# Europium, Yttrium, and Indium Recovery from Electronic Wastes
## 1 Introduction
There are important uses in the defense industry, such as in fighter aircraft engines, guided missile systems, missile defense units, and satellite communication systems. According to their use, these elements are used as mixtures, in addition to other chemical compounds, or in metallic and alloy forms [13]. The main mineral sources of rare earths are bastnastic [(Ce,La)(CO\({}_{3}\))F], monazite [(Ce,La)PO\({}_{4}\))], and xenotime (YPO\({}_{4}\)). These minerals are submitted to a communution process followed by a gravimetric concentration process [14; 15]. The processed minerals that are enriched in rare earths are leached with acids or alkalis. Generally, hydrochloric (HCI) or sulfuric acid (H\({}_{2}\)SO\({}_{4}\)) is employed at this stage, and according the leaching agent concentration, pulp density, and temperature, rare earths may have selective solubilities that may make their separation from other metals possible [16; 17]. Rare earths that are dissolved after leaching are recovered by several hydrometallurgical techniques such as: solvent extraction, ion exchange, and crystallization. The most used technique is solvent extraction, since extractants such as di-(2-ethylhexyl) phosphoric acid (DEHPA) enable obtaining products of high purity [18]. Re-extraction is accomplished mainly with the employment of acids such as HCl. Several parameters must be optimized, such as the molar ratio between the organic and aqueous phase, extractants concentration, and the contact time between the two phases, among others. In the case of yttrium, it can be recovered from the aqueous phase by precipitation after the addition of oxalic acid or sodium oxalate. Its transformation to an oxide can be achieved by calcination at 750 \({}^{\circ}\)C or, it can be heated to 750 \({}^{\circ}\)C with the addition of hydrofluoric acid in order to obtain yttrium fluoride [19]. Another group of electronic waste that has several elements of commercial interest is liquid crystal displays (LCDs). These devices are used in the manufacture of televisions, laptops, cameras, and mobile phones, among others [20]. The LCD screens have an approximate lifespan of up to eight years, so once their use is over, they must be properly disposed due to the presence of several toxic elements in their structure, such as mercury. Approximately 72% of LCD screens have fluorescent cathode lamps that contain mercury in their structure. One of the elements that has great commercial value and is inside the LCD screens is indium. This element is a semiconductor that has several properties such as electrical conductivity, chemical stability, and high hardness. The indium that is found in the liquid crystal panel has great commercial appeal when it is sold as indium oxide [20; 21]. Indium is commonly found in the veins of minerals that are associated with sulfides and have a high content of tin. Several places have been discovered where indium deposits are found, such as: Kidd Creek, Timmins, ON, Canada (0.027% \(w/w\) indium); Polaris, Nunavut, NT, Canada (0.010% \(w/w\) indium); Balmat, NY, USA (0.004% \(w/w\) indium); and Toyoha, Japan (6% \(w/w\) indium) [22]. There is also a massive sulfur deposit housed in the volcano of the Kidd Creek mine, with an indium content between 1-870 ppm, with an average of 106 ppm that can be exploitable. Tin and tungsten deposits house the highest concentrations of indium [23]. Indium is produced mainly from the waste obtained from the zinc refining and the recycling of chimney dust, slag, and gases generated during the zinc smelting, where the degree of indium is 0.027%. These residues are subjected to a process of leaching through the use of hydrochloric (HCl) or sulfuric acid (H\({}_{2}\)SO\({}_{4}\)). The solutions are concentrated by solvent extraction, and the indium is recovered by electrodeposition as 99% metal. The low grade of indium is then refined to a standard grade metal (99.99%) of higher purity [24].