6 commonly used refractories for coke ovens

Refractories for coke ovens

01. 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 working 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, chutes and regenerators) are all built with silica bricks.

During the baking 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 crystal transformation points of 117°C, 163°C, 180~270°C and 573°C during the oven process, among which the volume expansion caused by cristobalite is the largest between 180~270°C.

The key to determine the thermal stability of silica bricks is the true density, and the true density is one of the important signs to determine its quartz transformation. The smaller the true density of silica bricks, the more complete the lime conversion, and the smaller the residual expansion 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 softening temperature under load, and is the most stable form of quartz. . Among the well-fired silica bricks, the content of tridymite is the highest, accounting for 50% to 80%; cristobalite takes the second place, accounting for only 10% to 30%; while the content of quartz and glass phase fluctuates between 5% and 15%.

When the working temperature is lower than 600~700°C, the volume of silica bricks changes greatly, the performance of resistance to rapid cooling and rapid 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 be easily cracked and damaged.

02. Clay bricks

Clay bricks refer to clay products with Al2O3 content of 30%~40% aluminum silicate materials. Clay bricks are made of 50% soft clay and 50% hard clay clinker, according to certain particle size requirements, and after molding and drying, 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%~7% impurities (oxides of potassium, sodium, calcium, titanium, iron). The firing process of clay bricks is mainly a process in which kaolinite decomposes continuously to form mullite (3Al2O3?2SiO2) crystallization. SiO2 and Al2O3 in clay bricks form eutectic low-melting silicate with impurities during the firing process, surrounded by mullite crystals.

Clay bricks are weakly acidic refractory products, which can resist the erosion of acidic slag and acidic gas, and are slightly less resistant to alkaline substances. Clay bricks have good thermal performance and are resistant to rapid cooling and rapid heating.

In the temperature range of 0~1000℃, the volume of clay bricks expands uniformly with the increase of temperature, the linear expansion curve is similar to 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 then continues to heat up, its volume will start to shrink from the maximum expansion. The residual shrinkage of clay bricks leads to the cracking of masonry mortar joints, which is a major disadvantage of clay bricks. When the temperature exceeds 1200 ° C, the low melting point in the clay bricks gradually melts, and the particles are close to each other due to the action of surface tension, resulting in volume shrinkage.

Because clay bricks have a low load softening temperature and shrink at high temperatures, their thermal conductivity is 15% to 20% lower 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 sealing wall, small flue lining bricks and regenerator checker bricks, furnace door lining bricks, furnace roof and riser lining bricks, etc.

03. High alumina brick

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 for acid 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 bottom brick are built with high-alumina bricks, and the effect is better; 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 .

04. Refractory motar

Refractory motar is an amorphous refractory material composed of powdery materials and binders for preparing mud. It is mainly used as a binder and coating material for building refractory brick masonry. Most of the refractory mud is mixed with water (or aqueous solution) to make mud. It should have the corresponding properties of masonry bricks, and refractory mud for coke ovens should meet the following requirements:

(1) After construction and during use, it should have the necessary cohesiveness to ensure that it is integrated with the masonry or the surrounding layers, so that it should have the effect of resisting external force, gas resistance, and slag erosion.

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

(3) It has the same chemical composition as the material of the masonry or the surrounding layer, so as to avoid the first destruction of the refractory mud and avoid adverse chemical reactions between different materials.

(4) It has the same thermal expansion as the material of the masonry or the surrounding layer, so as to avoid separation from each other and the cracking of the mud layer.

(5) The volume should be stable and have small 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 fireclay should be selected according to the brick type and operating temperature, that is, clay fireclay is used when building clay bricks, and silicon fireclay is used when building silica bricks. Concentrate powder must be added to the fire clay for all masonry parts that are in contact with metal embedded parts. When laying bricks on the top surface of coke ovens, hydraulic binders—Portland cement and quartz sand—that can increase the strength should be added to the clay mortar.

