Glass-making is the process of melting a charge in special kilns. In many respects, the quality of the final product depends on the production stage. In glass melting kilns, the uniformity of the glass mass is ensured and as a consequence the amount of defects in that mass.
As this is one of the final stages of the production process, already enriched raw materials are fed into the glass melting kilns. Those materials are collectively called the charge. It contains both main (quartz sand, limestone, soda) and additional (clarifying agents, colorants, darkening agents) components.
The charge that enters the kiln has the necessary moisture content, uniformity and consistency.
Glass melting kiln is a piece of special equipment, whereby a charge is melted using gas, solid fuel or electric current.
Most glass melting kilns have identical structure. They consist of:
- a pool
- a hearth
- a crown
In the pool consisting of walls and bottom, melting, clarifying, cooling down and forming of the glass mass occurs. According to the pool design, there are kilns with a common pool, in which there is no zonal division, and kilns with sharply pronounced zones.
The part of the pool, where the melting and clarifying zones are located, is called heating one, while the part with cooling and forming zones is called forming one.
The fired part of the kiln is called the hearth. It is located between the walls and the crown. Both in the walls and in the crown, there are special openings intended for fuel and air supply, as well as for the removal of hot gases.
The crown of a glass kiln is lined with special refractory bricks. They are called Dinas bricks.
Bricks for the glass melting kilns should contain not less than 93% of silica. The thickness of the walls of that kind of equipment is 500 to 600 mm, while the thickness of the crown is 300 to 450 mm.
Since melting of the glass mass is carried out at extremely high temperatures, many chemical reactions occur during that production process, which is accompanied by the release of harmful vapors. That led to the appearance of such a precautionary measure as complete sealing of the glass melting kiln. Also, special ventilation equipment is installed in the production shops, which is intended to remove all impurities and combustion products from the ambient air.
Smoke extraction systems of modular glass melting kilns perform a similar function with the only difference that they ensure the necessary composition of the atmosphere inside the equipment proper, not outside it.
Types of glass melting kilns
There are two classifications of glass melting kilns.
According to the first of them, two types of kilns are distinguished as per the design of the working chamber, viz. pot-type and bath-type ones.
The division of kilns into groups using the second classification is done according to the method employed for heating the glass mass. Based on that principle, glass melting kilns can be:
- firing ones
- electric ones
- gas-electric ones
Pot-type kilns are today considered economically not viable, which is due to their high power consumption. Nowadays, they are used only by small companies producing high-quality fancy, lighting-equipment and special glass, as well as optics items.
High power consumption arises due to the fact that pot-type kilns are of intermittent action, because, except for the melting of the charge proper, a lot of thermal energy is spent on heating up the equipment.
Unlike the pot-type kilns, the bath-type ones can be of continuous action. The continuous operation of such equipment significantly reduces the resources spent on creating and maintaining the temperature necessary for melting the glass mass.
The bath for the glass melting kiln is a large reservoir containing a tin melt. Due to the fact that the density of the glass mass is much lower than that of the tin, those materials do not mix with each other.
The tin makes it possible to gradually cool down the glass mass from 1,600 ºС to 600 ºС (the melting point of that metal) without creating internal stresses that adversely affect the strength of the finished product. In addition, the uniform distribution of the glass mass makes it possible to obtain a perfectly flat sheet.
The tin also does thermal polishing of the glass, which has a positive effect on the strength and transparency of the latter.
According to the second classification, the equipment for melting the glass mass is divided according to the mode of heating.