Classification of refractory materials for cement kilns
The main function of refractory materials for cement kilns is to serve as the lining of cement kilns. They should maintain the high temperature required for burning cement clinker, protect kiln equipment and reduce kiln heat dissipation losses. Refractory materials have an important impact on the output, quality, energy consumption and equipment life of cement production. In terms of refractory technology, refractory materials for cement kilns have developed from a few varieties such as clay bricks, high-alumina bricks, and alumina cement blocks to a family of nearly 100 materials. Today, refractory materials for cement kilns include sintered alkaline refractory materials, sintered alumina-silica refractory materials, amorphous refractory materials and prefabricated parts, and thermal insulation refractory products.
In terms of sintered alkaline materials, there are ordinary magnesia-chrome bricks, directly bonded magnesia-chrome bricks, low-chrome magnesia-chrome bricks, periclase-magnesia-alumina spinel bricks (hereinafter referred to as magnesia-alumina spinel bricks), magnesia-alumina spinel zircon bricks, magnesia-alumina spinel zircon lanthanum bricks, magnesia-iron spinel bricks, iron-alumina spinel bricks, chromium-containing magnesia-alumina spinel bricks, magnesia-manganese spinel bricks, dolomite bricks, magnesia-dolomite bricks, dolomite zircon bricks, magnesia-dolomite zircon bricks, periclase calcium zirconate bricks, composite alkaline bricks and other varieties.
In terms of shaped aluminosilicate refractory materials, there are alkali-resistant clay bricks, high-strength alkali-resistant bricks, vault alkali-resistant bricks, alkali-resistant insulation bricks, phosphate-bonded high-alumina bricks, phosphate-bonded wear-resistant bricks, steel fiber-reinforced phosphate-bonded wear-resistant bricks, high-load soft bricks, kyanite bricks, anti-stripping high-alumina bricks, silica-molybdenum bricks, silica-molybdenum red bricks and other special high-alumina bricks.
In terms of shaped thermal insulation refractory materials, there are ordinary calcium silicate thermal insulation products, hard calcium silicate thermal insulation refractory products, refractory fiber products, and various thermal insulation products made of floating beads, ceramsite, diatomite, expanded perlite, hollow balls, other lightweight raw materials and added combustible materials.
In terms of amorphous refractory materials, there are ordinary refractory castables, low cement refractory castables, ultra-low cement refractory castables, cement-free refractory castables, steel fiber refractory castables, anti-explosion castables, anti-scaling castables, phosphate refractory castables, alkali-resistant castables, and thermal insulation castables. In addition, there are various new amorphous refractory materials such as self-flowing materials, pumping materials, and injection materials.
In developed countries, the use of amorphous refractory materials has accounted for 50% of the total refractory materials. To a large extent, many amorphous refractory materials need to achieve their special properties by controlling interface reactions. Taking low-cement castables as an example, cement is the most active substance, and α-Al2O3 powder and silica fume are potentially active substances. The hydration of cement, the reaction between cement and particles, and the composition and structure of castables can be controlled by raw material composition, particle size, dosage and admixtures. The conditions for achieving dispersion-agglomeration transition are: (1) the dissolution rate of admixtures is absolutely dominant; (2) the dosage of curing agent is absolutely dominant; (3) in the early stage of hydration, admixtures control the liquid phase properties, and their active functional groups can be adsorbed on the surface of solid particles, changing the zeta potential and producing a shielding effect, so that the castable has good fluidity; (4) after the admixture is consumed, the curing agent takes control, causing the castable to condense and harden rapidly. Amorphous refractory materials originated from ordinary concrete, and their subsequent development also borrowed a lot from the achievements of modern cement concrete materials.
Configuration of refractory materials for various parts of cement kiln
The development of cement production technology is mainly concentrated in the kiln process of cement clinker calcination. In the rotary kiln used for cement production, the configuration of lining refractory materials for various parts is as follows:
1. Discharge port and discharge belt of the kiln
The lining materials used for the discharge port and discharge belt of the kiln are subject to severe mechanical wear and chemical erosion, so the material's abrasion resistance and temperature change resistance are required to be high. The discharge belt generally uses high-alumina bricks with an Al2O3 content of 70-80%, heat-resistant high-alumina bricks, spinel bricks and magnesia-chrome bricks; the discharge port uses heat-resistant concrete or silicon carbide bricks with corundum as aggregate. Corundum, alumina low-cement refractory castables and steel fiber reinforced refractory castables are also used for the front and rear kiln ports, and the kiln head cover uses 16B steel fiber high-alumina refractory castables.
