(1) Melting Characteristics of High-borosilicate Glass High-borosilicate glass has a wide range of uses in scientific experiments and daily life. Its products include: various burners, measuring devices, various laboratory analytical foods, and various glass chemical pipelines. . Various kinds of products made of high-borosilicate glass can be seen in research laboratories, industrial and mining enterprises, and solar energy comprehensive utilization equipment. Due to the particularity of the use of high borosilicate glass products, it is determined that this glass must have good thermal stability and stability. At present, the chemical compositions of high borosilicate glass at home and abroad are basically listed in Table 1 below.
Table 1 Chemical composition and coefficient of thermal expansion α of borosilicate glass at home and abroad
Grade Country Chemical composition Coefficient of thermal expansion a (0-300 °C)
SiO2
B2O3
Al2O3
CaO
K2O
Na2O
Pyrex-7740
United States 80.5
12.6
2.1
0.1
4.5
32×10-7/°C
Durang DuRuN-50
West Germany 80.5
12.5
2.5
0.5
4.0
32×10-7/°C
West Max SIMAX
Czech Republic 81
12.6
2.0
0.1
4.0
32×10-7/°C
Pyrex PYREX
France 80.5
13
2.25
0.1
1.15
3.15
32×10-7/°C
Beijing special hard material BT-TY (GG-17)
China 80.7
12.5
2.2
0.6
4.0
32×10-7/°C
As can be seen from Table 1, the glasses have one thing in common: they all contain more than 80% of SiO2 and more than 10% of B2O3. The high content of SiO2 and the low content of Na2O make the silica tetrahedron in the glass structure to exist in the form of a large tetrahedron, and tetrahedrons are mostly connected in the form of silicon-oxygen bonds. Moreover, a 12% to 13% content of B2O3 can in turn cause a large part of the network cut by Na2O in the structure to be connected by the BO trigonal structure. In this way, the viscosity of the borosilicate glass is large (see Table 2).
Table 2 Viscosity reference points of high borosilicate glass and common soda-lime-silica glass Reference point types Operation points
103 Pa. second softening point
106.6 Pa.s Annealing point
1012 Pa. second strain point
1013.5 Pa.s
High borosilicate glass 1252
821
560
510
Common soda-lime-silica glass 1005
696
514
473
(a) High melting temperature (temperature >1680°C at 10 Ïa·s). Because the melting speed is directly related to the melting speed of the quartz particles in the batch, the large viscosity of the melt slows the diffusion of SiO2 from the surface of the quartz particles to the surroundings. Therefore, in order to reduce the viscosity of the melt, only the temperature is increased. However, as far as the material of the furnace is concerned, it is not practical to make the melting temperature above 1700°C. Therefore, only the melting time can be extended. According to calculations, the melting rate constant of PyREx glass is as high as 7.9 and the thermal energy consumption is very high (12000 KCal/Kg glass). The melting rate of such glass in China is only about 0.2 to 0.3 T/m 2 ·d.
(b) Glass delamination during melting. Since there is no heat source in the lower layer of the conventional flame heating tank kiln, the temperature becomes lower and lower as the depth of the pool increases. In this way, the fluidity of the lower glass is getting worse and worse, so that components such as Al2O3 having a larger specific gravity are likely to sink, forming a metamorphic layer with a high amount of Al2O3. When temperature and liquid flow change, they can easily be trapped in the glass flow to form defects such as stripes. In addition, due to the high content of SiO2, in the lower temperature regions, crystallization can easily occur, resulting in the formation of stones.
(c) The volatilization of B2O3 is another feature of the borosilicate glass melting process. The volatilization of B2O3 forms a layer of silicon-rich glass on the surface of the glass, which not only affects the chemical composition of the glass, but also increases the volatilization of B2O3 with increasing temperature. This is undoubtedly disadvantageous for high-borosilicate glass requiring high-temperature melting. When the operating conditions change, this layer of silicon-rich glass, if brought into the forming flow, will produce defects such as streaks and stones in the product.
(d) Due to these characteristics, the infusibilty of the borosilicate glass is constituted. Therefore, so far, most domestic manufacturers are still confined to small-scale flame tank furnaces and kiln furnaces for melting and supplying artificial blowing and small batch production. The energy consumption is large, the yield is low, and the labor intensity of workers is high. In order to meet the market demand and improve the level of melting technology in China, the domestic glass industry has been trying to solve this problem.
