The types and materials of refractory materials used in the glass industry are different from those used in the steel industry. Refractory materials used in glass furnaces are mainly divided into fused-cast materials, siliceous materials, and magnesium-based materials, such as silica bricks, clay bricks, high-alumina bricks, sillimanite bricks, mullite bricks, electrofused mullite bricks, zirconia-corundum bricks, electrofused corundum bricks, and zirconium-containing refractory bricks. During use in glass furnaces, refractory materials are severely damaged due to high temperatures, flames, raw materials, atmosphere, airflow, and liquid flow, significantly affecting the service life of the furnace. The use of refractory materials in the furnace begins from the kiln firing process. Improper operation can also cause significant, even severe, damage to the refractory materials, requiring special attention. Several types of damage are described below.
01 Corrosion
Raw materials, molten glass, and flame gases in the furnace all corrode refractory materials at high temperatures. Sodium carbonate, sodium sulfate, borates, fluorides, and oxides in the batch materials react with the surface of the refractory materials at high temperatures, forming eutectic mixtures or loose substances. These substances then penetrate and diffuse into the brick body through the pores or interfaces of the refractory material, causing the refractory material to gradually dissolve, peel off, thin, deteriorate, and undergo recrystallization. The corrosion mechanisms of these various salts and compounds are different; sodium sulfate has a much stronger corrosive effect than sodium carbonate.
The corrosive effect of raw materials on refractory materials is mainly manifested in the corrosion of the refractory materials by alkaline vapors evaporated from the powder at high temperatures, such as surface erosion and internal "rat holes" in silica bricks, and the nephelinization effect in checker bricks. Furthermore, the accumulation of ultrafine powder from the raw materials in the checkerwork of the regenerator forms lumps, blocking the checker holes, and in severe cases, causing the checker bricks to collapse and be damaged, forcing hot repairs. The corrosive effect intensifies with increasing temperature; every 50-60°C increase in melting temperature shortens the service life by approximately one year. The front wall, charging port, front space of the melting zone, tank walls, forehearth, and upper checkerwork of the regenerator are all subject to erosion by the batch materials.
The corrosive effect of molten glass on refractory materials is much less than that of the batch materials. The phase reaction at the interface between the molten glass and the refractory material is complex. The molten glass first dissolves the free SO₂ in the refractory material. The dissolution rate of mullite is relatively low; it accumulates at the interface between the molten glass and the refractory material. Although small mullite crystals dissolve, large mullite crystals may even grow during use. After the refractory material is corroded, the molten material in contact with it is enriched with SO₂ and Al₂O₃ components. The molten material will diffuse into the rest of the molten glass. During the diffusion process, the composition of the molten material changes, with increased SO₂ and alkali content, and the aggregation of β-Al₂O₃ crystals occurs at the interface. Therefore, at the contact surface between the refractory material and the molten glass, there is first a mullite layer, then a β-Al₂O₃ layer, and then the uncorroded refractory material. After the refractory material dissolves, the viscosity of the molten glass increases, promoting the formation of a less mobile protective layer on the surface of the refractory material, thus reducing further corrosion.
02 Burning Damage
Under high temperature and prolonged exposure, refractory materials can be melted (also known as burning flow) or softened and deformed, leading to damage. Local overheating in certain parts of the kiln or insufficient refractoriness of the refractory materials used can cause the refractory materials to melt. Sometimes, even if the refractoriness is adequate, but the load softening temperature is low, the refractory materials will soften and deform during long-term use, affecting the stability and service life of the entire structure. The severity of burning damage depends on the temperature and the properties of the refractory material. The forehearth arch, forehearth legs, tongue, regenerator arch, melting zone kiln arch, and breast wall are susceptible to burning damage.
03 Cracking Damage
Cracking damage mainly occurs during the kiln firing stage. During kiln firing, a certain temperature difference occurs inside the refractory bricks, generating corresponding mechanical stress. If the heating rate is too fast, exceeding the allowable limit strength of the refractory material, cracks will appear, or even the material will break into fragments. Electrofused, highly sintered, dense refractory materials are most susceptible to damage. In addition to stress caused by temperature differences, expansion or contraction caused by crystal structure changes in the refractory material also generates stress. When heating too quickly, the crystal structure changes rapidly, leading to excessive volume changes and excessive stress, causing the refractory material to crack. Therefore, during kiln firing, the temperature must be increased according to a pre-determined firing curve. After firing, the refractory material is subjected to high temperatures for a long period. The mechanical strength of the refractory material at this operating temperature is much lower than at room temperature. If the mechanical load applied to the refractory material is too large, the refractory material will undergo non-elastic deformation (similar to the flow of a highly viscous liquid), leading to failure.
