| Summary: | In this paper, the effects of CaO on the phase evolution mechanism of vanadium slag during slagging, direct roasting, and (NH<sub>4</sub>)<sub>2</sub>CO<sub>3</sub> leaching processes are investigated. Results indicate that with the increase in CaO content, vanadium is always concentrated as (Fe, Mn, Mg)V<sub>2</sub>O<sub>4</sub> in spinels, part of titanium is concentrated and transformed into CaTiO<sub>3</sub>, and phosphorus is concentrated in 3CaO·P<sub>2</sub>O<sub>5</sub> (C<sub>3</sub>P) and transformed into n·2CaO·SiO<sub>2</sub>-3CaO·P<sub>2</sub>O<sub>5</sub> (nC<sub>2</sub>S-C<sub>3</sub>P). During the direct roasting process, a part of the vanadium-containing spinel phase oxidizes and reacts with Ca<sub>2</sub>SiO<sub>4</sub> to produce calcium vanadate (Ca<sub>3</sub>V<sub>2</sub>O<sub>8</sub>, Ca<sub>10</sub>V<sub>6</sub>O<sub>25</sub>, and Ca<sub>2</sub>V<sub>2</sub>O<sub>7</sub>), which is soluble in (NH<sub>4</sub>)<sub>2</sub>CO<sub>3</sub> aqueous solution. However, a part of the vanadium-containing spinel phase is oxidized and decomposed to vanadium oxides (V<sub>2</sub>O<sub>5</sub> and V<sub>6</sub>O<sub>13</sub>), which are insoluble in (NH<sub>4</sub>)<sub>2</sub>CO<sub>3</sub> aqueous solution. This is not beneficial for vanadium extraction using (NH<sub>4</sub>)<sub>2</sub>CO<sub>3</sub> aqueous solution. In addition, (NH<sub>4</sub>)<sub>2</sub>CO<sub>3</sub> aqueous solution can restrain the leaching of C<sub>3</sub>P from the nC<sub>2</sub>S-C<sub>3</sub>P solid solution in the directly roasted vanadium slag with high CaO content.
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