## 1 Introduction  
There has been a huge change in production systems and, specifically, in sectors such as energy, Information Technology and Communications (ITC), and transport over recent years. The basis of these changes is the development of the silicon industry, which, by introducing electronic components, has made an impressive breakthrough in all sectors, from process control to computing. These new products are based on the consumption of previously unused material; therefore, new challenges have arisen in the identification, the extraction, and the use of elements that were virtually unknown until now [1, 2]. The most significant case is the development of the group known as rare-earth elements (REEs), which were virtually unknown and underused until a few decades ago. However, currently, rare earths are a basic element for the development of mobile batteries or vehicles [3], computers and electronic equipment [4], energy-efficient lighting [5], fluorescent lamps [6], aerospace and military applications [7], permanent magnets [8], refined fuels, glasses, eyeglasses, lasers [9], and even wind turbine blades [10, 11, 12]. The REEs are sought after in both mature and stable markets, including metallurgy, lighting, glass manufacturing, catalysts, etc., which occupy 41% of global consumption [13, 14, 15], as well as growing emerging markets (alloys for batteries, permanent magnets, etc.) [16, 17, 18, 19]. REEs are present over practically the entire globe. Nevertheless, it is very difficult to find them at levels with a sufficient concentration to be technically and commercially exploitable. The rapid growth of this market has created a demand that far exceeds the supply, which is highly centralized in only a few mines and countries and consequently entails a strategic risk as well as a supply shortage [7; 20; 21; 22; 23]. This risk has already been reflected in serious problems for some of the best-known international companies such as Hitachi, Samsung, and Apple, which have moved their factories to China, the main supplier, causing a bottleneck in the distribution chain. China has limited exports and only allows 39,800 tons, less than 10% of the total. Moving factories to China guarantees the constant supply of rare earths for these companies. Western countries concerned about this strategic dependence have taken steps to try to solve the problem and to at least identify resources that could be exploited if needed. Among these actions is the search for new sources to obtain these elements. Finding new sources of supply that are widely distributed worldwide will reduce the strategic risk and secure the freedom of opportunities in terms of access to such materials. On the other hand, in the most developed countries, a strategy of minimizing the extraction of materials in order to ensure sustainability is proposed. The possibility of using waste to obtain this type of material would be a step forward in the commitment of care to the planet adopted in the summits of Rio and Paris [24; 25] following the concept of the circular economy [26], which pursues two goals--reducing the generation of waste as far as possible and making efficient use of those wastes for which their generation has not been avoided. In this way, the available resources, both material and energy, are optimized so that they remain in the production process for as long as possible [27]. According to the International Union of Pure and Applied Chemistry (IUPAC) [28], the rare earths are a set of 17 elements, 15 of which are transition metals belonging to the lanthanide group (which comprises the chemical elements from lanthanum to lutetium in the periodic table); the two elements left are scandium and yttrium, which have similar physical and chemical characteristics and are usually found together in nature [7; 14; 29]. [PERSON] [15] identified the REEs in two groups:  
1.