1 Introduction
Flat glass for construction and automobiles, glass substrates for display equipment, special glass for displays, solar glass, glass bottles for beverage containers, fiber glass, enamel, physical chemistry, medical glass, lighting glass, and even decorative glass, etc. , From people's livelihood to industry, glass is being used in large quantities.
Glass is manufactured through many processes of melting, forming, and cooling. The device for melting is a glass melting furnace, which operates at a high temperature of about 1400-1600°C depending on the type of glass. Moreover, the composition of glass involves many aspects such as soda lime glass, borosilicate glass, alkali-free glass, and lead glass. Therefore, refractory materials used in glass melting furnaces are very important in terms of high-temperature corrosion resistance to molten glass, contamination of molten glass, and stability of the furnace structure. The selection of appropriate refractory materials has a great impact on the quality, cost, and furnace life of the glass. Therefore, the selection of refractory materials is an important technical issue and the technical improvement of refractory materials must also be considered.
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On the other hand, taking the opportunity of the environmental protection of the earth and energy prices, we have recently implemented technical improvements to glass melting furnaces and achieved great results.
This article introduces the materials and characteristics of refractory materials used in glass melting furnaces, the erosion of refractory materials by molten glass, the pollution of molten glass, and the simulation results of the most suitable glass bottle furnace design.
2 The development process of glass melting furnace and refractory materials
Approximately 4 million tons of glass are produced in Japan each year, and flat glass and glass bottles account for 80% of the total. With the rapid decrease in the output of CRT displays, the glass substrates used for liquid crystal and plasma displays have been mass-produced instead.
The progress of glass melting furnace is inseparable from the development of refractory materials. In 1926, the United States developed aluminum-silicon fused refractory bricks, which brought a revolution in glass melting technology. In the late 1930s, the United States and France developed aluminum-zirconium-silicon (AZS) fused refractory bricks, which made great contributions to the long life of glass melting furnaces and the improvement of glass quality. The oxidation method was introduced in the manufacturing and melting process of fused refractory bricks, and 33% ~41% AZS was developed. In the 1980s, Japan developed high-zirconium fused refractory bricks with a ZrO2 content of more than 90%. Because of its good performance in corrosion resistance and pollution to molten glass, it is widely used in the manufacture of high-quality glass.
Glass melting furnaces include crucible kilns with a daily melting capacity of several tons and continuous production tank kilns with a daily melting capacity of hundreds of tons. According to different combustion methods, it can be divided into vertical flame kiln and horizontal flame kiln.
For the purpose of environmental protection, oxy-fuel combustion kilns have increased. Oxygen combustion kilns are widely used because they can save energy and reduce CO2, NOx and particulate emissions.
On the other hand, the amount of combustion-supporting air is reduced to approximately 1/4, and the concentration of vaporized components in the exhaust gas is greatly increased, which significantly corrodes the refractory materials. In the past, the big moraine used silica bricks, while the big moraine of the oxy-combustion kiln used fused bricks. There are also recent examples of using special sintered bricks in the superstructure. In order to pass the corrosive exhaust gas through the oxy-fuel flue, there are cases where high-grade refractory materials such as fused refractory bricks are used.
3 Electric melting characteristics of fused bricks and high zirconium bricks
3. 1 Fused brick
Currently, typical fused refractory bricks used in general glass melting furnaces are classified into alumina, alumina, zirconia, silica, and zirconia. Table 2 shows the representative chemical composition, glass phasor, glass exudation amount and applicable parts. In addition, the relationship between the temperature and the erosion amount of various fused refractory bricks relative to soda glass is shown in Figure 1.
Table 2. Typical quality examples of fused bricks
Figure 1. The relationship between the temperature and erosion of each fused brick and soda-lime glass
Alumina has two products: α/β-alumina and β-alumina. α/β-alumina has the dense structure of the two crystals, zirconium, and has good alkali resistance. Compared with AZS, the corrosion resistance is relatively poor at high temperatures above 1350℃, and it is corrosion resistant at temperatures below 1350℃. The same sex. Moreover, even if it is in contact with molten glass, bubbles and impurities are rarely generated, and the pollution to the molten glass is small. Therefore, α/β-alumina is used in the work section and the forehearth. β-alumina is a structure of columnar crystals with zirconium cross, with good heat exfoliation and alkali resistance, and can be used for superstructures.
Alumina, zirconia, and siliceous oxide can be divided into three major parts based on the 33% -41% zirconia content. As shown in Figure 1, the higher the zirconia content, the higher the corrosion resistance. Generally speaking, 33% AZS is used for superstructures and underlays, 35% AZS is used for large bricks, and 41% AZS is used for liquid cavities and other severely corroded parts. However, after AZS refractory is heated to above 1400°C, the glass will extrude and seep out to the outside of the brick. This oozing glass phase will contaminate the glass liquid and become the cause of glass defects such as bubbles, ribs, and lines. Based on this phenomenon, it is necessary to take corresponding measures in the design and operation of the furnace. Recently, in order to reduce the amount of glass oozing out and improve the creep characteristics at high temperatures, new products with a high SiO2/Na2O glass phase or a reduced amount have been developed and used in large quantities.
More than 90% zirconia crystals and high zirconia fused bricks (ZFC, ZFCR) composed of a small amount of glass phase contain a lot of ZrO2 with excellent corrosion resistance. Because there is less glass, it has good corrosion resistance and Fewer bubbles.
3.2 The characteristics of high zirconia fused refractory bricks
Among the above-mentioned high zirconia electroformed bricks, there is ZFCR with ZFC (about 95% ZrO2) and high resistance characteristics. ZFC and AZS, which are widely used in high zirconia fused refractory bricks, especially introduce the characteristics of 41% AZS fused refractory bricks with excellent corrosion resistance.
