70% of the consumption of
refractory materials is in the iron and steel metallurgical industry, and the remainder is the main industry of cement and glass. As of the end of November 2019, there were a total of 297 float glass production lines nationwide, with 239 in production. In the first half of 2020, the output of daily-use glass products and glass packaging containers was 11.42 million tons.
The types and materials of refractory materials used in the glass industry are also different from those in the steel industry. The refractory materials used in glass kilns are mainly divided into casting materials, siliceous 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, refractory bricks containing zirconium, etc.
Float glass is formed in the tin bath, that is, the molten glass continuously flows into the tin bath from the overflow channel and the launder, spreads on the surface of the tin liquid and drifts forward under the traction of the driving roller, at a certain temperature Under the system, relying on surface tension and gravity to complete flattening and thinning. After cooling, the glass is lifted by the transition roller table, leaves the tin bath and enters the annealing kiln, and then undergoes cross-cutting, inspection, and packing.
When refractory materials are used in glass furnaces, they will be severely damaged due to the effects of high temperature, flame, powder, atmosphere, air flow and liquid flow, which greatly affects the service life of the furnace. Starting from the baking kiln, the use of refractory materials in the kiln begins. Improper operation will also cause great or even serious damage to the refractory materials, which requires special attention. Here are several damage situations.
The material powder, glass liquid and flame gas in the kiln will corrode refractory materials at high temperatures. The soda ash, Glauber's salt, borate, fluoride, and oxides in the batch react with the surface of the refractory at high temperatures to form eutectic or loose substances, and continue to move forward with the help of the metasomatic reaction of the voids or interfaces of the refractory itself. The brick body penetrates and diffuses, and the refractory material is gradually dissolved, peeled off, thinned, deteriorated, and recrystallized. The corrosion mechanism of the above-mentioned various salts and compounds is different. Glauber's salt has a much stronger corrosion effect than soda ash.
The corrosion effect of powder on refractory materials is mainly manifested in the erosion of refractory materials by alkaline vapor evaporated at high temperature, such as the erosion of the surface of silica bricks, internal "rat holes", etc., and the antinephelineization in checker bricks Role etc. Furthermore, the fly material of ultra-fine powder in the powder material accumulates in the grid body of the regenerator, forming tumors, blocking the grid holes, and in severe cases, causing the grid bricks to collapse and damage, and they are forced to be repaired. The effect of corrosion is aggravated with the increase of temperature. Each increase of the melting temperature by 50~60℃ will shorten the service life by about one year. The front wall, the charging port, the front space of the melting part, the pool wall, the small furnace, the upper grid body of the regenerator and other parts will be corroded by the powder.
The corrosion damage of molten glass to refractory materials is much smaller than that of powdered materials, and the phase reaction between molten glass and refractory materials is complicated. The molten glass first dissolves the free SO? in the refractory. The dissolution rate of mullite is relatively small. It gathers at the interface between molten glass and refractory material. Although the small crystalline mullite is dissolved, the large crystalline mullite even grows during use. After the refractory material is corroded, SO? and Al?O? are added to the melt in contact with it. The melt will diffuse into the rest of the glass. During the diffusion process, the composition of the melt changes, SO? and lye increase, and β-Al?O? crystal aggregation occurs at the interface. Therefore, on the contact surface between the refractory material and the molten glass, the first is Molai The stone layer, then the β-Al?O? layer, then the uncorroded refractory material. After the refractory material 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 material, and weakens the effect of continued erosion.
The corrosive effect of molten glass on refractory materials depends on its viscosity and surface tension and other physical properties. Glass liquid with low viscosity and low surface tension is the easiest to infiltrate the refractory material, and suck into the inside from the surface pores, causing the entire refractory material to be strongly corroded. High-alkali glass has a lower viscosity, and the surface tension of borosilicate glass is small, so their corrosive effect is severe. Increasing the melting temperature will reduce the viscosity and surface tension of the molten glass, thereby also accelerating the erosion. Liquid glass containing boric acid, phosphoric acid, fluorine, aluminum, and barium compounds has a violent corrosive effect on refractory materials. Strong convection of the glass liquid and unstable liquid surface will wash away the protective layer and accelerate corrosion. For the refractory material itself, the degree of corrosion damage is mainly related to its chemical composition, mineral composition and structural state. Generally, the structure of refractory materials is composed of one or more crystal phases, glass phases and gas phases. The pores, especially the open pores, are channels for the corrosive agent to penetrate into the refractory material and increase the erosion surface. Compared with the crystalline phase, the glass phase is a weak link and its chemical stability is poor. To improve the corrosion resistance of refractories, it must increase the high-temperature stable crystalline phase, reduce the glass phase content, and increase the softening temperature and viscosity. The rate is as low as possible. In addition, the crystal phase is required to be fine and evenly distributed in the glass phase to form a uniform and dense structure. The surface unevenness and cracks of refractory materials will increase the erosion. The pool wall bricks and the pool wall brick joints at the liquid level are in places that are easily corroded by liquid glass. The corrosion damage of horizontal joints is more serious than vertical joints, so the surface of masonry is required It is smooth, with small cracks, and should be built in one piece.
