GB2204861A - Preparation of inorganic metal compounds of ultrahigh purity - Google Patents

Description

A 22 Cl 4 8 61 PREPARATION OF INORGANIC METAL COMPOUNDS OF ULTRAHIGH

PURITY This invention relates to a method for obtaining water soluble metal salts of ultrahigh purity, which can be converted into insoluble or hardly soluble metal compounds such as oxides or fluorides of ultrahigh purity by simple treatments. Metal compounds obtained by using this method are extremely low in the contents of transition metal impurities and include ones very suitable for use in optical, electronical and optoelectronical materials.

The recent developments of industries in the fields of optics and optoelectronics including lasers and optical fibers are remarkable and are still expanding. With the development of more and more sophisticated devices there is a trend toward severer requirements as to the levels of impurities in chemical compounds for use in optical, electronical and optoelectronical materials.

For instance, recently keen attention is paid to fluoride glasses which are excellent in transmittance in the infrared region and accordingly are expected as superior materials for future optical communications. For extreme reduction in transmission loss of optical fibers using fluoride glasses, it is important to develop a novel technique to extremely enhance purity of fluorides for use as raw materials of fluoride glasses. Especially, existence of iron ions or other transition metal impurities constitutes a serious obstacle to a success in extremely reducing transmission loss of fluoride glass optical fibers. Since metal fluorides prepared by conventional processes are liable to contain relatively large amounts unwanted transition metals, efforts have been devoted to preparation of metal fluorides very low in the contents of transition metal impurities.

For example, JP-A 57-51146 shows a method of preparing a metal fluoride in which the contents of metallic impurities are of the order of 10 ppm by vapor phase fluorination of a sublimable or evaporable raw material. A merit of this method is that a metal fluoride of very high purity can be obtained by a single-stage reaction without need of supplemental refining operations such as recrystallization. However, in this method it is essential to use a sublimable or evaporable compound as the starting material though such a compound is usually costly, and extreme care should be taken to prevent contamination of the product resulting from the action of hydrogen fluoride gas, which is usually used as the fluorinating agent, on the materials of the apparatus. Besides such inconveniences, this method does not seem to give full satisfaction as to the quality of the product since according to the examples 1 1 in the published specification the purities of the obtained fluorides are not higher than the level of five-nine.

Meanwhile, solid state lasers are widely used in combination with optical fibers for precision working of various materials and also for medical treatments. As materials of solid state lasers metal oxides such as yttrium oxide, neodymium oxide and aluminum oxide of more than five-nine purity are keenly demanded.

Besides, as raw materials of piezoelectric and pyroelectric materials some metal compounds such as lithium carbonate, zirconium oxide and lanthanum oxide of similarly high purity are demanded. However, as far as we know no industrially practicable technique has been developed yet for drastic reduction in the contents of transition metal impurities in the metal compounds mentioned above or other metal compounds for similar uses to the level of nine-nine or ppb.

It is a fundamental object of the present invention to provide a relatively simple method for easily preparing inorganic metal compounds, mainly oxides and fluorides, of ultrahigh purity such as nine-nine purity.

From a different aspect, it is an object of the invention to provide a relatively simple method for easily and extremely enhancing purity of water soluble metal salts with a view to converting the purified metal salts into other kinds of metal compounds, such as oxides and fluorides, of ultrahigh purity.

As a basic part of the present invention, there is provided a method of extremely enhancing the purity of a water soluble salt of a metal selected from alkali metals, alkaline earth metals, aluminum, yttrium, zirconium, indium, lanthanum and the lanthanide metals, the method 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 water soluble diethyldithiocarbamate (abbreviated to DDTC) or ammonium pyrrolidiendithiocarbamate (abbreviated to APDC), to the aqueous solution while maintaining the pH of the solution within the range from 1 to 11, the amount of the chelating agent being at least 0.005 wt% of the water soluble metal salt, and separating a precipitate produced by the addition of the chelating aqent from the solution.

By this metahod it is easy to decrease transition metal impurities contained in the treated metal salt to less than 1110 of the initial amounts, and therefore it is easy to obtain a metal salt of nine-nine purity or, in some cases, of still higher purity.

