FR2614213A1 - PROCESS FOR THE PREPARATION OF ULTRA-HIGH PURITY METAL INORGANIC COMPOUNDS - Google Patents

Abstract

THE INVENTION RELATES TO A PROCESS FOR EXTREMELY IMPROVING THE PURITY OF AN INORGANIC SALT SOLUBLE IN WATER OF A METAL CHOSEN FROM THE GROUP CONSISTING OF ALKALINE METALS, ALKALINE EARTH METALS, ALUMINUM, YTTRIUM, ZIRCONIUM, INDIUM, LANTHANE, LANTHANE LANTHANIDE METALS. ACCORDING TO THE INVENTION, THE PROCESS INCLUDES THE STEPS OF: PREPARING AN AQUEOUS SOLUTION INCLUDING THE INORGANIC SALT SOLUBLE IN WATER OF THE SELECTED METAL, ADDING A CHELINATOR, WHICH IS CHOSEN FROM THE GROUP CONSIST OF DIETHYLDITHIOCARAMIDITHIUM-PYROLITHIOCARAMIDIUM-DYROLITHIOCRAMIDITHATE-DYROLITHIOCRAMIDITHATE-DYROLITHIOCRAMIDITHATE SOLUBLE IN WATER, IN THE SAID AQUEOUS SOLUTION, WHILE MAINTAINING THE PH OF THE SAID SOLUTION BETWEEN 1 AND 11, THE QUANTITY OF SAID CHELING AGENT BEING AT LEAST 0.005 BY WEIGHT OF SAID METAL SALT; AND SEPARATE A PRECIPITE FORMED BY THE ADDITION OF THE SAID CHELTING AGENT, OF THE SAID SOLUTION. THE INVENTION APPLIES IN PARTICULAR TO OPTICAL, ELECTRONIC AND OPTO-ELECTRONIC MATERIALS.

Description

The present invention relates to a process for obtaining salts of

  ultra-high purity water-soluble metal, which can be converted into insoluble or barely soluble metal compounds such as oxides or fluorides of ultra-high purity by simple treatments. The metal compounds obtained using this process have extremely low levels of transition metal impurities and include those very suitable for use with optical materials,

  electronic and opto-electronic.

  Recent developments in the optical and optoelectronics industry, including lasers and fiber optics, are remarkable and are still expanding. With the development of increasingly sophisticated devices, there is a trend toward increasingly severe conditions regarding the levels of impurities in chemical compounds for use in materials.

  optical, electronic and opto-electronic.

  For example, recent attention has been focused on fluoride glasses which are excellent in transmittance in the infrared region and, therefore, can be expected to be superior materials for future optical communications. For the extreme reduction of the transmission loss of optical fibers using fluoride glasses, it is important to develop a new technique to extremely improve the purity of the fluorides to be used as raw materials for fluoride glasses. In particular, the existence of iron ions or other impurities of a transition metal constitutes a serious obstacle to success in the extreme reduction of transmission loss of fluoride glass optical fibers. Since metal fluorides prepared by conventional processes may contain relatively large amounts of unwanted transition metals, efforts have been devoted to the preparation of metal fluorides having very low levels of

  impurities of transition metals.

  For example, JP-A 57-51146 shows a process for the preparation of a metal fluoride, where the content of metallic impurities is of the order of 10 ppm, by vapor phase fluorination of a sublimable or evaporable raw material. . This process has the merit that a very high purity metal fluoride can be obtained by a single-stage reaction without requiring additional refining operations such as recrystallization. However, in this process, it is essential to use a sublimable or evaporable compound as a raw material although such a compound is usually expensive and extreme care must be taken to prevent contamination of the resulting product by the action of the product. hydrogen fluoride gas, which is usually used as a fluorinating agent, on the materials of the apparatus. In addition to these disadvantages, this process does not seem to give full satisfaction as to the quality of the products because, according to the

  examples in the published description, the purities of

  fluorides obtained are not greater than the level of

five to nine.

  Moreover, solid state lasers are widely used in combination with optical fibers for precision work of various

  materials and also for medical treatments.

  As the materials of the solid state lasers, metal oxides such as yttrium oxide, neodymium oxide, and aluminum oxide with a purity of more than five-nine are in great demand. In addition, as raw materials for piezoelectric and pyroelectric materials, certain metal compounds such as lithium carbonate, zirconium oxide and

  lanthanum of similar high purity are required.

