Analysis and Countermeasures of Refractory Bubbles in Ultra-White Glass Production
Abstract: During the production of ultra-clear glass in a float glass production line, a large number of bubble defects appeared. There was no obvious pattern in the distribution of the strips, and the thickness was located in the middle and lower part of the glass plate. After detection and analysis, the bubble components mainly include nitrogen (N2), carbon dioxide (CO2), and argon (Ar). It is determined that the bubble defect comes from the corrosion of refractory materials at the bottom of the furnace clarification part. By taking measures to reduce the temperature at the interface between the refractory material at the bottom of the furnace clarification section and the molten glass and the fluidity of the molten glass, the purpose of solving the bubble defect was achieved. After the glass water was discharged into the furnace, the erosion of the refractory material at the bottom of the clarification section of the furnace was checked, which confirmed the determination of the source of the bubble defect.
0 Preface
Bubbles are a common defect in glass production. Ultra-white glass is more likely to have bubbles in the clarified area than ordinary glass. When producing ultra-clear float glass, the main problem is the difficulty in clarifying the glass liquid. Because the iron content in ultra-white glass is low and the thermal conductivity is high, which is 34 times that of ordinary glass, ultra-white float glass has good heat permeability, high temperature and low viscosity of the glass liquid, strong convection intensity in the horizontal direction, and forming circulation in The residence time in the clarification zone is short, so that the remaining bubbles in the glass liquid have no time to be discharged. Due to the low iron content, the vertical temperature gradient in the entire depth of the pool is significantly smaller than that of ordinary float glass. The temperature at the bottom 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 liquid is relatively small, and the convection is reduced, causing bubbles. Drainage is more difficult than ordinary float glass. On the other hand, the temperature of the refluxing molten glass below the forming circulation continues to rise during the advancement process, so that the microbubbles that have been absorbed by the molten glass are released into the molten glass again under the action of thermochemistry. At the same time, the viscosity of low-iron glass liquid is low, and microbubbles can easily rise into the surface flow, causing the bubbles in the glass liquid in the forming flow to rise significantly. Because ultra-white glass has good heat permeability, the temperature at the bottom of the pool is high, and the horizontal convection intensity is strong. It seriously corrodes the refractory materials on the bottom and wall of the pool, and it is easy to form refractory bubbles.
1. Bubble problems in ultra-white glass production
A certain float glass production line has good melting quality and stable production process during the production of ordinary white glass. After switching from plain white glass to ultra-white glass for a period of time, the quality of the glass strips gradually declined, bubble defects gradually increased, and the yield was greatly affected. The bubbles are randomly distributed throughout the glass plate, and there is no obvious pattern in the longitudinal distribution. The distribution of defects on the glass plate is shown in Figure 1. The defects represented by various symbols shown in the figure are all bubble defects after sampling verification.
2. Bubble defect detection and source analysis
After randomly selecting defective samples for inspection, it was found that the bubbles were located in the middle and lower parts of the thickness direction of the glass plate, the diameter of the bubbles was distributed between 0.5-2.0mm, and there were inconspicuous sediments in the bubbles.
The bubbles mainly contain N2, CO2, Ar, etc. The volume proportion of N2 is 79%-83%, the volume proportion of CO2 is 15%-20%, and the volume proportion of Ar is 0.7%. It can be judged that the generation of bubbles is related to the erosion of the refractory material and the subsequent release of residual air from the pores of the refractory material.
There is a high content of N2 (79%-83%) in the bubbles of the defective sample, and the N2/Ar ratio is consistent with that of ambient air, indicating that the bubbles are air-related. The pores of refractory materials usually contain air. When refractory materials are melted under reducing conditions, the 02 in the pores will be converted into CO2 or a mixture of CO2 and CO. Once the pores are opened, the O2 in the pores comes into contact with the glass melt and is preferentially absorbed by the reducing components in the glass melt. The bubbles will contain high concentrations of N2 (perhaps even as high as 100%), 0-20% by volume of CO2, and approximately 1% by volume of Ar.
The bubble diameter is 0.5∽2.0mm, the CO2 content is low (15% to 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 , In the medium-high temperature or medium-temperature area of the furnace, localized refractory material erosion is most likely to occur between the end of the clarification area and the neck. It is also possible that the molten glass penetrates the gaps in the refractory material and contacts the underlying refractory material that is more likely to generate bubbles, causing large bubbles. Temperature fluctuations in the furnace, or changes in technical conditions such as changes in glass flow, may produce such bubbles. The characteristic of ultra-white glass is strong convection in the horizontal direction. The fast-flowing glass flow forms a strong backflow after being blocked by the neck, which seriously washes away the refractory materials.
Based on the bubble analysis results and the actual operation of the furnace, it is determined that the location of the bubbles is in the clarification area of the furnace. Bubbles are generated due to erosion of the refractory material at the bottom of the furnace. The bubbles from the bottom pass through the middle and lower layers of glass liquid and rise to In the formed glass flow, the temperature of the glass liquid in this area is already relatively low, and 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 and small furnace
On the premise of ensuring the melting quality, the temperature of the glass liquid in the clarification zone should be appropriately reduced. By reducing the fuel consumption of the final pair of small furnaces to reduce the temperature of the molten glass downstream of the furnace, the purpose of lowering the temperature of the molten glass at the bottom of the clarification zone of the melting furnace and reducing the fluidity of the molten glass is achieved, and the damage of the molten glass to the refractory materials at the bottom of the clarification zone is slowed down. erosion.
3.2 Reduce the thickness of the insulation layer at the bottom of the furnace clarification area
The original calcium silicate board insulation layer at the bottom of the clarification area was removed, and ventilation measures were used to assist in cooling. The surface temperature of the inner refractory material was reduced from 200°C to 50°C to achieve the purpose of reducing the temperature of the silicone refractory material at the contact point with the glass liquid. Reduce the interface temperature, reduce the fluidity of the glass liquid, and slow down the erosion of the refractory material at the bottom of the clarification area by the glass liquid.
3.3 Replace the stuck neck water bag
The originally used clamp-neck water bag with a pressing depth of 340mm was changed to a clamp-neck water bag with a pressing depth of 280mm, which can reduce the backflow 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 area.
3.4 Adjustment results and verification
After a series of adjustments, the purpose of lowering the bottom temperature of the furnace clarification zone and reducing the fluidity of the glass liquid was achieved. The bubble defects in the glass liquid are gradually reduced, and the quality of the glass strip gradually returns to normal levels, and can remain stable for a long time.
When the production line was discharged with glass water for cold repair after the end of the kiln period, an inspection of the inside of the kiln revealed that the glass liquid at the end of the clarification area had caused serious erosion of the refractory materials at the bottom, and the AZS paving bricks at the bottom of the pool were partially eroded, and the glass The liquid has come into contact with the large clay bricks at the bottom of the pool. If no adjustment measures are taken, large-scale bubble defects will occur, affecting the quality of the glass plate, which confirms the analysis and judgment made at the time when bubbles appeared.
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