# A Review on the Recovery and Separation of Gallium and Indium from Waste
## 3 Methods of Recovering Gallium and Indium from Secondary Sources
### Discussion  
The mentioned specificity of e-waste is closely related to the multitude and diversity of uses of gallium and indium, and this complicates the selection of the method. Each has several advantages, but also some disadvantages (Table 4), arising from the fact that not every method is equally suitable for each form of gallium or indium, as well as from the difficulties related to the gradual loss of the reagent, its toxicity, limited access, or price. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Method** & **Advantage** & **Disadvantage** \\ \hline Extraction & high availability of selective extractants & gradual loss of solvent, environmentally unfriendly, time-consuming, ineffective for low metal concentrations \\  & high availability of sorbents, low time consumption, process effective for high and low metal concentrations, possibility of using waste and bio-waste to produce sorbents \\ Precipitation & the simplicity of the process, satisfactory purity of indium satisfactory recovery of gallium, indium and arsenic from GaAs and InAs waste, obtaining a less toxic form of arsenic \\  & & not suitable for gallium recovery \\  & & low recovery efficiency for gallium from GaN waste \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Advantages and disadvantages of gallium and indium recovery methods. The pretreatment stage itself must be adapted to the type of waste that will be processed. A decision must be made whether dismantling will be carried out manually or with a machine and to what extent. It is also necessary to take into account preliminary chemical treatment and calculate whether the reduction in metal losses will compensate for the costs associated with its implementation. The selection of the grinding technique is also important, in terms of the obtained grain size, grinding time, and the accompanying material losses that occur as a result of the formation of the dusty fraction. The HEBM method, which uses the high-energy ball mill and allows for the obtainment of grains with a diameter of up to 1 \(\upmu\)m in just a minute, seems to be a very good solution. However, it requires thorough cleaning of the material. Hammer and ball mills, which can obtain grains with a wide range of diameters, but in a much longer time (e.g., in the case of the ball mill this time is 6 h), do not have such limitations. Moreover, the obtained grain size should be adapted to further processes, because, although a high degree of material fragmentation greatly improves the washing efficiency, it also reduces the efficiency of magnetic, electrostatic, and eddy current separations. The selection of the physical separation method depends on the other components accompanying gallium and indium in the ground material. Magnetic separation is used to separate ferrous metals, light components are separated with air classification, and the eddy current method is used to separate nonferrous metals with a high conductivity to density ratio. The remaining nonferrous metals are removed from the mixture via the electrostatic method or by using permanent magnets. Gravity separation allows for the separation of components that differ significantly in density, and flotation allows for the separation of components that differ in wettability. Knowledge of the physical properties of the material components is therefore necessary when selecting a processing method. Nitric acid is the best choice for leaching gallium from materials containing GaAs, because, even though it is the most expensive of the mineral acids considered, it gives the best results. Using one of the three combinations of conditions presented in Table 1 enables the achievement of a leaching efficiency of up to 99%. Leaching gallium from GaN-containing materials requires more aggressive conditions: a pressure of 15 atm and a temperature of 200 \({}^{\circ}\)C. The alternative is to roast the material with alkaline salts.