JP2003095648A - Method for producing high purity calcium fluoride and intermediate thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing a high purity calcium fluoride usable as an optical member in the range of a vacuum ultraviolet wavelength and having an extremely small amount of metal impurity content, and also provide a method for producing the intermediate thereof, and a high purity calcium fluoride. SOLUTION: The production method comprises a process for adding an alkali to a calcium compound to produce calcium hydroxide in the range of pH 12 to 13.5 (process 1), a process for adding an acid to calcium hydroxide produced via the process 1 to adjust the value of pH 1 to 7 and then adding oxalic acid to the mixture to produce calcium oxalate (process 2) and a process for adding hydrogen fluoride to calcium oxalate produced via the process 2 to produce calcium fluoride (process 3).

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing high-purity calcium fluoride, which is suitable as a raw material for producing calcium fluoride crystals and can be used as an optical member in the vacuum ultraviolet wavelength region, and which contains extremely few impurities metals. It relates to a method for producing the intermediate.

[0002]

2. Description of the Prior Art Calcium fluoride crystals have high transmittance in a wide wavelength range from vacuum ultraviolet to infrared, and lenses, window materials, prisms, etc. for cameras, microscopes, telescopes, reduction projection exposure devices (steppers), etc. Widely used in optical members. In particular, with the recent high integration of semiconductors, calcium fluoride crystals have attracted attention as useful optical members for steppers in semiconductor manufacturing.

In the stepper, it is necessary to shorten the wavelength of light used to increase the resolution of the transfer pattern formed by photolithography. Therefore, as a light source for photolithography, g-line (436 nm) to i-line (365 nm)
nm), and a KrF excimer laser (248n
m) and the ArF excimer laser (193 nm) have been shortened, and at the development level, the F ₂ excimer laser (157 nm) is used. It is known that oxides such as quartz glass which have been conventionally used as an optical member of a stepper largely absorb light having a wavelength of 200 nm or less, that is, a so-called vacuum ultraviolet wavelength range. In particular,
F ₂ excimer laser light (157 nm) having a short wavelength does not pass through the silica glass at all. Therefore, in a stepper using an ArF excimer laser and an F ₂ excimer laser as a light source, a calcium fluoride crystal that efficiently transmits light in the vacuum ultraviolet wavelength range is used as an optical member.

Calcium fluoride crystals are produced mainly by powdery high-purity calcium fluoride by the Bridgman method (called the Stockburger method or the crucible lowering method). However, JP-A-10-597
As disclosed in Japanese Patent Application Laid-Open No. 99, Japanese Patent Application Laid-Open No. 11-15798, and the like, when the content of the impurity metal and the content of oxygen in the calcium fluoride crystal is large, the transmittance of light in the vacuum ultraviolet wavelength range is greatly reduced. This is a serious problem when used as an optical member in the vacuum ultraviolet wavelength range.

Therefore, in order to sufficiently bring out the excellent optical characteristics of the calcium fluoride crystal, it is essential to use a high-purity calcium fluoride powder raw material containing extremely few contained impurity metals and oxygen. Specifically, in the calcium fluoride raw material, the content of the specific impurity metal element is preferably 0.1 mass ppm or less and the content of the oxygen element is preferably 100 mass ppm or less.

[0006] Generally, high-purity calcium fluoride is synthesized by reacting highly purified calcium carbonate with hydrofluoric acid by various methods. However, in the step of highly purifying the calcium salt, it is extremely difficult to separate and remove the alkaline earth metal, which is a homologue of calcium, and in particular, a method of reducing strontium, which has similar physical properties to calcium, to a concentration of 0.1 mass ppm or less Has not been reported so far.

For example, in JP-A-62-3602, quicklime is brought into contact with water to produce a slaked lime slurry, and solid-liquid separation is performed to elute and remove strontium into an aqueous phase.
Subsequently, a method for dissolving high-purity calcium carbonate by dissolving slaked lime in an ammonium nitrate or ammonium chloride aqueous solution to separate and remove insoluble matter containing strontium, and then contacting carbon dioxide with an aqueous ammonium salt solution of slaked lime is described. . By this method, it is possible to produce calcium carbonate in which the residual strontium is reduced to 20 mass ppm or less.

Further, in Japanese Patent Laid-Open No. 63-156012, in a method for producing high-purity calcium carbonate by reacting a calcium salt with a carbonate or carbon dioxide, the molar ratio of CO ₂ / Ca to be reacted is 0.1. A method for producing high-purity calcium carbonate by adjusting the range to 2 to 0.9 is described. Strontium in calcium carbonate produced by this method can be reduced to 10 mass ppm or less. However, when calcium fluoride produced by these methods is reacted with hydrofluoric acid to synthesize calcium fluoride, the strontium of several mass ppm to several tens mass ppm contained in calcium carbonate remains almost unchanged in the synthetic calcium fluoride. Since it is mixed in at a concentration of, and the strontium is separated and removed by further purifying the calcium fluoride synthesized by such a method, the physical properties such as solubility and melting point of the fluoride of calcium and strontium are close to each other. Have difficulty.

On the other hand, as described above, the contained oxygen absorbs light in the vacuum ultraviolet wavelength range, so it is necessary to reduce the oxygen element in the calcium fluoride crystal. For example, JP-A-11
As described in JP-A-15798, etc., in the step of producing a calcium fluoride crystal, a scavenger such as lead fluoride powder is mixed with calcium fluoride powder, and the mixture is heated and melted to be crystallized. There is a method of lowering the oxygen concentration in calcium fluoride crystals. However,
As described in JP-A-2000-235101, when lead fluoride mixed with calcium fluoride powder as a scavenger remains in the calcium fluoride crystal, an absorption band due to lead fluoride is formed and It has been found to cause problems with the optical properties of calcium fluoride crystals.

Therefore, in the step of producing a calcium fluoride crystal, the calcium fluoride powder may be used as an optical member in the vacuum ultraviolet wavelength range without mixing the scavenger or by reducing the amount of the mixed scavenger. There has been a demand for a method capable of industrially producing a calcium fluoride crystal containing a very small amount of oxygen, which is suitable as a raw material for producing a high-quality calcium fluoride crystal.

