Asynchronous extraction of vanadium and chromium from vanadium slag by stepwise sodium roasting–water leaching

• Stepwise roasting method has been realized to asynchronously extract V and Cr.
• Toxic tailings are recycled into the production of V–Cr slag for cleaner purpose.
• Core/shell distribution of Cr/V in spinel grain of V–Cr slag has been discovered.
• V is extracted by fractional roasting while Cr is extracted by secondary roasting.
• 98.9% of V and 96.6% of Cr can be extracted from V–Cr slag by the proposed method.

A novel environment-friendly method based on stepwise roasting has been proposed to extract vanadium and chromium separately from vanadium slag with high chromium content (V–Cr slag), which consists of two stages of sodium roasting–water leaching. Fractional sodium roasting–water leaching is firstly conducted to extract vanadium from the V–Cr slag and 87.9% of vanadium is extracted in optimal conditions. Leaching residue of the fractional roasting stage is then roasted secondarily to extract chromium and 96.4% of chromium is extracted from the leaching residue in optimal conditions. Overall extractions of vanadium and chromium are 98.9% and 96.6%, respectively. Evolution mechanisms of V-bearing and Cr-bearing phases were elucidated by XRD, TG–DSC and SEM-EDS. It is elucidated that V(III) existing as V-spinels in the V–Cr slag is oxidized to water-soluble NaVO3 while Cr(III) remains in the form of Cr-spinels or R2O3 phase (R: Cr, Fe) during the fractional roasting; in secondary roasting, Cr(III) existing as Cr-spinels and R2O3 phase in the leaching residue of fractional roasting stage is oxidized to water-soluble Na2CrO4. Vanadium and chromium have thus been extracted separately and thoroughly at the extraction procedure by controlling the roasting extent of V–Cr slag via the proposed stepwise roasting method. The established method provides new insights for comprehensive utilization of complex minerals containing multiple valuable elements.

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