Compositions: ceramic – Ceramic compositions – Refractory
Patent
1996-07-09
1998-02-17
Bell, Mark L.
Compositions: ceramic
Ceramic compositions
Refractory
501 14, 501103, 501106, 106450, C04B 3548, C03C 820
Patent
active
057190917
DESCRIPTION:
BRIEF SUMMARY
This invention relates to the formation of a glaze opacifier from zirconia bearing feedstocks.
In a particular embodiment the present invention provides a process for the production of a glaze opacifier from zircon mineral or other zirconia bearing materials. In a general aspect the process of the invention comprises two basic steps, namely: either as fine particles of zirconia in other phases or into a phase which is readily chemically accessible. increasing the concentration of zirconia, removing deleterious impurities or decomposing chemically accessible phases into new forms which may have greater influence on the opacifying effect.
Additional steps may be employed as will be described below.
It is common when glazing or enamelling items such as tiles, tableware and sanitaryware for an opacifier to be added into the glaze. The opacifier has the effect of completely obscuring from visual observation the surface below the glaze layer. While a number of additives are known to have an opacifying effect the most commonly used materials contain zirconia. It is common for glazes where an opacification effect is desired to have 1-10% by weight of zirconia as determined by chemical analysis.
The main mechanism which is active in imparting opacity to glazes is in the distribution of micron to submicron particles of a material having high refractive index in the glassy matrix of the glaze. This arrangement is excellent for scattering light within the glaze layer, and thereby preventing incident light from reaching the underlying body. Zirconia bearing additives form zircon (zirconium silicate) particles in the glaze, which if properly sized and distributed can have a strong opacifying effect, since zircon has a much higher refractive index than the glassy matrix of the glaze and has very limited solubility in the glassy matrix.
The major aim of techniques for the production of zirconia bearing opacifying additives for use in glazes is the production and distribution within the glaze of micron to submicron sized particles of zircon. Two techniques have been found to be generally effective. In the first zircon is "micronised", i.e. milled to produce a size distribution in which a considerable proportion of the material is in the 0.1 micron to 2 micron particle size range. This zircon is then added by various application methods with other glaze constituents (either in a separate layer or in admixture, or both) to the body to be glazed. The body and the glazing layer are then fired together, during which the glaze fuses to a glassy matrix within which the fine zircon particles are suspended. In the second technique zirconia which has been dissolved in a glass formed at high temperature is chemically precipitated as zircon upon curing of the glass during subsequent firing at lower temperature. Hence incorporation of the glass into glaze formulations can cause precipitation of zircon during firing, resulting in an ultrafine suspension of well distributed zircon particles.
Both of these methods for producing opacity based on zircon suspension in glazes are expensive per unit of zircon having an opacifying effect. In the case of milling to produce micronised zircon the process lacks intensity, owing to the high milling energy requirements for zircon generally, and the fine grinding duty. Further, radioactive elements present in the zircon in low concentrations can represent a health hazard through inhalation in handling of the very finely ground material. Capital charges and operating costs are high. In the case of zircon dissolution into glass the solubility of zircon is low, requiring high dilution with other glass constituents and high glass formation temperatures. Typically such operations are small, also having high capital and operating costs per unit of production. Costs for each of these techniques are sufficiently high that the cost of the zircon which acts as the opacifying agent is usually only a small proportion of the price of opacifier, taken per unit of zircon.
Clearly there is a considerable incentive to
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Hollitt Michael John
Liddy Matthew Jon
McClelland Ross Alexander
Bell Mark L.
Troilo Louis M.
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