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

The kinetics of zinc vapour reoxidation in the Imperial Smelting Process was investigated in the system Zn-ZnO-CO-CO₂(-H₂-H₂O) by passing the gases through a heated quartz reactor tube containing zinc oxide pellets. The temperature profile could be controlled accurately and a mass balance was subsequently performed on the system. The morphological features of the zinc oxide deposit were investigated by scanning electron microscopy. The basic reoxidation reaction is

Zn(v) + CO₂(g) = ZnO + CO(g)

At temperatures, T, less than 750°C a distinct correlation was found between the deposition rate, R, excess zinc partial pressure (p_Zn - p^e _Zn) and zinc oxide morphology. Furthermore, the deposition rate at >850°C was significantly greater than that at <750°C, the greater thermodynamic driving force losing out to the reduced temperature kinetic effect. Two distinct types of crystals were produced for the two temperature ranges, indicating two different mechanisms of formation. At 750°C < T < 850°C a complex mixed morphology was observed. At >800°C 5 vol% hydrogen added to the gas stream increased the deposition rate by one order of magnitude—an effect that is attributed to the alternative reoxidation reaction

Zn(v) + H₂O(v) = ZnO + H₂(g)

At <760°C the increase is much less pronounced and is thought to be due to elevated CO₂ levels arising from the reaction

CO(g) + H₂O(g) = H₂(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 two 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 the process.