(e) An important way to melt the high-quality borosilicate glass is to increase the temperature of the glass. However, considering the flame capacity of flame space refractory materials, it is difficult to heat the glass liquid above 1650°C by simply relying on the flame heating. Abroad, the melting of this glass has been generally adopted two kinds of melting methods: electric melting and full melting. (2) Advantages of melting borosilicate glass with electric melting furnace Most electric melting furnaces for melting borosilicate glass in China For the small kiln, the cold-top fused kilns have significant advantages over the flame kiln such as energy saving, quality improvement, and cost reduction.
(1) Small electric furnaces have better economical performance. Take a high-borosilicate glass ball mill with a capacity of 2.0 tons per day as an example, and compare them with the two tops of fuel and boron volatilization.
table 3
Production Method Project Flame Kiln Heat Top Electric Furnace Cold Top Electric Furnace
Boron volatile %
18
10
3
Kiln thermal efficiency%
6.5
40
60
(2) The quality of the glass is good. In the flame tank kiln, changes in the process conditions such as kiln temperature, kiln pressure, atmosphere, and yield all cause changes in the volatility of boron, making the glass non-uniform. At the same time, the volatilization of boron seriously corrodes the upper structure in the flame kiln and the hot-top furnace, which not only shortens the life of the furnace, but also causes dripping into the glass kiln, which affects the quality of the glass. The cold top electric melting furnace can completely avoid the above drawbacks and obtain high quality glass.
3.3.4 All-electric furnace for melting fluoro opaque glass
(1) Characteristics of the melted milk glass (a) The milk glass generally contains a large amount of highly volatile components such as fluoride. In a conventional flame-heating furnace, when the flame passes over the surface of the glass, a considerable amount of volatile components will be carried away. After passing through the flue, they will run up the chimney and run away. This will result in loss of valuable raw materials. Caused air pollution. At the same time, due to volatilization losses, the composition of the surface glass becomes very different from that of the underlying deep glass, resulting in non-uniform glass composition.
(b) Fluoride glass seriously erodes electrodes and refractories.
(2) The melting furnace glass is the best choice for the electric melting furnace. When using the all-electric melting process, the heat is released under the batch material, and the gas generated from each batch component must escape upward through the batch material layer. Due to the lower temperature of the batch, the volatile gases will condense in the cold batch and will not evaporate, so that the molten glass that flows out through the stream hole can be basically mixed with the batch materials fed into the melting furnace. Keep the same, and make the chemical composition of the product stable. On the other hand, due to the reduction of volatilization, valuable raw materials are saved and raw material costs are reduced. In the case of conventional fuel flame heating, approximately 40% of the fluoride in the batch is lost due to volatilization. With all electrofusion, the volatilization of fluoride is only 2%.
(3) Selection of fining agent for melting milk glass in electric melting furnace As oxidizing ability of As2O3 and Na2SO4, which are commonly used in common glass, is strong at high temperature, it can oxidize molybdenum electrode, especially Na2SO4 decomposes O2 and SO2 at high temperature. The mixture reacts with molybdenum to form MoO3 and MoS4, which seriously erode the molybdenum electrode. Therefore, the main advantages of nitrates as clarifiers are as follows: (1) Nitrate has a low melting point and a low decomposition temperature, and it can interact with the surface of waste glass during the sintering process of the batch materials. Therefore, the clarifying effect of waste glass is obvious. (2) The introduction of nitrate can promote the conversion of Fe2+ to Fe3+, which is beneficial to the decolorization of waste white glass. (3) The decomposition reaction of nitrate is carried out at the interface of the batch glass. Therefore, the released oxygen is only concentrated in the upper part of the glass, and the erosion of the molybdenum electrode is small.
(4) Selection of Opacifying Agent for Melting White Glass in Electric Melting Furnace Milky glass can be obtained by introducing fluoride, phosphate, and high refractive oxide. Taking into account the characteristics of the furnace and source of raw materials, fluoride is selected as an opacifier. In order to ensure that the product has a certain degree of whiteness, the content of the opacifying agent (in terms of F-calculation) is preferably 4 to 5%. When the amount of opacifying agent is too low, the glass is translucent and the whiteness is low. When the amount is too high, the whiteness of the glass does not increase significantly. Instead, the glass tends to crystallize. In severe cases, the surface of the product becomes rough.
Since the volatilization of fluoride increases sharply with the increase of the melting temperature, generally, the R2O content of the opal glass is high to reduce the melting temperature and reduce the volatilization of fluoride. In order to promote opacification, improve crystallization ability, improve product gloss, introduce a certain amount of K2O. Taking into account the requirements of electrofusion on the resistance of the glass liquid and the requirements of the forming process, the content of R2O in the white glass is about 18-19%.
