1. Silica Bricks
Silica bricks are acidic refractory materials with good resistance to acid erosion. They have good thermal conductivity and high load softening temperature, generally above 1620℃, which is only 70~80℃ lower than their refractoriness. The thermal conductivity of silica bricks increases with the increase of working temperature, and there is no residual shrinkage. During the baking process, the volume of silica bricks increases with the increase of temperature. Therefore, silica bricks are ideal refractory products for coke ovens. Important parts of modern large and medium-sized coke ovens (such as combustion chambers, ramps and regenerators) are built with silica bricks.
During the baking process, the maximum expansion of silica bricks occurs between 100~300℃, and the expansion before 300℃ is about 70%~75% of the total expansion. The reason is that SiO2 has four crystal transformation points of 117℃, 163℃, 180~270℃ and 573℃ during the baking process. Among them, the volume expansion caused by quartz is the largest between 180~270℃.
The key to determining the thermal stability of silica bricks is the true density, which is one of the important indicators for determining its quartz transformation. The smaller the true density of silica bricks, the more complete the lime transformation, and the smaller the residual expansion generated during the furnace baking process.
Among silica bricks, tridymite crystals have the smallest true density, small linear expansion rate, better thermal stability than cristobalite and quartz, strong resistance to slag erosion, good thermal conductivity, high load softening temperature, and are the most stable form of quartz. In well-fired silica bricks, tridymite has the highest content, accounting for 50%~80%; cristobalite is second, accounting for only 10%~30%; and the content of quartz and glass phase fluctuates between 5%~15%.
When the working temperature is lower than 600~700℃, the volume of silica bricks changes greatly, the resistance to rapid cooling and heating is poor, and the thermal stability is not good. If the coke oven works at this temperature for a long time, the masonry will easily crack and break.
2. Clay bricks
Clay bricks refer to clay products made of aluminum silicate materials with an Al2O3 content of 30%~40%. Clay bricks are made of 50% soft clay and 50% hard clay clinker, which are mixed according to certain particle size requirements, formed and dried, and fired at a high temperature of 1300~1400℃. The mineral composition of clay bricks is mainly kaolinite (Al2O3・2SiO2・2H2O) and 6%~7% impurities (potassium, sodium, calcium, titanium, and iron oxides). The firing process of clay bricks is mainly the process of kaolinite continuously dehydrating and decomposing to form mullite (3Al2O3・2SiO2) crystals. During the firing process, SiO2 and Al2O3 in clay bricks form eutectic low-melting silicates with impurities, surrounding the mullite crystals.
Clay bricks are weakly acidic refractory products that can resist the erosion of acidic slag and acidic gases, but have a slightly poor resistance to alkaline substances. Clay bricks have good thermal properties and are resistant to rapid cooling and heating.
The refractoriness of clay bricks is comparable to that of silica bricks, up to 1690~1730℃, but the load softening temperature is more than 200℃ lower than that of silica bricks. Because clay bricks contain not only highly refractory mullite crystals, but also nearly half of the low-melting-point amorphous glass phase.
In the temperature range of 0~1000℃, the volume of clay bricks expands evenly with the increase of temperature, and the linear expansion curve is close to a straight line. The linear expansion rate is 0.6%~0.7%, which is only about half of that of silica bricks. When the temperature reaches 1200℃ and continues to rise, its volume will begin to shrink from the maximum expansion value. The residual shrinkage of clay bricks leads to the loosening of masonry mortar joints, which is a major disadvantage of clay bricks. When the temperature exceeds 1200℃, the low-melting-point substances in clay bricks gradually melt, and the particles are close to each other due to surface tension, resulting in volume shrinkage.
Since clay bricks have a low refractoriness under load, they shrink at high temperatures, and their thermal conductivity is 15% to 20% lower than that of silica bricks. Their mechanical strength is also worse than that of silica bricks. Therefore, clay bricks can only be used in minor parts of coke ovens, such as heat storage chamber sealing walls, small flue lining bricks and heat storage chamber checker bricks, furnace door lining bricks, furnace roofs, and riser lining bricks.
