Preparation of highly pure vanadyl sulfate electrolyte from vanadium slag leach solution with the complexing effect of EDTA on Fe(III)

Abstract Direct acid leach solution of converter vanadium slag containing tetravalent vanadium is considered to be a low-cost raw material to produce highly pure vanadyl sulfate electrolyte for vanadium battery. The presence of Fe(III) in the primary stripped solution is an obstacle for the further purification process since a) it is hard to separate Fe(III) from V(IV) by solvent extraction b) it is difficult to completely reduce Fe(III) to Fe(II) in the presence of oxygen from the air. A series of complexing agents, i.e. ethylenediaminetetraacetic acid (EDTA), 5-sulfosalicylic acid (SSA) and citric acid were initiatively employed to complex the Fe(III) in the aqueous phase. The impact of Fe(III) complexation on the V(IV) extraction process by D2EHPA (Di-(2-ethylhexyl)phosphoric acid) were investigated and compared. The ligand number within each complex were calculated to instruct the complexant amount needed for complete Fe(III) complexation. Experimental results confirmed that EDTA is the preferred complexing agent compared to SSA and citric acid. >98% of Fe(III) in the aqueous phase can be complexed in a liquid phase by EDTA with a mole ratio of EDTA to Fe(III) of 1.5. Moreover, the complexation degree of Fe(III)-EDTA strongly depends on solution pH, and the maximum separation coefficient (β V/Fe  = 729) between V(IV) and Fe(III) takes place at pH = 1.8 in the D2EHPA extraction system. A highly pure vanadyl sulfate electrolyte product can be obtained by two-stage extraction with the complexing function of EDTA.