Al-O Alumina Fused-Casting Refractory for Glass Furnace
Fused cast refractory materials are highly valued by experts from all over the world because they are particularly resistant to corrosion by glass liquid, slag, etc. and have a long service life. They have been rapidly developed. The biggest advantage of alumina-based fused cast refractory materials (except fused mullite bricks) is that they contain very little glass phase, so they are widely used in glass melting furnaces. In fact, fused corundum refractory materials should include two varieties, namely fused corundum sand and fused cast products. The former is combined to produce sintered corundum products and amorphous refractory materials, while the latter is a product directly cast into a certain shape after melting.
Composition of Alumina-Based Fused Cast Refractory Materials
Mullite fused cast bricks can be made from ferroalumina and other SiO2-Al2O3 natural raw materials due to their high SiO2 content. There are also re-sintered fused mullite made from fused mullite as aggregate.
The alumina raw material used in fused corundum bricks is chemically treated industrial alumina with a small amount of soda ash introduced. Adding a small amount of sodium carbonate to industrial alumina can make α (alpha)-corundum fused cast bricks. Adding a little more sodium carbonate can make α·β-mixed crystal corundum fused cast bricks. Adding a larger amount of sodium carbonate can make β (beta)-corundum fused cast bricks. Industrial alumina and chromium oxide can be mixed to make aluminum-chromium fused cast bricks.
Alumina fused cast bricks are mainly composed of Al2O3, with good crystal development and dense structure. Al2O3 precipitates α-corundum after melting and solidification, and β-corundum precipitates when a small amount of Na2O coexists. Its chemical formula is Na2O·11Al2O3, in which the weight percentage of Na2O to Al2O3 is 5:95. In addition to a large amount of Al2O3 and a small amount of Na2O, α-corundum bricks, β-corundum bricks and α·β corundum bricks contain very few other oxides. There are only two crystal phases: α corundum and β corundum.
The SiO2 content of mullite electrofused cast bricks is above 15%. Therefore, in addition to the formation of corundum crystals, a considerable amount of mullite crystals are also generated. A small amount of other oxides such as Fe2O3 and TiO2, part of which forms a solid solution with mullite, and the other part forms a glass phase with a small amount of SiO2 and Al2O3. The main crystal phases in the brick are corundum and mullite. The glass phase is filled between the crystal phases, and there are also a small amount of aluminosilicate crystal nuclei in the glass phase.
In aluminum-chromium electro-fused cast bricks, part of Cr2O3 dissolves in the corundum solid solution, and the other part forms composite spinel with Al2O3, MgO, and FeO in a certain proportion. There is very little glass phase, and due to the small amount of SiO2, the glass phase does not contain crystal nuclei.
Characteristics of Alumina Electro-Fused Casting Refractory
General Characteristics
The main crystal phase of Alumina Electro-Fused Casting Refractory is corundum, which belongs to neutral refractory.
The specific gravity of α corundum is 3.99, and the specific gravity of β corundum is 3.2, so the bricks containing more α corundum have a higher specific gravity. Mullite electro-fused casting bricks contain mullite crystals with a lower specific gravity. Therefore, the specific gravity is lower, which is similar to that of β-corundum bricks. Alumina-chrome bricks have a higher specific gravity than α-corundum bricks because the corundum crystals contain Cr2O3 solid solution and the specific gravity of spinel is greater than that of corundum.
α-corundum bricks (rarely used) and αβ-corundum bricks have less pollution to glass liquid
There are almost no metal oxides with strong coloring properties such as Cr2O3, Fe2O3, Ti02 in these two refractory materials. Therefore, when in direct contact with glass, the glass liquid will not be colored at all. The glass phase content in the material is below 1.0%, which is the lowest among all refractory materials in contact with glass. The glass phase is the weak link in the refractory. After the refractory is eroded, the glass phase is softened and lost first. As a result, the main crystal phase loses its binding material and becomes loose, entering the glass to become stones and streaks. Moreover, the glass phase often contains bubbles, and as the glass phase is eroded and lost, the bubbles also enter the glass liquid. Therefore, bricks with less glass phase have better corrosion resistance and less pollution to glass liquid.
