Effect of calcium carbonate on the thermodynamics and kinetics of lead sulfide carbothermic reduction

Abstract The effect of calcium carbonate on the thermodynamics and apparent kinetics of lead sulfide (PbS) carbothermic reduction was investigated in the PbS–Na 2 CO 3 –C and PbS–Na 2 CO 3 –CaCO 3 –C systems at 800–1100 °C. Thermodynamic analysis included Gibbs free energy calculations and multiphase equilibrium modeling, while kinetic behavior was evaluated through time-resolved gas analysis and TG–DTG–DSC measurements. Equilibrium calculations revealed predominant Na 2 S formation in the CaCO 3 -free system and CaS formation in the CaCO 3 -containing system. At 800 °C, the calculated equilibrium amount of metallic lead increased from 804.29 to 999.83 mol upon CaCO 3 addition, based on 1000 mol of initial PbS. Sodium carbonate remained largely unconsumed in the CaCO 3 -containing system. Experimental results demonstrated a pronounced temperature-dependent effect of CaCO 3 . At 800 °C, the apparent reaction extent (α app ) was 70.6% with CaCO 3 and 74.9% without it. At 900, 1000, and 1100 °C, α app reached 96.4%, 97.9%, and 99.5%, respectively, in the CaCO 3 -containing system, accompanied by higher apparent reaction rates. The corresponding apparent activation energies were 32.0 and 52.9 kJ·mol − 1 , indicating stronger temperature sensitivity with CaCO 3 addition. These findings demonstrate that CaCO 3 modifies sulfur distribution and the apparent kinetic behavior of PbS-containing systems.