As the requirements for environmental protection are getting higher and higher, the quality requirements for glass products are also getting higher and higher. The use of electrofusion in glass factories has become an inevitable trend. The author has engaged in technical research and promotion of glass electrofusion for more than ten years. He has participated in the design, introduction, and refurbishment of dozens of glass melting furnaces, electric heating channels, and electrically assisted heated glass furnaces, helping glass factories to solve problems. There are many difficult problems in electrofusion and electric heating of glass.
There are dozens of companies and individuals who design electric melting and electric heating of glass. The design of electric melting kilns has its own characteristics and the level is uneven. Here is a summary.
1. Ten years ago, the glass electric heating material channel and the gas multi-nozzle heating material channel were promoted at the same time as two new technologies. Some manufacturers also require the inspection of the electric heating material channel technology of glass. . It is only some pioneers who use this technology. After continuous efforts in the past decade, the electric heating material channel of glass has been widely accepted by the majority of glass factories and has become a popular technology. Some units and individuals have made it into a product for sale.
However, some special requirements and special electric glass heating material channel still need professionals to design. The electric heating material of the glass is roughly divided into a silicon carbon rod radiation electric heating material channel, an electric heating of a plate glass path, an electric heating material channel of a plate-shaped molybdenum electrode, an electric heating rod of a rod-like molybdenum electrode, a hybrid electric heating material channel, The thermal jacketing method electrically heats the material, the electric heating of the material pot, the tin oxide electrode electric heating material channel and the like.
2. Electrically-assisted heating of glass furnaces Electric-fusion heating of glass furnaces is mainly used to:
(1) The melting rate is greatly increased. The foreign large-scale fuel glass plate kilns generally have a melting rate of 2T/m2·d, and China has a rate of about 1.6T/m2·d. After the electric assisting technology is used, the melting rate can be increased to 3.2T/m2·d, or even Up to 4.2T/m2·d. The melting rate can be increased by 60% or even 100%.
(2) Improve the melting quality of the glass. Using electric fluxing in any case improves the quality of the glass. This is because the flow of molten glass is enhanced so that the uniformity of the glass liquid is improved. The introduction of electric energy in the glass liquid raises the temperature of the glass liquid, so that the viscosity of the glass liquid is reduced, the clarification process is accelerated, and the amount of gas dissolved in the glass liquid is significantly reduced, which has a good influence on the forming and processing of the glass.
In some small and medium-sized glass factories in China, due to the poor quality of the fuel, the melting temperature cannot be burned, and the quality of the glass liquid is very poor. If the condition of using electric fluxing technology, the quality of the glass will be greatly improved.
(3) Flexible adjustment of discharge volume. The tank furnace with electric flux heating can quickly adjust the output of the tank furnace according to the market demand. Without increasing the size of the tank, the melting capacity of the tank furnace can be increased by 30 50%. Electric fluxing is particularly suitable for kiln furnaces that need to change their discharge regularly. The fossil fuel consumed by melting the glass during the use of the kiln is not changed, and the electric melting can maximize the melting capacity of the kiln.
(4) The electric fuser device is particularly suitable for colored glass. In the case of dark glass having poor radiant heat radiation, it is extremely advantageous to use electric fluxing in the melting section.
(5) Electric fluxing device is especially suitable for refractory glass.
The key design point for electric melting heating of glass kilns is the electrode arrangement and power distribution of the kilns. The electrode arrangement for borosilicate glass and colored glass kilns is to enhance the energy at the bottom of the tank, in order to increase the output or to adjust The output of the glass furnace electrode arrangement is to enhance the convection of the glass.
After the electrode arranged at the bottom of the cell is energized, the molten glass is a conductor. According to the thermal effect principle of resistance, the glass liquid between the two electrodes will generate heat. At the same time, due to the edge effect of the electrode tip, the temperature of the glass liquid near the end of the electrode is the highest, and the temperature here can even reach above 1700°C. Because of the difference in specific gravity, a glass flow is formed near the electrodes, and the glass in each layer of the depth direction of the pool fully participates in this flow, thereby eliminating the problems caused by the delamination of the high-borosilicate glass. In addition, the “exothermic†phenomenon of the electrode tip in the molten glass and the resulting glass flow increase the temperature of the underlying glass solution, accelerates the dissolution rate of the quartz particles, and promotes clarification and homogenization of the glass.
