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 recover y of lead from battery residue by carbothermic reduction with soda ash (Na,CO,) was performed. Suitable models were selected to represent th e thermodynamic behaviour of the bullion, matte, slag and gas phases. Equilibrium considerations and mass balances were performed in order t o 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 reduci ng, 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 mode l was adapted to the rotary furnace process to diminish the lead in th e discarded matte-slag from 5-8 wt.% Pb to about 1-2 wt.% Pb. A 20% ex cess 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 fro m 1/9 to 7/3 the slag viscosity diminished and the furnace operation b ecame closer to the equilibrium condition. (C) 1997 Published by Elsev ier Science Ltd.