Application of Polycrystalline Mullite Fiber in Bell Kiln

Application of Polycrystalline Mullite Fiber in Bell Kiln
1. Introduction
The selection of masonry materials for high temperature kiln is one of the important factors affecting the thermal efficiency of the kiln. In order to improve the thermal efficiency of the kiln, there is a trend from dense heavy type to light or light combination type in the world. Among them, the use of ultra-lightweight ceramic fiber as the kiln lining has gone from the experiment to the practical stage. The use of ceramic fibers in industrial kilns, especially intermittent kilns, is also a major technological reform in industrial kilns.

2. Development of polycrystalline mullite fiber
Ceramic fiber is known as a new energy-saving material for its good heat insulation performance and easy construction, and it is also one of the new technologies and new materials that the country focuses on. Because of the characteristics of the material itself, it greatly reduces the energy consumption and attracts people's attention. After ordinary aluminum silicate fiber, high purity aluminum silicate fiber, high aluminum fiber and high purity aluminum silicate fiber containing chromium appeared one after another, in the early 1970 s, foreign countries successfully trial-produced crystal fiber (polycrystalline alumina fiber and polycrystalline mullite fiber, etc.). Polycrystalline mullite fiber (PMF) is a kind of high temperature refractory fiber composed of mullite (MeIIite) micro crystal, which integrates crystal material and fiber material, has high thermal stability and fiber material in one, has high thermal stability and elasticity, can withstand 1500 and high temperature, is a new type of ultra light heat insulation material. The bulk density of fiber products is 1/25 of that of refractory bricks, the thermal conductivity is 1/6 of that of refractory bricks, and the chemical performance is stable. The masonry of high-temperature kilns and thermal equipment can save energy by 5-40%.

The development of crystalline ceramic fibers was developed in the early 1970 s. Due to the technological development of the chemical, ceramic, metallurgical and aerospace industries, fiber materials can be used at higher temperatures. The first company to develop and build a large-scale factory was the British I .C. I Company, which produced the brand "Saffil" alumina fiber, A1Ot and SiO, with contents of 95% and 5% respectively. The main crystal phase was corundum phase, with an output of 500 tons in 1980. After that, the United States, Japan, the Soviet Union and other industrialized countries have also developed and built a certain scale of factories. For example, Britain and the United States jointly organized emery company (also translated as silicon carbide company), one of the largest refractory fiber companies in the world, to introduce polycrystalline mullite fiber with the brand "Fiber-Max", which contains A120372 and Sio227% and has a service temperature of 1600-1700oC. Accordingly, they have developed mixed products made of polycrystalline fiber and amorphous fiber, which have been successfully used in ceramic kilns of about 1500oC such as steel and chemical industry. The production of this kind of crystal fiber is difficult. At present, only the United States, Britain, the Soviet Union and Japan can produce it in the world, with an annual output of 1400 tons.

3. Main characteristics of polycrystalline mullite fiber
Polycrystalline mullite fiber and ordinary aluminum silicate fiber, alumina fiber are A1203-Sio2 refractory fiber, these fibers in 980oC above will produce quartz, mullite, corundum three crystals. Fibers composed of different crystals have different service temperature characteristics, see Table 1.

Under high temperature conditions, the huge stress caused by the volume change of cristobalite is enough to break and pulverize the fiber structure, which is the main reason why the aluminum silicate fiber cannot withstand the high temperature above 1000oC. Polycrystalline fibers composed of mullite and corundum crystals effectively avoid the defects of friendly matches on the job and increase the use temperature limit to 1450-1500oC.