# A Review on the Recovery and Separation of Gallium and Indium from Waste
## 3 Methods of Recovering Gallium and Indium from Secondary Sources
### Ionic Liquid Use  
With the rising trend towards green technologies, recent years have seen an increase in the use of ionic liquids (ILs) in the extraction of important metals from aqueous solutions. Ionic liquids (ILs) based on phosphonium cations can be promising extractants for metals, including gallium and indium. The use of tributydlecylphosphonium tribromide ionic liquids [P\({}_{44410}\)][Br\({}_{3}\)] for the recovery of gallium, indium, and arsenic from waste containing GaAs and InAs turns out to be very effective. The process consists of two stages: leaching of metals from semiconductor materials using ILs, and their selective recovery from the ILs solutions using appropriate regenerating substances. In addition to the high efficiency of 95% or more for all three metals, the great advantage of this method is the fact that arsenic is obtained in the form of H\({}_{2}\)AsO\({}_{4}\). It is much less toxic than the AsH\({}_{3}\) form usually obtained in other recovery methods. In the leaching process, the molar ratio of GaAs, InAs, and ionic liquid [P\({}_{44410}\)][Br\({}_{3}\)] is 1:1:20, respectively. As a result of the ionic liquid regeneration process, over 99% of metals can be recovered. To remove arsenic from the ionic liquid, a 4 M NaBr solution can be used, which allows for the obtainment of H\({}_{2}\)AsO\({}_{4}\) in a three-stage process. Gallium can be removed by washing the ionic liquid solution three times with distilled water and then indium can be precipitated in the form of indium(III) hydroxide by contacting the ionic liquid with a sodium hydroxide solution. The ionic liquid can be regenerated with the addition of molecular bromine and then reused. Figure 15 shows a schematic diagram of the process along with the exact concentrations of the separated metals in the solutions obtained after the specific steps [55]. In another study, acid-base bifunctional ionic liquids (Bif-ILs) in both solvent extraction and supported liquid membrane (SLM) processes were examined to separate and recover gallium from aqueous solutions. The SLM containing Bif-ILs demonstrated a highly selective transfer of 96.2% of Ga(III) from the feed into the stripping solution, containing Al(III), Mg(II), Cu(II), and Fe(II). After five cycles, the process efficiency showed only a slight decrease to 88.1% [92]. Figure 15: Schematic diagram of the recovery of gallium, indium, and arsenic using tribromide ionic liquids. Indium recovery from discarded LCD screens is possible using Cyphos IL 101 diluted in kerosene as the organic phase. The maximum efficiency of indium recovery was 99.7% at optimum conditions which were as follows: 4 M aqueous phase acidity, 197.79 ppm of indium, 0.009 M ionic liquid, and 1.58 mol/dm3 of aqueous to organic phase ratio [93]. In a solution in which the HCl concentration ranges from 1 M to 3 M and the accompanying ions are Al(III) and Ni(II), complete selectivity towards gallium is ensured by the use of trihexyl(tetradecyl)phosphonic chloride (trade name Cyphos IL 101). However, if Cu(II), Zn(II), or Fe(III) ions are also present in the same solution, it would not be possible to separate gallium from them with good efficiency. As with previous cases, this extractant can be regenerated using an HCl solution [65]. To solve the difficulty in the traditional solvent extraction process of stripping Ga(III) from the loaded organic phase, a low acidity aqueous solution with a pH of 2 can be introduced as the stripping agent of the Aliquat 336-based supported liquid membrane (SLM). This method allows for the selective separation of low-content Ga(III) ions from mixed solutions of concentrated hydrochloric acid that contain other metal ions like Fe(II), Al(III), Mg(II), and Cu(II) under optimal process conditions [94]. Some ILs were synthesized in a reaction of Aliquat 336 with organophosphorus acids (Cyanex 272, D2 EHPA, and PC88A), resulting in the formation of ALi-CY, ALi-PC, and ALi-D2.