Acid leaching of black shale for the extraction of vanadium

The feasibility of vanadium recovery from black shale was investigated using sulfuric acid and hydrofluoric acid solutions. The chemical characterization of black shale from Guizhou, China showed that vanadium mainly existed in free oxide and mica. According to E–pH diagram for V–H2O at 25 °C, the dissolved species of vanadium in aqueous solution mainly present in form of VO2+ and VO2+ at high potential and low pH value. The effects of leaching time, sulfuric acid concentration, hydrofluoric acid concentration, sodium hypochlorite concentration, temperature and liquid to solid ratio on the vanadium leaching were studied. The optimum parameters obtained were a reaction time of 6 h, sulfuric acid concentration of 87.5 g/L, hydrofluoric acid concentration of 15 g/L, sodium hypochlorite concentration of 1 g/L (added after a reaction time of 3 h), leaching temperature of 95 °C and a liquid to solid ratio of 4 mL/g. Under these conditions, up to 86% vanadium was recovered. HF has an important role in the leaching vanadium process. Compared to our previous two-step countercurrent leaching, the vanadium recovery only decreased 4% despite the leaching temperature was decreased from 150 °C for the previous tests to 95 °C for the present work.