Melting of bottle glass

Glass melting is a very complex process. The batch materials will undergo a series of physical, chemical and physicochemical changes and reactions at high temperatures. The results of these changes and reactions turn the mechanical mixture of various raw materials into a complex melt, namely glass liquid.
According to the changes and reactions that occur in the batch materials during the glass melting process, the glass melting process can be divided into five stages, namely silicate formation, glass formation, clarification, homogenization and cooling.

Most of the common bottle glass is composed of silicate, and the formation reaction of silicate is largely carried out in the solid state. In this stage, the composition of the powder undergoes a series of physical and chemical changes. A large amount of gaseous substances in the powder volatilize. Then the silicon dioxide and other components begin to interact. At the end of this stage, the main solid-state reaction ends, and the powder becomes a sinter composed of silicate and silicon oxide. For most glasses, this stage basically ends at 800~900℃.

Continue heating, the sinter generated in the silicate formation stage begins to melt, the low-melting mixture begins to melt first, and the silicate and the remaining silicon dioxide melt and diffuse each other, and the sinter becomes a transparent glass liquid. This process is called the glass formation stage. At this time, there is no unreacted batch material, but there are still a lot of bubbles and streaks in the glass, and the chemical composition and properties are also uneven. The temperature of ordinary glass at this stage is 1200~1250℃.

At the end of the glass formation stage, there are still many bubbles and streaks in the glass. When the glass liquid is heated further, the viscosity of the glass liquid will decrease. The process of eliminating visible bubbles in the glass liquid is the clarification process of the glass liquid.
During the silicate formation and glass formation stages, a large amount of gas is precipitated due to the decomposition of the batch materials, the volatilization of some components, the redox reaction of oxides, the interaction between glass and gas medium and refractory materials. Most of these gases escape into space, and most of the remaining gases will dissolve in the glass liquid. A small part of the gas still exists in the glass liquid in the form of bubbles. There are three main states of gas in the glass, namely visible bubbles, dissolved gases, and gases that form chemical bonds with glass components. The latter two are invisible and will not affect the appearance quality of the glass. The clarification process of the glass liquid is mainly the process of eliminating visible bubbles.
During the clarification process, the elimination of visible bubbles is carried out in the following two ways. 1. Increase the volume of the bubbles, accelerate their rise, float out of the glass surface, break and disappear. 2. Make the gas components in the small bubbles dissolve in the glass liquid, and the bubbles are absorbed and disappear.
In order to speed up the clarification of glass liquid, in addition to adding certain clarifiers to the batch, the method of increasing the temperature of glass liquid is generally adopted. This stage of most glasses is completed at 1400~1500o℃, which is often the highest temperature area in glass melting. The viscosity of glass liquid during clarification is 1~10Pa·s.

The role of homogenization is to eliminate stripes and other inhomogeneities in glass liquid, so that the chemical composition of each part of glass liquid is uniform. In this stage, due to the thermal motion and mutual diffusion of glass liquid, the stripes in the glass liquid gradually disappear, and the chemical composition of each part of the glass liquid gradually tends to be consistent. This uniformity is often characterized by whether the refractive index of each part of the glass liquid is the same. This stage of most glasses is completed at a temperature slightly lower than the temperature of the clarification stage.

The homogenized glass liquid cannot be molded into products immediately, because the temperature of the glass liquid at this time is high and the viscosity is lower than that during molding, which is not suitable for glass molding operations. It needs to be cooled and the temperature of the glass liquid is gradually reduced to increase the viscosity of the glass liquid to meet the needs of molding. The value of the glass liquid temperature reduction varies with the composition of the glass and the molding method. Generally, soda-lime glass usually needs to be cooled by 200~300o℃. The cooled glass liquid requires a uniform temperature to facilitate molding.
During cooling, the clarified glass liquid should prevent the re-precipitation of bubbles. The small bubbles that appear at this stage are called secondary bubbles or regenerated bubbles. The secondary bubbles are evenly distributed throughout the cooled glass liquid, with a diameter generally below 0.1mm, and the number can reach thousands per cubic centimeter of glass. Since the temperature of the glass liquid has been reduced at this stage, it is very difficult to eliminate the secondary bubbles. Therefore, the generation of secondary bubbles should be particularly prevented during the cooling process.
The five stages in the above glass melting process are different from each other, but they are also interrelated. These stages do not actually occur in a strict order, but often occur simultaneously.

The temperature at each point along the length of the continuous operation tank kiln is different, but it is constant over time, so it is possible to establish a stable temperature system. The correctness of the melting process system not only affects the quality of the melted glass, but also determines the output of the melted glass. Figure 2-10 shows the melting temperature system for bottle glass in a continuous operation tank kiln.

Whether it is a horizontal flame pool kiln or a road flame pool kiln, its temperature system has an impact on the baking degree of glass liquid, the flow of glass liquid, molding operations, fuel consumption and kiln age. For bottle glass, the glass bottles and cans on the market are mainly divided into four categories according to color: colorless, light blue, emerald green and brown. When the color of the glass changes or the concentration of the glass color changes, it has a vital impact on the heat transfer form and heat transfer efficiency. In terms of the melting process, the impact of glass color changes on the process conditions is much more obvious and serious than the impact of glass composition changes. There is a big difference in the temperature distribution of different colored glasses in the furnace.

It can be seen from Table 2-24 that at the same melting temperature, there are obvious differences in the liquid surface temperature and pool bottom temperature of glasses of different colors. There are three forms of heat transfer in the glass melting furnace: radiation, convection, and conduction. For glasses of different colors, the stronger the ability to absorb radiation light, that is, the stronger the ability to absorb high-temperature radiation heat, the more heat the glass surface absorbs, and the less heat is transferred through the glass body in the form of radiation. From the perspective of liquid surface temperature, brown glass has the strongest heat absorption capacity and the highest liquid surface temperature; emerald green glass is second, and light blue glass is third. From the perspective of pool bottom temperature, the problem becomes a little complicated: light blue glass has a poor ability to absorb radiation light, and more heat is transferred to the pool bottom through the glass body in the form of radiation, so the pool bottom temperature is higher; emerald green glass has a strong ability to absorb radiation light, and less heat is transferred to the pool bottom through the glass body in the form of radiation, so the pool bottom temperature is lower. However, brown glass has a strong ability to absorb radiation light, and the temperature at the bottom of the pool is much higher than that of emerald green glass. The reason may be: the glass in the pool is divided into several liquid layers. Since the light transmittance of brown glass is weak, the temperature difference between the liquid layers is large, and there should be a large temperature gradient along the depth of the pool. However, due to the strong heat absorption capacity of brown glass, after the upper glass liquid absorbs heat, the temperature rises, the volume expands, and a thrust toward the surrounding is generated in the horizontal direction. This thrust is changed by the pool wall and transferred to the lower liquid layer, forming a convection force. The enhancement of convective heat transfer makes up for the lack of radiation heat transfer, so the temperature at the bottom of the brown glass pool is higher.
Generally speaking, under the same process conditions and temperature system, for glasses with the same components but different colors, melting brown glass can obtain better glass uniformity and higher melting rate. The reason is precisely due to the strong convection caused by the strong heat absorption capacity of brown glass. Of course, the intervention of the bubbling device will change the heat transfer conditions. When melting emerald green glass, if you want to improve the bottom temperature, glass uniformity and melting efficiency, installing a bubbling device is an effective measure. When you want to change different colors of liquid in the same furnace, the process elements of the melting part, working part and feeding channel must be adjusted accordingly to adapt to the process state changes caused by the "heat transfer difference" of the glass color.