Title of Invention

"A PROCESS FOR THE PREPARATION OF ZIRCON BASED COLOURED CERAMIC PIGMENTS"

Abstract

A process for the preparation of zircon based coloured ceramic pigments . The present invention relates to the preparation of zircon based coloured pigments i.e., vanadium doped blue, iron doped coral and praseodymium doped yellow from plasma dissociated zircon. The new principle relates to the preparation of coloured ceramic pigments by calcining the plasma dissociated zircon which contains very fine zirconia crystallites of nanometric to submicron size in a highly reactive glassy silica matrix instead of a mixture of zirconia and silica with selected group of dopants and mineralizers.

Full Text

This invention relates to a process for the preparation of zircon based coloured ceramic pigments. These pigments find extensive applications as glazes, colourants in ceramic industry. Hitherto known process for preparation of zircon based coloured pigments follow the usual powder calcination route. Zirconia and silica with certain mineralizers, such as some of the alkali or alkaline earth halides in presence of any of the dopants ( V4+, Pr4+ or Fe3+) of predetermined amounts are thoroughly milled in the dry/wet state. The dried raw material mixtures are subjected to high temperature calcination depending on the type of dopants used. The temperature of calcination being 800-1000 °C for preparation of vanadium based blue, while it is 1000-1200 °C for praseodymium based yellow and iron based coral. The time required for calcination of a pigment varies depending on temperature as well as nature of calcining atmosphere. For example, 24 hours is required for vanadium blue a at temperature of 1050°C in oxygen atmosphere while it is 24 to 72 hours at temperature of 1000- 1330°C in hydrogen atmosphere. During calcination the colour is developed by sintering reaction as the metallic oxides are converted into well defined crystal structure. The hitherto known process has following drawbacks: The conventional process for preparation of zircon based coloured ceramic pigments involves use of chemical grade zirconia and silica. It may be mentioned here that zirconia is produced from zircon sand following various chemical unit operations. Also in order to develop desired end effects it is required to mill the mixture of zirconia and silica for size reduction and intimate contact. This process step allows the reactants to have larger surface area to react to form desired crystal structure during calcination. This process step is absolutely essential. It usually requires calcination at a temperature in the range of 1000-1330°C and the duration of 24 to 72 h to impart the proper colour in the product. The main object of the present invention is to prepare zircon based coloured ceramic pigments from a raw material, namely, plasma dissociated zircon, wherein zirconia of nanometric to submicron size is intimately associated with glassy silica phase and hence do not require extensive milling as stated in the preceding paragraph and no separate addition of silica is envisaged. It is also the object of the invention to reduce the calcination temperature as well as the duration of calcination and hence overall energy requirement. The present invention relates to the preparation of zircon based coloured pigments i.e., vanadium doped blue, iron doped coral and praseodymium doped yellow from plasma dissociated zircon. The new principle relates to the preparation of coloured ceramic pigments by calcining the plasma dissociated zircon which contains very fine zirconia crystallites of nanometric to submicron size in a highly reactive glassy silica matrix instead of a mixture of zirconia and silica with s® ected group of dopants and mineralizers. The intimate association of high surface area fine grained zirconia with highly reactive silica phase in plasma dissociated zircon readily reaction sinter with dopants during calcination and subsequently mineralizes to zircon. The presence of fine grained zirconia in reactive silica in plasma dissociated zircon also helps to lower the calcination temperature and the duration of calcination while preparing the coloured ceramic pigments. The plasma dissociated zircon is prepared by thermal dissociation of zircon sand in a plasma reactor (patented, vide application no. 190/DEL/91 dtd.11.3.91) following a patented process (patented vide application no. 188/DEL/91 dtd.11.3.91). By controlling process parameters during plasma dissociation, zirconia crystallites size can be varied in the dissociated zircon. The new result is the preparation of zircon based coloured ceramic pigments from plasma dissociated zircon wherein zirconia crystallites of nanometric to submicron sizes are intimately associated with the glassy and highly reactive silica phase which readily reaction sinter with dopants and mineralizes to zircon during calcination. The new invented process replaces raw materials, namely, zirconia and silica by plasma dissociated zircon. Use of alternate raw material reduces the calcination temperature to 800-1000°C and eliminates extensive milling of raw materials prior to calcination for final colour