  However, as far as is known, no industrially usable technique has yet been developed for a very significant reduction in the contents of transition metal impurities in the aforementioned metal compounds or other metal compounds for similar uses. at the level of nine-nine or

parts per billion.

  The present invention has as its fundamental objective a relatively simple process for the easy preparation of inorganic metal compounds, mainly oxides and fluorides, of an ultra high

  purity, like a purity of nine-nine.

  In a different aspect, the subject of the present invention is a relatively simple method for easily and extensively improving the purity of metal water soluble salts in order to convert the purified metal salts to other kinds of metal like

  oxides and fluorides, ultra-high purity.

  As a basic part of the present invention, there is provided a method for extremely improving the purity of a water-soluble salt of a metal selected from alkali metals, alkaline earth metals, aluminum, yttrium, and the like. , zirconium, indium, lanthanide and lanthanide metals, the process comprising the steps of preparing an aqueous solution comprising a water-soluble salt of the selected metal, adding a chelating agent, which is either a diethyldithiocarbamate (abbreviated as DDTC) soluble in water or ammonium pyrrolidinedithiocarbamate (abbreviated as APDC) to the aqueous solution while maintaining the pH of the solution between 1 and 11, the amount of the chelating agent being at least 0.005% by weight of the water-soluble metal salt, and separating a precipitate produced by the addition of the agent

chelator, solution.

  By this process, it is easy to reduce the transition metal impurities contained in the treated metal salt to less than 1/10 of the initial amounts and therefore it is easy to obtain a metal salt with a purity of nine. nine or, in some cases, still

higher.

  Success in such remarkable purification by simple operation can be attributed to the use of a specific chelating agent, DDTC or APDC, for the precipitation of metals as impurities as coordinating compounds. Both DDTC and APDC have high stability constants for forming coordination compounds with various transition metals and these chelating agents can coordinate simultaneously with many types of metals.

  transition compounds such as Fe, Co, Ni, Cr, Cu and / or Mn.

  Different types of chelating agents such as dimethyl-

  glyoxime, cupferone and dithizone are used in conventional purification processes. Each of these chelating agents becomes stable by selectively coordinating with a type of a transition metal or only a few types of transition metals. Therefore, in conventional processes, it is necessary to use

  together several types of chelating agents.

  The above-mentioned method of purification according to the invention gives an aqueous solution containing few transition metals as impurities. From this solution, an ultra-high purity metal salt can easily be recovered by recrystallization or any other operation. The present invention relates to the various treatments of the water-soluble metal salt

26 14,213

  highly purified, in the form of an aqueous solution or after separation of water, for its conversion to a different type of ultra-high purity metal compound. Depending on the type of the purified metal salt, the subsequent treatment may be a thermal decomposition reaction in an oxidizing atmosphere to obtain an oxide, a liquid phase fluorination or a different type of halogenation, carbonation, sulfation or nitration. In each case, the purity of the resulting metal compound is remarkably high, and it is easy

  to achieve a purity nine-nine or even higher.

  Accordingly, metal compounds suitable as raw material of advanced materials for optics, electronics or optoelectronics can be advantageously prepared using the present invention. Water-soluble metal salts to be purified by the process according to the invention include hydroxides, carbonates, hydrogen carbonates, nitrates, sulphates, chlorides and acid chlorides. The metals important in the practice of the metal-water-soluble salts are Li, Na and K among the alkali metals, Ce, Ba, Mg and Be among the alkaline earth metals, Al, Y, Zr, In, La, and Nd, Gd, Tb and Yb among the lanthanides

  (see elements of atomic numbers 58 to 71).

  Diethyldithiocarbamate as one of the two chelating agents specified in this invention is usually sodium or potassium diethyldithiocarbamate. The aqueous solution containing a selected water-soluble metal salt is not necessarily prepared by dissolving the water-soluble metal salt in water. Alternatively, a water-insoluble salt such as an oxide of the same metal can be converted to a water-soluble salt in an aqueous acid solution, followed by adjustment of the pH of the solution. To add DDTC or APDC to the aqueous solution, it is necessary to maintain the pH of the solution between 1.0 and 11. If the pH is less than 1.0 or greater than 11, it is difficult to accomplish a complete precipitation of unwanted transition metals, so the dissolved metal salt can not be purified to a desired extent. In the case where DDTC is used, it is preferable to maintain the pH of the solution between 4 and 8 and when using APDC,

  It is better to keep the pH between 1.5 and 7.

