Introduction of refractory materials commonly used in coke ovens
1. Silica brick
The refractory bricks used in coke ovens are mainly silica bricks, which account for about 70% of the total refractory materials. Silica brick for coke oven is an acidic refractory material with tridymite as the main crystal phase, which has good acid corrosion resistance and good thermal conductivity. With the increase of the working temperature, the thermal conductivity of the silica brick will increase, and in the oven process, the volume of the silica brick will increase with the increase of the temperature. Therefore, silica brick is an ideal refractory product for coke ovens. Nowadays, important parts of the combustion chamber, chute and regenerator of many large and medium-sized coke ovens are built with silica bricks.
Silica bricks for coke ovens should have the following characteristics:
(1) The softening temperature under load is high, and the long-term use volume is relatively stable at high temperature. The coke oven silica brick is required to withstand the dynamic load of the coal loading truck on the top of the furnace at high temperature, and requires long-term use without deformation, so the coke oven silica brick is required to have a high softening temperature under load.
(2) High thermal conductivity. Coke is made of coking coal by conduction heating by the wall of the combustion chamber in the carbonization chamber, so the silica bricks used to build the combustion chamber wall should have high thermal conductivity.
(3) Good thermal shock resistance. Since the coke oven needs to periodically charge coal and discharge coke, the temperature of the silica bricks on both sides of the combustion chamber wall changes drastically, so the coke oven silica bricks are required to have good thermal shock resistance. True density can determine the thermal stability of silica brick. The smaller the true density of the silica brick, the more complete the conversion of lime, and the smaller the residual expansion will be generated during the baking process.
(4) High temperature volume stability. When heated to 1450 ℃, the total volume expansion is about 1.5~2.2%. This residual expansion will make the cutting seams close and ensure that the masonry has good air tightness and structural strength.
It is mainly used for the partition walls of the carbonization chamber and the combustion chamber of the coke oven, the regenerator and the slag chamber of the steel-making open-hearth furnace, the soaking furnace, the refractory material of the glass melting furnace and the firing kiln of the ceramics, etc. load bearing part. It is also used for high temperature load-bearing parts of hot blast stoves and acid open hearth furnace tops.
2. Clay bricks
Clay bricks refer to clay products with Al2O3 content of 30%-40% aluminum silicate materials. The main mineral components of clay bricks are kaolinite, namely refractory clay and kaolin, and the rest are impurities such as K2O, Na2O, CaO, MgO, TiO2 and Fe2O3, accounting for about 6% to 7%. After high temperature sintering. Clay bricks are resistant to acid slag and acid gases, but slightly less resistant to alkaline substances. Clay bricks have good thermal properties and can withstand rapid cooling and rapid heating. Clay bricks can only be used in secondary parts of coke ovens such as regenerator sealing walls, small flue lining bricks and regenerator checkered bricks.
Characteristics of Clay Bricks for Coke Oven
① Refractoriness. With the increase of alumina content, the refractoriness of clay bricks also increases. The refractoriness of clay-based refractory products is low, but increases with the increase of Al2O3 content in the products, and its refractoriness is generally 1580-1750℃.
② load softening point. The starting temperature of clay bricks softening under load is about 1350℃. The reason is that the bricks contain glass phase, resulting in low softening point temperature under load. The softening temperature of clay refractory products under load is relatively low, usually lower than 1300℃. From the beginning of softening to deformation (compression) of 40%, the temperature interval is between 150 and 250 ° C.
③ volume stability. The use temperature of clay bricks cannot exceed the firing temperature, otherwise the volume will shrink and the masonry brick joints will increase. The manufacture of clay-based refractories uses soft clay as the binder. Due to the incomplete mineralization of the binder and clinker during the firing process, the long-term use of clay bricks at high temperatures will cause irreversible volume shrinkage due to recrystallization (residual volume shrinkage). Shrinkage), this phenomenon is called reburning line change, for clay brick materials, the residual shrinkage rate is required not to exceed 1%. The re-firing line of clay bricks changes. The manufacture of clay-based refractory products uses soft clay as a binder. Due to the incomplete mineralization of the binder and clinker during the firing process, the long-term use of clay bricks at high temperatures will cause refractory damage. Crystallization produces irreversible volume shrinkage (residual shrinkage) or volume expansion. This phenomenon is called reburning line change, and for clay brick products, the residual shrinkage rate is required not to exceed 1%.
