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 due
to the presence of uranium and its decay radio-isotope products like 226Ra, 214Pb and 214Bi. The γ-activity of natural 235U
was 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 main
minerals 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 clay
minerals 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 organic
carbon rich and contained 19.60% C, 1.25% N and 2.50% H, which indicated the presence of nitrogenous hydrocarbon
compound(s). Uranium was present in the tetravalent oxidation state (U4+) in the shale matrix. A series of leaching
experiments were conducted in shake flasks and percolation column for uranium dissolution from black shale with and
without acidic ferric sulfate solutions of pH 1.5 and pH 2.0 at 30±2 °C. Uranium leaching efficiency was mainly attributed
to the concentrations of H2SO4 and Fe3+ ions in the leaching solutions. Uranium dissolution from the weathered black
shale mainly attributed to redox reaction of U4+ to U6+ with Fe3+ as an oxidant in acidic leaching environment.