  The amount of DDTC or APDC addition depends on the Fe and other unwanted transition metals in the metal salt to be purified. Considering the contents of the transition metals as impurities in the currently available metal salts, DDTC or APDC must be at least 0.005% by weight of the metal salt. In most cases, it is preferred that DDTC or APDC be at least 0.05% by weight of the metal salt. From another point of view, the proportion of DDTC or APDC in total of the transition metals as impurities contained in the metal salt should be at least 50: 1 in moles. From an economic point of view, it is preferable not to use an excessively large amount of DDTC or APDC although the presence of an excess of DDTC or APDC does not adversely affect the efficiency of the purification. The form of DDTC or APDC to add to the salt solution of

  metal can be either a powder or an aqueous solution.

  The addition of DDTC or APDC to the solution of the metal salt, while stirring, results in the transfer of transition metals as impurities from the liquid phase to a precipitate which is composed of very fine

7 26 142 13

  particles. This precipitate is separated from the solution by performing filtration with a precision filter such as a membrane filter. The filtrate is a solution of a very pure metal salt, so the metal salt in the form of a crystalline powder of ultra-high purity can easily be obtained by recrystallization or by removing the water by a suitable process. . The metal salt thus purified and recovered can be converted to a different salt of the same metal by a known reaction. It is also possible to carry out this conversion by adding a suitable reagent to the filtrate mentioned above. In each case, it is possible to obtain a desired ultra-high purity metal compound comparable to the purity of the water-soluble metal salt.

first purified.

  If necessary, the water-soluble metal salt purified by the method described above can be further refined using a known technique. For example, after filtration of the precipitate formed by the addition of the chelating agent, the filtrate can be subjected to solvent extraction, followed by removal of the solvent from the extracted metal salt, or the metal salt can be precipitated by concentration. of

  filtrate then refined by recrystallization.

  An important application of the present invention is the preparation of an ultra-high purity metal oxide such as, for example, Al 2 O 3, ZrO 2, Y 2 O 3 or La 2 O 3. To obtain a metal oxide, a water-soluble metal salt which undergoes easy thermal decomposition and oxidation is used as the starting compound. The water-soluble salt is purified by the method described above and the purified salt is heated in an oxidizing atmosphere. Since the thermal decomposition of the metal salt can produce a corrosive gas, such as chlorine gas, it is appropriate to heat the metal salt in a corrosion resistant reactor through which oxygen is passed. or air. It is optional to perform thermal decomposition under reduced pressure or under vacuum to lower the decomposition temperature and

  promote the dissipation of corrosive gas.

  Another important application of the invention

  is the preparation of a metal fluoride of an ultra-

  high purity, suitable for use in new glasses of fluoride, although the invention is also applicable to the preparation of other types of metal halides such as chlorides, bromides and iodides or still different types of metal salts such

  as carbonates, sulphates and nitrates.

  When the desired metal fluoride is insoluble in water, the fluoride is formed by directly adding a fluorinating agent to the aqueous solution

  purified metal salt by the method described above.

  The precipitated fluoride of metal is separated from the solution

and is washed.

  Examples of fluorinating agents useful for this process are hydrofluoric acid, gaseous hydrogen fluoride, ammonium fluoride, acidic ammonium fluoride, fluorine gas,

fluorine and nitrogen trifluoride.

  When the desired metal fluoride is soluble in water, one way is to first add a fluorinating agent to the solution of the water soluble and purified metal salt and then precipitate the metal fluoride formed by concentration of the solution. It is also possible to modify this process starting from the concentration of the solution of the purified metal salt and adding a fluorinating agent during the concentration to thereby accomplish the fluorination reaction and the concentration simultaneously. As a different modification, the purified metal salt in the form of an aqueous solution may first be converted to an insoluble metal salt such as carbonate and then the precipitated metal salt may be reacted with a fluorinating agent. Another way is to first separate the water soluble and purified metal salt, water, and dry it and then react the dried metal salt with a fluorination gas. In the case of the wet process, the fluorination reaction is usually carried out at a temperature below 100 C. In the case of the dry process, it is appropriate to perform the fluorination at -6000C although an optimum temperature depends on

type of metal fluoride wanted.

  The metal fluoride obtained in this manner has a sufficiently high purity for use, for example, in optical fibers. However, if necessary, the resulting metal fluoride may be subject

  to a known refining treatment.

  It is often desirable to obtain an extremely low metal fluoride, not only by its content of transition metals as impurities but also by its oxygen content. To satisfy this desire, it is effective to extremely reduce the residual hydroxyl group in the water-soluble metal salt purified by the process according to the invention by crystallization, refinement and drying of the metal salt before reaction with a fluorinating agent. . It is likewise effective to heat the metal fluoride obtained by a fluorination reaction in a stream of a fluorination gas at a lower temperature than the

  decomposition temperature of metal fluoride.

