The Oxidation of Zinc Vapor in CO-CO 2 -N 2 Gas Mixtures J.M. OSBORNE, W.J. RANKIN, D.J. McCARTHY, and D.R. SWINBOURNE The kinetics of oxidation of zinc vapor in the Zn-CO-CO 2 -N 2 system was investigated for zinc partial pressures of 0.01 to 0.09 atm, carbon monoxide partial pressures up to 0.5 atm, and carbon dioxide partial pressures up to 0.6 atm at 730 °C to 900 °C. The experimental apparatus consisted of a flow reactor and a multitemperature zone furnace. Known gas compositions were generated and the rate of oxidation of zinc vapor was determined from the mass of zinc oxide deposited under controlled conditions. The rate of oxidation of zinc was found to be a function of temperature and of the partial pressures of zinc, carbon monoxide, and carbon dioxide. It was autocatalytic with respect to carbon monoxide and independent of the total mass of zinc oxide deposited. The reactions occurring in parallel for this mechanism are Zn (g) + CO 2 (g) = ZnO (s) + CO (g) and Zn (g) + CO 2 (g) + CO (g) = ZnO (s) + 2 CO (g) The two oxidation reactions occur simultaneously, both involving carbon dioxide and one with carbon monoxide as a catalyst. The autocatalysis of the reaction by carbon monoxide is explained by this mechanism, as is the observation that the effect of the partial pressure of carbon monoxide cannot be accounted for by a single p CO term (the rate expression). The experimental results fitted a rate expression of the form r = (k 1 + k 3 p CO )  p Zn p CO 2 - p CO K eq  (1 + k 4 p Zn ) (moles Zn/s) over a wide range of conditions, with an accuracy of 25 pct. Values of k 1 , k 3 , and k 4 were calculated and expressed as a function of temperature. The term K eq is the equilibrium constant for the reaction Zn (g) + CO 2 (g) = ZnO (s) + CO (g) I. INTRODUCTION and ZnO morphology were also studied and will be reported subsequently. BECAUSE of its low boiling point, zinc is produced The oxidation of zinc vapor has been studied by several as a vapor in smelting processes. The potential for zinc to workers over the past 25 years, [1–6] and the gas compositions be reoxidized by CO 2 and H 2 O in the smelting gases during and temperature ranges investigated are summarized in Table cooling prevented for many years the blast furnace smelting I. Stott and Fray [1] investigated the rate of oxidation of zinc of zinc calcine/sinter. Even today, zinc produced in slag- vapor in CO-CO 2 -N 2 mixtures, at compositions typical of fuming processes is recovered as zinc oxide. An improved those produced in the ISP, using a flow technique to improve understanding of the mechanism and kinetics of the oxida- on the static technique used by earlier workers for other tion of zinc vapor is important for improving the design of metals. In Stott and Fray’s method, dry CO, CO 2 , and N 2 zinc condensers for existing smelting processes, particularly were passed through a bed of zinc oxide pellets in a horizon- the Imperial Smelting Process (ISP), and for the development tal tube furnace at 1050 °C to generate zinc vapor, then of new intense and/or direct smelting processes. The work passed into a lower-temperature zone (at 800 °C to 930 °C). reported here was undertaken as part of a larger investigation A by-pass allowed the gases from the hotter zone to be into the oxidation of zinc vapor in smelter off-gases, particu- diluted with argon. The zinc oxide produced was deposited larly gases generated in intensive smelting vessels. The oxi- on the walls of a silica tube. After an experiment, the tube dation of zinc vapor in H 2 -H 2 O-N 2 and CO-CO 2 -H 2 -H 2 O- was sectioned and the mass of deposit was determined as a N 2 mixtures and the effect of sulfur on the rate of oxidation function of time. This permitted the composition of the gas to be calculated along the length of the oxidation zone. However, this method suffers from a stoichiometric compo- J.M. OSBORNE, Senior Metallurgical Engineer, is with Rio Tinto Tech- sition constraint, in that the partial pressure of zinc is the nical Services, Melbourne, Victoria, 3083 Australia. W.J. RANKIN, Deputy same as the partial pressure of CO 2 due to the method of Chief, is with CSIRO Division of Minerals, Clayton, Victoria, 3169 Austra- lia. D.J. McCARTHY is a Consultant with Glen Waverley, Victoria, 3150 zinc vapor generation. Australia. D.R. SWINBOURNE, Associate Professor, is with the Depart- Clarke [2] extended the flow technique of Stott and Fray ment of Chemical and Metallurgical Engineering, RMIT, Melbourne, 3001 to study the oxidation of zinc vapor in CO-CO 2 -Ar, H 2 -H 2 O- Victoria, Australia. Manuscript submitted January 12, 1999. Ar, and CO-CO 2 -H 2 -H 2 O-Ar mixtures in composition ranges METALLURGICAL AND MATERIALS TRANSACTIONS B VOLUME 32B, FEBRUARY 2001—37