Comparison of direct acid leaching process and blank roasting acid leaching process in extracting vanadium from stone coal

• We compare direct acid leaching process and blank roasting acid leaching process.
• Vanadium leaching mechanism is similar in direct and blank roasting acid leaching.
• Forms of vanadium and iron have key effects in choosing vanadium leaching process.
• Blank roasting increased vanadium (bound to free oxides) and facilitated leaching.
• Blank roasting transformed pyrite into hematite and hindered leaching.

Comparison of direct acid leaching process and blank roasting acid leaching process in recovering vanadium from a certain stone coal was carried out. The vanadium leaching efficiency in direct acid leaching process was 92.39% while in blank roasting acid leaching process was 85.43% under the comparative leaching condition of 15% (v/v) H2SO4, 1 mL/g, 4 h, 95 ± 1 °C and 5% (w/w) CaF2. In essence, the mechanism of vanadium leaching was similar in direct acid leaching process and blank roasting acid leaching process. First, calcium fluoride reacted with sulfuric acid to generate hydrofluoric acid. Second, the hydrofluoric acid thoroughly broke down silicoaluminate minerals due to the generation of [SiF6]2 − and [AlF5]2 − . However, the difference was that blank roasting contributed to changes in the minerals. On the one hand, blank roasting benefited the vanadium leaching because it transformed vanadium (bound to organics) into vanadium (bound to free oxides) and broke vanadium-bearing silicoaluminate minerals down. On the other hand, it also transformed pyrite into acid-consuming hematite, which increased the consumption of hydrofluoric acid. It was the generation of hematite covering the benefits from blank roasting that decreased the leaching efficiency of vanadium. It was reasonable for this stone coal where vanadium and iron mainly existed in forms of silicoaluminate and pyrite respectively to adopt direct acid leaching process with calcium fluoride.