The combustion products of coal gas and heavy oil (containing corrosive gases such as SO?, V?O?) and volatiles of individual batch components will also corrode the refractory materials in the flame space, small furnace, regenerator, etc. Under high temperature, different building materials will react with each other and cause damage. For example, clay bricks and silica bricks will react violently at 1600~1650°C, high alumina bricks and silica bricks will react moderately, and fused zirconia corundum bricks and silica bricks will react violently and cause severe eutectic. Fused zirconia corundum bricks react moderately with quartz bricks and white swelling stones, but react with corundum bricks in contact. Therefore, corundum bricks can be used as transition materials.
The lattice body used in the regenerator is also damaged by the effect of the redox atmosphere. The damage mechanism is mainly due to the different valence and coordination states of the variable ion in the oxidation and reduction states, resulting in volume changes, resulting in reduced product strength and cracking.
Cracked
Under the action of high temperature for a long time, refractory materials will be damaged by melting (also called burning) or softening and deformation. If a certain part of the kiln is overheated or the refractory material is not enough, the refractory material will be melted. Sometimes, if the refractoriness is qualified, but the softening temperature under load is low, the refractory material will soften and deform during long-term use, which affects the stability and service life of the entire masonry. The severity of burning depends on the temperature and the nature of the refractory material. Small furnace flares, small furnace legs, tongues, regenerators, melting part kilns and breast walls are easily burned parts.
Cracking mainly occurs in the kiln stage. When baking in the kiln, there is a certain temperature difference inside the refractory bricks, resulting in corresponding mechanical stress. If the heating rate is too fast and exceeds the allowable ultimate strength of the refractory material, cracks will appear or even break into pieces. Electrofusion, highly sintered dense refractories are the most vulnerable. In addition to the stress caused by the temperature difference, the expansion or contraction caused by the change in the crystal form of the refractory will also cause stress. When the temperature rises too fast, the crystal form changes quickly and the volume changes too drastically, resulting in excessive stress and cracking of the refractory. Therefore, the temperature must be raised according to the preset kiln curve when baking. After the kiln is baked, the refractory is exposed to high temperature for a long time, and 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 material is too large, the refractory material will produce inelastic deformation (similar to the flow of extremely viscous liquid), which will cause damage.
Wear
When the molten glass flows along the refractory material, it has the effect of dripping water through the stone, grinding the refractory material into grooves, which is mechanical wear. The main wear part is at the glass surface. In addition, it is also clearly visible in 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), wear intensifies.
Types of chemical attack
There are mainly the following four types of chemical attack.
①Corrosion caused by the reaction of molten glass and refractory
This kind of erosion is represented by the pool wall tiles in contact with the molten glass. The most important glass is soda lime silica glass. General bottle glass and flat glass belong to this category. This glass contains SO? as the main component, with a content of about 70%, Na?O content of about 15%, CaO content of about 10%, and a small amount of Al?O? and MgO. In order to improve the performance of the glass, K?O, L?O, BaO, PbO and other oxides can be introduced based on soda lime silica glass. Although there are many types of these glasses, they can all be simplified into SO? content, alkali metal oxide (Na?O+K?O+L?O) content, and alkaline earth metal oxide content (CaO+MgO+BaO). As long as the contents of the above three oxides are basically the same, the chemical attack on refractory materials is also basically the same. However, the chemical attack 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.