④ thermal stability. Clay bricks have good thermal stability because the bricks contain mullite and have a small expansion coefficient. In addition, the brick contains glass phase, which plays a buffering role, so that the stress cannot be developed. It can withstand water cooling 10 times at 850°C. Clay refractory products have good thermal stability. When heated to 1100 °C, the total volume expansion is small and the change is uniform, so the ability to resist temperature changes is strong. The water-cooling times of ordinary clinker clay bricks are 10 to 20 times, and the water-cooling times of multi-clinker clay bricks are generally more than 50 times, and some are as high as 100 times.
⑤ Slag resistance. Clay bricks contain a certain amount of SiO2 and belong to weak acid refractories. It can resist the erosion of acid slag and has poor resistance to alkali. The main chemical components of clay refractory products are SiO2 and Al2O3, and the content of SiO2 is greater than Al2O3, so the clay brick is weakly acidic, so the ability to resist acid slag erosion is stronger than that of alkaline slag erosion.
Due to the above characteristics of clay bricks, for modern large-volume coke ovens, clay bricks are not suitable for high-temperature parts, but are mainly used for parts with low temperature and large fluctuations, such as furnace doors, rising pipe lining bricks, and small flues. Lining bricks, furnace roofs, regenerator sealing walls and lattice bricks, etc. Due to the wide range of raw materials, easy manufacture and low cost of refractory bricks, some small coke ovens can be built with clay bricks, but during operation, the temperature should be strictly controlled to prevent premature damage to the top of the furnace.
3. High alumina brick
A coke oven is a high-temperature kiln used to produce coke. The main refractory materials are silica brick, clay brick and high alumina refractory brick. The structure of the coke oven is relatively complex, and the furnace body is mainly composed of a combustion chamber, a carbonization chamber, a regenerator and an inclined flue. The coke oven carbonization chamber is an area where coal is coked. Coal enters from the furnace head and begins to carbonize. After carbonization, the coke is released through the furnace door. At this time, cold air will enter the furnace door area to reduce the temperature, and this part needs to be resistant to thermal shock. High-alumina bricks with good performance and high load softening temperature are used to build the furnace door to ensure the long-term normal operation of the furnace head. The function of the coke oven is to decompose, dry distillation and carbonize the coking coal into coke under the condition of isolating the air.
High alumina bricks are refractory products of aluminum silicate or alumina with Al2O3 content greater than 48%. According to the composition of the main minerals in the products, high-alumina refractory products are divided into four categories: mullite-corundum, corundum-mullite, mullite, and corundum. According to the level of Al2O3 content, general-purpose high-alumina bricks are divided into three grades, that is, the first grade Al2O3> 65%; the second grade Al2O3 55% to 65%; the third grade Al2O3 48% to 55%.
From the point of view of refractoriness and softening temperature under load, the thermal stability of high alumina bricks is not as good as that of clay bricks. High alumina bricks have high density, low porosity, high mechanical strength and good wear resistance. High-alumina bricks can be used in the burner head of the coke oven combustion chamber and the burner head of the bottom brick in the carbonization chamber.
Characteristics of high alumina bricks for coke ovens
(1) Refractoriness. The refractoriness of high alumina bricks is relatively high among aluminum silicate refractories. With the increase of Al2O3 content in the product, the refractoriness will also increase, generally not lower than 1750 ~ 1790 ℃. For example, corundum products with Al2O3 content greater than 95% have a refractoriness of up to 1900-2000 °C.
(2) softening temperature under load. The softening temperature of high-alumina bricks under load will decrease with the increase of SiO2 and metal oxide content in the product. Generally, the softening temperature of high-alumina bricks under load is 1420-1530℃. For corundum products with Al2O3 content greater than 95%, the softening temperature under load is above 1600℃.
(3) Slag resistance. Because the Al2O3 content in the product is high and neutral, it has strong corrosion resistance to acidic or basic slag. With the increase of Al2O3 content, the slag corrosion resistance of high alumina bricks will also increase.
(4) Thermal shock resistance. In its products, two crystals of corundum and mullite coexist, and the linear expansion coefficient of corundum is larger than that of mullite, so the stress concentration is caused by the difference in expansion in the product when the temperature changes, so the high alumina brick The thermal shock resistance is poor.
