Fused refractory brick is a product made by melting raw materials such as alumina bauxite in an electric furnace and then cooling and solidifying. It has the advantages of simple process, short process and low product cost. Due to the different varieties of fused corundum, different reducing agents can be added during the torch process. The structure of the alumina melt close to the melting point of corundum (2100°C) is similar to that of corundum bricks. This melt crystallizes into corundum bricks when cooled. When the corundum brick is melted, the molar volume of the melt increases by 23.5%, and the heat of fusion is 109.2KJ/mol. As the temperature of the melt increases, its density decreases.
Generally speaking, "fused alumina" generally refers to fused white corundum. It is made of industrial oxidized name or calcined alumina as raw material, and is digitalized at high temperature in an electric arc furnace. Its Al2O3 content is generally greater than 98.5%, white, huge crystals, porous, and a significant porosity of 6%-10% The crystal phase is a-Al2O3, and the crystals are elongated and rhombic. It is an important raw material for making high-grade refractories and is also widely used in the abrasive industry.
Although the Al2O3 content of industrial alumina is above 98.5%, there is still a small amount of Na2O, SiO2 and a trace amount of Fe2O3. Although the electrofusion process has a certain purification and purification effect on the preparation of white corundum, these impurities cannot be completely eliminated. Among them, Na2O and Al2O3 produce Al2O3 in a molten state, which has a low melting point and low density. Therefore, the frit is segregated in the upper middle part of the frit when it is cooled and crystallized. Although it can be eliminated by crushing, there will still be a small amount of residual corundum in the corundum melt, which will seriously affect the fire resistance of the corundum. Therefore, the content of Na2O in industrial alumina must be strictly controlled.
In order to eliminate or reduce the harmfulness of Na2O, quartz sand or aluminum fluoride is added. The former makes β Al2O3 into nepheline, and the latter promotes the volatilization of Na2O.
Dense fused corundum is made from industrial alumina as raw material, added with additives and melted in an electric arc furnace. Its appearance is gray, gray-black or gray-white, Al2O3 is not less than 98%, the apparent porosity is lower than that of fused white corundum, generally not more than 4%, and the bulk density is not less than 3.8g/cm³. Dense fused corundum brick is a new type of refractory material, which is acid and alkali resistant, stable in volume at high temperature, and has the characteristics of dense particles and low porosity.
Due to improper operation and refining time, Al2O3 may be over-reduced during industrial alumina smelting to form carbides. Carbide is a harmful impurity in the corundum brick clinker. It will undergo a hydrolysis reaction to release methane (CH4) when it meets water or stored in a humid side of the atmosphere, which will pulverize the corundum particles; it will undergo acidolysis when it reacts with acid. Therefore, castables or ramming materials made of corundum raw materials containing carbides will crack during grilling.
Fused sub-white corundum is also called alumina-based fused corundum brick or high alumina corundum brick. It is made from light-burned super-grade or first-grade bauxite loose bulk material in an electric arc furnace under reducing atmosphere and controlled conditions, adding reducing agent, settling agent and decarburizing agent to remove impurities such as SiO2, Fe2O3 and TiO2 , A corundum material obtained after smelting. Its physical and chemical indicators are equivalent to dense fused corundum bricks, and the color is also similar. The content of Al2O3 is generally above 98.0%, and the apparent porosity is less than 4%; corundum crystals are generally granular, with a size of 1-15mm; the main impurity mineral is calcium hexaaluminate (CA6), rutile, aluminum titanate and its solid solution; it is a new type of refractory material second only to fused white corundum bricks and fused dense corundum bricks.
The production of fused corundum bricks has nearly doubled in the past ten years. In addition to the increased demand for corundum bricks in the petrochemical, fertilizer and other industries, the steel industry uses long nozzles for continuous casting, tundish integral stoppers and immersion Shuipinkou and others have also turned to Al2O3-C refractory materials. Most of the alumina raw materials are fused corundum, and amorphous refractories also use a large amount of fused corundum as aggregates and powders.
As a refractory material, fused corundum brick has high refractoriness, corrosion resistance, slag resistance, erosion resistance and high temperature strength. It is widely used in various metallurgical equipment and high temperature furnaces, various coating materials, ramming The punching material is also added with fused corundum as a raw material to improve its performance. The production of fused-cast corundum products, especially the production of fused-cast corundum bricks, is more widely used. However, the production process of fused-cast corundum bricks: smelting-product casting-cooling annealing-mechanical processing, each of these processes will have a great impact on the quality of production, and directly affect the service life of high-temperature thermal equipment. Therefore, the production process of fused corundum and fused-cast corundum bricks needs to be further developed and improved, moving towards the direction of mechanized production.