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More than 95% of the mineral components of acidic pellets are hematite. Quartz does not react with hematite in an oxidizing atmosphere, so independent quartz particles are visible. The hematite is recrystallized and the grain is long in Dalian. A small amount of additive - bentonite has melted and adhered to the surface of hematite grains. Only by magnifying the microscopic magnification, it is possible to occasionally find large particles of bentonite that have not been fully melted. Due to the consolidation of the pellets, the hematite single phase The solid phase reaction is dominant and the amount of liquid phase is extremely small. Its pores are irregular in shape, with many connected pores, and the total porosity and open porosity are not much different. The pellet of this structure has a relatively high compressive strength and a good low temperature and medium temperature reducing property. Most of the world's pellets currently belong to this category.
The use of magnetite concentrate to produce pellets, if the oxidation is not sufficient, its microstructure will be inconsistent inside and outside, and can be divided into three regions along the radius:
The surface layer is fully oxidized, just like the general acidic pellets. Hematite undergoes recrystallization and grain growth and is joined into pieces. A small amount of unmelted gangue, as well as a small amount of molten silicate mineral, is sandwiched between hematite grains.
The main mineral in the intermediate transition zone is still hematite. The hematite crystals are filled with liquid phase of iron silicate and vitreous silicate, and there is still unoxidized magnetite in this area.
The central magnetite belt, the unoxidized magnetite is recrystallized at high temperature, and is bonded by the liquid phase of iron silicate and vitreous silicate, and the pores are mostly circular atmospheric pores.
A pellet having such a microstructure generally has a low compressive strength. Because there are many liquid phases in the center, the volume is reduced during condensation, and concentric cracks are formed, so that the pellets have a two-layer structure. That is, a porous shell dominated by hematite, and a solid core mainly composed of a magnetite and a silicate liquid phase, separated by a crack. Therefore, when producing pellets from magnetite, it is necessary to make it fully oxidized.
For self-fluxing pellets, under normal conditions, the main mineral is hematite. The amount of calcium ferrite varies with alkalinity, in addition to a small amount of calcium silicate. Among the pellets containing higher MgO, there is also magnesium ferrite. Since FeO can replace MgO, it is actually mafic ore, which can be written as (Mg, Fe)O•Fe 2 O 3 .
The microstructure of the self-fluxing pellets, when the calcination temperature is low, and the residence time at a high temperature is short, the hematite crystals can be seen, and the calcium ferrite formed by the solid diffusion in a local area can be seen. When the calcination temperature is high and the residence time is high at a high temperature, an interlaced structure of hematite and calcium ferrite is formed. Since the calcium ferrite system can be in the liquid phase at the calcination temperature, the pores are round.
Experiments have shown that ferrite can only be stabilized at lower temperatures when silicates are present. At 1200 ° C, ferrite is solid solution in the corresponding silicate. Above 1250 ° C, ferrite is hard to find in the melt, and vitreous silicate appears in the binder phase of the pellet.
The use of magnetite to produce self-fluxing pellets, if the oxidation is not sufficient, along the radius of the pellets, there will also be a distinct layered structure. ,
The outer mineral composition and structure are similar to those of general self-fluxing pellets, mainly hematite and calcium ferrite, and even a small amount of silicate gangue minerals that are not involved in the reaction.
In the intermediate transition zone, there are hematite, magnetite that is not oxidized, and a large amount of vitreous silicate liquid phase.
In the central region of the pellet, unoxidized magnetite is bonded to the silicate glass and recrystallizes at high temperatures.
Compared with acidic pellets, self-fluxing pellets have a complex mineral composition. In addition to hematite, there are also calcium ferrite, calcium silicate, calcium olivine and so on. There are many liquid phases generated during the roasting process, so the pores are round atmospheric pores, and the average compressive strength is lower than that of acidic pellets. ,
In summary, it can be seen that there are two factors affecting the mineral composition and microstructure of the pellets: one is the type and composition of the raw materials, and the Other is the calcination process conditions, mainly the temperature, the atmosphere and the time maintained at high temperature. . Mineral composition and microstructure of pellets, metallurgical nature of its impact is enormous.
Mineral composition and microstructure of pellets
Compared with sintered ore, the mineral composition of pellets is relatively simple. Because the raw material pellets with high iron grade. Less impurities. The pellets are also relatively simple to make, almost a single iron concentrate powder, with only a small amount of additives. Flux is only added when producing self-fluxing pellets. In addition, the roasting process is also relatively simple, generally a high temperature oxidation process.