Investigação do processo de passivação da superfície de arsenopirita (FeAsS) - uma abordagem teórica

Abstract

The mineral extraction sector is one of the main sources of economic development in Brazil, and the state of Minas Gerais is highly dependent on this industry. The main ores mined are iron and gold; the second is usually found associated with sulfide minerals that end up as tailings. The process known as acid mine drainage (AMD) occurs by the oxidation and dissolution of these sulfide minerals that are exposed to a humid and oxidizing medium, leading to an important environmental impact caused by the potential for soil and water contamination. In this dissertation, a method for mitigating AMD, the carrier micro-encapsulation (CME), applied to arsenopyrite (FeAsS), a sulfide mineral that, when oxidized, also leads to the dissolution of toxic arsenic species. Using DFT methodologies with plane waves and localized bases, the calculations were performed to obtain structural and electronic properties of arsenopyrite, both in bulk and on the surface (001), and of the passivating agents of the CME method. From the characterizations carried out for arsenopyrite, structural parameters were obtained, in addition to evidence regarding its semiconductor profile, with a significant contribution from the iron atom both as receptor and electron donor. The passivating agent studied was the [Al(cat)(OH)] complex, in which (cat) represents catechol, a compound sensitive to redox interactions, which decomposes selectively on mineral sulfides. Structural and electronic analyzes of these systems were carried out. The CME method had its mechanism studied, with the adsorption of the complex on the surface (001) of arsenopyrite previously oxidized by the action of water and oxygen molecules, which dissociate to form hydroxyl groups. The main steps analyzed were the adsorption of the complex, its oxidative decomposition, accompanied by the exit of the catecholate ligand and the formation of aluminum oxy-hydroxide species which has the function of passivating the arsenopyrite, avoiding AMD. The occurrence of each stage was evidenced by the results of the electron localization function (ELF), by the difference in charge density - showing where electronic densities shared by atoms in the system increase and decrease -, and by integration of the projected density of states on the atoms (PDOS).