Comprehensive recovery of Ca, V, Zn, and Si from black shale using a novel hydrochloric acid selective leaching-decarburization process

By traditional sulfuric acid leaching, the comprehensive recovery of valuable elements from black shale is difficult to achieve, and the leaching residue containing large amounts of silicon and calcium, is an environmentally harmful waste. A two-stage hydrochloric acid leaching procedure and a subsequent purification process of the leaching residue were designed to produce a silicon-rich residue. Hydrochloric acid selectively eliminates acid-consuming minerals like calcium carbonate and pyrite in the first leaching stage, while the metals vanadium and zinc were leached in amounts of only 0.15 % and 9.82 %, respectively. Mica was severely degraded in the second leaching stage due to the combined effects of sodium fluoride and hydrochloric acid, with leaching percentages of 90.3 % for vanadium and 63.2 % for zinc, while only 25.6 % of the hazardous element iron was leached. After roasting the leaching residue of the second stage for 2 h at 750 °C to remove carbon, silicon-rich residue with a silica purity of 95.5 % was obtained. The proposed method is expected to achieve waste-free utilization of black shale.