The quality of sinter is mainly manifested in strength and reductibility. They are related to the development degree of binder phase, crystallization conditions, number and characteristics of pores, cracks, etc., and also related to the strength and reducibility of binder phase minerals. .
In recent years, domestic and foreign sinter researchers have conducted many studies on the strength and reduction of various minerals in sinter.
Determination of compressive strength: A single mineral was made into a cube having a side length of 10 mm, which was carried out on a universal testing machine. The abrasion resistance was carried out in a small ball mill with a test time of 55 minutes, and a sieve percentage of more than 5 mm and less than 1 mm after the test was used as an index for evaluating the mechanical strength. Brittleness is carried out in a ΠMT-3 instrument with different loadings on its diamond pyramids, as a function of the number of impressions before the occurrence of cracks on the mineral flakes.
Reducing the mineral composition of the sinter point of view, a magnetic iron ore, hematite, ferrite hemi-calcium, monocalcium easily reduced iron, dicalcium iron, aluminum ore reducing low brown needles, glass, calcium iron olives Stone, calcium iron pyroxene, especially fayalite, is difficult to reduce. The reduction of thousands of sintered ore is carried out by reducing gas through the pores of the sintered ore, so that for acidic sintered ore, most of the pore walls are composed of fayalite and glass, while the self-fluxing sintered ore is mostly composed of calcium and iron. Olivine, glass, calcium ferrite. Therefore, the reducibility of the acid sintered ore is also worse than that of the self-fluxing sintered ore under the same porosity. Of course, another reason for the poor reductibility of the acid sinter is that the silicate constitutes a low-melting liquid phase, softens prematurely, reduces the porosity, and causes difficulty in reduction. However, the sinter with particularly low alkalinity has a small reduction in the liquid phase, and the exposed surface of the iron-containing mineral is large, so that it has good reducibility.
As the alkalinity increases (exceeding the acid sinter, generally above 1.0), the porosity of the sinter increases, and the hard-reducing fayalite is replaced by calcium olivine, and the reduction of the sinter becomes better. Some sinter mines are resistant to alkalinity of 2.0 or higher, and the reductive properties are deteriorated, which may be due to the occurrence of relatively poorly reduced ferric calcium silicate.
When the sintering material increases the carbon content, for the quartz gangue self-fluxing sinter, the melting property of the sinter is increased, the porosity is lowered, and the hard-reduced binder phase is increased, thereby reducing the reducing property of the product. However, the effect of low quartz gangue sinter is not significant, because increasing the carbon content does not increase the silicate phase which is difficult to reduce. For example, the test of Shandong Zhangjiayu Mine (TFe61.68%, SiO 2 4.0%) shows that when the fixed carbon is increased from 3.5% to 4.5%, the FeO is increased from 16.65% to 20.18%, and the drum index is greatly improved. There is no change in sex.
The mineral structure of the sinter has a great influence on the reductibility. The magnetite which is finely packed and has a small binder phase is easily reduced, which generally occurs in the sinter of the low quartz gangue. Large magnetites are either difficult to reduct or simply surface-reduced when coated with silicates. In addition, the high porosity (macroporous and microporous) crystal interlacing relaxation and the structure with many cracks are easily destroyed and easily restored, but the strength is also poor.
The reduction of single mineral crystals is analyzed from the viewpoint of crystallization chemistry, and the mineral crystals with low character properties are more easily reduced. The more lattice energy (ie, the decomposition of the lattice into its constituent units and the repelling of these units to the energy consumed at infinity), the more difficult it is to destroy and the less reductive.
The strength of the sinter is also affected in many ways. For example, the raw materials are crushed to a large particle size, and they are not likely to be melted during the sintering process, especially the residue of lime, which forms Ca(OH) 2 after water, causing the sintered ore to rupture. Another cause of poor sinter strength is the strength of the minerals in the sinter mineral composition. Magnetite, hematite, and ferric acid have higher compressive strength, followed by calcium olivine and dicalcium ferrite, where x = 0.5, ie CaO 0.5 FeO 1.5 SiO 2 , regardless of compression resistance, The test indexes of wear resistance and micro-brittleness are close to or exceed those of the former category. The k-iron olivine with x=1.5 is quite low in strength and easy to form cracks. Its lattice constant is close to 2CaO•SiO 2 . The glass crucible has the lowest strength. From the structure of the sinter, it is found that the dendritic and granular structure of magnetite in the accumulated glass phase helps to reduce its brittleness. The most dangerous is the overall and single distribution between the calcium olivine and the sapphire crystal. Glassy. Therefore, the task of increasing the strength of the sinter is to remove these vitreous materials from the structure and convert it into crystals as much as possible.
When the liquid phase begins to solidify, the microstructure of the sinter is formed, and the mineral composition is different. Due to the crystallization process of the liquid phase and the recrystallization of the solid state, a relatively wide temperature range is formed, and the mineral composition is formed in this interval. The mineral composition and the microstructure of the sinter depend not only on the liquid phase composition but also on the crystallization ability of the mineral as it solidifies, and on the cooling rate.
The influence of mineral composition on the strength of sinter is not limited to the strength of crystallized individual or vitreous separated in sintering. The strength depends to some extent on the mineral composition of the sinter and the internal stress formed by its cooling. The necessary condition for generating internal stress is uneven deformation at different points in the object, and the cause of such uneven deformation may be different properties, thermal expansion (or shrinkage) or volume expansion due to phase change, and the like.
After liquid phase condensation, the cooling process of the sinter is accompanied by different types of internal stress:
1) thermal stress due to the presence of a temperature difference between the surface of the sintered ore and the center;
2) the stress between the mineral phases caused by the different coefficients of thermal expansion of the phases;
3) The corresponding force caused by the polycrystalline transformation of calcium orthosilicate. The thermal stress is mainly determined by the cooling conditions and can be eliminated by slow cooling or heat treatment. The stress between the phases is mainly due to the different volume expansion coefficients of the various phases. The volume expansion coefficients of some minerals are listed below.
The stress generated by the above factors is mainly between the contact faces of the two minerals. Sinter ore is a multi-phase structure, which consists of minerals and glass bodies with different expansion coefficients. It is one of the reasons for the decrease of the strength of sinter which promotes certain alkalinity under cooling or heating conditions.
The corresponding force caused by the polycrystalline transformation of calcium orthosilicate is very serious for destroying the strength of the sintered ore. Studies on its polymorphic transformation have been introduced in Chapter 7. When βC 2 S into γC 2 S, the volume expansion of 10%. This is an important reason of sintered ore. The transition temperature can be carried out between 525 ° C and 20 ° C due to the presence of impurities and the difference in cooling rate.
In order to prevent the conversion of βC 2 S, it can be realized by the following methods:
1) The object is quenched at a temperature equivalent to the stable region of the a' phase. Under high-speed cooling, the lattice ions are not ready to be rearranged, so that the high temperature phase is fixed at normal temperature.
2) A substance having a corresponding ion which can enter a high temperature deformed crystal lattice to form a solid solution to stabilize the phase transition of βC 2 S.
3) Adding a certain amount of addition of glassy crystals, which surrounds the crystal particles of C 2 S, while forming a film that prevents the particles from expanding, and mechanically suppresses the transformation of β→γ. Some people think that in the sinter with higher alkalinity (alkalinity 3 ~ 4) there are 20 ~ 25% C 2 S, so the reason why no pulverization occurs because of the mechanical blocking of the calcium ferrite binder phase, some people think that C 2 S is stable because the excess CaO stabilizes the β phase transition to some extent.

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