Selective indium extraction from zinc oxide dust leachate by microwave-assisted solvent extraction with P507 and stripping with HCl: Thermodynamics and kinetics

Abstract

In this work, indium was extracted from zinc oxide dust leachate using a microwave-assisted P507 extractant, and microwave-assisted indium stripping with hydrochloric acid. The effects of extractant concentration, pH, volume ratio of organic phase to aqueous phase, contact time, temperature and microwave power on indium extraction were investigated. The optimum conditions for indium extraction were obtained as follows: extractant concentration, 25 % (v/v) in sulphonated kerosene; pH = −0.2; O/A (volume ratio of organic phase to aqueous phase) = 1:2; contact time, 2 min; temperature, 40 °C; and microwave power, 70 W. Under these conditions, the extraction efficiencies of indium, zinc, iron and aluminium were 97.6 %, 2.50 %, 0.48 % and 0.89 %, respectively. The separation coefficients of indium for zinc, iron and aluminium were 1333, 7996 and 4442, respectively, which indicated that P507 has good selectivity for indium over zinc, iron and aluminium. The optimum conditions for indium stripping were a hydrochloric acid concentration of 1.5 mol/L, O/A = 2:1, contact time of 3 min, temperature of 40 °C and microwave power of 90 W, under which the indium stripping efficiency was 99.1 %. Additionally, McCabe–Thiele diagrams were constructed for indium extraction by P507 and indium stripping by hydrochloric acid, which theoretically predicted two stages for indium extraction by P507 and only one stage for indium stripping by hydrochloric acid. The advantage of microwave extraction over conventional extraction was that it reduced the time required for the extraction to reach equilibrium. The stoichiometric data for the extraction reactions were determined using the slope method. Thermodynamic studies showed that microwave-assisted indium extraction was feasible, with all negative standard Gibbs free energies, and the introduction of microwaves increased the entropy change of the extraction reaction. Kinetic studies showed that microwave-assisted In(III) extraction followed a pseudo-second-order kinetic model. The mechanism of In(III) extraction involves ion exchange with hydrogen in -P-O-H in P507. The future studies will focus on the amplification of microwave extraction equipment to promote industrialisation.