the magnetite grains recrystallization (at temperature above 900
o
C), on the hematite
grains recrystallization (temperature above 1100
o
C) and on the slag formation.
The strengthening of pellets is explained by the phenomenon of the hematite
grains recrystallization while the formation of the melt does not significantly increase
their strength. However, it is later shown that the recrystallization occurs to a small
extent, increasing the size of grains and it slightly affects the pellets strength. The
process that determines the pellets strength is the sintering of grains: the increase in
strength is carried out by creating and enlarging the area of the contact between the
particles along with the decrease in the porosity.
Under the influence of high temperatures and due to reactions between iron
oxides, waste rock and flux, one observes the presence of iron silicates, calcium ferrites
and other complex compounds, they have some mobility in the softened state. The
volatile matter release occurs along with the significant increase in the pellets porosity
before the beginning of the noticeable grains sintering in the solid and liquid states,
namely at the temperatures of 900–1200
o
C.
By adding less than 3% of the limestone to the content of the non-fluxed pellets,
we reduce the porosity of the pellets being fired. The optimal limestone additive is 7-
8%, since at these amounts the limestone is not capable of producing an adverse effect
either on the porosity or the strength of the pellets.
Otherwise, the porosity and the strength of the pellets are influenced by the
heating rate and the duration of the firing, which directly effect the productivity of the
process. The increase in the content of slag-forming oxides contributes to the
consolidation and the strengthening of pellets. With the porosity of more than 30–35%,
the crucial factor among all the indicators to influence the pellets strength is the quality
and the quantity of grain bonds, they are of a primary importance here. The optimal
ratio with regard to the open porosity value is the total porosity of 26 %, that is within
the range of 0.4–0.6.
The factors influencing the strength and the porosity of the pellets are presented
below in order of the capability to influence the stated properties of pellets, starting
with the most influential factors: the firing temperature; the content and the
composition of slag-forming substances; the heating rate and the firing duration.
The influence of principle parameters revealed for the firing process on the
specific productivity. The specific productivity of the firing machine depends on the
specific productivity of all its technological zones. The factors influencing the specific
productivity can be classified into general factors and zonal factors.
The general factors improve the gas permeability of the layer by changing its
granulometric composition - sieving the fines and large sizes, increasing the average
diameter of pellets, and decreasing layer depth. These factors operate throughout the
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