  The invention will be better illustrated by the

  non-limiting examples that follow.

Example 1

  In a 2-liter beaker made of polytetramethylene

  Fluoroethylene (PTFE), 10Og of commercial grade reagent grade AlC13.6H20 was dissolved in 1 kg of ultrapure water and the pH of the solution was adjusted to 2.5. 0.5 g of APDC was then added to the solution and stirring continued. A precipitate formed by this treatment has been

  removed by filtration using a membrane filter.

  The filtrate was condensed to precipitate crystals of

AlC13.6H20.

  The analysis of metals as impurities in aluminum chloride prior to the purification treatment and purified aluminum chloride gave the results

shown in Table 1.

Example 2

  The purification treatment of Example 1 was repeated. The filtrate containing purified AlCl 3 H 2 O was extracted with solvent using chloroform, and NH 3 was added to the extract to precipitate Al (OH) 3. The precipitate was filtered off with a PTFE filter, washed

  with ultra-pure water and dried. Then we have -

  18 g of dried Al (OH) 3 were introduced into a platinum tube and heated at 700 ° C. for 2 hours. As a result, A1203 was obtained as a powder. The analysis of this

  produced gave the result shown in Table 1.

Comparison example 1.

  The procedure of Example 2 was repeated except that the treatment of AlC13.6H20 with APDC was omitted. The analysis of A1203 obtained gave the results

shown in Table 1.

TABLE 1.

  Compound Impurities (ppb) * Fe Cu Ni Co AlCl3.6H20 before treatment 500 70 60 30 AlCl3.6H20 purified in ex.1 4 5 55 4 5 A1203 obtained in ex.2 20 t 5 (5 <5 A1203 obtained at ex comp 1 300 50 30 20 * ppb = part per billion

Example 3

  In a 2 liter PTFE beaker, 50 g of ZrOC12 was dissolved. 8H 2 O of commercially available reagent grade in 1 kg of ultra pure water, and the pH of the solution was adjusted to 2. 0.5 g of APDC was then added to the solution, and stirring was continued. A precipitate formed by this treatment was filtered in

using a membrane filter.

  The filtrate was extracted with solvent using methyl isobutyl ketone (MIBK) as the solvent. Then NH3 was added to the extract and a resulting precipitate was collected by filtration with a membrane filter, washed with ultrapure water and dried. Then, 16 g of the dried precipitate was introduced into a platinum tube and heated at 900 ° C. for 2 hours. As a result, a crystalline powder of ZrO 2 was obtained. The analysis of this oxide gave the result shown

in Table 2.

  Comparison Example 2. The method of Example 3 was repeated except that the treatment of ZrOCl 2 .8H 2 O using

APDC has been omitted.

  The ZrO2 analysis obtained gave the results

shown in Table 2.

TABLE 2.

  Compound Impurities (ppb) Fe Cu Ni Co ZrOC12.8H20 before treatment 2000 100 100 140 ZrO2 obtained in ex. 3 ZrO 2 obtained in ex. comp. 2 600 60 50 100

Example 4

  In a 2 liter PTFE beaker, 100 g of commercial grade reagent grade Y (NO3) 3 was dissolved in 1 kg of ultrapure water, and the pH of the solution was adjusted to 5. Then 0.5 g of DDTC was added to the solution, and stirring was continued. A precipitate formed by this treatment was filtered using a

membrane filter.

  NH3 was then added to the filtrate and a resulting precipitate was collected by filtration with a PTFE filter, washed with ultrapure water and dried. Then, 16 g of the dried precipitate was introduced into a platinum tube and heated at 900 ° C. for 2 hours. As a result, a Y 2 O 3 powder was obtained. The

  The result of the analysis is shown in Table 3.

Comparative example 3.

  The procedure of Example 4 was repeated except that the treatment of Y (NO 3) 3 with DDTC was omitted. The analysis of Y203 obtained gave the results

shown in Table 3.

TABLE 3.

  Compound Impurities (ppb) Fe Cu Ni Co Y (N03) 3 before treatment 600 100 50 50 Y203 obtained in ex. 4 30 4 5 <5 <5 Y203 obtained in ex. comp. 3 200 40 30 20

Example 5

  The procedure of Example 4 was repeated except that 100 g of commercial grade reagent LaCl 3.7H20 was used as the starting compound in place of Y (NO 3) 3. As a result, a La.sub.2 O.sub.3 powder was obtained. The result of the analysis is shown in Table 4.

