April 29, 2021
Refractories commonly used in cement kilns
1. Alkaline refractories
Alkaline refractories have excellent properties of high temperature calcination and chemical corrosion resistance, which are the key lining materials for cement kilns to achieve high quality, high yield, low consumption and long-term safe operation. However, it also has the main shortcomings of easy to be affected with damp metamorphism, high thermal expansion rate, high thermal conductivity and poor thermal shock resistance. At present, the imported and domestic basic refractories suitable for cement kilns mainly include direct bonded magnesia-chrome brick, semi-direct bonded magnesia-chrome brick, ordinary magnesia-chrome brick, dolomite brick, zirconium-containing and non-zirconium-containing special magnesia-chrome brick, spinel and chemically bonded non-burned magnesia-chrome brick.
With the change of coal instead of oil in clinker calcination, the improvement of energy saving requirements, the improvement of corrosion resistance requirements of new dry cement kiln lining and the demand of reducing or even preventing chromium pollution and strengthening environmental protection, in recent years, the development and utilization of low chromium or even chromium free alkaline refractory for cement kiln has been presented.
Low chromium and chromium free basic refractories. Ordinary magnesia chrome brick containing CR ₂O₃ often up to 8%~10%, directly combined with magnesia chrome brick 10%~16% larger. The favorable effect of adding chromium ore is to improve the thermal shock resistance of brick. However, they are used in cement kilns, especially in the new dry method cement kilns, which are vulnerable to alkali erosion, generating K₂CrO₄ containing 6 valent chromium and so on. The ₄ minerals are released to the environment. That is to say, the residue bricks can contaminate water equivalent to 50,000 to 250,000 times their own weight, seriously damaging the environment and threatening the health of the population. In order to improve the corrosion resistance of alkaline brick and reduce or even eliminate the chromium pollution to the environment, foreign countries have been devoted to the development and utilization of low cost magnesia-chrome brick, special magnesia-chrome brick and new type of dolomite brick and other alkaline refractory materials for many years, and have achieved success. China's chromium ore resources are poor, and remote western Xinjiang and Xizang, has been a large number of imports containing chromium alloy steel manufacturing, so the price soared. In cement kilns, it is more urgent to replace high chromium magnesia-chrome brick with low chromium and no-chromium basic brick and direct bonded magnesia-chrome brick.
Low chrome magnesia chrome brick. Seawater magnesia is the main raw material for foreign direct - bonded magnesia - chrome brick. Brick chrome ore content is more, containing ω (CR ₂O+ FE ₂O₃) up to 16%~25%, chrome ore and magnesite direct combination, combining with containing compound spinel (Mg, Fe)O -(Mg, Fe)O· (Al, CR) ₂O₃ is a "rigid" combination. Its biggest advantage is the ability of high temperature, high flexural strength under 1500 ℃ hot and creep rate is small, thus make the thermal shock resistance of brick, alkali corrosion resistance and oxidation - reduction atmosphere change has quite a sacrifice, especially in open stop more frequently on the kiln and the use of alkali is higher in the original, fuel, had to shorten its service life.
Has come out of a low chromium magnesia chrome brick is to coarse crystal high iron natural magnesia as raw material, coarse crystal magnesia interwoven in magnesite - magnesium ferrite - spinel matrix, (Mg, Fe) O-MgO· FE ₂O₃ combination still has better flexibility and plasticality at 1400℃. Brick with only a small amount of chrome ore particles, containing CR ₂O₃+ FE ₂O₃ only 10%~11%. Therefore, this kind of product in the thermal shock resistance, creep rate and alkali corrosion resistance between the more satisfactory balance, the brick lining due to heat and kiln body ellipticity caused by the internal stress lining ability is stronger, can not withstand very high temperature is its inherent characteristics.
2. High aluminum brick
High alumina brick has higher compressive strength, softening temperature under load and better thermal shock resistance. Because of its low price and high quality, it is widely used in various cement kilns. Various types of phosphate-bonded high-alumina bricks are characterized by high strength (not less than 60MPa) and good thermal shock resistance, but they creep more under high temperature, so it is better to calcine high-alumina bricks when used in vault parts.
In high aluminum brick, the introduction of a small amount of ZRO ₂, using the transformation between ZRO ₂ monoclinic and square type, can lead to the existence of microcracks and improvement of thermal shock resistance. The selection of appropriate grain size distribution makes the brick have higher apparent porosity, but also higher strength, and lower its thermal conductivity and thermal expansion. In the use of cement kiln, a thin layer of glaze film is formed on the brick surface to protect the brick from further alkali erosion.
High-alumina brick suitable for cement kiln mainly includes phosphate bonded high-alumina brick, phosphate bonded high-alumina brick wear-resisting high-alumina brick, spalling high-alumina brick, chemically bonded (special) high-alumina brick, and ordinary high-alumina brick, etc.
3. Series alkali resistant brick
Alkali-resistant brick has excellent alkali corrosion resistance, can react with alkali compounds in kiln material and kiln gas at a certain temperature and quickly form a closed and dense protective glaze layer on the brick surface, which can prevent the continuous infiltration of alkali and the damage of brick "alkali crack". It is one of the indispensable kiln lining materials for cement rotary kiln, especially for new type dry kiln.
The alkali - resistant brick series include ordinary alkali - resistant brick, high strength alkali - resistant brick, alkali - resistant insulation brick and arch - resistant brick.
4. Series refractory castables
Refractory castable has been widely used in cement kiln system, especially in preheater system with complex structure, because of its simple production process, low energy consumption and flexibility in use. The refractory castables suitable for cement kilns mainly include rigid jade castables, high aluminum castables, alkali resistant castables and light castables.
5. Performance of heat insulation material
The commonly used insulation materials for cement kiln systems are insulating bricks, insulating plates and insulating (lightweight) castables. In order to ensure the convenience, good quality and good use effect of brick lining, in addition to the internal quality and uniformity of refractory materials, the quality of brick appearance must be strictly controlled. This is especially important for large kilns.