6 types of refractory materials commonly used in coke ovens

1. Silica brick

Silica brick is an acidic refractory material with good acid corrosion resistance. It has good thermal conductivity and high load softening temperature, generally above 1620°C, which is only 70~80°C lower than its refractoriness. The thermal conductivity of silica bricks increases with the increase of operating temperature, and there is no residual shrinkage. During the oven process, the volume of silica bricks increases with the increase of temperature. Therefore, silica bricks are an ideal refractory product for coke ovens. Important parts of modern large and medium-sized coke ovens (such as combustion chambers, ramps and regenerators) are all built with silica bricks.

During the oven process, the maximum expansion of silica bricks occurs between 100 and 300°C, and the expansion before 300°C is about 70% to 75% of the total expansion. The reason is that SiO2 has four crystalline transformation points of 117°C, 163°C, 180~270°C and 573°C during the oven process. Among them, between 180°C and 270°C, the volume expansion caused by cristobalite is the largest.

The key to determining the thermal stability of silica bricks is true density. The true density is one of the important indicators for determining its quartz transformation. The smaller the true density of silica bricks, the more complete the lime conversion, and the smaller the residual expansion produced during the oven process.

Among silica bricks, tridymite crystal has the smallest true density, small linear expansion rate, better thermal stability than cristobalite and quartz, strong slag erosion resistance, good thermal conductivity, high load softening temperature, and is the most stable form of quartz in volume. . Among well-fired silica bricks, tridymite has the highest content, accounting for 50% to 80%; cristobalite follows, accounting for only 10% to 30%; and the content of quartz and glass phases fluctuates between 5% and 15%.

When the working temperature is lower than 600~700℃, the volume of silica bricks changes greatly, the resistance to rapid cooling and rapid heating is poor, and the thermal stability is also poor. If the coke oven operates at this temperature for a long time, the masonry will be easily cracked and damaged.

2. Clay bricks

Clay bricks refer to clay products made of aluminum silicate material with an Al2O3 content of 30% to 40%. Clay bricks are made of 50% soft clay and 50% hard clay clinker, which are mixed according to certain particle size requirements. After being shaped and dried, they are fired at a high temperature of 1300~1400°C. The mineral composition of clay bricks is mainly kaolinite (Al2O3?2SiO2?2H2O) and 6% to 7% impurities (potassium, sodium, calcium, titanium, iron oxides). The firing process of clay bricks is mainly a process in which kaolinite continuously loses water and decomposes to form mullite (3Al2O3?2SiO2) crystals. SiO2 and Al2O3 in clay bricks form eutectic low melting point silicates with impurities during the firing process, surrounding the mullite crystals.

Clay bricks are weakly acidic refractory products that can resist the erosion of acidic slag and acidic gases, but have slightly less 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 in addition to high refractoriness mullite crystals, clay bricks also contain nearly half of the low melting point amorphous glass phase.

In the temperature range of 0~1000°C, the volume of clay bricks expands uniformly as the temperature increases. The linear expansion curve is approximately a straight line, and the linear expansion rate is 0.6%~0.7%, which is only about half of that of silica bricks. When the temperature reaches 1200°C 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°C, the low-melting point substances in the clay bricks gradually melt, and the particles are brought very close to each other due to surface tension, resulting in volume shrinkage.

Since clay bricks have a low softening temperature under load and shrink at high temperatures, their thermal conductivity is 15% to 20% smaller than that of silica bricks, and their mechanical strength is also worse than that of silica bricks. Therefore, clay bricks can only be used in secondary parts of coke ovens. Such as regenerator wall sealing, small flue lining bricks and regenerator checker bricks, furnace door lining bricks, furnace roof and riser pipe lining bricks, etc.

3. High alumina bricks

High alumina bricks are aluminum silicate or alumina refractory products with an Al2O3 content greater than 48%, collectively referred to as high alumina refractory products.

The refractoriness and load softening temperature of high-alumina bricks are higher than those of clay bricks, and their slag corrosion resistance (especially against acidic slag) is better. These properties increase with the increase of Al2O3 content, but their thermal stability is not as good as clay bricks. High alumina bricks have high density, low porosity, high mechanical strength and wear resistance. The burner head of the coke oven combustion chamber and the bottom brick of the carbonization chamber are built with high alumina bricks, which has better results; however, it is not suitable to be used on the walls of the carbonization chamber because high alumina bricks are prone to curling and warping at high temperatures. .

4. Refractory motar

Refractory motar is an amorphous refractory material composed of powdery materials and binders used to prepare motar. It is mainly used as a binder and coating material for refractory brick masonry. Refractory mud is mostly made into slurry by adding water (or aqueous solution). It should have corresponding masonry brick properties, and 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 can resist external forces and gas and slag erosion.

(2) It must have good fluidity and plasticity to facilitate construction.

(3) Have the same chemical composition as the masonry or surrounding layer material to prevent the refractory mud from being destroyed first and avoid adverse chemical reactions between different materials.

(4) Have the same thermal expansion as the masonry or surrounding layer material to avoid mutual separation and rupture of the mud layer.

(5) The volume must be stable and have less shrinkage to ensure the integrity and tightness of the masonry.

(6) Sintering can occur at the service temperature to increase the mechanical strength of the masonry.

(7) It has a certain refractoriness and load softening point.

The corresponding refractory mud should be selected according to the type of brick and operating temperature, that is, clay refractory mud should be used when laying clay bricks, and silicon refractory mud should be used when laying silica bricks. For all masonry parts in contact with metal embedded parts, concentrate powder must be added to the fire clay. When building coke oven top bricks, Portland cement and quartz sand, a hydraulic cement that can increase strength, should be added to the clay fire motar.

