In the field of non-metallic and refractory materials, especially refractory materials, for the sake of convenience, the three minerals of kyanite, sillimanite and andalusite are collectively referred to as the “three stones”, which are all high-aluminum mineral raw materials.
The main applications of three stones in amorphous refractories are:
(1) as refractory aggregates, such as using coarse-grained andalusite as refractory aggregates;
(2) as refractory powders, such as using andalusite and sillimanite concentrates as powders;
(3) as admixtures, mainly as expansion agents. Among the three stone minerals, kyanite is the most widely used. Because the expansion value associated with the mullite reaction of kyanite is the largest, kyanite is a good expansion agent for amorphous refractories, which is used to offset the shrinkage of refractory materials at high temperatures and improve their high-temperature performance.
1. Application of three stones in refractory castables
In recent years, there are four main aspects of research on the application of three stones in refractory castables:
1. Application of kyanite in refractory castables.
Kyanite from Yinshan and Shuyang Hanshan is mainly used as an expansion agent for refractory castables. In terms of expansion, the kyanite from Tongbai has a larger expansion value and is more suitable as an expansion agent for amorphous refractory materials. In the ladle castable, Nanyang Yinshan kyanite is added as an expansion agent to improve the linear change rate of the castable, thereby eliminating the shrinkage cracks generated by the material during high temperature and cooling, and increasing the service life of the material. During the use of the ladle, the bottom of the ladle is repaired as necessary, and the entire use of a ladle is 1200 to 1300 times. The high-aluminum castable prepared with kyanite as an expansion agent improves the linear change of the castable after firing. When kyanite is not added to the castable, the line changes after firing are all negative, and the shrinkage increases with the increase of temperature. It is -0.09% at 1300℃ and -1.05% at 1500℃. However, after adding kyanite, the castable shrinkage decreases or expands positively. For example, when kyanite of different particle sizes (0.175mm or 0.09mm) is added at 8% and 10%, the castable is positive at 1300 ~ 1500℃, that is, after offsetting the shrinkage of the material at high temperature, it still expands.
In production, the index requirements of the comprehensive materials are considered, and the particle size of the kyanite concentrate is reasonably selected to ensure that the material does not shrink (or shrinks very little) at high temperature, while ensuring high strength. Generally, the particle size of 0.174~0.074mm is selected as moderate.
2. Effect of adding andalusite on castable performance
(1) Application of andalusite in iron ditch material With the large-scale development of blast furnaces, the scouring force of molten iron and slag increases, and the wear and tear increases. In particular, the working conditions of the main ditch are becoming increasingly harsh, and the service life is reduced. Andalusite is used in the production of iron ditch main ditch materials, and the characteristics of andalusite are used to improve the performance of the iron ditch main ditch castable. In production, according to different usage requirements, adding andalusite iron ditch castables of different particle sizes (0~1mm, 0.074mm) is beneficial to reduce the apparent porosity of the castable, improve the compressive strength at room temperature, and improve the thermal shock stability of the castable. The better the quality of andalusite, the better its effect and high temperature performance.
Using iron ditch castables with andalusite added, the one-time iron throughput of a single taphole of a 250m3 blast furnace reaches 80,000 to 120,000 tons. After spraying and sleeve pouring repairs, the service life can reach more than 1.5 million tons, reducing production costs. The mechanism of use is that when andalusite decomposes at high temperature, it can produce a certain amount of mullite and liquid phase. The generated mullite helps to improve the thermal shock stability and load softening temperature of the castable. The generated liquid phase can not only promote sintering to make the matrix and aggregate tightly assembled, but also close the pores, reduce the apparent porosity of the product, and improve the compressive strength of the castable.
(2) Pretreatment of castable iron for desulfurization gun The desulfurization gun is always in a drastic change of hot and cold temperatures. At this time, the general damage to the gun is not erosion, but cracks caused by thermal stress, which penetrate into the molten iron and cause damage. By improving the thermal shock resistance of the castable, cracks and peeling of the gun body are prevented. By adding andalusite, shrinkage is compensated, the volume stability of the gun body is guaranteed, and its performance is improved. During the trial at a steel plant in Ningbo, it reached more than 200 times at a time, and a small amount of repairs were performed in the middle.
3. The effect of adding sillimanite and kyanite on the performance of castables.
Adding sillimanite or sillimanite and kyanite concentrate to the castable improves the linear change of the product after firing. In addition, the load softening temperature and compressive strength of the product are also significantly improved. The higher the grade of sillimanite concentrate, the more obvious the effect. For example, the SC-3 sample with sillimanite with an Al2O3 content of 59% and an aggregate of first-grade bauxite has a load softening temperature (4%) greater than 1600℃; when the Al2O3 content of sillimanite is 48%, the aggregate is still first-grade bauxite, and its load softening temperature (4%) is significantly reduced, such as the SC-12 sample, which is 1565℃.
4. The effect of adding natural sillimanite and andalusite composite mineral powder on the performance of castables.
When composite mineral powder is added to the castable, the thermal shock resistance of the sample is improved and the linear change rate after firing is reduced. The main technical indicators of low-cement high-alumina castables with appropriate addition of natural sillimanite andalusite composite ore have been improved. The main reason is that a large amount of mullite is generated in the matrix. The composite mineral powder forms a liquid phase at a relatively low temperature (1000 ~1300℃), and the liquid phase is conducive to the formation of in-situ mullite and secondary mullite, thus having a beneficial effect on the performance of the castable. The appropriate addition amount of the composite mineral powder is 5%.
