Problems Need to be solved in production of ultra-white float glass
Ultra-white float glass is an ultra-transparent low-iron glass. It is a new type of high quality, versatile glass. The content of Fe2O3 is less than 150×10-6, and the transmittance (3 mm glass) can reach more than 91.5%. Compared with ordinary float glass, due to low iron, the glass has good thermal conductivity, the convection strength of the glass in the horizontal direction is large, and the temperature gradient and viscosity gradient in the vertical direction are small, so the production process is more common than the ordinary float glass. Float glass is much more difficult. Our company produces omni-oxygen combustion ultra-white glass. Although oxy-fuel combustion has the advantages of good melting quality, energy saving and emission reduction, the content of water vapor in the oxy-combustion process is high, and the glass liquid is difficult to clarify; the ultra-white glass is susceptible to mildew. In order to solve these problems, according to the characteristics of the ultra-white float glass and the characteristics of the oxy-fuel combustion process, its control method should be explored.
1 Raw material control problems
The main feature of ultra-white glass is that Fe2O3 is low, below 0.015% (150×10-6), so it has strict requirements on the types of raw materials, chemical composition, particle composition, moisture content, weighing accuracy, etc. On the strict control of the introduction of mechanical iron.
2 Clarify quality control
2.1 Clarifying the Cause of Difficulties
When producing ultra-white float glass, the main problem is the clarification of the glass. Ultra-white glass has low iron content and high thermal conductivity, which is 3 to 4 times that of ordinary glass. This brings great difficulties to the melting of ultra-white float glass. The glass liquid is difficult to clarify and the bubbles are difficult to discharge: 1 Because of low iron content As a result, ultra-white float glass has good thermal conductivity, high glass temperature, low viscosity, large convection intensity in the horizontal direction, and short residence time of annular flow in the refining zone. 2 As the iron content is low, the vertical temperature gradient in the entire depth direction of the pool is obviously smaller than that of ordinary float glass. The bottom temperature 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. Discharge is more difficult than ordinary float glass. The temperature of the reflowed glass under the annular flow continues to increase during the advancement of the flow, so that the microbubbles that have been absorbed by the glass liquid are released to the glass liquid again under the action of thermochemistry. At the same time, the viscosity of the low-iron glass liquid is relatively low. Low, microbubbles can easily rise into the surface stream, causing bubbles to rise significantly in the glass. 4 In the oxy-fuel furnace where natural gas is combusted, the content of water vapor is relatively large, accounting for 1/3 to 2/3 of the space gas composition of the flame, and the content is highest at the hot spot of the glass surface, reaching 1/2. With increased water content, the glass is difficult to clarify and there are more microbubbles.
(1) Adjust the heat load
Reduce the heat load of the melting circuit. Take 6 pairs of small furnaces as an example, reduce the heat load of 1# to 3# small furnaces, increase the heat load of 4# to 5# small furnaces, and slightly increase the number of 6# small furnaces. White float glass requirements. The ultra-white float glass has good thermal conductivity, which makes the compounding material easier to melt. Therefore, the heat load in the melting circuit is reduced. And it is difficult to clarify, especially the characteristics of micro-bubbles are not easy to absorb, so that the heat load of the clarification circuit increases accordingly, but the increase of the end-to-hearth cannot be too high, or the micro-bubbles that have been absorbed by the glass under the forming flow can easily rise to the surface. Secondary bubbles are formed in the flow.
(2) Using supplementary measures
Bottom bubble bubbling technology, addition of compound clarifiers in batches, addition of defoamers, etc., as well as other advanced melt fining techniques, can be used to reduce the number of bubbles in the glass liquid. Our company adopts bubbling technology at the bottom of the melting section and uses defoaming technology to inject the antifoaming liquid into the kiln, which acts on the glass foam layer. Because the antifoaming liquid has the function of suppression and breaking foam, the board surface bubbles Significantly reduced, the quality has improved significantly.
3 Glass mold problem
Because of the composition and thermal history of ultra-white glass, it is more prone to mildew than ordinary float glass.
3.1 The mechanism of easy mildew in ultra-white float glass
(1) Effect of high alkali
Glass moldy is the result of the exchange of alkali metal ion R+ with external ions. The more R+ content, the more serious the moldy tendency. The content of alkali metal oxides of ultra-white float glass is as high as 15%, and that of ordinary float glass is about 14%. Therefore, ultra-white float glass is more prone to mildew than ordinary float glass.
