# A Review on the Recovery and Separation of Gallium and Indium from Waste
## 3 Methods of Recovering Gallium and Indium from Secondary Sources
### Solvent Extraction
#### 3.3.3 Separation of Ga and In  
In many cases, gallium and indium occur together in the solutions. They can be isolated in two ways: by using one common extractant or two extractants selective for ions of one of these metals. In the first case, the extract obtained contains both gallium and indium, and their actual separation occurs because two different solutions are needed for their re-extraction, differing, for example, in concentration. The second method involves carrying out two extractions: during the first one, the solvent used will be selective towards one of the metals, and during the next one the solvent used will be selective towards the other. Of course, it is also necessary to consider other ions present in the solution.  
Extractants used for the separation of gallium and indium can be acidic, basic, or neutral. There is a wide range of them available on the market, and the choice depends on the individual case [31]. Some examples will be presented.  
The use of Cynaev 272 allows for the isolation of gallium and indium from a solution of hydrochloric acid. As described earlier, the efficiency of the process decreases when the acid concentration exceeds 0.1 M, and with concentrations of 1 M or more, it starts to increase again. A dilute acid solution is used to wash out gallium, while the quantitative washing of indium requires the use of a concentrated solution [68].  
Another example may be the use of D2 EHPA for the extraction of gallium and indium from solutions containing significant amounts of Zn(II). Depending on the solution pH at which the process is conducted, it is possible to isolate both at once or separately. If the pH is 2.2, the extract will contain both gallium, indium, and a small amount of zinc. They are then separated using appropriate solutions. Extraction at a pH of 1.48 ensures selectivity towards indium, while extraction of the raffinate at a pH of 1.99 allows for selective isolation of gallium. Studies have shown that the re-extraction of these metals from the extract is more effective using the H\({}_{2}\)SO\({}_{4}\) solution compared to the HCl solution [69].  
Valuable metals can be effectively extracted from nitric acid solutions after leaching of CIGS solar panels through a sequential extraction process (Figure 8). The first step involves extracting In using di-(2-ethylhexyl) phosphoric acid, while Cu and Ga remain in the aqueous phase. After extracting In, Ga is extracted using the same extracting agent but under different conditions. The residual aqueous solution contains nearly pure Cu. To form metal hydroxide precipitates, ammonium hydroxide is added to the three solutions. Under optimal conditions, a recovery efficiency of over 90% can be achieved for In, Ga, and Cu. Additionally, the formed hydroxides can be recycled and converted into metal oxides, with a purity of over 99%, through calcination [36].  
Another, perhaps slightly unusual, but certainly worth mentioning, possible extraction method is an extraction using scCO\({}_{2}\)--carbon dioxide under supercritical conditions. Furthermore, the addition of a chelating agent is also necessary. This method was used to recover indium from a solution after leaching of ITO and was able to reach over 90.8% extraction efficiency (in relation to the indium content in the ground material). The optimal conditions established during the testing of this method were as follows: the chelating agent was HFOD and the extraction was carried out at 80 \({}^{\circ}\)C, under a pressure of 20.7 MPa, and for 30 min. The efficiency of indium recovery from the solution was 93.3% [60].  
Solvent extractants yielding high extraction efficiencies and separation of metals are listed in Table 3.  
Solvent extractants and optimal conditions for the extraction of gallium and indium from leaching solutions.  
Figure 8: An exemplary process of the recovery of gallium and indium via extraction.  
\begin{table}
\begin{tabular}{c} **Stripping** \\ **Stripping** \\ **Efficiency**