FR2568240A1 - PROCESS FOR EXTRACTING ZIRCONIA ZIRCON DISSOCIA - Google Patents

Abstract

THIS PROCESS CONSISTS OF EXTRACTING THE ZIRCONIA IN THE FORM OF ZIRCONIUM SULFATE FROM THE DISSOCIATED ZIRCON, BY GRINDING THIS MATERIAL, FOLLOWING A REACTION AT A TEMPERATURE OF 200 TO 350C WITH AN EXCESS OF CONCENTRATED SULFURIC ACID.

Description

It is well known to dissociate zircon zirconia and silica by

  heating at high temperature in an arc furnace, resistor or plasma, followed by rapid cooling. During this treatment, some of the silica is lost in the vapor state, particularly in the case of the arc furnace. It is also known that it is possible

  to recover the zirconia of the dissociated zircon by extracting the

  by treatment with caustic soda. This process

  has the disadvantage that the recovered zirconia

  lost in the residue may contain an unacceptable

high in silica.

  Thus, a typical dissociated zircon sample produced in a plasma furnace which has been examined by the Applicant has been found, by X-ray fluorescence analysis, to have the following composition, in parts by weight: ZrO 2 + HfO 2 68.3% SiO2 31.8%

A1203 0.33

  CaO 0.02 TiO 2 0.26 Fe 2 O 0.05 After stirring 50 g of this material with a solution of caustic soda at 50% by weight for about 6 hours at 95 ° C., it was observed, by X-ray fluorescence analysis, that the residue contained: ZrO2 + HfO2 98.05% SiO2 1.04 However, for many industrial uses of zirconia, for example, in the ceramic, electronic and engineering industries, a silica content is required

less than 0.1%.

  359 The plaintiff therefore attempted to separate the

  256e240 killing zircon dissociated in a different way, ie, separating the zirconia from the dissociated product by leaching and leaving the silica in the residue. This has proved to be extremely difficult work because the dissociated zircon is a vitreous product composed of small pieces of silica in which zirconia particles are coated. The plaintiff tried to obtain an extraction

  selective zirconia of this product by leaching by treatment.

  by acids but without success. Only fluorhy-

  drique managed to extract the zirconia but this extra acid

was also silica.

  Finally, the plaintiff came to a result when she discovered that if we grind zircon

  dissociated to a grain size sufficiently small to

  the zirconia of the vitreous silica which surrounds it, the zirconia

  could be successfully leached by means of

  sulfuric acid, leaving the silica.

  The present invention relates to a process for extracting zirconia from dissociated zircon, which consists in grinding the dissociated zircon to a grain size sufficiently small to release the zirconia from the vitreous silica which coats it and to heat the ground product to a temperature range of 200 R 350 C with an excess of concentrated sulfuric acids to

  convert zirconia to zirconium sulphate.

  The degree of extraction of the zirconia depends on the degree of fineness to which the dissociated zircon is ground, the excess of acid

  concentrated sulfuric acid (98%) in relation to the stoichiometric

  metric required to react with the zirconia present, the time allotted to the reaction and the temperature of the

the reaction.

  In original experiments of the Applicant, zircon having the composition indicated in Example 1 below was milled by a one-dimensional wet process.

  between 1 and 8 microns, according to an estimate of

  under the light microscope. By heating, under the conditions of Example 1, a 256824 amount of zirconia (and hafnium oxide) of 85% was dissolved. With a longer grinding, to produce a powder containing a considerable proportion of particles of less than 1 micron

  diameter and few particles larger than 5 microns, these proportions

  Here, also, microscopy was used, and with heating as above, 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 data, which were obtained this time on the different dissociated zircon having the following composition: ZrO 2 + HfO 2 68.3% SiO 2 31.8% Al 2 3 0.33% 2 3 CaO 0.02% TiO2 0.26% Fe203 0.05%

  illustrate the advantage of fine grinding. If you crushed

  For the different charges of this material to different average particle sizes (approximate), the yield varied as follows:

  Average particle size Percentage of ZrO2 extracted.

  Not milled 0.0 microns 36.3 15 microns 56.4 3 microns 88.6

  -Subsequent experience has shown that a temperature

  higher temperature and a longer heating period resulted in satisfactory extraction by

  a lower stoichiometric excess of acid.

  Thus an industrially milled material having the following particle size distribution (determined by sedimentation): 80% below 38 microns% below 35 microns

% "15"

"7"

% "3"

  and heated for 3 hours at 300 ° C., provided 90.3% extraction of the zirconia contained using a fourfold excess of acid, and 85% extraction with a double excess. This last figure could be increased to 91.7% in

heating for six hours.

