Microstructural changes on the reduction of imperial smelting furnace sinters
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I.
INTRODUCTION
D E S P I T E extensive studies of many operational aspects of the Imperial Smelting Furnace (ISF) process, tl-51 relatively little work has been carried out on the characterization of the sinter feedstock.t6.T] Many aspects of the reaction product morphologies, phase transformations, and reaction kinetics during the reduction process at various gas mixtures and temperatures are poorly understood. Zinc sinter consists of a complex arrangement of phases and microstructures, which makes it difficult to establish and interpret the phase relationships in the system and the reaction mechanisms which take place during the reduction reaction. In the ISF process, the CO content in the blast furnace gas mixtures progressively decreases as the reducing gas ascends to the top of the furnace. Heat- and mass-transfer processes, phase transformations, and chemical reactions occur simultaneously in various furnace zones. The control of blast furnace operation is highly dependent on the physical and chemical properties of the ISF sinter. A detailed microstructure study into the behavior of the mineral constituents in the sinter microstructure is therefore essential in establishing structure/property relationships. In the present investigation, the behavior of constituent mineral phases within the sinter microstructure before, during, and after reduction of ISF sinters has been studied, in order to obtain a deeper understanding of the factors defining the reducibility, strength, and softening points of the sinter in the ISF smelting operation. The reducing gas mixtures employed in these experiments were selected to match closely those gas compositions occurring in various temperature zones of an actual ISF zinc blast furnace. F.T. LEE, Postdoctoral Research Fellow, and P.C. HAYES, Associate Professor in Extractive Metallurgy, are with the Department of Mining and Metallurgical Engineering, The University of Queensland, Queensland, Australia 4072. Manuscript submitted December 30, 1991. METALLURGICAL TRANSACTIONS B
II.
EXPERIMENTAL
A. Materials
Two types of ISF sinter samples, supplied by the Sulphide Corporation, Boolaroo, Australia, were used for this study, and the chemical analyses of the sinters are given in Table I. Type 6 ISF sinter had a CaO/SiO2 ratio ->1.3, F e O / SiO2 >2.5, and a SiO2 content between 3.5 and 5.0 wt pct. The microstrncture of type 6 ISF sinter (Figure 1(a)) shows structures consisting of zincite, franklinite, CaPb-silicates, and lead oxide. Type 6 sinter has softening point ranging from 1323 and 1373 K. Type 7 ISF sinter is chemically similar to type 6 but has CaO/SiO2 -->1.6 and FeO/SiO2 > 4 . 0 ratios and is produced by using higher sintering temperatures (-> 1723 K). This sinter has the eutectic structure, which acts as a refractory link between the zincite crystals (Figure I(h)). The eutectic in the ISF sinters consists of two separate compounds, a Ca-Pb silicate rodlike structure in a zinc ferrite (franklinite) honeycombed-type matrix, and has a melting point in excess of 1523 K
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