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Refractory castables include acidic, alkaline, and neutral castables; they are divided into heavy castables and light castables according to density; they are divided into aluminum silicate, magnesia, magnesium chromium, corundum castables, and silicon-based materials. So how to choose refractory castables under different circumstances?
First of all, we must consider the kiln environment, usage location, usage temperature, corrosion properties, construction precautions, etc. Only based on these basic requirements can we select appropriate refractory castables.
1. Kiln environment
First of all, we need to look at the overall kiln environment of the kiln we want to build, including the medium in the kiln, the characteristics of the medium, the operating temperature of the kiln, and a characteristic of the kiln when it is working. Based on these environments, we can correct the selection of refractory castables.
2. Parts of use
If it is the working layer, heavy refractory castables with high-temperature resistance should be selected. If the erosion is acidic, castables that are resistant to acid erosion should be selected. If the erosion is alkaline, castables that are resistant to alkaline erosion should be selected. If the parts in contact with slag should use castables with strong resistance to slag erosion; if the parts with severe material impact should use high-strength and wear-resistant castables; if the parts with frequent temperature changes should use castables with good thermal shock resistance.
Lightweight refractory castables should be used as the castables for the insulation layer. The main considerations are load-bearing structure, lowering the temperature of the furnace shell, and reducing heat loss. Then choose thermal insulation castables with different volume densities. Generally, 1.2 volume density is used. For thermal insulation castables, if the user has weight requirements, they should choose thermal insulation castables with a body density of 0.6-1.0.
3. Using temperature
Different grades of castables are used at different temperatures, and the castables used at different temperatures are also different. Generally speaking, clay castables are used below 1200°C, and high-aluminum castables are used between 1300°C and 1400°C. If it exceeds 1450°C It is necessary to choose corundum castable;
4. Construction thickness
The construction thickness of refractory castables is different. If it is less than 50mm thick, you must use smearing material or spray paint. If it is for repair or furnace top use, choose plastic. If the thickness of the castable is greater than 100mm, large particle aggregates are needed to improve the wear resistance of the castable. Generally, the best thickness of the castable is 200mm.
5. Precautions for construction of refractory castables
Formwork: The formwork for castable construction can be made of steel plate or hardwood board (plywood). The formwork must have sufficient strength, good rigidity, no deformation, no displacement, and no leakage of slurry. The rigid formwork must be coated with a release agent, and the wooden formwork must be coated with a release agent. To apply a waterproofing agent, reused templates must be cleaned first and then painted before use. Inspection holes and cleaning holes should be made of regular wood moldings. The metal tie bars supporting the wooden magic board should be more than 4 per square meter (to prevent strong heat transfer) and the tie bars should be welded based on the anchors (that is, the tie bars of the formwork should be welded on the anchors).
Water-cement ratio: The amount of water added when mixing castables must be strictly controlled by the instructions and must not exceed the limit. On the premise of ensuring construction performance, the amount of water added should be less rather than more.
Stirring: The mixing time of the castable should be no less than 5 minutes (subject to the requirements of the instruction manual), and a forced mixer should be used during operation. When stirring, it is advisable to mix dry first, then add 80% of the amount of water to stir, and then slowly add the remaining water and continue stirring depending on the degree of dryness and wetness until a suitable working level is obtained. When mixing different castables, the mixer must be cleaned.
Use of castables: Castables must be used in full buckets and bags. The mixed castables must generally be used up within 30 minutes. In a high-temperature and dry operating environment, this time should be shortened appropriately, as the castables have initially been set or even agglomerated. The castables shall not be poured into the mold frame, nor shall they be mixed with water before use.
Vibration: The castables poured into the mold frame should be immediately vibrated in layers with a vibrator. The height of each layer should be no more than 300mm, and the vibration spacing should be about 250mm. When vibrating, try to avoid touching the anchors, damage the insulation layer, and do not vibrate or re-vibrate at the same position for a long time. After seeing the slurry on the surface of the castable material, the vibrating rod should be slowly pulled out to avoid segregation and occurrence of the castable material layer. Hollow.
Large-area construction: During large-area construction, construction must be carried out in blocks. The area of each pouring area is preferably about 1.5m2. Expansion joints must be left by the design requirements and must not be omitted. Expansion joints should be left at anchor intervals. In the middle position, a 6~8mm thick wooden board can be used for the expansion joint. The distance between the cross-section of the wooden board and the formwork should be 40~60mm. The expansion wooden board should be effectively fixed to prevent dislocation during the construction of the castables.
Regarding the welding of anchors: draw grid lines on the working surface (according to the requirements of the drawing), weld the anchors at the grid line contact points, and the roots of the anchors must be fully welded. The castable body after the construction of the castable material is completed must not be subjected to pressure or shock before solidification.
The above is how to correctly select refractory castables. Only by correctly selecting refractory castables can our kiln work normally and stably. At the same time, strict construction can ensure that the kiln has a long service life.
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