The chemical attack of glass on refractory materials, if there is no physical attack at the same time, will proceed very slowly. The upper structure near the feeding port is chemically attacked by the batch dust. The composition of the batch dust here is basically the same as that of the molten glass. That is to say, the refractory material and the pool wall brick are basically the same chemical attack here. But the damage to the pool wall tiles is much more serious than the superstructure. The main reason for this difference is that the physical erosion conditions are different. In addition to the chemical corrosion of the glass, the wall tiles are also physically corroded by the scouring action of the glass flow. The scouring of the liquid stream continuously washes away the products of chemical attack, so that the glass liquid can continuously chemically attack the fresh surface of the refractory material. As a result of the combined action of these two types of erosion, the pool wall tiles are damaged very 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 by the liquid flow. Therefore, the product of chemical attack stays on the surface of the refractory material, which plays a protective role and prevents further erosion of the refractory material by the batch dust. It can be seen that the damage degree of chemical erosion has a great relationship with physical erosion.
②Corrosion caused by chemical reaction between glass batch dust and refractory materials
This chemical attack mainly occurs in the upper structure of the melting pool and the regenerator. In different parts, the batch dust is also different. The composition of the batch dust near the feeding port is basically the same as that of the glass. Due to the higher density of silica sand particles, the farther away from the feeding port, the lower the SO? 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 feeding method. Adding water to the batch, pressing cake or making balls can greatly reduce the amount of dust in the batch.
③Chemical attack caused by the reaction of glass batch volatiles and refractory materials
The volatiles of glass and batch materials exist in the upper space of the pool furnace and the middle of the regenerator, and chemically attack the refractory materials in these parts. The volatile components are mainly alkali metal oxide compounds and boron compounds, as well as fluorides, chlorides and sulfur compounds. These volatiles not only chemically react with refractory materials in the gas phase, but also condense into a liquid phase to chemically react with refractory materials when the temperature is low. The sodium compound is at 1400°C. It will condense at times. The condensed liquid penetrates into the pores of the refractory material through infiltration and diffusion. Especially when the superstructure masonry has cracks and joints not filled with mud, it will cause great damage to the refractory.
④Chemical corrosion caused by the chemical reaction of fuel ash and combustion products with refractory materials
When burning heavy oil and natural gas, ash content basically does not exist, and although V?O? and NO are severely corroded to refractory materials, the content of heavy oil in general is very small, and it has little effect on the production of pool furnaces. The sulfur in heavy oil and producer gas generates SO? during combustion and reacts with R?O in the volatile component to form sodium sulfite. The chemical reaction between sodium sulfite and refractory materials is strong. This influencing factor must be considered in the glass production process.
Types of 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 zone, the scouring action of the molten glass stream will double the rate of chemical attack. In the low temperature area, chemical erosion is very small, mainly physical erosion caused by liquid flow. In the high temperature zone of the melting pool, the viscosity of the glass flow is low and the liquid flow is strong. Especially after using electric fluxing and bubbling, the liquid flow becomes stronger. The combination of strong scouring and chemical erosion will cause great damage to refractory materials.
The gravity damage caused by the load mainly occurs in the checker bricks of the regenerator. With the advancement of pool furnace technology, the height of the regenerator continues to increase. The weight of the lattice body exerts great pressure on the lower checker bricks and grate slats. When chemical erosion damages them, the damaged parts will be destroyed due to stress concentration. Will cause the entire grid to collapse.
Refractory material pollution to glass
The composition of the refractory material and the composition of the reaction product with glass are different from the glass composition. This different composition can be solid, gas or liquid. From the appearance point of view, the defects produced by refractory materials mainly include the following three types: stones and streaks, coloring, and bubbles.
The pollution caused by refractory materials is in most cases the result of corrosion of refractory materials. The more severe the corrosion of refractory materials, the more glass defects will be caused. There are three types of stones caused by refractory materials: one is the original crystal phase of the refractory, the second is the metamorphic crystal formed by the reaction between the refractory and the glass, and the third is the recrystallization of the refractory after being melted. Streaks produced by refractory materials usually coexist with stones. Some stones are crystallized in the stripes, and some stripes are caused by the melting of the stones. Another kind of streak without stones, which is mostly caused by the glass phase in the refractory material. Most of the stones and streaks produced by refractory materials are the result of physical erosion and chemical erosion.
Since the
refractory contains Fe?O?, Cr?O? and other substances, these are all strong colorants. Therefore, after the refractory material is corroded, these oxides enter the glass and cause coloration.
There are two main types of bubbles caused by refractory materials. One is that the refractory material itself has pores, and the gas in it is replaced by glass. The other is bubbles formed by the reaction of glass and refractory materials. The former is caused by physical action, and the latter is the result of physical erosion and chemical erosion.
