Glass melting furnaces are important equipment for glass production. The temperature inside the glass melting furnace is extremely high, so special refractory materials are needed to protect the kiln body. Among them, insulating refractory bricks are very important. So why do glass melting furnaces use these 5 kinds of insulating refractory bricks?
First of all, these five kinds of insulating refractory bricks have good high temperature resistance. The temperature inside the glass melting furnace is extremely high, and general refractory materials cannot withstand such high temperatures. However, these five kinds of insulating refractory bricks can remain stable under high temperature conditions due to their special materials and manufacturing processes, and will not melt or deform.
Secondly, these five kinds of insulating refractory bricks have good thermal insulation properties. In a high temperature environment, the transfer of heat is very fast. If there are no adequate insulation measures, the kiln body is easily damaged by the heat. However, these five kinds of insulating refractory bricks can effectively prevent the transfer of heat and reduce the temperature of the kiln body due to their internal pore structure.
Thirdly, these five kinds of insulating refractory bricks have good wear resistance. During the use of the glass melting furnace, the refractory material will be eroded and worn by the glass liquid. If it does not have sufficient wear resistance, it is easy to be damaged. These five types of insulating refractory bricks can effectively resist this wear due to their high hardness and good toughness.
Lightweight refractory bricks
The main indicators of lightweight refractory materials are: bulk density, porosity, thermal conductivity, use temperature and mechanical strength.
There are many types of lightweight refractory materials. According to the use temperature, they are divided into low-temperature type (less than 900℃) such as diatomite bricks, etc.; medium-temperature type (900~1200℃) such as calcium silicate, lightweight clay bricks, aluminum silicate fibers, etc.; high-temperature type (above 1200℃) such as lightweight high-alumina bricks, alumina hollow balls, etc. According to the shape, it is divided into fibrous, porous and granular. According to the volume density, it is divided into sub-lightweight (1.0~1.3g/cm3), lightweight (0.4~1.0g/cm3) and ultra-lightweight (below 0.4g/cm3). According to the production method, it is divided into the method of adding combustibles, the foam method and the chemical method.
Lightweight refractory bricks
Types and manufacturing processes of lightweight insulating refractory bricks Lightweight insulating refractory bricks are block-shaped insulating refractory products with internal porous structures and insulating properties. Among them, brick-shaped products are widely produced and used, so they are also called lightweight insulating refractory bricks. Lightweight insulating refractory bricks can be divided into clay, silica, aluminum and mullite according to the raw materials used. Lightweight refractory bricks have low compressive strength, poor slag resistance, poor thermal shock resistance, and cannot be in direct contact with glass liquid or flames. Table 1 lists the types and characteristics of lightweight insulating refractory bricks according to the raw materials and components used to manufacture lightweight refractory bricks.
1. The reason why lightweight bricks can insulate is that the bricks contain a large number of pores (including closed pores and open pores). The methods of creating pores are: adding combustibles, foaming, and chemical methods.
Light clay insulation bricks
Light clay insulation bricks are insulation refractory products with an Al2O3 content of 30% to 48% made of refractory clay as the main raw material. The production process adopts the burnout and foaming method. Refractory clay, floating beads, refractory clay clinker are used as raw materials, binders and sawdust are added, and the products with a volume density of 0.3 to 1.5 g/cm3 are obtained through batching, mixing, molding, drying and firing. Light clay insulation bricks are widely used, mainly used as insulation layer refractory materials in various industrial kilns that do not contact molten materials and have no corrosive gas effects, and the use temperature is 1200 to 1400°C.
Light silica insulation bricks
Light silica insulation bricks are insulation refractory products with an SiO2 content of not less than 91% made of silica as the main raw material. The volume density of lightweight silica insulation bricks is 0.9~1.1g/cm3, and the thermal conductivity is only half of that of ordinary silica bricks. It has good thermal shock resistance, and its load softening temperature can reach 1600℃, which is much higher than clay insulation bricks. Therefore, the maximum use temperature of silica insulation bricks can reach 1550℃. It does not shrink at high temperatures, and even has a slight expansion.
Lightweight silica bricks generally use crystalline quartzite as raw material, and flammable substances such as coke, anthracite, sawdust, etc. are added to the batch, or a porous structure is formed by gas foaming. Similar to the production industry of ordinary silica bricks, some mineralizers (CaO, Fe203) are added to the batch to promote the conversion of quartz, and pulp waste liquid is used as a binder to give the bricks a certain strength. When coke or anthracite is used as a combustible, since the ash will bring in Fe203 and Al2O3, no additional iron and phosphorus need to be added to the batch. When firing lightweight silica bricks, a strong oxidizing flame should be maintained before 1200℃ to completely burn out the added materials.
