Currently, the existing technologies for commercial large-scale production of
fired pellets include the following: the shaft furnace, the grate-kiln, and the straight
grate firing systems (Zhu et al., 2015). Figure 1 shows the flow sheets for modern
straight grate and grate-kiln pelletizing processes.
The straight grate process consists of a single furnace, where the different is zones
can be distinguished for drying, green ball preheating, green ball indurating and the
zones for fired pellets cooling. However, the grate-kiln system consists of three
separate process units connected in a series: a traveling grate for drying and preheating
green balls, a rotary kiln for indurating preheated pellets, and an annular cooler for
cooling the fired pellets.
Certain characteristics allow the substitution of sinter and calibrated lump ore in
the blast furnace burden with iron ore pellets. They are as given below. Spherical shape,
open pores of pellets and with their uniform size range (generally within a range of 8-
16 mm) give better and uniform permeability resulting in a high furnace drive (i). Very
high value of pellet cold crushing strength generates negligible amounts of fines during
storing in a stock house (ii). High porosity (greater than 18 %) leads to faster chemical
reduction (iii). High strength of pellets (around 250 daN/p) provides good resistance to
disintegration during the descent of burden within the metallurgical unit (iv). The iron
ore pellets have better tumbling index when compared with the calibrated iron ore (v)
and more uniform chemical composition compared to calibrated lump ore (vi).
Absence of LOI (loss on ignition at 950
o
C in air atmosphere) is another advantage of
the pellets (vii).
In this chapter, the analysis of the experimental data on magnetite pellets
oxidation and calcite dissociation, as well as the theoretical analysis of the dedicated
processes, have showed that these processes occur simultaneously within certain
temperature ranges under heat treatment conditions created for iron ore pellets. The
development of technological requirements for the pellets induration requires
analyzing the mutual character of magnetite pellets oxidation and calcite dissociation
and places this topic within the scientific and practical interest. This issue can be solved
by the analytical study on the stated processes using the mathematical model of mass
and heat exchange specially developed for heterogeneous reactions of magnetite
oxidation, calcite dissociation and reduction.
Based on the heat treatment phenomenon of the pellets moving layer and
considering the main heat and mass exchanging processes, the model, which has been
developed, allows establishing the influence of various parameters of the technological
process and the characteristics of individual pellets on the induration and the quality of
pellets.
The crucial parameters of iron ore pellets induration process. Determination
of the principle parameters in the process of pellets drying. Generally, the drying
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