Kinetics and Mechanism of Hydrogen Reduction of Lead-Silicate Slag

A systematic study on the microstructure evolution and kinetics analysis of PbO–SiO2 slag reduction using hydrogen was conducted. The reduction was carried out on PbO–SiO2 pellets (70 wt pct PbO–30 wt pct SiO2; or 38.6 mol pct PbO and 61.4 mol pct SiO2) using 15 pctH2/85 pctN2 gas, with a flowrate of 500mL/min for various reaction time (30 minutes to 3 hours) isothermally at 300 °C to 700 °C. The kinetics and reaction mechanism were assessed by measuring the weight loss over reaction time and applying kinetics models on the data; supported by detailed samples characterizations. The results from microstructure observation show a viscous and blackish glass structure formed on the pellets when the reduction was carried out above softening point (glass transition temperature). This viscous structure appeared to reduce the overall reduction rate. The kinetics analysis shows that the reduction appeared to be a diffusion-controlled process. The activation energy, Ea, was calculated to be 70.7 kJ/mol at temperature range of 300 °C to 500 °C; while between 500 °C and 700 °C the kinetics were found to decrease with increasing temperature due to the formation of the viscous glass. These results suggest that for a complete reduction in industrial process, the formation of the viscous glassy state should be avoided.