In the past, the main function of kyanite in refractory castables was to act as an expansion agent to offset the shrinkage of the material at high temperature. With the improvement of understanding, the application of andalusite, sillimanite, or the composite of three stones can effectively improve and enhance the quality of Al2O3-SiO2 materials, both in shaped refractory materials and amorphous refractory materials, showing the unique properties of three stones.
2. Application of three stones in refractory plastics
In refractory plastics, the linear changes after firing at 1400℃ are compared with those without kyanite. The former has a larger linear change and the latter has a smaller linear change. Generally, the expansion increases and after adding kyanite, the linear changes of each sample after firing at 1400℃ are all in an expansion state. This is beneficial to the use of plastics, enhancing the stability of the structure, reducing cracks, and slowly peeling. The linear change at 1600℃ is slightly expanded compared with that at 1400℃.
3. Application of three stones in refractory ramming materials
After adding three stone minerals, the linear change of high-alumina refractory ramming materials after firing changes from linear contraction to linear expansion. Among them, kyanite has the best effect, and the linear change after firing at 1400℃ increases from -0.40% to + 1.60%. This shows the expansion agent effect of three stone minerals.
Adding tri-stone minerals has no significant effect on the compressive strength of high-alumina refractory ramming materials at 1400 ~ 1500℃, because at this temperature stage, kyanite and andalusite decompose rapidly and are not completely mullitized.
4. Application of tri-stone in refractory spray coatings
Refractory spray coatings are amorphous refractory materials constructed by mechanical spraying using pneumatic tools. They are composed of refractory aggregates, powders, and binders (or admixtures). According to the volume density of the material, there are three types of light, medium, and heavy spray coatings. Light spray coatings (0.5~1.39g/cm3) are often used as thermal insulation linings, medium and heavy spray coatings (1.3~1.8g/cm3) can be used as both thermal insulation linings and working linings for low and medium temperature kilns, and heavy spray coatings (greater than 1.89g/Cm3) are mainly used as working linings.
Different types of refractory spray coatings can be used on the inner side of the iron sheet above the middle of the blast furnace belly, the combustion chamber, heat storage chamber, mixing chamber and the inner wall of various hot air ducts of the hot blast furnace. Its function is to insulate and heat-preserve, improve the air tightness of the furnace body, and protect the iron sheet of the furnace body. The hot blast furnace of the largest blast furnace in my country (volume greater than 5500m3) also uses refractory spray coatings. The introduction of three stones can not only improve the linear change rate of the refractory spray coating, but also improve the index value of the material due to the addition of the new phase of mullite. The added three stones are both single varieties and composite types.
In amorphous refractory materials such as refractory spray coatings or refractory castables, the addition of three stone materials has achieved good results. According to the index requirements of shaped or amorphous refractory materials, select the variety or composite variety of three stones.
5. Application of three stones in refractory mud
With the development of amorphous refractory materials, refractory mud has been widely developed in research, production, inspection and other aspects. With the expansion of the application scope of refractory mud, ordinary refractory mud can no longer fully meet the requirements of construction kilns in terms of performance. The addition of kyanite plays a great role in the new refractory mud. In the refractory mud, kyanite concentrate is added to compensate for the shrinkage of the mud at high temperature due to the mullite expansion reaction of kyanite.
6. Application of three stones in high-strength slag retaining wall prefabricated parts
The high-strength slag retaining wall prefabricated parts for the blast furnace tapping ditch are added with expansion agents to make the material expand evenly at various temperatures (especially in the temperature range of 1000 ~ 1500℃), offsetting or weakening the shrinkage of the material at various temperature stages. The material with expansion agent can fill the microcracks caused by the shrinkage of the material matrix and its own internal stress, and improve the volume stability of the material.
In summary, various types of amorphous refractory materials, such as castables, plastics, ramming materials and refractory slurries, all use kyanite, andalusite and sillimanite to varying degrees, especially kyanite, which is more widely used. The main mechanism is to use the volume expansion of the decomposition of tri-stone minerals at high temperature and the mullite reaction to compensate for the shrinkage of amorphous refractory materials at high temperatures, so that the linear expansion tends to be positive, reduce structural spalling, and enhance the volume stability of the material.
In addition, the decomposition of tri-stone minerals to form mullite minerals has a good effect on the load softening temperature and strength of the material.
Quartz (SiO2) is also used as an expansion agent for amorphous refractory materials. The expansion generated by the homogeneous multiphase transformation of quartz is used to compensate for the shrinkage of amorphous refractory materials at high temperatures. Among them, the expansion of α-quartz (high-temperature quartz) into α-cristobalite is mainly used because it has the largest volume change.
However, compared with quartz, kyanite has a better effect. First, the expansion value is relatively large, and second, due to the decomposition of kyanite, mullite crystal phase is obtained, which is beneficial to the high-temperature performance of the material. Therefore, kyanite is often used as an expansion agent in amorphous refractory materials, or it is used as an additive in combination with andalusite or sillimanite.
The decomposition and mullitization reaction of kyanite, andalusite and sillimanite has a good effect on the performance of the material. It is necessary to select the grade, dosage and particle size according to the operating temperature, otherwise the internal part of the material will deteriorate, causing expansion cracks and a decrease in density and strength of the material.
The above method is to use the decomposition of three-stone minerals, the expansion reaction accompanied by mullitization and the expansion reaction generated by the transformation of quartz crystal form to compensate for the shrinkage of amorphous refractory materials at high temperatures and enhance the volume stability. However, the good effect of expansion reaction on materials is more than that. For example, low creep bricks for hot blast furnaces and modified high alumina brick series use the expansion reaction inside the material to improve the load softening temperature and creep resistance, thermal shock resistance and other properties. So we should make good use of three stones to improve the performance of refractory materials.
