Al-O aluminum oxide series electrofusion casting refractory for glass kiln
In 1928, the American Corhart Rcfracries.Co. began to develop fused cast refractory materials. Because such refractory materials are particularly resistant to corrosion by molten glass and molten slag, and have a long service life, they have attracted the attention of experts from all over the world. And get rapid development.
The biggest advantage of alumina-based fused cast refractories (except fused mullite bricks) is that there are very few glass phases, so they are used in glass melting furnaces. In fact, fused corundum refractories should include two varieties, namely fused corundum sand and fused casting products. The former is combined to produce sintered corundum products and unshaped refractory materials, and the latter is a product that is directly cast into a certain shape after melting.
Due to the high content of SiO2 contained in mullite electrofusion cast bricks, iron bauxite and other natural raw materials of the SiO2-Al2O3 series can be used. There are also re-sintered fused mullite made of fused mullite as aggregate.
The aluminum oxide raw material used in the fused corundum brick is chemically treated industrial alumina and introduces a small amount of soda ash. Adding a small amount of sodium carbonate to industrial alumina can make α (alpha)-corundum fused cast brick; adding a little more sodium carbonate can make α·β-mixed crystal corundum fused cast brick; adding more sodium carbonate can Make β (beta)-corundum electrofusion cast bricks. Industrial alumina and chromium oxide can be mixed into aluminum chromium electrofusion cast bricks.
Al2O3 fused cast bricks are mainly composed of Al2O3, with well-developed crystals and dense structure. Al2O3 precipitates α-corundum after melting and solidification. When a small amount of Na2O coexists, β-corundum is precipitated. Its chemical formula is Na2O·11Al2O3, and the weight of Na2O and Al2O3 The percentage is 5:95. The α-corundum bricks, β-corundum bricks and α·β corundum bricks contain a large amount of Al2O3 and a small amount of Na2O, but few other oxides. There are only two crystal phases of α corundum and β corundum.
The SiO2 content of mullite electrofusion cast bricks is above 15%. Therefore, in addition to the formation of corundum crystals, a considerable amount of mullite crystals are formed. A small amount of other oxides such as Fe2O3, TiO2, one part 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 a small amount of aluminosilicate crystal nuclei in the glass phase.
A part of Cr2O3 in the aluminum-chromium fused cast brick is dissolved in corundum solid solution, and the other part is combined with Al2O3, MgO, and FeO to generate composite spinel in a certain proportion. There is very little glass phase, and because there are few SiO2 components, the glass phase does not contain crystal nuclei.
2. Features
1. General characteristics
The main crystal phase of the aluminum-oxygen fused cast refractory is corundum, which is a neutral refractory.
Alpha corundum has a specific gravity of 3.99 and beta corundum has a specific gravity of 3.2, so bricks with more alpha corundum have a heavier weight. Mullite fused cast bricks contain mullite crystals with a smaller specific gravity. Therefore, the specific gravity is relatively low, which is similar to that of β-corundum bricks. Aluminium chrome bricks have a solid solution of Cr2O3 mixed in the corundum crystals, and the specific gravity of spinel is also greater than that of corundum, so it is really larger than α-corundum bricks.
2. α-Corundum bricks (rarely used) and αβ-Corundum bricks have less pollution to molten glass
There are almost no metal oxides with strong coloring properties such as Cr2O3, Fe2O3, and TiO2 in these two refractories. Therefore, the glass liquid will not be colored at all when it is in direct contact with the glass. The content of glass phase in the material is below 1.0%, which is the lowest containing glass phase among all refractory materials in contact with glass. The glass phase is the weak link in refractories. After the refractory is corroded, the glass phase is the first to be softened and lost. As a result, the main crystal phase loses its combination and becomes loose, and enters the glass to become stones and stripes. Moreover, the glass phase often contains bubbles. 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 molten glass.
The main crystal phase of fused corundum brick is corundum, and the viscosity of the metamorphic layer formed after being corroded by soda lime glass at high temperature is lower than that of cast corundum brick. Therefore, the corrosion resistance at high temperature is not as good as that of zirconium corundum bricks, and it is generally not used in places such as the wall of the melting part and the liquid hole. It is mainly used on the wall of the working pool and the material channel. The temperature of these parts is low, and the bricks are mainly required to pollute the molten glass. These two kinds of refractory materials have less pollution to molten glass than zirconium corundum bricks when used in these low temperature parts. Compared with α-corundum bricks and αβ-mixed crystal corundum bricks, if the porosity is the same, the α-corundum bricks are more resistant to the corrosion of molten glass. In fact, due to production reasons, some α-corundum bricks have higher porosity. The α-corundum bricks with higher porosity have worse corrosion resistance than αβ-corundum bricks with lower porosity. Pay special attention to this point. (Alphaβ-corundum bricks are often used for pool wall tiles in the work department)
3. β corundum fused cast brick has good stability to alkali vapor
β-corundum bricks are pure β-corundum crystals, namely Na2O·11Al2O3 crystals. The crystal shape is plate-like, with large crystals, and crosses with each other. The surface of the brick is flexible, 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.