The main crystal phase of fused corundum bricks is corundum, and the viscosity of the metamorphic layer generated after being eroded by soda-lime glass at high temperature is lower than that of cast corundum bricks. Therefore, the corrosion resistance is not as good as that of zirconium corundum bricks at high temperatures, and it is generally not used in the melting pool wall and liquid flow hole. It is mainly used on the working pool wall and material channel. The temperature in these parts is relatively low, and the main requirement is that the bricks have less pollution to the glass liquid. When these two refractory materials are used in these low-temperature parts, they have less pollution to the glass liquid than zirconium corundum bricks. Compared with α-corundum bricks and αβ-mixed crystal corundum bricks, α-corundum bricks are more resistant to glass liquid corrosion if the porosity is the same. In fact, due to production reasons, some α-corundum bricks have a higher porosity. The corrosion resistance of α-corundum bricks with higher porosity is worse than that of αβ-corundum bricks with lower porosity. Pay special attention to this point. αβ-corundum bricks are often used for the working pool wall bricks.
β-corundum electric fused cast brick has good stability to alkali vapor
β-corundum brick is pure β-corundum crystal, that is, Na2O·11Al2O3 crystal. The crystal is plate-shaped, large, and cross-bonded. The surface of the brick is soft and light-colored, translucent, brittle and fragile. This brick has two major characteristics. The first is good resistance to alkali vapor erosion. The second is good thermal stability.
Alumina will generate β-corundum after contacting with alkali metal oxides at high temperature. Because β-corundum bricks themselves are composed of corundum crystals, they will not react with alkaline oxides. But another point to pay special attention to is that SiO2 will react with corundum as follows:
2SiO2+Na2O·11Al2O3→Na2O·Al2O3·2SiO2+10Al2O3
That is, nepheline and α-corundum are generated. This will make the structure of β-corundum bricks loose and destroyed. Therefore, β-corundum bricks cannot be in contact with SiO2 at high temperatures. This makes this brick unsuitable for use in areas with large dust content, in the upper space near the charging port in a horizontal flame furnace, and in the upper space of the melting pool in a horseshoe flame furnace.
β-corundum bricks have the best thermal stability among all electro-cast refractory materials due to their well-developed crystals, close bonding and cross-linking between crystals, and high porosity. They are very suitable as refractory materials for the rear section of the melting section of a horizontal flame pool furnace (away from the charging port) and the upper space of the working pool. But one thing to note is that their compressive strength is low. If it is used as a sill brick, the span of the sill cannot exceed 6 meters.
Mullite electro-cast refractory has poor performance
Mullite electro-cast brick is the oldest electro-cast refractory. It is inferior to other electro-cast refractory materials, but better than combined refractory materials. It was used as the lower pool wall brick a long time ago to balance the life of the upper and lower pool wall bricks, so as to reduce the cost.
When electro-cast mullite bricks are manufactured, corundum crystals are first precipitated during the cooling process, followed by mullite crystals. These two crystal phases consume most of the SiO2 and Al2O3 in the brick composition. The remaining small amount of SiO2 and Al2O3 coexists with the remaining components, namely Fe2O3, TiO2, CaO, MgO, and Na2O, and finally becomes a glass phase. The glass phase accounts for a large proportion of the brick, and the glass phase contains so many low-melting-point substances. Therefore, the corrosion resistance is worse than other electro-cast refractory materials. In addition, the glass phase contains bubbles and reducing substances, so the generation of bubbles will pollute the glass liquid. On the other hand, electro-fused mullite bricks have better corrosion resistance than sintered refractory materials with the same composition. This is because the mullite in the refractory material will generate corundum and nepheline liquid phases when it is corroded by alkali. The nepheline liquid phase has low viscosity and is easy to lose. As a result, the brick body is damaged. Due to the high porosity of sintered refractory materials, this reaction can penetrate into the interior of the brick body, so it is very destructive. Due to the high density of mullite electro-fused bricks, this reaction can only occur on the surface of the brick, so it has higher corrosion resistance.
