After the glass melting furnace adopts oxy-fuel combustion technology, the volume concentration of water vapor in the flame space increases greatly, and the volume concentration of alkali volatiles increases by 3 to 6 times. In the case of more upper space in traditional air-assisted combustion furnaces, siliceous materials will be used. High concentrations of water vapor and high concentrations of alkali volatiles form a strong alkaline atmosphere, which harms the performance of silica bricks in the upper structure.
SiO2+2NaOH+O2→Na2SiO4+H2O
Compared with the arch, the concentration of alkali vapor in breast walls, gables and other parts will be slightly lower, but they are also facing acid-base neutralization reactions. The erosion rate beyond the norm makes the life of the kiln unpredictable. In addition, the eroded and peeled materials fall on the surface of the glass to form scum, which further affects the melting performance and glass quality.
1. Factors to be considered in material selection
In the principle of refractory selection, in addition to high temperature resistance, processability, economy, and systematization, suitable refractory materials should also be selected according to the characteristics of oxy-fuel combustion. Whether it is fused AZS or fused alumina products, domestic oxy-fuel combustion glass furnaces have accumulated relatively rich successful experience. According to the characteristics of the furnace and the characteristics of the product, there are also cases of using other refractory materials in China.
At present, the refractory materials used in the upper structure of oxy-fuel combustion glass furnaces are mainly fused casting materials, such as fused AZS series, fused alumina series, fused chrome corundum series (currently there is a tendency to use sintered chrome corundum instead), etc. In the fused AZS series, in addition to the common ER1681, ER1685 and ER1711, it also includes ER1851 and ER1195. The fused alumina series includes fused αβ alumina and fused β-alumina materials.
2. Low glass phase product-ER1851
This is a low glass phase fused cast AZS series product. Compared with ordinary AZS products, the content of silica and sodium oxide in its composition is reduced to a certain extent, so that the glass phase content in the product is usually less than 14%. When used in kilns, it can not only reduce the pollution of glass phase to glass, but also has strong resistance to alkali vapor, flying material and good creep resistance, making it an excellent refractory material for anti-drop pollution.
The products of the ER1851 series have been used in some oxy-fuel combustion projects and achieved satisfactory results. They have good performance in low seepage and erosion resistance. Some domestic manufacturers are also actively developing such products, and the market prospects of low glass phase products are broad.
3. High zirconium series refractory materials (1195 or 950 Series)
High zirconium series products usually refer to products with a zirconium oxide content of 85% to 95%. Although it is difficult to produce and has a high price, it is widely used in many glass industries due to its superior performance.
Due to its excellent properties, it reduces the generation of stones and bubbles in glass production, so it is more suitable for the parts in contact with glass in glass kilns, and has good performance in melting furnaces for borosilicate glass, aluminosilicate glass, glass ceramics and display glass. According to different requirements, there are also subdivided products with high resistivity.
ER1195 has a higher load softening temperature, a higher corrosion resistance level, and low permeation characteristics close to ER1851. It is widely used in pool walls, electrode bricks, and flow holes in oxy-fuel kilns. It is also the preferred choice for flow hole bricks in oxy-fuel electronic microcrystalline glass kilns.
4. Electric fused α-β alumina products
It is composed of about 50% α-alumina and β-alumina, and the crystals of the two are interlaced to form a very dense organizational structure, with excellent alkali resistance and excellent corrosion resistance below 1350℃. Because it does not contain harmful impurities such as iron and titanium, the glass phase content is very small, and the flakes have little pollution to the glass. In addition to being used in common flow channels, clarification tanks, and lip bricks of float glass melting furnaces, it can also be used as the top of the kiln (mainly considering the long life and the pollution of glass liquid by hanging drops).
5. Fused β-alumina products
It is composed of 100% β-alumina, with large flat β-alumina crystal structures, the crystals are interlaced and the crystal phase is very coarse, the porosity is high, and the strength is low. However, it has good anti-stripping properties, especially high corrosion resistance to strong alkali vapor. Since β-alumina is easy to break with α-alumina and sodium oxide after reacting with silica, it must be used in parts with lighter dust flying. As the top of the kiln, it is generally used at the back end.
Fused α-β or β-alumina products have shown excellent corrosion resistance and little pollution to glass in kilns that have been used in China, and the application effect is very good. The high price is the biggest obstacle to its widespread use.
6. Sintered α-β alumina products
Sintered α-β alumina products are relatively cheap, and have been successfully used in domestic kilns in recent years. Due to its special composition, it has become a better choice for medium and low temperature channels. For smaller furnaces or when the requirements for crown drop pollution formed by the crown are not very high, it can be considered as a crown material.
7. Application of new refractory materials
7.1 High-purity magnesium aluminum spinel
The chemical formula of magnesium aluminum spinel is MgO-Al2O3, containing 28.3% MgO and 71.7% Al2O3 (atypical data, for reference only). Its main advantages are strong corrosion resistance to reducing atmospheres such as free CO2, free SOX and free K2O/Na2O, and good thermal stability and wear resistance. Magnesium aluminum spinel has a high melting point, small thermal expansion coefficient, low thermal stress, good thermal shock stability, and a smaller volume density (2.7g/cm3) than fused materials, and has strong resistance to alkaline atmosphere.
7.2 Isostatically pressed high zirconium bricks
Isostatically pressed high zirconium bricks have been used in oxy-fuel kilns due to their excellent density, corrosion resistance and lower price than fused high zirconium bricks, and the application effect is good.
7.3 Dense chromium oxide bricks
Dense chrome bricks have been widely used in glass fiber kilns due to their excellent thermal shock resistance, alkali resistance and erosion resistance, including the flame space of oxy-fuel glass fiber kilns. The chromium content in dense chrome bricks is different according to different use parts, generally Cr2O3>90%, bulk density>4.0g/cm3.
7.4 Fused and sintered mullite
The typical product data of fused and sintered mullite bricks are: Al2O376%, Fe2O30.1%, SiO223%. Load softening temperature T0.5>1700℃, volume density 2.66g/cm3, room temperature compressive strength 1100N/mm2, glass phase content <4%. Due to its relatively neutral characteristics, it is currently used more in the large arches of all-oxygen combustion fiberglass kilns.
7.5 Calcium-free silica bricks
The advantages of its products are: ① It uses an electric fused silica bonding phase, does not contain calcium oxide, and the silica content can reach more than 98.5%; ② The density is smaller (1.8g/cm3), which reduces the weight of the entire arch; ③ The price is lower than that of the same level of electric fused materials; ④ It has a lower expansion coefficient (0.6% at 650℃) and a higher operating temperature (T0.5 is 1690℃) than silica bricks; ⑤ Better volume stability. It has been used in a domestic all-oxygen combustion kiln, but the arch has deformation problems that threaten the safety of the kiln.
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