Zinc reoxidation in the shaft of a zinc-lead Imperial Smelting Furnace - 1: zinc-carbon-oxygen system with deposition initiated on a quartz substrateand subsequent propagation on zinc oxide

The kinetics of zinc vapour reoxidation in the Imperial Smelting Process wa s investigated in the system Zn-ZnO-CO-CO2(-H-2-H2O) by passing the gases t hrough a heated quartz reactor tube containing zinc oxide pellets. The temp erature profile could be controlled accurately and a mass balance was subse quently performed on the system. The morphological features of the zinc oxi de deposit were investigated by scanning electron microscopy. The basic reo xidation reaction is Zn(v) + Co-2(g) = ZnO + CO(g) At temperatures, T, less than 750 degreesC a distinct correlation was found between the deposition rate, R, excess zinc partial pressure (p(Zn) - P-Zn (e)) and zinc oxide morphology. Furthermore, the deposition rate at >850 de greesC was significantly greater than that at <750<degrees>C, the greater t hermodynamic driving force losing out to the reduced temperature kinetic ef fect. Two distinct types of crystals were produced for the two temperature ranges, indicating two different mechanisms of formation. At 750 degreesC < T < 850 degreesC a complex mixed morphology was observed. At >800 degreesC 5 vol% hydrogen added to the gas stream increased the deposition rate by o ne order of magnitude-an effect that is attributed to the alternative reoxi dation reaction Zn(v) + H2O(v) = ZnO + H-2(g) At <760<degrees>C the increase is much less pronounced and is thought to he due to elevated CO2 levels arising from the reaction Co(g) + H2O(g) = H-2(g) + CO2(g) Carbon deposition was observed separately in the presence of furnace sinter and sulphuric acid. This was most probably caused by sulphur from these tw o sources. When they were absent carbon deposition via the reaction Zn(v) + CO(g) = ZnO + C was not observed, indicating that CO is not a viable oxidizing species in t he process.