Bubbles are a common defect in glass production. Compared with ordinary glass, ultra-white glass is more prone to bubbles in the clarification area. When producing ultra-white float glass, the main problem that exists is the difficulty of clarifying the glass. Because ultra-white glass has low iron content and high thermal conductivity, which is 3 to 4 times that of ordinary glass, the ultra-white float glass has good heat permeability, high temperature of glass liquid, low viscosity, high convective strength in the horizontal direction, and forming The circulation stays in the clarification zone for a short time, making it too late for the remaining bubbles in the glass to be discharged. Due to the low iron content, the vertical temperature gradient in the depth of the entire pool is obviously smaller than that of ordinary float glass. The bottom temperature of the pool is about 6% higher than that of ordinary float glass. The temperature difference between the upper and lower sides of the glass is relatively small, and the convection is reduced, causing bubbles. It is more difficult to discharge than ordinary float glass. On the other hand, the temperature of the refluxing molten glass below the forming loop continues to rise as it progresses, causing the microbubbles that have been absorbed by the molten glass to be released into the molten glass again under the action of thermochemistry. At the same time, the viscosity of the low-iron glass liquid is low, and the microbubbles can easily rise to the surface flow, which causes the bubbles in the forming flow glass liquid to rise significantly. Because the ultra-white glass has good heat permeability, high temperature at the bottom of the pool, and high convection intensity in the horizontal direction, it will seriously corrode the bottom and wall of the refractory, and it is also easy to form refractory bubbles.
1 Bubble problem in the production of ultra-clear glass
In a float glass production line, during the production of ordinary white glass, the melting quality is good and the production process is stable. After changing from ordinary white glass to ultra-white glass for a period of time, the quality of the glass strip gradually decreases, bubble defects gradually increase, and the yield is greatly affected. The bubbles are randomly distributed throughout the glass strip, and there is no obvious regularity in the longitudinal distribution.
2 Bubble defect detection and source analysis
After randomly selecting defective samples for inspection, it is found that the bubbles are in the middle and lower part in the thickness direction of the glass plate, the diameter of the bubbles is distributed in the range of 0.5 to 2.0 mm, and there are inconspicuous deposits in the bubbles. Use a mass spectrometer to analyze the gas composition of the bubble sample, and the bubble mainly contains N?, CO?, and Ar. The volume ratio of N? is 79%～83%, the volume ratio of CO? is 15%～20%, and the volume ratio of Ar is 0.7%. It can be judged that the generation of air bubbles is related to the residual air released from the pores of the refractory after the corrosion of the refractory. The high content (79% to 83%) of N? in the bubbles of the defective sample, and the ratio of N?/Ar is consistent with the ambient air, indicating that it is air-related bubbles. Refractory materials usually contain air in the pores. When refractory materials melt under reducing conditions, the O? in the pores will be converted into CO? or a mixture of CO? and CO. Once the pores are opened, the O? in the pores contacts the glass melt and is preferentially absorbed by the reducing components in the glass melt. The bubbles will contain high concentrations of N? (may be as high as 100%), 0-20% volumetric CO?, and approximately 1% volumetric Ar. The bubble diameter is 0.5～2.0mm, the CO? content is low (15%～20%), and the bubbles are located in the middle and lower part of the thickness of the glass plate, indicating that the bubbles stay in the furnace for a short time, and it can be judged that the bubble generation location may be downstream of the hot spot area , The medium-high or medium-temperature area of ??the melting furnace is most likely to be between the end of the clarification zone and the collar, where local refractory corrosion has occurred. It is also possible that the glass liquid penetrates the gap of the refractory material and contacts the lower
refractory material that is more likely to generate bubbles, resulting in a large number of bubbles. Changes in technical conditions such as temperature fluctuations in the furnace or changes in the flow of glass, etc., may produce such bubbles. Ultra-white glass is characterized by strong convection in the horizontal direction. The glass flow with faster flow rate will form a strong backflow after the collar meets resistance. The refractory material will be washed seriously. According to the results of bubble analysis and the actual operation of the furnace, determine the generation of bubbles It is located in the clarification zone of the melting furnace. The refractory material at the bottom of the melting furnace is corroded to produce bubbles. The bubbles from the bottom pass through the middle and lower layers of molten glass and rise to the forming glass flow. In this area, the temperature of the molten glass is already relatively low. , The bubbles cannot be discharged or absorbed by the glass liquid, and will remain in the glass liquid.
3 Furnace process adjustment
3.1 Adjust the fuel consumption of the furnace end to the small furnace
Under the premise of ensuring the quality of melting, appropriately lower the temperature of the glass in the clarification zone. Reduce the temperature of the molten glass downstream of the furnace by reducing the fuel consumption of the final small furnace, so as to reduce the temperature of the molten glass at the bottom of the clarification zone of the melting furnace and reduce the fluidity of the molten glass, and slow down the effect of the molten glass on the refractory materials at the bottom of the clarification zone. Erosion.
3.2 Reduce the thickness of the insulation layer at the bottom of the furnace clarification zone
Remove the originally used silica-calcium board insulation layer at the bottom of the clarification zone, and assist ventilation measures to cool down. The surface temperature of the inner refractory material is reduced from 200℃ to 50℃ to achieve the purpose of reducing the temperature of the refractory material at the contact position of the glass liquid. The interface temperature reduces the fluidity of the molten glass and slows the erosion of the molten glass to the refractory at the bottom of the clarification zone.
3.3 Replace the card neck water bag
The original card neck water bag has a pressing depth of 340mm, and it is changed to a 280mm card neck water bag, which can reduce the reflux of the molten glass in the furnace, reduce the temperature and flow rate at the interface between the molten glass and the refractory material, and slow down the molten glass Erosion of refractory materials at the bottom of the clarification zone.
3.4 Adjustment results and verification
After a series of adjustments, the purpose of lowering the bottom temperature of the clarification zone of the melting furnace and reducing the fluidity of the glass liquid was realized. The bubble defects in the molten glass are gradually reduced, and the quality of the glass strip gradually returns to the normal level, and can remain stable for a long time. After the kiln period of the production line was finished, when the glass water was put in for cold repair, the inside of the furnace was inspected and found that the glass at the end of the clarification zone had caused serious erosion to the refractory at the bottom. The AZS paving tiles at the bottom of the pool were partially corroded and the glass Has touched the clay bricks at the bottom of the pool. If no adjustment measures are taken, a large number of bubble defects will be generated, which will affect the quality of the glass surface, which confirms the analysis and judgment made at the time when the bubbles appeared.
4 Conclusion
Due to the decrease in iron content of ultra-white glass, compared with ordinary white glass, the thermal conductivity and viscosity during the melting process will be very different. During the production process, the temperature of the bottom of the pool will increase and the fluidity of the glass will increase significantly. , The erosion of refractory materials is intensified, and bubbles are prone to corrosion of refractory materials. In the design of the ultra-white glass furnace, it is necessary to consider the parts with strong liquid glass flow and severe erosion, optimize the design size, use refractory materials that are more resistant to erosion and erosion, and consider reducing the insulation effect of these parts. Reduce the corrosion of refractory materials during operation and avoid the generation of bubbles. In the melting process, in order to achieve energy saving in the furnace, it is usually necessary to increase the pressing depth of the neck water bag and strengthen the heat preservation of the furnace. However, these measures will affect the reflux and convection of the molten glass, causing the local temperature of the furnace to rise and increase the melting Local erosion of kiln refractories. In the production process of ultra-clear glass, it is necessary to comprehensively consider the balance of furnace energy saving, furnace protection and long-term melting quality stability, in order to achieve the long-term high-quality production goal.