(5) The reburning line changes. The change of the re-fired line of high-alumina bricks depends on the firing degree of the product. If the sintering temperature is sufficient and the sintering time is sufficient, the volume will be stable and the refiring line change will be small. Conversely, residual shrinkage occurs due to recrystallization inside the product.
In view of the above characteristics of high-alumina bricks, it is mainly suitable for masonry coke oven combustion chamber burner head and carbonization chamber bottom burner head and other parts. Carbonization room wall.
4. Refractory clay
Refractory mud is composed of powdery materials and binders for preparation of mud. Refractory clay for coke oven is divided into silicon clay, clay clay and high alumina clay. Silicon fire clay is used when building silica bricks, clay fire clay is used when building clay bricks, and high alumina refractory clay is used when building high alumina bricks.
Silica fire clay is formulated from silica, waste silica bricks and bonded clay. Silica is the main component, and the higher the SiO2 content in silica, the higher the refractoriness. The purpose of adding waste silica brick powder is to improve the high temperature bonding ability of silica fire clay and silica brick. Because the expansion of waste silica brick powder and silica brick is basically the same, the brick joints can be attached to the brick surface. The powder content is between 20% and 30%. Adding bound clay to silicon fire mud can increase the plasticity of fire mud, reduce air permeability and water loss rate, and the general addition amount is about 15% to 20%.
Clay fire clay is formulated from calcined clay (clinker), waste clay brick powder and bound clay (raw meal). Clinker is the main component of clay fire mud, accounting for about 75% to 80%. Raw clay is a binder. It is added to increase the plasticity of clay fire mud and reduce air permeability and water loss rate. However, the dosage should not be too large, otherwise cracks will easily occur. It is advisable to add 20% to 25%. Clay fire clay is used in coke ovens with temperatures below 1000°C.
When building a coke oven, different refractory clays are used according to the different types of bricks and temperatures in each part. From the top of the combustion chamber to the middle and upper part of the chute area, the medium temperature silicon fire mud is used, from the lower part of the ramp area to the middle of the regenerator, the low temperature silicon fire mud is used, and the lower part of the regenerator adopts the low temperature silicon fire mud with water glass. Clay fire clay is mainly used for laying clay bricks and repairing coke ovens.
High alumina refractory clay is a dry bulk material prepared from high alumina bauxite clinker, soft clay, binder and admixture, and is a semi-finished product. High-alumina refractory clay is a hydraulically combined refractory clay, which needs to be stirred with water when used, and has a certain strength and refractory temperature after heating. The particle size of high-alumina refractory clay varies according to the requirements of use, and its limit particle size is generally less than 1mm, and some are less than 0.5mm or finer. The refractory properties of the masonry should be considered when using it. High-alumina refractory mud is usually used in combination with high-alumina bricks.
The refractory mud used in masonry coke ovens is often used as a binder and coating material for masonry refractory bricks. It is required to have good plasticity and cohesion at room temperature to facilitate construction; small shrinkage after drying to prevent cracking; sintering or consolidation can occur at the operating temperature, and it has a certain strength to ensure the furnace body. Firmness and tightness; its refractoriness and load softening temperature should be higher than the service temperature.
5. Heat-resistant concrete
Heat-resistant concrete is a mud material prepared by refractory aggregate, cementitious material and water in a certain proportion. It is a kind of special concrete, which can withstand long-term high temperature. The aggregates used are often bauxite, blast furnace slag, etc., and the cementing materials are generally bauxite cement, Portland cement, phosphoric acid and water glass.
6. Insulation material
We usually refer to building materials with thermal conductivity less than 0.8kJ/mh°C as thermal insulation materials. The thermal insulation material has small pores, large porosity, low mechanical strength and small bulk density.
In the masonry construction of coke ovens, in addition to the application of refractory bricks and castables, thermal insulation materials are also one of the furnace linings that must be used in coke oven masonry construction. Different types of thermal insulation materials (insulation materials) have different thermal conductivity. The material composition of the thermal insulation material is different, and its physical and thermal properties are also different; the thermal insulation mechanism is different, and its thermal conductivity or thermal conductivity is also different.
