# A Review on the Recovery and Separation of Gallium and Indium from Waste
## 3 Methods of Recovering Gallium and Indium from Secondary Sources
### Leading
\[\text{In}_{2}\text{O}_{3}+6\text{H}^{+}\to 2\text{In}^{3+}+3\text{H}_{2} \text{O}, \tag{5}\]  
\[\text{SnO}+2\text{H}^{+}\to\text{Sn}^{2+}+\text{H}_{2}\text{O}. \tag{6}\]  
The leaching efficiency is influenced by many factors, such as: grain size, temperature, acid concentration, and S/L ratio. Pulp density is also important, and its increase causes a decrease in the leaching efficiency. Research shows that the optimal temperature for the process ranges from 80 \({}^{\circ}\)C to 90 \({}^{\circ}\)C [21]. Various types of mineral acids and their mixtures are used to leach indium from ITO. The most popular ones are sulfuric acid and hydrochloric acid, although sometimes nitric acid is also used [23; 24; 29; 31; 32; 33; 37]. Much depends on the way the material is previously processed. Hydrochloric acid has many benefits: a high value of the dissociation constant, high reactivity, thanks to which it easily dissolves many different compounds, and its use does not generate problematic waste (wastewater is neutralized with sodium hydroxide, which leads to the production of sodium chloride) [38]. Moreover, it has a lower tendency to leach toxic components of the material, such as arsenic and antimony [23]. However, it is very corrosive and requires equipment resistant to its corrosive properties [37]. The use of hydrochloric acid at a concentration of 3.2 M allows for the obtainment of a leaching efficiency of 90% [23]. It is possible to obtain higher yields with lower concentrations of hydrochloric acid; however, at elevated temperatures a recovery efficiency of \(>\)90% can be achieved with 0.5 M HCl at 80 \({}^{\circ}\)C or 2 M HCl at 55 \({}^{\circ}\)C [39]. According to the research, using sulfuric acid can result in a similar performance as other options, while also being less expensive and less dangerous to the equipment [40]. A study found that a 0.25 M solution of H\({}_{2}\)SO\({}_{4}\) was the most effective leaching agent for relatively fast reactions and high selectivity, with a recovery efficiency of over 99% for indium from ITO layers. At the same time, more than 95% of the tin remained in a solid residue [41]. Nitric acid is the most expensive of these three acids, so its application is reduced as much as possible [23]. Many different mixtures can be used to achieve high efficiencies. They are not limited to just the three acids mentioned above; they may also contain various other compounds, such as hydrogen peroxide. For example, a mixture containing 90% of 5 M HCl and 10% of hydrogen peroxide (by volume), at 75 \({}^{\circ}\)C, at atmospheric pressure, with an S/L phase ratio of 500 g/dm\({}^{3}\) and a contact time of 2 h, allows for the obtainment of a solution in which the indium concentration will be around 76 mg/dm\({}^{3}\)[38]. An oxidizing mixture of H\({}_{2}\)SO\({}_{4}\) and H\({}_{2}\)O\({}_{2}\) can be used to recycle waste containing InP. This process involves using an oxidizing mixture of H\({}_{2}\)SO\({}_{4}\) and H\({}_{2}\)O\({}_{2}\). When the mixture comes into contact with InP, it causes the phosphorus to oxidize to the valence state of +5, forming a stable H\({}_{3}\)PO\({}_{4}\). Meanwhile, indium is dissolved and becomes soluble in the solution. It has been found that under optimized conditions--which consist of an H\({}_{2}\)SO\({}_{4}\) concentration of 3 mol/dm\({}^{3}\), an H\({}_{2}\)O\({}_{2}\)/InP mole ratio of 4, an S/L phase ratio of 1:4, a temperature of 80 \({}^{\circ}\)C, and a reaction time of 3 h--the leaching efficiency for indium can reach over 99% [42]. It is also possible to improve the washing process by using ultrasound, increased pressure, and mixing [38; 43]. The technique of ultrasonic washing has gained importance in previous years [44]. [PERSON] et al. [45] investigated the recovery of In from LCD waste using HCl leaching and ultrasonication without the need to perform the primary crushing of the material. At low HCl concentrations, and without extra heating, a leaching efficiency of 96.80% for In can be achieved. [PERSON] et al.