  It is therefore preferable that the average particle size of the dissociated zirconia is not greater than microns, it is preferable to use the sulfuric acid in an amount of at least twice the stoichiometric amount, and that the reaction temperature at least 300 C and the

  reaction time of more than 3 hours.

  After completion of the reaction, the zirconium sulfate can be recovered by removing the sulfuric acid by evaporation and adding cold water to the resulting paste.

  to produce an acidic solution containing zirconium sulphate

  conium and a residue that contains untreated zirconia and silicon. Zirconium sulfate tetrahydrate can be isolated

  of this solution, by selective crystallization, for the separation

  small amounts of sulphates from other metals that are

  also extracted by sulphuric acid.

  The process thus produces chemically pure zirconium sulfate which can be converted into other zirconium salts and zirconium oxide as needed. Thus, by heating the zirconium sulfate tetrahydrate to

  1100 C, it is converted into high purity zirconia.

  The process according to the invention will now be described with reference to the following examples:

EXAMPLE 1

  Melt-dissociated zircon in an arc furnace and by quenching the product in water was wet-milled in a conventional laboratory mill under the following conditions: Inner mill length 100 mm Inner mill diameter 95 mm Agents Grinding media 19 mm high density beads Charge per sample 400 g Water volume 250 cm3 Grinding time 24 hours Particle size of starting material 1 to 4 mm Particle size of crushed material> 1.0 to 8pum ( that is to say, maximum size 8 microns, very little or no particles of less than 1 micron) The analysis, in terms of weight, of the starting material, carried out by X-ray fluorescence spectrometry was as follows : ZrO2 + HfO2 84.87 SiO2 13.1

A1203 1.2%

  CaO 0.03% TiO 2 0.14% Fe 2 0.29% After drying, the ground dissociated zircon (20 g) was stirred in a closed vessel equipped with a reflux condenser, at 220 ° C., with 100 cm 3 of concentrated sulfuric acid for three hours, after which the reflux condenser was removed

  and most of the sulfuric acid was removed

  only by evaporation to leave a paste. When this paste has cooled, 300 cm3 of water have been added and the mixture has been stirred and filtered. After evaporation of a sufficient amount of water, crystals of zirconium sulfate tetrahydrate were recovered. On the amount of zirconia and hafnium oxide present in the starting material, a proportion of 85

  reacted with sulfuric acid and was extracted into the

  tion. The residue (6.6 g) was analyzed by X-ray fluorescence spectrometry: ZrO 2 + HfO 2 43.4% SiO 2 46.96%

A1203 1.27%

  CaO 0.13% TiO2, 0.31% Fe203 0.26% The crystals of zirconium sulfate tetrahydrate were analyzed by the same technique: ZrO 2 + HfO 2 81.5% SiO 2 0.02%

A1203 0.08%

CaO 0.04% TiO2 0.00% Fe203 0.03

  This represented an extraction of 86 t from the weight of the zirconium

  contained in dissociated zircon.

EXAMPLE 2

  The ground material obtained by grinding the dissociated zircon as described above was subjected to a new

  grinding under the same conditions for 24 hours.

  Particle size before 2nd milling:> 1.0 to 8.0JUm Particle size after 2nd milling: <1.0 to 5.0μm

  (ie the powder resulting from the second grinding

  a considerable proportion of particles less than 1 μm in diameter and a small number of particles of

  more than 5pm). This powder was reacted with the acid

  concentrated sulfuric acid as described in Example 1.

  Of the zirconia and hafnium oxide contained in the starting material, a proportion of 96 l reacted with

1, sulfuric acid.

  The residue (3.7 g) was analyzed as before: ZrO 2 + HfO 2 8.02% SiO 2 71.9%

A1203 4.72%

  CaO 0.23% TiO 2 0.31% Fe 2 O 3 o, 27% The crystals of zirconium sulfate tetrahydrate obtained from the solution by evaporation of the amount of water required had the following analysis: ZrO 2 + HfO 2 81.76% SiO2 0.03%

A1203 0.04%

  CaO 0.05% TiO2 0.00% Fe203 0.02% While unmilled dissociated zircon does not react with

  concentrated hot sulfuric acid, the process according to the invention.

  This solution makes it possible to solubilize 96% of the zirconia contained, the zirconium sulphate crystals obtained containing only

0.02 to 0.03% silica.

