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The promotion of strengthening of vanadium oxidation by compound additive of sodium salts is described from the changes of the vanadium valence state and mineral phase. The results are as follows: during the roasting process, dehydroxy mica converts to the melt, which the composite system mainly contains Na, K, Al, Si, and O. Under the action of NaCl-Na2CO3, ion exchange between sodium and potassium promotes the crystallization of albite from the melt. Na2CO3 enhances the reactivity of quartz. The albite reacts with activated quartz, which promotes the migration of sodium ions and the generation of vanadate. Under the action of NaCl-Na2SO4, The crystallization of spinel in the melt is promoted and that of the albite is inhibited. Under the action of Na2SO4-Na2CO3, the permeability of ore is deteriorated. The ion exchange process and the vanadium oxidation is inhibited from the melt. The composite additive with three sodium salts shows a stronger promoting effect on vanadium oxidation. The contribution of NaCl and Na2CO3 to V(V) in roasted slag was the highest among three sodium salts. With a certain ratio of NaCl and Na2CO3, the low content of Na2SO4 did not significantly affect the V(V) content in roasted slag. [ABSTRACT FROM AUTHOR]