FR2641774A1 - PROCESS FOR THE PREPARATION OF LITHIUM ZIRCONATE, AND LITHIUM ZIRCONATE AND INTERMEDIATE PRODUCTS PREPARED THEREOF - Google Patents

Abstract

To prepare lithium zirconate, on the one hand, a clear solution, free from dispersed solid particles, of a water-soluble lithium compound is mixed, and on the other hand, a dispersion of lithium monoxide powder is mixed. zirconium in water, then the mixture obtained is dried until a product in powder form is obtained and this product in powder form is subjected to calcination.

Description

-Process for the preparation of lithium zirconate, and zirconate of

lithium and intermediate products

prepared according to this.

  The present invention relates to a process for the preparation of lithium zirconate, according to which zirconium oxide and a lithium compound are mixed, which is then reacted by means of a

  heat treatment, hereinafter calcination.

  Lithium zirconates and more particularly lithium metazirconate, are likely to be used as culture broth in future

fusion reactors.

  It is obvious that for such applications the

  zirconate must be of high quality and, more particularly

  Slightly, the microhomogeneity of the oxide mixture

  zirconium - lithium compound must be excellent.

  It is known from the publication "Preparation, characterization and properties of Li2ZrO3 fabricated from Li2C03 and ZrO2" by AJ Flipot and others, published by the applicant in connection with the 15th "Symposium on Fusion Technology" in Utrecht in September 1988, lithium carbonate as a lithium compound for

  the preparation of lithium metazirconate.

  In this case, the lithium carbonate is mixed and

  dry zirconium oxide followed by wet molding.

  During the preparation of a superstoichiometric lithium metazirconate (that is to say with a predominant amount of lithium), starting from lithium carbonate, it is discovered, after sintering, the presence of residues of a liquid phase at the granulation limits. This liquid phase, being the result of the presence of low-melting compositions, is very disadvantageous for obtaining a fine grain and for

  the homogeneity of the microstructure.

  -2- In addition, it is impossible to train others

  lithium zirconates, such as Li8ZrO6 and L16Zr207.

according to this known process.

  This can be done if lithium peroxide is used as the lithium compound. It has been discovered that, in this case, the formation of the above-mentioned low-melting compounds and therefore the appearance of the liquid phase during

  sintering of lithium zirconate can be avoided.

  In "The Preparation, characterization and properties of lithium oxide and lithium metazirconate specimens irradiated in HFR Petten in second and third EXOTIC experiments" by P. Kennedy in "Proceedings of the 14th Symposium on Fusion Technlology", Pergamon Press (1986) editors (pages 1013 - 1018), the dry mixing of

  lithium peroxide and zirconium oxide is described.

  In addition, in order to prepare lithium zirconate from such a dry mixture, high calcination temperatures are required. The powdered lithium zirconate obtained by P. Kennedy was difficult to concrete or not at all. This powder could only be transformed into non-deformable components by means of hot pressing, which is of little interest, from the industrial point of view, due to the relatively low manufacturing capacity of this forming technique. The object of the present invention is to eliminate all these disadvantages and to present a process for the preparation of lithium zirconate capable of producing a good quality, fine grain lithium zirconate with a homogeneous structure, from which if necessary, zirconates other than lithium metazirconate can be prepared, and capable of forming a powder-shaped lithium zirconate being

  particularly suitable for sintering.

  -3- For these purposes, on the one hand, a clear solution, free of dispersed solid particles, of a water-soluble lithium compound is prepared, and, on the other hand, a dispersion of oxide powder of zlrconium in water, this solution and this dispersion are mixed, the mixture obtained being dried until a product in the form of powder and this product in the form of

  powder then being subjected to calcination.

  By preparing the circumate according to the wet method, a microhomogeneous and uncontaminated zirconate is obtained which can be produced in the desired form according to

  the usual pressing and sintering techniques.

  The process includes a limited number of steps and the composition of the powdered product before

  calcination is easily reproducible and precise.

  This product has a high degree of purity since little contamination arises due to contact with foreign materials such as containers, mixing devices, etc. and that a molding step

  is not * other, not essential.

  The mixture before calcination has a high microhomogeneity. The lithium compound in solution is homogeneously dispersed in the dispersed solid zirconium oxide. During drying the lithium compound crystallizes homogeneously in, between, on

  and around the zirconium oxide powder particles.

  Without having to use a molding step, a homogeneous mixture is obtained producing a microhomogeneous zirconate

uncontaminated.

  To obtain a clear solution of the lithium compound, the lithium compound cannot exceed a certain solubility limit. Exceeding this limit could produce a solution with remaining solid particles of the lithium compound that does not

  will not produce a microhomogeneous mixture after drying.