[0011]

Under the circumstances described above, the present invention provides a method for producing high-purity calcium fluoride containing a very small amount of impurity metals contained therein which can be used as an optical member in the vacuum ultraviolet wavelength region. This is an issue. Moreover, this invention makes it a subject to provide the method of manufacturing the intermediate body which can be used when manufacturing high purity calcium fluoride. A further object of the present invention is to provide high-purity calcium fluoride that can be used as an optical member in the vacuum ultraviolet wavelength range and is suitable as a raw material for producing calcium fluoride crystals.

[0012]

Means for Solving the Problems As a result of intensive studies to solve the above problems, the present inventors have found that the pH is adjusted by adding an alkali to a calcium compound or a solution or suspension of the calcium compound to adjust the pH. A step (step 1) of forming a precipitate of calcium hydroxide in a region between 12 and 13.5 and separating the solid-liquid to obtain calcium hydroxide having a reduced content of impurity metals; At least one selected from the group consisting of oxalic acid, oxalates and oxalic acid compounds is prepared by adding an acid to calcium hydroxide or a solution or suspension of the calcium hydroxide thus obtained to adjust the pH to 1 to 7. Adding a seed compound to produce a precipitate of calcium oxalate, separating the solid-liquid to obtain calcium oxalate having a reduced content of impurity metals (step 2), Precipitation of calcium fluoride by adding at least one selected from the group consisting of hydrogen fluoride, fluorine gas and ammonium fluoride to the calcium oxalate or the solution or suspension of calcium oxalate obtained through step 2. Is produced and the solid-liquid is separated to obtain calcium fluoride in which the content of impurity metals is reduced (step 3). The present invention has been completed and the present invention has been completed. The present invention relates to a method for producing high-purity calcium fluoride, a method for producing an intermediate thereof, and high-purity calcium fluoride shown in the following (1) to (41).

(1) Addition of an alkali to a calcium compound or a solution or suspension of a calcium compound to adjust the pH
To produce calcium hydroxide precipitates in a pH range of 12 to 13.5 and separate the solid-liquid to obtain calcium hydroxide having a reduced content of impurity metals (step 1). And an acid is added to the calcium hydroxide or the solution or suspension of the calcium hydroxide obtained through the above-mentioned step 1 to adjust the pH to 1 to 7, and comprises oxalic acid, an oxalate and an oxalic acid compound. A step of adding at least one compound selected from the group to form a precipitate of calcium oxalate, separating the solid-liquid to obtain calcium oxalate having a reduced content of impurity metals (step 2)
And fluorination by adding at least one selected from the group consisting of hydrogen fluoride, fluorine gas and ammonium fluoride to the calcium oxalate or the solution or suspension of the calcium oxalate obtained through the above step 2. A method for producing high-purity calcium fluoride, comprising a step (step 3) of forming a precipitate of calcium and separating a solid-liquid to obtain calcium fluoride having a reduced content of impurity metals. (2) In the step 1, the alkali is added to the calcium compound or the solution or suspension of the calcium compound to add p.
A step of adjusting H to generate a precipitate of a metal hydroxide contained as an impurity in the calcium compound in a pH range of 10 to 12.5, and separating the solid-liquid to obtain a solution of the calcium compound (step The method for producing high-purity calcium fluoride according to (1) above, which comprises 4). (3) The method according to (1) above, which includes a step (step 5) of drying the calcium fluoride obtained through the step 3 and then contacting it with fluorine gas to reduce oxygen contained in the calcium fluoride. Method for producing pure calcium fluoride. (4) The method for producing high-purity calcium fluoride according to the above (1) or (2), wherein in step 1, the alkali is an organic alkali. (5) The method for producing high-purity calcium fluoride according to the above (2), wherein in step 4, the alkali is an organic alkali.

(6) The method for producing high-purity calcium fluoride according to the above (4) or (5), wherein the organic alkali is tetramethylammonium hydroxide. (7) The method for producing high-purity calcium fluoride according to the above (1) or (2), wherein in step 1, the alkali is a hydroxide of an alkali metal. (8) The method for producing high-purity calcium fluoride according to the above (2), wherein in step 4, the alkali is a hydroxide of an alkali metal. (9) The method for producing high-purity calcium fluoride according to the above (7) or (8), wherein the alkali metal hydroxide is sodium hydroxide and / or potassium hydroxide. (10) The high purity according to (1) above, wherein in step 2, the acid added to the calcium hydroxide or the solution or suspension of calcium hydroxide obtained through step 1 is hydrochloric acid and / or nitric acid. A method for producing calcium fluoride.

(11) In the step 2, oxalic acid,
The method for producing high-purity calcium fluoride according to (1) above, wherein the oxalate salt and the oxalic acid compound are purified by contacting with a solid substance having an ion exchange property. (12) The method for producing high-purity calcium fluoride according to the above (11), wherein the solid substance having an ion-exchange property is an ion-exchange resin, a chelate resin, zeolite and / or activated carbon. (13) In the step 2, the oxalic acid compound is methyl oxalate, and after the addition of the methyl oxalate, the step (step 6) of heating the solution to 40 to 100 ° C. Method for producing pure calcium fluoride. (14) In the step 2, the concentration of the solution of oxalic acid, oxalate salt or oxalic acid compound added is 0.1% or more and 10% or less in terms of oxalic acid. A method for producing calcium fluoride. (15) After carrying out the step 3, a step (step 7) of recovering and purifying oxalic acid from the solution in which calcium fluoride is separated is included, and the purified oxalic acid is reused in the step 2 ( The method for producing high-purity calcium fluoride according to 1).

(16) High-purity calcium fluoride characterized in that the content of strontium contained as an impurity is 0.1 mass ppm or less. (17) Potassium, sodium contained as impurities,
Magnesium, barium and iron contents of 0.1 each
The high-purity calcium fluoride according to (16), which has a mass ppm or less. (18) The content of oxygen element contained as an impurity is 10
The high-purity calcium fluoride according to the above (16) or (17), which is 0 mass ppm or less. (19) Consists of calcium fluoride having a content of strontium, potassium, sodium, magnesium, barium and iron contained as impurities of 0.1 mass ppm or less and an oxygen element content of 100 mass ppm or less. Calcium fluoride single crystal for steppers. (20) Calcium fluoride having a content of strontium, potassium, sodium, magnesium, barium and iron contained as impurities of 0.1 mass ppm or less and an oxygen element content of 100 mass ppm or less Calcium fluoride lens for steppers.

