In the process of glass melting, the interaction between refractories and molten glass at high temperature will cause corrosion damage to refractories and even defects to molten glass. In the tank kiln, the corrosion effect of the batch components on the refractory is much greater than that of the molten glass. Mirabilite batch has stronger erosion than soda ash batch. Usually, the corrosive effect of molten soda ash is limited to the vicinity of the feeding port, while mirabilite can attack almost all the tank walls. It contains boric acid, phosphoric acid, fluorine, chlorine, lead, barium and other compounds, as well as a batch with high alkalinity. Its glass liquid has a particularly strong corrosive effect on refractory materials. Although the furnace arch, breast wall, small furnace, regenerator and other structures in the flame space of the tank furnace do not directly contact with the glass liquid, they are also eroded by the mixture dust and the volatiles on the glass liquid surface to varying degrees.
The corrosion strength of the molten glass to the refractory mainly depends on the physical properties such as the viscosity and surface tension of the molten glass, and the chemical reaction in the process of corrosion only plays a subordinate role. Molten glass with low viscosity and low surface tension is the easiest to infiltrate the refractory, and it can penetrate into the refractory along the capillary system on the surface of the refractory. The glass containing more alkali has low viscosity and low surface tension of borosilicate glass, so their corrosion to refractory is very strong.
During the heating process of the batch, the most fusible polyacid compound starts to form and flows on the glass surface. Then, these melts gradually dissolve with relatively refractory components, so the refractory materials in the tank furnace melting zone are eroded by polyalkaline silicates. Especially when Glauber’s salt batch glass is melted, the molten nitrate water floating on the glass surface directly interacts with the refractory material. Sodium sulfate melts at 885 ℃ and participates in the glass formation reaction. The reaction is not complete until about 1440 ℃. Nitrate water, alkali liquor and polyalkaline silicate are easy to be absorbed into the pores on the surface of refractory materials, so that refractory materials are strongly eroded.
When refractories for tank kilns are subject to physical and chemical erosion, the erosion rate is a function of temperature. The erosion rate increases logarithmically with the increase of temperature.
Raising the melting temperature will reduce the viscosity of molten glass, accelerate the erosion of refractory materials, and greatly shorten the service life of refractory materials. Every 50 ~ 60 ℃ increase in melting temperature in the tank kiln will shorten the life of refractory by about 1 / 2. In the crucible kiln, as long as the melting temperature increases by 20 ~ 40 ℃, the service life of the crucible will be shortened by 1 / 2.
The physical and chemical corrosion resistance of refractories is mainly determined by the types, distribution and bonding states of their constituent phases. General refractories are composed of one or more crystal phases, glass phases and gas phases (pores). The chemical stability of glass is worse than that of crystal, and the pores are the channels (especially open pores) through which the aggressive agent penetrates into the refractory. The penetration depth of the components of the molten glass or batch into the pores of the refractory material is directly proportional to the fourth power of the pore diameter. The corrosives first act on the glass phase in the refractory and react with each other. After the solution penetrates into the refractory and dissolves the glass phase, the crystals in the refractory will be eroded by the glass liquid flow, and new parts that continue to be eroded may appear. Most of the pores and glass phase exist in the combination of sintered refractories, so the combination becomes the weak link of physical and chemical corrosion resistance of refractories.
The higher the viscosity of the molten material formed by the corrosion of the refractory material, the denser the material, the fewer the open pores, and the smaller the degree of corrosion. Since the refractory material is dissolved and the viscosity of the molten glass is increased, a protective film with little movement can be formed on the surface of the refractory material, thereby weakening the erosion.
In order to obtain refractory materials with good corrosion resistance, in addition to stable crystal phase, high softening temperature, large melt viscosity, less glass phase and low porosity, it is also required that the crystal form of crystal phase be fine, evenly distributed in the glass phase, uniform structure and tight bonding, so that the glass phase can be strengthened.
The uneven surface of refractory materials, gaps and cracks will deepen the erosion, especially the transverse joints. The denser the masonry is and the finer the gap is, the smaller the erosion effect of the glass on the pool wall brick is.
The convection of liquid glass and the instability of liquid glass can aggravate the corrosion of refractory materials. This is mainly because the liquid flow will accelerate the physical and chemical interaction between the glass melt and the refractory material. However, the friction between the molten glass and the refractory material is very small, so the effect of mechanical wear is relatively light.
