Why are sodium chloride or sodium carbonate added during the roasting of vanadium ores?

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During the vanadium processing of various ores and slags, sodium chloride or sodium carbonate is added to the material before it is roasted at approximately 850° C (1,550° F). The purpose of this addition is to convert the vanadium oxides present in the ore into sodium metavanadate. This resulting compound is water-soluble, which allows the vanadium to be effectively recovered through leaching in hot water for further purification.1