Discussion on the defects of float glass forming
[China Glass Net] The molten glass enters the tin bath through the runner and the launder, and is formed into the annealing kiln by the transition roller after being formed in the tin bath. In this process, the glass liquid (plate) is to be connected with the gate and the lip brick. Direct contact with tin liquid, edger, protective gas transition roller, etc., and closely related to tin bath water bag, top cover brick, bottom brick, etc., it is easy to form various defects related to forming, including cassiterite, tin point (top tin), optical distortion points (shedding), sticky tin, iridescent, fog, bubbles, etc., other than bubbles can be collectively referred to as tin defects, these forming defects seriously restrict the quality grade and processing properties of the glass. This article discusses its causes and preventive measures to help improve the quality of float glass.
1. Analysis of the causes of tin defects
1.1. Properties of tin compounds in tin and tin baths
Pure tin has a melting point of 232 ° C and a boiling point of 2271 ° C. Tin has a lower melting point and a higher boiling point, a lower saturated vapor pressure, and a higher density in the temperature range of 600 to 1050 ° C. It is easy to reduce, and has a large infiltration angle (175°) between tin liquid and glass liquid, and it is almost completely infiltrated. Tin is used as a good carrier for glass forming.
Tin oxide SnO2, density 6.7 ~ 7.0g / cm3, melting point 2000 ° C, the vapor pressure at high temperature is very small, insoluble in tin liquid, normal production in the temperature of the tin bath is solid, often in the form of scum in the low temperature On the surface of the zone, usually the scum is concentrated near the outlet end. If the oxidation is severe, the scum will extend for a long time and it is easy to form a scratch on the lower surface of the glass plate.
Stannous oxide SnO, melting point of 1040 ° C, boiling point of 1425 ° C, solid blue-black powder, can be dissolved in tin liquid, SnO molecules are generally in the form of its polymer (SnO) x. In a neutral atmosphere, SnO is stable only above 1040 ° C, and decomposition reaction occurs below 1040 ° C. SnO may be present in the reducing atmosphere of the tin bath, which is often dissolved in the tin bath and present in the atmosphere in the form of steam.
Stannous sulfide SnS, density 5.27g/cm3, solid blue crystal, melting point 865 ° C, boiling point 1280 ° C, with a large vapor pressure, 81.3Pa at 800 ° C, normal production, high temperature area volatile Entering the atmosphere, the low temperature zone is easy to condense and drip.
1.2. Sulfur and oxygen pollution cycle in tin bath
Oxygen pollution is mainly caused by traces of oxygen and water vapor in the atmosphere and oxygen that leaks in and diffuses from the tin bath gap. In the tin bath condition, they oxidize tin to SnO and SnO2 scum. SnO dissolves in the tin liquid and volatilizes into the atmosphere, and condenses and collects on the top cover and water bag to fall onto the glass surface. In addition, the glass itself is also a source of pollution. The oxygen in the glass enters the tin liquid, which also oxidizes the tin. Water vapor enters the atmosphere on the upper surface of the glass, increasing the oxidizing atmosphere in the atmosphere.
Sulfur pollution is mainly carried by glass when using nitrogen and hydrogen shielding gas. One is derived from the glass component and the furnace atmosphere, and the other is derived from the sulfur dioxide-treated glass lower surface technology at the outlet of the tin bath. In the tin bath condition, the upper surface of the glass is released into the atmosphere in the form of H2S. On the lower surface of the glass, sulfur enters the tin liquid and is oxidized to SnS. The H2S in the atmosphere reacts with tin to form SnS. These SnS are dissolved in tin and partially volatilized. Into the atmosphere, the SnS vapor also causes tin defects in the glass. This is a sulfur pollution cycle, as shown in Figure 2. The main chemical reactions are:
The chemical reactions related to oxygen and sulfur pollution maintain a dynamic balance in different temperature zones of the tin bath.
The balance state is closely related to the composition of the shielding gas and the working conditions of the tin bath. When the oxidation component is high, the reduction component is low and the oxidation reaction is intense; when the reduction component is high, the oxidation component is low, and the oxidation of tin can be avoided or reduced.
2. Discrimination and treatment of tin defects
2.1, cassiterite
The appearance of cassiterite is white or off-white, and is generally biased to the upper surface in the glass plate. The main component is SnO2. It tends to accumulate in the side of the flow channel, the front and back of the sluice, the surface of the bridge brick, etc., and gather to a certain extent or flow, temperature, airflow and other changes will fall on the glass surface to form cassiterite.
The number and period of formation of cassiterite are closely related to the working conditions of tin bath. The tin bath is seriously polluted, the seal near the runner is poor, and the probability of cassiterite is large. Therefore, in order to control the defects of the cassiterite, it is necessary to keep the tin tank and the flow channel sealed well, ensure the stable tank pressure and the pressure of the furnace, and ensure the stability of the pulling amount, especially the flow rate when the board is changed. Secondly, the flow path is periodically purged. Around the ram, the condensate near it is detached at one time. Generally, it is required to purge the flow path once with high pressure nitrogen for no more than three months in one to two months.
