Introduction of production process and performance of fused white corundum refractory

According to the manufacturing process, fused white corundum refractory fused white corundum materials can be divided into fused and cast corundum bricks and recombined (semi-recombined) production of fused white corundum. Fused chrome sand cast corundum brick is melted in an electric arc furnace in an industrial oxidation furnace and a small amount of alkali and quartz powder, and then goes through the processes of casting and annealing. After Z, it is machined into the required shape and size. Recombined (semi-combined) fused corundum bricks use crushed fused corundum particles and powders, add binders, and after thorough mixing, they are formed by a brick press, and the bricks are dried and fired at high temperature.

 Fused Corundum Products

 After the alumina raw material batch is melted at a temperature higher than the melting temperature, it is poured into a prefabricated refractory model, and the product formed by cooling and solidifying to make the crystal grow up is called fused cast corundum refractory material, which is generally melted by electrofusion. The fused-cast corundum refractory fused with industrial oxidation furnace as the main raw material is melted and refined by batch materials, and then poured into the model. The castings are solidified, annealed, cut, ground and pre-assembled, which are called qualified products. There are three main types according to their chemical mineral composition.

Fused chrome sand

 Fused cast α-A1203, also known as fused α-A1203 brick, is a fused cast refractory with α-A1203 as the main phase, about 95% of α-A1203, about 99.5% of β-A1203, and less glass phase, accounting for less than 0.5% , inert to alkali vapors. In an alkali-free atmosphere, α-A1203 is easily separated from alkali, and α-A1203 is converted, accompanied by volume shrinkage, which may cause cracking of bricks. Casting β-A1203 bricks have good thermal shock resistance, and the air circulation from 700 degrees to room temperature can reach about 40 times. It is generally suitable for the upper structure of masonry glass furnaces.

 Fused and cast α, β-A1203 bricks, also known as fused α, β-A1203, are fused and cast products containing α-A1203 and β-A1203. α-A1203 accounts for about 40%-50%, β-A1203 accounts for about 45%-60%, and there is very little glass phase. Compared with β-A1203 type products, it has stronger corrosion resistance to glass liquid, and also has better resistance to alkali steam at high temperature.

 Production of fused white corundum

 The manufacturing process of fused and cast corundum products is generally the same as that of fused and cast refractory materials. Just due to the difference in material properties, the process parameters are slightly different: such as high melting temperature (2300-2500 degrees), high pouring temperature (1900-1980 °C). These specially determine the process points of the molten corundum products as follows:

 Raw materials and batches The raw materials used for casting corundum products are mainly industrial industrial alumina and quartz sand. The batching material is composed of industrial alumina 97% (requires A1203 content greater than or equal to 98.5%) and quartz sand 3%, and a small amount of additives are added. After 30-40min mixing on the wheel mill.

 In the case of no additives, the temperature of the molten pool of the melting furnace should reach 2200-2300 degrees. In this case, the melt has a large expansion, and a large shrinkage cavity will inevitably occur after pouring and cooling. It is difficult to obtain dense and uniform At the same time, because the viscosity of the alumina melt is very low and the crystallization ability is very strong, it will solidify before exhausting, and a large number of micropores will be formed in the casting. In order to reduce the shrinkage of the cast brick, it is necessary to add a small amount of extra In order to reduce the melting temperature bottom, and then reduce the volume expansion of the melt, different materials are added according to the product type.

 In addition to the batch materials, there are recycled waste materials such as waste castings, risers, etc., as well as foreign clean corundum fragments, but the amount of the charging material does not exceed 30% of the charging amount. Smelting and casting of products, the fused and cast alumina products produced in my country contain C0.03%-0.55%, while foreign similar products are only 0.05%. The existence of C will reduce the density, thermal shock resistance and corrosion resistance of products, increase Shrinkage cracks and the tendency to form bubbles in the glass increase, so the low carbon content needs to be reduced as much as possible, and the content of dyed oxides (Fe2O3, TiO2) and H2O, SO2, N2, etc. must be limited, so the oxidative melting production process is adopted. The amount is reasonable, and the method of adding oxidant and oxygen blowing is also added. During the melting process, oxygen blowing can play the dual role of stirring and decarburization.

 The oxidation electric melting method was first invented in France. It is characterized by the use of a long arc. The graphite electrode uses the arc arc radiation on the upper part of the material to melt the material at high temperature, avoiding contact with the solution, and the working space is in an oxidizing atmosphere. During the melting process, attention should be paid to oxidation. The molten arc should not be too long, generally 30-50mm, because the arc voltage has a linear relationship with the arc length, and as the arc voltage increases, the corundum crystal grain size decreases. When it reaches 230v, the density of the casting will decrease; at the same time, the melting time will be reduced. Shorten it to the limit to avoid carbon increase; secondly, there is flue gas dust removal and ventilation to ensure the oxidizing atmosphere in the furnace.