First introduce the corrosion resistance of various glasses (resistance to glass erosion). Table 3 (1) shows the glass composition used for the corrosion test; Table 3 (2) shows the results of the corrosion test. Borosilicate glass and aluminosilicate glass have very little corrosion to ZFC and have good corrosion resistance. Figure 2 shows the microscopic observation results of the interface after the corrosion test of borosilicate glass. There is no reaction layer in the cross section of ZFC. ZrO2 is eroded by dissolution and diffusion. It is a smooth damage. It can be inferred that it is difficult to produce stones in the glass. On the other hand, 41% AZS has a completely different reaction layer, and it can be observed that the ZrO2 particles separate and fall off in the cross section of the glass. If the ZrOz particles dissolve and diffuse incompletely, stone defects may be formed. However, ZFC is difficult to produce stone defects and has little pollution to molten glass. The bubble characteristics of 41% AZS and the rate of bubble generation from the kiln material in the high temperature of ZFC are shown in Figure 3.
Because ZFC has good corrosion resistance and low pollution to molten glass, it is mostly used in large bricks, flow holes, and bottoms of high-quality glass melting furnaces such as display glass, and has produced good results. In addition, it has good corrosion resistance to borosilicate glass, and is also used in physicochemical glass and heat-resistant glass furnaces.
ZFCR is a high zirconia brick with high resistivity at high temperature. As shown in Figure 4, because the resistivity of ZFCR is higher than that of various glass liquids, it is often used in electric furnaces for electric auxiliary heating, such as electrode bricks.
Table 3(1)
Table 3(2) Corrosion test results of various glasses on two types of fused bricks
4 Sintered bricks and unshaped refractory materials
4..1 Bricks for regenerator
In the contact part between the glass melting furnace and the glass, fused refractory bricks are mainly used. Use sintered bricks and monolithic refractories for upper structures, fire walls, and regenerators that are not in contact with glass. The quality of unshaped refractories (mortar, ramming material) is shown in Table 4. Here is a description of sintered bricks.
Table 4. The quality of unshaped refractory materials for glass melting furnace
The regenerator is a heat recovery device that temporarily stores the heat of high-temperature combustion exhaust gas in checker bricks, and uses the heat to preheat the secondary air for combustion. It is roughly divided into the frame part of the roof, side walls, and bottom of the furnace, and the checker bricks for heat recovery and the combustion channel part. The quality of commonly used bricks is shown in Table 5. The top moraine and upper side wall of the exterior wall are made of high-purity magnesia bricks with high corrosion resistance and good creep resistance against high temperature and strong alkali steam and flying materials. Of course, there are also silica bricks used in the roof.
Table 5.
Checker bricks are an important part of controlling the performance of the regenerator. In the uppermost section, 98% magnesia bricks with good corrosion resistance to strong alkalis and flying materials are used. As the temperature gradually drops from the upper section to the middle section, it is suitable to use 95% grade magnesia bricks.
Magnesia bricks were previously used in the area of Glauber's salt solidification and liquefaction, but they are now replaced by magnesia and spinel bricks for environmental protection reasons, and have shown good practical effects. In Europe, magnesium oxide and spinel bricks are widely used in this field.
4.2 Special bricks
Representative special bricks are zircon, alumina, zircon, sillimanite, pure spinel, mullite bricks, and dense carammel bricks. Table 6 describes its quality examples.
Diamond bricks are acid bricks that are stable at high temperatures. It is mostly used in borosilicate glass kiln. The decomposition of zircon is about 1540%: start slowly, because the volume expansion will cause peeling, so pay special attention to the operation. Alumina and zircon bricks have good alkali resistance and heat-resistant peeling resistance, and are mostly used in the upper structure and auxiliary bottom of the front wall and back wall of the furnace. Sillimanite bricks made of high-purity raw materials have good corrosion resistance and pollution resistance to molten glass, and are mostly used in the forming part of the glass contact part and the upper structure of the forehearth.
The upper structure of the oxy-combustion kiln basically uses fused refractory bricks. Depending on the glass type, location, and operating conditions, pure spinel or mullite bricks can also be used. Dense zircon bricks have a bubble rate below 1% and ultra-dense zircon bricks have high resistivity at high temperatures, so they can be used as materials for electric melting kilns.
Table 6. The quality of special sintered bricks for glass melting furnace
5 Bottle glass melting furnace design examples
Most glass furnaces can maintain long-term continuous operation for more than 10 years. The selection of refractory materials used in glass furnaces and the structural design are important aspects for the life of the furnace and the quality of the glass.
The simulation technology of molten glass flow and combustion has made significant progress, and many results have been achieved through the application of design technology. Moreover, due to the introduction of bubbling technology in the melting technology, the sill furnace material with a forced cooling structure, etc., the quality of the glass is improved. By setting bubbling and barriers in the melting furnace, the homogenization of the molten glass and the quality of the molten glass in the high-temperature area of the melting furnace are improved.
6 Concluding remarks
This article introduces the refractory materials used in the glass melting furnace, the erosion of glass, the contamination of molten glass, and design examples.
Even in the future, glass is an important basic material for mankind. With the development of glass materials, operational changes, and environmental changes, the demand for refractory materials will involve many aspects and become more advanced. The glass industry is a high-temperature industry that consumes more energy. There are many technical issues in terms of energy saving. RCF (refractory ceramic fiber) has not been used recently, and the development of environmentally friendly super-insulation materials is entering the actual stage. In the future, the use of raw materials for refractory materials will be further developed.
