APPLIED PROCESSES AND TECHNIQUES IN CERAMIC

MANUFACTURING

Raw materials

A wide range of materials is employed by the ceramic industry to match the diversity of its product range. Both natural and synthetic materials are used, many produced within Europe but a few are imported. Sectoral needs are different and details of raw materials are listed in the chapters regarding the different sectors. Formulas (or bodies) of clay based ceramics may consist of one single clay or more clays, mixed with mineral modifiers, so-called ‘non-plastics’, such as powdered quartz and feldspar. Common clay minerals (‘plastic clays’) are hydrated aluminium silicates that have resulted from the weathering of rocks and there are two structural units involved in most clay mineral lattices. One is the ‘silica sheet’ formed of tetrahedra consisting of a Si4+ surrounded by four oxygen ions. The other structural unit is the ‘aluminium hydroxide or gibbsite sheet’, consisting of octahedra in which an Al3+ ion is surrounded by six hydroxyl groups. These octahedral sheets condense with the silica sheets to form the clay minerals. There are a number of mineral species called clay minerals, but the most important are ‘kaolinite’, Al2O3·2SiO2·2H2O, ‘montmorillonite’, Al2O3·4SiO2·H2O and ‘halloysite’, Al2O3·2SiO2·3H2O.

Advanced ceramic products, which contain only a small fraction of clay or none at all, are based on the following materials: oxides, carbides, nitrides and borides of Al, Mg, Mn, Ni, Si, Ti, W, Zr and other metal ions. Typical examples are Al2O3 (alumina), MgO (periclase or dead burned magnesia), SiC (silicon carbide), TiN (titanium nitride).

Clays or argilliferous materials (sedimentary clays, schistous clay, loamy clay, marl) are mostly used for the manufacture of bricks, roof tiles and clay pipes. Organic additives (sawdust, paper binding substances, formed polystyrene) or inorganic auxiliary agents such as kieselguhr or perlite can be added in order to obtain a greater pore volume. In the facing brick and roof tile industry, metallic oxides such as MnO2, TiO2, Fe2O3, chromites, and minerals such as CaCO3, CaMgCO3 can be added to obtain the desired colour and/or to enhance porosity of the finished product. BaCO3 can be added to reduce efflorescence. Such additives are added just before the

shaping process, or in the raw material preparation, either in solid or liquid form.

Refractory products consist of clay, chamotte (calcined and crushed raw plastic clay) and certain natural rocks such as quartzite, dolomite, bauxite and magnesite, but also of the abovementioned synthetic materials, as for example sintered corundum, silicon carbide or spinels. In order to produce compressible masses, binders and aggregates are added to the milled raw materials.

The expanded clay industry uses clays with a specific expandability. Additives (e.g. flux, expansion aids, and release agents) are used for promoting expansion.

Products manufactured by the wall and floor tiles, household ceramics and sanitaryware industry predominantly consist of aluminium silicates and the clay minerals serve as plastic raw materials. However, technical ceramics and inorganic bonded abrasives often contain only a small amount of clay or in the case of technical ceramics none at all and are, therefore, based on the metal borides, carbides, oxides and nitrides as mentioned above.

Quartz, feldspar, in some cases chalk, dolomite, wollastonite and steatite are used as fillers and fluxing materials. Various ceramic products are glazed or engobed on the visible or whole surface with glaze frits or engobing slips. In addition to the above-mentioned raw materials and auxiliary agents, additional auxiliary agents, firing auxiliaries, fuels and sorption agents are necessary for the manufacture of ceramics. Organic slip additives and binding agents are employed as auxiliary agents in the

shaping process. The firing auxiliaries consist of re-usable fire-proofed capsules, plates and stanchions. Sorption agents such as calcium carbonate, calcium hydroxide and fine chalk are employed in flue-gas treatment.

Date: 2016-01-05; view: 802