1. Scope

The main raw materials used by the glass industry are sand, lime, dolomite, feldspar, as well as soda, borosilicates and numerous additives which embrace practically the entire periodic system of elements. Its products are a large number of glasses with different properties, many of them further processed after manufacture (Table 1).

Table 1 - Glass products

In the modern glass industry the raw materials are no longer generally extracted by the companies themselves but are purchased in the desired chemical and physical composition, e.g. in terms of granulation, moisture content, impurity (for the environmental relevance of the extraction of raw materials refer to the environmental brief Surface Mining). The substantial differences between the materials to be dosed and mixed necessitate the use of mixing and processing plants where the mixtures are melted in tank furnaces, more rarely in pot furnaces, or special furnaces. Cupola furnaces are still sometimes used for mineral fibres, and electric melting systems are used for manufacturing ceramic fibres. The flue gases formed during melting are nowadays cooled by regenerative or recuperative plants, thereby reducing the specific fuel consumption.

After melting, the glasses are moulded. Most glasses must then be cooled according to the subsequent application, to avoid glass stresses. Glasses are frequently further processed by thermal, chemical and physical post-treatments, such as clamping, pouring, bending, gluing, welding and grinding. Hollow glassware is frequently decorated. Fibres are drawn, centrifuged, blown or extruded after melting, using a variety of technologies.

The capacities of the individual glass-producing companies vary considerably, and it is often the case that several melting systems with different production programmes are combined in one works. Pot furnaces have a capacity of 3-8 t/day, whilst the tank capacities for special glasses range from 8 to 15 t/day in most cases. In specialist fields, however, the outputs are much higher, e.g. tanks for container glass melt between 180 and 400 t/day, float glass tanks attain melting capacities of between 600 and 1000 t/day.

The melting temperatures of the glass generally range between 1200 and 1500°C, the temperature depending for the most part on the mixture and the product to be manufactured. The amount of energy required to melt 1 kg of glass is between 3700 and 6000 kJ. The capacities and energy consumptions indicated above are average values which depend on the design and operating time of the tank, the production programme and the actual tank load. The specific energy consumption should be reduced by the use of waste fragments wherever possible.