This article describes the formation and properties of streaks, analyzes the changes in the refractive index and expansion coefficient of streaks and the stress generated. Points out the reasons why streaks are generated in various areas of glass melting furnaces. Analyzes the reasons for the generation of streaks in domestic melting furnaces and proposes improvement measures.
Harm and source of streaks
Streaks are heterogeneous glass inclusions in the main body of glass. It is a common defect of glass inhomogeneity. It is different from the main glass in chemical composition and physical properties. Streaks can be understood as the mutual diffusion of various components, mutual penetration and fusion of organizational structures between several mixed glass liquids through chemical reactions, physical changes, and physical and chemical reactions. Before reaching complete uniformity, it is cooled to form glass with uneven structural organization inside. It has a different chemical composition, structural state and can produce different physical and chemical properties from the main glass.
From the appearance of the product, the streaks are distributed in different parts of the product, some inside the glass products, and some on the surface of the glass products. The interface between the streaks and the main glass presents different shapes. Taking bottle glass as an example, most of the streaks are curved strips, twisted lines and fibers. Most of the stripes start from the bottle neck, passing through the bottle shoulder, bottle body and all the way to the bottom of the bottle. Obvious stripes are relatively thick and wide, with a width of more than 10mm; less obvious stripes are relatively thin, like thin lines, with a width of less than 0.5mm. More obvious stripes protrude from the surface of the glass and can be felt by hand. Less obvious stripes are vaguely visible to the naked eye under the irradiation of light, and some are even invisible to the naked eye and can only be observed with the help of optical instruments. Stripes are called differently by many manufacturers, some are called "lines", some are called "screw lines", and some are called "cat paw prints", etc. Stripes are a common uneven defect in glass products.
From the perspective of the harmfulness of stripes, after the appearance of stripes, the appearance of stripes first affects the beauty of the product and seriously affects the appearance quality of the product. It is a relatively obvious appearance defect, which greatly affects the qualified rate of the product. According to the different standards for stripe defects required by different products, the degree of influence on the qualified rate of the product is also different. For some high-end daily glass products, when stripes appear, they almost 100% affect the qualified rate of the product.
When some stripes appear, characteristic stresses of different sizes are generated inside the product, usually called structural stress. This stress cannot be eliminated during product annealing and can cause the product to break by itself. The appearance of this stress has a great impact on the physical and chemical properties of the product, such as the mechanical strength, thermal stability and chemical stability of the glass. For some products with relatively high requirements, the reduction of these physical and chemical properties will greatly reduce the qualified rate of the product, such as beer bottles with relatively high requirements for compressive strength and impact strength.
The structural stress generated in the glass is related to the composition of the stripes. When the composition of the stripes differs more from that of the main glass, the thermal expansion coefficient differs more, and the characteristic stress generated is greater.
Areas where streaks are generated in glass melting furnaces
Through the analysis of the melting process of glass container production, it is not difficult to find that the process of streaks should be generated in two areas. One area is the area in the melting pool before the glass liquid passes through the flow hole of the glass melting furnace. This area is called the melting area; the other area is the area where the glass liquid has passed through the flow hole and entered the working pool (or the distribution channel) and the area in the supply channel. This area is called the cooling area.
01Reasons for streaks in the melting area
(1) Streaks caused by poor melting of glass liquid;
(2) Streaks caused by the kiln crown dripping into the glass melting pool without sufficient homogenization;
(3) Streaks caused by the erosion of the refractory material and the dissolution of some refractory components into the glass;
(4) Streaks caused by the melting of various stones into the glass liquid.
Analysis of the melting process of glass in the melting area shows that the first step of the glass melting process is to put the batch materials composed of quartz sand, soda ash, limestone, broken glass, etc. into the glass melting furnace. Under the action of high temperature, some raw materials immediately decompose to produce gas, and various solid-phase reactions are carried out at the same time to form silicate bodies. Subsequently, part of the silicate body melts and a liquid phase appears, which is then transformed into a melt with different components and viscosities. This is the glass liquid in the initial stage.
