These are the Types of Erosion that Glass Furnace Refractory Suffers During the Production Process!
The types and materials of refractory materials used in the glass industry differ from those used in the steel industry. Glass furnace refractory are mainly divided into casting materials, silica materials, and magnesium metal materials, such as silica bricks, clay bricks, high-alumina bricks, sillimanite bricks, mullite bricks, fused mullite bricks, zirconia-corundum bricks, fused corundum bricks, and zirconium-containing refractory bricks, etc. When used in glass furnaces, refractory materials are severely damaged by high temperatures, flames, powder, atmosphere, airflow, and liquid flow, significantly affecting the furnace’s service life. The use of refractory materials in the furnace begins from the start of firing. Improper operation can cause significant, even severe, damage to the refractory materials, requiring special attention. The types of erosion that refractory materials in glass tank furnaces experience during production include:
Corrosion
The powdered feedstock, molten glass, and flame gases within the kiln all corrode refractory materials at high temperatures. Soda ash, sodium sulfate, borates, fluorides, and oxides in the batch react with the refractory surface at high temperatures, forming eutectic or porous substances. These substances then continue to penetrate and diffuse into the brick body through the pores of the refractory material itself or through interfacial replacement reactions, gradually dissolving, peeling off, thinning, deteriorating, and recrystallizing the refractory material. The corrosion mechanisms of the various salts and compounds mentioned above are different; sodium sulfate is much more corrosive than soda ash.
The corrosive effect of the powdered feedstock on refractory materials is mainly manifested in the erosion of the refractory material by the alkaline vapors emitted from the powder at high temperatures. Examples include the melting and corrosion of the surface of silica bricks, internal “rat holes,” and the de-nephelineization in checker bricks. Furthermore, ultrafine powder particles accumulate in the regenerator checker, forming nodules that block the checker pores, and in severe cases, causing the checker bricks to collapse and be damaged, requiring 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 section, pool walls, small furnace, and upper grid of the regenerator are all susceptible to corrosion from the refractory powder.
The corrosive effect of molten glass on refractory materials is much smaller than that of molten glass. The phase reaction at the interface between molten glass and refractory materials is complex. Molten glass first dissolves free SO₂ in the refractory material. Mullite dissolves at a slower rate, accumulating at the interface between the molten glass and refractory material. Although small mullite crystals dissolve, larger mullite crystals may even grow during use. After the refractory material is eroded, the molten material in contact with it gains SO₂ and Al₂O₃. The molten material then diffuses into the rest of the molten glass. During diffusion, the composition of the molten material changes, with increased SO₂ and alkali content, while β-Al₂O₃ crystals aggregate at the interface. Therefore, at the interface between the refractory material and the molten glass, the first layer is mullite, followed by the β-Al₂O₃ layer, and then the uncorroded refractory material. The dissolution of the refractory material increases the viscosity of the molten glass, promoting the formation of a more difficult-to-move protective layer on the refractory surface, thus reducing further erosion.
Burn-off
Under prolonged exposure to high temperatures, refractory materials can be damaged by melting (also known as burn-through) or softening and deformation. Localized overheating in a part of the kiln or insufficient refractoriness of the refractory material can cause it to melt. Sometimes, even if the refractoriness is acceptable, a low load softening temperature can lead to softening and deformation over long-term use, affecting the stability and service life of the entire masonry. The severity of burn-off depends on the temperature and the properties of the refractory material. Small furnace burner arches, small furnace legs, tongues, regenerator arches, melting section kiln arches, and breast walls are prone to burn-off.
Cracking
Cracking mainly occurs during the kiln firing stage. During firing, a temperature difference exists within the refractory bricks, generating corresponding mechanical stress. If the heating rate is too rapid, exceeding the allowable ultimate strength of the refractory material, cracks will appear, even breaking into fragments. Electrofused, highly sintered, dense refractory materials are most susceptible to damage. Besides stress caused by temperature differences, the expansion or contraction resulting from changes in the crystal structure of refractory materials also generates stress. When the temperature rises too quickly, the crystal structure changes rapidly, and the volume change is too drastic, generating excessive stress and causing the refractory material to crack. Therefore, the temperature must be increased according to a pre-defined firing curve during kiln firing. After firing, the refractory material is under high temperature for a long time, and its mechanical strength at this operating temperature is much lower than at room temperature. If the mechanical load acting on the refractory material is too large, it will undergo inelastic deformation (similar to the flow of a highly viscous liquid), leading to failure.