Comparative example 4.

  The procedure of Example 5 was repeated except that the treatment of LaCl 3. 7H 2 O with DDTC was omitted. The analysis of La203 obtained gave the result

shown in Table 4.

Table 4.

  Compound Impurities (ppb) Fe Cu NiCo LaCl3.7H20 before treatment 600 60 100 50 La203 obtained in ex. 5 20 <5 5 <5 5 La203 obtained in ex. comp. 4 300 50 80 30

Example 6

  In a 2 liter PTFE beaker, 100 g of ZrOC12 was dissolved. 8H20 of commercial reagent grade in 1 kg of ultrapure water and the solution was concentrated and then cooled to cause a

recrystallization of the solute.

  In a 2-liter PTFE beaker, 70 g of recrystallized ZrOC12 was dissolved in 1 kg of ultrapure water and the pH of the solution was adjusted to 5. 2 g of APDC were then added to the solution. and stirring continued. A precipitate formed by this treatment was filtered using a membrane filter. Then, 70 g of 50% hydrofluoric acid was added to the filtrate, and the resulting reaction liquid was concentrated by heating and then cooled to precipitate a crystalline reaction product. The crystalline product was collected by filtration, washed with ultrapure water and dried. Then, 60 g of the dried crystalline product was placed in a platinum tube and heated at 400 ° C. for 2 hours. As a result, a ZrF4 powder was obtained. The result of

the analysis is shown in Table 5.

Comparison Example 5.

  The procedure of Example 6 was repeated with the exception that 2 g of cupferone and 2 g of

  dimethylglyoxime in place of APDC of Example 6.

  The analysis of ZrF4 obtained gave the result shown in

table 5.

TABLE 5.

  Compound Impurities (ppb) Fe Co Ni Cu Cr Mn O ZrOC12 8H20 before treatment 600 200 100 50 50 50 ZrF4 obtained in ex. 6 10 - - - - - 20 ppm ZrF4 obtained in ex. comp. 5 60 50 20 10 40 30 25 ppm -: undetected

Example 7

  The procedure of Example 6 was repeated except that during the final heating of ZrF4 a stream of gaseous HF was passed through the platinum tube. This modification produced little difference in the metal contents as impurity in ZrF finally obtained, but in ZrF4 obtained in this

  for example, the oxygen content was less than 1 ppm.

Example 8.

  To prepare NaF, BaF2, LaF3, YF3 and InF3, NaHCO3, BaCl2, LaCl3, YCl3 and InCl3 were used as raw materials in commercially available reagent grade. In each case, 50 g of the metal salt was dissolved in 1 kg of ultrapure water. After adjusting the pH of the solution to 7, 1 g of DDTC was added to the solution, and stirring was continued. A precipitate formed by this treatment was filtered using a membrane filter. The filtrate was concentrated and then 50% hydrofluoric acid was added. The resulting reaction liquid was cooled to cause precipitation of the metal fluoride formed by the fluorination reaction. The precipitate was collected by

  filtration, washed and then dried in a clean oven.

  The metal fluoride analysis obtained gave the

results shown in Table 6.

Example 9.

  First 50 g of commercial grade reagent AlCl were dissolved in 500 g of ultrapure water and purified using 0.5 g of APDC. A precipitate formed by the purification treatment was removed by filtration using a membrane filter. The filtrate was extracted with solvent using MIBK as the solvent and the extract was concentrated to precipitate purified AlCl3. Precipitated AlCl3 was recovered and washed and reacted

  with hydrofluoric acid at 80 ° C for 2 hours.

  As a result, an A1F3 powder was obtained. The result of

  the analysis of this fluoride is shown in Table 6.

18 64213

Example of comparison 6.

  NaF, BaF2, LaF3, YF3 and InF3 were prepared by repeating the method of Example 8 except that the amount of DDTC was decreased to 0.002 g (only 0.004 wt% of the metal salt to be purified). In addition, AlF3 was prepared by repeating the procedure of Example 9 except that the amount of APDC was decreased to 0.002 g. The analysis of the metal fluorides obtained gave

  the results shown in Table 7.

TABLE 6.

  Fluoride obtained Impurities (ppb) in ex. 8 or ex 9 Fe Co Ni Cu Cr Mn

NaF 40 10 - - - -

BaF2

BaF2 20 5 - - - -

LaF3 20 10 - - - -

YF3 20 10 - - - -

InF3 30 20 - - - -

AlF3 5 - - - - -

TABLE 7.

