# A Review on the Recovery and Separation of Gallium and Indium from Waste
## 3 Methods of Recovering Gallium and Indium from Secondary Sources
### Leading  
Leaching is the first stage of hydrometallurgical treatment used in the processing of low-grade raw materials and WEEE. It involves transferring metals into solutions using appropriate chemical reagents. The type of reagent used in a specific case depends strongly on the chosen method of separating metals from the solution; however, its availability and price should also be considered. Additionally, it is necessary to know which gallium and indium compounds dominate in each material, because some of them may react reluctantly with a given solution, which will reduce the efficiency of the leaching process and result in losses. In 2019, in the United States, as much as 81% of gallium was used to produce gallium arsenide, gallium nitride, and gallium phosphide [14]. Gallium arsenide is by far the most in-demand product; in recent years, it accounted for up to 95% of the annual global consumption of gallium [16]. The reagent most often used for leaching gallium from GaAs is nitric acid because it yields the best results. The efficiency of the process depends on the concentration, temperature, solid-to-liquid phase ratio (S/L), and leaching time. With various combinations, it is possible to achieve up to 98-99% leaching efficiency [21; 22; 29]. Leaching is carried out in reactors equipped with a stirrer, usually at normal pressure. After the process is completed, the mixture is filtered, solid residues are separated, and the filtrate containing gallium ions is sent for further processing [29]. The form in which Ga\({}^{3+}\) ions appear in the solution depends on the pH and other components present in the solution. Figure 6 shows the speciation of Ga(III) as affected by pH, calculated using the data generated in [PERSON] et al. 's study [30]. The dominant species of soluble Ga(III) is Ga\({}^{3+}\) at pH \(<\) 3.0. In the pH range of 2-8, the species of cations Ga(OH)\({}^{2+}\) and Ga(OH)\({}_{2}\)\({}^{+}\) and neutral Ga(OH)\({}_{3}\) are present. When the pH is above 8, Ga speciation is strongly dominated by the negatively charged species, Ga (OH)\({}_{4}\)\({}^{-}\). However, the research has shown that in the case of supersaturated and concentrated Ga(NO\({}_{3}\))\({}_{3}\) solutions, the vast majority of gallium ions will occur in the form of ion pairs separated via solvation: Ga(H\({}_{2}\)O)\({}_{6}\)NO\({}_{3}\)\({}^{2+}\). However, a small amount of bound ion pairs can also be found: Ga(H\({}_{2}\)O)\({}_{5}\)ONO\({}_{2}\)\({}^{2+}\)[31]. Figure 6: Speciation of Ga(III) aqueous species as a function of pH at 25 \({}^{\circ}\)C. Reprinted with permission from [30]. GaN is much more chemically resistant than GaAs. At atmospheric pressure, in the temperature range from 25 \({}^{\circ}\)C to 90 \({}^{\circ}\)C, the highest leaching efficiency achieved does not exceed 10%. Of all the tested acids, hydrochloric acid turned out to be the best, reaching 8.9% leaching efficiency at 90 \({}^{\circ}\)C, while sulfuric acid, nitric acid, and hydrofluoric acid did not even exceed 4%. Effective transfer of gallium from GaN into solution can be achieved using a high pressure. An HCl solution is used for washing, with a concentration of 0.25 M. The ratio of the S/L phase is 1 g/30 cm\({}^{3}\), the pressure is 15 atm, and the temperature is 200 \({}^{\circ}\)C. With this method, the achieved leaching efficiency can be around 98% [32]. Other methods have proposed roasting Ga-containing materials with hydroxoides or basic salts such as LiBO\({}_{2}\), NaOH, and Na\({}_{2}\)CO\({}_{3}\), as a prewashing step. The salts are mixed with the ground waste in a mass ratio of 10:1. Roasting is usually carried out for about 8 h at a temperature of about 1000 \({}^{\circ}\)C [32,33]. The salts react with GaN, resulting in compounds that can react more easily with the above-mentioned acids. In addition, a certain amount of the compound also reacts with oxygen. For example, during roasting with Na\({}_{2}\)CO\({}_{3}\), the reactions presented in equations 1, 2, 3, and 4 occur [33].