Ceramics raw materials

Fig. 2. Examples of traditional ceramics manufactured usiag traditional ceramic raw materials clay, flint, Si02 and feldspar.

The processes employed in manufacturing a ceramic are defined and controlled to produce a product with properties suited to a specific appHcation. Processing—microstmcture—property relationships are deterrnined by characterizing the ceramic raw materials, mixes, and the formed ceramic body intermittently during processing and after final thermal consoHdation. It is possible to modify and optimize processes to optimize properties and to identify and correct processing deficiencies when less than optimal properties are obtained. Examples of some process—microstmcture—property relations in advanced ceramics are outlined in Figure 4. 

If the ore consists of separate grains containing the desired material, it can be separated from undesired minerals by physical methods such as flotation, sedimentation, or magnetic separation. For metals this step can lead to 80 to 95 percent concentration of the value of the ore. Ceramic raw materials such as sand and clay can often be found pure enough in nature so that no concentration is needed. 

Traditionally ceramic raw materials have been dug out of the ground and used with little or no treatment or purification. Sand, fireclay, talc, and gypsum are examples. The energy expenditure for producing these materials is therefore small. Some of these materials can be found naturally in high purity. Silica sands (SiO,) with less than 100 ppm (parts per mil-... 

Raigon-Pichardo, M., Garcia-Ramos, G. Sanches-Soto, P. J. 1996. Characterisation of a waste washing solid product of mining granitic tin-bearing sands and its application as ceramic raw material. Resources, Conservation and Recycling, 17, 109-124. 

Why are geology and mineralogy discussed in a book on ceramics The main reason is the fact that nature shows us which materials might be suitable ceramic materials for various applications. Over the past millions of years nature has produced an enormous variety of rocks and minerals with numerous properties. Researching these products has taught us how nature shaped them and what their structure are like. Nowadays these production processes can be imitated in the laboratory, very often within a short period of time. In addition, most ceramic raw materials are obtained from nature and sometimes altered chemically or physically in the laboratory. This means that the ceramicist must have a good knowledge of these raw materials. 

Gypsum CaS04 is used in the fine ceramic raw material mass... 

Table 9.1 Some commonly used synthetic, ceramic raw materials...

Ceramic raw materials are powders. Together with a mixture of additives these powders are moulded into a particular shape and subsequently dried and baked. After the baking or sintering process it is possible to apply a finishing coat if necessary and then the ceramic object is ready. The entire route from raw material to finished product was elaborately discussed in chapter 9. 

According to literature sources mainly oxides, nitrides, borides and carbides are used as ceramic raw materials of chemical and structural applications and the most common elements in these compounds are Be, Mg, Ca, Ti, V, Cr, Y, Zr, La, Hf, W, B, Al, Si and Sn. These elements can be found in rather small area of the periodic table, i.e. in the groups 2 up to and including 6, 13 and 14. So apparently relatively few ingredients are used in this branch of ceramics to produce a wide range of products. The tricks of the trade are in the preparation. The properties are determined by a number of factors, such as the nature of the building blocks, the kind of bonds, the strength of the bonds, the crystal structure and the reactivity of the material. 

Natural ceramic raw materials have the disadvantage that they are not soluble in water and consequently unsuitable for wet chemical analysis. That is why they are first destructed in the laboratory. Destruction means that a substance or a mixture of substances is heated together with a substance or a mixture of substances and as a result the component to be destructed is largely separated into ions and consequently dissolves in water. The destruction method and the substances are specific for the substance undergoing the destruction. Different methods applied to the same substance lead to different results. 

Usually, hardness is only measured on baked objects. This subject was already discussed in chapter 9 (see also no. 19 of this chapter). However, very little has been published about the hardness measurement of ceramic raw materials, i.e. powders. Such powders consist of miniature crystals and are sometimes used as abrasives. Obviously this use requires an extreme hardness. 

It is very understandable that people know so little about ceramics. Ever since time immemorial, clay has been an important ceramic raw material. Not until approximately 1850 were other (synthetic) raw materials introduced in ceramics. The objects which are made of the latter often have specific properties which clearly differ from those of the clay ceramics and which meet the requirements of the sophisticated high-tech world of the nineties. So ceramics involve much more than simply clay ceramics and are, in my view, well worth writing a book about. 

Worrall, W.E. Clays and Ceramic Raw Materials, Applied Science Publishers London,... 

Since the Bronze Age both natural ceramic raw materials and synthetic raw materials have been used. Today synthetic raw materials are referred to as industrial minerals or specialty chemicals. Natural raw materials are those to which only physical separations are performed (e.g., clay soils from which organic raw materials are floated, feldspar rock ground to a particular size distribution). With this classification, a description of common ceramic raw materials will be given in the next part of this chapter. 

Clays were probably the first ceramic raw materials. Clay minerals are fine-particle hydrous aluminum silicates, like those shown in Figure... 

In all ceramic raw materials, both natural and synthetic, a powder with a particular chemical formula is the primary objective. Chemical... 

Almost all the main ceramic raw materials belong to the groups of oxides, silicates and carbonates. They were dealt with, especially as regards their behaviour during technological operations, in Chapter I. Further necessary data will be specified in connection with the individual types of ceramics. The problems of ceramic raw materials have been dealt with in detail in specific monographs, for example, by Radczewski (1968) and Grimshaw (1971). 

Kaolin and various types of clays are important ceramic raw materials. With water, they form plastic, easily worked pastes which can be dried and fired without undergoing significant deformations, and therefore form a basis of the traditional types of ceramics. 

Natural kaolin is treated by washing, in order to remove the coarser particles and impurities such as quartz, feldspars, etc. For some purposes, use is also made of crude kaolin (e.g. in fireclay manufacture). Clays are more widespread in nature they are usually contaminated with larger amounts of quartz, feldspar, mica, etc. Clays containing larger or smaller amounts of aluminium hydroxides (diaspore or gibbsite) occur and are used as ceramic raw materials in some countries. 

From this aspect, the basic ceramic raw materials can be divided into the following three groups, according to their main function. 

Clay materials impart to moist clay ceramic raw material batches the plasticity important for their processing. Quartz is used as a lean clay and reduces the shrinkage upon firing. The feldspar acts as a flux due to its alkali content. 

This entry will be divided into sections covering the classification of ceramics, a brief history of ceramics, raw materials for ceramics, properties and applications of ceramics, processing of ceramics, and a brief commentary on future trends. The field of ceramics is very broad and encompasses not only well-known, conventionally used materials and technology but also much newer compositions, processing methods, and applications. More detailed information on all of these sections is available in printed format and on the web.[ °- "l... 

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