The success in such a remarkable purification by a simple operation is attributed to the use of a specific chelating agent, D= or APDC, for precipitating impurity metals as coordination compounds. Both D= and APDC are high in their stability constants in forming coordination compounds with various transition metals, and these chelating agents can make simultaneous coordination with many kinds of transition metals such as Fe, Co, Ni, Cr, Co and/or Mn. Different kinds of chelating agents such as dimethylglyoxime, cupferron and dithizone are used in conventional purification methods. Each of these chelating agents becomes stable by making selective coordination with one kind of transition metal or only a few kinds of transition metals. Therefore, in the conventional methods it is necessary to jointly use several kinds of chelating agents.

The above stated purification method according to the invention gives an aqueous solution containing little transition metal impurities. From this solution a metal salt of ultrahigh purity can easily be recovered by recrystallization or any other alternative operation. The present invention includes variously treating the extremely purified water soluble metal salt, in the form of aqueous solution or after separating from water,to convert it into a different kind of metal compound of ultrahigh purity. Depending on the kind of the purified metal salt, the subsequent treatment may be a thermal decomposition reaction in an oxydizing atmosphere to obtain an oxide, liquid phase fluorination or a different type of halogenation, carbonation, sulfation or nitration. In every case the purity of the obtained k metal compound is remarkably high, and it is easy to realize nine-nine purity or still higher purity. Accordingly, metal compounds suitable as raw materials of advanced optical, electronical and optoelectronical materials can advantageously be prepared by utilizing the present invention.

Water soluble metal salts to be purified by the method according to the invention include hydroxides, carbonates, hydrogen carbonates, nitrates, sulfates, chlorides and acid chlorides. Practically important metals of the water soluble metal salts are Li, Na and K among alkali metals, Ca, Ba, Mg and Be among alkaline earth metals, Al, Y, Zr, In, La, and Nd, Gd, Tb and Yb among lanthanides (viz. elements of Atomic Nos. from 58 to 71).

The diethyldithiocarbamate as one of the two chelating agents specified in this invention is usually sodium or potassium diethyldithiocarbamate.

The aqueous solution comprising 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 oxide of the same metal may be converted into a water soluble salt in an aqueous acid solution, followed by adjustment of pH of the solution.

In adding D= or APDC to the aqueous solution, it is necessary to maintain the pH of the solution within the range from 1.0 to 11. If the pH is below 1.0 or above 11, it is-difficult to accomplish complete precipitation of unwanted transition metals so that the dissolved metal salt cannot be purified to a desired extent. In the case of using DDTC it is preferred to maintain pH of the solution within the range from 4 to 8, and when using APDC it is preferred to maintain the pH within the range from 1.5 to 7.

The amount of addition of DDTC or APDC depends on the contents of Fe and other unwanted transition metals in the metal salt to be purified. Considering the contents of transition metal impurities in commonly available metal salts, DDTC or APDC needs to amount to at least 0.005 wt% of the metal salt. In most cases it is preferable that D6TC or APDC amounts to at least 0.05 wt% of the metal salt. From another aspect, the proportion of DDTC or APDC to the total of transition metal impurities contained in the metal salt should be at least 50:1 by mol. From an economical point of view it is favorable not to use an excessively large amount of DDTC or APDC though the presence of an excess of DDTC or APDC does not adversely affect the purification efficiency. The form of DDTC or APDC to be added to the metal salt solution may be either powder or aqueous solution.

The addition of DDTC or APDC to the metal salt solution, with stirring, results in transfer of transition metal impurities from the liquid phase to a precipitate which consists of very fine 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 that the metal salt in the form of crystalline powder of ultrahigh purity can easily be obtained by recrystallization or by removing water by a suitable method. The thus purified and recovered metal salt can be converted into a different salt of the same metal by a known reaction. Also it is possible to carry out such conversion by adding a suitable reagent to the aforementioned filtrate. In either case it is possible to obtain a desired metal compound with ultrahigh purity comparable to the purity of the firstly purified water soluble metal salt.

If necessary, the water soluble metal salt purified by the above described method may further be refined by using a known technique. For example, after filtering out the precipitate formed by the addition of the chelating agent the filtrate may be subjected to solvent extraction, followed by removal of the solvent from the extracted metal salt, or the metal salt may be precipitated by concentrating the filtrate and then refined by recrystallization.