Silicon refractory clay is a powder prepared from silica, waste silica bricks and refractory clay (raw clay). Silica is the main component of silicone fire clay. The higher the SiO2 content in the silica, the higher the refractoriness of the fire clay. Adding waste silica bricks can improve the high-temperature bonding performance of fire motar and silica bricks. The reason is that silica brick powder has a similar thermal expansion curve to that of silica bricks. When the quartz crystal form changes in volume, the fire motar is less likely to separate from the silica bricks. Good adhesion to silica bricks. Generally, the silica brick powder content is 20% to 30%. Adding raw clay to silicone fire clay can increase plasticity, reduce air permeability and water loss rate, but the amount added should not be too large, otherwise the refractoriness of silicone fire clay will be reduced and the shrinkage rate will increase. Generally, it should not exceed 15% to 20 % is appropriate.

The requirements for particle size are: no more than 3% for particles above 1mm, and no less than 50% for particles smaller than 0.074mm. The particle composition of silicon fire mud affects the performance of the fire mud. If the particles are too large, the motar will lose water quickly, making the bricklaying operation difficult, and sedimentation and segregation will easily occur in the ash tank; if the particles are too small, the mud will easily ferment, and the mortar joints will be tight. get worse. Generally, after the mortar is applied to the bricks, the bricks can be easily kneaded and beaten for about 15-20 seconds. This time is related to the composition of the particles. Therefore, the performance of silicon fire clay can be expressed by this time.

Fireclay motar is made from clinker or crushed clay bricks from the calcination process plus refractory clay (raw clay). Clinker is the main component of clay fire clay, accounting for about 75% to 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 will easily cause cracks, so the ingredients account for about 20% to 25%.

The use temperature of clay fire clay is generally below 1000℃. Clay fire mud for coke ovens is generally fine-grained and medium-grained, and the percentage of particles passing through 0.5mm and 1mm sieves should be greater than 97%. Fireclay motar is used not only for building clay bricks, but also for repairing coke ovens.

5. Heat-resistant concrete

Heat-resistant concrete is a special concrete that can withstand high temperatures for a long time. It is a mud made of refractory aggregates, cementitious materials (sometimes with mineral admixtures or organic admixtures) and water in a certain proportion. Refractory products with a certain strength obtained by pounding or vibrating into shape, hardening, curing and drying.

Bauxite, waste refractory bricks, blast furnace slag, etc. are usually used as aggregates, and alumina cement, Portland cement, phosphoric acid, and water glass are used as cementing materials. Heat-resistant concrete is divided into many types according to the different aggregate materials and cementitious materials. Their compositions are different, their properties are different, and therefore their scope of use is also different. This kind of refractory product has the following advantages compared with refractory bricks:

(1) Strength develops rapidly at room temperature and does not decrease at operating temperature.

(2) There is no need to go through firing before use, which reduces the complex process of manufacturing refractory bricks. The preparation process is simple and can be cast into various shapes on-site. It can reduce the joints of masonry bricks, simplify the structure, and simplify the brick type, thereby innovating the masonry bricks. construction work to speed up construction.

Heat-resistant concrete has been used in coke ovens for many years. It is mainly used as riser pipe and furnace door lining bricks, furnace roof track sleeper bricks to replace clay bricks, and is also used as coke oven roof paving. The ingredients vary depending on the parts used.

Although the trial time of heat-resistant concrete in coke ovens is not long, it has shown some advantages, but there are also some shortcomings, such as the softening temperature under load is not high enough, and there is delamination and spalling during use.

6. Insulating materials

Generally, building materials with thermal conductivity less than 0.8kJ/m.·h·℃ are called thermal insulation materials. Generally, it has the characteristics of large porosity, small pores, low mechanical strength, and low volume density. There are many ways to classify thermal insulation materials. They can generally be classified according to use temperature, volume density and manufacturing method, but they are often classified according to use temperature and volume density.

Insulation materials are divided into 3 types according to volume density:

(1) Low-temperature insulation materials: used with temperatures below 900°C, such as diatomite, asbestos, slag, slag wool, vermiculite, perlite, etc.

(2) Medium temperature insulation materials: The use temperature is 900~1200℃, such as diatomaceous earth bricks, lightweight clay bricks, etc.

(3) High-temperature insulation materials: The use temperature is higher than 1200°C, such as high-aluminum lightweight insulation bricks, floating bead bricks, lightweight silica bricks, etc.

Lightweight clay bricks are clay bricks made from clay and added with a certain proportion (30%~35%) of sawdust. They are available in a variety of grades, with a bulk density of 0.4~1.3g/cm3 and a refractory degree of 1670 ~1710℃.

Diatomite bricks are products made of diatomite as raw material, into which a certain amount of combustibles can be added to increase the porosity of the product and improve the heat insulation capability. Diatomaceous earth bricks can only be used in locations below 1000°C. They will shrink and melt when the temperature is too high. Diatomite bricks can also be divided into several levels according to physical and chemical indicators. Their bulk density is 0.5~0.7g/cm3, refractoriness is 1280℃, apparent porosity is 73%~78%, and compressive strength is 0.5~1.1Mpa.

Diatomite is divided into two types: 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 materials.

Other insulation materials, such as slag wool, vermiculite (hydrated biotite) and perlite, are materials that contain many tiny pores. The more and smaller the pores are, the lower the thermal conductivity is. Using perlite, vermiculite, and lightweight clay bricks as aggregates, cement, water glass, phosphoric acid, etc. as cementing materials, and clay powder and ceramsite powder as admixtures, various lightweight resistant bricks can also be made. Hot concrete, used as thermal insulation material.