When alumina is in contact with alkali metal oxides at high temperatures, beta corundum is formed. Because β-corundum brick itself is composed of corundum crystals, it will not react with alkaline oxides. But another point to pay special attention to is that Si02 will react with corundum as follows:
2SiO2+Na2O·11Al2O3→Na2O·Al2O3·2SiO2+10Al2O3
That produces nepheline and α-corundum. This will loosen the β-corundum brick structure and destroy it. Therefore, β corundum bricks cannot be in contact with SiO2 at high temperatures. This makes this kind of bricks unable to be used in places with large batch material dust, in horizontal flame furnaces in the upper space near the charging port, and horseshoe flame furnaces in the upper space of the melting pool.
Beta-corundum bricks have the best thermal stability among all fused cast refractory materials due to the well-developed crystals, the tight bonding between crystals and the high porosity. It is very suitable to be used as the refractory material for the rear section of the melting part of the horizontal flame tank furnace (far away from the feeding port) and the upper space of the working tank. But it should be noted that its compressive strength is low. If it is used as a dome brick, the span of the dome cannot exceed 6 meters.
4. Poor performance of mullite electrofusion casting refractories
Mullite electrofusion cast brick is the oldest electrofusion cast refractory material. It is inferior to other electrofusion cast refractories, but better than combined refractories. Long ago, it was used as the lower pool wall brick to balance the life of the upper and lower pool walls and reduce the cost.
When the electrofusion cast mullite brick is manufactured, corundum crystals are first precipitated during the cooling process, and then mullite crystals are precipitated. These two crystal phases consume most of the SiO2 and Al2O3 in the brick composition. The remaining small amount of SiO2, Al2O and other components, namely Fe2O, TiO2, CaO, MgO, Na2O coexist, and finally become the glass phase. The glass phase occupies a large proportion in the brick, and the glass phase contains so many low-melting point substances, so the corrosion resistance is worse than other electrofusion cast refractory materials. In addition, the glass phase contains bubbles and reducing substances, so bubbles will contaminate the glass. On the other hand, electrofusion cast mullite bricks have better corrosion resistance than sintered refractories of the same composition. This is because the mullite in the refractory material is corroded by alkali to generate corundum and nepheline liquid phase. Nepheline has a low liquid phase viscosity and is easy to lose. As a result, the brick body is destroyed. Due to the high porosity of sintered refractories, this reaction can penetrate into the interior of the brick body, so it is highly destructive. Because of its high compactness, mullite electrofusion cast bricks can only carry out this reaction on the surface of the bricks, so they have high corrosion resistance.
5. Aluminum chromium (chromium corundum) electrofusion cast refractory material with particularly good corrosion resistance
The glass phase content of aluminum-chromium electrofusion cast refractories is very low, below 5%. It is surrounded by two crystal phases with good corrosion resistance. One of the crystal phases is corundum reinforced by containing Cr2O3. The second crystal phase is a composite spinel composed of Cr2O3, MgO, Al2O3, etc. Both of these crystal phases are very stable. Therefore, the corrosion resistance of the whole brick is particularly good, which is three times higher than the high temperature corrosion resistance of No. 41 zirconium corundum brick. It is ideal for making liquid hole bricks.
However, because Cr2O3 has extremely strong colorability, 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 do not require color. In addition, the thermal stability of this kind of brick is not good, and its use is also restricted.
3. Matters needing attention
Poor thermal shock resistance. Due to its compactness and low porosity, electrofusion cast refractories have no pores for buffer adjustment when subjected to thermal shock and uneven heating, so they are very easy to explode. Since the glass bath furnace is operated continuously for a long time, the temperature does not change much, so this shortcoming does not affect the use. However, in the vicinity of the vent, the temperature will change somewhat due to the reversal, so be careful. In addition, pay attention to the temporary cooling and repair of the furnace. The furnace temperature cannot change too much. This type of brick has poor thermal stability. But there are still differences between different varieties.
The thermal stability sequence is as follows (when the porosity is the same):
β-Corundum fused cast brick>α-corundum fused cast brick=mullite fused cast brick>aluminum chromium fused cast brick>α.β-fused cast brick
