Fused refractory materials are precisely prepared mixtures that are melted in an electric arc furnace, then cast in a sand mold, and the ingot is heat treated before mechanical processing. The smelting process can obtain complete and coarse crystals and dense structures. Compared with sintered refractory materials, it has the characteristics of dense structure, very low porosity, high volume density, high mechanical strength and high-temperature structural strength, and strong resistance to glass liquid erosion. The main varieties currently produced are: fused mullite bricks, fused zirconium corundum bricks, fused chromium zirconium corundum bricks, fused quartz bricks, fused corundum bricks, etc.
1Fused mullite bricks
Mullite bricks are aluminum-silicon system refractory products with mullite as the main crystal phase. The refractory temperature is around 1850℃, the load softening temperature is high, the high-temperature creep rate is low, and the thermal shock resistance is good.
Fused mullite bricks use high-alumina bauxite as raw material, and different bauxites are mixed into the composition of mullite (3Al2O3·2SiO2, with mass fractions of Al2O3 72%, SiO2 28%). It melts at around 2300℃, is poured into a sand mold at 1850℃, and then annealed to eliminate stress. The main crystal phases are mullite and corundum, and the glass phase is filled between the crystal phases. Its resistance to glass liquid erosion is stronger than that of sintered refractory materials, but not as good as other fused refractory materials. Adding a small amount of zirconium dioxide (7%~8.5%) can make the mullite crystals smaller and the brick structure dense, and the amount of mullite increases to 60%~70%, which relatively reduces the content of the glass phase and reduces the cracks of the product. Fused mullite bricks have now been basically replaced by fused zirconium bricks.
2 Fused zirconium corundum bricks
Fused zirconium corundum bricks belong to the Al2O3-ZrO2-SiO2 system (abbreviated as AZS bricks). According to the content of ZrO2, it is divided into three levels, and the corresponding grades are: AZS-33 (containing 33% ZrO2), AZS~36 (containing 36% ZrO2), AZS-41 (containing 41% ZrO2).
Fused zirconium corundum bricks are made of zircon and industrial alumina as the main raw materials, and a small amount of zircon-rich sand (in order to increase the content of ZrO2), soda ash and borax are added.
According to the different atmospheres during melting, there are two methods: reduction melting method and oxidation melting method. At present, the oxidation method is mainly used. The oxidation melting method can eliminate the pollution caused by graphite electrodes in the molten liquid, and the carbon content is low (0.005%~0.02%), which can reduce the bubble content in the glass and improve the quality of the glass liquid.
There are several aspects to pay attention to when using fused zirconium corundum bricks.
①Irregular changes in thermal expansion
The characteristics of fused zirconium corundum bricks are stable and dense organization, and strong resistance to glass liquid erosion.
②Electrical insulation
Fused zirconium corundum bricks have good electrical insulation and decrease with increasing temperature. It can be seen that fused zirconium corundum bricks can be used as electrode bricks in the production of soda-lime glass melting furnaces.
③Thermal conductivity
The thermal conductivity of fused zirconium corundum bricks is more than twice that of clay bricks. Therefore, when used as pool wall bricks, the cooling air volume required near the liquid surface should be 1 times larger than when clay bricks are used as pool walls.
1—AZS bricks containing 40% to 42% ZrO2;
2—AZS bricks containing 31% to 33% ZrO2
3Fused corundum bricks
Fused corundum bricks are made of high-purity alumina as raw material, and a small amount of soda ash is introduced, and melted at 2000 to 2200°C. The production process is similar to that of high-aluminum products. There are fused α-corundum bricks, fused α, β-corundum bricks, fused β-corundum bricks, etc. Fused α-corundum bricks are mainly suitable for melting borosilicate glass, opal glass and sulfate-containing melts. Fused α, β-corundum bricks contain only a small amount of glass phase, which will not seep out during use, have little pollution to the glass liquid, and will not color the glass liquid when in direct contact with the glass liquid. They have good corrosion resistance and are ideal materials for the cooling pool wall, kiln bottom and float glass furnace launder of the glass melting furnace. Fused corundum bricks are suitable for the upper structure of the working pool of the melting furnace and the burner at the rear of the melting zone and the breast wall and small furnace flat sill nearby.
4 Fused chromium-zirconium corundum bricks
10%~30% Cr2O3 is introduced into AZS-33 bricks, and fused chromium-zirconium corundum bricks (abbreviated as AZCS bricks, French grade ER2161) are produced by shrinkage-free casting method. The surface and interior of the brick are dark green. The brick contains 4.5% shrinkage pores and no connected pores. Due to the formation of aluminum-chromium spinel solid solution, the viscosity of the glass phase is increased, and the resistance to glass liquid erosion is greatly improved. It is 3.4 times that of fused AZS-33 bricks and 2.6 times that of fused AZS-41 bricks.
5 Fused quartz bricks
The production process of fused quartz bricks is the same as that of quartz glass. High-purity quartz sand (SiO2 content above 99.5%) is used as raw material, a small amount of Na20 is added as a mineralizer, and it is melted in a graphite resistance furnace (1750~1800℃). After melting, the whole molten material is quickly taken out, rolled into bricks, and then mechanically processed after cooling in the air. The thermal expansion coefficient is extremely small and the thermal shock resistance is good. It has strong resistance to boron glass liquid erosion, but not to sodium-lime-silicon glass liquid erosion.
6 Zircon bricks
The main component of zircon products is ZrO2·SiO2. Zircon is decomposed into ZrO2 and SiO2 in large quantities when heated at 1680℃. Zircon products have good corrosion resistance to liquid glass. Aluminum-zirconium-silicon (AZS) cast bricks and fired bricks have good resistance to erosion by glass liquid and can be used for the pool wall and upper structure of glass melting furnaces.
Zircon bricks are refractory products fired with zircon as the main raw material. Zircon is the only compound in the ZrO2-SiO2 system, and its chemical formula is ZrSiO4. The theoretical chemical composition of zircon is: ZrO2 67.2%, SiO2 32.8%. Zircon bricks have excellent properties such as high temperature resistance, good thermal vibration resistance and resistance to glass liquid erosion.
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