Silica fire clay is a powder made of silica, waste silica bricks and refractory clay (raw clay). Silica is the main component of silicon 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 clay and silica bricks. The reason is that silica brick powder has a thermal expansion curve similar to that of silica bricks. When the volume of quartz crystal transformation changes, the possibility of fire clay breaking away from silica bricks is small. The ability to adhere to silica bricks is good. Generally, the content of silica brick powder is 20%~30%, which is more suitable. Adding raw clay to silicon 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 silicon fire clay will be reduced and the shrinkage rate will be increased. Generally, it should not exceed 15%~20 % is appropriate.

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

Clay fireclay is made by adding refractory clay (raw clay) to clinker or pulverized clay bricks from the calcination process. Clinker is the main component of clay fire mud, 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 is prone to cracks, so the ingredients account for about 20% to 25%.

The use temperature of clay fire clay is generally lower than 1000 ℃. Clay mortar 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%. In addition to being used for masonry clay bricks, clay fire mortar is also used for repairing coke ovens.

05. Heat-resistant concrete

Heat-resistant concrete is a kind of special concrete that can withstand high temperature for a long time. It is a mud made of refractory aggregate, cementitious material (sometimes with mineral admixture or organic admixture) and water in a certain proportion. A refractory product with a certain strength obtained by pounding or vibrating, hardening, curing, and drying.

Usually, bauxite, blast furnace slag, etc. are used as aggregates, and bauxite cement, Portland 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 its scope of use is also different. Compared with refractory bricks, this refractory product has the following advantages:

(1) Rapidly develops strength at room temperature and does not decrease at operating temperature.

(2) It does not need to be fired before use, which reduces the complicated process of making refractory bricks. The preparation process is simple, and it can be cast into various shapes on the spot, which can reduce the joints of masonry bricks, simplify the structure, and simplify the brick shape, thereby innovating masonry. Construction work, speed up construction.

Heat-resistant concrete has been used in coke ovens for many years. It is mainly used as riser pipes, furnace door lining bricks, and furnace top rail sleeper bricks instead of clay bricks. It is also used as coke oven roof pavement. According to different parts of use, its ingredients are also different.

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

06. Heat insulation material

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 small bulk density. There are many classification methods for thermal insulation materials, generally classified by use temperature, bulk density and manufacturing method, but often classified by use temperature and bulk density.

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

(1) Low-temperature insulation materials: use temperature below 900°C, such as diatomaceous earth, asbestos, water slag, slag wool, vermiculite, perlite, etc.

(2) Medium-temperature insulation materials: the use temperature is 900~1200°C, 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 pearl bricks, lightweight silica bricks, etc.

Lightweight clay bricks are clay bricks fired with clay as raw material and a certain proportion (30%~35%) of sawdust. There are various grades, the bulk density is 0.4~1.3g/cm3, and the refractoriness is 1670 ~1710°C.

Diatomite brick is a product made of diatomite, in which a certain amount of combustibles can be added to increase the porosity of the product and improve the heat insulation capacity. Diatomite bricks can only be used in parts below 1000°C, and they will shrink and melt when the temperature is too high. Diatomite bricks can also be divided into several grades according to physical and chemical indicators. Their bulk density is 0.5~0.7g/cm3, their refractoriness is 1280℃, their apparent porosity is 73%~78%, and their compressive strength is 0.5~1.1Mpa. The product size has two specifications: 250×123×65mm and 230×113×65mm.

Diatomaceous earth 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 boards are boards made of asbestos and bonding materials.

Other insulating materials, such as slag wool, vermiculite (hydrous biotite) and perlite, are materials that contain many fine pores. The more pores the smaller, the lower the thermal conductivity. Perlite, vermiculite, lightweight clay bricks are used as aggregates, cement, water glass, phosphoric acid, etc. are used as cementing materials, and clay powder and ceramsite powder are used as admixtures. Hot concrete, used as insulation.

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