Cause analysis of defects in 1 tin
Properties of 1. 1 tin and tin compounds in tin bath
Pure tin has a melting point of 232℃ and a boiling point of 227 1℃. In the temperature range of 600 ~ 65,438+0,050℃, tin has a low melting point, a high boiling point, a low saturated vapor pressure, a high density and easy reduction, and the wetting angle between tin liquid and glass liquid is large (65,438+0,75).
The density of SnO2 is 6.7 ~ 7.0g/cm3, the melting point is 2000℃, and the vapor pressure is very small at high temperature, so it is insoluble in tin liquid. In normal production, it is solid at the temperature of tin bath, and often appears on the liquid surface of low temperature area in the form of scum, usually gathering near the outlet end. If the oxidation is serious, the scum will extend for a long time, and it is easy to form scratches on the lower surface of the glass plate.
Stannous oxide SnO, melting point 1040℃ and boiling point 1425℃, is a blue-black powder, which can be dissolved in tin solution. The molecule of SnO generally exists in the form of its polymer (SnO) X. In neutral atmosphere, SnO is stable only above 1040℃, and will decompose below 1040℃. SnO can exist in the reducing atmosphere of tin bath, often dissolved in tin liquid and exists in the atmosphere in the form of water vapor.
SnS sulfide, density 5.27g/cm3, blue crystal, melting point 865℃, boiling point 1280℃. Its vapor pressure is very high, which is 8 1.3Pa at 800℃. During normal production, it is easy to volatilize into the atmosphere in high temperature area and condense and drip in low temperature area.
1.2 sulfur and oxygen pollution cycle in tin bath
Oxygen pollution mainly comes from trace oxygen and water vapor in the atmosphere and oxygen leaked and diffused from the gap of tin bath. Under the working conditions of tin bath, tin is oxidized into SnO and SnO2 scum, and SnO is dissolved in tin solution and volatilized into the atmosphere, condensed and aggregated at the top cover and water sac, and then falls on the glass surface. In addition, glass itself is also a source of pollution. When oxygen in glass enters tin liquid, it will also oxidize tin, and water vapor will enter the atmosphere on the upper surface of glass, increasing the oxidizing atmosphere in the atmosphere.
When nitrogen and hydrogen are used as shielding gas, sulfur pollution is mainly brought in by glass. One is from the glass composition and kiln atmosphere, and the other is from the technology of treating the lower surface of glass with sulfur dioxide at the outlet of tin bath. Under the working condition of tin bath, the upper surface of glass is released into the atmosphere in the form of H2S, and the sulfur on the lower surface of glass enters the tin solution and is oxidized into SnS. H2S in the atmosphere reacts with tin to generate SnS, which is dissolved in tin solution and partially volatilized into the atmosphere. SnS steam can also produce tin defects in glass. This is the pollution cycle of sulfur, as shown in Figure 2. The main chemical reactions are: (omitted)
The chemical reactions related to oxygen and sulfur pollution keep dynamic balance in different temperature regions of the tin bath.
The equilibrium state is closely related to the composition of shielding gas and the working conditions of tin bath. When the oxidation component is high, the reduction component is low and the oxidation reaction is fierce; The higher the reduction component, the lower the oxidation component, which can avoid or reduce the oxidation of tin.
Identification and treatment of two kinds of tin defects
2. 1 cassiterite
Cassiterite is white or grayish white in appearance, generally leaning towards the upper surface in glass plates, and its main component is SnO2. Often gathered in the runner side wall, before and after the gate, bridge brick surface, etc. When it gathers to a certain extent or changes in flow, temperature, airflow, etc. It will fall on the glass surface to form cassiterite.
The quantity and period of cassiterite formation are closely related to the working conditions of tin bath. The tin bath is seriously polluted, and the sealing near the flow channel is poor, so the probability of cassiterite is high. Therefore, in order to control the defects of cassiterite, we must first keep the tin bath and runner well sealed, ensure the stable bath pressure and furnace pressure, and ensure the stability of drawing quantity, especially the stability of flow when changing plates; Secondly, the runner and gate are periodically purged, so that the condensed water nearby falls off at one time. In general, every one to two months, up to three months, it is necessary to purge the runner with high-pressure nitrogen.
2.2 Tin spot (top tin) and light distortion spot (shedding)
Tin dots (top tin) are silvery white or black dots stuck on the upper surface of glass plate. According to the shape of the tin spot and the depth of the embedded glass plate, it can be judged whether it comes from the hot end or the cold end of the tin bath. If the tin spot is round, the embedded glass is easy to peel off if it is not deep, and the tin spot comes from the top of the cold end groove; If it is oval, the embedded glass is deep and difficult to peel off. Even if it is forcibly taken out, it will leave a deep pit on the glass surface, so the tin spot comes from the top of the hot-end groove.
