Effect of CaO on the Phase Evolution of Vanadium Slag during Crystallization and Roasting-Leaching Processes for Selective Extraction of Vanadium

In this paper, the effects of CaO on the phase evolution mechanism of vanadium slag during slagging, direct roasting, and (NH4)(2)CO3 leaching processes are investigated. Results indicate that with the increase in CaO content, vanadium is always concentrated as (Fe, Mn, Mg)V2O4 in spinels, part of titanium is concentrated and transformed into CaTiO3, and phosphorus is concentrated in 3CaO center dot P2O5 (C3P) and transformed into n center dot 2CaO center dot SiO2-3CaO center dot P2O5 (nC(2)S-C3P). During the direct roasting process, a part of the vanadium-containing spinel phase oxidizes and reacts with Ca2SiO4 to produce calcium vanadate (Ca3V2O8, Ca10V6O25, and Ca2V2O7), which is soluble in (NH4)(2)CO3 aqueous solution. However, a part of the vanadium-containing spinel phase is oxidized and decomposed to vanadium oxides (V2O5 and V6O13), which are insoluble in (NH4)(2)CO3 aqueous solution. This is not beneficial for vanadium extraction using (NH4)(2)CO3 aqueous solution. In addition, (NH4)(2)CO3 aqueous solution can restrain the leaching of C3P from the nC(2)S-C3P solid solution in the directly roasted vanadium slag with high CaO content.