# A Review on the Recovery and Separation of Gallium and Indium from Waste
## 3 Methods of Recovering Gallium and Indium from Secondary Sources
### Leading
\[4\text{GaN}_{(g)}+3\text{O}_{2(g)}\to 2\text{Ga}_{2}\text{O}_{3}+2\text{N}_{2}, \tag{1}\]  
\[2\text{Na}_{2}\text{CO}_{3(g)}\to 2\text{Na}_{2}\text{O}+2\text{CO}_{2}, \tag{2}\]  
\[4\text{GaN}_{(g)}+2\text{Na}_{2}\text{O}+3\text{O}_{2(g)}\to 4\text{Na} \text{Ga}\text{O}_{2}+2\text{N}_{2}, \tag{3}\]  
\[4\text{GaN}_{(g)}+2\text{Na}_{2}\text{CO}_{3(g)}+3\text{O}_{2(g)}\to 4\text{ Na}\text{Ga}\text{O}_{2}+2\text{N}_{2}+2\text{CO}_{2}. \tag{4}\]  
After roasting, a washing step is carried out. Studies have shown that the best washing efficiency (73.3%) is obtained while using NaOH and HCl [32]. Nevertheless, one of the developed methods, focusing on GaN contained in LEDs, uses Na\({}_{2}\)CO\({}_{3}\). It is based on a double washing process: prewashing and washing after incandescence. Each time it is done at 100 \({}^{\circ}\)C, using HCl at a concentration of 4 M. After the first washing, the dust is dried and mixed in a 1:1 mass ratio with Na\({}_{2}\)CO\({}_{3}\), then ground in a ball mill all day and dried and heated at 1000 \({}^{\circ}\)C for 4 h. After the second washing, the solid phase is filtered from the mixture. The filtrate is mixed with the one from the first leaching and directed for further processing, while the precipitate is analyzed to check the amount of gallium remaining in it. If the content turns out to be significant, it is returned to the process and, together with a new portion of material, introduced to the ball mill once again [33]. In a study documented in reference [34], CIGS materials were treated through a roasting process, which facilitates a transformation of selenides to oxides and a volatilization of Se. Afterward, a hydrochloric acid solution was utilized to leach the oxides in the experiments. The optimal leaching conditions consisted of an acid concentration of 4 mol/dm\({}^{3}\), a temperature of 80 \({}^{\circ}\)C, a leaching time of 3 h, and an S/L phase ratio of 1 g/10 cm\({}^{3}\). The leaching rates of Cu, In, and Ga were 99.98%, 93.40%, and 96.86%, respectively. One way to transform the elements in spent CIGS into complex selenides is through an oxidative leaching process using nitric acid solution, during which separation of indium can be achieved. After leaching, copper, gallium, and selenium can be completely recovered from the solution by using magnesium oxide as a precipitant. The residue and precipitates can then be heated to 800 \({}^{\circ}\)C for 90 min to separate selenium. The recovery efficiencies of copper, indium, gallium, and selenium are high, at 99.23%, 96.82%, 98.08%, and 96.38%, respectively [35]. In solutions containing SO\({}_{4}\)\({}^{2-}\) ions, gallium may occur in the following forms: GaSO\({}_{4}\)\({}^{+}\) and Ga(SO\({}_{4}\))\({}_{2}\)\({}^{-}\), with the former dominating when the acid concentration in the solution is low. In the hydrochloric acid solutions, gallium can be found in the form of GaCl\({}^{2+}\), GaCl\({}_{2}\)\({}^{+}\), GaCl\({}_{3}\), and GaCl\({}_{4}\)\({}^{-}\), where the last form dominates in the concentrated acid solutions [31]. The spent GZO and IGZO targets were leached with three kinds of inorganic acids. The highest leaching efficiency using H\({}_{2}\)SO\({}_{4}\) only reaches 85%, while the efficiency using HCl can be 99.34%, but it takes 6 h to leach the material. HNO\({}_{3}\), as the leaching agent, has the highest leaching efficiency for a reaction time of 2 h. Under optimal conditions, a leaching efficiency of over 99% can be achieved for indium, gallium, and copper [36]. In the case of indium, most of the annual consumption for several years has been attributed to the production of ITO [4]; therefore, most attention will be given to it. Indium occurs primarily in the form of In\({}_{2}\)O\({}_{3}\), and tin as SnO\({}_{2}\). Because tin dioxide reacts reluctantly with acids, Equation (5), together with Equation (6), will constitute the two leading reactions of the indium leaching process with acid solutions [27].