GB2162831A - Process for extracting zirconia from dissociated zircon - Google Patents

Abstract

Zirconia is extracted as zirconium sulphate from dissociated zircon by fine grinding of this material followed by reaction at a temperature of 200-350 DEG C with an excess of concentrated sulphuric acid.

Description

SPECIFICATION Process for extracting zirconia from dissociated zircon It is well known to dissociate zircon into zirconia and silica by heating it to a high emperature in an arc, resistance or plasma furnace followed by quenching. During this treatment some silica is lost as vapour, especially in the case of an arc furnace. It is also known that it is possible to recover zirconia from the dissociated zircon by extracting the silica from it by treatment with caustic soda. This procedure suffers from the disadvantage that the zirconia recovered in the residue can contain an unacceptably high content of silica.

Thus a typical sample of dissociated zircon produced in a plasma furnace which we have examined was found buy X-ray fluorescence analysis to have the following composition in parts by weight : ZrO2 + HfO2 68.3% SiO2 31.8% A1203 0.33 CaO 0.02 TiO2 0.26 Fe2Os 0.05 When 50 g of this material was stirred with a 50% by weight solution of caustic soda for about 6 hours at 95"C the residue was found by X-ray fluorescence analysis to contain : ZrO2 + HfO2 98.05% SiO2 1.04 For many industrial uses of zirconia, however, e.g. in the ceramic, electronics and engineering industries, a silica content of less than 0.1% is required.

We have accordingly attempted to separate the constituents of dissociated zircon in a different way, i.e. by leaching the zirconia from the dissociated product and leaving the silica behind in the residue. This has proved to be an extremely difficult task because the dissociated zircon is a glassy product consisting of small pieces of silica in which particles of zirconia are encapsulated.

We attempted to obtain selective leaching of the zirconia from this product by treatment with acids but without success. Only hydrofluoricacid effected extraction of zirconia, but this acid extracted the silica as well.

At last we achieved success when we discovered that by milling the dissociated zircon to a sufficiently small size to expose the zirconia from the surrounding vitreous silica the zirconia could be successfully leached with sulphuric acid, leaving the silica behind.

The present invention provides a process for extracting zirconia from dissociated zircon, which comprises grinding the dissociated zircon to a sufficiently small-size to free the zirconia from the surrounding vitreous silica and heating the ground product at a temperature in the range of 200" - 350"C with an excess of concentrated sulphuric acid to convert the zirconia into zirconium sulphate.

The degree of extraction of zirconia depends upon the degree of fineness to which the dissociated zircon is ground, upon the excess of concentrated sulphuric acid (98%) over the stoichiometric amount required for reaction with the zirconia present, the time allowed for reaction and the reaction temperature.

In our original experiments dissociated zircon of the composition stated in Example 1 below was ground wet to a particle size, as estimated by the optical microscope, of between 1 and 8 microns. On heating under the conditions of Example 1, the amount of zirconia (plus hafnia) taken into solution was 85%. More prolonged grinding, to produce powder with a considerable proportion of particles below 1 micron in diameter and little above 5 microns, again as estimated by microscopy, heated as before, the yield was 96%.

In the above experiments the amount of acid used was 6.7 times the stoichiometric amount. Using the same considerable excess, and the same conditions, the following further data, this time obtained on a different dissociated zircon, of the composition : ZrO2 + HfO2 68.3% SiO2 31.8% Al203 0.33% CaO 0.02% TiO2 0.26% Fe203 0.05% illustrate the benefit of fine grinding. By grinding different portions of this material to different (approximate) mean particle sizes, the yield varied as follows : Average particle size Percentage ZrO2 extracter Unground 0.0 20 microns 36.6 15 microns 56.4 3 micron- 88.6 Laterwork indicated that a higher temperature and a more prolonged heating period enabled a satisfactory extraction to be obtained using a lower stoichiometric excess of acid.

