Silica refractory for glass kiln
In the 1980s, my country introduced the manufacturing technology of advanced silica bricks for glass kilns from the United States. In order to distinguish it from the original product, the product manufactured with the imported technology is called high-quality silica brick. The phase composition of high-quality silica bricks (w): cristobalite 45%, phosphorite 50%, glass phase 1%-2% and a small amount of residual quartz; W(sio2)≥96%, melting index w(2R2O+Al2O3)< 0.5%.
After adopting the oxy-fuel technology, the life span of silica bricks has been drastically reduced from 5-10 years to 2-3 years. Rat holes and erosion are the main causes of damage. The rat hole is caused by the alkali in the leaking gas from the kiln that condenses on the joints of the bricks and corrodes the silica bricks. The methods to improve the life of silica bricks are as follows:
(1) Improve the dimensional accuracy of bricks, dimensional tolerance ≤0.5 mm, and brick gap width ≤1.5mm. The cracks between the bricks are narrowed, the kiln gas leaks less, the erosion is light, and the mouse hole is also small.
(2) Reasonably design the kiln lining structure so that the condensation temperature zone (954～788℃) of alkali is moved to the sealing layer behind the silica brick.
(3) Use low-calcium silica bricks or high-purity silica bricks with good corrosion resistance. Among them, the w (CaO) in the low-calcium silica bricks is about 0.8%. It can be seen from the schematic diagram of the mouse hole on the glass kiln in Figure 1 that the gas in the kiln leaks outward along the cracks in the bricks. As the temperature decreases, the alkali vapor in the gas condenses in the cracks of the bricks. Subsequently, the alkali corrodes the siliceous refractory material, and the cracks between the bricks are enlarged under the corrosive flow, resulting in larger gaps, until rat holes are generated and the dome roof is damaged. Therefore, it is necessary to lay an amorphous material layer and a thermal insulation layer on the back of the silica brick roof. By design, the alkali vapor condensation zone is located in the amorphous refractory layer behind the silica brick. As a result, the life of the dome roof can be greatly increased. The SiO₂ content of high-purity silica bricks can be increased to more than 98% (w). The main technical route:
1) The matrix uses quartz powder with w(SiO₂) ＞99%;
2) Use tricalcium silicate and superfine iron oxide powder as mineralizers, and use third-generation water reducer as dispersant. Due to the greatly improved efficiency of the mineralizer, very few CaO and Fe₂O₃ mineralizers can be used for mineralization. High-purity silica bricks have excellent physical and chemical properties, and their appearance is pale yellow or even white, without the "iron spots" and "calcium holes" like leopards.
2. Alkaline refractory materials for glass kiln
2.1 The configuration of refractory materials in the regenerator of the glass furnace
Generally, the uppermost layer of the regenerator lattice body uses high-grade magnesia bricks, w(MgO)>97%; the upper layer uses medium-grade magnesia bricks, w (MgO) is about 95%～96%; the middle layer uses directly combined with magnesia chrome bricks; the lower layer uses Low-stomata clay bricks.
The important properties of the lattice body are corrosion resistance and creep resistance. To improve the creep resistance, it is necessary to use high-purity, low-iron, low-porosity and large-grain raw materials, and use high temperature to fully sinter. The stability and heat exchange effect of cylindrical checker bricks are better than those of strip checker bricks. For example, the specific heat exchange area of cylindrical bricks is 16m⁻¹, while the bar bricks have only 10.4～12.7m⁻¹.
2.2 The configuration of refractory materials in the regenerator when using alternative fuels
After using petroleum coke as fuel, a large amount of SO₃ and V₂O₅ is brought in. Free SO₃ may corrode alkaline refractory materials; it may also generate a reducing atmosphere due to incomplete combustion, causing a large amount of alkali to volatilize and corrode acidic and neutral refractories.
In a neutral environment, refractory materials will also be severely corroded. M-95 magnesia bricks suffered the most damage. Figure 8 is a photo of the microstructure of an M-95 magnesia brick damaged after a company uses petroleum coke fuel. The gray round particles are periclase with eroded corners and the dark gray matrix is forsterite (M₂S). The bright gray matrix is forsterite (CMS), and the black part is stomata. It can be seen from Figure 8 that a large amount of SiO₂ and CaO invaded the brick body. On the one hand, it destroyed the original MgO-M₂S combination, forming a continuous CMS-M₂S low-melting-point binding phase; on the other hand, because petroleum coke is difficult to catch fire and burn Slow, after using petroleum coke, the unburned components enter the regenerator and continue to burn in the regenerator, increasing the temperature of the regenerator. In this way, under the action of high temperature and erosion, the lattice body softens and then collapses. For this reason, glass companies have expanded the use of M-97 magnesia bricks and direct-bonded magnesia chrome bricks in the regenerator, and replaced the poorer performance of M-95 magnesia bricks, extending the life of the regenerator from 1 year to 3 years. In 2009, it achieved initial success. If a longer service life is obtained, composite spinel materials can be used in the middle of the regenerator of the glass furnace. At present, this type of material has achieved good results.
Generally, the top layer of the regenerator of the glass kiln uses a magnesium material with a MgO content of 97% to 98% (w). If the magnesia material cannot meet the demand, magnesia-zirconium, aluminum-chromium and electrofusion can be combined with magnesia-aluminum spinel materials, or even fused-cast materials. If the use environment is alkaline and the alkali vapor concentration is high, it is not appropriate to use fused cast zirconium corundum checker bricks, because the glass phase in the brick will seep out a lot; if the use environment is alkaline or afraid of polluting the glass, aluminum chrome bricks are not suitable. Because the Cr₂O₃ in the brick may react with Na₂O to form volatile Na₂CrO₄; if the regenerator fly contains a large amount of CaO and SiO₂, it is not appropriate to use magnesia-zirconium or magnesia-aluminum spinel bricks, because this may form CMS or C -M-A-S glass phase.
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