Extraction of base and precious metals and other valuable elements from mining tailings by hydrometallurgical processes

Abstract

The generation of a great volume of harmful wastes causes many environmental issues, such as AMD (Acid Mine Drainage) generation or metals release, concerning mining and metallurgical activity. Also, several valuable metals are lost in these tailings. However, the reprocessing of these wastes presents several troubles due to the content of pyrite that hinders the process feasibility, nonetheless, the recovery of iron and sulphur from pyrite could increase the incomes of tailings reprocessing. In this work, the bioleaching of mining wastes followed by two precipitation steps and a mineral carbonation step was proposed to recover metals, Fe and S, whilst the main environmental issues are mitigated. Non-ferrous metals were dissolved through bioleaching and brine leaching. Also, bioleaching enabled the dissolution of FeS2 and S0, obtaining an acidic ferric ion liquor with a low metal concentration. After this step, Fe and S were removed through the addition of limestone at pH 3.5 while non-ferrous metals were precipitated later by lime addition at pH 9.5. Cu-Zn-Co hydroxides were used to generate rich liquor by an acidic dissolution enabling the use of conventional purification techniques with a low treatment volume. Mineral carbonation of the Fe/S precipitate was used for the recovery of S as a concentrated sulphate ion solution, while the resulting solid containing Fe(OH)3 and CaCO3 was recirculated to the Fe/S precipitation step as the alkali compound, generating a solid with a higher Fe content after each recirculation step. Some impurities, such as As, P, S, and Cu, were also accumulated during the iron precipitate recirculation. Cu and P were satisfactorily removed through a bioleaching step at low ORP and the modification of basal salts composition, respectively. However, the removal of As and S required further steps, sulphur was easily removed by thermal treatment (pelletising of fine particles). Instead, arsenic must be washed from precipitates by an alkaline washing or precipitated by the promotion of jarosite formation during bioleaching. After these steps, the iron-bearing compound accomplished the impurities limit to be used in the steel-making process. According to a preliminary feasibility assessment, the reprocessing of these tailings required the FeS2 flotation to reach a positive IRR (Internal Rate or Return) and NPV (Net Present Value). The removal of arsenic by alkaline washing or jarosite formation changed substantially the economic results, achieving a greater IRR when the alkaline washing step was selected (10.2 vs. 6.9%).