Sequential extraction of vanadium and chromium from chromium-bearing vanadium slag through two-stage soda roasting-water leaching

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Abstract
The sequential extraction of vanadium and chromium from the chromium-bearing vanadium slag through two-stage soda roasting-water leaching was proposed. The precipitation order and thermodynamic stability of Cr-spinel and V-spinel were thoroughly calculated with FactSage 6.3 software. The mechanism of the sequential extraction process is further determined by X-ray diffraction (XRD). Thermodynamic calculations show that the precipitation order of main phases in the chromium-bearing vanadium slag is as follows: Cr-spinel → V-spinel → Fe-olivine, while the priority of the reaction between main phases with Na2CO3 follows the order of Fe-olivine → V-spinel → Cr-spinel. XRD results reveal that the vanadium-bearing phase in roasted slag is water-soluble NaVO3, while the chromium-bearing phase in roasted slag is Cr-spinel ((Mn, Fe) Cr2O4) and solid solutions ((Fe0.6Cr0.4)2 O3) with the soda amount of 21% and temperature of 800 °C. The different evolution route of vanadium-bearing phase and chromium-bearing phase during the roasting process ensures preferential extraction of vanadium from the chromium-bearing vanadium slag. The leaching rates of vanadium and chromium reach 89.36% and 4.91% with optimized experimental conditions. The high leaching rate of vanadium and low leaching rate of chromium show good results of preferential extraction of vanadium from the chromium-bearing vanadium slag.

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