EXAMPLE 3

  In this example, the starting material was dissociated zircon produced in a resistance electric furnace,

  with a particle size before grinding of 1 to 2 mm.

  This material was wet milled in the same manner as described in the first two examples to produce a milled material in which most of the particles were in the range of 2 to 7 microns in size. Its chemical analysis was as follows, expressed as a percentage by weight: ZrO 2 + HfO 2 66.4 SiO 2 32.9 Al 0 0.41 A 123 CaO 0.05 TiO 2 0.20 Fe 0, 2 v 2 3 0.28 react a charge of 20 g at 220 ° C. for 3 hours with 100 cm 3 of concentrated sulfuric acid. The residue, after the recovery of zirconium sulphate in an aqueous solution, corresponded to the following analysis: ZrO 2 + HfO 2 11.0 SiO 2 84.0

A1203 2.77

CaO 0.15 TiO2 0.20 Ti2 Fe203 0.10

EXAMPLE 4

  An amount of a second batch of dissociated zircon that had been dry milled industrially was characterized by

  the following particle size distribution and by chemical analysis

  chemical analysis (percentages by weight) ZrO2 + HfO2 76.7

S O 2 21.6

A1203 0.48

CaO 0.06 TiO2 0.12 Fe203 0.09

  Particle size distribution (t in weight of the pas-

health) O1OGm 100

90

50

30

10 30

6 18

3 10

2 7

  A load of 1 kg was heated with 2500 cm3 of sulfuric acid.

  than concentrated (98%) (four times the stoichiometric amount) at 300 C for three hours, with stirring, in a flask equipped with a condenser arranged to remove the water produced during the reaction. Zirconium sulphate produces rare

  The reaction was dissolved in the water and the percentage of extractions

  zirconium was evaluated by solution analysis.

The extraction was 90.3%.

EXAMPLE 5

  A load of 2 kg of the same zircon

  milled compound as used in Example 4, with stirring, at 300 C with 2500 cm3 of concentrated sulfuric acid (98%) (twice the stoichiometric amount) for four hours

  and half, here also to eliminate the water of reaction.

  The extraction of zirconia, evaluated by the analysis of the solu-

  after dissolution of zirconium sulphate in water,

was 85.3%.

EXAMPLE 6

  Ground dissociated zirconia and sulfuric acid were reacted in the same amounts and in the same manner as in Example 5 except that the heating period was increased to 6 hours. The proportion of zirconia entering solution

has been increased to 91.7%.

  The zirconium sulfate solutions produced in Examples 4, 5 and 6 were combined. Part of the solution was evaporated to concentrate the zirconium sulfate.

tetrahydrate.

  A charge of 3.5 kg (dry weight) of the crystals was dried and calcined for one hour at 1200 ° C. and i kg of zirconia was produced with the following analysis: HfO 2 2.33 SiO 2 0.04

A1203 0.02

CaO 0.01 TiO2 0.04

F203 0.03

  S03 less than 0.04 10 In the above examples, zirconium sulfate tetrahydrate contains a small proportion of hafnium sulphate, derived from the original mineral zircon, which is difficult to separate but whose presence is not detrimental. . Apart from the metallic zirconium used for the nuclear industry, it is usual to consider zirconium and hafnium jointly, because the chemical properties of these two elements and their compounds are so similar.

  that we consider them virtually identical.

R E V E ND I C AT I 0 N S

  1. Process for extracting zirconia from dissociated zircon, characterized in that the dissociated zircon is ground to a sufficiently small particle size to release the zirconia from the vitreous coating silica and the ground product is heated to a temperature of range from 200 to

  350 C with an excess of concentrated sulfuric acid to convert

  shooting the zirconia in zirconium sulphate.

  2. Method according to claim 1, characterized in that the average particle size of zircon

  dissociated ground does not exceed 10 microns.

  3. Method according to one of claims 1 and 2,

  characterized in that a quantity of sulfuric acid is used

  that at least equal to twice the stoichiometric amount.

  4. Method according to any one of the claims

  1 to 3, characterized in that the reaction temperature is at least 300 ° C. 5. Process according to claim 1, characterized

  in that the heating time is at least three hours.

  6. Process according to any one of the claims

  preceding, characterized by the complementary phases

  both to remove the sulfuric acid from the reaction mixture by evaporation, to add cold water to the resulting paste to produce an acidic solution and to crystallize the sulfate

  of zirconium tetrahydrate from this solution.