(21) Add an alkali to the calcium compound or the solution or suspension of the calcium compound, and p
A step of adjusting H to generate a precipitate of calcium hydroxide in a region having a pH of 12 to 13.5, and separating the solid-liquid to obtain calcium hydroxide having a reduced content of impurity metals (step 1 ] The manufacturing method of the high purity calcium hydroxide characterized by including these. (22) In the step 1, the alkali is added to the calcium compound or the solution or suspension of the calcium compound to adjust the pH, and the metal contained as an impurity in the calcium compound in the range of pH 10 to 12.5. The method for producing high-purity calcium hydroxide according to the above (21), which comprises a step (step 4) of forming a precipitate of the hydroxide and separating the solid-liquid to obtain a solution of a calcium compound. (23) The method for producing high-purity calcium hydroxide according to the above (21) or (22), wherein in step 1, the alkali is an organic alkali. (24) The method for producing high-purity calcium hydroxide according to the above (22), wherein in step 4, the alkali is an organic alkali. (25) The method for producing high-purity calcium hydroxide according to the above (23) or (24), wherein the organic alkali is tetramethylammonium hydroxide.

(26) In the above step 1, (21) or (2), wherein the alkali is a hydroxide of an alkali metal.
The method for producing high-purity calcium hydroxide according to 2). (27) The method for producing high-purity calcium hydroxide according to (22) above, wherein in step 4, the alkali is a hydroxide of an alkali metal. (28) The method for producing high-purity calcium hydroxide according to the above (26) or (27), wherein the alkali metal hydroxide is sodium hydroxide and / or potassium hydroxide. (29) The pH is adjusted by adding an alkali to the calcium compound or the solution or suspension of the calcium compound,
a step (step 1) of forming a precipitate of calcium hydroxide in a region having a pH of 12 to 13.5 and separating the solid-liquid to obtain calcium hydroxide having a reduced content of impurity metals; The pH is adjusted to 1 to 7 by adding an acid to calcium hydroxide or a solution or suspension of the calcium hydroxide obtained through 1, and is selected from the group consisting of oxalic acid, oxalates and oxalic acid compounds. At least one compound is added to form a precipitate of calcium oxalate, and a solid-liquid separation is performed to obtain calcium oxalate having a reduced content of impurity metals (step 2). A method for producing high-purity calcium oxalate. (30) In the step 1, the alkali is added to the calcium compound or the solution or suspension of the calcium compound to adjust the pH, and the metal contained as an impurity in the calcium compound in the range of pH 10 to 12.5. The method for producing high-purity calcium oxalate according to (29) above, which includes a step (step 4) of producing a precipitate of hydroxide and separating a solid-liquid to obtain a solution of a calcium compound.

(31) The method for producing high-purity calcium oxalate according to the above (29) or (30), wherein in step 1, the alkali is an organic alkali. (32) The method for producing high-purity calcium oxalate according to (30) above, wherein in step 4, the alkali is an organic alkali. (33) The method for producing high-purity calcium oxalate according to the above (31) or (32), wherein the organic alkali is tetramethylammonium hydroxide. (34) The method for producing high-purity calcium oxalate according to the above (29) or (30), wherein in step 1, the alkali is a hydroxide of an alkali metal. (35) The method for producing high-purity calcium oxalate according to the above (32), wherein in step 4, the alkali is a hydroxide of an alkali metal.

(36) The method for producing high-purity calcium oxalate according to the above (34) or (35), wherein the alkali metal hydroxide is sodium hydroxide and / or potassium hydroxide. (37) The high purity according to the above (29), wherein in the step 2, the acid added to the calcium hydroxide or the solution or suspension of the calcium hydroxide obtained through the step 1 is hydrochloric acid and / or nitric acid. A method for producing calcium oxalate. (38) The high-purity calcium oxalate according to the above (29), which is purified by bringing the oxalic acid, the oxalate salt, and the oxalic acid compound into contact with a solid substance having an ion exchange property in the step 2. Production method. (39) The method for producing high-purity calcium oxalate according to the above (38), wherein the solid substance having an ion-exchange property is an ion-exchange resin, a chelate resin, zeolite and / or activated carbon. (40) In the step 2, the oxalic acid compound is methyl oxalate, and after the addition of the methyl oxalate, the step (step 6) of heating the solution to 40 to 100 ° C. Method for producing high purity calcium oxalate. (41) In the step 2, the concentration of the oxalic acid, oxalate salt or oxalic acid compound solution added is 0.1% or more and 10% or less in terms of oxalic acid. Calcium acid production method.

[0021]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in detail below. First, the method for producing high-purity calcium fluoride of the present invention will be described. In the method for producing high-purity calcium fluoride of the present invention, the pH is adjusted by adding an alkali to a calcium compound or a solution or suspension of the calcium compound, and calcium hydroxide is added in a region between 12 and 13.5. And a solution of the calcium hydroxide obtained through the above step 1 (step 1), in which a precipitate is generated and solid-liquid is separated to obtain calcium hydroxide having a reduced content of impurity metals. Alternatively, an acid is added to the suspension to adjust the pH to 1 to 7, and at least one compound selected from the group consisting of oxalic acid, oxalate and oxalic acid compounds is added to precipitate calcium oxalate. A step (step 2) of producing and separating the solid-liquid to obtain calcium oxalate with a reduced content of impurity metals;
At least one selected from the group consisting of hydrogen fluoride, fluorine gas and ammonium fluoride is added to the calcium oxalate or the solution or suspension of the calcium oxalate obtained through the above step 2. The method is characterized by including a step (step 3) of forming a precipitate and separating the solid-liquid to obtain calcium fluoride having a reduced content of impurity metals.

In the method of the present invention, copper, cadmium,
Calcium oxide (quick lime), calcium hydroxide (slaked lime), calcium carbonate, calcium chloride, calcium nitrate containing impurity metals such as iron, manganese, cobalt, nickel, magnesium, sodium, potassium, strontium, barium, aluminum, lead and zinc. Any calcium salt such as can be used as a raw material. The concentration of each impurity metal element contained varies greatly from several mass ppm to several mass% depending on the raw material to be obtained, but the method of the present invention is not limited to the concentration of the impurity metal contained. Therefore, an inexpensive calcium salt having a high content of impurity metals can be used as a raw material, and the raw material cost can be suppressed to a low level.

