# 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
#### 3.3.2 Extraction of In  
In the case of indium, many different types of extractants are currently used to extract indium from solutions. One of the most popular is D2 EHPA. It is valued for its high selectivity towards indium in the presence of other ions such as Zn(II), Fe(II), Cd(II), As(III), and Cu(II). In solutions with a low acid concentration, extraction is the most efficient for HNO\({}_{3}\) as the leaching agent, followed by H\({}_{2}\)SO\({}_{4}\), while the lowest efficiency is when the acid was HCl. However, this relationship reverses with an increase in the concentration of the given acid due to the change in the dominant form of indium in the solution [37]. If Fe(III) ions are present in the solution, it is necessary to reduce them to Fe(II) before the extraction process [66]. However, D2 EHPA is not suitable for the selective extraction of indium from a solution that also contains tin, for example from ITO leaching. Tin passes into the extractant together with indium and it is necessary to separate it with tributyl phosphate (TBP). Another effective method to separate indium and tin is to wash indium away from the extractant with an HCl solution. Many studies have also been carried out on the extraction of metals from solutions after leaching of ITO with a mixture of D2 EHPA and TBP, but they usually show greater selectivity towards tin. The developed extraction method enables the separation of tin from the solution using such a mixture or TBP alone, while indium remains in the raffinate [37; 31]. Indium that was dissolved in the bioleaching process was extracted using an organic solvent containing 20% (\(v/v\)) of D2 EHPA in kerosene. To selectively recover indium instead of iron, a stripping step was performed after extraction. The optimal volume ratio of the aqueous to organic phase was found to be 1:1 and the ideal time was determined to be 30 min, resulting in a 91.5% recovery efficiency for indium. The use of a 5 M sulfuric acid solution also proved to be effective in the stripping process. Finally, sodium hydroxide was added to precipitate the indium, resulting in a final precipitate containing 94% (\(w/w\)) of indium [58]. An organic mixture of 15% of D2 EHPA and 85% of kerosene can be used for indium-selective solvent extraction from the In-Ge residue leaching using a sulfuric acid solution. The indium-loaded organic phase can be washed with 0.5 M H\({}_{2}\)SO\({}_{4}\) and then stripped with a 4 M HCl solution to obtain an indium extraction efficiency of 99% and a stripping efficiency of 98.3%. This process can be also used to separate indium from germanium which can be precipitated as a tannin-germanium complex, allowing for the selective recovery (99%) of In from the In-Ge residue leaching solutions [67]. After treating LCDs with UAL and SLM extraction at a 300 W ultrasound power level and 0.75 M sulfuric acid solution, the highest efficiency of indium extraction achieved was about 82%. This was accomplished using a 20% (\(v/v\)) D2 EHPA solution, a 2 M HCl stripping solution, and a feed solution with a pH of 3. The indium recovery efficiency was nearly 80%, and the purity of indium in the final solution was close to 100% [46]. Another valued acid extractant is Cyanex 272, which allows for the quantitative extraction of indium from dilute (maximum 0.1 M) solutions of sulfuric and nitric acids. As their concentration increases, the efficiency decreases, while in the case of hydrochloric acid, it increases above the concentration of 1 M. LIX 973N, which is classified as a chelating extractant, is used for the extraction of indium from solutions containing sulfate, with moderately acidic pH and low ionic strength. Although solvating solvents, such as TPB, can be used for indium extraction, they are not used on a large scale because they give worse results than other available substances [37]. Basic and neutral solvents also give good results if neutral or anionic forms of indium dominate in the solution (e.g., in hydrochloric acid solutions).