3. High-alumina bricks
High-alumina bricks are refractory products made of aluminum silicate or alumina with an Al2O3 content greater than 48%, collectively referred to as high-alumina refractory products.
The refractoriness and refractoriness under load of high-alumina bricks are higher than those of clay bricks, and their slag corrosion resistance (especially for acidic slag) is better, and these properties increase with the increase of Al2O3 content, but their thermal stability is not as good as that of clay bricks. High-alumina bricks have high density, low porosity, high mechanical strength and wear resistance. The furnace head of the coke oven combustion chamber and the furnace head of the carbonization chamber are built with high-alumina bricks, which has a better effect; but it is not suitable for the wall of the carbonization chamber because high-alumina bricks are prone to curling and warping at high temperatures.
4. Refractory mud
Refractory mud is an amorphous refractory material composed of powdered materials and binders for preparing mud. It is mainly used as a binder and coating material for masonry of refractory bricks. Refractory mud is mostly mixed with water (or aqueous solution) to make mud. It should have the corresponding properties of masonry bricks. Refractory mud for coke ovens should meet the following requirements:
(1) It should have the necessary adhesion after construction and during use to ensure that it is integrated with the masonry or surrounding layers, so that it should have the function of resisting external forces and gas and slag erosion.
(2) It must have good fluidity and plasticity to facilitate construction.
(3) It must have the same chemical composition as the masonry or surrounding layer material to avoid the refractory mud from being destroyed first and to avoid adverse chemical reactions between different materials.
(4) It should have the same thermal expansion as the masonry or surrounding layer material to avoid separation and cracking of the mud layer.
(5) The volume should be stable and have a small shrinkage to ensure the integrity and tightness of the masonry.
(6) It should be able to sinter at the operating temperature to increase the mechanical strength of the masonry.
(7) It should have a certain refractoriness and load softening point.
The corresponding refractory mud should be selected according to the brick type and operating temperature, that is, clay fire mud should be used for masonry of clay bricks, and silica fire mud should be used for masonry of silica bricks. All masonry parts that come into contact with metal embedded parts must be added with concentrate powder in the fire mud. When laying coke oven roof bricks, hydraulic binders that can increase strength - silicate cement and quartz sand should be added to the clay fire mud.
Silica fire mud is a powder made of silica, waste silica bricks and refractory clay (raw clay). Silica is the main component of silica fire mud. The higher the SiO2 content in silica, the higher the refractoriness of the fire mud. Adding waste silica bricks can improve the high-temperature bonding performance of fire mud and silica bricks. The reason is that silica brick powder has a thermal expansion curve similar to that of silica bricks. When the volume changes during the quartz crystal transformation, the fire mud is less likely to detach from the silica bricks and has good adhesion to the silica bricks. Generally, the silica brick powder content is 20%~30%, which is more appropriate. Adding raw clay to silica fire mud can increase plasticity, reduce air permeability and water loss rate, but the amount added should not be too large, otherwise the refractoriness of silica fire mud will be reduced and the shrinkage rate will increase. Generally, it is appropriate not to exceed 15%~20%.
The requirements for particle size are: no more than 3% for particles above 1mm, and no less than 50% for particles less than 0.074mm. The particle composition of silica fire mud affects the performance of fire mud. If the particles are too large, the mud will lose water quickly, bricklaying operation will be difficult, and precipitation and segregation will easily occur in the ash trough; if the particles are too small, the mud will ferment easily and the density of the ash joint will deteriorate. Generally, after the good mortar is hit on the brick, the brick can be kneaded and knocked at will for about 15-20 seconds. This time is related to the particle composition. Therefore, the performance of silicon fire clay can be expressed by this time.
Clay fire clay is made of clinker or crushed clay bricks in the calcination process plus refractory clay (raw clay). Clinker is the main component of clay fire clay, accounting for about 75%~80%. Raw clay is a binder. Adding raw clay can increase plasticity, reduce air permeability and water loss rate, but increase shrinkage. Too much raw clay is prone to cracks, so the ingredients account for about 20%~25%.
The use temperature of clay fire clay is generally below 1000℃. Clay fire clay for coke ovens is generally fine and medium particle size, and the percentage of particles passing through 0.5mm and 1mm sieves should be greater than 97%. In addition to being used for masonry of clay bricks, clay fire clay is also used to repair coke ovens.