  - 4 - A suitable lithium compound for a clear solution is lithium peroxide (hydrate and / or perhydrate). Such a lithium compound makes it possible to maintain the lithium concentration in the clear solution

  below 30 g. of lithium per liter of solution.

  Lithium carbonate, on the other hand, is not suitable since lithium carbonate is poorly soluble

at the water.

  Effectively, the clear solution of lithium peroxide (hydrate and / or perhydrate) is produced in situ by reacting lithium hydroxide or lithium hydroxide monohydrate with an excess of hydrogen peroxide for the formation a viscous and aqueous dispersion followed by sufficient addition of water to bring the lithium concentration below the solubility limit and thus obtain a clear solution. In a preferred applied embodiment of the present invention, the mixture obtained is dried by

spray drying.

  The powder can optionally be subjected to a granulation operation before calcination in order to obtain in this way a product having the appropriate powdery rheological properties (which means here the fluidity of the product necessary for the proper filling of a mold in the case o the product is subjected to a cold pressing treatment for the

  preparation of non-deformable components).

  The lithium zirconate powder or granulate can then be subjected to a sintering treatment under an oxidizing or neutral atmosphere where the sintering takes place at temperatures above 900 ° C. A sintered powder or a sintered granule having a specific gravity of 95% of the theoretical specific gravity can be produced in this way. The invention also relates to lithium zirconate prepared according to the process according to one of the

previous achievements.

  The invention also relates to the intermediate product, that is to say the mixture of zlrconium oxide and of a lithium compound obtained during the process according to one of the preceding embodiments.

  until inclusion of the drying of the mixture.

  Other features and advantages of the invention

  will appear in the following description of a process

  for the preparation of lithium zirconate and an intermediate product according to the invention; this

  description only serves as an example.

  For the preparation of a lithium metazirconate.

  on the one hand, a clear solution of lithium peroxide (hydrate and / or perhydrate) is prepared, and, on the other hand, a dispersion of zirconium oxide in water followed by

  mixing of said clear solution and of said dispersion.

  drying the mixture obtained until a powder-shaped product is obtained and subjecting this powder-shaped product to calcination to make

  react lithium peroxide and zirconium oxide.

  To prepare a clear solution of lithium peroxide (hydrate and / or perhydrate), lithium hydroxide or solid lithium hydroxide monohydrate is reacted with an excess of hydrogen peroxide in order to obtain a viscous dispersion and

aqueous Li2O2. (H202) x. (H20) y.

  A sufficient amount of water is added to this dispersion in order to bring the concentration of Li 2 O 2 (H 2 O 2) x '. (H 2 O) y' (x 'and y' depending on the concentration) below the solubility limit producing a completely homogeneous and clear solution completely free of remaining quantities of dispersed solid material. To this end, every precaution will be taken to maintain the lithium concentration

  below 30 g of lithium per liter.

  -6- For the preparation of the dispersion of zirconium oxide in water, the desired amount of zlrconium oxide is moistened with water followed by the addition of the desired amount of water as a liquid. dispersion Until the desired concentration of zlr: onium oxide particles in water is obtained. In order (to avoid sedimentation of the zirconlum oxide particles, the

  dispersion is slightly agitated.

  The aforementioned clear solution of a lithium compound and the homogeneous dispersion of zirconium oxide are then mixed so that the lithium peroxide (hydrate and / or perhydrate) is dispersed in a completely homogeneous state in a state soluble in the particles of zlrconium oxide in dispersion. In order to avoid sedimentation of the zirconium oxide particles, the

  mixture is stirred continuously.

  The mixture of zirconlum oxide and the lithium compound is then dried by spray drying until a powder-like product is obtained comprising the original particles of zirconium oxide and around, on and between these particles of lithium peroxide (hydrate and / or perhydrate) dispersed and crystallized

  extremely homogeneously on a micro scale.

  During the actual drying, as long as a solution is still resident, it is a question of stabilizing the lithium compound in the solution by the additional addition of hydrogen peroxide, since ZrO2 has a

  catalyzing effect on the decomposition of H202.

  Finally, the powder obtained after drying is subjected to calcination at a temperature between 500 and 900 ° C. under an oxidizing or neutral atmosphere. Zirconium oxide reacts with lithium peroxide to form lithium metazircon according to the following reaction ZrO2 + Li2O2 -> Li2ZrO3 + 1/2 02 -7- The powder obtained can optionally be subjected to a granulation operation before calcination to obtain a product having appropriate powdery rheological properties and more particularly a desired fluidity to achieve good filling of the mold in the case where the product is subjected to a cold pressing treatment for the manufacture of non-deformable components. Since the preparation takes place according to the wet process and the process includes a limited number

  stages, it is interesting from an industrial point of view.