An important application of the present invention is preparation of an ultrahigh-purity metal oxide such as, for example, Al 2 0 3, ZrO 2' y 2 0 3 or La 2 0 3 To obtain a metal oxide, a water soluble metal salt which readily undergoes thermal decomposition and oxidation is used as the starting compound. The water soluble salt is purified by the above described method, and the purified salt is heated in an oxidizing atmosphere. Since themal decomposition of the metal salt is liable to generate a corrosive gas, e.g.

chlorine gas, it is suitable to carry out heating of the metal salt in a corrosion resistant reactor through which oxygen or air is passed. It is optional to perform the thermal decomposition under reduced pressure or under vacuum to lower the decomposition temperature and promote dissipation of the corrosive gas.

Another important application of the invention is preparation of a metal fluoride of ultrahigh purity suitable for use in new fluoride galsses, though the invention is applicable also to preparation of other kinds of metal halides such as chlorides, bromides and iodides or still different kinds of metal salts such as carbonates, sulfates and nitrates.

When the aimed metal fluoride is insoluble in water, the fluoride is formed by directly adding a fluorinating agent to the aqueous solution of the metal salt purified by the above described method. The precipitated metal fluoride is separated from the solution and washed.

Examples of fluorinating agents useful for this method are hydrofluoric acid, hydrogen fluoride gas, ammonium fluoride, acidic ammonium fluoride, fluorine gas, fluorine halides and nitrogen trifluoride.

When the aimed metal fluoride is soluble in water, one way is first adding a fluorinating agent to the solution of the purified water soluble metal salt and then precipitating the formed metal fluoride by concentrating the solution. It is also possible to modify this process by first starting concentration of the solution of the purified metal salt and adding a fluorinating agent in the course of concentration to thereby accomplish the fluorinating reaction and concentration simultaneously. As a different modification, the purified metal salt in the form of aqueous solution may be first converted into an insoluble metal salt such as carbonate, and then the precipitated metal salt may be reacted with a fluorinating agent. Another way is first separating the purified water soluble metal salt from water and drying it and then reacting the dried metal salt with a fluorinating gas. In the case of the wet process the fluorinating reaction is usually carried out at a temperature below 100 0 C. In the case of the dry process it is suitable to carry out the fluorination at 200- 600 0 C, though an optimum temperature depends on the kind of the aimed metal fluoride.

11 The metal fluoride obtained in this way has a sufficiently high purity for use, for example, in optical fibers.. If necessary, however, the obtained metal fluoride may be subjected to a known refining treatment.

Often it is desired to obtain a metal fluoride extremely low not only in the contents of transition metal impurities but also in the content of oxygen. To satisfy such desire, it is effective to extremely reduce residual hydroxyl group in the water soluble metal salt purified by the method according to the invention by crystallizing, refining and drying the metal salt before reacting with a fluorinating agent. Also it is effective to heat the metal fluoride obtained by a fluorination reaction in a stream of a fluorinating gas at a temperature lower than the decomposition temperature of the metal fluoride.

The invention is further illustrated by the following nonlimitative examples. EXAMPLE 1 In a 2-liter beaker made of polytetrafluoroethylene (PTFE) 100 g of AlCl 30 6H 2 0 of commercial reagent grade was dissolved in 1 kg of ultrapure water, and pH of the solution was adjusted to 2.5. Then 0.5 g of APDC was added to the solution, and stirring was continued. A precipitate formed by this treatment was filtered out by using a membrane filter. The filtrate was condensed to precipitate crystals of AlCl 3' 6H 2 0.

Analysis of impurity metals in the aluminum chloride before.the purification treatment and the 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 6H 2 0 was subjected to solvent extraction using chloroform, and NH 3 was added to the extract to cause precipitation of Al(OH) 3 The precipitate was separated by filtration with a PTFE filter, washed with ultrapure water and dried. Next, 18 g of the dried Al(OH) 3 was packed in a platinum tube and heated at 700 0 C for 2 hr. As the result Al 2 0 3 in powder form was obtained. Analysis of this product gave the result shown in Table 1.

COMPARATIVE EXAMPLE 1 The process of Example 2 was repeated except that the treatment of AlCl 3 6H 2 0 with APDC was omitted. Analysis of the obtained Al 2 0 3 gave the result shown in Table 1.