5. Heat-resistant concrete
Heat-resistant concrete is a special type of concrete that can withstand high temperatures for a long time. It is a clay material made of refractory aggregate, cementitious material (sometimes with mineral admixtures or organic admixtures) and water in a certain proportion. It is a refractory product with a certain strength obtained by ramming or vibration molding, hardening, curing and drying.
Alumina, waste refractory bricks, blast furnace slag, etc. are usually used as aggregates, and alumina cement, silicate cement, phosphoric acid and water glass are used as cementitious materials. According to the different aggregate materials and cementitious materials, heat-resistant concrete is divided into many types. Its composition is different, its properties are different, and therefore its scope of use is also different. Compared with refractory bricks, this refractory product has the following advantages:
(1) It quickly generates strength at room temperature and does not decrease at operating temperature.
(2) It does not need to be fired before use, which reduces the complex process of manufacturing refractory bricks. The preparation process is simple and can be cast in various shapes on site. It can reduce masonry brick joints, simplify the structure, and simplify the brick type, thereby innovating masonry operations and speeding up construction.
Heat-resistant concrete has been used in coke ovens for many years, mainly as lining bricks for risers and furnace doors, and roof rail sleeper bricks to replace clay bricks. It is also used as the paving of coke oven roofs. Its ingredients vary according to the different parts of use.
Although the trial time of heat-resistant concrete in coke ovens is not long, it has shown some advantages, but there are also some disadvantages, such as the load softening temperature is not high enough, and there is stratification and peeling during use.
VI. Thermal insulation materials
Building materials with a thermal conductivity coefficient of less than 0.8kJ/m.・h・℃ are usually called thermal insulation materials. Generally, it has the characteristics of large porosity, small pores, low mechanical strength, and low bulk density. There are many ways to classify thermal insulation materials. Generally, they can be classified according to the use temperature, bulk density and manufacturing method, but they are often classified according to the use temperature and bulk density.
Thermal insulation materials are divided into three types according to bulk density:
(1) Low-temperature thermal insulation materials: The use temperature is lower than 900℃, such as diatomaceous earth, asbestos, slag, slag wool, vermiculite, perlite, etc.
(2) Medium temperature insulation materials: The use temperature is 900~1200℃, such as diatomite bricks, lightweight clay bricks, etc.
(3) High temperature insulation materials: The use temperature is higher than 1200℃, such as high-alumina lightweight insulation bricks, floating bead bricks, lightweight silica bricks, etc.
Lightweight clay bricks are clay bricks made of clay as raw material, with a certain proportion (30%~35%) of wood chips added and fired. There are many brands, with a volume density of 0.4~1.3g/cm3 and a refractoriness of 1670~1710℃.
Diatomite bricks are products made of diatomite as raw material, to which a certain amount of combustibles can be added to increase the porosity of the product and improve the insulation capacity. Diatomite bricks can only be used in areas below 1000℃. When the temperature is too high, they will shrink and melt. Diatomite bricks can also be divided into several levels according to physical and chemical indicators. Their volume density is 0.5~0.7g/cm3, the refractoriness is 1280℃, the apparent porosity is 73%~78%, and the compressive strength is 0.5~1.1Mpa. The product size is 250mm×123mm×65mm and 230mm×113mm×65mm.
Diatomite is divided into raw material and clinker. The former is used for bricklaying and insulation layer plastering, and the latter is used as insulation layer filler.
Asbestos rope is made of asbestos yarn, thread (or metal wire), and is divided into asbestos twisted rope, asbestos braided rope and asbestos square rope according to shape and weaving method. Asbestos board is a board made of asbestos and bonding material.
Other insulation materials, such as slag wool, vermiculite (hydrous biotite) and perlite, are materials containing many fine pores. The more and smaller the pores, the lower the thermal conductivity. Various lightweight heat-resistant concretes can be made using perlite, vermiculite, and lightweight clay bricks as aggregates, cement, water glass, phosphoric acid, etc. as gelling materials, and clay powder and ceramsite powder as admixtures for use as thermal insulation materials.
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