Abstract : This paper summarizes the development of polycrystalline mullite fibers at home and abroad, focusing on the formation mechanism and characteristics of polycrystalline mullite fibers, indicating that it is on the bell jar kiln.
The application was successful, the energy-saving effect was analyzed in detail, and the improvement suggestions on the structure and installation of the fiber were put forward. Finally, the development prospect of polycrystalline mullite fiber was predicted.
1 Introduction
The choice of high temperature kiln masonry materials 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 of development from a dense heavy type to a light or light weight combination. Among them, ultra-light ceramic fiber is used as the lining of the kiln, which has gone from the experiment to the practical stage. The use of ceramic fibers in industrial kilns, especially in intermittent kilns, is also a major technological change in industrial kilns.
2. Development of polycrystalline mullite fiber
Ceramic fiber is known as a new energy-saving material because of its good thermal insulation performance and easy construction. It is also one of the new technologies and new materials promoted by the state. Due to the nature of the material itself, the energy consumption has been greatly reduced and people have paid attention to it. After ordinary aluminum silicate fiber, high-purity aluminum silicate fiber, high-alumina fiber and high-purity chromium-containing aluminosilicate fiber appeared in succession, in the early 1970s, foreign crystal fiber (polycrystalline alumina fiber and polycrystalline molybdenum) was successfully produced abroad. To the stone fiber, etc.), at present, Zhejiang Oushiman Crystal Fiber Co., Ltd. has successfully developed high-performance HPMF polycrystalline alumina fiber. Polycrystalline mullite fiber (PMF) is a high-temperature refractory fiber composed of mullite (MeIIite) microcrystals. It integrates crystal material and fiber material, and has high thermal stability and fiber material. It has high thermal stability and elasticity and can withstand 1500 high temperature. It is a new type of ultra-light 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. The chemical properties are stable. The masonry of high temperature furnace and thermal equipment can save 5-40% energy.
The development of crystalline ceramic fibers was in the early 1970s, due to the technical development of the chemical, ceramic, metallurgical and aerospace industries required to use fiber materials at higher temperatures. The earliest developed and built-scale factory was the British ICI company, which produced the grade “Saffil†alumina fiber, A1Ot and SiO, with content of 95% and 5% respectively. The main crystal phase was corundum phase, and the output in 1980 was 500 tons. After that, industrialized countries such as the United States, Japan, and the Soviet Union have also developed and built factories of a certain scale. For example, the United Kingdom and the United States jointly organized the Emery Corporation (also translated as SiC), one of the world's largest refractory fiber companies, and launched the "Fiber-Max" polycrystalline mullite fiber, which contains A120372% and Sio227%. The use temperature of 1600-1700oC, and the corresponding development of polycrystalline fiber and amorphous fiber to make a mixed product, has been successfully used in steel, chemical and other ceramic furnaces around 1500oC. This kind of crystal fiber production is very difficult. At present, only the United States, Britain, the Soviet Union and Japan can produce in the world, with an annual output of 1,400 tons.
On September 28, 1986, Zhejiang Oushiman Crystal Fiber Co., Ltd.'s “10-ton polycrystalline mullite fiber pilot project†passed the technical appraisal of Zhejiang Science and Technology Commission. It was adopted in Hangzhou, Zhejiang Province on December 28, 1990. The application of polycrystalline mullite fiber technology and research technology in the Ministry of Metallurgy has become the only company in China that produces polycrystalline mullite fibers. The main technical indicators of the product (including fiber diameter, capacity, high temperature shrinkage), by the Zhejiang University on-site test, equivalent to the "Fibermax" polycrystalline mullite fiber index produced by the US Emery company, has reached the international advanced level in the early 1980s.
3. Main characteristics of polycrystalline mullite fiber
Polycrystalline mullite fiber and common aluminum silicate fiber and alumina fiber belong to the same A1203-Sio2 refractory fiber. These fibers will produce quartz, mullite and corundum crystals above 980oC. Fibers composed of different crystals have different use temperature characteristics, as shown in Table 1.
