Comparison and evaluation of vanadium extraction from the calcification roasted vanadium slag with carbonation leaching and sulfuric acid leaching

• A vanadium recovery of 73.67% requires 200 g/L (NH 4 ) 2 CO 3 leaching at 353 K. • Sulfuric acid leaching reactions progress rapidly in the initial 10 min. • Carbonation leaching performs better than sulfuric acid leaching in selectivity of vanadium. • The diffusion resistance of ammonium carbonate leaching is from silicate and rich iron phases. • The sulfuric acid leaching process is controlled by diffusion across the solid product CaSO 4 . Calcification roasting process for vanadium extraction has advantages in environmental protection, low cost of roasting additives, easy reutilization of the tailings. During the calcification roasting process, the vanadium spinel contained in the converter vanadium slag transforms into calcium vanadate. Carbonation leaching and sulfuric acid leaching used for extracting vanadium from the calcification roasted slag were compared and evaluated in detail, and effects of leaching parameters and the leaching kinetics were investigated. It was found that grinding the roasted slag particles below 94 μm can ensure a higher vanadium recovery. Leaching temperature, concentration of the ammonium carbonate, solid to liquid ratio all impacted the recovery of vanadium significantly. Under the conditions of temperature of 353 K, ammonium carbonate concentration of 200 g/L and solid to liquid ratio of 1:20 g/mL, 73.67% of the vanadium was extracted in 60 min. In the sulfuric acid leaching process, the aqueous pH value played a vital role and it was better to conduct the leaching at temperatures below 323 K. The sulfuric acid leaching reactions progressed fast and under a solid to liquid ratio of 1:10 the leaching efficiency of vanadium reached around 84% at 313 K and pH 2.50 in less than 30 min. Both leaching processes can be well described by the kinetic equation of 1/5(1 − α ) −5/3 − 1/4(1 − α ) -4/3 + 1/20 = k t, which represents the leaching process is controlled by diffusion of the two reactants in both directions. The difference lies in that the solid layer which the reactants pass through is mainly composed of silicate phases and Fe-rich phases in the ammonium carbonate leaching process, while in the sulfuric acid leaching process, calcium sulfate which instantaneously generates and tightly wraps the unreacted core, plays a dominant role in limiting the diffusion of the reactants. The ammonium carbonate leaching performs better in selectivity over the vanadium than the sulfuric acid leaching, but collection of ammonia and enrichment of vanadium-bearing solution should be considered. .