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

Abstract

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) + H 2 O(v) = ZnO + H 2 (g)At <760°C the increase is much less pronounced and is thought to be due to elevated CO 2 levels arising from the reaction CO(g) + H 2 O(g) = H 2 (g) + CO 2 (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.

Thermal characteristics and fate of heavy metals during thermal treatment of Sedum plumbizincicola, a zinc and cadmium hyperaccumulator

“…Further reactions that have to be considered are the subsequent homogeneous reactions in the gaseous phase. The oxidation of volatile zinc by carbon dioxide to solid zinc oxide and carbon monoxide (reaction 1) and especially the homogeneous oxidation of gaseous zinc by steam to solid zinc oxide and hydrogen (reaction 2) proceed at temperatures above 750°C and are described for pyrometallurgic processes [36,37].…”

Splitting Water and Carbon Dioxide via the Heterogeneous Oxidation of Zinc Vapor: Thermodynamic Considerations

“…Zinc aerosol reactors have achieved no more than 20% conversion of the zinc aerosol (a=0.2) with hydrolysis temperatures near 700 K [18][19][20][21][22][23][24]. The heterogeneous oxidation of zinc vapor, however, has demonstrated the potential for achieving complete zinc conversion in short reaction times [25][26][27][28][29][30][31][32][33]. The energy fiows and process irreversibility for the water splitting cycle with ^8=0.6, a= 1, and Thydroiysis=1185 K (case 1: heterogeneous zinc vapor oxidation path) and /3=0.6, a=0.2, and hydroiysis=700 K (case 2: zinc aerosol oxidation path) are shown in Table 1, and the resulting efficiencies for both water and carbon dioxide splitting cycles are noted by (i) the points for the zinc aerosol path and by (ii) the points for the heterogeneous zinc vapor oxidation path in Figs.…”

“…Empirically derived rate expressions work well within the experimental parameters for which the expression was found, but extrapolation introduces error [28]. Also, while solid carbon is a thermodynamically favored product, experimentalists have observed the exclusive deposition of zinc oxide [12,[26][27][28][29][30][31][32][33]. There is a kinetic barrier to carbon fomiation, and it was therefore neglected as a product in the oxidation of zinc by carbon dioxide in the present study.…”

“…First, the reaction rates are not limited by the diffusion of reactive ions through the solid ZnO product. Rather, the heterogeneous oxidation of zinc vapor is rapid with sustained rates of up to and possibly greater than I X 10"' mol s"' cm"- [25][26][27][28][29][30][31][32][33]. Second, the heterogeneous oxidation of zinc vapor promises complete zinc conversion.…”

“…Kinetic studies in tube flow reactors have also been completed on the oxidation of zinc vapor by CO2-CO mixtures for temperatures between 973 K and 1273 K [26][27][28][29][30][31][32][33]. Reaction rates as high as 5 X 10"^ mol s~' cm~-were observed.…”