The mineral composition of lightweight silica bricks is: phosphotarcite 78%~86%, cristobalite 78%~86%, and quartz 78%~86%. The compressive strength of general lightweight silica bricks is 2.0~9.5MPa, and the thermal conductivity is 0.35~0.42W/(m.K). Lightweight silica bricks have good refractory properties, and the load softening temperature is close to that of dense silica bricks, reaching more than 1620℃, with only a small amount of residual expansion, and its thermal stability is better than that of dense silica bricks. Therefore, lightweight silica bricks can be used for a long time at 1500~1550℃ under high temperature conditions without contact with slag. Table 2 lists the technical conditions of lightweight silica bricks for industrial kilns in my country (YB/T 386-1994).
2. Lightweight alumina insulation bricks
Lightweight alumina insulation bricks are thermal insulation refractory products made of fused corundum, sintered alumina and industrial alumina as the main raw materials. There are two production processes: foam method and burnout addition method. The physical and chemical indicators of alumina insulation bricks are listed in Table 3.
3. Foam method to produce alumina insulation bricks. The clinker is ground and mixed with a binder and a foaming agent to prepare mud, which is cast into a mold and fired at 1350~1500℃. The product has high refractoriness and good thermal shock resistance, and is often used as a high-temperature insulation layer of kilns. The operating temperature is 1350~1500℃, and the operating temperature of high-purity products can reach 1650~1800℃.
Lightweight mullite insulation bricks
Lightweight mullite insulation bricks are thermal insulation refractory products made of mullite as the main raw material. Mullite insulation bricks are resistant to high temperatures, have high strength, low thermal conductivity, can be directly exposed to flames, and are suitable for the lining of various industrial furnaces.
Diatomaceous earth and its products
Diatomaceous earth is a natural porous insulation material. It is a sedimentary mineral formed by the corpses of aquatic plants diatoms deposited on the bottom of the sea or lake. When observed under a microscope, it can be seen that diatomaceous earth is composed of tiny algae shells of 5 to 400um, containing a large number of its fine pores. Diatomaceous earth has good thermal insulation properties.
The main component of diatomaceous earth is amorphous hydrous silicate. Diatomaceous earth is white and the SiO2 content can reach 90% to 98%. Ordinary diatomaceous earth often contains clay, volcanic ash, organic matter and insoluble substances, and its color is light gray, light yellow or dark green. The SiO2 content is between 70% and 90%; different methods are used in industry to remove impurities in diatomite, such as organic matter by roasting; silica sand by elutriation; iron oxide by adding salt and roasting at 700-800°C.
The main component of diatomite, hydrous silicic acid, has good heat resistance and can be used as a raw material for medium and high temperature insulation materials. During the heating process, the temperature is between 50 and 250°C, residual moisture is removed and gel is aged, and 0.1% to 0.3% shrinkage occurs. At a temperature of 500-800°C, all moisture evaporates, countless pores are formed inside, and the volume of absorbed liquid can reach 5 times its own volume. Above 800°C, sintering begins to occur between diatom shells, and shrinkage occurs. When the temperature rises to above 1000°C, the diatom shells are transformed into quartz and shrink significantly.
Because diatomite is porous and has a small bulk density, its main component, silicon dioxide, has good chemical stability and good heat resistance, so it is widely used as a thermal insulation material. In addition, diatomite has many other uses, such as absorbent, adsorbent, degreasing agent, filter material, etc.
When raw diatomite is used as a thermal insulation material, it can be directly used for thermal insulation coating, and after drying, it can be used as a thermal insulation filler. Because diatomite has strong hygroscopicity, it is not suitable for use as a thermal insulation material for low temperatures.
Usually, diatomite is used in large quantities to make diatomite lightweight insulation bricks and products. When the diatomite ore contains enough clay, it has sufficient plasticity after being crushed into fine powder and mixed with water. It can be squeezed into mud strips by a mud extruder, cut into bricks by steel wire cutting, or formed by plastic method; when the raw materials are relatively pure, a certain proportion of binding clay must be added to make the bricks have sufficient strength. In order to improve the thermal insulation performance of diatomite bricks, ceramic fiber materials, sawdust and other combustibles are sometimes added to the ingredients. After the bricks are dried, they are fired at 900-1000℃.
The firing temperature has a great influence on the performance of diatomite insulation bricks. Bricks fired at a higher temperature will produce a large volume expansion at about 200°C when reheated. This is because when fired at above 1000°C, many diatom shells have been transformed into quartz, and when reheated, quartz begins to undergo a crystal transformation at about 200°C. Bricks fired at a lower temperature will shrink more when reheated.
Therefore, the firing temperature of diatomite bricks should be 900-1000℃. The characteristics of diatomite lightweight insulation bricks are: compared with other insulation bricks with the same bulk density, it has a lower thermal conductivity because the pores in diatomite are very small and have a good shielding effect on heat.
The use temperature of diatomite insulation bricks varies with purity. The use temperature of products made of general diatomite is generally below 1000℃, because the shrinkage and deformation of the products are large at high temperatures. When using pure diatomite raw materials, it needs to be fired at a higher temperature to convert the silica shell into phosphotquartz. Lime can also be added as a binder and mineralizer to promote the conversion of phosphotquartz during firing, which is beneficial to improve the heat resistance of the product and reduce the shrinkage under high temperature.
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