In the process of erosion, the refractory material is dissolved, and a thin film is formed on the surface. When it is washed by the liquid glass flow, the protective film, which was rarely moved, moves, exposing the refractory material to a new surface part, which provides favorable conditions for further erosion. When the clay pool wall brick is eroded by sodium calcium silicate glass, the damage to the glass surface is much faster than that not too deep below the liquid surface, and it is easy to form deep trenches.
The fluctuation of glass liquid level will strengthen the scouring effect on the damaged refractory layer. When the glass level drops, the softened film can no longer be kept on the inner surface of the refractory. When the glass level rises again, the peeled film can not return to its original position and is taken away by the liquid flow. A new layer of refractory material is exposed again and is further eroded by the rising glass liquid to accelerate the damage. Sometimes, the high viscosity liquid glass layer formed due to the dissolution of refractory materials is peeled off, and there is no time for diffusion and homogenization, which will cause stripes on the glass.
Due to the temperature difference of the liquid glass, the liquid glass near the pool wall flows downward, and the corrosion and dissolution of the refractory materials on the pool wall will change the density of the liquid glass, which will affect the speed of the liquid flow near the pool wall and strengthen the corrosion. Ventilation and cooling of the pool wall can help to reduce erosion, but it can only be realized if the thickness of the pool wall brick is not large. Sometimes it can also enhance the convective circulation of liquid glass at the pool wall, which will strengthen the corrosion of refractory materials.
The temperature fluctuation in the tank furnace will lead to the destruction of the balance of the refractory glass system. If the temperature rises, the viscosity of the protective film covering the surface of the pool wall brick will decrease, which is easy to be washed away by the liquid glass flow, and will also accelerate its erosion and destruction.
Modern tank kilns often adopt auxiliary electric melting and bubbling clarification processes to improve the melting rate. But it also strengthens the convection of the molten glass and raises the temperature of the deep molten glass, which also strengthens the erosion effect on the refractory.
When the batch is added into the tank kiln, the powder is easily taken away by the gas flowing in the kiln, and the dust contains a lot of alkali, which often deposits on the upper surface of the tank wall brick, generates glaze, and flows down along the surface of the brick, forming deep grooves on the brick surface, and even drops into the molten glass, causing defects such as stripes in the molten glass.
The upper structure of the tank kiln is often eroded by the mixture dust and volatiles. However, the dust reacts with the refractory material chemically, and its products are mostly left on the surface of the refractory material to form a thin film, which has a protective effect and can prevent further erosion of the refractory material by the batch dust.
The volatile matter of the batch and the molten glass also carries out chemical attack on the refractory materials. The volatile components are mainly alkali metal oxides and boron compounds, as well as fluoride, chloride and sulfide. These volatiles react chemically with the refractory material in a gaseous state, penetrate into the pores or gaps of the refractory material, condense into a liquid phase at a lower temperature, and react chemically with the refractory material. The condensed liquid of these compounds corrodes the refractory more strongly. They penetrate deeply into the pores of the refractory through infiltration and diffusion, especially when there are cracks and gaps in the superstructure, which will cause great damage to the refractory.
The mixture dust and volatile matter jointly erode the refractory materials under most cleaning conditions. The farther away from the feeding port, the less the dust of the batch, and the upper structures of the working tank and the feeding trough are only eroded by the volatiles of the liquid glass.
These two kinds of chemical erosion can induce and promote the interaction between different kinds of refractory materials, reduce the strength of refractory materials, and even cause damage under physical action.
The atmosphere in the kiln also affects the corrosion of refractory materials. If it is operated in a reducing atmosphere, or when generator gas is used, CO and H2 in the gas will reduce iron oxide in the brick, thus accelerating the corrosion of refractory materials.
Batch dust, glass liquid volatiles and their condensates as well as the atmosphere in the kiln also play a considerable role in destroying the refractory materials of the regenerator.
The metasomatic reaction of molten glass, powder and volatiles in contact with refractory materials is mainly carried out by diffusion in the presence of solid phase. Due to the high viscosity of molten glass, the reaction is usually difficult to reach equilibrium and the reaction speed is slow. Dissolution, recrystallization and the formation of new equivalents occur during the alteration process. The process of erosion change is not the same for refractory materials with different properties. The erosion process of various refractory materials is mainly related to the type of erosives, chemical reaction speed, concentration of reaction products, brick structure and temperature, action time, physical and chemical erosion conditions.
Post time: Sep-02-2022