3. Full-melt glass furnace
3.1. Mechanism of reducing volatilization Full electrosulfur thick layer vertical deep electrofusion process, the surface of the melt pool covers the cold batch material, the batch material is heated under the cover layer, from the heating to the glass formation in the four stages, all in the same A place, different times and different vertical heights are completed. Therefore, it can basically avoid the existence of boron volatilization in the flame tank furnace. The mechanism is:
(a) The surface of the batch material is stable. When the full-thick layer is melted, the surface temperature of the layer can be as low as 200°C, and the surface of the layer does not have any flame and high-temperature high-speed airflow, and the plate is in a stable state. Basically, the physical loss of "flying material" can be avoided.
(b) Cold roof can recover volatiles. Thick material layer thickness is generally 100 ~ 200mm, the surface temperature below 200 °C. From the top of a typical melting borosilicate glass cold furnace in Figure 1, you can see the 0 ~ 40mm layer of the batch material surface, the temperature is stable at 120 °C, only the water in the evaporation, can be called cold batch materials Floor. Depth at 40 ~ 80mm, temperature increased from 120 °C to 250 °C, boric acid began to decompose, known as thermal compound layer, the depth of the most dramatic change in temperature of 80 ~ 110mm, rose from 250 °C to 1000 °C, completed borosilicate The salt reaction process, known as the borosilicate reaction zone, can also be referred to as a light layer. The next layer is a molten glass containing a large number of bubbles and unsmelted sand, called the glass melting process, also called semi-molten layer. Below this layer is the area of ​​clarification and homogenization of the glass. The borosilicate batch material from the surface layer to the light layer, through the thermal process temperature from 120 to 1000 °C, will inevitably occur boric acid and borax in the dehydration period of boron volatilization, volatiles from the bottom up when the case of Upon condensation on the cold surface batch layer, boron recoagulation occurs, and the CO2 and NOx gases generated during the reaction easily escape through the loose batch layer. Therefore, the boron volatiles can be recovered here. Using this principle, it is also possible to recover other volatile substances containing fluorine, lead and selenium glass. It is also easy to explain why the boron evaporation rate is high (about 10%) in the thin layer all-electrosmelting process.
(c) Clarification and homogenization are performed in the vertical deep direction. In the vertical deep electric kiln, under the action of the work flow, the molten glass moves vertically downwards from the semi-fused layer to the flow hole (see Figure 1), and the bubbles in the glass depend on the principle of Stokes. The upward movement, discharged from the liquid surface, completes the clarification and homogenization process. This process is quite different from the horizontal flame tank furnace melting. It seems to be performed in a vertical pipeline, there is no free surface in contact with air, so it is impossible to produce the problem of boron volatilization from the glass bonded state.
From the above-mentioned melting mechanism, it can be clearly seen that since the loss of boron is reduced to a very small extent, the fluctuation of the glass composition is small and stable, which is one of the important process conditions for obtaining a high-quality borosilicate glass.
3.2. Thick material layer vertical deep-level electrofusion technology If the thick material layer is not properly handled, not only can not obtain high-quality glass, but will produce stones and streaks in the glass, making the quality of the glass worse. To achieve stable operation, the basic process technology for thick layer electrofusion is as follows:
(a) Strictly controlling the thickness of the material layer: The thickness of the material layer is a very important process parameter in a thick material layer of a fully electric melting furnace. The thickness of the normal operation material layer is 100 to 200 mm. Too thick material layer causes deeper layers to cause more cold batch materials, the molten glass surface temperature decreases, and the glass does not melt well. At the same time, due to the thermal insulation of the material layer, the temperature of the deep batch material increases, softens the raw material, forms a semi-rigid shell, and may extend to overlap with the surrounding pool walls to form a material arch. Bubbles accumulate under the material arch and do not go out, and the glass surface drops quickly, resulting in the phenomenon of voiding. In severe cases, an accident of glass flow occurs. In order to prevent the material arch phenomenon and make the bubbles in the glass can be discharged smoothly, the compound material can not completely cover the surface of the molten pool. According to Sorg's experience in the VSM's all-electric furnace, the area covered by compound material accounts for 90% of the liquid surface, leaving a 10% area of ​​smooth surface. The ordinary glass is melted and the smooth side is left on the six corners of the hexagon. The smooth surface of the melted milk glass stays in the center. For a rectangular electric furnace, leave a distance of 75 to 150 mm around the wall. If the material layer is too thin, the surface temperature of the batch material will rise quickly, the insulation effect of the material layer and the phenomenon of boron reversion will disappear, and the purpose of the thick material layer melting process cannot be achieved. In addition, the addition must be evenly covered on the glass surface. If a stockpile is formed in a local place, the bottom of the stockpile will sink, and the hot glass liquid will envelop the cold raw material, and defects such as stones and air bubbles will be easily generated in the glass.