In step 1, for example, calcium oxide or calcium carbonate is used as a raw material. As described above, there is no limitation on the concentration of the impurity metal contained in these calcium raw materials. First, water such as water is added to the calcium raw material, and an acid such as hydrochloric acid or nitric acid is added to and dissolved in the suspension. When a water-soluble raw material such as calcium chloride or calcium nitrate containing an impurity metal is used, water can be added to calcium chloride or calcium nitrate to obtain an aqueous solution. The pH of the solution is adjusted to 12 to 13.5 by adding an alkali to the calcium aqueous solution obtained by the above method.
Preferably the pH is adjusted to 13. As the alkali to be added in step 1, an alkali metal hydroxide such as sodium hydroxide or potassium hydroxide can be used, but a high-purity organic alkali such as tetramethylammonium hydroxide having a low metal content is used. It is preferable. When a compound having a high solubility in water such as calcium chloride is used, the alkali may be added directly.

The pH of the solution is adjusted to 12 to 1 by adding alkali.
Adjusting to 3.5 produces calcium hydroxide precipitates. Alkali metal ions such as sodium and potassium contained in the calcium salt used as the raw material, alkaline earth metal ions such as strontium and barium, and ions of metals that produce amphoteric hydroxides such as aluminum, lead and zinc are It remains on the solution side without forming a hydroxide precipitate. The solid solution is separated from the solution containing the precipitate of calcium hydroxide obtained by the above-mentioned method by a usual method such as filtration, sedimentation separation and centrifugation. Ions of metals such as sodium, potassium, strontium, barium, aluminum, lead, and zinc are separated and removed from calcium hydroxide on the solution side to obtain high-purity calcium hydroxide having a low content of impurity metals.

In the step 1, the alkali is added to the calcium compound or the solution or suspension of the calcium compound to adjust the pH, and the metal contained as an impurity in the calcium compound in the range of pH 10 to 12.5. It is preferable to include a step (step 4) of forming a precipitate of hydroxide and separating the solid-liquid to obtain a solution of a calcium compound. As the alkali to be added in step 4, a hydroxide of an alkali metal such as sodium hydroxide or potassium hydroxide can be used, but an organic alkali such as high-purity tetramethylammonium hydroxide having a low metal content should be used. Is preferred.

The pH of the solution is adjusted to 10 to 1 by adding an alkali.
In a calcium compound solution whose pH is adjusted to 2.5, preferably 12, hydroxides of impurities such as copper, cadmium, iron, manganese, cobalt, nickel and magnesium contained in the calcium salt used as the raw material Precipitates. At the same time, some calcium in the solution also precipitates as hydroxide. Obtained by the above method,
The liquid containing the metal hydroxide precipitate is separated into solid and liquid by a usual method such as filtration, sedimentation separation, and centrifugation to obtain a solution of calcium compound. On the solution side, in addition to calcium ions as the main component, ions such as sodium, potassium, strontium, barium, aluminum, lead, and zinc are contained as impurity metals. When step 4 is carried out, following step 4, the pH of the solution is adjusted to 12 to 12 by adding alkali again to the solution of the calcium compound obtained by the above method.
Adjust the pH to 13.5, preferably 13.

By carrying out step 1, preferably step 4, calcium hydroxide precipitates can be selectively produced in the region where the pH of the solution is between 12 and 13.5. It is possible to reduce the content of each impurity metal element such as copper, cadmium, iron, manganese, cobalt, nickel, sodium, potassium, barium, aluminum, lead and zinc contained in calcium to 0.1 mass ppm or less. . For magnesium and strontium, the pH range in which precipitation of hydroxide is generated is close to the pH range in which precipitation of calcium hydroxide is generated. Inclusion of magnesium and strontium hydroxide during the precipitation of calcium hydroxide is unavoidable. Therefore, in the manufacturing method of the present invention, a step of further separating and removing magnesium and strontium is performed after the above step.

In step 2, for example, pure water is added to the calcium hydroxide obtained through step 1 to form a suspension, and an acid is added to this suspension to dissolve the calcium hydroxide again. The acid to be added is preferably hydrochloric acid or nitric acid. However,
Since commercially available acids such as hydrochloric acid and nitric acid contain impurity metals such as iron, it is preferable to use them after purification by distillation or the like. After the calcium hydroxide is dissolved, the acid is added in excess to lower the pH of the solution to a range of 7 or less, preferably 1 to 4, more preferably pH 2. Subsequently, oxalate ions are added to the calcium solution obtained by the above method to selectively generate a calcium oxalate precipitate.

Since the solubility of magnesium and strontium oxalate is higher than that of calcium oxalate, magnesium and strontium ions are separated and removed on the solution side, and the calcium hydroxide obtained in step 1 is removed. It is possible to separate magnesium and strontium, which are mixed in about several ppm by mass, from the precipitate of calcium oxalate. Add acid before adding calcium oxalate ion to produce calcium oxalate precipitate
It is possible to further increase the solubility of magnesium and strontium and improve the accuracy of separation of calcium oxalate from the precipitate by adjusting the pH of the oxalate to an acidic region. However, lowering the pH below 1 is not preferable because the solubility of calcium oxalate also increases and the yield decreases.

The oxalate ion added to the above calcium solution is preferably at least one compound selected from the group consisting of oxalic acid, oxalates and oxalate compounds. These compounds may be solids or aqueous solutions. For example, since commercially available oxalic acid contains an impurity metal such as iron, it is preferable to use it after purifying it. The metal element contained in oxalic acid can be separated and removed by bringing oxalic acid into contact with a substance having a metal ion adsorbing property such as a cation exchange resin, a chelate resin, zeolite, or activated carbon. Specifically, for example, the above-mentioned metal ion-exchangeable substance is packed in a column, washed with an acid such as hydrochloric acid and pure water, and then an oxalic acid aqueous solution containing metal ions is passed through.