TABLE 1

Compound Impurities (Ppb) Fe Cu Ni Co AlCl 3' 6H 20 before treatment 500 70 60 30 AlCl 3' 6H 2 0 purified in Ex. 1 <5 <5 <5 <5 Al 2 0 3 obtained in Ex. 2 20 <5 <5 <5 Al 203 obtained in Comp. Ex. 1 300 50 30 20 EXAMPLE 3

In a 2-liter PTFE beaker 50 g of ZrOC12 8H 2 0 of commercial reagent grade was dissolved in 1 kg of ultrapure water, and pH of the solution was adjusted to 2. Then 0.5 g of APDC was added to the solution, and stirring was continued. A precipitate formed by this treat- ment was filtered out by using a membrane filter.

The filtrate was subjected to solvent extraction using methyl isobutyl ketone (MIBK) as solvent. Then NH 3 was added to the extract, and a resultant precipitate was collected by filtration with a membrane filter, washed with ultrapure water and dried. Next, 16 g of the dried precipitate was packed in a platinum tube and heated at 9000C for 2 hr. As the result a crystalline powder of ZrO 2 was obtained. Analysis of this oxide gave the result shown in Table 2.

COMPARATIVE EXAMPLE 2 The process of Example 3 was repeated except that the treatment of ZrOC1 2 08H 2 0 with APDC was omitted.

Analysis of the obtained ZrO 2 gave the result shown in Table 2.

TABLE 2

Compound Impurities (ppb) Fe Cu Ni C0 ZrOC1 2. 8H 2 0 before treatment 2000 100 100 140 ZrO 2 obtained in Ex. 3 30 <5 <5 <5 ZrO 2 obtained in Comp. Ex.2 600 60 50 100 EXAMPLE 4

In a 2-liter PTFE beaker 100 g of Y(NO 3)3 of commercial reagent grade was dissolved in 1 kg of ultrapure water, and pH of the solution was adjusted to 5. Then 0.5 g of DDTC was added to the solution, and stir- ring was continued. A precipitate formed by this treatment was filtered out by using a membrane filter.

Then NH 3 was added to the filtrate, and a resultant precipitate was collected by filtration with a PTFE filter, washed with ultrapure water and dried. Next, 16 g of the dried preicipitate was packed in a platinum tube and heated at 900 0 C for 2 hr. As the result a powder of Y 2 0 3 was obtained. The result of analysis is shown in Table 3.

COMPARATIVE EXAMPLE 3 The process of Example 4 was repeated except that the treatment of Y(NO 3)3 with DDTC was omitted.

Analysis of the obtained Y 2 0 3 gave.the result shown -is- in Table 3.

TABLE 3

Compound Impurities (Ppb) Fe Cu Ni co Y(NO 3)3 before treatment 600 100 50 50 Y203 obtained in Ex. 4 30 <5 <5 <5 Y203 obtained in Comp. Ex. 3 200 40 30 20 EXAMPLE 5

The process of Example 4 was repeated except that 100 g of LaCl 3. 7H 2 0 of commercial reagent grade was used as the starting compound in place of Y(NO 3)3 As the result a powder of La 2 0 3 was obtained. The result of analysis is shown in Table 4.

COMPARATIVE EXAMPLE 4 The process of Example 5 was repeated except that the treatment of LaCl 3 7H 2 0 with DDTC was omitted. Analysis of the obtained La 2 0 3 gave the result shown in Table 4.

TABLE 4

Compound Impurities (Ppb) Fe Cu Ni- CO, LaCl 3 7H 2 0 before treatment 600 60 100 50 La 2 0 3 obtained in Ex. 5 20 <5 <5 <5 La 2 0 3 obtained in Comp. Ex. 4 300 50 80 30 EXAMPLE 6

In a 2-liter PTFE beaker 100 g of ZrOC1 2 8H 2 0 of commercial reagent grade was dissolved in 1 kg of ultra pure water, and the solution was concentrated and then cooled to cause recrystallization of the solute.

