Study of moisture transfer during the strand sintering process
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I.
INTRODUCTION
A. The Strand Sintering Process
IN ironmaking
as well as in zinc metallurgy, the use of a blast furnace requires presintering of the mixture of ores to give them good mechanical (strength and permeability) and chemical (reducibility) properties. The sintering operation is usually carried out on a moving grate (Figure 1). Pellets composed of ores, return sinter, fluxes, coke (in the case of iron), and water are loaded on a moving grate. After ignition under a hood, air blowing through the bed maintains a strongly exothermal reaction (coke combustion in the case of iron, sulfide roasting in the case of zinc). The bed attains a temperature of approximately 1300 ~ which causes partial melting of the charge and sintering of the pellets. The iron ore sintering and zinc ore sinter-roasting processes are fundamentally similar, differing only in the details of gas flow direction, ignition process, fuel, pellet characteristics, and the chemical reactions involved.
B. Importance of Drying The feed mixture to a sintering operation contains both constitutive water (water chemically bound to the solids) and free water. Constitutive water comes mainly from ores and exists in low concentrations; its removal occurs by dehydration reactions. Free water comes from ores, fluxes, and mainly from water added to aid the pelletizing process. It represents 6 to 12 wt pct of the feed mixture for iron ore, and 4 to 6 wt pet for zinc ore. Free water removal is an important step in the sintering operation because of the high thermal energy consumption required for drying (up to 25 pet of the requirements for iron ore sintering) and the reduced permeability of the
F. PATISSON, Research Scientist from Centre National de la Recherche Scientifique (CNRS), J.P. BELLOT,Assistant Professor, and D. ABLITZER, Professor, are with the Laboratoire de Science et G6nie des Mat6daux M6talliques (LSG2M), Ecole des Mines, 54042 Nancy Cedex, France. Manuscript submitted June 13, 1988. METALLURGICAL TRANSACTIONS B
overmoistened (i.e., with excess water, after condensation) zone, which obstructs gas flow. As shown in Figure 2, when the hot gases leave the reaction zone and enter the moist zone, they dry the solids and become loaded with water vapor. But, as will be shown, when these now very moist gases come in contact with the colder charge in the deep layers of the bed, water condenses on the solids. Therefore, the term "moisture transfer" will be used to include both drying and condensation.
C. Previous Descriptions In the past, moisture transfer has been described in a rather simplified manner in various mathematical models of the sintering process. For drying, the simplest approach considers that it takes place entirely at a fixed temperature of 100 ~ Other authors introduce a twostage drying model: first, drying occurs at a constant rate and, after, at a rate that decreases linearly with the moisture content, t4-9] Yoshinaga and Kubo [1~ mention a threestage drying process but give a single equation where the drying rate decreases co
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