(b) To ensure the electric heat balance under normal conditions: When changing the discharge amount of the melting furnace, if the electrical parameters are not adjusted in time, the electric heat balance condition of the original working state is quickly broken, and the unstable state is transferred to the internal temperature distribution in the kiln. The trajectory of the curve and the glass will change. If the output power is increased and the input power is constant, the temperature of the glass surface drops and the hot spot sinks. In the kiln, "supercooling" occurs, forcing the glass surface to decrease, the material layer thickening, and a large amount of batch material sinking. Raw material is too late to melt. Conversely, when the discharge amount becomes small, the temperature of the glass surface rises. The hot spots moved upwards, overheating occurred in the kiln, the raw material melting speed was accelerated, the material layer became thinner, the liquid surface increased, and the refractories and the electrode accelerated erosion. Therefore, when changing the discharge amount, the input power and the discharge amount should be matched well, and the electric heat balance of the melting process conditions should be kept as far as possible.
(c) Establishing a reasonable melting temperature profile and stable glass flow: The difference in the temperature of the molten glass in the kiln is the cause of the convection of the molten glass. In the thick layer vertical and deep layer electrofusion process, if the temperature field is not reasonable, the surface Layers (half-fused layers) Unmelted glass is often degraded by a small amount of vertical convection into the working stream. Under normal circumstances, the small convection of the molten glass in the semi-fused layer floats the unmelted material and fixes the position of the semi-molten layer and the cold layer. When the semi-fused glass enters the heating zone in the highest temperature zone, the glass melting process is completed and enters into the glass homogenizing zone where strong vertical convection does not occur and all the glass passes through the same thermal process. , to melt out a high degree of uniformity of the glass. Therefore, the temperature distribution curve at the highest temperature zone should be flat, and no large temperature difference should occur. The temperature distribution curve of the electric furnace designed based on this principle helps to obtain a glass with good thermal uniformity.
(d) Adequate depth of molten glass: Melting borosilicate glass in an all-electric melting kiln, the melting rate is 2-3 times higher than in a kiln, so that the high melting rate requires the clarification of the glass in the vertical and deep direction. Homogenization must have a sufficient depth to meet the residence time of the glass in the kiln. The depth of the pool is related to factors such as the production capacity and melting rate of the melting furnace, and the liquid depth is generally more than 900 mm. For example, in a borosilicate glass 6m2 electric melting furnace, the depth of the pool is 1.38m. In a 15-20m2 VSM electric furnace, the depth of the pool is 2.2m.
3.3.1 Melting soda lime glass All electric melting kilns are mainly used in the following places:
(1) Located in the scenic area, the smoke generated from the burning of fossil fuels and the dust generated during the glass melting process seriously affect the landscape. The EPA requested the plant to be closed for rectification or to take the place where the smoke was removed.
(2) Types with particularly high glass quality requirements.
(3) Places where electricity prices are particularly low. Like some small hydroelectric plants nearby.
3.3.2 All-electric melting furnace for melting lead glass
(1) Fusion of lead glass A flame-heated tank kiln is used to melt lead glass. One of the major disadvantages of this process is the volatilization of lead oxide during the melting process. The amount of volatilization is 10 more than when melting in an electric furnace. %. For many years, lead crystal glass was melted in a single crucible kiln, a multiple crucible kiln, or a daily pond kiln. Since the 1960s, unit kiln and horseshoe flame tank kiln have been used, and the glass output and quality have been greatly improved. When melting glass with a crucible, it is difficult to eliminate refractory stones caused by clay crucibles. Richien Kiln due to fluctuations in the liquid level of the glass, scouring the refractories also causes severe refractory stones and streaks. Heat-exchange continuous melting of the tank kiln, the disadvantage is the large amount of volatilization of lead oxide in the melting process, the glass is easy to layer reduction; glass erosion of refractories is more serious, easily lead to streak and air pollution. This tank kiln is generally fueled with natural gas or city gas. If heavy oil is used, it will contaminate the glass.
The first electric heated kiln was built before 1959 and used SiC or MoSi2 heating elements (the former is installed horizontally and the latter is installed vertically). Its power consumption is quite high. Afterwards, it used electric-assisted heating in a daily pond kiln. However, the above-mentioned electrofusion method cannot overcome the major drawbacks of the severe volatilization of lead oxide, the serious pollution of refractory glass, and environmental pollution.
