Vanadium extraction from black shale: Enhanced leaching due to fluoride addition

During the direct acid-leaching process of vanadium extraction from black shale, the participation of fluoride ions greatly improves the leaching efficiency of vanadium. Its enhanced effect is influenced by the sulfuric acid concentration, amount of added fluoride, leaching temperature, and leaching time. The migration of fluoride ions during leaching is critical for the dissolution of muscovite and the release of vanadium. In this work, we employed a central composite design of response surface methodology to design the leaching experiment. Results show that the CaF2 dosage is highly significant, and the H2SO4 concentration, CaF2 dosage, and leaching temperature all exhibit interactive effects with each other during acid-leaching. The higher concentration and reactivity of H+ and F+ in solution result in a higher vanadium leaching efficiency for direct acid-leaching. The role of F - in the reaction mechanism was investigated using density functional theory simulation. After the removal of structural oxygen of tetrahedra by H+, F+ closely bonds with the exposed structural aluminum atom and drags it away from the muscovite surface in the form of [ALF(4)](-), F- continues to coordinate with [AlF4](-) to form the stable [AlF5](2-) in solution. The enhanced effect of F- is mainly reflected in its smaller volume and higher electronegativity as compared to those of HSO4- and SO42-.