The Melting Process of Enamel Frit
The melting process of enamel frit is to melt the enamel frit material at high temperature, and after a series of physical and chemical reactions, it becomes a enamel melt that meets the quality requirements, and then rolls or water agitates to form an enamel frit. The melting of enamel glaze is usually carried out at a high temperature of about 1250°C. The melting temperature of industrial acid-resistant enamel glaze can reach 1400°C, while the melting temperature of low-temperature aluminum enamel glaze is generally about 1150°C.
The physical and chemical reactions in the process of enamel glaze melting can be roughly divided into the following four stages.
1. Dehydration stage
The temperature in the dehydration stage is below 260°C, mainly the drying of the enamel glaze batch and the dehydration of the borax crystal water, but there is still a part of the raw material crystal water that can only be discharged at a higher temperature.
At the same time as dehydration, some solid-phase reactions have begun, forming part of the low-temperature eutectic. The stage below 260°C is dominated by physical reactions.
2. The primary reaction stage
The temperature in the primary reaction stage is 260~700°C, which mainly includes the decomposition, melting and oxidation of raw materials. A large amount of solid phase reaction in the batch starts to proceed, and a liquid phase appears at the same time. Carbonate starts to react with quartz in a solid phase at 500°C and gradually turns into a liquid state. When the melting temperature rises to 573°C, the quartz transforms into α-quartz.
At this stage, a large number of low-temperature eutectics appeared, and the melting process was accelerated.
Part of the opacifier participates in the solid phase reaction to form a low-temperature eutectic, which promotes the melting of batch materials and the acceleration of chemical reactions. The start time and degree of the solid-phase reaction have a great influence on the melting speed and quality of the batch.
3. Silicate formation stage
The temperature of the silicate formation stage is 700~870°C. The refractory raw materials in the batch and the flux raw materials form the enamel glaze matrix, that is, the carbonate and quartz react to form silicate. At this time, the melt is in a boiling state, and the fluoride can greatly reduce the melting point of the batch, thereby speeding up the melting speed of the enamel glaze batch.
4. Mixed melting stage
The temperature of the mixed melting stage is 870~1300°C, and the reactions in this stage are all carried out in the liquid phase. With the increase of the melting temperature and the extension of the time, the melt temperature rises rapidly, the melt viscosity decreases, the gas continues to escape, the density increases, and the colorant particles (such as CoO, NiO, etc.) quickly dissolve.
The opacifier in the batch is also dissolved in a large amount in the melt, and the melt gradually becomes transparent, which promotes the uniform progress of various physical and chemical reactions until the reaction is complete. However, if the melting temperature and time exceed the process requirements, the turbidity, elasticity, and enameling performance of the enamel will be reduced.
The mixing stage of industrial acid-resistant enamel glaze takes longer than that of daily enamel glaze. This is mainly because the gas in the industrial acid-resistant glaze batch needs to escape fully, and the melting becomes more uniform, that is, the degree of vitrification is higher.
1. Dehydration stage
The temperature in the dehydration stage is below 260°C, mainly the drying of the enamel glaze batch and the dehydration of the borax crystal water, but there is still a part of the raw material crystal water that can only be discharged at a higher temperature.
At the same time as dehydration, some solid-phase reactions have begun, forming part of the low-temperature eutectic. The stage below 260°C is dominated by physical reactions.
2. The primary reaction stage
The temperature in the primary reaction stage is 260~700°C, which mainly includes the decomposition, melting and oxidation of raw materials. A large amount of solid phase reaction in the batch starts to proceed, and a liquid phase appears at the same time. Carbonate starts to react with quartz in a solid phase at 500°C and gradually turns into a liquid state. When the melting temperature rises to 573°C, the quartz transforms into α-quartz.
At this stage, a large number of low-temperature eutectics appeared, and the melting process was accelerated.
Part of the opacifier participates in the solid phase reaction to form a low-temperature eutectic, which promotes the melting of batch materials and the acceleration of chemical reactions. The start time and degree of the solid-phase reaction have a great influence on the melting speed and quality of the batch.
3. Silicate formation stage
The temperature of the silicate formation stage is 700~870°C. The refractory raw materials in the batch and the flux raw materials form the enamel glaze matrix, that is, the carbonate and quartz react to form silicate. At this time, the melt is in a boiling state, and the fluoride can greatly reduce the melting point of the batch, thereby speeding up the melting speed of the enamel glaze batch.
4. Mixed melting stage
The temperature of the mixed melting stage is 870~1300°C, and the reactions in this stage are all carried out in the liquid phase. With the increase of the melting temperature and the extension of the time, the melt temperature rises rapidly, the melt viscosity decreases, the gas continues to escape, the density increases, and the colorant particles (such as CoO, NiO, etc.) quickly dissolve.
The opacifier in the batch is also dissolved in a large amount in the melt, and the melt gradually becomes transparent, which promotes the uniform progress of various physical and chemical reactions until the reaction is complete. However, if the melting temperature and time exceed the process requirements, the turbidity, elasticity, and enameling performance of the enamel will be reduced.
The mixing stage of industrial acid-resistant enamel glaze takes longer than that of daily enamel glaze. This is mainly because the gas in the industrial acid-resistant glaze batch needs to escape fully, and the melting becomes more uniform, that is, the degree of vitrification is higher.