The principle parameters for the operation of pellets preheating. As pellets
preheating, there are two main physical and chemical processes occur parallel in time,
namely, they are dissociation of limestone and oxidation of magnetite. Consider the
preheating temperature of roughly 1000 °C, it allows you to see that the preheating
time of the material at this temperature greatly affects its porosity (the sizes and the
shapes of the pores), the structure and sizes of its crystals, the material bulk density,
and its specific surface. The first step of the magnetite oxidation is observed at the
temperatures of 150–200
o
С, when the energy supply to particles is sufficient for their
transition into the crystalline lattice. However, appreciable oxidation of magnetite
occurs only at the temperature of 400
о
C. At the temperature of 600
o
C, the oxidation
rate of magnetite is higher and pure hematite is formed. With increasing the
temperature, the degree of oxidation also increases and at the temperature of 1000
o
C,
the oxidation of magnetite into hematite completes within 10 minutes. The temperature
when the oxidation begins and the final degree of oxidation depend both on the iron
ore type, the presence of impurities, and the iron ore grain size. The processes of
carbonates dissociation and the magnetite oxidation are largely influenced by the fact
whether the pellets drying process is completed, since in pellets having residual
moisture, these processes occur with a delay, even at temperatures as high as in the
firing zone.
Eventually, the preheating process is determined greatly by the following
parameters: the temperature of the drawn off gas at the pellets preheating; the gas flow
rate for the pellets preheating; the residual moisture amount within the pellets.
The principle parameters for pellets firing. The temperature and the time
conditions for iron ore pellets firing are the main factor to determine the quality of the
final product. The value of the firing temperature, the heating rate and the cooling rate
for pellets are the most important under these conditions.
The temperature is more influential than the other factors, it intensifies the process
of sintering, that is why it is advisable to firing the pellets at the maximum possible
temperature, which is determined by the temperature of pellets softening and melting.
As the blend compositions typically used for the production of pellets vary largely, one
cannot identify a certain value of the firing temperature for all these conditions.
However, the firing temperature below 1200 °C does not provide the pellets with that
strength required for the transportation needs.
The presence of the liquid phase significantly intensifies the process when the
pellets become denser and more strong. In this case, the role of the liquid phase is not
limited to the function of making pellets denser and accelerating this process. It also
changes the shape of the pores that become rounded and this brings a greater strength
for pellets in comparison with the structures with irregular pores (the equal porosity
values have been regarded in the both cases).
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