(2) Status Quo and Development Prospect of Lead Crystal Glass Electric Furnace Since lead crystal glass is characterized by volatility, delamination, and reduction, it is very sensitive to process parameters such as temperature, atmosphere, kiln pressure, and convection during the melting process of the kiln. . Through practice and comprehensive analysis of various fuels and kiln types, the countries of the United Kingdom, Sweden, the Czech Republic, and West Germany all agreed that all-electric furnaces are the most reasonable and relatively economical.
In recent decades, many countries have conducted research on all-electric furnaces and achieved major breakthroughs. There are already many electric melting furnaces in the world. Its superiority has been paid more and more attention.
The former Soviet Union and Eastern European countries have vigorously developed fused-crystal glass technology. The Soviet Union conducted industrial tests in the early 1970s and promoted it in 1974. In Kiev, Gush, Leningrad, Ukraine, Minsk and other art glass factories used to obtain good technical and economic benefits.
In the normal operation of the electric melting furnace, when the temperature in the vicinity of the electrode in the molten glass in the molten bath is 1370°C, the upper space of the melting bath is only about 120°C. This is due to the fact that there is a few centimeters thick layer of batch cold material on the surface of molten glass, which eliminates the volatilization of lead oxide (typically the volatilization of lead oxide in the flame melting tank kiln is as high as 10%) and makes the glass composition in the kiln uniform and stable. The cost of powder material is reduced, and environmental pollution is reduced. In short, although the electric power consumption of the electric melting furnace is large, a large amount of lead oxide is saved, and the electric melting furnace is therefore economical.
(3) Selection of internal electrode of all-electric melting furnace of lead crystal glass There are two types of electrodes used in all-electric furnaces of lead glass. One is tin oxide electrode, which is characterized by poor resistance to reduction and is suitable for oxidizing glass. The other is a molybdenum electrode, characterized by poor oxidation resistance, suitable for reducing glass. Tin oxide electrodes are very suitable for melting lead glass. In an electric melting furnace that melts lead glass, if molybdenum electrodes are used, they will be oxidized to varying degrees, and lead oxide in the glass will be reduced to produce metal lead sinks to the bottom of the melting tank. This design of the electric furnace must be daily Drain at the bottom of the pool.
(4) Current situation and development prospects of lead glass electric furnaces in China The lead glass electric furnaces in China are mainly used in the following areas:
Lead crystal glassware.
Optical glass.
Gem glass.
Lighting glass. The latter three must all be used in conjunction with Platinum.
(5) The kiln types used in China's lead glass electric melting furnaces The kiln types used in China's lead glass electric melting furnaces are:
(a) T-type kiln: The shape of the upper and lower kiln, the tin oxide electrode is vertically mounted on the steps on both sides, and single-phase power supply or three-phase SCOT power supply is used.
(b) Rectangular kilns: As with the upper and lower size, the tin oxide electrodes are horizontally mounted on the walls of both sides and are supplied with single-phase or three-phase SCOT.
(6) China's lead glass electric melting furnaces need to be improved. Some problems often occur in the use of lead glass electric furnaces in China:
(a) The water jacket is disconnected from the silver rod. Due to improper design or operation, all six silver rods are broken in the tin oxide electrode, resulting in no power supply.
(b) The head of the tin oxide electrode is broken in the furnace.
Designers who have experienced the above problems have not mastered some of the features of lead glass furnaces.
3.3.3 The borosilicate glass in all-electric melting kiln melting borosilicate glass In the traditional flame tank furnace melting process, the loss of boron's volatilization emission is often encountered. The volatilization of boron not only makes the glass liquid non-uniform, deteriorates the quality of the glass, but also loses the expensive boron raw material in vain. Therefore, foreign countries have conducted long-term research on reducing the volatilization of boron. To date, boron has a volatilization rate of 10 to 15%. If anhydrous borax or boric anhydride is used as a raw material, it can be reduced to 5 to 10%.
Since the emergence of a thick vertical deep-buried all-electric furnace, a new method has been brought to the melting of borosilicate glass, which has greatly reduced the volatilization of boron. With this method, the boron evaporation rate is reduced to 1 to 2%. The production of borosilicate glass in China basically uses a flame kiln. In a borosilicate glass system, the glass quality does not reach the desired level.
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