# A Review on the Recovery and Separation of Gallium and Indium from Waste
## 3 Methods of Recovering Gallium and Indium from Secondary Sources
### Precipitation  
Precipitation is a classic method, used primarily in the separation of indium and tin from solutions after leaching of ITO glass. Increasing the pH of the solution causes indium to precipitate in the form of indium(III) oxide.  
One method involves the addition of an oxidant to the solution (for example, hydrogen peroxide), in the presence of which tin precipitates completely from the solution at a pH of 3.5, while indium remains in the solution. When the pH increases above 4-5, indium(III) oxide then precipitates. Chemical precipitation alone makes it possible to obtain indium with a purity of 99.99%, and if the method is combined with solvent extraction, it is possible to obtain a recovery efficiency of 99.99%.  
Precipitation of indium from solutions containing zinc and iron ions can be carried out at 90\({}^{\circ}\)C using an ammonia solution, at a pH of 6. Metals accompanying indium do not precipitate in such conditions [37].  
The precipitation method can be also used to separate and recover indium from Pb-Ag tailings. Following the leaching of the roasted material in water, Pb, Ag, and most of the Fe remain in the solid residue. The leaching solution can be treated further with precipitation using NaOH, where In and the remaining Fe are precipitated, in comparison to Cu and Zn which remain in the solution. In(OH)\({}_{3}\) and Fe(OH)\({}_{3}\) precipitates can be dissolved further in a diluted H\({}_{2}\)SO\({}_{4}\) solution, and then a cementation of In with Al can be performed. Next, HCl can be used to separate Al from In, after which raw In is obtained [89].  
Separation of Ga and In with the Precipitation Method  
No research papers on the separation of gallium and indium using the precipitation method were found. To explore this possibility, the use of a Pourbaix diagram can be very helpful. This diagram visually displays the ionization equilibrium of various ions in the system and indicates the conditions for their stable existence. Figure 14 shows the Pourbaix diagrams for the Ga-H\({}_{2}\)O system and In-H\({}_{2}\)O system, both with a concentration of 1 mol/dm\({}^{3}\) at 25 \({}^{\circ}\)C and 60 \({}^{\circ}\)C [30]. Figure 14a shows that for the Ga-H\({}_{2}\)O system, as the pH of the system increases, the form of gallium changes from gallium ions to gallium hydroxide precipitate. The dissolution of the Ga(OH)\({}_{3}\) precipitate and the formation of the highly soluble Ga(OH)\({}_{4}\)\({}^{-}\) complex ion can be observed at pH levels around 8 or higher. On the other hand, Figure 14b shows that for the In-H\({}_{2}\)O system, as the pH of the system increases, the form of indium changes from indium ions to indium hydroxide precipitate. Even at a strongly alkaline pH of 14, no dissolution of the In(OH)\({}_{3}\) precipitate is observed. Other references indicate that above a pH of 12, In\({}_{2}\)O\({}_{3}\) is transformed into the InO\({}_{2}\)  
Figure 14: Potential pH equilibrium diagrams for the gallium–water system (**a**) and indium–water system (**b**) reprinted with permission from [30].  
The Pourbaix diagrams indicate that gallium and indium exist as Ga(OH)\({}_{4}\)\({}^{-}\) and In(OH)\({}_{3}\), respectively, in a wide alkaline environment, and their dominant regions expand with the increase in temperature of the system. This implies that by raising the pH and temperature of the system, the precipitation method can potentially be used to separate gallium and indium.