The optical distortion point (shedding) is that the upper surface of the glass plate has obvious deformation, but the core is very small or there is no obvious core defect. From the color and composition of the falling objects, it can be judged whether it comes from the hot end or the cold end of the tin bath. If it is white or off-white, it seems to feel greasy when wiping, and the ingredients contain elements such as Cl, Sn and Na. , a water bag from the top or front area of the hot-end water tank; If the shedding is black or brown, the core is slightly obvious and the deformation is small, it comes from the water bag at the top of the cold end water tank or in the back area.
The key to prevent such tin defects is to prevent oxygen and sulfur from entering the tin bath and reduce tin liquid pollution. The first is to strengthen the sealing of tin bath, taking sealing as the daily work of molding section, which should be done every day, every shift and every hour; Regularly check the sealed nitrogen bag at the inlet and outlet of the tin bath to ensure its deterrent effect; In addition to mud sealing, observation holes and movable sealing edges should also be sealed with nitrogen to ensure gas consumption; It is also necessary to reasonably adjust the amount of tank protective gas in each district, and the tank pressure in the front and rear areas must be higher than that in the middle area; In addition to the movable sealing edge that must be used, try to use the fixed sealing edge; Multi-layer sealing curtain between tin bath outlet door and transition roller table. Secondly, to stabilize the airflow in the tin bath and keep the draft tube unobstructed, one or two pairs of small chimneys are set at the entrance end of the tin bath to exhaust tin oxides and sulfides along the airflow as much as possible to reduce the chance of caking. Third, the tin bath should be purged and cleaned regularly. Tin bath purging refers to purging the top of the tin bath with high-pressure nitrogen, including electric heating elements, especially in corners and directly above the water bath. Generally, it is required to purge once every one to two months, at most once every three months. Generally, the water bag is pumped once a week or two and cleaned once.
2.3 paste tin and rainbow colors
Sticking tin is silvery white metal tin or gray tin ash attached to the lower surface of glass plate, which can't be removed seriously, or has caused pits on glass after removal. Sticking tin is a defect of glass itself, which will also damage the surface of transition roller and cause glass scratch. Pure tin liquid hardly wets the glass liquid and will not stick to the glass. When the tin liquid contains impurity elements such as oxygen, sulfur, magnesium and aluminum, the surface tension of the tin liquid changes and tin sticking will occur. Rainbow refers to the interference color of light on the tin surface when float glass is tempered or hot bent. The main reason is that a small amount of tin oxide and tin sulfide in the tin bath infiltrated into the glass. In the process of tempering or hot bending, bivalent tin and tetravalent tin are transformed into each other. Because the radius of tetravalent tin ions is larger than that of bivalent tin ions, tiny cracks are generated on the tin surface of glass during the conversion process, and interference rainbows are formed under illumination.
The main measures to control tin sticking and rainbow color are still to strengthen the sealing of tin bath and runner to prevent and reduce air from entering tin bath. The sealing method is as described above. The second is to ensure that the tin surface in the triangle area at the outlet of the tin bath is clean, and the linear motor is required to operate normally here. At the same time, it is necessary to regularly clean up the liquid level in the triangle area and the dust accumulated along the mouth, especially after changing plates, adding tin and tin bath accidents. The third is to ensure the purity of tin liquid. When the tin bath is well sealed, the newly added tin must meet the standard, and the reused tin after cold patching should be purified to avoid the pollution of magnesium, aluminum, lead, bismuth, oxygen and sulfur. In normal production, tin liquid can be purified, and trace metal elements such as sodium, potassium and iron, which are more active than tin, can be added to react with impurities such as oxygen and sulfur to generate scum, which can be removed manually. Fourthly, improve the purity of protective gas, prevent O2, NH3, H2O and other gases from entering the tank to pollute the atmosphere, and make the gas dew point in the tank normally below -50℃ and the outlet below -30℃.
