Acid Leaching of Uranium from Weathered Uraniferous Black Shale

Abstract

This paper describes the chemical leaching of uranium from weathered black shale by using sulfuric acid(H2SO4) and acidic ferric sulfate (Fe2 (SO4)3 solutions as lixiviants. The black shale sample was radioactive in nature dueto the presence of uranium and its decay radio-isotope products like 226Ra, 214Pb and 214Bi. The γ-activity of natural 235Uwas 97.6 ± 1.34 Bq Kg-1. The shale sample showed 48±1% porosity with bulk density of 1.38 ± 0.015 g/ cm3. The mainminerals identified in the weathered shale sample were quartz, illite, microcline (K-feldspar), anorthite (Ca-feldspar),gypsum and hematite. A mixed layer illite-smectite phase at 10.8Å was also detected indicating the weathering of clayminerals present in the shale sample. The shale sample was polymetallic in nature and contained 0.005% U3O8, 0.24%V2O5, 0.038% NiO, 0.012% CuO, 0.037% CeO2 and 0.019% ZnO on dry matter basis. The shale sample was organiccarbon rich and contained 19.60% C, 1.25% N and 2.50% H, which indicated the presence of nitrogenous hydrocarboncompound(s). Uranium was present in the tetravalent oxidation state (U4+) in the shale matrix. A series of leachingexperiments were conducted in shake flasks and percolation column for uranium dissolution from black shale with andwithout acidic ferric sulfate solutions of pH 1.5 and pH 2.0 at 30±2 °C. Uranium leaching efficiency was mainly attributedto the concentrations of H2SO4 and Fe3+ ions in the leaching solutions. Uranium dissolution from the weathered blackshale mainly attributed to redox reaction of U4+ to U6+ with Fe3+ as an oxidant in acidic leaching environment.