Oxidation of magnetite and oxidation of sulfur in pellets
Magnetic iron ore powder ore is the main raw material to produce pellets, during firing, should seek to fully oxidized to hematite, magnetite, it is important for the consolidation of pellets.
First, the oxidation of magnetite to hematite is accompanied by changes in the lattice structure. The magnetite crystal is equiaxed, and the hematite is hexagonal. The lattice change during oxidation and the atomicity of the surface of the nascent crystal have higher migration ability, which is favorable for forming crystal bonds between adjacent particles. .
Second, the oxidation of magnetite to hematite is an exothermic reaction. It emits almost half the total heat of the calcined pellets. Therefore, it is possible to save energy by ensuring that the magnetite is fully oxidized.
Third, if the magnetite is not sufficiently oxidized, there is still magnetite remaining in the center of the pellet. If it enters the high temperature calcination zone, the temperature rises, the partial pressure of oxygen decreases, and the magnetite is not stripped. In this case the magnetite will react with the gangue SiO 2 to form a low melting point compound with a liquid slag phase inside the pellet. It shrinks when cooled, causing cracks inside the pellets, which not only affects the strength of the pellets, but also deteriorates its reducibility.
The oxidation of magnetite starts at 200 ° C and is carried out in two stages.
4Fe 3 O 4 +O 2 =6γ-Fe 2 O 3 (1)
γ-Fe 2 O 3 =a-Fe 2 O 3 (2)
In the first step, only chemical changes occur, and there is no crystal transformation because γ---Fe 2 O 3 is still an equiaxed crystal system; the second step is only a crystal transformation, and finally becomes a hexagonal a-Fe 2 O 3 . If it is oxidized at a higher temperature, it can also be oxidized in one step to a hexagonal a-Fe 2 O 3 from an equiaxed system of Fe 3 O 4 .
The oxidation of magnetite pellets advances from the surface to the center; it is controlled by diffusion factors. It conforms to the adsorption-diffusion theory. First, the oxygen in the medium is adsorbed on the surface of the magnetite particles, reacts with Fe +2 to form Fe +3 , and then Fe +3 diffuses into the interior of the crystal grains.
The oxidation reaction of magnetite takes place very quickly in the first few minutes, and then the oxidation rate drops sharply. See the above figure as the temperature rises, not only the oxidation rate increases, but also the oxidation degree increases rapidly. When the temperature exceeds 900 ° C, the reaction is fast enough, and then the temperature is increased, and the effect on the oxidation process is not so obvious. When the temperature exceeds 1200 ° C, the oxidation rate and oxidation degree are no longer increased.
The oxidation time of magnetite pellets under isothermal conditions can be expressed by the following formula:
Where d———the diameter of the pellet;
K———the oxidation rate coefficient;
ω———degree of oxidation.
The oxidation rate coefficient k is related to the combustion medium containing O 2 , if it is air:
K = (1.2 ± 0.2) • 10 -4 cm 2 / sec (4)
If it is pure oxygen:
K=(1.4±0.1)•10 -3 cm/sec (5)
The calcination medium contains O 2 which varies, but is always lower than O 2 in the air, so the k value is smaller than the formula (4). Therefore, it is necessary to fully oxidize the magnetite in the preheating stage. In addition, according to thermodynamic analysis, hematite can be decomposed in air at 1383 °C. Since the partial pressure of oxygen in the calcining medium is much lower than the partial pressure of oxygen in the air, the temperature of hematite decomposed into magnetite during calcination is lower than 1383 ° C. In order to fully oxidize magnetite to hematite roasting The temperature should not be too high.
Since the roasting process of the pellets is a strong oxidation process, it is very advantageous for the desulfurization reaction. Magnetite is generally higher sulfur content, the sulfur chemical speciation is often FeS 2 - pyrite or CuFeS 2 - brass .FeS 2 ore began to decompose at 200 ° ~ 300 ℃, the decomposition pressure reaches 688 deg.] C sulfur 1 Atmospheric pressure (98066.5 Pa), the reaction formula is as follows:
FeS 2 =FeS+S (6)
S+O 2 =SO 2 (7)
4FeS 2 +11O 2 =2Fe 2 O 3 +8SO 2 (8)
3FeS+5O 2 =Fe 3 O 4 +3SO 2 (9)
FeS+10Fe 2 O 3 =7Fe 3 O 4 +SO (10)
Generally, more than 90% of the sulfur in the ore is removed during the roasting of the pellets.
The affinity of sulfur for oxygen is greater than the affinity of Fe +2 , so sulfur has an effect of inhibiting the oxidation of magnetite. If high-sulfur magnetite is used to produce pellets, it should be noted that the magnetite is fully oxidized during the preheating period, otherwise the quality of the pellets will be affected.
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