# A Review on the Recovery and Separation of Gallium and Indium from Waste
## 2 Primary and Secondary Sources of Gallium and Indium  
In obtaining gallium from its natural sources, the Bayer process currently has the largest importance. The main purpose of this process is to produce metallic aluminum from bauxite, while gallium is one of the by-products, forming an insoluble precipitate called \"red mud\". It is separated from the mixture by subjecting the sludge to two electrolysis processes. The first one, using a mercury cathode, aims to concentrate the mixture. During the second one, using a stainless-steel cathode, gallium is obtained. It is most often subjected to further processing and purification, including the use of high temperatures, acids, and oxygen [17]. The second primary source is the process of hydrometallurgical treatment of zinc ores, in which gallium, along with several other elements, is again only an additional benefit and not the main product. It is also obtained through the electrolysis of the process residues, which is preceded by a series of steps aimed at removing or masking the elements present in the solution (cadmium, copper, iron, aluminum, germanium, and others). Depending on the method used, these stages may include, among others: leaching, precipitation, filtration, acidification, ion exchange, or extraction [18; 19; 20]. On a smaller scale, gallium is also obtained from coal ashes [7]. Secondary sources of gallium consist primarily of electronics: all kinds of damaged and used equipment, such as smartphones, TV sets, microwave ovens, computers, and laptops, as well as thermometers, diodes, and plates. As can be seen in Figure 3, approximately 54 megatons of WEEE are produced annually. Asia (24.9 megatons and 5.6 kg per person), the Americas (13.1 megatons and 13.3 kg per person), and Europe (12 megatons and 16.2 kg per person) lead the way in waste production. It is worth noting that in Europe about 42.5% of this waste is appropriately recycled, in Asia 11.7%, and in both of the Americas, only 9.4% [3]. It is estimated that in recent years \"large equipment\" used all over the world, i.e., mainly household appliances, accounted for approximately 24.5% of WEEE--see Figure 4. Another 32.4% was \"small equipment\", which included both small household appliances (e.g., vacuum cleaners and fans) and electronics not included in the IT and telecommunication category (e.g., cameras). Waste from the IT and communication category, the share of which amounted to 8.7%, was mainly smartphones, computer parts, and devices equipped with transmitters (usually of the newer generation). Among them, pen drives, CDs, and portable memory devices could also be found. Lamps and light bulbs accounted for about 1.7%, while screens and monitors accounted for 12.5%. A category generally called \"heat exchangers\", which includes, among others, thermometers, air conditioners, refrigerators, and freezers, consisted of 20.2%. In all categories, except for \"screens and monitors\", there was an increase in percentage of content compared to the previous year. The number of used screens and monitors decreased by 1%, but only seemingly; the number of components continues to grow, but the new screens are getting lighter [3]. About 95% of indium manufactured from natural sources is a byproduct of obtaining zinc from minerals containing it (mainly sulfides). The remaining 5% comes mainly from the processes of obtaining copper and tin, but the production of other base metals, such as iron or lead, also has a small part [17]. Indium is obtained from zinc smelting residues, dusts, slags, and exhaust fumes, in refineries specially adapted for this purpose, which can operate independently or be  
Figure 4: E-waste categories and their percentage contribution (% (m/m)). Figure 3: Global production of e-waste by continents: total generated e-waste in Mt (**a**) and kg of e-waste generated per capita (**b**). E-waste collected and properly recycled in brackets. located on the premises of smelters.