A facile method for efficient synergistic oxidation of Fe2+ in phosphorus-sulfur mixed acid system with a mixture of oxygen and ozone

The recovery of iron phosphate involves the addition of oxidizer to oxidize Fe2+ in the spent LiFePO4 (LFP) material to Fe3+ and the agent commonly used is hydrogen peroxide (H2O2). Nevertheless, H2O2 has disadvantages of high price, easy decomposition and low utilization efficiency. In this manuscript, a facile method is proposed for efficient synergistic oxidation of Fe2+ in spent LFP leachate with a mixture of oxygen and ozone. Specifically, we found by thermodynamic computations that the dominant oxidation groups of ozone during oxidation varied with acidity. The oxidation would produce a large number of iron-phosphate complex groups (\({\rm{F}}{{\rm{e}}_3}{{\rm{H}}_6}\left( {{\rm{P}}{{\rm{O}}_4}} \right)_4^{3 + }\), \({\rm{Fe}}{{\rm{H}}_8}\left( {{\rm{P}}{{\rm{O}}_4}} \right)_4^ - \) and \({\rm{F}}{{\rm{e}}_2}{\rm{HPO}}_4^{4 + }\)) in the phosphorus-sulfur mixed acid system, leading to a paradoxical pH drop. The optimized conditions for H2O2 oxidation were explored. It was determined experimentally that oxidation by gas mixture and O2 belonged to the firstorder and second-order reactions with activation energies of 28.68 kJ/mol and 34.61 kJ/mol, respectively, which were both controlled by a mixture of chemical reaction and diffusion. The optimized oxidation method was finally determined by evaluating the cost and oxidation rate of the oxidizers. The results in this study offer a promising method for new low-cost and efficient Fe2+ oxidation for industrial production.