Leaching of Copper from Waste-Printed Circuit Boards (PCBs) in Sulfate Medium Using Cupric Ion and Oxygen

Abstract

In the present paper, the leaching of copper from printed circuit boards (PCBs) using sulfuric acid with Cu²⁺ and O₂ is proposed. The effects of various process parameters such as agitation speed, temperature, the type and the flow rate of gas, initial Cu²⁺ concentration, and pulp density were investigated to examine the dissolution behavior of Cu from PCBs in 1 mol/L sulfuric acid. The kinetic studies were performed using the obtained leaching data. The leaching rate of Cu from PCBs was found to be higher on addition of Cu²⁺ and O₂ to the leachant in comparison with the addition of O₂ or both Cu²⁺ and N₂ in the leachant. The leaching efficiency of Cu was found to be increased with increasing agitation speed, temperature, O₂ flow rate, and initial Cu²⁺ concentration and decreasing pulp density. The 96% of Cu leaching efficiency was obtained under the following conditions: sulfuric acid concentration, 1 mol/L; temperature, 90 °C; agitation speed, 600 rpm; pulp density, 1%; initial Cu²⁺ concentration, 10,000 mg/L; and O₂ flow rate, 1000 cc/min. The leaching data and analyses indicate that the Cu leaching from PCBs followed the reaction-controlled model satisfactorily and determined that the activation energy was found to be 23.8 kJ/mol. Therefore, these results indicate that the sulfuric acid solution with Cu²⁺ and O₂ as a mild leach medium without strong oxidants such as HNO₃, H₂O₂, and Fe³⁺ is valid for Cu leaching from PCBs.

1. Introduction

Printed circuit boards (PCBs) are core parts of electric and electronic equipment (EEE) [1,2,3,4,5]. The market of PCBs in Korea grew to 13 trillion Korean won by 2016, and 60% of waste PCBs was recycled, while the rest was exported as used PCBs [6]. Waste PCBs have hazardous pollutants as well as valuable resources, which were recovered by smelters in Korea. However, only three smelters are in operation in Korea, and there is an increasing concern about hazardous gas emission from smelters.

Hydrometallurgical recycling processes have been recognized as alternative methods; generally, the processes consist of pretreatment (liberation), leaching, purification, and electrowinning. Some wastes need pretreatment such as grinding for liberation, which could facilitate contact between leaching reagent and metal components [1]. Even recently, a number of pretreatments and leaching processes for waste PCBs recycling have been proposed, such as the following examples. The pretreatment processes such as soaking in NaOH solution [7] and microwave pyrolysis [8] and mineral processing processes [9] such as flotation [10] were investigated to enhance the dissolution efficiency of valuable metals from PCBs. Leaching processes of PCBs have been examined using hydrogen peroxide (H₂O₂) [11,12,13,14], ammonia solution [10,14,15,16], HNO₃ [16,17,18], halogen [16,17,19,20], glycine [12,13], and ionic liquid [21]. Bacterial leaching processes of PCBs have also been reported as environment-benign processes, where Phanerochaete chrysosporium [22], Acidithiobacillus ferrooxidans [23], Leptospirillum ferriphilum or its mixed culture with Acidithiobacillus ferrooxidans [24,25,26], and Frankia sp. [27] were used for Cu leaching. The fact that many processes are still being investigated and tried as above means that bio-hydrometallurgical recycling processes cannot be commercialized easily, despite more than 30 years of research.

