Hydrothermal desulfurization of spent lead paste based on comproportionation reaction

Abstract A hydrothermal method was adopted to strengthen the desulfurization reaction of PbSO4 with the aim of addressing the problem of incomplete desulfurization and large reagent consumption in the pretreatment process of spent lead paste. Mineral liberation analysis showed that lead sulfate disseminates in lead oxide and they become mixed with each other, which is an important reason for incomplete desulfurization via conventional methods. E-pH diagrams of the Pb-SO42--H2O system at different temperatures indicate that the pH value of PbSO4 converted to PbO decreased with an increase in temperature. Experimental results showed that increases in time, the NaOH excess coefficient, temperature, stirring rate, and the liquid-solid ratio are beneficial for improving the desulfurization ratio. Under hydrothermal conditions, PbO and PbO2 react to form Pb3O4, and this comproportionation reaction destroys the encapsulation of the solid product layer and is conducive to the progress of the desulfurization reaction. Under the optimized reduction conditions (i.e., time of 60 min, sodium hydroxide coefficient of 1.1, temperature of 150 °C, liquid-solid ratio of 10 mL/g, and stirring rate of 600 rpm), a 98.95% desulfurization ratio was achieved. The phases of lead in the desulfurization residue were mainly PbO, PbO2, and Pb3O4, and these can be recovered via low-temperature reduction smelting.