The glass liquid produced in the initial melting stage is rich in melts with different components and viscosities. The melt is mixed with unmelted residual quartz sand particles and a large number of bubbles. The melts with different components and viscosities are not easy to merge with each other, and their boundary layers are relatively stable. Some of the glass bodies rich in silica or alumina have great viscosity and surface tension, which are mixed in the entire glass melt. If the glass body is originally spherical, it will remain unchanged in the absence of other external forces, and a region rich in silica or alumina will be formed in the entire glass melt. This is the initial area of the stripes.
Through the above analysis of the glass melting process, it is not difficult to find the reasons for the streaks in the melting area.
(1) In terms of batch materials, the quality of batch materials is the prerequisite for ensuring the smooth elimination of streaks in the glass melting stage. When the following factors occur, streaks will be generated or even more serious. First, the uneven mixing of batch materials, the stratification of batch materials during transportation, the excessive particle size of some raw materials in the batch materials, the unstable and large changes in the moisture content of raw materials such as quartz sand, the unreasonable feeding method or the low moisture content of the batch materials cause some components of raw materials to fly. Secondly, the proportion of broken glass in the batch materials is relatively large, the difference between the selected broken glass and the main glass components is large, and the uniformity of mixing between broken glass and powder is poor. In addition, when the batch material weighing system fails, serious streaks will be generated.
(2) In terms of melting process, through the analysis of the melting process, it is not difficult to see the importance of various process parameters of batch materials in the melting process. Correct and reasonable process parameters must be formulated and strictly controlled. First of all, the melting temperature, the pressure of the melting furnace during the melting period, the length of the flame during the combustion, and the atmosphere of flame oxidation and reduction will all affect the viscosity and surface tension of the clarified glass. Any factor that can affect the viscosity and surface tension of the glass will affect the homogenization of the glass and produce streaks.
In addition, the streaks caused by the change in the flow state of the glass in the molten pool. When the overall discharge volume of the melting furnace, the melting furnace temperature, the glass liquid level, the bubbling and stirring processes change greatly, the overall flow state of the glass liquid will change greatly, which will cause the glass liquid at the bottom of the melting furnace or the dead corner to participate in the flow, thus causing the streaks.
(3) The streaks caused by dripping, erosion of refractory materials, and melting of stones. The streaks caused by these three aspects are all due to the fact that the glass melting furnace, under high temperature, burns, erodes, and melts other heterogeneous materials that are not related to the glass batch composition (such as the refractory materials used in the masonry melting furnace), forming a melt with a composition, viscosity, and surface tension that is seriously different from the main glass, and then mixed and melted into the main glass liquid to form streaks. When the dripping produces stripes, it is often accompanied by the formation of nodules.
02 Reasons for the formation of stripes in the cooling area
Analysis of the fully separated and semi-separated cooling section structures shows that in the five stages of glass formation, when the glass liquid enters the working pool (distribution channel) through the flow hole, it enters the last stage of glass liquid formation - the cooling stage. According to the different structures of the melting furnace, the flame separation form of the working section can be divided into semi-separated form and fully separated form. The products of the semi-separated working section require a relatively high molding temperature, and the temperature of the glass liquid in the working section is completely heated by the radiation heat of the flame space of the melting section. In addition, the working pool is generally semicircular, and the several supply channels supplied by the melting furnace at the same time directly discharge materials from different parts of the semicircular working pool. The glass liquid comes out of the flow hole and flows into the supply channel immediately after entering the working pool. In such a cooling process stage, stripes are rarely produced. This type of melting furnace structure was used more in China before the 1990s. When producing corresponding wine bottles and beverage bottles, stripes were rarely found in the working pool. When producing bulb shell products, this kind of working pool melting furnace structure is still used, and stripes are rarely seen on the products.
After the 1990s, due to the introduction and replacement of a large number of molding equipment, double-drop array machines with strict molding temperature requirements appeared, and advanced technologies of foreign melting furnaces were also introduced. In the cooling stage of the glass liquid, a distribution channel type working section structure appeared. This kind of working section flame is completely separated, and the glass liquid temperature in the working pool is not affected by the temperature of the melting section flame space. At the same time, inside this working section flame space, according to the outlet position of the feed channel, the working section is divided into several different flame spaces, and each flame space is equipped with a complete and advanced temperature control system. Before the glass liquid enters each feed channel, according to the molding temperature requirements of each feed channel, the temperature is controlled to the required index through the temperature control system in the distribution channel. Similarly, each feed channel is also divided into several areas, and each area has a complete and advanced temperature control system to ensure the molding temperature of the glass liquid entering the material basin. For such a perfect cooling structure of the distribution channel and the supply channel, it is also difficult to have streak defects. For example, many foreign-funded and joint-venture glass enterprises introduced into China after the 1990s rarely found streaks caused by their distribution channels and supply channels.
