The recovery of gallium from gallium arsenide (GaAs) scraps is essential for sustainable resource utilization and environmental protection. Traditional recovery methods, such as acid leaching and pyrometallurgy, often suffer from high energy consumption, excessive waste generation, and difficulties in selectively extracting gallium while preventing arsenic contamination. Swain et al. (2015) established a highly efficient and environmentally friendly process for gallium recovery using alkaline oxidative leaching, followed by cooling crystallization for arsenic removal and cyclone electrowinning for final gallium metal extraction. Compared to conventional techniques, this approach significantly improves selectivity, enhances current efficiency during electrowinning, and minimizes waste by enabling the reuse of process solutions and byproducts.
The leaching process was carried out using a sodium hydroxide (NaOH) and hydrogen peroxide (H₂O₂) system, which enabled the selective dissolution of gallium and arsenic while preventing the leaching of aluminum and silicon. The optimized leaching conditions were determined to be an NaOH concentration of 33.3 g/L, an H₂O₂ excess coefficient of 1.29, a rotation speed of 150 rpm, a liquid-to-solid (L/S) ratio of 4:1, and a reaction time of 40 minutes. Under these conditions, the gallium leaching efficiency (LE) reached 98.74%, demonstrating the effectiveness of this alkaline oxidative system. However, despite the high extraction efficiency, the initial gallium concentration in the lixivium was only 8.17 g/L, which was insufficient for direct electrowinning.
To increase gallium concentration, cyclic leaching was performed. Over five cycles, the leaching efficiency remained relatively stable, only slightly decreasing from 98.74% to 96.49%, while gallium concentration in the lixivium progressively increased. After five cycles, the gallium concentration reached 30.65 g/L, making it suitable for downstream electrowinning. The arsenic concentration also increased in parallel, reaching 27.55 g/L, necessitating effective arsenic removal before gallium recovery. Additionally, after three leaching cycles, a noticeable decline in arsenic leaching efficiency was observed due to the limited solubility of As(V), leading to the formation of white arsenic crystals during filtration (Swain et al., 2015).
Since arsenic contamination in the lixivium could interfere with gallium purity during electrowinning and potentially lead to the formation of highly toxic arsine gas (AsH₃) at the cathode, its removal was a critical step. Two methods were explored: neutralization precipitation and cooling crystallization.
Neutralization precipitation was initially considered based on the Pourbaix diagram of the Ga-H₂O and As-H₂O systems, which suggested that gallium should precipitate as Ga(OH)₃ while arsenic remained in solution when the pH was adjusted to neutral (2.8–8.8). However, experimental results showed that as the pH approached 4, both gallium and arsenic precipitated simultaneously, deviating from theoretical expectations. Further analysis using XRD, SEM, and EDS revealed that gallium hydroxide precipitates had a colloidal nature and were capable of adsorbing arsenic ions from the solution. The precipitate contained 28.67 wt% gallium and 22.54 wt% arsenic, proving that neutralization was ineffective for selective arsenic removal.
To overcome this issue, cooling crystallization was employed, leveraging the temperature-dependent solubility of sodium arsenate in alkaline solutions. Research showed that at an NaOH concentration of 1.03 mol/kg, sodium arsenate’s solubility was 1.254 mol/kg at 85°C but dropped to 0.268 mol/kg at 25°C, suggesting that cooling could effectively precipitate arsenic while keeping gallium in solution. In the process, the lixivium was first heated to 60°C to dissolve arsenic fully, preventing premature crystallization during filtration. It was then gradually cooled to 20°C over four hours, leading to the precipitation of white sodium arsenate crystals. XRD, XRF, and SEM analyses confirmed that these crystals were composed primarily of Na₃AsO₄·xH₂O, with minimal gallium contamination. The cooling crystallization process successfully removed most of the arsenic, making the lixivium suitable for gallium electrowinning (Swain et al., 2015).
Electrowinning is a crucial step in recovering high-purity gallium metal from the dearsenicized lixivium. However, gallium’s standard electrode potential is more negative than that of the hydrogen evolution reaction (HER), leading to low current efficiency in traditional flat-plate electrowinning processes. To address this issue, cyclone electrowinning was employed, a technique that enhances mass transfer and reduces concentration polarization through forced convection.
The dearsenicized electrolyte, prepared through pilot experiments, contained 45,050 mg/L gallium and only 2,511 mg/L arsenic, making it highly suitable for electrowinning. Cyclone electrowinning was conducted with an electrolyte volume of 2 L, a current density of 500 A/m², a circulation flow rate of 300 L/h, a temperature of 45°C, and a deposition duration of 12 hours. Titanium sheets were selected as the cathode material due to their high hydrogen evolution overpotential, which minimized side reactions and prevented gallium adhesion.
After electrowinning, gallium metal was deposited in the form of small droplets on the titanium cathode, with some accumulating at the bottom of the electrolytic cell. The deposited gallium was easily detached and underwent sequential acid-washing and water-washing processes to achieve a final purity of 99.993%, as verified by ICP–MS analysis. The process achieved a gallium recovery rate of 90.46% and a current efficiency of 34.43%, representing a substantial improvement over traditional electrowinning methods (Swain et al., 2015).
The newly developed process for gallium recovery from GaAs scraps offers several distinct advantages over conventional methods. The use of alkaline oxidative leaching enables selective gallium and arsenic extraction while preventing the dissolution of aluminum and silicon, reducing impurities in the final product. Cooling crystallization effectively removes arsenic without requiring additional chemical reagents, resulting in a highly pure sodium arsenate byproduct that can be further processed or safely disposed of. Cyclone electrowinning enhances gallium deposition efficiency, lowering energy consumption and improving recovery rates.
Another key sustainability benefit of this process is its resource recycling potential. The post-electrowinning alkaline solution, still rich in NaOH, can be reused in the leaching stage, minimizing chemical waste and gallium loss. Additionally, the primary solid residues from leaching, which consist of ZrSiO₄ and Al₂O₃, can be repurposed for producing AZS (alumina-zirconia-silica) refractory materials, further contributing to waste reduction and resource optimization.
Overall, Swain et al. (2015) present a comprehensive and highly efficient process for the separation and recovery of gallium from GaAs scraps. By integrating alkaline oxidative leaching, cooling crystallization, and cyclone electrowinning, the process achieves high selectivity, minimal environmental impact, and excellent gallium purity. These findings demonstrate the potential for industrial application, offering a scalable and cost-effective method for gallium recovery in the electronics and semiconductor industries.