Method for producing porcelain, porcelain and ceramic...

Compositions: ceramic – Ceramic compositions – Clay containing

Reexamination Certificate

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C501S130000, C501S141000, C501S143000

Reexamination Certificate

active

06475941

ABSTRACT:

BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
The invention relates to a process for producing porcelain, in particular for applications in electrical insulation, in which bauxite is used as a starting material. In the text that follows, porcelain of this type is also referred to as bauxite porcelain. The invention also relates to bauxite porcelain and to a ceramic insulator made from the porcelain.
Nowadays, alumina porcelain is customarily used as an industrial ceramic for use in electrical insulation. In this context, the term alumina porcelain is understood as meaning a sintered mixture of alumina, clay, kaolin, feldspar, and, if appropriate, sintering aids and fluxes. In this context, the term alumina denotes a high-purity aluminum oxide and is obtained in a complex manner, using the Bayer process, from the raw material bauxite. Alumina should in particular not be confused with clay, which is usually understood as meaning the weathering product of feldspar-containing rocks that is to be found at secondary deposits. For its part, kaolin is used to refer to the weathering product of feldspar-containing rocks that remains at primary deposits.
Alumina porcelains, which have a high tensile strength, a high bending strength and a high internal compressive strength are used in particular for strength-tested high-voltage insulators. High-strength alumina porcelains have bending strengths, measured on a standardized, glazed bending bar made from the alumina porcelain, of over 170 N/mm
2
. Depending on the desired bending strength, the amount of alumina to be introduced varies between 27 and 55% by weight, the strength rising as the alumina content increases.
High-strength alumina porcelains are known, for example, from Published, European Patent Application EP 0 189 260 A3, Published, British Patent Application GB 2 056 431 A, U.S. Pat. No. 4,183,760 and European Patent EP 0 522 343 B1.
However, alumina is a relatively expensive raw material that—as has been stated—has to be obtained in a complex manner from naturally occurring alumina oxide, such as for example bauxite. For this reason, a particularly high-strength alumina porcelain is relatively expensive, which entails drawbacks in particular for mass production for applications in electrical insulation. The price of the alumina represents a considerable burden on the manufacturing and product costs.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a method for producing porcelain, porcelain and a ceramic isolator formed from porcelain which overcome the above-mentioned disadvantages of the prior art methods and devices of this general type, which can be used in particular for highly mechanically loaded components used in electrical insulation. A further object of the invention is to provide porcelain that is less expensive than those used in the prior art while achieving the same mechanical properties. Furthermore, it is an object of the invention to provide a ceramic insulator that is less expensive than conventional insulators known in the prior art while having the same mechanical properties.
With the foregoing and other objects in view there is provided, in accordance with the invention, a process for producing porcelain. The process includes mixing calcined bauxite, clay containing more than 5% by weight of first foreign metal oxide inclusions, kaolin containing more than 5% by weight of second foreign metal oxide inclusions, feldspar and magnesium silicate resulting in a mixed compound. The mixed compound is milled and processed into a slurry and the slurry is further processed into a shapeable starting compound. The starting compound is then dried and sintered to produce the porcelain.
According to the invention, the first object is achieved by a process for producing the porcelain, in which calcined bauxite, clay which contains more than 5% by weight of foreign metal oxide inclusions, kaolin which contains more than 5% by weight of foreign metal oxide inclusions, feldspar and magnesium silicate are mixed, milled and processed into a slurry. The slurry is processed further to form a shapeable starting compound, and the starting compound is dried and finally sintered to produce the porcelain. If appropriate, conventional auxiliaries can be added when required.
In other words, in the process described the use of alumina is dispensed with altogether. Instead of alumina, a calcined bauxite is employed, which can be obtained at significantly lower cost than alumina. Calcined bauxite is a raw material that is in the natural state up until the calcining operation. The calcining converts some of the aluminum hydrate contained in the bauxite into aluminum oxide. The use of calcined bauxite allows production costs to be drastically reduced compared to alumina.
The invention is based on the discovery that corundum (&agr;-Al
2
O
3
), which is formed from the alumina or the bauxite during firing of the porcelain, is a major factor in ensuring the mechanical strength of the porcelain. Since alumina provides more corundum than calcined bauxite (bauxite still contains impurities), when replacing alumina with calcined bauxite, correspondingly more bauxite has to be employed in order to achieve the same mechanical strength. However, the higher quantity of calcined bauxite required results in that, compared to alumina porcelain, the amount of the plastic components kaolin and clay and of the feldspar that forms the vitreous phase, has to be reduced. However, this in turn entails drastic changes in the mechanical properties of the porcelain.
Extensive tests have shown that the adverse effect of reducing the levels of feldspar and plastic components on the mechanical strength of the porcelain can be compensated for if the plastic components used are a clay and a kaolin, in each case containing more than 5% by weight of the foreign metal oxide inclusions, and magnesium silicate is additionally admixed with the starting materials.
In the case of clays and kaolins, the foreign metal oxides are included in what are known as clay minerals. Examples of clay minerals are sheet silicates, such as kaolinite, illite or montmorillonite.
Surprisingly, it has been found that the foreign metal oxides (impurities) that are included in the clay or kaolin promote the formation of eutectic molten phases during the sintering of the porcelain. The molten phase of the mixture occurs at lower temperatures than the molten phase of the individual components. The sintering temperature of the porcelain can be reduced, which in turn reduces production costs. The particular feature is that the foreign metal oxides that are incorporated in the lattice of the clay minerals have a particularly favorable influence on the formation of the advantageous or aggressive molten phase.
As a result of the aggressive molten phase forming at lower temperatures, it is possible to achieve virtually complete dissolution and conversion of the quartz containing feldspar and kaolin into the vitreous phase. In contrast, in conventional alumina porcelains, there is always a certain proportion of residual quartz. Since inclusions of quartz form imperfections in the microstructure of the porcelain, the porcelain often fractures at locations where quartz particles are included. Therefore, quartz particles per se are undesirable in the porcelain microstructure. Therefore, complete conversion of the harmful quartz into the vitreous phase leads to a considerable improvement in the mechanical strength of the porcelain. The better microstructural properties make the scatter of the strength parameters narrower. The higher damage tolerance also makes the microstructure more stable in terms of long-term performance, which is particularly important for high-voltage insulators.
The use of clays and kaolins which contain more than 5% by weight of foreign metal oxide inclusions therefore results in an aggressive molten phase, which leads to there being scarcely any quartz particles remaining in the finished porcelain. The content of silicon dioxide is virtually exclus

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