Under high temperature conditions, the large stress generated by the volume change of cristobalite is enough to cause the fiber structure to be fragmented and powdered, which is the main reason why the aluminum silicate fiber can not withstand the high temperature above 1000oC. The polycrystalline fiber consisting of two crystals of mullite and corundum effectively avoids the defect of the friendly match and raises the use temperature limit to 1450-1500oC.
Table 1 High temperature characteristics of various crystal fibers
Type of crystal | Volume change of crystal phase at high temperature | composition | Fiber type | Al 2 O 3 (%) | Operating temperature (oC) |
Quartz (SiO 2 ) | Alpha quartz→α-square quartz +15.4% (1020oC) | → ↗ → ↘ → | Aluminum silicate fiber | 45-60 | <1100 |
Mullite (3Al 2 O 3 · 2SiO 2 ) | Single mullite phase | Mullite fiber | 72-75 | -1500 | |
Corundum (α- Al 2 O 3 ) | Homogeneous evolution | Alumina fiber | 80-95 | -1400 |
A simple method for comprehensively evaluating the high-temperature performance of fibers is to determine the relationship between the high-temperature heating rate and the heating time of the products, and the change in shrinkage rate is small. The relationship between the heat shrinkage rate of mullite fiber and the heating time is shown in Fig. 1.
The difference between polycrystalline mullite fiber and aluminum silicate fiber is that polycrystalline mullite fiber effectively solves the existence of registered ore-like cristobalite which cannot be solved by aluminum silicate fiber, thereby increasing the use temperature of the fiber. 500oC. The difference between polycrystalline mullite fiber and aluminum silicate fiber is shown in Table 2.
Table 2 Comparison of polycrystalline mullite fiber and aluminum silicate fiber
name | Production method | Mineral phase composition | Purity (Al 2 O 3 + SiO 2 )% | Damage temperature () | Operating temperature() |
Aluminum silicate fiber | High temperature melting method (melt) | Glassy mullite, cristobalite | <98 | 1050-1100 cristobalite precipitation | <1000 |
Polycrystalline mullite fiber | Chemical colloid method (colloid) | Crystalline mullite | >99.5 | >1600 grain growth | <1500 |
There are two series of polycrystalline mullite fibers produced by Zhejiang Oushiman Crystal Fiber Co., Ltd.: PMF-100 series and PMF-300 series. The former is made of polycrystalline mullite fiber cotton blanket, which is cut and compressed, wrapped into a block, and does not contain any binder. The latter is made of polycrystalline mullite fiber cotton by adding a special bonding material, and is formed into a flexible felt, a mat, a ring, or the like by a vacuum forming method, or a shape of a hard board, a cylinder, a block, a box, or the like. See Table 3.
Table 3 Comparison of PMF two series performance
Project | category | PMF-100 two series | PMF-300 series |
characteristic | Thermal shock resistance, light weight, low heat storage, low thermal conductivity, good sound absorption | Light weight, low strength, low heat storage, thermal shock resistance, low thermal conductivity, easy machining | |
performance | Composition: Al 2 O 3 72-75%. SiO 2 25-28% Crystal phase: single mullite phase melting point: 1840oC Use temperature: electric furnace <1500oC. flame furnace <1400oC Thermal conductivity (W/mK): 0.226 (1000oC) . 0.264 (1200oC) . 0.336 (1300oC) . 0.384 (1400oC) Reburning line shrinkage: 1500 × 6h <1.0% Volume weight: 100kg/m Dimensions (length × width × thickness): 200mm × 100mm × (30-50) mm | Operating temperature: 1450oC Bulk density: 200-380kg/m Rewinding shrinkage: 1500 × 1h - 0.22 (1250oC) . 0.25 (1350oC) Size: 300mm or <300mm | |
Be applicable | It is used as a heat lining for various high temperature furnaces. Such as steel, chemical, machinery and other heating furnaces, soaking furnaces, forging furnaces, heat treatment furnaces; ceramics, building materials, electronics and other sintering kiln, firing kiln | High temperature resistance furnace, induction furnace, ladle, glass melting furnace, kiln car, high temperature radiant tube, high temperature vessel, burner, boiler, etc. Special high temperature parts such as military industry, scientific research, aviation, and nuclear melon piles. | |
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