The Korean government has also supported the development of hydrometallurgical recycling processes of waste PCBs for small-sized businesses for the last two decades [28]. Several pilot plants of waste PCBs recycling were operated to investigate the feasibility of the pre-treatment followed by hydrometallurgical recycling processes, but they did not reach the demo-plant or commercialization stage for two reasons. Firstly, precious metals, i.e., gold and silver were lost during extensive grinding for liberation and, secondly, capital and environmental costs increased by using nitric acid or hydrogen peroxide as leaching reagents for copper (Cu) dissolution. The amount of copper contained in waste PCBs found to be 5–20% [1,29,30], and Cu should be removed or recovered before the precious metal recovery process because Cu as an impurity could interfere with the recovery of precious metal such as gold [5,31]. Generally, strong oxidants such as HNO₃, H₂O₂, and Fe³⁺ are required to dissolve copper (Cu) from waste PCBs because the standard electrode potential of copper is +0.34 V (Cu²⁺ + 2e = Cu) [32], which indicates Cu metal will not be dissolved in sulfuric acid solution.

Conventional oxidants have disadvantages such as NO_x gas emission due to HNO₃ leaching [33,34,35,36], increase in reagent consumption due to instability of H₂O₂ [37,38], and requirement of additional separation process due to the use of Fe³⁺ [38,39,40]. These drawbacks could increase the capital and environmental costs as mentioned above. Therefore, the development of mild leaching condition and minimization in reagent consumption and wastewater generation should be fulfilled for successful hydrometallurgical recycling processes. The use of sulfuric acid-based leaching medium could reduce the capital cost because sulfuric acid is less corrosive than nitric and hydrochloric acids, and sulfuric acid is favorable for purification and electrowinning processes.

In our previous study [40], although a new copper dissolution process in sulfuric acid solution using Cu²⁺ and O₂ was proposed, the leaching tests were carried out to dissolve Cu from reagent-grade Cu metal powder. Therefore, in the present study, the leaching tests of PCBs were performed in sulfuric acid solution with Cu²⁺ and O₂ to investigate the effects of agitation speed, temperature, the type and the flow rate of gas, initial Cu²⁺ concentration, and pulp density for Cu leaching from PCBs in 1 mol/L sulfuric acid solution and, further, activation energy was calculated based on the leaching data and kinetic study.

2. Materials and Methods

All waste PCBs were collected from spent hard disk drives of the same model, which were obtained from a recycling company. As shown in Figure 1, the electronic components on the waste PCBs were removed by dissolving solders [1,41], which was achieved by HCl leaching with Sn⁴⁺. The bare waste PCBs were cut with a cutting mill (SM100, Retsch GmbH & Co., Haan, Germany) equipped with a 5 mm sieve, and the product was screened with a 1 mm sieve. The 1 mm oversized product was ground further with a mixer mill (MM400, Retsch GmbH & Co., Haan, Germany). Therefore, all waste PCBs samples with less than 1 mm were used in the leaching tests. The waste PCBs samples contain 31.7% of Cu, which was determined by 2-hour aqua-regia digestion (over 100 °C, less than 1 mm PCBs, and 5 g PCBs/100 mL aqua regia) before the leaching tests. All reagents used in this study were of reagent grade.

The leaching tests of the ground waste PCBs were performed in a 500 mL three-necked Pyrex glass reactor using a heating mantle to maintain the temperature. The reactor was fitted with a reflux condenser and an agitator. The reflux condenser was inserted into one port to avoid solution loss at high temperatures. In a typical run, 200 mL of 1 mol/L sulfuric acid solution with 0, 3000, 5000, 7000, or 10,000 mg/L Cu²⁺ was poured into the reactor with an agitation speed of 200–800 rpm. After the temperature of solution reached the thermal equilibrium (30–90 °C), 200–1000 cc/min of oxygen, air, or nitrogen gas was introduced into the reactor and 2 g of the ground waste PCBs was added to the reactor in all the experiments. During the leaching tests, 3 mL of the solution was withdrawn periodically at a desired time interval (15−120 min) by a syringe. The sample was filtered using a 0.45 µm membrane filter, and the obtained filtrate was diluted with 5% HNO₃ solution. The leach residue was digested with aqua regia and the solution was diluted with distilled water. The concentration of Cu was measured using optima 8300 ICP-OES (inductively coupled plasma optical emission spectrometer, PerkinElmer Inc., Waltham, MA, USA). The leaching efficiency of Cu was calculated using the following equation.

