What are monolithic refractories? What are the types of monolithic refractories?
Publish Time:
2023-07-11
I. What are monolithic refractories?
Monolithic refractory materials are refractory materials composed of refractory aggregates and powders with a certain particle size distribution, binders, and additives. They have no fixed shape and can be made into pastes, muds, and loose forms, so they are also called loose refractory materials. They are used for the lining of thermal equipment and are used directly after baking without a firing process. Compared with refractory bricks, monolithic refractory materials have the advantages of simple process (because the firing process is omitted), energy saving, low cost, and easy mechanization. In some thermal equipment, the effect is better than refractory bricks.
II. What are the types of monolithic refractory materials?
According to the construction method and material properties, monolithic refractory materials can be divided into castables, plasticized materials, ramming materials, spray-patching materials, dry-mixed materials, cast materials, and coatings.
1. Castables
A material with good fluidity after being mixed with water. After molding, it needs to be properly cured to allow it to solidify and harden, and then baked according to a certain system before it can be used. Castables use alumina-silicate clinker, corundum materials, or basic refractory clinker as aggregates; lightweight castables use expanded perlite, vermiculite, ceramsite, and hollow alumina balls as aggregates. Binders include calcium aluminate cement, water glass, ethyl silicate, polyaluminum chloride, clay, or phosphates. Additives vary depending on the application and serve to improve workability and enhance physicochemical properties.
Construction methods for castables include vibration, pumping, pressing, and spraying. When using castables for integral linings, they are often used in conjunction with metal or ceramic anchors. If further reinforced with stainless steel fibers, their resistance to mechanical vibration and thermal shock can be improved. Castables are used as linings for various heat treatment furnaces, ore roasting furnaces, catalytic cracking furnaces, conversion furnaces, etc., and are also used as linings for melting furnaces and high-temperature molten material flow channels, such as lead-zinc melting furnaces, tin baths, salt bath furnaces, steel or iron tapping troughs, steel ladles, and nozzles for steel vacuum circulation degassing devices.
2. Plasticized Materials
Plastic clays or clay blanks. When subjected to appropriate external force, they are easy to deform without cracking; after stress relief, they no longer deform. Plasticized materials include semi-siliceous, clay, high-alumina, zircon, carbon, and lightweight plasticized materials. Plasticized materials must contain plasticizing materials, which are mostly highly plastic clays. Plasticizers can also be used to improve the plasticity of this clay. Plasticizers include carboxymethyl cellulose, dextrin, and lignin sulfonate. Binders used in plasticized materials include plastic clay, phosphoric acid, aluminum dihydrogen phosphate, and aluminum sulfate. Alumina plasticized materials containing phosphoric acid or phosphate binders will react chemically with alumina during storage, forming insoluble aluminum orthophosphate and causing the clay to harden. Therefore, preservatives such as oxalic acid, citric acid, and acetylacetone must be added.
Construction methods generally use ramming or vibration methods. When using plasticized materials to build integral furnace linings, metal or ceramic anchors must be used. Plasticized materials are used as linings for heat treatment furnaces, heating furnaces, boilers, and other thermal equipment, and are also used for wrapping water-cooled tubes in heating furnaces.
3. Spray-Patching Materials
A type of refractory mixture used for spraying or coating with a spraying machine. According to the spraying method, it can be divided into wet spraying (or slurry spraying), semi-dry spraying, and fire (flame) spraying. Wet spraying uses compressed air to spray a slurry containing 20-40% refractory powder, which can achieve high atomization, high adhesion, and relatively uniform thin-layer spraying. Semi-dry spraying wets the refractory powder sprayed by compressed air at the nozzle with water, with a water content of 11-14%, lower adhesion, and thicker layer spraying. Fire spraying is a dry spraying method, where the spray material is sent to the flame of a fuel-oxygen gun, and the spray material is partially melted in the nozzle flame and adheres to the brick lining.
Spray-patching materials include aluminosilicate, aluminosilicozirconate, magnesia, magnesia-lime, and magnesia-chrome materials. The binders used include sodium silicate, phosphates, polyphosphates, asphalt, and resins. To improve adhesion, clay, bentonite, and lime are added. To ensure that the spray material achieves good sintering, sintering aids such as serpentine, olivine, lime, refractory clay, and iron oxide are added.
4. Refractory Coatings
Materials coated on refractory brick linings. Depending on the requirements and construction methods, refractory coatings can be prepared in the form of pastes and slurries. The binders used vary depending on the material. For example, phosphate, polyphosphate, and magnesium sulfate are used for preparing alkaline coatings for continuous casting tundishes; for preparing high-alumina coatings, clay, aluminum dihydrogen phosphate, aluminum chromium phosphate, and water glass are used. To improve the spreadability of the coating, plasticizers and other additives are generally added. Coatings are mainly used as protective coatings for linings of various thermal equipment or for repairing locally damaged brick linings.
5. Ramming Materials
A type of loose refractory material with very low or no plasticity. Materials include siliceous, clay, high-alumina, corundum, zircon, silicon carbide, carbon, and magnesia materials. Depending on the material and use conditions of the ramming material, inorganic binders or organic binders similar to castables can be used, such as water-soluble dextrin, carboxymethyl cellulose, lignin, sulfonate, polyvinyl alcohol; water-resistant and thermoplastic paraffin wax, asphalt, tar, phenolic resin, and atactic polypropylene.
Ramming materials are constructed using forced ramming, resulting in low porosity and high density. Therefore, among monolithic refractory materials, ramming materials are particularly suitable for use as melting furnace linings and linings for various containers holding high-temperature metal melts, such as open-hearth and electric furnace hearths, various induction furnace linings, blast furnace tapholes, and steel ladles.
6. Gunning Materials
A semi-dry, projected refractory material used primarily for constructing monolithic steel tank linings. Materials include siliceous, waxstone, clay, high-alumina, and zircon. High-silica and high-alumina materials are most commonly used.
7. Dry Mix
Dry mix, also known as dry vibrated material or dry material, refers to monolithic refractory materials constructed in a dry state using vibration or ramming. These materials often contain a temporary binder that is removed during baking.
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