In order to suppress inclusion of magnesium and strontium oxalate in the precipitation of calcium oxalate, the oxalic acid aqueous solution added to the calcium solution preferably has a low oxalic acid concentration. In particular,
A method in which a solution having an oxalic acid concentration of 10% or less, preferably 1% or less is added dropwise to the above calcium solution little by little is preferable. Further, in order to suppress inclusion of magnesium and strontium oxalate in the precipitation of calcium oxalate, a compound such as methyl oxalate that produces oxalate ion by thermal decomposition was used as a raw material of oxalate ion. A homogeneous precipitation method can be applied. Specifically, after adding methyl oxalate to the above calcium solution, step 6 of gradually heating the solution in a water bath in the range of 40 to 100 ° C. is performed, and the methyl oxalate is thermally decomposed to form a solution. Oxalate ions are generated at a uniform concentration therein, and combine with calcium ions in the solution to selectively form a precipitate of calcium oxalate. By gradually increasing the amount of oxalate ion in the solution by this method, inclusion of magnesium and strontium oxalate in the precipitation of calcium oxalate can be suppressed.

Subsequently, the liquid containing the precipitate of calcium oxalate obtained by the above method is separated into solid and liquid by a usual method such as filtration, sedimentation and centrifugation. Ions of magnesium and strontium can be separated and removed from calcium oxalate to obtain high-purity calcium oxalate having a low content of impurity metals. The contents of magnesium and strontium contained in the calcium oxalate obtained by the above method can each be reduced to 0.1 mass ppm or less.

In step 3, for example, pure water is added to the calcium oxalate obtained through step 2 to form a suspension, and hydrogen fluoride generated by volatilizing anhydrous hydrofluoric acid into the suspension,
At least one selected from the group consisting of fluorine gas and ammonium fluoride is introduced to generate calcium fluoride precipitate. The higher the purity of hydrogen fluoride, fluorine gas, and ammonium fluoride used, the better, and the content of impurity metals is 0.01 mass pp for each metal element.
It is preferably m or less.

Subsequently, the solid-liquid separation is carried out by a usual method such as filtration, sedimentation separation, centrifugation or the like, whereby calcium fluoride containing a small amount of impurity metals can be obtained.
Copper, cadmium, iron, manganese, cobalt, nickel, magnesium, sodium, potassium, strontium, barium, aluminum is carried out by sequentially carrying out the above steps 1 to 2 and step 3, and preferably step 4. It is possible to obtain calcium fluoride in which the content of each of impurity metal elements such as lead, zinc and the like is 0.1 mass ppm or less.

The step (step 5) of reducing the oxygen contained in the calcium fluoride by washing the calcium fluoride obtained through the above step 3 preferably with pure water and then drying it and then contacting it with fluorine gas is carried out. It is preferable to include. Specifically, a reactor made of nickel or monel is filled with calcium fluoride obtained through the step 3, and the reactor is heated to 50 ° C or higher and 600 ° C or lower, preferably 300 ° C or higher 4
Fluorine gas is circulated by heating to a temperature of 00 ° C. or lower.
The higher the purity of fluorine gas, the better. Especially, the content of impurity metals is 0.01 mass ppm or less, and the content of oxygen is 1
It is preferably 0 mass ppm or less.

In step 5, fluorine acts on the moisture contained in the calcium fluoride and the oxygen element in the form of oxide chemically bound to calcium, and the moisture reacts with the fluorine to disappear, resulting in the oxide form. It is considered that the oxygen content of oxygen can be reduced by substituting the elemental oxygen with the elemental fluorine.
It can be reduced to 00 mass ppm or less.

Further, the method for producing calcium fluoride of the present invention includes the step 7 of carrying out the step 3 and then recovering and purifying the oxalic acid from the solution in which the calcium fluoride is separated. Is preferably reused in step 2. Oxalic acid generated when a precipitate of calcium fluoride is produced contains a trace amount of impurity metal elements separated from the precipitate of calcium fluoride. Aqueous solution of oxalic acid, cation exchange resin, chelate resin,
It can be easily purified by passing it through a column packed with a substance having a metal ion adsorbing property such as zeolite or activated carbon. The purified oxalic acid can be reused again in the second step.

Further, the method for producing calcium fluoride of the present invention comprises the step of selectively producing a precipitate of calcium oxalate in the above step 2 and then separating the solid-liquid from a solution containing hydrochloric acid or nitric acid. It is preferable to collect and purify the acid by an operation such as distillation. Acids such as hydrochloric acid or nitric acid, which are generated when calcium oxalate precipitates are produced,
It contains impurity metal elements such as magnesium and strontium that are separated from the precipitation of calcium oxalate. The acid such as hydrochloric acid or nitric acid can be easily purified by a distillation operation, and the purified acid such as hydrochloric acid or nitric acid can be reused again in step 2.

As described above, the high-purity calcium fluoride obtained by the production method of the present invention is characterized by having a low content of impurity metal elements, and in particular, the content of strontium contained as an impurity is 0. It is characterized by being 1 mass ppm or less. Further, the contents of potassium, sodium, magnesium, barium and iron contained as impurities are each 0.1 mass ppm or less, and the content of oxygen element is 100 mass ppm or less. Impurity metals can be obtained, for example, by inductively coupled plasma emission spectrometry (ICP-AE) by dissolving or diluting a sample to be measured in pure or acid.
S), inductively coupled plasma mass spectrometry (ICP-MS) and the like can be used for analysis. The oxygen element can be measured, for example, by putting a sample to be measured in a carbon crucible
Carbon monoxide and / or carbon dioxide produced by heating to high temperatures up to 000 ° C. can be analyzed using infrared spectroscopy.

Next, a method for producing intermediate calcium hydroxide and calcium oxalate which can be used in producing the high-purity calcium fluoride of the present invention will be described.
In the method for producing high-purity calcium hydroxide according to the present invention, the pH is adjusted by adding an alkali to a calcium compound or a solution or suspension of the calcium compound so that the pH is 12 to 1
The method is characterized by including a step (step 1) of producing a precipitate of calcium hydroxide in a region between 3.5 and separating the solid-liquid to obtain calcium hydroxide having a reduced content of impurity metals.

In the step 1, the alkali is added to the calcium compound or the solution or suspension of the calcium compound to adjust the pH, and the metal contained as an impurity in the calcium compound in the range of pH 10 to 12.5. It is preferable to include a step (step 4) of forming a precipitate of hydroxide and separating the solid-liquid to obtain a solution of a calcium compound.

