Recovery of rare earth, uranium and thorium from industry residue leaching experiments

This paper comprises the study of the recovery of rare earth elements (REE), uranium, and thorium from an industrial residue. REE are important in many areas of materials science, and their application in industry has grown rapidly in recent years. On the other hand, thorium and uranium are elements of great interest in the nuclear area. Thus, the recovery of these elements from secondary sources is important both from the point of view of sustainability and the environmental aspects. The residue used in this work was obtained by the liming treatment of an acid mine water generated in a closed uranium mining site. The main constituents of the residue are REE (4.55%), U (0.20%), Th (0.03%), Ca (15.8%), Al (9.63%), S (6.09%), Fe (4.13%), P (0.59%), and Si (1.92%). Additionally, Mn, Zn and K are present. The leaching experiments were carried out under mechanical agitation, with temperature and final acidity control, using hydrochloric and sulphuric acid as leaching agent. The variables investigated were solids percentage, reaction time, temperature, and the acid/sample ratio. For the conditions evaluated, the recovery of REE was more efficient when hydrochloric acid was used. The dissolution of U and Th was practically the same for both acids. However, due to technical problems, such as solid – liquid separation, purity of the liquor and the fact that HCl is more aggressive than H₂SO₄, sulphuric acid was selected as the leaching agent. Using HCl in an acid/sample ratio of approximately 600 kg t^-1, 10% of solids and 2 hours of mechanical agitation at room temperature, a recovery of 99 % of rare earths, 90 % of Th and 98 % of U was obtained. When sulphuric acid was used with acid/sample ratio of 700 kg t^-1, the recovery of the metals was 93.6 % of rare earths, 81.1 % of thorium and 96.7 % of uranium. After the optimization of the leaching parameters, a sulphuric liquor containing 0.082 gL^-1 Th, 0.35 gL^-1 U, 9.1 gL^-1 REE, 16 gL^-1 Al, 7.8 gL^-1 Fe, 0,47 gL^-1 Ca, 0.75 gL^-1 Zn and 0.1 gL^-1 Si was obtained.