A mechanism of calcium fluoride-enhanced vanadium leaching from stone coal

•Presentation of the whole process of vanadium leaching from stone coal•Essence of the mechanism of CaF2 in enhancing vanadium leaching•Fluorine existed in acid leachate in the forms of [AlF5]2 − and [SiF6]2 −.•Introducing CaF2 decreased the ∆G° of vanadium-bearing phlogopite disintegration.•CaF2 declined the effect of the control of chemical reaction on vanadium leaching.

A comparison between unassisted and calcium fluoride-enhanced leaching demonstrated that calcium fluoride can markedly boost the efficiency and accelerate the rate of vanadium leaching from stone coal. Analysis methods were adopted to identify the mechanism of calcium fluoride-enhanced vanadium leaching from stone coal, including Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES), valence state of vanadium, X-ray Diffractometry (XRD), X-ray Photoelectron Spectroscopy (XPS), 19F liquid Nuclear Magnetic Resonance (NMR), Scanning Electron Microscope (SEM) and thermodynamics. The whole calcium fluoride-enhanced vanadium leaching process was that calcium fluoride reacted with sulfuric acid and then generated HF(aq); calcite was dissolved into the acid; chlorite and phlogopite were thoroughly disintegrated and subsequently generated quartz, K+, Mg2 +, [SiF6]2 −, [AlF5]2 − and Al3 +; the released V(III) was oxidized to VO2 + by O2 from the air. It is the procedure that fluorine combined with aluminum and silicon in the lattice of vanadium-bearing phlogopite to generate [SiF6]2 − and [AlF5]2 − that facilitated the vanadium leaching from stone coal. This generation decreased the ∆G° of phlogopite disintegration, declined the effect of chemical reaction on vanadium leaching, accelerated the leaching rate of vanadium and boosted the leaching efficiency of vanadium.

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