In step 1 and step 4, the alkali to be added may be an alkali metal hydroxide such as sodium hydroxide or potassium hydroxide, but high purity tetramethyl hydroxide having a low metal content. It is preferable to use an organic alkali such as ammonium.

The method for producing high-purity calcium oxalate of the present invention comprises adjusting the pH by adding an alkali to a calcium compound or a solution or suspension of the calcium compound,
a step (step 1) of forming a precipitate of calcium hydroxide in a region having a pH of 12 to 13.5 and separating the solid-liquid to obtain calcium hydroxide having a reduced content of impurity metals; The pH is adjusted to 1 to 7 by adding an acid to calcium hydroxide or a solution or suspension of the calcium hydroxide obtained through 1, and is selected from the group consisting of oxalic acid, oxalates and oxalic acid compounds. At least one compound is added to form a precipitate of calcium oxalate, and a solid-liquid separation is performed to obtain calcium oxalate having a reduced content of impurity metals (step 2). .

In the step 1, the alkali is added to the calcium compound or the solution or suspension of the calcium compound to adjust the pH, and the metal contained in the calcium compound as an impurity in the pH range of 10 to 12.5. It is preferable to include a step (step 4) of forming a precipitate of hydroxide and separating the solid-liquid to obtain a solution of a calcium compound.

In step 1 and step 4, the alkali to be added may be an alkali metal hydroxide such as sodium hydroxide or potassium hydroxide, but high purity tetramethyl hydroxide having a low metal content. It is preferable to use an organic alkali such as ammonium.

In the step 2, the acid added to the calcium hydroxide or the solution or suspension of the calcium hydroxide obtained through the step 1 is preferably hydrochloric acid and / or nitric acid.

In the step 2, it is preferable that the oxalic acid, the oxalate salt and the oxalic acid compound are purified by contacting with a solid substance having an ion exchange property, and the solid substance having an ion exchange property is preferable. Is preferably an ion exchange resin, a chelate resin, zeolite and / or activated carbon.

In the above step 2, the oxalic acid compound is methyl oxalate, and it is preferable to include a step (step 6) of heating the solution to 40 to 100 ° C. after adding the methyl oxalate. , The concentration of the solution of oxalic acid, oxalate or oxalate compound to be added,
It is preferably 0.1% or more and 10% or less in terms of oxalic acid.

[0049]

EXAMPLES The present invention will be described below with reference to examples and comparative examples, but the present invention is not limited to these examples.

Example 1 High-purity calcium fluoride was synthesized using 98% pure calcium oxide as the raw material calcium salt. Impurity metals contained in this raw material calcium oxide (sodium, potassium, magnesium,
The concentrations of strontium, barium, copper, cadmium, iron, manganese, cobalt, nickel, aluminum, lead and zinc) are shown in Table 1.

Water was added to calcium oxide (200 g) as a raw material to prepare a suspension of 0.6 liter, and 0.8 liter of 35% hydrochloric acid was added and dissolved therein. A commercially available aqueous solution of high-purity 20% tetramethylammonium hydroxide was added to this calcium solution to adjust the pH of the solution to 12.
After stirring, the mixture was allowed to stand at room temperature for 24 hours, and a solid liquid was separated by filtration. Subsequently, while stirring the obtained calcium solution, a high-purity aqueous solution of 20% tetramethylammonium hydroxide was added again to adjust the pH of the solution to 13.
A white precipitate of calcium hydroxide formed. The obtained liquid containing the precipitate of calcium hydroxide was allowed to stand at room temperature for 1 hour and then filtered again to separate a solid liquid. Part of the obtained calcium hydroxide was dried at 110 ° C., and the result of measuring the impurity metal concentration of calcium hydroxide (calcium hydroxide after step 1) is shown in Table 1.

Pure water was added to a portion (20 g) of the calcium hydroxide obtained through the step 1 to form a suspension of 0.3 liter. 35% hydrochloric acid of high purity obtained by distilling and purifying commercially available hydrochloric acid was added to dissolve calcium hydroxide,
The pH of the solution was adjusted to 2. On the other hand, a commercially available cation exchange resin (made by Organo) was packed in a polytetrafluoroethylene column, and after passing through hydrochloric acid and pure water to wash it, a commercially available 2% aqueous solution of oxalic acid was placed in the column. The solution was passed to purify the oxalic acid aqueous solution. While stirring the calcium solution obtained by the above method, the high-purity 2% oxalic acid aqueous solution (1.2 liters) obtained by the above method was added dropwise little by little, and a white precipitate of calcium oxalate was obtained. Was generated. After standing at room temperature for 1 hour, the obtained liquid containing a precipitate of calcium oxalate was filtered to separate a solid liquid. Part of the obtained calcium oxalate was dried at 110 ° C., and the result of measuring the impurity metal concentration of calcium oxalate (calcium oxalate after step 2) is shown in Table 1. In addition, regarding the 2% oxalic acid aqueous solution purified by passing through a commercially available oxalic acid raw material, a commercially available ion exchange resin (manufactured by Organo), a chelate resin (manufactured by Sumitomo Chemical), and activated carbon (manufactured by Kureha), Table 2 shows the measurement results
It was shown to.

Pure water was added to a portion (10 g) of the calcium oxalate obtained through the step 2 to form a suspension of 0.3 liter. While stirring this suspension at room temperature, a hydrogen fluoride vapor generated by heating a container filled with anhydrous hydrofluoric acid at 50 ° C. was diluted with nitrogen to a concentration of 20% to prepare a mixed gas, and the mixture gas was adjusted to 0. When introduced at a flow rate of 2 liters, a white precipitate of calcium fluoride was produced. Then, the solid-liquid was separated by filtration to obtain high-purity calcium fluoride (5 g) containing less impurity metal. Part of the obtained calcium fluoride was dried at 110 ° C., and the result of measuring the concentration of the contained impurity metal (calcium fluoride after step 3) is shown in Table 1.