The leaching efficiency of Cu (%) = M_solution/(M_solution + M_residue) × 100,

(1)

where M_solution (g) and M_residue (g) represent the mass of Cu in the leach solution and residue, respectively, and M_solution does not contain the initial Cu²⁺, which was added before the leaching tests.

3. Results and Discussion

Generally, it has been found that PCBs contain various metals, but the metals, except Au and Cu, are contained in electronic components such as IC chips, capacitors, and resistors, while Au and Cu are distributed in both electronic components and bare PCBs. In the recycling of bare PCBs, Cu could act as an impurity in the hydrometallurgical Au recovery process using cyanide, and Cu must be removed before the Au recovery from PCBs. Therefore, in this study, the electronic components were removed by HCl leaching with Sn⁴⁺, and then the leaching behaviors of Cu were investigated as follows.

The leaching tests of Cu from PCBs were carried out to investigate the effect of Cu²⁺ and O₂ addition on the improvement of Cu leaching. Figure 2 shows the leaching behaviors of Cu from PCBs in 1 mol/L sulfuric acid solution with or without 10,000 mg/L Cu²⁺ at a temperature of 90 °C and agitation speed of 600 rpm with 1% pulp density and 1000 cc/min N₂ or O₂ introduction. The fastest leaching efficiency of Cu was obtained by adding both O₂ and Cu²⁺, and when only O₂ was introduced in the solution, the leaching efficiency was higher than that with Cu²⁺ and N₂ introduction. In the previous study [40], the mechanism of Cu leaching with Cu²⁺ and O₂ was explained as follows. The added cupric ion (Cu²⁺) oxidizes Cu metal into cuprous ion (Cu⁺), and then the added O₂ oxidizes Cu⁺ ion further into Cu²⁺. The reactions are expressed in Equations (2) and (3).

$${\mathrm{Cu}}^{2+}+\mathrm{Cu}=2{\mathrm{Cu}}^{+},$$

(2)

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

(3)

Equations (2) and (3) could be rearranged into Equation (4):

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

(4)

As shown in Figure 2, when Cu²⁺ and N₂ were added to sulfuric acid solution, the dissolution efficiency of Cu increased gradually to only 18%. This dissolution would result from partly oxidized surface of Cu during grinding, and an oxidized part of Cu could be dissolved in sulfuric acid solution [30,40]. When only Cu²⁺ ions are added without O₂, even though Cu⁺ ions are formed, as shown in Equation (2), because Cu⁺ ions are unstable in sulfuric acid solution, the Cu⁺ ions are changed into Cu²⁺ or Cu as follows [30]:

$$2{\mathrm{Cu}}^{+}={\mathrm{Cu}}^{2+}+\mathrm{Cu},$$

(5)

If no any other oxidant is added in the solution, when one of the two Cu⁺ oxidizes to Cu²⁺ by losing an electron, the remaining Cu⁺ must reduce to elemental copper (Cu⁰) by receiving the electron. When only O₂ is added without Cu²⁺, the leaching efficiency of Cu increases gradually, and then increases rapidly after 30 min. Although Cu²⁺ was not added, because Cu ions were dissolved out in the beginning of leaching, the Cu leaching was achieved by O₂ addition with a small amount of Cu²⁺, which was dissolved from PCBs.

As discussed above, Cu⁺ ions are extremely unstable in sulfuric acid solution and it is impossible to detect Cu⁺ as an intermediate in Equations (1) and (2). Therefore, three conditions—Cu²⁺/O₂ addition, only O₂ addition, and Cu²⁺/N₂ addition—were compared and the synergetic effect of simultaneous addition of Cu²⁺ and O₂ on the improvement of Cu leaching was confirmed as shown in Figure 2.

