When refractory materials are used in glass kilns, they will be severely damaged due to high temperature, flame, powder, atmosphere, air flow and liquid flow, which greatly affects the service life of the kiln. The use of refractory materials in kilns begins from the time of kiln baking. Improper operation will also cause great or even serious damage to refractory materials, so special attention should be paid. The following are several damage situations.
PART 01 Corrosion
The powder, glass liquid and flame gas in the kiln will corrode the refractory materials at high temperatures.
The erosion effect of powder on refractory materials is mainly manifested in the erosion of refractory materials by alkaline vapor evaporated from the powder at high temperature, such as melting of silica bricks, internal "rat holes", and anti-nepheline in the checker bricks. In addition, the flying materials of ultrafine powder in the powder gather in the checker body of the regenerator to form tumors and block the checker holes. In severe cases, the checker bricks will collapse and be damaged, and forced to be hot repaired. The corrosion effect intensifies with the increase of temperature. Every increase of 50-60℃ in melting temperature will shorten the service life by about one year. The front wall, charging port, front space of melting section, pool wall, small furnace, upper grid body of heat storage chamber and other parts will be corroded by powder.
The corrosion effect of glass liquid on refractory materials is much smaller than that of powder. The phase reaction on the interface layer between glass liquid and refractory materials is complicated. Glass liquid first dissolves the free SiO2 in refractory materials. Mullite dissolves slowly and gathers at the interface between glass liquid and refractory materials. Although small crystal mullite is dissolved, large crystal mullite even grows during use. After the refractory material is corroded, SiO2 and Al2O3 components are added to the melt in contact with it. The melt will diffuse into the rest of the glass liquid. During the diffusion process, the composition of the melt changes, SiO2 and alkali solution increase, and the aggregation of β-Al2O3 crystals occurs on the interface. Therefore, on the contact surface between the refractory and the glass liquid, the first is the mullite layer, followed by the β-Al2O3 layer, and then the uncorroded refractory. After the refractory is dissolved, the viscosity of the glass liquid increases, which promotes the formation of a protective layer that is difficult to move on the surface of the refractory, weakening the effect of continued corrosion.
The erosion of glass liquid on refractory depends on its physical properties such as viscosity and surface tension. Glass liquid with low viscosity and low surface tension is most likely to infiltrate refractory and be sucked into the interior from its surface pores, causing the entire refractory to be strongly corroded. High-alkali glass has a lower viscosity and borosilicate glass has a small surface tension, so their erosion is severe. Increasing the melting temperature will reduce the viscosity and surface tension of the molten glass, thereby accelerating the erosion. Glass liquid containing boric acid, phosphoric acid, fluorine, aluminum, and barium compounds has a severe corrosive effect on refractory materials. Strong glass liquid convection and unstable liquid surface will wash away the protective layer and accelerate corrosion. For refractory materials themselves, the degree of corrosion is mainly related to their chemical composition, mineral composition, and structural state. Uneven surface and cracks of refractory materials will aggravate the corrosion. The pool wall bricks and pool wall brick joints at the liquid surface are in places that are easily corroded by glass liquid. The corrosion of horizontal joints is more serious than that of vertical joints. Therefore, the masonry surface is required to be smooth, the joints are small, and the whole piece must be erected.
The combustion products of coal gas and heavy oil (containing corrosive gases such as SO2 and V2O5) and the volatiles of individual batch components will also corrode the refractory materials in the flame space, small furnace, heat storage chamber, etc. Different furnace materials will react with each other at high temperatures, resulting in damage. For example, at 1600-1650℃, clay bricks and silica bricks will react violently, high alumina bricks and silica bricks will react moderately, and fused zirconium corundum bricks and silica bricks will react violently and severely eutectic. Fused zirconium corundum bricks react moderately with quartz bricks and white foam stone, and react in contact with corundum bricks. Therefore, corundum bricks can be used as transition materials.
The lattice used in the regenerator is also damaged by the redox atmosphere. The damage mechanism is mainly that the variable ion has different valence and coordination states in the oxidation and reduction states, resulting in volume changes, which leads to reduced strength and cracking of the product.
PART 02 Burning
Under high temperature and long-term action, refractory materials will be damaged by melting (also known as burning flow) or softening and deformation. If a part of the kiln is locally overheated or the refractoriness of the refractory material is not enough, the refractory material will be melted. Sometimes, if the refractoriness is qualified but the load softening temperature is low, the refractory material will soften and deform during long-term use, affecting the stability and service life of the entire masonry. The severity of the burn depends on the temperature and the nature of the refractory material. The small furnace blast hole arch, small furnace leg, tongue, heat storage chamber arch, melting kiln arch and breast wall are parts that are easily burned.
PART 03 Cracks
Cracks mainly occur during the kiln baking stage. During the kiln baking, a certain temperature difference occurs inside the refractory bricks, generating corresponding mechanical stress. If the heating rate is too fast and exceeds the limit strength allowed by the refractory material, cracks will appear, and even break into pieces. Electric melting, highly sintered dense refractory materials are most susceptible to damage. In addition to stress caused by temperature difference, expansion or contraction caused by changes in the crystal form of refractory materials will also generate stress. When the temperature rises too fast, the crystal form changes quickly, the volume changes too drastically, and the stress is too large, causing the refractory to crack. Therefore, the temperature must be raised according to the pre-set kiln baking curve during kiln baking. After the kiln is baked, the refractory is under high temperature for a long time. The mechanical strength of the refractory at this operating temperature is much lower than that at room temperature. If the mechanical load acting on the refractory is too large, the refractory will produce non-elastic deformation (similar to the flow of extremely viscous liquid), which will lead to damage.
