Water–Rock reactions in the acid leaching of Uranium: Hydrochemical characteristics and reaction mechanisms

Abstract

Acid in–situ leaching (AISL) mining is used to recover uranium from sandstone using sulphuric acid. However, the evolution of groundwater hydrochemistry, the water–rock reaction mechanisms, and their relationship remain elusive. To effectively address these issues, ore samples were obtained from a sandstone uranium deposit in Inner Mongolia in this work. Soaking tests at different sulphuric acid concentrations were performed to quantify the mining aquifer’s water–rock reaction processes (sulphuric acid and minerals) according to the hydrogeochemical composition analysis and inverse modelling. From our results, it was demonstrated that the water–rock interactions between the sulphuric acid and gangue minerals primarily affect the leachate’s chemical composition. The introduction of sulphuric acid efficiently facilitates the gangue mineral dissolution and the groundwater total dissolved solids (TDS) increases and enhances the formation of uranyl sulphate (predominantly UO2SO4 and UO2(SO4)22– and UO22+, thus stimulating uranium mobility. Mineral precipitation (mirabilite) is primarily responsible for the elevated TDS in the leachate. The leachate’s oxygenation mainly depends on the sulphuric acid concentration and Fe3+ concentration produced by hematite dissolution. During the AISL process, the dissolution of dolomite, calcite, FeO, hematite, pyrite, K–feldspar, anorthite, chlorite, and alunite take place in the groundwater system. In parallel, the precipitation of kaolinite, illite, Ca–Montmorillonite and mirabilite, accompanied by Na–K exchange and Na–Mg exchange occur. The dissolution of dolomite, calcite and illite and Na–Mg exchange mostly affects the hydrochemical evolution, as well as the leachate of Fe mainly from dissolved chlorite. The predominant precipitates were kaolinite and mirabilite, with their precipitation ability increasing with acidity. These findings highlight the importance of clarifying the relationship between the hydrochemical characteristics and the water–rock reaction mechanisms in AISL geochemical processes to elucidate the hydrochemical evolution and mineral corrosion characteristics of ore–bearing aquifers. Finally, K–feldspar, alunite, chlorite, anorthite and dolomite were obtained as the major acid–consuming minerals. Our work provides a novel evaluation method for the prediction of acid consumption that can be used for actual production.