Figure 3 shows the effect of agitation speed on the leaching efficiency of Cu to examine the effect of liquid film boundary diffusion surrounding the solid particles on the dissolution efficiency in 1 mol/L sulfuric acid solution at 90 °C with 10,000 mg/L Cu²⁺, 1% pulp density, and 1000 cc/min O₂ introduction. The dissolution efficiency increased gradually with leaching time, and the higher leaching efficiency was obtained at a higher agitation speed. Similar dissolution behaviors of Cu were observed in the leaching test with 600 rpm and 800 rpm. Therefore, in all subsequent leaching tests, a working agitation speed of 600 rpm was selected to provide effective particle suspension in the solution. The effect of temperature (30–90 °C) was investigated in 1 mol/L sulfuric acid solution at 600 rpm with 10,000 mg/L Cu²⁺, 1% pulp density, and 1000 cc/min O₂ introduction, and the result is presented in Figure 4. The elevated temperatures yielded higher dissolution rates of Cu from PCBs, and the leaching efficiency of Cu at 90 °C increased gradually to 96.0% within 120 min.

As discussed above, the leaching of Cu was improved by adding Cu²⁺ and O₂, where the O₂ used in the leaching reaction indicates dissolved oxygen. Generally, the concentration of dissolved oxygen in water decreases with increasing temperature, and the oxygen was introduced continuously into the reactor during the leaching tests. The effect of the O₂ gas flow rate was investigated in 1 mol/L sulfuric acid solution at 90 °C and 600 rpm with 10,000 mg/L Cu²⁺, 1% pulp density, and 200–1000 cc/min O₂ introduction. The results presented in Figure 5 indicate that the leaching efficiencies of Cu increased gradually up to 95% in all flow rates, but the tests with less than 400 cc/min showed a slower leaching rate. As oxygen plays a role of oxidant for the oxidation of cuprous ion (Cu⁺) to cupric ion (Cu²⁺), as shown in Equation (4), it is important to provide sufficient oxygen into the leaching system. Therefore, 1000 cc/min flow rate of oxygen was selected in further leaching tests.

Leaching experiments were carried out to investigate the effect of initial cupric ion concentration as an oxidant for the dissolution of Cu metal in 1 mol/L sulfuric acid solution, temp. 90 °C, agitation speed 600 rpm, Cu²⁺ concentration 0–10,000 mg/L, pulp density 1%, and flow rate of O₂ in solution1000 cc/min. Equation (4) represents the chemical reaction of Cu leaching, where Cu²⁺ ion acts as an oxidant for the dissolution of Cu metal. Figure 6 indicates that the leaching efficiencies of Cu increased gradually with time and the Cu²⁺ concentration; therefore, the use of Cu²⁺ is favorable for Cu metal leaching in sulfuric acid solution. When no Cu²⁺ was added to the leaching system (0 mg/L initial Cu²⁺ in Figure 6), the leaching efficiency of Cu increased up to 64.7% and was comparatively slower. As discussed above, partly oxidized Cu could be dissolved in sulfuric acid solution and the dissolved Cu ions could act as an oxidant for Cu metal leaching.

In Figure 7, the effects of pulp density on the leaching behaviors of Cu were investigated in 1 mol/L sulfuric acid solution at a temp. of 90 °C and agitation rate of 600 rpm with Cu²⁺ concentration 10,000 mg/L, pulp density 1–5%, and flow rate of O₂ addition 1000 cc/min. The leaching efficiency of Cu increased rapidly with the decreasing pulp density. The leaching efficiency with 1% pulp density increased slower in the beginning of leaching than that with 3% pulp density, but they show similar leaching behaviors after 60 min. The leaching efficiency with 5% pulp density was retarded. If all Cu metals in the PCBs samples dissolved, the Cu concentrations of sample were found to be 3170 mg/L, 9510 mg/L, and 15,850 mg/L at 1%, 3%, and 5% pulp density, respectively. In Equation (4), 1 mol of Cu²⁺ should be required to oxidize 1 mol of Cu metal (the stoichiometric mole ratio is 1). Because the initial Cu²⁺ concentration was 10,000 mg/L, in the case of 5% pulp density, the initial Cu²⁺ concentration was not enough to oxidize all the Cu metal from PCBs samples, but the dissolved Cu could be changed into Cu²⁺ ion, as shown in Equation (4), and the leaching efficiency with 5% increased to 85.8% in 120 min, even though it was slow.