  The composition of the dried mixture being subjected to calcination is easily reproducible and with precision. This mixture has a high degree of purity and is microhomogeneous. Upon drying, the lithium peroxide, which is homogeneously distributed in the solid zirconium oxide dispersed in the solution, will crystallize homogeneously in, on and around the zirconium oxide powder particles. An additional molding step is not necessary to

  It obtains a homogeneous mixture.

  The lithium zirconate powder or granulate obtained after calcination can be subjected to a sintering treatment under an oxidizing or neutral atmosphere at a temperature above 900 ° C. In the case of a sintered granule, the individual granulated particles can reach a density of up to 95% of the theoretical density. The powder or the aggregate can be subjected to a compacting operation after sintering such as cold compression by means of a mechanical, hydraulic or cold isostatic press. The components obtained in this way are then subjected to the aforementioned sintering in order to obtain non-deformable objects of a desired shape with a higher density of

  X and up to 95% of the theoretical density.

  Cold compression and sintering are preferable to hot compression from the point of view of

manufacturing speed.

  The sintered parts obtained have a grain size of less than 2 μm and preferably less than 1 μm and a fully open porosity o

  the pore diameter is less than 0.6 µm.

  The invention will be illustrated in more detail by the following examples:

Example 1

  73.12 g of lithium hydroxide monohydrate from the firm Baker are added to an excess of 150 ml of hydrogen peroxide (30% H 2 O 2 in H 2 O). The viscous dispersion is stirred for 60 minutes. After conversion of the lithium hydroxide monohydrate into lithium peroxide (hydrate and / or perhydrate), the viscous dispersion is diluted with water until a clear solution

  containing 15 g of lithium per liter is obtained.

  We also disperse 105.00 g of zirconium oxide powder, with a specific surface of 25 m2 / g and a density

  0.2 g / cm3 apparent from the Criceram firm in France.

  in water until a concentration of g per liter of zirconium oxide powder particles is obtained. The first solution is added to the zirconium oxide dispersion and the mixture is homogenized by stirring. The mixture is dried by spray drying. During drying, the mixture is stirred to avoid sedimentation of the zlrconium oxide particles in the dispersion. Additional hydrogen peroxide is added at regular intervals to compensate for the decomposed hydrogen peroxide

  catalytically by zirconium oxide.

  After spray drying, a white powder is obtained composed of an intimate mixture of lithium peroxide (hydrate and / or perhydrate) and oxide

lithium.

- 9 -

  After precompaction under a pressure of 20 MPa.

  the compacted powder is granulated into more particles

small than 1 mm.

  This aggregate is calcined at 650-C for 20 hours

  under a dry nitrogen atmosphere.

  Finally, the calcined aggregate is mixed with an appropriate lubricating product and compacted using a hydraulic press in a tungsten carbide mold under a pressure of 100 MPa into pellets with a diameter of 18 mm. The green pastilles thus obtained have a

  density of 44.5% of the theoretical density.

  The green pellets are finally sintered at a temperature of 1014 C for 15 hours in an atmosphere

dry air.

  The pellets finally obtained have a

  density of 80% of the theoretical specific weight.

  According to an X-ray diffraction analysis, it appears that the structure consists of pure lithium metazirconate. The fluorine content of the sintered pellets is less than 5 ppm and the chlorine content is

less than 10 ppm.

  The contamination with metallic impurities is not higher than in the starting materials. The grain size of 1.1 µm is measured using

  S.E.M. The pores are fully open.

Example 2

  A mixture of lithium peroxide and zirconium oxide is prepared in an identical manner to that described in Example 1, but the powder obtained after spray drying is now directly calcined, without

  granulation, during I hour at 750'C.

  The calcined powder is pressed in the form of cylinders by means of an isostatic cold pressing under a pressure of 300 MPa. Green cylinders

  have a density of 54% of the theoretical density.

- 10 -

  The cylinders are sintered under an atmosphere

  dry air at a temperature of 950 ° C for 15 hours.

  The density after frlttage is 73% of the

theoretical density.

Example 3

  73.12 g of lithium hydroxide monohydrate from Baker are added to an excess of 150 ml of peroxide

hydrogen (30% H202 in H20).

  The viscous dispersion is stirred for 60 minutes. After conversion of the lithium hydroxide monohydrate into lithium peroxide, the viscous dispersion is elongated with water until a

  clear solution of 20 g of lithium per liter.

  103.02 g of zirconium monoxide powder with a specific surface of 16 m2 / g and an apparent density of 0.32 g / cm3 from the company Criceram in France are dispersed in water until water is obtained. a concentration of

g of zirconium oxide per liter.

