Optimizing the operation of straight-grate iron-ore pellet induration systems using process models
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I. INTRODUCTION
SINCE the 1960s, some 200 million metric tons per year of pellet production capacity has been constructed, covering most parts of the world where iron ore is mined; this product is used as a feedstock for most ironmaking processes. The pellet induration process consists of a number of key stages; concentrate is fed, along with a mixture of bentonite and other chemical additives (such as limestone or coke breeze) and moisture, into balling devices (drums or discs) to produce what are known as green pellets. These pellets are loaded onto a moving grate and form a packed bed; increasingly hot process gas is blown through it to dry and fire (to 1300 8C) the pellets, then ambient air cools them back to room temperature. There are two predominant forms of the iron-ore pellet induration process. The straight-grate process is as indicated previously. To protect grate bars (upon which the pellets are laid), a hearth layer of fired pellets is used. The fresh pellets are charged onto the hearth layer of fired pellets; the objective is then to fire the fresh pellets to a temperature which ensures they have appropriate physical and chemical properties (Figure 1(a) provides an illustration of this process). The other process involves the use of a long rotary kiln to fire the pellets. For these so-called grate-kiln systems, illustrated in Figure 1(b), the green pellets are charged to a moving grate, but no hearth layer of fired pellets is required because, in this case, the bottom of the bed does not reach a high-enough temperature to damage the grate bars before the burden is charged into the kiln, where it is fired. After this, the pellets are charged into an annular cooler to form MARK CROSS, Professor, is with the Centre for Numerical Modelling and Process Analysis, University of Greenwich, London, SE18 6PF United Kingdom. PHILIPPE BLOT, Process Engineer, is with Process Engineering Resources Inc., Salt Lake City, UT 84106. Manuscript submitted June 25, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS B
a deep-packed bed, where ambient air is blown through it to cool them. In this article, we will focus our attention on the straightgrate version of the pellet induration process. Typically, this process includes five key phases: drying (D), preheating (PH), firing (F), after-firing (AF), and first- and then secondphase cooling (C). The primary gas stream passes through the pellet bed two or three times; initially, it flows updraft through the first phase of cooling. This stream is then distributed and is pulled downdraft through the bed in the AF, F, and PH zones. Typically, some of this gas is recuperated and used either in the updraft mode in the initial drying phase or in the downdraft in the second phase of drying. The secondary gas stream passes updraft, through the second phase of cooling, and thence is used either in the first or second phase of drying (to complement the route of the primary gas stream). As the primary gas stream leaves the first stage of cooling, it has an average temperature of about 1
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