Mathematical Model of Lead Behaviour in the Zinc Slag Fuming Process

Abstract

A mathematical model of zinc slag fuming, based on rate phenomena such as mass transfer, chemical kinetics and heat transfer has been improved and extended to include the behaviour of lead in the bath. In the model, a fraction of the pulverized coal injected into the slag is entrained by the bath where reduction of ZnO, PbO and ferric iron proceeds. Most of the coal not entrained is combusted by blast air while a small quantity bypasses the bath unreacted. Lead is eliminated from the slag as a vapour in secondary bubbles which result from the reaction of the entrained coal with the slag. These secondary bubbles are carried to the bath surface where they release their contents to the furnace freeboard. The major portion of the lead reduced, however, results in liquid metallic particles which remain in the bath where they are re-oxidized to PbO by ferric iron. Other improvements to the model include the incorporation of a three-suffix Margules equation to calculate the activity coefficients for FeO and Fe2O3 in the slag. The model has been fitted to slag composition and temperature profiles obtained from six industrial fuming cycles. Thus it has been shown that the fraction of coal entrained is greater with increasing nominal tuyere exit velocity and that oxygen utilization increases as the bath depth is raised. The model also predicts that the settling and collection of metallic lead will bear importantly on the removal of lead from high-lead slags produced by the QSL or flash smelting processes.