PART 04 Wear
When the glass liquid flows along the refractory, it has the effect of dripping water through the stone, and the refractory is worn out with grooves. This is mechanical wear. The main wear part is at the glass liquid surface. In addition, it is also clearly visible at the circulating liquid flow (especially the turbulent liquid flow). When the liquid level fluctuates and the liquid flow changes (such as affected by temperature fluctuations), the wear is aggravated.
PART 05 Chemical erosion
① Erosion caused by the reaction of molten glass and refractory
This erosion is represented by the pool wall bricks in contact with the glass liquid. The most important glass is soda-lime-silica glass. General bottle glass and flat glass belong to this category. This kind of glass is mainly composed of SO₂, with a content of about 70%, Na₂O content of about 15%, CaO content of about 10%, and a small amount of Al₂O₃ and MgO. In order to improve the performance of 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 to SO₂ content, alkali metal oxide (Na₂O+K₂O+L₂O) content, and alkaline earth metal oxide content (CaO+MgO+BaO). As long as the content of the above three oxides is basically the same, the chemical erosion of refractory materials is basically the same. However, the chemical erosion of borosilicate glass on refractory materials is different from that of soda-lime-silica glass. In particular, low-alkali or alkali-free borosilicate glass has a high acidic oxide content and a high melting point. Therefore, special refractory materials should be used.
The chemical erosion of glass on refractory materials proceeds very slowly if there is no physical erosion at the same time. The upper structure near the charging port is chemically eroded by the batch dust. The composition of the batch dust here is basically the same as that of the glass liquid. That is to say, the chemical erosion of the refractory and the pool wall bricks here is basically the same. However, the damage to the pool wall bricks is much more serious than that of the upper structure. The reason for this difference is mainly due to the different physical erosion conditions. In addition to being chemically eroded by glass, the pool wall bricks are also physically eroded by the scouring effect of the glass liquid flow. The scouring of the liquid flow continuously washes away the products of chemical erosion, so that the glass liquid can continuously chemically erode the fresh surface of the refractory. As a result of the combined action of these two types of erosion, the pool wall bricks are damaged quickly. However, the upper structure is only eroded by the batch dust with the same composition as the glass, and there is no physical erosion of the liquid flow here. Therefore, the products of chemical erosion remain on the surface of the refractory, which plays a protective role and prevents the batch dust from further eroding the refractory. It can be seen from this that the degree of damage caused by chemical erosion is closely related to the physical erosion.
② Erosion caused by chemical reaction between glass batch dust and refractory materials
This chemical erosion mainly occurs in the upper structure of the pool furnace melting pool and the regenerator. In different parts, the batch dust is also different. The composition of the batch dust near the charging port is basically the same as that of the glass. Due to the high density of silica sand particles, the farther away from the charging port, the lower the SO₂ content in the batch dust. The amount of batch dust is related to many factors. For the same glass batch, the amount of dust is closely related to the raw material density, particle size, and charging method. Adding water to the batch, pressing cakes or making balls can greatly reduce the amount of batch dust.
③ Chemical erosion caused by the reaction of glass batch volatiles with refractory materials
The volatiles of glass and batch exist in the upper space of the pool furnace and the middle of the regenerator, chemically corroding the refractory materials in these parts. The components of volatiles are mainly alkali metal oxide compounds and boron compounds, as well as fluoride, chloride and sulfur compounds. In addition to chemically reacting with refractory materials in gas phase, these volatiles will also condense into liquid phase and react with refractory materials at low temperature. Among them, sodium compounds will condense at 1400℃. These condensed liquids penetrate into the pores of refractory materials through infiltration and diffusion. Especially when there are cracks in the upper structure masonry and the masonry joints that are not filled with mud, it will cause great damage to the refractory materials.
④ Chemical erosion caused by the chemical reaction between the ash content and combustion products of the fuel and the refractory materials
When burning heavy oil and natural gas, ash is basically non-existent, and although V₂O₅ and NO have serious erosion on refractory materials, their content in heavy oil is generally very small, and they have little effect on pool furnace production. The sulfur content in heavy oil and generator gas generates SO₂ during combustion, and reacts with R₂O in the volatile components to generate sodium sulfite. The chemical reaction between sodium sulfite and refractory materials is strong. This influencing factor should be considered in the glass production process.
PART 06 Physical erosion
Physical erosion has a great relationship with time and temperature. The most important physical erosion is the scouring effect of the glass flow and the gravity effect of the refractory load.
In the high temperature area, the scouring effect of the molten glass flow will multiply the chemical erosion rate. In the low temperature area, the chemical erosion is very small, and it is mainly the physical erosion of the flow scouring. In the high temperature area of the melting pool, the glass flow viscosity is low and the flow is strong. Especially after the use of electric melting and bubbling, the flow is more intense. The strong scouring effect combined with chemical erosion will cause great damage to the refractory material.
The gravity damage caused by the load mainly occurs in the regenerator checker bricks. With the advancement of pool furnace technology, the height of the regenerator is constantly increasing. The deadweight of the grid body puts a lot of pressure on the lower checker bricks and the grate arch. When it is damaged by chemical erosion, it is damaged due to stress concentration at the damaged part, resulting in the collapse of the entire grid body.
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