In a 2-liter PTFE beaker 70 of the recrrystallized ZrOC1 2 was dissolved in 1 kg of ultrapure water, and pH of the solution was adjusted to 5. Then 2 g of APDC was added to the solution, and stirring was continued. A precipitate formed by this treatment was filtered out by using a membrane filter. After that 70 g of 50% hydrofluoric acid was added to the filtrate, and the resultant reaction liquid was concentrated by heating and then cooled to cause precipitation of a crystalline reaciton product. The crystalline product was collected by filtration, washed with ultrapure water and dried. Next, 60 g of the dried crystalline product was packed in a platinum tube and heated at 400 0 C for 2 hr. As the result a powder of ZrF 4 was obtained. The result of analysis is shown in Table 5.

COMPARATIVE EXAMPLE 5 The process of Example 6 was repeated except that 2 g of cupferron and 2 g of dimethy1glyoxime were used in place of APDC in Example 6. Analysis of the obtained ZrF 4 gave the result shown in Table 5.

k 4 TABLE 5

Compound Impurities (ppb) Fe Co Ni Cu Cr Mn 0 Zrocl 2 8H 2 0 before treatment ZrF 4 obtained in Ex. 6 ZrF 4 obtained in Comp. Ex. 5 600 200 100 50 50 50 - 2 0 ppm 50 20 10 40 30 25 ppm -: not detected EXAMPLE 7

The process of Example 6 was repeated except that during the final heating of ZrF 4 a stream of HF gas was passed through the platinum tube. This modification produced little difference in the contents of impurity metals in the finally obtained ZrF 4' but in ZrF 4 obtained in this example the content of oxygen was less than 1 ppm.

EXAMPLE 8

To prepare NaF, BaF 2' LaF 3' YF 3 and InF 3 p NaHCO 3' BaCl 2' LaCl 31 Ycl 3 and InCl 3 of commercial reagent grade were alternately used as starting materials.

In every case, 50 g of the starting metal salt was dissolved in 1 kg of ultrapure water. After adjusting pH of the solution to 7, 1 g of D= was added to the solution, and stirring was continued. A precipitate formed by this treatment was filtered out by using a membrane filter. The filtrate was concentrated, and 1 then 50% hydrofluorice acid was added. The resultant 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. Analysis of the obtained metal fluorides gave the results shown in Table 6. EXAMPLE 9 First, 50 g of A1C1 3 of commercial reagent grade was dissolved in 500 g of ultrapure water and purified by using 0.5 g of APDC. A precipitate formed by the purifying treatment was filtered out by using a membrane filter. The filtrate was subjected to solvent extraction using MIBK as solvent, and the extract was concentrated to cause precipitation of purified A1C1 3 The precipitated A1C1 3 was recovered and washed and then was reacted with hydrofluoric acid at 80 0 C for 2 hr. As the result a powder of A1F 3 was obtained. The result of analysis of this fluoride is shown in Table 6.

COMPARATIVE EXAMPLE 6 NaF, BaF 21 LaF 31 YF 3 and InF 3 were prepared each by repeating the process of Example 8 except that the quantity of DDTC was decreased to 0. 002 g (only 0.004 wt% of the metal salt to be purified). Besides, A1F 3 was prepared by repeating the process of Example 9 except that the quantity of APDC was decreased to 0.002 g. Analysis of the obtained metal fluorides gave the results shown in Table 7.

TABLE 6

Fluoride obtained in Ex. 8 or Ex. 9 Impurities (Ppb) Fe Co Ni Cu Cr Mn NaF BaF 2 LaF., YF 3 InF 3 A1F 3 Fluoride obtained in Comp. Ex. 6 10 5 10 10 20 - TABLE 7

Impurities (ppb) Fe Co Ni Cu Cr Mn NaF 400 10 70 20 30 20 BaF 2 500 70 80 30 90 70 LaF 3 600 50 50 80 80 80 YF 3 600 40 100 30 100 50 InF 3 500 50 80 50 50 100 AlF 3 200 40 50 50 10 10 EXAMPLE 10

First 50 g of BaCl 2' 2H 2 0 of commercial reagent grade was dissolved in 500 g of ultrapure water and purified by using 0.5 g of DDTC. The precipitate formed by the purifying treatment was filtered out by using a membrane filter. Then 28% NH 3 was added to the filtrate, and CO 2 gas was blown into the filtrate to i i 1 cause precipitation of BaCO 3 The thus obtained carbonate was reacted with 50% hydrofluoric acid to form BaF 2 as a precipitate. This precipitate was collected by filtration, washed and then dried in a clean oven. Analysis of BaCO 3 and BaF 2 formed in the above process gave the results shown in Table 8. COMPARATIVE EXAMPLE 7 The process of Example 10 was repeated except that the quantity of DDTC was decreased to 0.002 9 (0.004 wt% of the dissolved barium chloride). Table 8 contains the results of analysis of BaCO 3 and BaF 2 obtained in this comparative example.