  The first solution is added to the zirconlum oxide dispersion and the mixture obtained is homogenized

by agitation.

  The solution is then dried by spray drying while, during drying, the mixture is stirred to avoid sedimentation of the oxide particles.

of zirconium.

  An excess of hydrogen peroxide is added at regular intervals to compensate for the hydrogen peroxide decomposed by catalysis with zlrconlum oxide. This powder is compacted under a pressure of

  MPa, the compacted powder is broken and granulated.

  The aggregate is then fractionated through a sieve until a fraction is obtained with

  particles with dimensions between 355 µm and 100 µm.

- He -

  1 61 This fraction is then calcined under an atmosphere of dry nitrogen at a temperature of 675 ° C

for 20 hours.

  The calcined granulate is then sintered under an atmosphere of dry air at a temperature of 1000 ° C. for 20 hours. X-ray diffraction analysis shows that

  the sintered aggregate is pure lithium metazirconate.

  After sintering, it appeared that the shape of the original aggregate had been maintained so that the lithium metazirconate in particles is capable of being used in a fluidized bed or for applications comprising

  the transport of particles with fluidized gas.

  The present invention is in no way limited to the embodiments described above and, within the framework of the patent application, several modifications can be applied to the embodiments described, inter alia with regard to the form, the composition and the number of components being used when carrying out the invention. More particularly, the procedure is not essentially limited to the manufacture of metazirconate. Other zirconates can be made

analogously.

  Also, it is not essential to take lithium peroxide as a starting point. So other lithium compounds easily soluble in water, among others, excluding lithium carbonate

  hardly soluble, can be used.

- 12 -

Claims (18)

Claims.   1. Process for the preparation of lithium zirconate according to which zlrconium monoxide and a lithium compound are mixed followed by a reaction between them by a heat treatment, called below calcination, characterized in that, on the one hand, a clear solution, free from dispersed solid particles of a water-soluble lithium compound and, on the other hand, a dispersion of zirconium oxide powder in water are prepared, said solution and said dispersion being mixed, the mixture obtained being dried until a powder-shaped product is obtained and this powder-shaped product is subjected to calcination. 2. Method according to claim 1, characterized in that lithium peroxide (hydrate and / or   perhydrate) is used as a lithium compound.   3. Method according to claim 2, characterized in that the clear solution of lithium peroxide is prepared in situ by the reaction of lithium hydroxide or lithium hydroxide monohydrate with an excess of hydrogen peroxide followed by adding a sufficient amount of water to bring the lithium concentration within the solubility limits to obtain a clear solution.   4. Method according to one of claims 2 and 3,   characterized in that the concentration of lithium in a clear solution is kept below 30 g of lithium per liter.   5. Method according to one of claims 1 to 4.   characterized in that the zirconium oxide dispersion is prepared by, firstly, moistening the zirconium oxide in powder form and adding thereto   then the desired amount of water. - 13 -   6. Method according to one of claims I to 5,   characterized in that the mixture obtained is dried by spray drying.   7. Method according to claim 6, characterized in that the mixture of the clear solution and the dispersion of zirconium oxide with water is established during spray-drying by the addition of hydrogen peroxide.   8. Method according to one of claims I to 7,   characterized in that the dry mixture obtained is calcined at a temperature between 500 and 900'C in an atmosphere oxidizing or neutral.   9. Method according to one of claims I to 8,   characterized in that the calcined powder is additionally sintered. 10. Method according to claim 9, characterized   in that the calcined powder is granulated before sintering.   11. Method according to one of claims I to 8,   characterized in that the mixture obtained is granulated before sintering.   12. Method according to one of claims) to   it, characterized in that the calcined powder or granulate is sintered at a temperature above 900'C.   13. Method according to claims 9 to 11,   characterized in that the calcined and dried powder or aggregate is formed by cold pressing and then sintered to   a temperature above 900 ° C.   14. Lithium zirconate prepared according to one of the previous claims.   15. Lithium zirconate prepared according to the process   according to one of claims 9 to 13, characterized in   that the sintered product obtained has a grain size less than 2 µm.   1 4 16. Lithium zirconate according to claim, characterized in that the sintered product has a   grain size less than 1 µm.   17. Lithium zirconate prepared according to the method according to claim 13, characterized in that the sintered product obtained has a density greater than X of the theoretical density and a fully open porosity in which the diameter of the pores is less at 0.6 µm.   18. Mixture in the form of a zirconium oxide powder and a lithium compound obtained according to the steps of   method according to one of the preceding claims until inclusion of drying.