3.3.5 All-electric furnace melting colored glass The dark glass has a strong absorption of heat rays, so when using conventional surface radiant heating methods, the heat penetration problem often occurs when melting dark glass. The reduction of radiant energy is related to the color of the glass. When the transparent window glass is melted, the radiant energy is only reduced by 10% below 30-60mm below the liquid surface of the glass. When the green glass is melted, the radiant energy is 4-6mm below the liquid surface. Reduced by 10%. The amount of iron contained in the glass has a great influence on the thermal conductivity, and the transmittance of 2 mm thick colorless glass is about 85%. When the Fe2O3 content reaches 1.5%, the glass transmittance can be reduced to 28%. The darker the glass, the worse the thermal conductivity. The surface glass liquid absorbs more heat, and the temperature gradient in the depth direction of the pool is larger. Therefore, the temperature of the deep glass is too low, which often causes difficulty in melting and crystallization.
The intense coloration of the glass reduces the thermal conductivity of the glass melt, and it is very difficult to uniformly heat the glass melt, and uniform heating of the glass melt is necessary to obtain high quality products. Therefore, colored glass is basically melted in the crucible, and at the prescribed temperature, the melt in the crucible furnace must be heated for a long time to meet the requirement.
In the fully fused state, heat energy is released from the glass body, and the current can pass through most of the glass fairly evenly, so only a small temperature difference will occur. When melting high iron content amber glass, the temperature of the glass near the bottom of the pool is only about 14°C lower than the glass near the surface. Glass with up to 12% iron oxide and glass with 1.3% chromium oxide can be melted smoothly. Electric melting furnaces created a new era for the production of such glass.
Due to the low thermal conductivity of the melt, the colored glass leads to overheating in the vicinity of the electrode layer. Therefore, the current density of the electrode is reduced to 0.5 A/cm2 as much as possible under the given conditions, and the current is distributed as evenly as possible. Usually, the plate electrode is used for electrofusion. kiln.
Author: Chen Jinfang (1963), male, Jiangsu Taixing, Master, Associate Professor of Jiangsu University, Expert of the Kiln Group of the Glass Specialty Committee of the Chinese Silicate Academy, "Electrical Melting and Electrical Heating of Glass". Mainly engaged in the research and design of glass furnaces
Table 1 Chemical composition and coefficient of thermal expansion α of borosilicate glass at home and abroad
Grade Country Chemical composition Coefficient of thermal expansion a (0-300 °C)
SiO2
B2O3
Al2O3
CaO
K2O
Na2O
Pyrex-7740
United States 80.5
12.6
2.1
0.1
4.5
32×10-7/°C
Durang DuRuN-50
West Germany 80.5
12.5
2.5
0.5
4.0
32×10-7/°C
West Max SIMAX
Czech Republic 81
12.6
2.0
0.1
4.0
32×10-7/°C
Pyrex PYREX
France 80.5
13
2.25
0.1
1.15
3.15
32×10-7/°C
Beijing special hard material BT-TY (GG-17)
China 80.7
12.5
2.2
0.6
4.0
32×10-7/°C
As can be seen from Table 1, the glasses have one thing in common: they all contain more than 80% of SiO2 and more than 10% of B2O3. The high content of SiO2 and the low content of Na2O make the silica tetrahedron in the glass structure to exist in the form of a large tetrahedron, and tetrahedrons are mostly connected in the form of silicon-oxygen bonds. Moreover, a 12% to 13% content of B2O3 can in turn cause a large part of the network cut by Na2O in the structure to be connected by the BO trigonal structure. In this way, the viscosity of the borosilicate glass is large (see Table 2).
Table 2 Viscosity reference points of high borosilicate glass and common soda-lime-silica glass Reference point types Operation points
103 Pa. second softening point
106.6 Pa.s Annealing point
1012 Pa. second strain point
1013.5 Pa.s
High borosilicate glass 1252
821
560
510
Common soda-lime-silica glass 1005
696
514
473
(a) High melting temperature (temperature >1680°C at 10 Ïa·s). Because the melting speed is directly related to the melting speed of the quartz particles in the batch, the large viscosity of the melt slows the diffusion of SiO2 from the surface of the quartz particles to the surroundings. Therefore, in order to reduce the viscosity of the melt, only the temperature is increased. However, as far as the material of the furnace is concerned, it is not practical to make the melting temperature above 1700°C. Therefore, only the melting time can be extended. According to calculations, the melting rate constant of PyREx glass is as high as 7.9 and the thermal energy consumption is very high (12000 KCal/Kg glass). The melting rate of such glass in China is only about 0.2 to 0.3 T/m 2 ·d.
