01
Impurity iron causes the melting temperature to rise
During the melting process of glass, the material absorbs heat from the flame, and the flame transfers heat to the material by radiation. When the melting pool temperature is 1500-1600℃, the radiation heat transfer of the flame and the top of the arch has a maximum value at about 1500nm; in addition, the absorption value of Fe2+ in soda-lime glass at 1050nm is the largest. Therefore, it can be understood that the temperature distribution of glass deep in the melting pool caused by radiation heat transfer in the kiln is directly related to the iron content in the glass. According to relevant measurements, the temperature difference between the surface and bottom of the pool kiln is 35℃ for glass liquid with a Fe2O3 content of 0.088%, while the temperature difference can reach 380℃ for glass liquid with a Fe2O3 content of 0.51%. It is precisely because of the presence of Fe2O3 and FeO that the temperature difference between the surface and deep layers of the glass in the kiln is enlarged, which will affect the melting and clarification of the glass liquid. This is caused by the strong absorption of heat radiation by iron oxide. Most of the radiant heat is absorbed by the surface glass liquid, forming a temperature difference between the upper and lower melts. If the glass raw materials at the bottom of the glass kiln are melted, the temperature of the entire kiln must be increased. Experiments have shown that within the normal melting operation temperature range, the erosion rate of refractory materials doubles for every 50°C increase in temperature. This not only consumes energy and shortens the service life of the kiln, but also causes uneven temperatures in various parts of the glass liquid. It is also easy to cause defects such as ribs on the glass plate surface.
02
Impurity iron causes erosion of the melting furnace
In recent years, with the development of glass melting technology, the melting capacity per unit area of modern glass melting furnaces has been greatly increased. Most of the pool bottoms adopt a multi-layer structure and have good thermal insulation, which greatly reduces heat loss. The pool bottom temperature is increased by about 200°C, which reduces the viscosity of the glass liquid at this location and intensifies the flow, which also strengthens the scouring and erosion of the pool bottom. Not only that, the glass liquid can easily penetrate into the poorly sealed brick joints, react with the refractory materials to produce bubbles, form three-phase interface conditions, and cause upward drilling.
When adding materials, the impure iron brought in with the batch and broken glass will cause more serious erosion; especially in the brick joints, the impure iron will invade the brick joints or pores to produce bubbles, and erode holes in the pool bottom, which is similar to the upward drilling caused by the glass liquid on the contact surface of the refractory material. The erosion of the pool bottom of modern glass melting furnaces comes from two aspects: first, direct erosion of the part in contact with the glass liquid; second, erosion of the lower sealing layer (lining), which damages the pool bottom bricks.
The entire erosion process is divided into four stages: glass liquid or impurities penetrate from the brick joints; the sealing layer begins to be damaged; the foaming glass liquid penetrates between the pool bottom paving bricks and the lining; "upward drilling" occurs, which damages the pool bottom paving bricks.
Therefore, the iron oxide in the glass raw materials is controlled to the lowest allowable limit. In particular, the iron oxide content of silica sand with a large amount is required to be less than 0.2%.
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