[0054]

[Table 1]

As is apparent from Table 1, sodium oxide, potassium oxide, magnesium oxide, strontium oxide, barium oxide, copper oxide, and calcium oxide are obtained by sequentially performing step 1 to step 2 and step 3 using calcium oxide containing a large amount of impurity metal as a raw material.
It was possible to produce high-purity calcium fluoride in which the content of each impurity metal element such as cadmium, iron, manganese, cobalt, nickel, aluminum, lead, and zinc was 0.1 mass ppm or less.

[0056]

[Table 2] As is clear from Table 2, the metal element contained in oxalic acid can be easily removed by bringing an oxalic acid aqueous solution containing an impurity metal into contact with a solid having ion exchangeability such as an ion exchange resin, a chelate resin, or activated carbon. It could be purified.

Example 2 The acid added to the suspension of calcium oxide in step 1 was nitric acid, and the acid added to the suspension of calcium hydroxide in step 2 was high-purity nitric acid purified by distillation. Calcium fluoride was produced by the same procedure as in Example 1 except for the above. Calcium hydroxide obtained in step 1,
Table 3 shows the concentration of each impurity metal contained in the calcium oxalate obtained in step 2 and the calcium fluoride obtained in step 3.

[0058]

[Table 3]

As is apparent from Table 3, calcium oxide containing a large amount of impurity metals is used as a raw material, and steps 1 to 2 and step 3 are sequentially performed, so that the content of each impurity metal element is 0.1 ppm. The following high purity calcium fluoride could be produced.

Example 3 Calcium fluoride was produced in the same procedure as in Example 1 except that the alkali added to the calcium solution in step 1 was 10% sodium hydroxide aqueous solution. Table 4 shows the impurity metal concentrations of the calcium hydroxide obtained in step 1, the calcium oxalate obtained in step 2, and the calcium fluoride obtained in step 3, respectively.

[0061]

[Table 4]

As is clear from Table 4, by using Step 1 to Step 2 and Step 3 sequentially by using calcium oxide containing a large amount of impurity metal as a raw material, the content of all impurity metal elements is 0.1 ppm each. The following high purity calcium fluoride could be produced.

Example 4 Calcium fluoride was produced in the same procedure as in Example 1 except that the alkali added to the calcium solution in step 1 was 10% potassium hydroxide aqueous solution. Table 5 shows the impurity metal concentrations of the calcium hydroxide obtained in step 1, the calcium oxalate obtained in step 2, and the calcium fluoride obtained in step 3, respectively.

[0064]

[Table 5]

As is apparent from Table 5, by using Step 1 to Step 2 and Step 3 sequentially using calcium oxide containing a large amount of impurity metal as a raw material, the content of each impurity metal element is 0.1 ppm. The following high purity calcium fluoride could be produced.

Example 5 Methyl oxalate (50 g) was used as the oxalate ion added to the calcium solution in step 2, and the solution containing methyl oxalate was added at 100 ° C. in a water bath.
Calcium fluoride was produced by the same procedure as in Example 1 except that Step 6 in which the mixture was heated to was performed. Table 6 shows the contained impurity metal concentrations of the calcium oxalate obtained in step 2 and the calcium fluoride obtained in step 3, respectively.

[0067]

[Table 6]

As is clear from Table 6, by using Step 1 to Step 2 and Step 3 sequentially using calcium oxide containing a large amount of impurity metal as a raw material, the content of all impurity metal elements is 0.1 ppm each. The following high purity calcium fluoride could be produced.

Example 6 Calcium fluoride was produced by the same procedure as in Example 1 except that the fluorine source introduced into the calcium oxalate suspension in step 3 was fluorine gas.
Table 7 shows the impurity metal concentrations of calcium fluoride obtained in step 3.

[0070]

[Table 7]

As is clear from Table 7, calcium oxide containing a large amount of impurity metal is used as a raw material, and step 1, step 2, and step 3 are sequentially performed, whereby the content of all impurity metal elements is 0.1 ppm. The following high purity calcium fluoride could be produced.

(Example 7) Example 7 except that ammonium fluoride was used as the fluorine source to be introduced into the calcium oxalate suspension in step 3, and ammonium fluoride (10 g) was added to the calcium oxalate suspension. Calcium fluoride was produced by the same procedure as in 1. Table 8 shows the concentration of metal impurities contained in calcium fluoride obtained in step 3.

[0073]

[Table 8]

As is clear from Table 8, by using Step 1 to Step 2 and Step 3 sequentially using calcium oxide containing a large amount of impurity metal as a raw material, the content of all impurity metal elements is 0.1 ppm each. The following high purity calcium fluoride could be produced.

Example 8 Calcium fluoride was produced by the same procedure as in Example 1 except that calcium carbonate having a purity of 99% was used as the raw material calcium salt. Regarding the calcium carbonate used as a raw material, the calcium hydroxide obtained in the step 1, the calcium oxalate obtained in the step 2 and the calcium fluoride obtained in the step 3, the respective impurity metal concentrations contained are shown in Table 9. .

[0076]

[Table 9]

As is clear from Table 9, by using calcium carbonate containing a large amount of impurity metal as a raw material and sequentially performing step 1 to step 2 and step 3, the content of all impurity metal elements is 0.1 ppm each. The following high purity calcium fluoride could be produced.

Example 9 Fluorination was carried out in the same manner as in Example 1 except that calcium chloride having a purity of 99% was used as the raw material calcium salt and the operation of adding hydrochloric acid to the suspension was omitted in step 1. Generated calcium. Calcium chloride used as a raw material, calcium hydroxide obtained in step 1,
Table 10 shows the concentration of each impurity metal contained in the calcium oxalate obtained in the step 2 and the calcium fluoride obtained in the step 3.

[0079]

[Table 10]

As is clear from Table 10, by using Step 1 to Step 2 and Step 3 sequentially using calcium chloride containing a large amount of impurity metal as a raw material, the content of all impurity metal elements is 0.1 ppm each. The following high purity calcium fluoride could be produced.

Example 10 Calcium fluoride having a purity of 98% was used as a raw material calcium salt, and calcium fluoride was prepared in the same procedure as in Example 2 except that the step of adding nitric acid to the suspension was omitted in Step 1. Was generated. Calcium nitrate used as a raw material, calcium hydroxide obtained in step 1,
Table 11 shows the contained impurity metal concentrations of the calcium oxalate obtained in step 2 and the calcium fluoride obtained in step 3, respectively.

