into the solutions, and the solutions were withdrawn at designated times. The solutions were filtered with a 0.45 µm membrane filter and subsequently diluted with 5% HNO₃ for ICP-OES (inductively coupled plasma optical emission spectrometer, PerkinElmer, Inc. Waltham, MA, USA) analysis.

3. Results and Discussion

In Equations (5)–(7), the Cu⁺ ions play a key role as an intermediate. However, it is difficult to detect the Cu⁺ ions in the sulfuric acid solution due to their instability. Therefore, the improvement in copper metal leaching was verified experimentally in sulfuric acid solution by adding the Cu²⁺ ions and introducing O₂. Four leaching conditions were used according to the addition of the Cu²⁺ ions and introduction of O₂, and the leaching behavior of Cu from Cu metal was investigated.

Figure 4 shows the leaching behavior of Cu in 1 mol/L sulfuric acid solution at 50 °C and 400 rpm with 1% pulp density and < 75 µm Cu metal powder. The leaching efficiencies of Cu increase rapidly when O₂ is introduced into the solution, while they increase gradually when N₂ is introduced into the solution. The leaching efficiency of Cu increases more rapidly in the case of the solutions containing 10,000 mg/L Cu²⁺ ions than in the case of the solutions not containing 10,000 mg/L Cu²⁺ ions. Moreover, the leaching efficiency of Cu increased to more than 99.9% within 60 min in the sulfuric acid solution containing 10,000 mg/L Cu²⁺ ions.

Figure 5 shows the leaching behavior of Cu in 1 mol/L sulfuric acid solution at 50 °C and 400 rpm with 1% pulp density and 150–420 µm Cu metal powder. The leaching of Cu is slower with 150–420 µm than with 75 µm Cu metal powder, as observed after comparing Figure 4 and Figure 5. When oxygen is introduced into the solution, the improvement in Cu metal leaching by adding the Cu²⁺ ions is remarkable, as shown in Figure 5. Further, the leaching efficiency of Cu in the solution containing 10,000 mg/L Cu²⁺ ions increases to 54.8% after 120 min, while that of the solution not containing 10,000 mg/L Cu²⁺ ions increases to 35.6%.

In Figure 4 and Figure 5, in the cases of N₂ gas introduction, the leaching efficiencies of Cu are lower than those with O₂ gas introduction. The leaching efficiencies with N₂ introduction and <75 µm Cu metal powder increase to 9.8% and 35.5% without and with 10,000 mg/L Cu²⁺ ions, respectively, after 120 min. In the case of 150–420 µm Cu metal powder, the leaching efficiencies were 2.0% and 8.8% without and with 10,000 mg/L Cu²⁺ ions, respectively, after 120 min. Although Cu metal cannot be dissolved theoretically by sulfuric acid, the dissolution of Cu would result from the following two reasons. (1) The surface of Cu metal powder could be partially oxidized into Cu₂O as reported before [25]; thus, even sulfuric acid could dissolve the oxidized copper surface. Or, (2) a vortex could be formed during agitation; therefore, a small amount of air (oxygen), which was introduced into the leach solution, could act as an oxidant. Further study will be required to investigate what factors affect the leaching efficiency under the condition.

Figure 6 shows the leaching efficiencies of Cu with different gases such as O₂, air, N₂, and without gas introduction under the following conditions: 1 mol/L H₂SO₄ concentration, 400 rpm agitation speed, 50 °C temperature, 1% pulp density, < 75 µm-sized Cu metal powder, and 10,000 mg/L Cu²⁺ concentration. When 10,000 mg/L Cu²⁺ ions are added, the leaching efficiency increases to more than 99.9% in 60 min with 100 cc/min of O₂ introduction. In addition, the leaching efficiency increases to 98.1% after 120 min with 100 cc/min of air introduction. The oxygen in Equation (7) indicates the dissolved oxygen, and the leaching efficiency is higher with O₂ than with air because the partial pressure is higher during O₂ introduction. The leaching efficiency increases to 79.1% after 120 min without gas introduction because it is caused by the inflow of air due to vortex formation during agitation. The dissolution of Cu was suppressed by introducing N₂ gas to avoid O₂ inflow.

4. Conclusions

The leaching process of copper metal in sulfuric acid solution with both Cu²⁺ and O₂ was proposed to avoid the use of nitric acid or hydrochloric acid and the addition of oxidants such as Fe³⁺ for copper metal leaching. The leaching of Cu metal was enhanced by adding Cu²⁺ ions and introducing O₂ in the sulfuric acid solution; e.g. the leaching efficiency of Cu increased to more than 99.9% within 30 min in 1 mol/L sulfuric acid solution with 10,000 mg/L Cu²⁺ at 400 rpm with 1% pulp density, <75 µm particle size, and 100 cc/min O₂. It was also observed that a smaller particle size increases the leaching efficiency of Cu. In this case, it is suggested that the added Cu²⁺ ions oxidize Cu metal into Cu⁺ ions, and the O₂ introduced into the sulfuric acid solution further oxidizes the Cu⁺ ions into the Cu²⁺ ions. The use of Cu²⁺ ions and O₂ is for Cu metal leaching would leave no impurity in solution after leaching.