The above is an analysis of the structure of the working part of the melting furnace. On this basis, some situations in the cooling process of the glass liquid in some self-designed cooling areas (working pools in the form of distribution channel structures) in China are analyzed, and it is not difficult to find the reasons for the streaks in this stage.
03 Stripes caused by temperature
The temperature of the glass liquid in the distribution channel and the supply channel is generally in the range of 1270-1320 ℃ before the glass liquid comes out of the flow hole and enters the distribution channel through the ascending channel. The glass liquid needs to be controlled by cooling and heat preservation in the distribution channel before entering the supply channel. The temperature of the glass liquid entering the supply channel is generally in the range of 1210-1250 ℃. The glass liquid in the supply channel also needs to be cooled, kept warm and heated before entering the material basin. The temperature of the glass liquid in the material basin is generally around 1100 ℃. The temperature control system is also needed in the material basin to adjust the dripping temperature to meet the product requirements.
In the normal production process, a part of the glass liquid in the distribution channel and the supply channel will always remain when it flows. The temperature of this part of the glass liquid is lower than that of the glass liquid that just flowed in, or even much lower. The glass liquid that just flowed in and has a relatively high temperature flows in the middle part of the distribution channel and the supply channel, and the glass liquid with a lower temperature that remains in the channel flows around. Therefore, the glass liquid must always go through a process of mixing, infiltration and homogenization of high-temperature glass liquid and low-temperature glass liquid while flowing through the distribution channel and the supply channel. Since temperature has a great influence on the viscosity of glass liquid, glass liquids of different viscosities in the distribution channel and the supply channel are always mixed together. If the viscosity difference is relatively large, stripes will be generated. The greater the temperature difference, the more obvious the stripes will be. In severe cases, the stripes will be very thick and wide, and can be felt by hand.
04 Stripes caused by other reasons
① In the distribution channel type working pool, there are some corners of the brick structure, where some non-flowing glass materials are often condensed, generally called "dead materials". Due to long-term condensation, the composition and viscosity of this part of the material are different from those of the normally flowing glass liquid. Secondly, when the glass liquid flows through the distribution channel and the supply channel for a long time, as time accumulates, some glass liquid with relatively high viscosity is deposited at the bottom. When the discharge volume and the temperature of the glass liquid change greatly in production, the immobile and deposited materials in these parts will participate in the flow and form stripes. In addition, when some iron debris is deposited in the corners of the distribution channel and the supply channel, yellow and brown stripes will be formed on the product.
② Under reducing conditions, the viscosity of the glass liquid is two orders of magnitude higher than that of the glass under oxidizing conditions, and the surface tension is also greater than that under oxidizing conditions. Substances with low surface tension tend to spread on the surface of substances with high surface tension and form a film. Therefore, for the distribution channel and supply channel of combustion gas and liquid fuel, when the surface of the glass liquid is in a reducing atmosphere, the surface tension will increase, the surface glass will be immediately squeezed out and the lower material will be turned over, so that two glass materials with different surface tensions and viscosities are mixed together to form stripes.
③ In the distribution channel and supply channel, because the surface of the glass liquid is in an open state, some volatile components evaporate, resulting in different surface glass liquid components from internal glass liquid components, which leads to different surface tensions of the inner and outer glass liquids, causing convection of the upper and lower glass liquids, forming stripes.
④ In the distribution channel and supply channel, the temperature of the glass liquid is low, and the glass liquid does not erode the material trough bricks much, but despite this, the glass liquid still washes and erodes the material trough bricks, and the eroded material has a large viscosity and is difficult to diffuse, and adheres to the liquid surface line. Every time the glass liquid level fluctuates, it will drive the glass liquid rich in corrosive substances to rise or fall. When the temperature and discharge volume of the distribution channel and the supply channel change greatly or when the glass liquid level fluctuates greatly, this part of the corrosive substances will participate in the flow and form stripes.
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