The ideal chemical composition of kyanite is a homogeneous multiphase variant with the same composition as andalusite and sillimanite. This high density is in a metastable state under normal pressure and can spontaneously decompose into a low-density mineral combination, namely mullite-quartz, at a certain temperature. Due to the unique expansibility of kyanite, it is very difficult to produce dense refractory materials with it as the main raw material. First, refractory materials with kyanite as the main component are difficult to sinter; second, its high-temperature expansion often causes cracking of the products. What should be paid attention to when producing mullite insulation brick with kyanite as the main raw material?
(1) Product cracking Due to the expansibility of kyanite itself and the expansion generated during the secondary mullitization process, although it is beneficial to reduce firing shrinkage, it is easy to cause cracking of the product.
The solution to this problem is:
① Grind the material fully to reduce the expansion amplitude of individual particles.
② Add a small amount of clay to buffer the expansion of kyanite. In addition, adding some extra-fine sawdust to create pores inside the brick after calcination can also alleviate the expansion of kyanite and prevent cracking of the product. Cracking is not only related to the raw materials, but also to the firing system and the uniformity of the brick. When using kyanite as the main raw material to produce mullite insulation brick, the temperature should be raised very slowly (0.5 (/min) within the temperature range of kyanite's mass transformation (1200~1300). Before the bricks are formed, the raw materials should be fully mixed and trapped to make the moisture content of each part uniform so that the density of the green bricks is uniform and there are no potential cracks.
(2) Selection of kyanite raw materials Kyanite can be divided into four types according to the main impurity minerals it contains: quartz type, corundum type, rutile type, and mica type. Quartz type and corundum type kyanite can be used as raw materials for producing lightweight mullite bricks, while mica type is the least desirable, and corundum type is the best. When the set operating temperature of mullite insulation bricks is below 1500℃, the NazO+K:O content in kyanite is allowed to be controlled at 0.5%~1%. When the set operating temperature is above 1600℃, the NazO+K:O content in kyanite is allowed to be controlled at 0.5%~1%. The Na:O+KO content should be controlled below 0.5%. When using a stirred mill, the particle size of the kyanite raw material purchased should be 500 μm. Below, otherwise the factory should be equipped with crushing equipment.
(3) Selection of alumina raw materials When producing lightweight mullite bricks, ALO₂ raw materials with the lowest possible Na:O content should be selected. Generally, the Na:O content should be controlled below 0.5%. Many alumina plants in my country can achieve this indicator. When the operating temperature of lightweight mullite insulation bricks is set above 1650°C, the Na:O content in alumina powder should be below 0.3%. If necessary, a-Al.O with low Na.O or fused corundum fine powder should be used. Using kyanite as the main raw material can produce high-performance lightweight mullite refractory bricks, breaking the traditional concept that kyanite cannot be used as the main raw material of refractory materials. The difficulties caused by the high-temperature decomposition and expansion problem of kyanite can be overcome by adjusting the particle size, pore structure and firing curve. Using kyanite as the main raw material for producing lightweight mullite refractory bricks can significantly reduce production costs and improve product performance.