 When all the ingredients are melted and the surface of the melt is very clean, alkaline pouring can be done. Before pouring, the mold should be prepared. The temperature of the melt should be controlled at 1960-1980 ° C. In order to improve the pouring density, it can be divided into Layer pouring, pour one layer in 10-20 minutes, pour the next layer when the surface temperature of the previous pouring is cooled to 1680-1700, and add an electric sintered block with a similar composition to the melt to increase the melt flow during pouring. Density, reasonable setting of the gate, and removal of the part of the casting with many pores can promote the increase of the density of the casting.

 When pouring the melt, it is necessary to ensure good fluidity and avoid overheating, because overheating will cause the melt to absorb gas, and the shrinkage rate after casting into the mold will increase accordingly, and the casting will also produce shrinkage and shrinkage. cracked.

 Casting cooling, the cooling and cooling process of castings is roughly divided into four stages: melt flow (casting), heat dissipation, melt solidification and hardening (crystallization), and cooling of hardened castings. In melt hardening engineering, its structure is being formed. In the initial stage, the skin temperature of the casting dropped sharply, and the melt quickly dissipated heat to the model to form a very dense part, and the crystal phase precipitated to form the microcrystalline and mesocrystalline regions of the casting. The remaining melt was mainly concentrated in the middle part of the casting near the pouring surface. A coarse-grained structure is formed. Since the crystallization begins to expand from the edge to the center, when the periphery of the casting crystallizes and hardens and begins to cool, the center of the casting is still a high-temperature liquid melt, and the temperature difference between the edge and the center is quite large, and cracks are bound to form in the casting. The greater the difference between the internal and external hardening speed, the greater the thermal stress. Therefore, the cooling process of castings is of great significance to the quality and structure of fused castings.

 The casting should be annealed after demoulding. The actual practice is to control the hardening (crystallization) and cooling rate of the casting. There are two ways of annealing: natural annealing and controlled annealing. Natural annealing depends on the good heat insulation layer outside the casting; Cool the casting smoothly and slowly, and the cooling rate can be adjusted by the thickness of the diatomite or expanded vermiculite layer outside the mold. Controlled annealing is to remove the hardened skin from the casting mold or put it into a small tunnel kiln, and slowly cool it according to the specified annealing curve.

 Machining of products, qualified products of annealed cast corundum refractories must be mechanically processed before being put into storage. Due to the high hardness and high strength of corundum, diamond tools are used for processing. The castings undergo cutting, grinding, drilling and other processes, so that the products have accurate geometric shapes, smooth and flat surfaces and installation requirements.

 Combined with fused corundum products

 The fused corundum clinker is used as the granular material, and the fused corundum fine powder or the sintered corundum fine powder is used as the matrix. Or when white corundum is used as the raw material, it is necessary to smash the frit and select it, and remove the ferrosilicon alloy entrained by the brown corundum block into other impurity components. The white corundum block should be removed from the flaky crystalline sodium peraluminate and other low-melting substances. These impurity minerals usually float on the surface of the corundum frit due to their low density and are easier to identify. Corundum contains a small amount of harmful components. It will cause poor sintering or cracking of the product, so it should be calcined in advance before use. The unselected ferrosilicon alloy is oxidized and decomposed into Fe2O3 and SiO2 at 500-1000 degrees, and titanium-containing minerals are oxidized into TiO2 (rutile), etc., all of which produce large volume expansion. The resulting failure stress is eliminated during the pre-sintering process to avoid cracking of the product due to the reaction expansion of these impurity minerals when the product is fired.

 The fused corundum block is large in size and needs to be smashed with a drop hammer or other methods, and then pulverized, and stored in different sizes. When it reaches <40um or finer <10um, electromagnetic iron is used for granular materials, and pickling is used to remove iron for fine grinding powder.

 In batching, mixing and molding, the particle matching should be based on the principle of close packing, using multi-stage proportioning, reducing intermediate particles and increasing the amount of fine powder (including a certain amount of ultrafine powder), which is conducive to improving the density and sintering of the product. Adding a certain amount of combination There are mainly aluminum phosphate, phosphoric acid, aluminum chromium phosphate, cellulose, pulp waste liquid, etc. Among them, the most promising is active phosphate. In recent years, ammonium phosphate has been tried and good results have been obtained. It is 3%-4%, after high pressure forming, dense brick body is obtained.

 Firing, fused corundum products have high purity and are difficult to sinter. They need to be fired at a temperature higher than 1800 degrees. The kiln used for firing depends on the scale of production. In small batch production, high temperature intermittent kiln is appropriate, and batches are stable. Small high-temperature tunnel kilns can be used in production.