production. Calcination temperature can be reduced further to 500-800°C by adding some oxidizing agents such as sodium metamolybdate etc., thus reducing the energy consumption. Accordingly, the present invention provides a process for the preparation of zircon based coloured ceramic pigments which comprises mixing plasma dissociated zircon with conventional chromophores and alkali or alkaline halides; optionally in presence of conventional oxidising agent such as herein described followed by wet mixing, drying and calcining at a temperature in the range of 500-1000 °C for duration 6-10 h. According to a feature of the present invention any of the chromophores, i.e., V4+, Pr4+ or Fe3+ bearing compounds is added in the range of 1-20 wt% of plasma dissociated zircon whereas alkali or alkaline earth halides can be selected from sodium, potassium, lithium halides etc. and is added in the range of 1-20 wt% of plasma dissociated zircon. According to another feature of the invented process, the calcination temperature and duration of calcination are lowered. According to yet another feature the invented process can be further made economical by adding some oxidizing agent which releases active oxygen during reaction sintering which in turn reduces the calcination temperature to 500- 800°C. The invention is now described here in detail. Zircon based coloured ceramic pigments are developed from plasma dissociated zircon. Plasma dissociated zircon is prepared in a plasma reactor described in our copending patent application no.190/DEL/91 dtd. 11.3.91 and a process described in the patent application no.188/DEL/91 dtd. 11.3.91, wherein zircon sand particles of narrow size range are fed into the reactor at a controlled rate. During their passage through the hottest zone of plasma, they get dissociated into zirconia and silica followed by a rapid solidification due to ultrafast cooling. This provides the necessary conditions to form very fine zirconia crystallites in glassy, active silica matrix. Dissociated zircon thus formed is mixed with any one of V4+, Pr4+, Fe3+ bearing compounds along with alkali or alkaline earth halides of predetermined amount; the said mixture is wet ground, dried and calcined in the temperature range of 800-1000 °C in ambient and/or oxidizing atmosphere. The preparation of vanadium doped zircon based coloured ceramic pigment is described here as a typical example. The following example is given to illustrate how the invention is carried out in actual practice and should not be construed to limit the scope of the invention. The preparation of vanadium doped zircon based coloured ceramic pigment is carried out in 200 gm batch as basis. Plasma dissociated zircon of percentage dissociation 80-90% is prepared following the process described above. The said product (88% by weight) is then thoroughly wet mixed with ammonium metavanadate (8% by weight) and sodium fluoride (4% by weight) in alcohol medium for duration of half an hour. The said powder was then dried at 110 °C for one hour and calcined in an air atmosphere at a temperature of 900 °C for six hours. During calcination, the blue colour is developed. The said product is then furnace cooled, crushed and washed in water. The powder is then characterized by X-ray diffraction for confirmation of zircon formation and 80-85% conversion is observed. The main advantages of the invention are: i) The invented process uses plasma dissociated zircon which contains a dispersion of zirconia crystallites of nanometric to submicron size in glassy and highly reactive silica matrix instead of mixture of chemical grade zirconia and silica. Use of this raw material avoids an extensive milling step employed in the hitherto known process. ii) High reactivity of plasma dissociated zircon allows lower calcination temperature for reaction sintering with dopants towards formation of zircon based coloured pigments. iii) By increasing oxygen potential during reaction sintering, the calcination temperature can be lowered further and hence the energy consumption. We claim: 1. A process for the preparation of zircon based coloured ceramic pigments which comprises mixing plasma dissociated zircon with conventional chromophores and alkali or alkaline halides; optionally in presence of conventional oxidising agent such as herein described followed by wet mixing, drying and calcining at a temperature in the range of 500-1000 0C for duration 6-10 h. 2. A process as claimed in claim 1 wherein the chromophores V4+, Pr4+ and Fe3+ bearing compounds are added in the range of 1 to 20 wt% of plasma dissociated zircon. 3. A process as claimed in claims 1 and 2 wherein the alkali earth halides are chosen from sodium, potassium, lithium halides and added in the range of 1 to 20 wt% of plasma dissociated zircon. 4. A process for preparation of zircon based coloured ceramic pigments substantially as herein described with reference to the example.

Documents:

335-del-1999-abstract.pdf

335-del-1999-claims.pdf

335-del-1999-correspondence-others.pdf

335-del-1999-correspondence-po.pdf

335-del-1999-description (complete).pdf

335-del-1999-form-1.pdf

335-DEL-1999-Form-19.pdf

335-del-1999-form-2.pdf