Refractory materials for glass kilns
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. Here are several types of damage.
PART 01 Corrosion
The powder, glass liquid and flame gas in the kiln will corrode 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 powder at high temperature, such as melting of silica brick surface, "rat holes" inside, and anti-nepheline in checker bricks. Furthermore, the flying materials of ultrafine powder in the powder gather in the lattice body of the regenerator to form tumors and block the lattice holes. In severe cases, the lattice 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 between glass liquid and refractory interface layer is complicated. Glass liquid first dissolves 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 corrosive effect 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 corrosive effect is severe. Increasing the melting temperature will reduce the viscosity and surface tension of the molten glass, thereby accelerating the corrosion effect. 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 building 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 ions have different valence states 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 certain part of the kiln is locally overheated or the refractoriness of the refractory materials is not enough, the refractory materials will be melted. Sometimes, the refractoriness is qualified, but the load softening temperature is low. In long-term use, the refractory materials will also soften and deform, affecting the stability and service life of the entire masonry. The severity of the burnout depends on the temperature and the nature of the refractory. The small furnace blast hole arch, small furnace legs, tongue, heat storage chamber arch, melting part kiln arch and breast wall are the parts that are easily burned.
PART 03 Cracks
Cracks mainly occur in 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 the stress caused by the temperature difference, the expansion or contraction caused by the change of the crystal form of the refractory material 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 material to crack. Therefore, when baking the kiln, the temperature must be raised according to the pre-established kiln baking curve. After baking the kiln, the refractory material is under the action of high temperature for a long time. The mechanical strength of the refractory material at this operating temperature is much lower than that at room temperature. If the mechanical load acting on the refractory material is too large, the refractory material 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 material, it has the effect of dripping water through stone, and the refractory material is worn out with grooves, which is mechanical wear. The main wear part is at the glass liquid surface. In addition, it is also clearly visible at the flow of circulating liquid (especially at 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 materials
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 attack on the refractory is basically the same. However, the chemical attack 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 temperature. Therefore, special refractory materials must be used.
The chemical attack of glass on refractory materials proceeds very slowly if there is no physical attack at the same time. The upper structure near the feed port is chemically attacked 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 attack on 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 different physical erosion conditions. In addition to chemical erosion 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 material. 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 material, which plays a protective role and prevents the batch dust from further eroding the refractory material. It can be seen from this that the degree of damage caused by chemical erosion is closely related to the physical erosion situation.
② Erosion caused by chemical reaction between glass batch dust and refractory material
This chemical erosion mainly occurs in the upper structure and regenerator of the pool furnace melting pool. The batch dust is also different in different parts. The batch dust near the feeding port has basically the same composition as the glass composition. Due to the high density of silica sand particles, the farther away from the feeding port, the lower the SO₂ content in the batch dust. The amount of dust in the batch 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 feeding method. Adding water to the batch, pressing cakes or making balls can greatly reduce the amount of dust in the batch.
③Chemical erosion caused by the reaction of volatiles in the glass batch with refractory materials
Volatiles of glass and batch materials 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 the gas phase, these volatiles will also condense into liquid phases and react with refractory materials at low temperatures. 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 mortar joints that are not filled with mud, it will cause great damage to the refractory materials.
④ Chemical erosion caused by chemical reaction between fuel ash and combustion products and refractory materials
When burning heavy oil and natural gas, ash is basically non-existent. Although V₂O₅ and NO have serious erosion on refractory materials, their content in heavy oil is generally very small and has little effect on pool furnace production. The sulfur in heavy oil and producer gas generates SO₂ during combustion and reacts with R₂O in volatile components to generate sodium sulfite. Sodium sulfite has a strong chemical reaction with refractory materials. 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 flushing effect of glass flow and the gravity effect of refractory load.
In the high temperature zone, the flushing effect of molten glass flow will double the chemical erosion rate. In the low temperature zone, chemical erosion is very small, mainly physical erosion caused by liquid flow flushing. In the high temperature zone of the melting pool, the glass flow has low viscosity and strong liquid flow. Especially after using electric fluxing and bubbling, the liquid flow is more intense. Strong scouring and chemical erosion can cause great damage to refractory materials.
Gravity damage caused by load mainly occurs in the regenerator lattice bricks. With the advancement of pool furnace technology, the height of the regenerator is increasing. The deadweight of the lattice body puts great pressure on the lower lattice bricks and grate arches. When chemical erosion damages them, the damaged parts are damaged due to stress concentration, resulting in the collapse of the entire lattice body.
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