AbstractsChemistry

Off-gas dust wastes from steelmaking processes contain a number of by-product metals, which are hazardous to dispose of by landfill. With the increased use of electric arc furnaces, zinc has become a significant by-product in the off-gas dust and a main inhibiter of off-gas dust disposal as common waste. Zinc’s low vaporization temperature (907 °C) causes the zinc to evaporate from the melt and become entrained in the off-gas stream. Zinc then deposits as zinc, zinc oxide, and zinc ferrite spinel, which are difficult to separate postprocess from ferrous and other entrained oxides. If the zinc could be separated from the residual entrained particulates through an in-process method, the cost of processing the dust for recycling would go down. In addition, if the zinc-rich portion of the separated dust has high enough zinc content the dust could be sold for a profit. Zinc recovery could increase the steelmaker’s profitability while reducing the environmental toll. The focus of this research was the condensation of zinc vapor to elemental zinc and zinc oxide solid under varying environments in order to investigate the feasibility of in-process separation of zinc from steelmaking off-gas dusts. Water vapor content, temperature, degree of cooling, gas composition, and initial zinc partial pressure were varied in order to simulate the possible conditions that can occur within steelmaking off-gas systems. Temperature of deposition and the effect of rapidly quenching the gas was specifically studied. Postexperimental analysis included the Zn:ZnO ratio in the condensate; homogeneous nucleation of zinc particles; and developing kinetic rate expressions that correlate condensation rates to gas composition, water content, temperature, and zinc partial pressure. It was determined that oxidation by H2O or CO2 does not occur below 835 °C for highly oxidizing streams (CO2:CO = 40/7). The rate of oxidation of zinc vapor by carbon dioxide and water vapor were determined. It was proven that cooling rate significantly increased the ratio of elemental zinc to zinc oxide, as well as increased the number of particles produced in the system. SEM analysis was performed of the zinc and zinc oxide samples collected. Homogeneous nucleation was modeled using Matlab and was compared to experimental data, proving that elemental zinc formation within the reactor was homogeneous in origin. The model was expanded to include the rates of oxidation by CO2 and H2O; the experimental data had a relatively good fit to the model. For the conditions used in this study, the reaction rates for both carbon dioxide and water vapor oxidation of zinc as well the homogeneous nucleation model of elemental zinc held true for various temperatures, zinc partial pressures, CO2:CO ratios, and H2O partial pressures.