# Optimization of Electric Field Assisted Mining Process Applied to Rare Earths in Soils
## 1 Introduction  
The rare earths elements (REE) form a group composed by the lanthanide series, as well as the elements yttrium and scandium. These species are distributed in the lithosphere, however some regions present higher concentrations of REE, including China, Brazil, Russia, India, and Australia [1; 2; 3; 4]. REE are employed in several applications, such as permanent magnets, catalysts in the fluidized catalytic cracking process, automotive catalysts, ceramic materials, electronic components, lasers, and metallic alloys energy [4; 5; 6; 7]. Additionally, the REE have an important role in the development of new technologies and in the production of clean renewable energy [5; 8; 9]. REE mining mainly occurs through hydrometallurgical processes, from minerals such as bastnaesite, monazite, and xenotime or through ion adsorption clays [10; 11; 12; 13; 14; 15]. However, these mining processes may cause great negative environmental impacts, in addition to the high generation of wastewater and high energy expenditure from the separation and purification steps [16; 17; 18; 19; 20; 21]. Another alternative for obtaining REE is fromsecondary resources, by the hydrometallurgical process of waste electric and electronic equipment [9; 17; 18]. Although this technique is efficient in removing species, the use of strong acids and the generation of non-biodegradable effluents result in an ecologically unfriendly method. China is responsible for more than 95% of the global production of REE. This monopoly condition allowed the country to limit export quotas and raise the price of these commodities. These actions, combined with growth in the consumption of REE, resulted in the increase of the criticality of some REE [19; 20; 21; 22; 23]. Due to the possibility of interruption in the supply of REE and considering the elevated environmental impact of the hydrometallurgical processes, the search for new mining routes for these elements is essential. Thereby, the Electric Field Assisted Mining (EFAM), also known as electromining (EM), is presented as an innovative alternative for REE extraction. The EFAM consists of the removal of the species in ionic form by the action of an electric field [19]. For this purpose, electrodes are inserted in soil that must be moistened with an ionic conductor fluid, the electrolyte [24; 25; 26; 27]. Due to the action of an electric potential gradient, the ions migrate to the opposing charge electrode, allowing the extraction of the elements [28; 29; 30]. This technique is considered promising [19], as it allows the extraction of REE from soils using weak and biodegradable electrolytes. Another advantage is the low water consumption of the process. Furthermore, when applied in-situ, on a larger scale, the technique has advantages over conventional mining, as there are no excavation steps to remove the surface layers of soil. Therefore, the environmental impact for the extraction of REE via EFAM is reduced. The extraction of yttrium ion (Y\({}^{3+}\)) from a spiked sandy soil using the EFAM technique was studied by [PERSON] et al. [19]. Different experimental configurations were performed, evaluating the different electric field strengths and electrolyte concentrations. The authors used a biodegradable electrolyte acetic acid and obtained low values of dissipated electric current in the process. It was observed that the Y\({}^{3+}\) extraction capacity is directly associated to the electric field applied to the system, which is the driving force to the mobility of this species. On the other hand, the increase in the electric field gradient resulted in the increase of the electric current, which contributed to the occurrence of undesired parallel reactions on the surface of the electrodes, such as water hydrolysis. In view of this problem, the aim of this paper is to apply optimization techniques to obtain the best electromining process configuration, presented by [PERSON] et al.