(b) Glass delamination during melting. Since there is no heat source in the lower layer of the conventional flame heating tank kiln, the temperature becomes lower and lower as the depth of the pool increases. In this way, the fluidity of the lower glass is getting worse and worse, so that components such as Al2O3 having a larger specific gravity are likely to sink, forming a metamorphic layer with a high amount of Al2O3. When temperature and liquid flow change, they can easily be trapped in the glass flow to form defects such as stripes. In addition, due to the high content of SiO2, in the lower temperature regions, crystallization can easily occur, resulting in the formation of stones.
(c) The volatilization of B2O3 is another feature of the borosilicate glass melting process. The volatilization of B2O3 forms a layer of silicon-rich glass on the surface of the glass, which not only affects the chemical composition of the glass, but also increases the volatilization of B2O3 with increasing temperature. This is undoubtedly disadvantageous for high-borosilicate glass requiring high-temperature melting. When the operating conditions change, this layer of silicon-rich glass, if brought into the forming flow, will produce defects such as streaks and stones in the product.
(d) Due to these characteristics, the infusibilty of the borosilicate glass is constituted. Therefore, so far, most domestic manufacturers are still confined to small-scale flame tank furnaces and kiln furnaces for melting and supplying artificial blowing and small batch production. The energy consumption is large, the yield is low, and the labor intensity of workers is high. In order to meet the market demand and improve the level of melting technology in China, the domestic glass industry has been trying to solve this problem.
(e) An important way to melt the high-quality borosilicate glass is to increase the temperature of the glass. However, considering the flame capacity of flame space refractory materials, it is difficult to heat the glass liquid above 1650°C by simply relying on the flame heating. Abroad, the melting of this glass has been generally adopted two kinds of melting methods: electric melting and full melting. (2) Advantages of melting borosilicate glass with electric melting furnace Most electric melting furnaces for melting borosilicate glass in China For the small kiln, the cold-top fused kilns have significant advantages over the flame kiln such as energy saving, quality improvement, and cost reduction.
(1) Small electric furnaces have better economical performance. Take a high-borosilicate glass ball mill with a capacity of 2.0 tons per day as an example, and compare them with the two tops of fuel and boron volatilization.
table 3
Production Method Project Flame Kiln Heat Top Electric Furnace Cold Top Electric Furnace
Boron volatile %
18
10
3
Kiln thermal efficiency%
6.5
40
60
(2) The quality of the glass is good. In the flame tank kiln, changes in the process conditions such as kiln temperature, kiln pressure, atmosphere, and yield all cause changes in the volatility of boron, making the glass non-uniform. At the same time, the volatilization of boron seriously corrodes the upper structure in the flame kiln and the hot-top furnace, which not only shortens the life of the furnace, but also causes dripping into the glass kiln, which affects the quality of the glass. The cold top electric melting furnace can completely avoid the above drawbacks and obtain high quality glass.
3.3.4 All-electric furnace for melting fluoro opaque glass
(1) Characteristics of the melted milk glass (a) The milk glass generally contains a large amount of highly volatile components such as fluoride. In a conventional flame-heating furnace, when the flame passes over the surface of the glass, a considerable amount of volatile components will be carried away. After passing through the flue, they will run up the chimney and run away. This will result in loss of valuable raw materials. Caused air pollution. At the same time, due to volatilization losses, the composition of the surface glass becomes very different from that of the underlying deep glass, resulting in non-uniform glass composition.
(b) Fluoride glass seriously erodes electrodes and refractories.
(2) The melting furnace glass is the best choice for the electric melting furnace. When using the all-electric melting process, the heat is released under the batch material, and the gas generated from each batch component must escape upward through the batch material layer. Due to the lower temperature of the batch, the volatile gases will condense in the cold batch and will not evaporate, so that the molten glass that flows out through the stream hole can be basically mixed with the batch materials fed into the melting furnace. Keep the same, and make the chemical composition of the product stable. On the other hand, due to the reduction of volatilization, valuable raw materials are saved and raw material costs are reduced. In the case of conventional fuel flame heating, approximately 40% of the fluoride in the batch is lost due to volatilization. With all electrofusion, the volatilization of fluoride is only 2%.
