Thermodynamic analysis of the soda ash smelting of lead acid battery residue in a rotary furnace

A thermodynamic analysis of the rotary furnace process for the recovery of lead from battery residue by carbothermic reduction with soda ash (Na₂CO₃) was performed. Suitable models were selected to represent the thermodynamic behaviour of the bullion, matte, slag and gas phases. Equilibrium considerations and mass balances were performed in order to determine the effect of input parameters such as carbon and soda ash additions on the lead recovery. The processing of battery residue is characterized chemically by two distinct regimes, oxidizing and reducing, the occurrence of which depends on the amount of reductants added (carbon and iron). A maximum in the lead is predicted by the model at a critical range of concentration of reductants, which depends on the Pb, PbSO₄, PbS and PbO₂ contents of the charge. The thermodynamic model was adapted to the rotary furnace process to diminish the lead in the discarded matte-slag from 5-8wt.% Pb to about 1-2wt.% Pb. A 20% excess of carbon was added to achieve the highest lead recovery. It was found that by increasing the Fe/Fe₂O₃ mass ratio in the iron chips from 1/9 to 7/3 the slag viscosity diminished and the furnace operation became closer to the equilibrium condition.