Studies on the leaching kinetics of Cu from PCBs were reported previously and investigations were carried out using HCl solution with Cl₂ [42], NH₄OH solution with H₂O₂ [14], HNO₃ solution [43], and H₂SO₄ solution with Fe³⁺ [44]. Activation energy values were calculated using shrinking core model [14,42,43,44] and were determined to be 20.7–24.5 kJ/mol in HCl solution with Cl₂ [42], 47.39 kJ/mol in NH₄OH solution with H₂O₂ [14], 23.35 kJ/mol in HNO₃ solution [43], and 14.87 kJ/mol in H₂SO₄ solution with Fe³⁺ [44], respectively. The shrinking core model can be presented by [45]

$$1-{(1-x)}^{\frac{1}{3}}=k_{r}t,$$

(6)

$$1-\frac{2}{3}x-{(1-x)}^{\frac{2}{3}}=k_{d}t,$$

(7)

where Equation (6) represents the shrinking core model basically based on the surface chemical reaction (reaction-controlled model), while Equation (7) is based on the diffusion reaction (diffusion-controlled model). The value for x in the equations represents the fraction reacted and is obtained from the leaching efficiencies, while k_r and k_d are rate constants.

The data in Figure 4 were fitted to the models as shown in Figure 8 (reaction-controlled model) and Figure 9 (diffusion-controlled model). The correlation coefficients (R²) are 0.9768 to 0.9879 in Figure 8 and 0.8019 to 0.9774 in Figure 9, respectively, which indicate that the reaction-controlled model is more suitable to this leaching behavior than the diffusion-controlled model. Generally, PCB has a multi-layered structure composed of copper foil and resin. However, as shown in Figure 10, because the copper in PCBs was exposed during extensive milling with cutting and mixer mills, the reaction-controlled model is suitable for this PCBs’ leaching. Rate constants (k) for different temperature were calculated by plotting Figure 8, and the Arrhenius plots using the rate constants over T⁻¹ are presented in Figure 11, where the activation energy is found to be 23.8 kJ/mol. The activation energy is similar to the data reported in conventional studies [14,42,43,44], which indicate that the leachate containing sulfuric acid with Cu²⁺ is effective for Cu leaching.

4. Conclusions

Mild leaching conditions using Cu²⁺ and O₂ in sulfuric acid solution were investigated to leach Cu from PCBs. When Cu²⁺ and O₂ were added to the leach solution, the leaching efficiency of Cu increased faster than that of the leaching test performed without Cu²⁺ and O₂. On the other hand, the simultaneous use of Cu²⁺ and O₂ improved Cu leaching from PCBs. The leaching efficiency of Cu increased with the increasing agitation speed, temperature, gas flow rate of O₂, and initial Cu²⁺ concentration and decreasing pulp density. The leaching efficiency of Cu increased to 96% in 1 mol/L sulfuric acid solution at temperature 90 °C, agitation speed 600 rpm, pulp density 1%, Cu²⁺ concentration 10,000 mg/L, and O₂ introduction flow rate 1000 cc/min. The kinetic data indicate that the leaching behavior of Cu followed the reaction-controlled model well rather than the diffusion-controlled model, and the activation energy was calculated to be 23.8 kJ/mol, which is similar to the data reported in the conventional studies. Therefore, these results suggest that the sulfuric acid solution with Cu²⁺ and O₂ addition will be a suitable leaching condition to leach Cu from PCBs.