(2) The influence of ultra-white glass divalent metal oxides
Ultra-white glass RO reduced the divalent metal oxides, weakening the suppression of the alkali metal ion R+, resulting in easy mildew. According to the theory of glass pressing effect, when a part of SiO in the ROO-SiO binary glass is replaced with the bivalent metal oxide RO, the diffusion coefficient of the alkali metal ion R+ becomes small because the bivalent metal ion R2+ is filled in the glass network structure. In the voids, the diffusion movement of the alkali metal ion R+ is blocked, and the monovalent alkali metal ion R+ is suppressed. The sum of divalent metal oxides CaO and MgO in ordinary float glass accounts for l2% to 15% of the content of glass components, while in ultra-white float glass compositions, the sum of CaO and MgO accounts for only about 11%. Reduced metal oxides. In addition, the low iron content of ultra-clear glass is also likely to cause mildew because about 30% of the total iron content exists in the divalent oxide state. The weakening of all these divalent metal oxides suppresses the high activity of alkali metal ions, and the ultra-white glass is naturally more prone to mold.
(3) Effect of Thermal History Effect
The dense glass structure has a strong retarding effect when weathered erosion is encountered, whereas the loose glass structure is easily weathered and eroded. The formation of a dense glass structure depends on the thermal history of the glass and is mainly achieved by the slow cooling of the forming process and the precise annealing process. Due to the current domestic design of the annealing furnace for float glass production line, which is mainly for ordinary float glass, the annealing is not accurate when producing ultra-white glass. This structure of ultra-white glass is not dense enough, leaving hidden dangers. Table 3 shows the cold stress values of the ultra-white and normal float glass with the same thickness in the same production line.
Table 3 Cold stress values of ultra-white and normal float glass
As can be seen from Table 3, the ultra-white glass has a cold state stress value that is 18% higher than ordinary float glass. Ultra-white glass has high radiation transmission, and its body absorbs less heat. In the annealing zone at the front of the annealing furnace, it cannot absorb the same amount of heat as ordinary float glass. The glass body is relatively cold, so the annealing cooling process is shortened, resulting in more residual stress in the internal structure. Affects the density of the glass structure.
3.2 Ultra-white glass anti-mildew measures
(1) The use of CaO and MgO is not reduced as much as possible in formula design to obtain a good suppressing effect of divalent metal oxides. However, the design of the formula should be considered in an integrated manner so as to prevent one from losing sight of one another.
(2) Adjust the annealing system properly to obtain good annealing performance
(3) Choose mildew-proof powder suitable for ultra-white glass. For the existing ultra-white glass, the composition and structural characteristics cannot be changed, and the effectiveness of the anti-mildew material becomes the key to reducing moldy risk. Because the ultra-white glass is easy to mold, simply using ordinary float glass mold powder on ultra-white glass is obviously not enough, must use anti-mold powder with higher anti-mold efficiency. Nanjing Greil Company specially researched the adaptability of acid powder and glass and the surface morphology characteristics of ultra-white glass. Based on this, it developed a special anti-mold isolation powder for ultra-white glass. After more than 2 years of customer application, this product has been widely recognized by the market.
(4) vacuum packaging. Similar to the principle of food vacuum packaging, it is effective even if it is impossible to achieve such a high degree of vacuum. This method uses a thicker plastic film or aluminum foil to wrap the glass during packaging, and the inner air is drawn to seal the mouth. In order to prevent the impact of a small amount of gas on the glass, a certain amount of desiccant is placed in the bag. Practice at home and abroad shows that this method has a significant effect on the anti-mold effect of glass. A domestic company adopts this method, and its ultra-white glass is basically free of mildew after being placed for one year.
Fully understand the importance of glass raw material control, strengthen raw material control, prevent secondary pollution, ensure the stability of iron content is the key to the production of ultra-white float glass; strengthen melt clarification control, reduce micro-bubbles in the glass, is to improve the quality of ultra-white glass The effective way to understand the ultra-white glass mold mechanism, to grasp its characteristics, and accordingly take targeted anti-mildew measures, can minimize the ultra-white glass mold risk, create more benefits for the enterprise.