Pellet mine quality control
The quality of pellets is evaluated mainly from two aspects: one is its strength, and the other is its reducibility.
Generally, the compressive strength of pellets should not be less than 200 kgf/piece (1961.33 N/piece), the drum index (+6.3 cm) is above 93%, and the wear index (-0.6 cm) is below 7%. In fact, blast furnace smelting does not necessarily require the strength of the pellets to be so high, because the pressure inside the blast furnace is not large. The foreign pellets are mostly produced in the mines, and they must be transported by water and land for long-term storage. The strength requirements are very high.
Furthermore, the strength of the pellets is also a sign of firing consolidation. Too low a strength indicator indicates that the firing consolidation is not good. The average strength of pellets is not as important as its intensity distribution. The average intensity of two similar pellets, poor metallurgical properties of a broad distribution of persons, who must as part of the wide impermeable burning, another portion of burnt. The former has low strength and is easy to form a powder. Although the latter has high strength, the reducing property is inferior.
The porosity of the pellets is generally between 20 and 30%. It has a direct linear relationship with the low temperature and medium temperature reduction of pellets. see picture 1.
Cryogenic pellets, the temperature is also cool is generally good, because it's easy to restore the main mineral hematite. Moreover, the porosity is high and there are many micropores. Conducive to reduction.
The high-temperature reductive property of the acidic pellets is not good because FeO reacts rapidly with SiO 2 at a high temperature of 1200 ° C or higher, and melts to deteriorate the reducing conditions. Experiments have shown that at 1250 ° C, the acid pellets are reduced, and only a metal shell is formed on the surface. The inside of the pellets is mostly FeO and molten silicate slag phase.
Pellets with addition of magnesium oxide, it is possible to improve the high-temperature reducing, MgO can be improved because the melting temperature of the slag phase. There are two ways to add magnesium oxide, one of which is dolomite, but dolomite will bring CaO. In order to improve the metallurgical properties of the blast furnace raw materials, the ore is sintered with high alkalinity, and the acidic pellets containing MgO are added. More ideal. If the pellet contains high CaO, it will affect the alkalinity of the sinter. In addition, dolomite will increase the liquid phase in the pellets, affecting its compressive strength. Furthermore, the addition of dolomite will cause more MgO to combine with Fe 2 O 3 to form mafic ore, which is associated with hematite grains. When reduced at low temperatures, hematite is first reduced to magnetite. Crystallization occurs, stress is generated, and cracks appear in the crystal grains, causing low-temperature pulverization of the pellets. To solve this problem, Swedish LKAB uses forsterite instead of dolomite.
Forsterite is a silicate with a high melting point. Only solid phase diffusion in the pellets does not melt, which can reduce the MgO entering the hematite, and can improve the high temperature reduction of the pellets. The disadvantage is brought part of SiO 2, so that the SiO 2 pellets increased.
The problem of reduction and expansion of pellets has attracted the attention of many metallurgical workers in the 1970s. Most pellets expand when they are reduced, and some expand abnormally. It is generally believed that the expansion ratio is below 20%, which is allowed by the blast furnace and is called normal expansion; those exceeding this boundary are called abnormal expansion. The expansion of individual pellets can reach 100%, which deteriorates the permeability of the blast furnace. The hemispherical diffusion-consolidated pellets change from 500 °C to 1000 °C during the reduction process as follows: see Figure 2. It can be seen from curve 1 that normal expansion mainly occurs from Fe 2 O 3 to Fe 3 O. The transformation of 4 , because of the crystal form change in the process, changed from hexagonal to cubic, and the lattice constant changed from 5.42 angstroms to 8.38 angstroms. Curve 2 is an abnormal expansion, which mainly occurs from the transition from FeO to metallic iron, and the iron whisker is the basic cause of abnormal expansion during the process.
FeO is reduced to metallic iron, and if a uniform metallic iron shell is formed on the surface, abnormal expansion does not occur. Under certain circumstances, the surface of FeO is partially covered by slag phase or contaminated by CaO, and the reduction will be carried out in the uncovered portion. The iron ions in FeO rapidly diffuse and move to the point where the reduction starts, like the seedlings that break the soil. Similarly, iron whiskers grow out and cause abnormal expansion. The ability of the floating body to absorb CaO is very strong, and CaO is unevenly distributed in the floating body, which is the cause of abnormal expansion. Na 2 O can also cause abnormal expansion. In addition, the anisotropy of hematite minerals, that is, the crystal axes of two adjacent hematite grains are perpendicular or inconsistent with each other, and the crystal change generates thrust during the reduction, which is also one of the causes of expansion. Appropriate increase of SiO 2 in the pellets, and more formation of some silicate liquid phase, can reduce the reduction expansion rate. In addition, reducing the iron concentrate size, increasing the roasting temperature, increasing the ratio of return to the mineral, etc., can reduce the reduction expansion ratio.
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