  Fluoride obtained Impurities (ppb) in ex. of Comp. Fe Co Ni Cu Cr Mn NaF 400 10 70 20 30 20 BaF2 500 70 80 30 90 70 LaF3 600 50 50 80 80 80

YF3 600 40 100 30 100 50

  InF3 500 50 80 50 50 100 AlF3 200 40 50 50 10 10

Example 10

  First 50 g of commercial grade reagent grade BaCl2.2H2O was dissolved in 500 g of ultrapure water and purified using 0.5 g of DDTC. The precipitate formed by the purification treatment was removed by filtration using a membrane filter. 28% NH 3 in the filtrate was then added, and CO 2 gas was blown into the filtrate to precipitate BaCO 3. The carbonate thus obtained was reacted with 50% hydrofluoric acid to form BaF2 as a precipitate. This precipitate was collected by filtration, washed and then dried in a clean oven. Analysis of BaCO3 and BaF2 formed in the above process gave the

results shown in Table 8.

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Example of comparison 7.

  The procedure of Example 10 was repeated except that the amount of DDTC was decreased to

  0.002 g (0.004% by weight of the dissolved barium chloride).

  Table 8 contains the results of the BaC03 analysis

  and BaF obtained in this comparison example.

TABLE 8.

  Compound Impurities (ppb) Fe Co Ni Cu Cr Mn

BaCo3 from ex. 10 15 5 - - - -

BaF ex 10 20 5 - - - -

  BaCO3 from ex. comp.

7,400 60 70 25 70 60 BaF2 from ex. comp. 7,500 70 80 30 90 70

Claims (10)

CLAIMS CATIONS   A process for greatly improving the purity of a water-soluble inorganic salt of a metal selected from the group consisting of alkali metals, alkaline earth metals, aluminum, yttrium, zirconium, indium, lanthanum and lanthanide metals, characterized in that it comprises the steps of: preparing an aqueous solution comprising said water-soluble inorganic salt of the selected metal, adding a chelating agent, which is selected from the group consisting of diethyl dithiocarbamates and ammonium pyrrolidinedithiocarbamate soluble in water; water, in said aqueous solution, while maintaining the pH of said solution between I and 11, the amount of said chelating agent being at least 0.005% by weight of said metal salt, and separating a precipitate formed by the addition of said   chelating agent, of said solution.   2. Method according to claim 1, characterized in that the chelating agent is a diethyldithiocarbamate, the pH of the solution being maintained between 4 and 8.   3. Method according to claim 1, characterized in that the chelating agent is ammonium pyrrolidinedithiocarbamate, the pH of the   solution being maintained between 1.5 and 7.   A process for preparing an ultra high purity inorganic metal compound, the metal of said compound being selected from the group consisting of alkali metals, alkaline earth metals, aluminum, yttrium, zirconium, indium, lanthanum and metals lanthanide, characterized in that it comprises the steps of: preparing an aqueous solution comprising an inorganic salt soluble in the water of the selected metal, 2 2614213   adding a chelating agent, which is selected from the group consisting of water-soluble ammonium diethyldithiocarbamates and pyrrolidinedithiocarbamate, in said aqueous solution, while maintaining the pH of said solution between 1 and 11, the amount of said chelating agent being at least 0.005% by weight of said inorganic salt soluble in water; separating a precipitate formed by the addition of said chelating agent, said solution, thereby to highly purify the inorganic salt soluble in water; and converting the highly purified metal salt to an inorganic metal compound desired by at least one chemical reaction.   5. Method according to claim 4, characterized in that the chelating agent is a diethyldithiocarbamate, the pH of the aqueous solution being maintained between 4 and 8.   6. Process according to claim 4, characterized in that the chelating agent is ammonium pyrrolidinedithiocarbamate, the pH of the   aqueous solution being maintained between 1.5 and 7.   7. Process according to claim 4, characterized in that the desired metal compound is a metal oxide, the chemical reaction comprising a   thermal decomposition and oxidation.   8. Process according to claim 7, characterized in that the metal oxide is chosen from   group consisting of A1203, ZrO2, Y203 and La203.   9. Process according to claim 4, characterized in that the compound sought is a metal fluoride, the chemical reaction comprising a reaction with a fluorinating agent.   The process according to claim 9, characterized in that the metal fluoride is selected from the group consisting of NaF, BaF2, AlF3, YF3, ZrF4, InF3 and LaF3.