Thus industrially ground material of the following particle size distribution (determined by sedimentation) 80% below 38 microns 60% below 35 microns 40% below 15 microns 20% below 7 microns 10% below 3 microns when heated for three hours at 3000C gave a 90.3% extraction of the contained zirconia using a fourfold excess of acid, and an 85% extraction with a twofold excess. This latter figure was increased to 91.7% by heating for six hours Accordingly it is preferred that the average particle size of the dissociated zircon should not exceed 10 microns, that at least twice the stoichiometric amount of sulphuric acid should be used, that the reaction temperature should be at least 300"C and the reaction time in excess of three hours.

After completion of the reaction, zirconium sulphate can be recovered by evaporating off the sulphuric acid and adding cold water to the resulting paste to produce an acidic solution containing zirconium sulphate and a residue containing untreated zirconia and silica. Zirconium sulphate tetrahydrate can be crystallised selectively out of this solution to separate it from minor amounts of sulphates of other metals extracted by the sulphuric acid.

The process thus produces chemically purezirconium sulphate which can be converted to other zirconium salts or oxide as required. Thus on heating to 11 000C zirconium sulphate tetrahydrate is converted into zirconia of high purity.

The process according to the invention will now be further described with reference to the following Examples Example 1 Zircon dissociated by melting in an arcfurnace and quenching the product in water was wet milled in a conventional laboratory mill underthe following conditions Internallength of mill 100mm Internal diameter of mill 95mm Grinding medium 3/4" high density balls Sample loading 400 grms.

Volume of water 250 cm3 Milling time 24 hours Particle size of starting material 1-4mm Particle size of ground material > 1.0- 8,um (i.e. max. size 8 microns very few if any below 1 micron) The analysis, in proportions by weight, of the starting material carried out by X-Xay fluorescence spectrometrywas as follows ZrO2 + HfO2 84.87% SiO2 13.1% Al203 1.2% CaO 0.03% TiO2 0.14% Fe203 0.29% After drying, the ground dissociated zircon (20grms) was stirred in a closed container fitted with a reflux condenser at 220with 100 cm3 of concentrated sulphuric acid for 3 hours, after which the reflux condenser was removed and the majority of the sulphuric acid evaporated off to leave a paste.When cool 300 cm3 of water was added, the mixture stirred and filtered. Upon evaporation of sufficient water crystals of zirconium sulphate tetrahydrate can be recovered. Of the contained zirconia plus hafnia present in the starting material 85% reacted with sulphuric acid and was extracted into solution. The residue (5.6 grms) was analysed by X-Ray fluorescence spectrometry : ZrO2 + HfO2 43.4% SiO2 46.96% At203 1.27% CaO 0.13% TiO2 0.31% Fe2O3 0.26% The crystals of zirconium sulphate tetrahydrate were analysed by the same technique ZrO2 + HfO2 81.5% - SiO2 0.02% Al203 0.08% CaO 0.04% TiO2 0.00% Fe2O3 0.03% This constituted extraction of 86% by weight of the zirconia in the dissociated zircon.

Example 2 The ground material obtained by milling dissociated zircon as described above was further ground under the same conditions for 24 hours.

Particle size before 2nd grinding - > 1.0 - 8.0 m Particle size after 2nd grinding - < 1.0 - 5.0 pWm (i.e. the powder resulting from the second grinding contained a considerable proportion of particles below 1 Wm in diameter and few with a diameter above 5 Clam). This powder was reacted with concentrated sulphuric acid precisely as described in Example 1.

Of the contained zirconia plus hafnia present in the starting material 96% reacted with the sulphuric acid.

The residue (3.7 grms) was analysed as before ZrO2 + HfO2 8.02% SiO2 71.9% Al203 4.72% CaO 0.23% TiO2 0.31% Fe2O3 0.27% Crystals of zirconium sulphate tetrahydrate obtained from the solution by evaporation of the necessary amount of water analysed as follows Zero2 + HfO2 81.76% SiO2 0.03% Al203 0.04% CaO 0.05% TiO2 0.00% Fe2O3 0.02% While unground dissociated Zircon does not react with hot concentrated sulphuric acid, the process according to the invention permits solubilisation of 96% of the contained zirconia, the crystals of zirconium sulphate obtained containing only 0.02- 0.03% of silica.

