# A Review on the Recovery and Separation of Gallium and Indium from Waste
## 3 Methods of Recovering Gallium and Indium from Secondary Sources
### Leading
[46] also used ultrasound in the process of indium leaching with H\({}_{2}\)SO\({}_{4}\) from the end-of-life LCDs, obtaining an In leaching efficiency of 92% after 20 min, which was higher than the leaching efficiency of 70% without the ultrasound. An additional improvement is the use of UAL and supported liquid membrane (SLM) extraction. The leaching efficiency and purity of indium were approximately 97% and 94%, respectively, when using a 0.75 M sulfuric acid solution and 300 W ultrasound power in the experiments [47]. Furthermore, the leaching efficiency for In from the secondary sources is improved when sonication is combined with microwaves. [PERSON] et al. [48] explored the recovery of In from waste LCDs using different inorganic acids (HCl, H\({}_{3}\)PO\({}_{4}\), and HClO\({}_{4}\)) in a new microwave-ultrasound heating system. It was found that HCl was the most effective leaching agent and the leaching efficiency for In reached 100%. The form of In\({}^{3+}\) ions in the solution also depends on its pH and other components present in it. Figure 7a shows the speciation of In(III) as affected by pH calculated using the data generated in [PERSON]'s doctoral thesis [49]. The dominant species of soluble In(III) is In\({}^{3+}\) at a pH < 4.0. In the pH range of 2-7, the species of cations In(OH)\({}^{2+}\) and In(OH)\({}_{2}\)\({}^{+}\) exist while neutral In(OH)\({}_{3}\) is present in the pH range of 3.5-11.5. When the pH is above 10, In speciation is strongly dominated by the negatively charged species, In(OH)\({}_{4}\)\({}^{-}\). In hydrochloric acid solutions, they take the form of [InCl\({}_{n}\)(H\({}_{2}\)O\({}_{6-n}\))\({}^{3-n}\), where n depends on the concentration of the solution and can have a value from 0 to 6. In the diluted solutions, the form with n = 2 dominates, simplifying InCl\({}_{2}\)\({}^{+}\)--see Figure 7b [50]. In the solutions where the concentration of the acid is from 1 M to 12 M, the forms with n = 3 and n = 4 (InCl\({}_{3}\) and InCl\({}_{4}\)\({}^{-}\), respectively) are the most apparent. In the sulfuric acid solutions, indium ions are mainly found in the forms of InSO\({}_{4}\)\({}^{+}\), In(SO\({}_{4}\))\({}_{2}\)\({}^{-}\), and In(SO\({}_{4}\))\({}_{3}\)\({}^{3}\) for low, medium, and high acid concentration, respectively (Figure 7c) [50]. In the dilute solutions of nitric acid, the InNO\({}_{3}\)\({}^{2+}\) form dominates, while the In(NO\({}_{3}\))\({}_{2}\)\({}^{+}\) form is more common in the solutions with acid concentrations ranging from 1 M to 3 M [31]. The recovery of gallium and indium present together in white LED bulbs, also containing significant quantities of Cu, Sn, Pb, and Al, was successfully carried out using nitric acid. Using a 4 M solution of HNO\({}_{3}\) at 80 \({}^{\circ}\)C with a L/S ratio of 10 caused Cu, Fe, Pb, and Ag to be washed from the waste, while Al passivated and Sn precipitated. After filtration, the LED beads, Al scrap, and Sn(OH)\({}_{4}\) precipitate can be separated via three-stage sieving. The LED balls separated on a 2 mm sieve were then leached with HNO\({}_{3}\) and ground in a mortar to an average grain size of 250-750 \(\upmu\)m. Roasting was then carried out to convert GaN and InN into their leachable forms, according to Reaction (1) for GaN and to Reaction (7) for InN:  
\[4\text{InN}_{(\text{s})}+3\text{O}_{2(\text{g})}\to 2\text{In}_{2}\text{O}_{3}+ 2\text{N}_{2} \tag{7}\]  
Application of an alkaline fusion with NaOH or roasting with Na\({}_{2}\)CO\({}_{3}\) yielded recovery efficiencies above 90%. Then, the leaching of the roasted material with a 6 M solution of NaOH (L/S ratio of 1) at 95 \({}^{\circ}\)C for 3 h can efficiently dissolve Ga, while In is dissolved in the next step with the more concentrated NaOH (10-11 M) [51]. A comparison of the most favorable conditions for the leaching of indium from e-waste is presented in Table 1.  
Gallium-based liquid metal (G-LM) waste was leached with a 3 M solution of HNO\({}_{3}\) for 2 h at 40 \({}^{\circ}\)C and an S/L ratio of 1 g/20 cm\({}^{3}\).