# Europium, Yttrium, and Indium Recovery from Electronic Wastes
## 1 Introduction  
During the last decades, there has been a vertiginous development of the technological industry, which has caused a significant increase in electronic waste. Electric and electronic wastes, which are also called waste electrical or electronic equipment (WEEE), is a general term that covers any device that has ceased to be useful [1]. The electronic waste that is generated due to the continuous advance of the technology is stored in deposits as scrap. However, many of these WEEE have high value-added metals in their structure in concentrations that are much higher than those within minerals [2]. Globally, WEEE is the waste that has shown the most growth in recent decades. WEEE constitutes approximately 8% of total urban waste [3]. It is estimated that 14 kg of WEEE per year are generated per person, and this tendency grows exponentially each year [4]. Despite this trend, there are no regulations or provisions for the proper disposal of WEEE in undeveloped countries due to economic constraints. In addition, it must be taken into account that the number of electronic devices and the recycling rate is lower than in developed countries. In the case of Europe, it is known that the amount of WEEE is approximately 7.5 million tons, and the annual growth of this trend ranges between 3-5% [5]. Another aspect to take into account is the negative environmental impact caused by WEEE if they are not properly disposed. When electronic devices end their useful life, they are destined for landfills or incinerators. However, it must be taken into account that WEEE has toxic metals such aslead, cadmium, or mercury, which are harmful to the environment and the health of living beings [6]. It should be noted that one of the main attractions of electronic waste is the high concentration of precious metals, base metals, and rare earths, which makes them potential sources of these elements. For instance, it is common to find 41% and 22% of iron and copper respectively within WEEE, since they are the major elements in electronic wastes. Precious metals and rare earths are in ppm concentrations instead [7; 8]. Much electronic waste has precious metals or rare earths in concentrations that make it possible to extract them. One of them is cathode ray tubes (CRTs), which present a powder composed of zinc sulfide and yttrium oxysulfide activated with europium. The CRTs are found on computer screens and televisions that are covered by layers of barium oxide and lead that protect users from the exposure of X-rays generated within the CRTs. It should be noted that the glasses that are part of the CRTs have up to 28% lead, which becomes a serious environmental problem when the CRTs are leached in the landfills where they are stored [6]. However, due to the presence of yttrium and europium in its structure, the CRTs are highly appreciated, since rare earths have a high commercial value [9]. The elements of rare earths are a group composed of 17 chemical elements, which includes the 15 metals corresponding to the series of lanthanides in addition to yttrium and scandium. All of these elements present similarities in their chemical, electrical, magnetic, and optical properties [10]. This similarity is mainly due to the electronic configuration of the rare earth elements. In the atoms of these elements, f orbitals are partially filled, and the electrons are added to sublayer 4f, which is surrounded by 6s sublayers. Since the last layer is completely filled, in the case of all of the elements, the electrons of sublayer 4f are so well protected that the chemical properties remain practically unchanged. In the case of lanthanum, there are no electrons in the f orbitals, but the 5d sublayer is the one that is protected by the filled external sublayers [11; 12]. Due to their exceptional physical, optical, electrical, and magnetic properties, these elements are used in a wide range of applications, from metal alloys, flat screens, hard drives, and oil-refining catalysts to medical equipment.