Phase and composition evolutions of iron-vanadium spinels containing chromium during calcium roasting

As a dominant resource for vanadium extraction, roasting of vanadium-bearing slags involves the destruction of vanadium-containing phases, mainly including iron-vanadium spinels and liquid phase, and the formation of soluble vanadate. Extraction of vanadium from iron-vanadium spinels determines the efficiency of the whole process due to their ultrastabilities. Here the phase and composition evolutions of iron-vanadium spinels containing chromium were investigated by both advanced experimental characterizations (EPMA, XRD and XPS, etc.) and thermodynamic analysis. The roles of roasting conditions were clarified, including roasting temperature, time and the presence of chromium. It was found that, upon roasting, the iron-vanadium spinels underwent a phase transition to form two distinct phases, namely a vanadium-enriched phase and a Me 2 O 3 solid solution phase. A relatively high roasting temperature with a relatively long roasting time could enhance the completeness of the roasting reaction and improve the separation of vanadium from iron and chromium. The evolution of iron-vanadium spinels with the current compositions (n(FeO): n(V 2 O 3 ): n(CaO): n(Cr 2 O 3 ) = 4: 1: 2: 0.2) followed a three-step mechanism under the current roasting conditions, i.e., an initial step of the oxidation decomposition of spinels, a reaction between CaO and V 2 O 5 to form CaV 2 O 6 , and a final reaction between residual CaO and CaV 2 O 6 to form a Ca 2 V 2 O 7 phase. Moreover, the presence of chromium resulted in an increase in the Fe 2 O 3 concentration within the vanadium-enriched phase due the formation of a solid solution with an infinite miscibility between Cr 2 O 3 and Fe 2 O 3 . The findings of this study offer not only scientific insight with regard to the evolution of iron-vanadium spinels upon roasting but also significant guidance for the optimization of roasting conditions toward target vanadium extraction from iron-vanadium spinels.