2.2, tin point (top tin), optical distortion point (shedding)
Tin dots (top tin) are silver-white or black dots that stick to the surface of the glass plate. According to the shape of the tin spot and the depth of the embedded glass plate, it can be judged whether it originates from the hot end or the cold end of the tin bath. If the tin point is round, the embedded glass is not deep and easy to peel off, the tin point is from the top of the cold end groove; if it is elliptical, the embedded glass is deep and not easy to peel off, that is, the force is removed to leave a deeper surface on the glass surface. In the pit, the tin point is derived from the top of the hot end slot.
The optical distortion point (shedding material) is a defect in which the surface of the glass plate is obviously deformed, but the core has little or no obvious core, and the color and composition of the exfoliated material can be determined from the hot end or the cold end of the tin bath. If the exfoliated material is white or grayish white, it seems to have a greasy feeling when wiping, and the composition contains Cl, Sn, Na and other elements, which are derived from the top of the hot end tank or the water in the front area; if the exfoliated material is black or brown, the core is slightly Obviously, the deformation is small, and the source is cold-end tank top or rear water pack.
The key to preventing such tin defects is to prevent oxygen and sulfur from entering the tin bath and reducing tin contamination. First of all, it is to strengthen the tin bath seal, the seal is used as the daily work of the forming section, every day, every shift, every time to do; regularly check the nitrogen seal of the inlet and outlet of the tin tank to ensure its blocking effect; In addition to the sealing of the mud, the movable edge seal should also be sealed with nitrogen gas to ensure the gas dosage; the amount of the shielding gas in each zone should be adjusted reasonably, and the groove pressure in the front and rear zones must be higher than the middle zone; Outside the seal, use the fixed edge seal as much as possible; the tin slot exit gate and the transition roller table are multi-layer sealing curtains. Secondly, it is necessary to stabilize the flow of the tin bath and keep the draft tube unobstructed. At the same time, one or two pairs of small chimneys are arranged at the inlet end of the tin tank, and the oxides and sulfides of tin are discharged along the air flow as much as possible, thereby reducing the chance of coagulation. Third, the tin bath cleaning and water bag cleaning should be carried out regularly. The tin bath purging is to purge the top of the tank with high-pressure nitrogen, including electric heating elements, especially the corners and the water bag directly above the water purging. Generally, one is required. It should be purged no more than three months in two months; the water bag is usually cleaned once in one to two weeks.
2.3, sticky tin, iridescent
The sticking tin is a silver-white metallic tin or an off-white tin ash adhered to the lower surface of the glass plate. When it is severe, it cannot be removed, or the glass is pitted after being removed. Sticky tin is a defect in the glass itself and can damage the surface of the transition roll, causing scratches on the glass. The pure tin liquid and the glass liquid are hardly wetted and do not stick to the glass. When the tin liquid contains oxygen, sulfur, magnesium, aluminum and other impurity elements, the surface tension of the tin liquid changes, and the sticking phenomenon occurs. . Rainbow refers to the interference color of the tin surface when the float glass is tempered or hot-bent, that is, the rainbow. The reason is mainly that a small amount of tin oxide and tin sulfide in the tin bath penetrate into the glass, and in the case of tempering or hot bending, the divalent tin and the tetravalent tin are converted to each other because the radius of the tetravalent tin ion is larger than that of the divalent tin ion. During the conversion process, tiny cracks are formed on the tin surface of the glass, and an interference rainbow is formed under illumination.
The primary measure for tacking tin and iridescence is still to strengthen the seal between the tin bath and the runner to prevent and reduce air entry into the tin bath. The sealing method is as described above. The second is to ensure that the tin liquid level in the triangle area at the exit of the tin bath is clean, and the linear motor is required to operate normally. At the same time, the liquid level in the triangle area and the ash accumulation along the mouth should be cleaned regularly, especially after the operation of the plate change, tinning and tin bath accident. It must be cleaned up in time. The third is to ensure the purity of the tin liquid. When the tin bath is well sealed, the newly added tin must meet the standard. The tin used for re-use after cold repair should be purified to avoid magnesium, aluminum, lead, antimony, oxygen, Sulfur and other pollution; in normal production, the tin liquid can be purified, and more trace elements such as sodium, potassium and iron, which are more active than tin, are added to preferentially react with impurities such as oxygen and sulfur to form scum and be manually removed. The fourth is to improve the purity of the shielding gas, reduce the O2, NH3, H2O and other gases into the tank, pollute the atmosphere, so that the gas dew point in the tank is normally below -50 °C, and the outlet end is below -30 °C.
