Mechanism of vanadium extraction from stone coal via hydrating and hardening of anhydrous calcium sulfate

In order to increase the muscovite/solution active regions in black shale during pressure acid leaching process, a novel reinforcing method is proposed: the hydrating and hardening of anhydrous calcium sulfate (CSA). By adopting this method, the vanadium extraction reached 95.45% after 5 h at a temperature of 150 °C with a sulfuric acid concentration of 20 vol% and potassium sulfate addition of 7 %wt. The addition of potassium sulfate accelerated the hydration of CSA to converts into calcium sulfate dihydrate (CSD). Under hydrothermal conditions, the CSD dissolved and crystallized to transform back into CSA. The transformation of CSD → CSA hardened CSA crystals and caused local stress and generated expension and cracking to muscovite particles. This produced more muscovite/solution active regions to make aluminum and vanadium ions exposed to the attack of hydrogen ions, prompting vanadium release from muscovite lattice.