Special refractories mainly include high melting point oxides, high melting point non-oxides, and the derived composite compounds, cermet, high-temperature coating, high-temperature fiber, and its reinforcing materials. Among them, non-oxides with a high melting point are usually called refractory compounds, which also include carbides, nitrides, borides, silicides, and sulfides.
Although the cost of special refractories is relatively high, they are indispensable and irreplaceable products in many industrial sectors due to their excellent properties. Special refractories replace other products in many key parts in many new technologies and fields, which can greatly improve service life and significantly increase economic benefits. The classification of special refractories can be roughly divided into the following five aspects according to the different properties of raw materials and products.
Corundum brick with high purity, high density, and high performance has been widely used in the large blast furnace and the ladle of the steel plant and achieved good results. It is the best lining material for the second stage furnace and gasifier of large ammonia synthesis plant (300,000t) and the cracking furnace of 300,000t ethylene project.
Besides, the corundum brick with high thermal stability is the preferred lining material for various ultra-high temperature furnaces such as special refractories and products firing furnace, molybdenum wire furnace, diffusion furnace, ceramic metallization furnace, molybdenum silicide electric furnace and so on.
Alumina hollow ball products and alumina fiber products belong to high-temperature energy-saving products, which are ideal lining material for energy saving and consumption reduction because of their small volume density, low thermal conductivity. Alumina precision ceramics can be made into crucible and gasket, high-temperature furnace tube, thermocouple protection tube and insulating porcelain tube for smelting or purifying non-ferrous metals and rare and precious metals, etc.
Magnesium oxide (magnesia)
Magnesia is an alkaline refractory with high load softening temperature and low creep. While it’s melting point can reach 2800 ℃, it is easy to volatilize in reducing atmosphere, so the maximum service temperature can only be 2000 ℃.
Magnesium oxide is a good structural material for ultra-high temperature furnace, as well as a good lining material for industrial frequency furnace, intermediate frequency furnace, and high-frequency heating furnace. Magnesium oxide ceramic products can be made into non-ferrous, rare metals and precious metals smelting, purification of magnesium oxide crucible, high-temperature furnace tube, ultra-high temperature tungsten-rhenium thermocouple protection tube, insulation porcelain beads.
Due to its high-temperature chemical stability, the maximum service temperature of zirconia can reach 2400 degrees, which can be used as vacuum furnace, molybdenum wire furnace, gas furnace, medium and high-frequency electric furnace, single crystal furnace and other high-temperature lining materials.
Zirconia hollow ball and zirconia fiber products are the most advanced heat preservation and energy saving lining materials in the oxide. The high-temperature zirconia ceramics can be made into crucibles for smelting nonferrous and rare metals, as well as high-temperature furnace tubes and thermocouple protection tubes. Besides, zirconia special refractories can also be made into high-temperature electric furnace heating body, thermistor, gas sensor, and spray coating and castable refractory and other amorphous.
Other special oxide refractories mainly include beryllium oxide (BeO), calcium oxide (CaO), and silicon oxide (SiO2).
Beryllium oxide is a kind of alkaline special refractories, especially those with high thermal conductivity, good thermal shock resistance, and low electrical conductivity. It has good nuclear properties, such as strong neutron deceleration ability and X-ray penetration, but it is restricted in production and application due to its great toxicity.
Beryllium oxide can be used to make containers for smelting rare metals and high purity metals Be, Pt and V, neutron moderator and anti-radiation material for atomic reactors, and high frequency, insulation, and heat dissipation devices for the electronics industry.
Calcium oxide is also alkaline special refractories. Although it has a high melting point, it is easy to hydrate and produce pulverization, which is greatly restricted in application. The products of calcium oxide can be used as special refractories for melting platinum, ultrapure uranium and nonferrous metals crucible and jet metallurgy.
Silicon oxide has an extremely low linear expansion coefficient and excellent thermal shock resistance. Fused quartz products can be used as the liner for conveying pipe, valve and pump in nonferrous metal smelting.
In the chemical industry, quartz is used as the lining material for acid-resistant and corrosion-resistant containers and reactors, while it is used as the melting pool brick, arch flow ring, plunger and hot repair material in the glass industry.
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