The reduction of ISF sinters

Abstract

The reduction of small, dense zinc oxide (ZnO) and zinc ferrite (ZnFe204) samples with and without impurity oxide additions in CO/CO2, CO/N2 and H2/N2 gas mixtures has been investigated at temperatures between 600 and 1100oC . Microstructural examination of fractured and polished samples using optical and scanning electron microscope (SEM-EDX) techniques has enabled the conditions for the formation of a range of product microstructures to be defined. The microstructural characterization of the reacted and partially reduced samples were complemented with measurements of the rates of the reactions.The mechanisms and kinetics of reduction of zinc oxide were found to be dependent upon the CO and H2 partial pressure, the oxygen potential of the gas mixtures, temperature, degree of reduction, and the type and amount of impurity oxides(i.e. FeO, PbO, MgO, CaO, SiO2. MnO, AI2O3) in the samples. The reduction of ZnO solid solutions resulted in the formation of a planar ZnO reaction interface and a porous product layer composed of impurity oxides whereas the reduction of pure ZnO resulted in selective attack of particular crystal planes and an irregular growth interface. From the analysis of the relative rates of the various chemical reactions and transport processes, the rate of reduction of ZnO with or without impurity addition were found to be controlled jointly by chemical reaction at the ZnO reaction interface and pore diffusion of reducing gas through the porous ZnO layer or reaction product layer.The reduction rates of zinc ferrites were also found to be dependent upon temperature, particle size, reaction gas composition, and the type and amount of impurity oxide (i.e. MgO, MnO, AI2O3, CaO) additions. The presence of CaO and MgO In ZnFe204 increased the region for porous Iron growth whereas AI2O3 and MnO reduces the range of gas conditions over which the porous iron microstructure may be formed. Five broad microstructural types formed during the reduction of pure and impure zinc ferrites in the system studied have been identified. These structures were porous iron (type A), porous wustite covered with dense iron (type B), dense wustite covered with dense iron (type C), wustite phase (type D), and porous magnetite grains (type E).The reaction mechanisms of ZnO, ZnFe2O4 and dissolved impurities were explained in terms of the relative rates of the various chemical reactions and mass transport processes both in and out of the solid. The changes in growth mechanisms and reduction kinetics which occur with changing reduction conditions are discussed.The results obtained for the reduction of ZnO and ZnFe204 both with and without dissolved impurity oxides have been used to explain the reactions involving zincite and franklinite phases within the sinter microstructure in the simulated blast furnace conditions. Initial and partially reduced samples were examined using optical, electron-probe microanalysis, SEM-EDX, and XRD to characterise the structural and compositional changes occurring during reduction reaction. The principal phases identified in the sinter microstructure were franklinite, zincite, copper oxides, lead oxide, Ca-Pb silicates, and calcium silicates.The reaction mechanisms and reduction sequences for the various oxide phases within the sinter structure during reduction of types 6 & 7 sinters under the system studied are discussed. The reduction of sinters resulted in structural modification of zincite, franklinite, slag phases, and the formation of new oxide and metallic phases. The rate and behaviour of these complex phase transformations and phase coarsening were found to be dependent upon reduction time, temperature and the reacting gas composition. The eutectic in the ISF sinters was positively identified to consist of two separate compounds, Ca-Pb silicate rod-like structure in a zinc ferrite (franklinite) honeycombed-type matrix.