Preparation of vanadyl sulfate electrolyte for vanadium flow battery from vanadium slag using calcium salt precipitation, sodium carbonate leaching and solvent extraction

The conventional process of recovering vanadium from vanadium slag produces residue and wastewater containing toxic V(V) and Cr(VI), which pollute the environment. To address this problem, herein, low valence vanadium (LVV) solution is produced from vanadium slag under controlled oxidation, additionally, V(IV), Fe(II), Cr(III), and Mn(II) are separated from the LVV solution through sodium diethyldithiocarbamate complexation (DDTC), based on previous studies. Subsequently, V(IV) is recovered as calcium salt of V(IV) (CaV2O5), which is subjected to sodium carbonate leaching and solvent extraction with Aliquat 336 in the form of tetramine to produce a high-purity VOSO4 solution. This solution serves as an electrolyte for the vanadium redox flow battery. During leaching, CaV2O5 is converted to CaCO3 as vanadium is recovered in solution up to 99.2%, wherein 88.2% vanadium is in the form of V(IV), thus indicating that vanadium is slightly oxidized after leaching for 60 min. Analysis of the leaching mechanism reveals that V(IV) is in the form of V4O92− and HV2O5− in the pregnant leach solution (PLS), and V(V) is in the forms of VO43−. After acidification with H2SO4, Aliquat 336 extracts V(IV) and V(V) via an anion-exchange mechanism, without adjusting the pH of the leach solution. Further, equations for the extraction and stripping reactions are deduced. The total vanadium extraction and stripping efficiencies exceed 99.3% and 97.2%, respectively. Finally, high-purity VOSO4 electrolyte is obtained after using activated carbon to remove organic, and the corresponding V(IV) ion concentration reaches 77.8 g/L. The cyclic voltammetry test results reveal identical electrochemical properties between the electrolyte product and commercial high-purity VOSO4. Therefore, this process of preparing high-purity VOSO4 electrolyte from vanadium slag without producing V(V) and Cr(VI) provides an effective method for comprehensively utilizing vanadium slag.