Abrasion
When molten glass flows along the refractory material, it has a “dripping water wears away stone” effect, grinding grooves into the refractory material; this is mechanical wear. The main wear area is at the surface of the molten glass. Additionally, it is also clearly visible in areas of circulating liquid flow (especially in turbulent areas). Wear is exacerbated by fluctuations in the liquid surface and changes in liquid flow (such as those caused by temperature fluctuations).
Chemical Erosion
Chemical erosion mainly includes the following four types:
① Erosion caused by the reaction between molten glass and refractory materials.
This type of erosion is exemplified by the wall bricks of the pool that come into contact with molten glass. The most important type of glass is soda-lime-silica glass. Common bottle and jar glass and flat glass belong to this category. This type of glass is mainly composed of SO₂ (approximately 70%), Na₂O (approximately 15%), CaO (approximately 10%), and 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 glass, they can all be simplified to consider the SO₂ content, the alkali metal oxide (Na₂O + K₂O + L₂O) content, and the alkaline earth metal oxide (CaO + MgO + BaO) content. As long as the contents of these three oxides are basically the same, the chemical erosion effect on the refractory materials will also be basically the same. However, the chemical erosion of refractory materials by borosilicate glass differs from that of soda-lime-silica glass. Especially low-alkali or alkali-free borosilicate glass, which has a high content of acidic oxides and a high melting temperature, requires the use of special refractory materials.
② Erosion caused by the chemical reaction between glass batch dust and refractory materials.
This chemical erosion mainly occurs in the upper structure of the melting pool and the regenerator in the furnace. The composition of batch dust varies in different locations. The batch dust near the charging port has a composition basically the same as that of the glass. Due to the high density of silica sand particles, the SO₂ content in the batch dust decreases the further away 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 raw material density, particle size, and charging method. Adding water, pressing into cakes, or forming into balls can significantly reduce the amount of batch dust.
③ Chemical erosion caused by the reaction between volatiles in the glass batch and refractory materials.
Volatile substances from glass and batch materials are present in the upper space of the furnace and the middle of the regenerator, chemically eroding the refractory materials in these areas. The volatile components are mainly compounds of alkali metal oxides and boron, as well as fluorides, chlorides, and sulfur compounds. These volatiles react chemically with the refractory materials not only in the gaseous phase but also condense into a liquid phase at low temperatures, reacting 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. This is especially damaging when the upper structure masonry has cracks or incompletely filled mortar joints.
④ Chemical erosion caused by the chemical reaction between fuel ash and combustion products and the refractory materials.
When burning heavy oil and natural gas, ash is virtually non-existent. While V₂O₅ and NO severely corrode refractory materials, their content in heavy oil is generally very low, having little impact on furnace production. Sulfur in heavy oil and producer gas generates SO₂ during combustion, which reacts with R₂O in volatile components to form sodium sulfite. Sodium sulfite reacts strongly with refractory materials, and this influencing factor must be taken into account during glass production.
Physical Erosion
Physical erosion is highly dependent on time and temperature. The most significant physical erosion effects are the scouring action of the molten glass flow and the gravitational force of the refractory material under load. In high-temperature zones, the scouring action of the molten glass flow can multiply the rate of chemical erosion. In low-temperature zones, chemical erosion is minimal, with physical erosion primarily caused by the scouring action of the molten glass flow. In the high-temperature zone of the melting pool, the glass viscosity is low, and the flow is intense. This is especially true with the use of electric flux and bubbling, which further intensifies the flow. The combination of intense scouring and chemical erosion can cause significant damage to the refractory material. Gravity-induced damage mainly occurs in the regenerator checker bricks. With advancements in pool furnace technology, the height of the regenerator has increased, resulting in significant pressure from the checker bricks and grate arches due to the weight of the checker. When chemical erosion damages these checker bricks, stress concentration at the damaged areas leads to further collapse, ultimately causing the entire checker to collapse.