TABLE 8

Compound Impurities (ppb) Fe Co Ni Cu Cr Mn BaCO 3 of Ex. 10 BaF 2 of Ex. 10 BaCO 3 of Comp. Ex. 7 BaF 2 of Comp. Ex. 7 5 - - - - 5 - - - - 400 60 70 25 70 60 500 70 80 30 90 70 t

Claims (13)

1. -A method of enhancing the purity of a water soluble inorganic salt of a metal selected from alkali metals, alkaline earth metals, aluminum, yttrium, zirconium, indium, lanthanum and the lanthanide metals, the method comprising 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 water soluble diethyldithiocarbamates and ammonium pyrrolidinedithiocarbamate, to said aqueous solution while maintaining the pH of said solution within the range from 1 to 11, the amount of said chelating agent being at least 0.005 wt% of said metal salt; and separating a precipitate formed by the addition of said chelating agent from said solution.

2. A method according to Claim 1, wherein said chelating agent is a diethyldithiocarbamate, said pH of said solution being maintained within the range from 4 to 8.

3. A method according to Claim 1, wherein said chelating agent is ammonium pyrrolidinedithiocarbamate, said pH of said solution being maintained within the range from 1.5 to 7.

1 h 1 1

4. A method of preparing an inorganic metal compound of ultrahigh purity, the metal of said metal compound being selected from alkali metals, alkaline earth metals, aluminum, yttrium, zirconium, indium, lanthanum and the lanthanide metals, the method comprising the steps of:

preparing an aqueous solution comprising a water soluble inorganic salt of the selected metal; adding a chelating agent, which is selected from water soluble diethyldithiocarbamates and ammonium pyrrolidinedithiocarbamate, to said aqueous solution while maintaining the pH of said solution within the range from 1 to 11, the amount of said chelating agent being at least 0.005 wt% of said water soluble inorgani salt; separating a precipitate formed by the addition of said chelating agent from said solution to thereby extremely purify said water soluble inorganic salt; and converting the extremely purified metal salt into an aimed inorganic metal compound by at least one chemical reaction.

5. A method according to Claim 4, wherein said chelating agent is a diethyldithiocarbamate, said pH of said aqueous solution being maintained within the range from 4 to 8.

6. A method according to Claim 4, wherein said chelating agent is ammonium pyrrolidinedithiocarbamate, 1 said pH of said aqueous solution being maintained within the range from 1. 5 to 7.

7. A method according to any of Claims 4 to 6, wherein the-aimed metal compound is a metal oxide, said at least one chemical reaction comprises thermal decomposition and oxidation.

8. A method according to Claim 7, wherein said metal oxide is selected from Al 2 0 3' ZrO 21 Y 2 0 3 and La 2 0 3

9. A method according to any of Claims 4 to 6, wherein the aimed compound is a metal fluoride, said at least one chemical reaction comprises a reaction with a fluorinating agent.

10. A method according to Claim 9, wherein said metal fluoride is selected from NaF, BaF 2' A1F 3' YF 3' ZrF 4' InF 3 and LaF 3

11. A method of extremely enhancing the purity of a water soluble inorganic metal salt, substantially as hereinbefore described in any of Examples 1 to 10.

12. A method of preparing a metal oxide of ultrahigh purity, substantially as hereinbefore described in any of Examples 2 to 5.

13. A method of preparing a metal fluoride-of ultrahigh purity, substantially as hereinbefore described in any of Examples 6 to 10.

1 Published 1988 at The Patent Office, State House, 66.171 High Holborn, London WC1R 4TP. Further copies may be obtained from The Patent Office, Sales Branch, St Mary Cray, Orpington, Kent BR5 3F.D. Printed by Multiplex techniques ltd, St Mary Cray, Kent. Con. 1/87.