2.4, fog point
The fog point makes the lower surface of the glass fog, and it seems to be a kind of misty thing with the naked eye. Sometimes it is mixed with visible bubbles. Under the microscope, it is a dense open bubble, which is dense and tiny makes the glass appear. Matte-like. The cause of the fog point is related to the dissolution, adsorption, and permeation of the gas in the tin liquid in the tank, and H2 and O2 have the characteristics of high temperature solubility and low temperature solubility. The oxygen content in the tin bath is high, tin is above 232 °C, and oxygen exists in the form of Sn3O4 in the tin solution. Due to the large convection and temperature fluctuation of the tin liquid, the tin liquid containing Sn3O4 in the low temperature region may enter the high temperature region. The reaction occurs, the oxygen is released by the heat, and the escape of oxygen destroys the lower surface of the glass, and a small open bubble can be formed. In addition, the hydrogen in the shielding gas is also dissolved in the tin liquid. When the temperature is lowered from 1000 ° C to 800 ° C, the hydrogen dissolved in the tin liquid will all escape, causing a fog point. Therefore, the top priority of avoiding the fog point is to strengthen the tin bath seal and improve the purity of the shielding gas; the second is to reasonably adjust the proportion of hydrogen in the protective gas in each zone of the tank, especially the ratio of H2 in the high temperature zone should not exceed 3%; It is necessary to ensure that the hydrogen diffusion index of the refractory material at the bottom of the tank meets the requirements, because the diffusion and penetration of the refractory material at the bottom of the tank will volatilize and escape to form a fog point when the equilibrium after the critical state is destroyed.
3, forming bubbles
The formed bubbles generally have obvious features on the glass plate, and are relatively fixed in the lateral position of the original plate, and are also easily recognized in the thickness direction of the original plate.
3.1, open bottom bubble
Under the normal production process conditions, open bubbles appear continuously on the lower surface of the glass plate. The bubble in the production line at the initial stage of production is sometimes not fixed in the lateral position of the original plate, sometimes with obvious laws; the production line continues to appear after a long period of production. The bottom open bubble is relatively fixed in the lateral position of the glass plate, and the position of the bubble changes relatively by adjusting the width of the plate and the position of the original plate in the tin bath. These features can be determined as the bottom bubble.
The main measures to prevent the bubble at the bottom of the tank are to accurately and reliably calculate the expansion joint of the bottom brick of the tank in the design and construction of the tin tank. The quality of the bottom brick should be strictly tested to ensure compliance with the standard. Proofreading and adjustment of the expansion joints, strictly in accordance with the requirements, the graphite sealing material of the bottom bolts should be tightly compacted, and the sealing and brick joints should be thoroughly cleaned. Second, in the tin bath baking process, according to the actual situation of the refractory material and tin bath installation at the bottom of the tank, adjust the heating curve of the tin bath baking, and take corresponding measures to maximize the volatile matter at the bottom of the tank. The third is to keep the tin working condition stable, especially to keep the temperature at the bottom of the tank stable. The fluctuation of the bottom temperature of each area should be less than 5 °C, and the higher temperature of the bottom of the tank should be lower than 120 °C. Check the bottom fan of the tank strictly according to the requirements of the process system. For the operation, pay attention to the temperature changes at various points at the bottom of the tank. The fourth is to reduce the convection of tin liquid in the high temperature zone and the low temperature zone of the tin bath, which can be achieved by adding a damper after the shrinking section and the edger and adding a graphite damper at an appropriate position.
3.2, lip brick bubbles
Lip brick bubble is another type of forming bubble. It is also located on the lower surface of the glass plate. Generally, it is a bubble band along the direction of glass pulling. There are large and small, some openings, some closed, and relatively fixed in the lateral position of the glass plate. The position of the bubble band generally does not change by adjusting the width of the plate and the position of the original plate in the tin bath. These features can be judged as lip brick bubbles, and in severe cases, the erosion of the corresponding position of the lip brick can be seen by opening the outer side seal of the tin-shaped stile. The treatment measures of the lip brick bubble are to reduce the pulling amount and reduce the temperature of the flow channel, which can reduce the hazard of the bubble but cannot be completely eradicated. To completely solve the problem that the lip brick must be replaced, this requires a preparation process, which may further affect the quality for several days. According to the author's experience, when the furnace runs to 70% to 80% of its life, even if there is no obvious lip brick bubble, it is better to have a planned replacement to ensure the stability of product quality.
3.3, impurity bubbles
The impurity bubbles discussed herein refer to larger bubbles located on the surface of the glass sheet having a diameter greater than 1 cm, which are relatively fixed in the lateral position of the glass sheet. The kiln heat repairs the refractory material in the kiln, the impurities brought in the cullet, the refractory minerals accumulated in the raw material converge at the glass surface of the ram and the side of the runner, the thermocouple at the runner, and the electric heating element are inserted. The glass liquid will form impurity bubbles. Dealing with such defects is to inspect the quality of the molten glass at the flow path in detail, hook out the debris here, check the condition of the thermocouple and the electric heating element here, and ensure the quality of the hot repair, ensure the use of raw materials that meet the quality requirements and glass.
4 Conclusion
Tin defects and formed bubbles are inevitable defects in the forming process of float glass. The technicians in the float glass industry can only take the corresponding formation mechanism and defect characteristics by accurate design, precise construction and careful operation. The measures to reduce their impact on quality can achieve satisfactory results.
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