NL8502128A - PROCESS FOR EXTRACTING ZIRCONIUM DIOXIDE FROM DISSOCIATED ZIRCONY. - Google Patents

Description

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The invention relates to a process for extracting zirconium dioxide from dissociated zircon.

It is well known to dissociate zircon into zirconia and silica by heating the raw material to high temperature in an arc, resistance or plasma oven followed by quenching. During this treatment, a certain amount of silica umdi oxide is lost as a vapor, especially in the case of a blast 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 sodium hydroxide (caustic soda).

This method has the drawback that the zirconium oxide recovered from the residue may contain an unacceptably high content of silicon dioxide.

For example, examination by the applicant of a typical plasma furnace sample prepared in a plasma oven revealed that the sample has the following composition by X-ray fluorescence analysis:

ZrOg + HfÖ2 68.3 wt%

SiO 2 31.8 wt% 25 A12O3 0.33 wt%

CaO 0.02 wt%

TiO2 0.26 wt% Ρβ2θ3 0.05 wt%

After 50 g of this material for about 6 hours at 95 ° C

was stirred with a 50 wt% solution of sodium hydroxide, the residue was found by X-ray fluorescence analysis to contain: 35 ZK> 2 + HfO 2 98.05 wt%

SiO2 1.04 wt%

However, many industrial applications of zirconia, e.g., in the ceramic, electronics, and construction industries, require a 40 silica content of less than 0.1% by weight.

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Therefore, attempts were made to separate the dissociated zircon components in other ways, e.g. by leaching the zirconia from the dissociated product and leaving the silica in the residue. This proved to be an extremely difficult task because the dissociated zircon is a glassy product consisting of small pieces of silicon dioxide encapsulating zirconia particles.

Therefore, the applicant has attempted to obtain a selective leaching of the zirconium dioxide from this product by treatment with 10 acids, but this has not been successful. Extraction of zirconia was accomplished only with hydrogen fluoride, but this acid also extracted the silica.

It has now been found that by grinding the dissociated zircon to a particle size sufficiently small to liberate the zirconium dioxide from the enveloping vitreous silicon dioxide, the zirconium dioxide could be successfully leached out with sulfuric acid, leaving the silica.

According to the present invention there is provided a method of extracting zirconia from dissociated zircon, the method being characterized by grinding the dissociated zircon to a particle size sufficient to expose the zirconia from the enveloping vitreous silica. and the milled product is heated at a temperature in the range of 200-350 ° C with an excess of concentrated sulfuric acid to convert the zirconium dioxide into zirconium sulfate.

The degree of extraction of zirconium dioxide depends on the degree of fineness, to which the dissociated zirconium is ground, from the excess of the concentrated sulfuric acid (98 wt%) relative to the stoichiometric amount required for the conversion with the zirconium dioxide present, the reaction time used and the reaction temperature.

In applicant's original experiments, dissociated zirconia with the composition shown in Example I below was ground wet to a particle size between 1 and 35 µm, measured with an optical microscope. When heated under the conditions of Example 1, the amount of zirconium dioxide (plus hafnium dioxide) contained in the solution was 85% by weight. At Longer grinding, in which powder with a substantial amount of particles with a diameter of less than 1 µm and a small amount of 40 particles with a diameter of more than 5 µm was obtained, in which case again 0502 1 2 8.

f *. Λ * 3 was measured with a microscope, the yield after heating was 96¾ in the above manner.

In the experiments described above, the amount of acid was 6.7 times the stoichiometric amount. Using the same excess and the same conditions, the following values were obtained, this time using another dissociated zircon of the composition:

ZrO + HfU2 68.3 wt% 10 SiOg 31.8 wt% AI2O3 0.33 wt%

CaO 0.02 wt%

TiO2 0.26 wt%

Fe2Ü3 0.05 wt% 15 and with the help of this composition the benefit of fine grinding is illustrated. By grinding different portions of this material into different (approximate) average particle sizes, the yield varied as follows: 20

Average particle size Extracted ZrÜ2 (wt%) unground 0.0 20 / um 36.6 15 / um 56.4 25 3 / um 88.6

Later research indicated that by using a higher temperature and longer heating time, a satisfactory extraction can be obtained using a lower stoichiometric excess of the acid.

For example, milled material with the following particle size distribution (determined by sedimentation) gave on an industrial scale 80 wt.% Less than 38 / um 35 60 wt.% Less than 35 / um 40 wt.% Less than 15 / um 20 wt.% Less than 7 / um 10 wt.% Less than 3 / um 3502 12S · 4 after heating for three hours at 300 ° C an extraction of 90.3 wt.% Of the zirconium dioxide present, yielding a fourfold excess of the acid and an extraction of 85% by weight when using a two-fold excess. The latter value was increased to 91.7% by weight by heating for six hours.