Aluminum-chromium (chrome corundum) electro-cast refractory with particularly good corrosion resistance
The glass phase content of aluminum-chromium electro-cast refractory is very low, below 5%. It is surrounded by two crystal phases with good corrosion resistance. One of the crystal phases is corundum enhanced by the presence of Cr2O3. The second crystal phase is a composite spinel composed of Cr2O3, MgO, Al2O3, etc. Both crystal phases are very stable. Therefore, the corrosion resistance of the entire brick is particularly good, three times higher than the high-temperature corrosion resistance of No. 41 zirconium corundum brick, and it is ideal for use as a flow hole brick.
However, due to the extremely strong coloring of Cr2O3, the coloring ability of trivalent Cr is dozens of times higher than that of trivalent Fe, so it can only be used in glass fiber tank furnaces and dark green bottle glass tank furnaces that have no requirements for color. In addition, the thermal stability of this brick is not good, which also limits its use.
Precautions
Poor thermal shock resistance. Electrofused refractory materials are dense and have low porosity. When subjected to thermal shock and uneven heating, there are no pores for buffering and adjustment, so they are very easy to burst. Since the glass pool furnace is operated continuously for a long time, the temperature does not change much, so this shortcoming does not affect the use. However, near the crater, the temperature will change somewhat due to reversing, so pay attention. In addition, pay attention to temporary cooling and furnace repair. The furnace temperature cannot change too much. This type of brick has poor thermal stability. But there are still differences between different varieties.
The order of thermal stability is as follows (when the porosity is the same):
β-corundum electrofused brick>α-corundum electrofused brick = mullite electrofused brick>aluminum chromium electrofused brick>α.β-electrofused brick
Corundum Mullite Refractory Bricks for Glass Kilns
Corundum mullite bricks for glass kilns are high-quality refractory products. This product uses corundum as the main raw material and mullite as the auxiliary raw material. By adding an appropriate amount of high-purity alumina, ultrafine silicon oxide powder, and additives, it is strongly pressed and formed by a 630T friction press. It is fired at high temperature in an oxidizing atmosphere and can be used for a long time in a high-temperature environment of 1700 degrees. The characteristics of corundum mullite bricks for glass kilns are as follows:
- (1) The Al2O3 of corundum mullite bricks for glass kilns is ≥82%, which has good high-temperature resistance. At the same time, the refractory temperature is high, and the load softening temperature is greater than 1700 degrees.
- (2) Resistant to chemical erosion, it has strong resistance to acidic solutions or slag.
- (3) The Fe2O3 content in the product is ≤0.3%, which is resistant to oxidation. It is not easy to react chemically with gases such as O2, H2, and CO.
- (4) Good thermal stability, high temperature volume stability, not easy to expand or shrink.
- (5) Good thermal shock resistance, 1100℃ water cooling ≥30 times, resistant to rapid cooling and heating, not easy to peel off.
- (6) High compressive strength at room temperature, ≥100Mpa, not easy to wear during transportation or unloading.
Corundum mullite bricks can directly contact flames, resist peeling, and withstand high temperatures. They can be used as working linings for high-temperature industrial furnaces. They are mainly used in petrochemical industry, large and medium-sized synthetic ammonia gasification furnaces and magnetic material gas furnaces, and supporting facilities for high-temperature industrial kilns. Rongsheng Refractory Material Manufacturer can customize refractory lining solutions according to the working conditions of glass kilns. It can also customize and process various shapes of corundum and mullite refractory bricks and products according to the customer’s high-temperature equipment requirements.