2.4 Fog point
Fog spots make the lower surface of the glass foggy, which is a foggy thing to the naked eye, sometimes mixed with visible bubbles; Observing under the microscope, it is a dense open bubble, which frosts the glass because of its density and fineness. The causes of fog spots are related to the dissolution, adsorption and permeation of gas in tin liquid in the tank. H2 and O2 have the characteristics of high solubility at high temperature and low solubility at low temperature. The oxygen content of tin bath is relatively high, and the tin is above 232℃. Oxygen exists in tin solution in the form of Sn3O4. Due to the large convection and temperature fluctuation of tin liquid, the tin liquid with high Sn3O4 content in the low temperature zone may enter the high temperature zone, react and decompose to release oxygen. The escape of oxygen will destroy the lower surface of glass and form open small bubbles. In addition, the hydrogen in the shielding gas will also dissolve in the tin liquid. When the temperature drops from 1000℃ to 800℃, all the hydrogen dissolved in the tin liquid will escape, resulting in fog spots. Therefore, the primary task of avoiding fog spots is still to strengthen the seal of tin bath and improve the purity of shielding gas; Secondly, the proportion of hydrogen in the protective gas in each area of the tank should be reasonably adjusted, especially the proportion of H2 in the high temperature area should not exceed 3%; The third is to ensure that the hydrogen diffusion index of the refractory at the bottom of the tank meets the requirements, because the diffusion and penetration of H2 by the refractory at the bottom of the tank will volatilize and escape when the balance is destroyed after reaching the critical state, forming fog spots.
3 bubble formation
Generally, the molded bubbles have obvious characteristics on the glass plate, which are relatively fixed in the transverse position of the original plate and easy to identify in the thickness direction of the original plate.
3. 1 tank bottom bubbling
Under normal production process conditions, open bubbles appear continuously on the lower surface of the glass plate, and the horizontal position of the bubbles in the original plate in the initial stage of production is sometimes not fixed, and sometimes it has obvious regularity; The production line has been put into production for a long time, and there are bubbles at the bottom of the plate, which are relatively fixed in the transverse position of the glass plate. After adjusting the width of the board and the position of the original board in the tin bath, the position of the bubble will change relatively. These characteristics can be judged as bottom foam.
In the design and construction of tin bath, it is one of the main measures to prevent bubbles at the bottom of the bath to accurately and reliably calculate the reserved expansion joints of the bricks at the bottom of the bath. The quality of tank bottom brick must pass strict inspection to ensure that it meets the standard. During the construction, the expansion joints should be proofread and adjusted, and the construction should be carried out in strict accordance with the requirements. The graphite sealing material for the bottom bolts should be dense, and the sealing and brick joints should be thoroughly cleaned. Second, the baking temperature curve of tin bath should be adjusted according to the actual situation of refractory materials at the bottom of the bath and the installation of tin bath, and corresponding measures should be taken to make the volatile substances at the bottom of the bath volatilize as completely as possible. The third is to maintain the stability of tin working conditions, especially the tank bottom temperature. The fluctuation of tank bottom temperature in each area should be less than 5℃, and the highest tank bottom temperature should be less than 120℃. Check the operation of the fan at the bottom of the tank in strict accordance with the requirements of the process system, and pay attention to the temperature changes at various points at the bottom of the tank. Fourthly, the convection of tin liquid between the high temperature zone and the low temperature zone of the tin bath can be reduced by adding baffles behind the shrinking section and the edge rolling machine and adding graphite weirs at appropriate positions.
3.2 lip brick bubble
Lip brick bubble is another kind of forming bubble, which is also located on the lower surface of the glass plate. Generally, it is a bubble belt along the direction of glass drawing, which is large and small, some are open and some are closed, and the lateral position of the glass plate is relatively fixed. After adjusting the width of the plate and the position of the original plate in the tin bath, the position of the bubble band generally does not change. These characteristics can be judged as lip brick bubbles, and when it is serious, the corrosion of the corresponding position of the lip brick can be seen by tearing the outer edge seal of the tin bath splayed brick. The measures to deal with the bubbles in lip brick are to reduce the drawing amount and the temperature of the runner, which can reduce the harm of bubbles but can not be completely eradicated. To solve the problem completely, the lip brick must be replaced, which requires a preparation process and may further affect the quality for several days. According to the author's experience, when the furnace runs to 70% ~ 80% of its service life, even if there is no obvious lip brick bubble, it is best to replace it in a planned way to ensure the stability of product quality.
3.3 impurity bubbles
The impurity bubbles discussed here refer to larger bubbles located on the upper surface of the glass plate with a diameter greater than 1cm, which are relatively fixed in the lateral position of the glass plate. Impurity bubbles will be formed by refractory materials gathered at the gate and the glass surface on the side wall of the runner, impurities brought by broken glass, refractory minerals gathered in raw materials, and thermocouples and electric heating elements inserted into the molten glass at the runner. To deal with this defect, it is necessary to check the quality of molten glass in the runner in detail, tick off the impurities here, check the condition of thermocouple and electric heating element here, ensure the quality of hot repair, and use raw materials and broken glass that meet the quality requirements.
4 conclusion
Tin defects and bubbles are inevitable defects in the forming process of float glass. Only through precise design, precise construction and careful operation, combined with the formation mechanism and characteristics of molding defects, can technicians in the float glass industry take corresponding measures to reduce their impact on quality, and then satisfactory results can be achieved.