(3) Selection of fining agent for melting milk glass in electric melting furnace As oxidizing ability of As2O3 and Na2SO4, which are commonly used in common glass, is strong at high temperature, it can oxidize molybdenum electrode, especially Na2SO4 decomposes O2 and SO2 at high temperature. The mixture reacts with molybdenum to form MoO3 and MoS4, which seriously erode the molybdenum electrode. Therefore, the main advantages of nitrates as clarifiers are as follows: (1) Nitrate has a low melting point and a low decomposition temperature, and it can interact with the surface of waste glass during the sintering process of the batch materials. Therefore, the clarifying effect of waste glass is obvious. (2) The introduction of nitrate can promote the conversion of Fe2+ to Fe3+, which is beneficial to the decolorization of waste white glass. (3) The decomposition reaction of nitrate is carried out at the interface of the batch glass. Therefore, the released oxygen is only concentrated in the upper part of the glass, and the erosion of the molybdenum electrode is small.
(4) Selection of Opacifying Agent for Melting White Glass in Electric Melting Furnace Milky glass can be obtained by introducing fluoride, phosphate, and high refractive oxide. Taking into account the characteristics of the furnace and source of raw materials, fluoride is selected as an opacifier. In order to ensure that the product has a certain degree of whiteness, the content of the opacifying agent (in terms of F-calculation) is preferably 4 to 5%. When the amount of opacifying agent is too low, the glass is translucent and the whiteness is low. When the amount is too high, the whiteness of the glass does not increase significantly. Instead, the glass tends to crystallize. In severe cases, the surface of the product becomes rough.
Since the volatilization of fluoride increases sharply with the increase of the melting temperature, generally, the R2O content of the opal glass is high to reduce the melting temperature and reduce the volatilization of fluoride. In order to promote opacification, improve crystallization ability, improve product gloss, introduce a certain amount of K2O. Taking into account the requirements of electrofusion on the resistance of the glass liquid and the requirements of the forming process, the content of R2O in the white glass is about 18-19%.
3.3.5 All-electric furnace melting colored glass The dark glass has a strong absorption of heat rays, so when using conventional surface radiant heating methods, the heat penetration problem often occurs when melting dark glass. The reduction of radiant energy is related to the color of the glass. When the transparent window glass is melted, the radiant energy is only reduced by 10% below 30-60mm below the liquid surface of the glass. When the green glass is melted, the radiant energy is 4-6mm below the liquid surface. Reduced by 10%. The amount of iron contained in the glass has a great influence on the thermal conductivity, and the transmittance of 2 mm thick colorless glass is about 85%. When the Fe2O3 content reaches 1.5%, the glass transmittance can be reduced to 28%. The darker the glass, the worse the thermal conductivity. The surface glass liquid absorbs more heat, and the temperature gradient in the depth direction of the pool is larger. Therefore, the temperature of the deep glass is too low, which often causes difficulty in melting and crystallization.
The intense coloration of the glass reduces the thermal conductivity of the glass melt, and it is very difficult to uniformly heat the glass melt, and uniform heating of the glass melt is necessary to obtain high quality products. Therefore, colored glass is basically melted in the crucible, and at the prescribed temperature, the melt in the crucible furnace must be heated for a long time to meet the requirement.
In the fully fused state, heat energy is released from the glass body, and the current can pass through most of the glass fairly evenly, so only a small temperature difference will occur. When melting high iron content amber glass, the temperature of the glass near the bottom of the pool is only about 14°C lower than the glass near the surface. Glass with up to 12% iron oxide and glass with 1.3% chromium oxide can be melted smoothly. Electric melting furnaces created a new era for the production of such glass.
Due to the low thermal conductivity of the melt, the colored glass leads to overheating in the vicinity of the electrode layer. Therefore, the current density of the electrode is reduced to 0.5 A/cm2 as much as possible under the given conditions, and the current is distributed as evenly as possible. Usually, the plate electrode is used for electrofusion. kiln.
Author: Chen Jinfang (1963), male, Jiangsu Taixing, Master, Associate Professor of Jiangsu University, Expert of the Kiln Group of the Glass Specialty Committee of the Chinese Silicate Academy, "Electrical Melting and Electrical Heating of Glass". Mainly engaged in the research and design of glass furnaces
Cast Iron Frying Skillet,Cast Iron Waxed Skillet Cookware,Cheese Cookware,Cast Iron Cheese Pot
Shijiazhuang Ever Fresh Trading Co., Ltd. , https://www.efhomedeco.com