# A Review on the Recovery and Separation of Gallium and Indium from Waste
## 3 Methods of Recovering Gallium and Indium from Secondary Sources
### Discussion
In both cases, hydrochloric acid is used and the choice between them requires calculating whether the process using a furnace and a ball mill operating for many hours or the process requiring the high-pressure equipment will be more expensive. Hydrochloric acid, sulfuric acid, and nitric acid, as well as their mixtures or other mineral acids, can be used to leach indium from ITO. Due to having the lowest cost and lower risk to equipment, the suggested acid is sulfuric acid. Hydrochloric acid is also used because it does not generate problematic waste and is less prone to leaching toxic components from the material. Due to the high price, it is recommended to reduce the consumption of nitric acid. Choosing a method for separating gallium and indium from the post-leaching solution requires considering what other components are present in it. Of course, issues such as price and availability of reagents are also important. Extraction is one of the most popular methods, and there are solvents available on the market with a high affinity for gallium and indium and a low affinity for the accompanying metals such as copper, zinc, or iron. When choosing the specific extractant, the acid used in the earlier stage and its concentration should be taken into account, because they may not have the same affinity for each of the forms in which gallium or indium may occur. The serious disadvantage of this method is the gradual loss of the solvent, part of which leaves the system together with the extract and raffinate. This not only generates losses related to the need to refill the reagent but very often poses a threat to the environment. Moreover, extraction is time-consuming due to the need to keep the appropriate contact time between the extractant and the raw solution and then to separate them in a centrifuge or settler. There is also a level of minimum concentration of metal that must be present in the solution (usually several hundred ppm) for it to be carried out successfully. Sorption methods solve the problem of solvent loss, as long as they are not impregnated with it. Many substances can be used as sorbents, liquids, gels, and solids, both synthetic and natural. Commercial sorbents (silica gels, chelating resins, and others) can be additionally modified to increase their affinity towards gallium and indium. Particularly promising sorption processes are those using living organisms or their fragments. Sorption methods are much less time-consuming than extraction. There is also no minimum concentration below which the sorption process is ineffective. As in the extraction method, the selection of the sorbent depends on the accompanying metals and the acid used in the leaching. It is also worth considering the type of regenerating agent to minimize the cost of the process. The precipitation method is recommended only for indium recovery. The advantage of precipitation is undoubtedly the simplicity and the degree of purity of indium obtained in this way, which can reach 99.99%. This method is mainly intended for the recovery of indium from ITO and combining it with extraction, which allows for a further increase in the purity of indium to 99.999%, seems to be a very optimal solution. The use of tribromide ionic liquids for the recovery of gallium, indium, and arsenic from waste containing GaAs and InAs allows for the quantitative isolation of all three elements. In addition to the high efficiencies of the element recovery (which are 95% or more for all), the additional advantage of this method is the fact that arsenic is obtained in the form of H\({}_{2}\)AsO\({}_{4}\). This form is much less toxic than arsenae, which is usually obtained through other recovery methods. However, so far it has not been possible to isolate gallium from the components containing GaN. The choice of method for separating the solid component (metal or metal oxide) depends largely on the cost of electricity in a given geographical location.