Example 3 In this example the starting material was dissociated zircon produced in an electrical resistance furnace with a particle size before grinding of 1 -2mm. It was wet-milled in a similar manner to that described in the first two examples to produce a ground material with most particles in the size range 2 to 7 microns. Its chemical analysis was as follows, expressed in percentage by weight: Zero2 + HfO2 66.4 SiO2 32.9 At203 0.41 CaO 0.05 TiO2 0.20 Fe2O3 0.28 A 20g portion was reacted at 2200C forthree hours with 100 cm3 of concentrated sulphuric acid.The residue, after taking the zirconium sulphate into aqueous solution, analysed as follows : ZrO2 + HfO2 11.0 SiO2 84.0 At203 2.77 CaO 0.15 TiO2 0.20 Fe203 0.10 Example 4 A quantity of a second batch of dissociated zircon that had been dry-ground industrially was characterised by the following particle size distribution and chemical analysis :: Chemical analysis (percentage by weight) ZrO2 + HfO2 76.7 SiO2 21.6 At203 0.48 CaO 0.06 TiO2 0.12 Fe2O3 0.09 Particle size distribution (% undersize by weight) 100m 100 50 90 20 50 10 30 6 18 3 10 2 7 A 1 kg portion was heated with 2500 cm3 of concentrated (98%) sulphuric acid (four times the stoichiometric amount) at 300"C for three hours with stirring, in a flask fitted with a condenser arranged to remove the water produced during the reaction. The zirconium sulphate produced by the reaction was dissolved in water, and the percentage extraction of zirconium assessed by analysing the solution. The extraction had been 90.3%.

Example 5 A 2kg portion of the same ground dissociated zircon as that used in Example 4 was heated with stirring at 300"C with 2500 cm3 of concentrated (98%) sulphuric acid (twice the stoichiometric amount) for four and a half hours, again in such a way as to remove the water of reaction. The extraction of zirconia, assessed by analysing the solution after dissolving the zirconium sulphate in water, was 85.3%.

Example 6 Ground dissociated zircon and sulphuric acid were reacted in the same quantities and in the same manner as in Example 5 exceptthat the period of heating was extended to 6 hours. The proportion of the zirconia taken into solution was increased to 91.7%.

The solutions of zirconium sulphate produced as a result of Examples 4,5 and 6 were combined. A portion of the solution was evaporated in order to concentrate the zirconium sulphate tetrahydrate.

A 3.5kg (dry weight) portion of the crystals was dried and calcined for one -hour at 1200 C, and produced 1kg of zirconia analysing as follows HfO2 2.33 SiO2 0.04 Al203 0.02 CaO 0.01 TiO2 0.04 Fe2O3 0.03 SO3 less than 0.04 In the above Examples the product zirconium sulphate tetrahydrate contains a small proportion of hafnium sulphate, derived from the original mineral zircon, which is difficult to separate but the presence of which is not objectionable. Apart from metallic zirconium used in the nuclear industry it is customary to consider zirconium and hafnium together because the chemical properties of these two elements and their compounds are so similar as to be virtually identical.

Claims (7)

1. A process for extracting zirconia from dissociated zircon, which comprises grinding the dissociated zircon to a sufficiently small size to free the zirconia from the surrounding vitreous silica and heating the ground product at a temperature in the range of 200" - 350"C with an excess of concentrated sulphuric acid to convert the zirconia into zirconium sulphate.

2. A process according to claim 1, in which the average particle size of the ground dissociated zircon does not exceed 10 microns.

3. A process according to claim 1 or claim 2, in which at least twice the stoichiometric amount of sulphuric acid is used.

4. A process according to any one of the precbding claims, in which the reaction temperature is at least 300"C.

5. A process according to claim 1 in which the time of heating is at least three hours.

6. A process according to any one of the preceding claims, which includes the further steps of evaporating off sulphuric acid from the reaction mixture, adding cold water to the resulting paste to produce an acidic solution and crystallising zirconium sulphate tetrahydrate from this solution.

7. A process for extracting zirconia substantialiy as described in any one of the foregoing Examples.