2. Firing zone
The firing zone has a high temperature, about 1200-1500℃, and generally uses magnesia-chrome bricks, direct-bonded magnesia-chrome bricks, sodium polyphosphate-bonded magnesia bricks, alkali-resistant bricks, spinel bricks, magnesia-zirconium bricks, and silicon-molybdenum bricks. These refractory materials usually have high strength in both cold and hot states, and have good thermal shock stability, and have been increasingly widely used.
Magnesia-chrome bricks: Direct-bonded magnesia-chrome bricks have high resistance to high temperature, SiO2 corrosion, and oxidation-reduction effects, as well as high high temperature strength and mechanical stress resistance, and good kiln skin performance. They are widely used in the firing zone.
When magnesia-chrome bricks are used in cement kilns, under the action of alkali (or sulfur), stable trivalent chromium is converted into hexavalent chromium with strong oxidation ability. The chromium content in the gas exceeds 10mg/m3, and the chromium content in the aqueous solution exceeds 0.5mg/l, which will cause extremely serious harm to the human body. Since the mid-1980s, industrialized countries have formulated a series of environmental protection and health regulations to comprehensively monitor the waste gas dust of cement kilns, the residual bricks of magnesia-chrome bricks and the drainage of cement plants. The use of magnesia-chrome bricks is subject to certain restrictions.
Spinel bricks: spinel bricks that appeared in the 1990s not only have strong kiln skin resistance, but also have a series of advantages in terms of resistance to alkali, sulfur melt and clinker liquid phase erosion, resistance to thermal shock and mechanical stress caused by kiln body deformation, and resistance to heat load. The performance is better than that of magnesia-chrome bricks and has become the mainstream of the development of alkaline brick technology in the world today.
Magnesia-zirconium bricks: The biggest feature of zirconium oxide is that microcracks are formed around the particles, thereby absorbing external stress, and have high fracture resistance under hot and cold conditions. In a series of comparative tests with spinel bricks, magnesia-zirconium bricks have obvious advantages in terms of corrosion resistance to harmful substances such as SO3, CO2, and alkali vapor, corrosion resistance to clinker liquid phase, the influence of redox atmosphere on it, and compressive strength. However, magnesia-zirconium bricks need to add a large amount of scarce zirconium oxide, which is expensive and the source of raw materials is not guaranteed.
3. Transition zone
The transition zone is adjacent to the firing zone. The environmental characteristics are high cylinder temperature and frequent temperature changes, kiln skin hanging and falling, and severe chemical erosion. Commonly used refractory materials include high-alumina bricks made of corundum and bauxite (50-80% AI203), directly bonded magnesia-chrome bricks, ordinary magnesia-chrome bricks, and spinel bricks. In recent years, silica-molybdenum bricks have been widely used in cement kiln transition zones. The bricks have high refractoriness point, high strength, high adhesion and abrasion, low thermal conductivity and good anti-stripping performance. They have strong anti-permeability and corrosion resistance to kiln materials, coal melts, and volatile components mainly composed of sulfate alkali and alkali oxide, and are better than alkaline bricks. At the same time, the thermal shock stability of this brick is better than that of alkaline bricks, and the structural strength is much higher than that of alkaline bricks. It has strong resistance to mechanical stress, thermal stress, chemical reaction, overheating, thermal fatigue and other comprehensive damage.
4. Cooling zone
The temperature of the cooling zone is still relatively high (about 1100-1300℃), but the chemical erosion is not as serious as the previous zone. Generally, high-alumina bricks, magnesia-chrome bricks, phosphate-bonded high-alumina bricks, magnesia-alumina spinel bricks and corundum high-strength low-cement refractory castables are selected.
5. Decomposition zone
In the part where the decomposition zone is connected to the preheating zone, the thermal stress and chemical stress are small, and clay bricks, high-alumina bricks, ordinary magnesia-chrome bricks, etc. can be selected; in the area where the decomposition zone is connected to the wave-passing zone, the wear resistance and high temperature resistance of the material are required to be high. High-alumina bricks with an Al2O3 content of 50-60%, ordinary magnesia-chrome bricks, spinel bricks, special high-alumina bricks and anti-stripping high-alumina bricks can be used, as well as calcium silicate boards and series of high-strength insulation bricks, high-alumina high-strength low-cement refractory castables, and 50S anti-scaling castables as insulation materials.