04 Abrasion
When molten glass flows along the refractory material, it has a "water dripping on stone" effect, wearing away grooves in the refractory material; this is mechanical abrasion. The main abrasion occurs at the glass liquid level. It is also clearly visible in areas of circulating liquid flow (especially in turbulent flow areas). Abrasion is intensified when the liquid level fluctuates and the liquid flow changes (e.g., due to temperature fluctuations).
05 Chemical Corrosion
Chemical corrosion mainly includes the following four types:
① Corrosion caused by the reaction between molten glass and refractory materials. This type of corrosion is exemplified by the furnace wall bricks in contact with the molten glass. The most important type of glass is soda-lime-silica glass. General bottle and flat glass belong to this category. This type of glass mainly contains SiO₂, with a content of about 70%, Na₂O content of about 15%, and CaO content of about 10%, as well as small amounts of Al₂O₃ and MgO. To improve the performance of the glass, oxides such as K₂O, L₂O, BaO, and PbO can be introduced based on soda-lime-silica glass. Although there are many types of these glasses, they can all be simplified by considering the SiO₂ content, alkali metal oxide content (Na₂O+K₂O+L₂O), and alkaline earth metal oxide content (CaO+MgO+BaO). As long as the content of the three oxides mentioned above is basically the same, the chemical corrosion of the refractory materials will also be basically the same. However, the chemical corrosion of borosilicate glass on refractory materials is different from that of soda-lime-silica glass. Especially low-alkali or alkali-free borosilicate glass has a high content of acidic oxides and a high melting temperature. Therefore, special refractory materials must be used.
② Corrosion caused by chemical reaction between glass batch dust and refractory materials: This chemical corrosion mainly occurs in the upper structure of the melting tank and the regenerator of the tank furnace. The composition of the batch dust also varies in different areas. The composition of the batch dust near the charging port is basically the same as the glass composition. Due to the higher density of silica sand particles, the SO₂ content in the batch dust decreases with increasing distance from the charging port. The amount of batch dust is related to many factors. For the same type of glass batch, the amount of dust is greatly related to the density, particle size, and feeding method of the raw materials. Adding water to the batch, pressing it into cakes, or forming pellets can greatly reduce the amount of batch dust.
③ Chemical corrosion caused by the reaction of glass batch volatiles with refractory materials: Volatiles from glass and batch materials exist in the upper space of the tank furnace and the middle of the regenerator, chemically corroding the refractory materials in these areas. The components of the volatiles are mainly compounds of alkali metal oxides and boron compounds, as well as fluorides, chlorides, and sulfur compounds. In addition to chemically reacting with refractory materials in a gaseous state, these volatiles will also condense into a liquid phase at lower temperatures and chemically react with the refractory materials. Sodium compounds, in particular, condense at 1400℃. These condensed liquids penetrate into the pores of the refractory materials through wetting and diffusion. Especially when there are cracks and unfilled mortar joints in the upper structure masonry, it will cause significant damage to the refractory materials.
④ Chemical corrosion caused by the chemical reaction of fuel ash and combustion products with refractory materials: When burning heavy oil and natural gas, ash is basically absent, and although V₂O₅ and NO cause serious corrosion to refractory materials, their content in heavy oil is generally very low, and their impact on tank furnace production is not significant. The sulfur content in heavy oil and producer gas generates SO₂ during combustion, which reacts with R₂O in the volatile components to form sodium sulfite. Sodium sulfite reacts strongly with refractory materials, and this influencing factor must be considered in the glass manufacturing process.
06 Physical Erosion
Physical erosion is highly dependent on time and temperature. The most important aspects of physical erosion are the scouring effect of the molten glass flow and the gravitational force of the refractory material load. In high-temperature areas, the scouring effect of the molten glass flow can multiply the rate of chemical erosion. In low-temperature areas, chemical erosion is minimal, and the main factor is physical erosion caused by the liquid flow. In the high-temperature zone of the melting tank, the viscosity of the glass flow is low, and the flow is strong. This is especially true after using electric boosting and bubbling. The strong scouring effect, combined with chemical erosion, can cause significant damage to the refractory materials. Gravitational damage caused by the load mainly occurs in the checker bricks of the regenerator. With the advancement of furnace technology, the height of the regenerator has continuously increased, and the self-weight of the checker body exerts great pressure on the lower checker bricks and furnace arch. When chemical erosion damages them, stress concentration at the damaged areas leads to further damage, ultimately resulting in the collapse of the entire checker body.
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