Chrome corundum platform bricks are mainly used on the push steel platform of the side-out steel heating furnace. The use conditions of this part are harsh, with both mechanical wear at high temperature and chemical erosion. Years of practical experience have proved that general refractory materials, such as clay, high alumina, and mullite, are difficult to meet the use requirements. Only chrome corundum materials can meet this use condition. At present, this type of chrome corundum platform brick is generally produced by two methods, namely the electric melting method and the pouring first and then firing method. The cost of chrome corundum bricks produced by the electric melting method is too high, and it is difficult for manufacturers to accept it; the latter has a more complicated production process and low production efficiency. Through many tests, our company has produced a chrome corundum brick. This brick adopts a new production process. The chrome corundum brick produced has better high-temperature performance, and the production efficiency is greatly improved, and the production cost is greatly reduced.
1 Experimental study
1.1 Selection of corundum
In order to enhance the wear resistance and corrosion resistance of this product, fused white corundum and industrial chromium oxide fine powder are selected as the main raw materials. In order to facilitate molding, the critical particle size of corundum is selected as 5 mm, and the ratio of particles to fine powder is controlled at about 55:45.
1.2 Selection of chromium oxide fine powder
In order to enhance the ability of chrome corundum platform bricks to resist the corrosion of iron oxide at high temperature, chromium oxide fine powder is added. Because chromium oxide has very high refractoriness and good chemical stability. Since this chrome corundum brick is used at a high temperature of about 1,300°C for a long time, it not only undergoes severe wear at high temperature, but also chemical corrosion by molten iron oxide. Properly increasing the content of chromium oxide can improve the chemical corrosion resistance of the brick body. However, experiments have shown that if the amount of chromium oxide added exceeds a certain limit, it is not conducive to the sintering of the brick. It is more appropriate to determine that the amount of chromium oxide added is 17%~20%. After the fine powder is added, it not only improves the chemical stability of the product at high temperature, but also has a favorable effect on the molding and sintering of the product. Its fineness requirement is less than 200 mesh and greater than 90%.
1.3 Selection of alumina fine powder
In order to improve the molding performance of the mud material, and to increase the volume density and high and room temperature strength of the chrome corundum platform brick, an appropriate amount of alumina ultrafine powder is added. Through many experiments, it is believed that the addition amount of alumina ultrafine powder is 5%~10% is appropriate.
1.4 Binders and additives
In order to facilitate the molding of the brick body without reducing the high temperature performance of the chrome corundum brick, sulfite pulp waste liquid is selected as the binder through experiments. Because in the semi-dry molding brick material, especially most of the lean material brick material, adding sulfite pulp waste liquid can effectively improve the strength of the brick blank, facilitate transportation and kiln loading, and determine its specific gravity to be 1.07, and the addition amount is about 4%. Since the raw materials of the bricks are all lean materials, molding is more difficult, so carboxymethyl cellulose is selected as a plasticizer, and its addition amount is determined to be 1.5%. Because this material has a large viscosity, it will not have a negative impact on the refractory properties of the brick.
1.5 Experimental method
Since this chrome corundum platform brick is used at a high temperature of about 1300℃ for a long time, it not only undergoes severe wear at high temperature, but also chemical erosion by molten iron oxide. Under such conditions, if the service life of the brick is to be significantly improved, then the brick must have sufficient strength at high temperature to withstand strong mechanical wear, so the high-temperature flexural strength index of the developed test block is tested. First, the aggregate and fine powder are mixed evenly, then added to a small mixer and stirred with the binder, and then formed on a 400 t friction brick press. After forming, the test block size is a standard brick of 230 mm×115 mm×65 mm. These test blocks are cut into test blocks of 115 mm×115 mm×65 mm and 160 mm×40 mm×40 mm. Finally, the room temperature compressive strength and high temperature flexural strength of this brick are tested, and the test block index is compared with the cast test block index. Through testing, the high-temperature flexural strength of cast chrome corundum bricks is 0.79 MPa and 0.12 MPa, while the high-temperature flexural strength of machine-pressed test blocks is 12.23 MPa, which is significantly greater than that of cast test blocks.
2 Production process
In terms of raw material selection, fused white corundum should be selected as high-quality products with dense sintering and low porosity, with particle sizes of 5~3 mm, 3~1 mm, and 1~0 mm, respectively. First, 180 mesh corundum fine powder, industrial chromium oxide fine powder, aluminum oxide fine powder and additives are mixed to prepare mixed fine powder. The main equipment for mixing is a mixer with a roller, and its minimum speed is 24r/min. The order of adding materials is to add granular materials first, stir for 1~2 min, then add about half of the binder, stir for about 2 min, add mixed fine powder, stir for 2 min, and finally add the remaining binder. Stir for about 5 min before discharging. The molding is carried out on a 600 t friction brick press. Due to the large volume of the bricks, the molding principle of light first and heavy later should be followed. The bricks should be handled with care when they are taken out to prevent the bricks from being damaged. The products are placed in a tunnel dryer for drying. The inlet temperature of the drying medium is about 120℃, and the outlet temperature is about 50~60℃. The residual moisture after drying is controlled to be less than 1%. The dried bricks are placed in a tunnel kiln for firing. The loading position is the middle and lower part of the kiln car. The highest firing temperature of the kiln is 1500℃, and the heat preservation is 4 hours. After firing, the bricks are dark brown.
3 Application
The chrome corundum platform bricks produced were used on-site in the heating furnace of Northwest Bearing in May 2015. The use position is the most demanding position of temperature and wear on the push steel platform, and the performance is compared with the chrome corundum bricks produced by a certain factory (same material, produced by casting molding). After use, it was observed that the surface erosion of this brick was significantly less than that of the control brick, which proved that this brick had excellent wear resistance and strong resistance to iron oxide corrosion, and could meet the use requirements of the push steel part of the heating furnace.
4 Conclusion
(1) This machine pressing method can produce chrome corundum bricks with relatively high strength at both room temperature and high temperature. Compared with the same type of bricks formed by casting, this brick has high volume density and uniform particle distribution. It can overcome the tendency of large particles generated by casting to settle downward and more fine powder on the upper part of the brick body, making the brick structure more uniform. (2) This new type of chrome corundum brick can greatly shorten the production cycle and significantly reduce the production cost due to the use of machine pressing, and the use performance of this product is better. (3) Through comparative use on the heating furnace, it can be proved that this new type of chrome corundum brick has excellent chemical corrosion resistance and wear resistance, which can fully meet the use requirements of the heating furnace.
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