Therefore, it is preferred that the average particle size of the dissociated zircon is not greater than 10 µm, that at least twice the stoichiometric amount of sulfuric acid should be used, that the reaction temperature should be at least 300 ° C, and that the response time should be more than three hours.

After the completion of the reaction, the zirconium sulfate can be recovered by evaporating the sulfuric acid and adding cold water to the formed paste to obtain an acidic zirconium sulfate-containing solution and an unconverted zirconium and silica containing residue. Zirconium sulfate tetrahydrate can be selectively crystallized from this solution, separating it from minor amounts of sulfates from other metals extracted by the sulfuric acid.

This process provides chemically pure zirconium sulfate, which can be converted to other zirconium salts or zirconium dipoxide if desired. For example, zirconium sulfate tetrahydrate is converted into high purity zirconium dioxide by heating at 1100 ° C.

The following examples further illustrate the invention.

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EXAMPLE I

Zircon, which was dissociated by melting in an arc furnace and quenching the product in water, was ground wet in a conventional laboratory mill under the following conditions:

Internal length of the mill 100 mm

Internal diameter of the grinder 95 mm 10 1.25 mm high density grinding balls

Quantity of the sample used 400 g

Amount of water 250 cm3

Meal 24 hours

Particle size of the starting material 1-4 mm

Particle size of the milled material ^ 1.0-8 µm (i.e. maximum size 8 µm-little or no material smaller than 1 µm) 20 '

The analysis of the starting material by X-ray fluorescence spectrometry was as follows:

ZrO 2 + HfO 2 84.87 wt% ^ SiO 2 13.1 wt%

Al203 1.2 wt%

CaO 0.03 wt%

TiO2 0.14 wt%

Fe2 O3 0.29 wt% 30

After drying, the ground, dissociated zirconium was refluxed at 220 ° C for 3 hours in a closed vessel equipped with reflux condenser with 100 ml of concentrated sulfuric acid, after which the reflux was removed coolly and the bulk of the Sulfuric acid was evaporated to leave a paste. Then 300 ml of cold water were added, and the mixture was stirred and filtered. After evaporation of a sufficient amount of water, zirconium sulfate tetrahydrate crystals could be recovered. The zirconia plus hafnium dioxide contained in the starting material is 3502 i.

6, 85 wt.% With sulfuric acid and this material was taken up into the solution by extraction. The residue (5.6 g) was analyzed by X-ray fluorescence spectrometry: * 5 ZrÜ2 + HfÜ2 43.4 wt%

SiO2 46.96% by weight

Al 2Ο3 1.27 wt%

CaO 0.13 wt%

Ti O2 0.31 wt.% Fe2Ü3 0.26 wt.%

The zirconium sulfate tetrahydrate crystals were analyzed by the same technique: ZrÜ2 + HfÜ2 81.5 wt%

SiO2 0.02 wt%

Al2Ο3 0.08 wt%

CaO 0.04 wt%

TiO2 "0.00 wt% 20 Fe2 O3 0.03 wt%

This means an extraction of 86% by weight of the zirconium dioxide contained in the dissociated zircon.

EXAMPLE II

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

Particle size for the second time grinding -> 1.0 - 8.0 / um 30 Particle size after the second time grinding - Ό, Ο - 5.0 / um (ie the powder obtained after the second time grinding significant amount of particles less than 1 µm in diameter and containing a small amount of particles more than 5 µm in diameter). This powder was reacted with concentrated sulfuric acid in exactly the same manner as in Example I.

96% by weight of the zirconium dioxide plus hafium dioxide contained in the starting material reacted with the sulfuric acid.

The residue (3.7 g) was analyzed as described above: 8502126

Si 7

Zr02 + HfO £ 8.02 wt%

SiO 2 71.9 wt.% Al 2 O 3 4.72 wt.% 5 CaO 0.23 wt.% HO2 0.31 wt.%

Fe2Ü3 0.27 wt%

Zirconium sulfate tetrahydrate crystals obtained from the solution by evaporation of the necessary amount of water were analyzed as follows:

ZrO + HfO2 81.76 wt%

SiO2 0.03 wt% 15 A12O3 0.04 wt%

CaO 0.05 wt%

TiO2 0.00 wt%

Fe2 O3 0.02 wt.% Although unground dissociated zirconium does not react with hot concentrated sulfuric acid, it is possible according to the process of the invention to dissolve 96 wt.% Of the zirconium oxide present, whereby the crystals of the zirconium sulfate obtained contain only 0.02-0.03 wt.% silicon dioxide.

