|Environmental Handbook Volume II: Agriculture, Mining/Energy, Trade/Industry (GTZ, 1995, 736 p.)|
|Trade and industry|
|47. Ceramics - Fine, utilitarian and industrial|
Fine, industrial and utilitarian ceramics cover the following industrial sectors:
- Ordinary ceramics: tiles, roof-tiles, earthenware,
expanded clay, wall tiles and floor slabs, refractory products
- Fine ceramics: earthenware, pottery, fine earthenware, porcelain, electrical porcelain, sanitary products, grinding discs and abrasive wheels
- Technical ceramics
Most ceramics companies are established in the vicinity of clay deposits. (This environmental brief deals only briefly with the extraction of raw materials; for further details refer to the environmental brief Surface Mining. Advice on processing and transportation of raw materials is also given in the relevant environmental brief. The size of ceramic plants and their daily throughputs vary from a few kilograms for technical ceramic plants, normally 10 to 50 t/day for fine ceramics, to as much as 450 t/day in the tile industry. Since many companies operate different types of production, the total output of the works is often higher than the typical daily output of a specific product.
The fine, industrial and utilitarian ceramics industries use all types of clays, kaolins and fireclays (burnt clay), feldspars and sands as a raw material base. The refractory, abrasives and technical ceramics industries also use numerous high-temperature-resistant and abrasion-resistant oxides such as corundum (Al203), zirconium oxide (ZrO2) and silicon carbide (SiC).
Besides using their own, readily available raw materials, many companies are increasingly purchasing ready-processed raw materials, particularly for refractory products, abrasives and technical ceramics, as well as the raw materials required for glazes and frits.
The following process sequence is typical of the production processes in industrial, utilitarian and fine ceramics:
- extraction, processing, forming, drying, partial glazing or enamelling, firing, sorting/packing and transportation.
Execution of the individual process stages varies according to the selected method. Generally speaking, casting, plastic or drying processes are employed, with smooth transitions between the process stages.
Table 1 - Production processes
Dry pressing processes
- Porcelain - Sanitary products - Electrical porcelain |- Refractory
- Tiles - Roof tiles - Expanded clay - Cleaving tiles - Electrical porcelain - Pottery - Earthenware
- Refractory products - Wall tiles, floor slabs - Pottery - Earthenware tiles - Technical ceramics - Steatite - Abrasive wheels
- In the casting process the raw materials are
dosed, wet-ground and poured into plaster moulds as so-called
slip. During pressure casting, the slip is shaped to produce the blank under
pressure in machines.
- In the plastic process the raw materials are normally prepared in the wet state, mixed and shaped with moisture content of 15 - 20% water.
- In the dry pressing process used in fine ceramics, the raw materials are frequently prepared in the wet state, then dried in a spraying tower to a residual moisture content of 5-7%. In the refractory industry the raw materials are mixed dry and are often processed with pressing moisture content of less than 2%, also using organic and inorganic binding agents.
The moulded products are dried and then fired. They are generally fired in high-power tunnel kilns; special products are fired mainly in individual, hood-type or batch kilns, while fast-burning products are fired in roller hearth kilns of various designs. In many countries, tile products in particular are often fired in self-built single-chamber and ring kilns or in charcoal kiln systems.
Many fine ceramic products are glazed or enamelled before firing.
Depending on the raw materials used, the firing temperatures in industrial, utilitarian and fine ceramics begin at 950°C for some tile products, for example, whereas most fine ceramic products are fired at between 1100°C and 1400°C. Refractory and technical ceramic products have firing temperatures of 1280 to 1900°C. (Pure glaze baking is done at lower temperatures.) The dual firing process is sometimes used for porcelain and very rarely for wall tiles.
Energy consumption depends on the product and the process; in the tile industry, because of the low firing temperatures, it is between 800 and 2100 kJ/kg of manufactured product, but in almost all other areas of industrial, utilitarian and fine ceramics it is on average much higher per manufactured product, and may be as much as 8000 kJ/kg of product.
After firing the products must be sorted and sometimes reworked, which will involve varying labour costs depending on the product.