6. Preheating zone
The lining of the preheating zone needs to have sufficient alkali resistance and thermal insulation performance. Alkali-resistant and thermally insulating clay bricks are mainly used in industrial applications. If lightweight bricks are used, the kiln shell temperature can be reduced by 60-100℃ compared with clay bricks of the same thickness. For dry kilns, the unit heat consumption can be reduced by 21-38kJ/kg clinker.
7. Preheater system
The preheater system needs to use lining materials with good alkali resistance and thermal insulation performance. Such as a series of alkali-resistant bricks and alkali-resistant castables. In the straight cylinder, cone part and connecting pipes of the preheater and decomposition furnace, alkali-resistant clay bricks are mainly used, and a thermal insulation composite layer is added. Fire mud masonry: the top cover can be hung with fire bricks, backed with mineral wool or concrete casting; castables are mostly used in various elbows; semi-silica clay bricks with dense structure are used in the rising pipes at the kiln tail to prevent alkali erosion.
Alkali-resistant castable: Under alkali erosion, a glaze layer appears on the surface of the alkali-resistant castable after use. The substance generated in the glaze layer is kAs2, which is surrounded by a glass phase matrix. At a lower temperature, a liquid phase appears on the surface of the castable. These viscous liquid phases can block the surface gaps and prevent alkali from penetrating into the interior of the refractory material.
8. Cooler system
The temperature of the material inside the cooler changes the most, and the degree of erosion of the refractory material is also uneven, especially in the cooler shrinkage and the area between the air inlet of the tertiary air duct and the cooling shrinkage. In addition, due to the accumulation restriction of dust and the expansion of the masonry itself, the side wall will also be damaged as a whole. The refractory materials used in the cooler system include mullite high-strength wear-resistant castables, refractory bricks, lightweight castables, insulation bricks, insulation panels, etc. Ordinary magnesia-chrome bricks and high-alumina bricks can be used in the throat area and high-temperature area of the material discharge; clay bricks can be used in the medium and low temperature areas.
For some large rotary kilns, the heat load on the lower part of the kiln head hood is relatively high. If the general high-alumina refractory castable is not well controlled during maintenance and temperature rise, it is easy to cause cracking blocks; the top of the kiln head hood is close to the tertiary air duct, and the dust-containing airflow scouring is relatively serious, and the construction of the top castable is relatively difficult, and the fluidity and early strength requirements of the material are relatively high.
9. Other parts
Other important equipment of cement kiln needs to be lined with refractory materials. The elbows and air valves of tertiary air ducts have large changes in temperature and are eroded by high-temperature clinker particles. The castables are prone to loosening and peeling. They are the most prone to wear during cement plant operation, and wear-resistant castables are usually used. The C4 and C5 cones and their feed pipes, decomposition furnace cones and smoke chambers are very easy to crust and are difficult to clean. Cleaning requires manual iron tools, which inevitably causes mechanical damage to the refractory castables. In addition, if severe crusting occurs, the kiln needs to be stopped for treatment. High-strength anti-scaling silicon carbide castables are required, with a maximum operating temperature of 1400℃, a bulk density of ≥2.50g/cm3 after baking at 110℃, a silicon carbide content of ≥55%, a flexural strength of ≥11MPa at 110℃×24h, and a compressive strength of ≥80MPa at 110℃×24h. In addition, C1-C3 should use high-strength alkali-resistant castables, C4 and C5 (except the cone part) use high-temperature high-strength alkali-resistant castables, the maximum use temperature is 1400℃, the volume density after baking at 110℃ is ≥2.20g/cm3, the alumina content is ≥45%, the flexural strength at 110℃×24h is ≥8MPa, and the compressive strength at 110℃×24h is ≥80MPa. The decomposition furnace uses high-strength wear-resistant castables, the maximum use temperature is 1600℃, the volume density after baking at 110℃ is ≥2.70g/cm3, the alumina content is ≥80%, the flexural strength at 110℃×24h is ≥10MPa, and the compressive strength at 110℃×24h is ≥100MPa.
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