25

EXAMPLE III

In this example, the starting material was dissociated zircon prepared in an electric resistance furnace with a particle size of 1-2 mm before grinding. It was ground wet in the same manner as in the two previous examples to obtain a ground material, most of the particles having a size in the range of 2-7 µm. Its chemical analysis was as follows:

ZrÜ3 + HfÜ2 66.4 wt% 35 SiO2 32.9 wt% A12Ü3 0.41 wt%

CaO 0.05 wt%

TiO2 0.20 wt%

FegOs 0.28 wt% Λ Λ ..

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A 20 g portion was reacted with 100 ml of concentrated sulfuric acid at 220 ° C for three hours. The residue after analysis of the zirconium sulfate in an aqueous solution was analyzed as follows: # 5 ZK> 2 + HfO 2 11.0 wt%

Si O2 84.0 wt% AI2O3 2.77 wt%

CaO 0.15 wt% O2 0.20 wt% 10 Fe2 O3 0.10 wt%

EXAMPLE IV

An amount of a second batch of dissociated zircon, which had been milled on an industrial scale in the dry state, was characterized by the following particle size distribution and chemical analysis:

Chemical analysis

ZrÜ2 + HfÜ2 76.7 wt%

SiO2 21.6 wt% 20 Al2Ο3 0.48 wt%

CaO 0.06 wt%

TiO2 0.12 wt%

Fe2 O3 0.09 wt% 25

Particle size distribution (wt%, lower limit) 100 µm 100 50 90 20 50 30 10 30 6 18 3 10 2 7 35 A 1 kg portion was heated with stirring at 2500 ° C for 3 hours at 300 ° C (98 wt. percent sulfuric acid (four times the stoichiometric amount) in a flask equipped with a cooler to remove the water formed during the reaction. The zirconium sulfate formed by the reaction was dissolved at 40 in water and the extract percentage of zirconium dioxide became 8502 '.

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9 determined by analyzing the solution. The extraction was 90.3% by weight.

EXAMPLE V

A 2 kg portion of the same ground, dissociated zir-5 koon as used in Example IV was heated with stirring with 2500 ml of concentrated (98% by weight) sulfuric acid (twice the stoichiometrically) at 300 ° C for four and a half hours. required amount), again operating in such a way that the reaction water was removed. The extraction of zirconium dioxide was 85.3% by weight, which was determined by analyzing the solution after dissolving the zirconium sulfate in water.

EXAMPLE VI

Ground dissociated zircon and sulfuric acid were converted in the same amounts and in the same manner as in Example V, except that the heating time was extended to six hours. The amount of the zirconium dioxide contained in the solution was increased to 91.7% by weight.

The solutions of zirconium sulfate obtained according to Examples IV, V and VI were combined. Part of the solution was evaporated to concentrate the zirconium sulfate tetrahydrate.

A 3.5 * kg (dry weight) portion of the crystals was taken. dried and calcined at 1200 ° C for one hour to obtain 1 kg zirconium dioxide with the following analysis: 25 HfOg 2.33 wt%

SiO2 0.04 wt% A12O3 0.02 wt%

CaO 0.01 wt%

TiO2 0.04 wt% 30 Fe2 O3 0.03 wt% SO3 less den 0.04 wt%

In the examples described above, the product zirconium sulfate tetrahydrate contains a small amount of hafnium sulfate from the original mineral zircon, which is difficult to separate but the presence of which is not objectionable. Apart from metallic zirconium, which is used in the nuclear industry, it is common to consider zirconium and hafnium as one because the chemical properties of these two elements and their compounds are so similar that they are nearly identical.

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Claims (6)

Process for extracting zirconia from dissociated zircon, characterized in that the dissociated zircon is milled to a particle size sufficient to release the zirconia from the enveloping glassy silica, and the ground product at a temperature heated in the range of 200-350 ° C with an excess of concentrated sulfuric acid to convert the zirconium dioxide into zirconium sulfate. 2. A method according to claim 1, characterized in that the mean particle size of the ground dissociated zirconium does not exceed 10 µm. Process according to claim 1 or 2, characterized in that at least twice the amount of sulfuric acid required stoichiometrically is used. Process according to one or more of the preceding claims, characterized in that the reaction temperature is at least 300 ° C. Method according to one or more of the preceding claims, characterized in that the heating time is at least three hours. 6. Process according to one or more of the preceding claims, characterized in that the following steps are further carried out: evaporation of sulfuric acid from the reaction mixture, adding cold water to the formed paste to form an acid solution and crystallizing zirconium sulphate tetrahydrate from this solution. ********** S3 © 2 128