[0082]

[Table 11]

As is apparent from Table 11, calcium nitrate containing a large amount of impurity metal is used as a raw material, and step 1, step 2, and step 3 are sequentially performed, whereby the content of each impurity metal element is 0.1 ppm. The following high purity calcium fluoride could be produced.

Example 11 A part (0.5 g) of the calcium fluoride obtained in Example 1 was dried at 110 ° C. for 2 hours and then charged into a nickel reaction tube having an inner diameter of 10 mm. Step 5 was performed by introducing fluorine gas into the reaction tube at a flow rate of 0.1 liter per minute. The temperature for heating the reaction tube is 10
Fluorine treatment of calcium fluoride was carried out under each condition with the temperature of 0 ° C. or 350 ° C. and the time of flowing the mixed gas for 1 hour or 5 hours. The results of measuring the oxygen content in the obtained calcium fluoride are shown in Table 12.

[0085]

[Table 12]

As is clear from Table 12, by treating calcium fluoride containing oxygen with fluorine, the oxygen concentration in calcium fluoride could be reduced to 100 ppm or less. Further, under the processing condition that the heating temperature is 100 ° C., the oxygen concentration contained in calcium fluoride is 100 pp.
While it took about 5 hours to lower the temperature to m or less, under the heating temperature of 350 ° C., the oxygen concentration in calcium fluoride was 100 ppm or less in only 1 hour. Could be reduced to.

(Comparative Example 1) The procedure of Step 1 and Step 2 of Example 1 was repeated except that the pH was adjusted to 8 when hydrochloric acid was added to the suspension of calcium hydroxide in Step 2. Calcium acid was produced. Table 15 shows the impurity metal concentrations of calcium oxalate obtained in step 2.

[0088]

[Table 13]

As is clear from the comparison between Table 1 and Table 13, when the pH before adding the oxalate ion to the calcium solution was increased in step 2, magnesium and strontium were not formed during the precipitation of calcium oxalate. It becomes easy to be included and it turns out that the precipitation of calcium oxalate cannot be selectively generated.

Comparative Example 2 Calcium oxalate is produced by the same procedure as the steps 1 and 2 of Example 1 except that the concentration of the oxalic acid aqueous solution added to the calcium solution in step 2 is 11%. did. Table 14 shows the impurity metal concentrations of calcium oxalate obtained in step 2.

[0091]

[Table 14]

As is clear from the comparison between Table 1 and Table 14, when the concentration of the oxalic acid aqueous solution added to the calcium solution in step 2 was increased, magnesium and strontium were included in the calcium oxalate precipitate. It is easy to see that calcium oxalate precipitates cannot be selectively formed.

(Comparative Example 3) Calcium fluoride was treated in the same procedure as in Example 11 except that the gas flowing through the reaction tube filled with calcium fluoride (0.5 g) in step 5 was nitrogen. . Table 15 shows the results of measuring the oxygen content in the obtained calcium fluoride.

[0094]

[Table 15]

As is clear from the comparison between Table 1 and Table 15, it is understood that when the calcium fluoride containing oxygen is treated with nitrogen, the oxygen concentration in calcium fluoride cannot be sufficiently reduced.

[0096]

As described above, according to the method of the present invention, each of the impurity metal elements contained in the raw material calcium salt is obtained by sequentially using the inexpensive calcium salt containing a large amount of the impurity metal as a raw material and sequentially performing each step. It is possible to efficiently produce high-purity calcium fluoride suitable as a raw material for producing a calcium fluoride crystal used as an optical member in the vacuum ultraviolet wavelength region, in particular, by sequentially separating and removing. Further, by reusing the substance used in each step, the manufacturing cost can be reduced and a practical manufacturing process can be established.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Hitoshi Atobe             Showaden 5-1 Ogimachi, Kawasaki-ku, Kawasaki-shi, Kanagawa             Kawasaki Production & Engineering Department F-term (reference) 4G076 AA05 AA10 AA26 AB06 AB12                       AB18 BA33 BD01 BE11 CA36                       DA11                 4G077 AA01 AA02 AB01 BE02 CB02                       EB01 EB06 EC04                 4H006 AA02 AC47 BB31 BC53 BE11                       BS10 BS70

Claims (41)

[Claims] 1. A pH is adjusted by adding an alkali to a calcium compound or a solution or suspension of a calcium compound to form a precipitate of calcium hydroxide in a region between pH 12 and 13.5. To obtain calcium hydroxide having a reduced content of impurity metals (step 1), and adding an acid to the calcium hydroxide or the solution or suspension of the calcium hydroxide obtained through the step 1 Then p
H is adjusted to 1 to 7 and at least one compound selected from the group consisting of oxalic acid, oxalate and oxalic acid compounds is added to form a precipitate of calcium oxalate,
A step (step 2) of separating the solid-liquid to obtain calcium oxalate having a reduced content of impurity metals; and a step or a solution or suspension of calcium oxalate obtained through the step 2, Calcium fluoride in which at least one selected from the group consisting of hydrogen fluoride, fluorine gas, and ammonium fluoride is added to generate calcium fluoride precipitate, and solid-liquid is separated to reduce the content of impurity metals. A method for producing high-purity calcium fluoride, comprising the step (step 3) of obtaining 2. The step 1 comprises adding an alkali to a calcium compound or a solution or suspension of a calcium compound to adjust the pH, and the calcium compound being contained as an impurity in the range of pH 10 to 12.5. The method for producing high-purity calcium fluoride according to claim 1, further comprising a step (step 4) of producing a precipitate of a metal hydroxide and separating the solid-liquid to obtain a solution of a calcium compound. 3. The method according to claim 1, comprising a step (step 5) of drying the calcium fluoride obtained through the step 3 and then contacting it with fluorine gas to reduce oxygen contained in the calcium fluoride. A method for producing high-purity calcium fluoride. 4. The method for producing high-purity calcium fluoride according to claim 1, wherein in step 1, the alkali is an organic alkali. 5. The method for producing high-purity calcium fluoride according to claim 2, wherein in step 4, the alkali is an organic alkali. 6. The method for producing high-purity calcium fluoride according to claim 4, wherein the organic alkali is tetramethylammonium hydroxide. 7. The method for producing high-purity calcium fluoride according to claim 1, wherein in step 1, the alkali is a hydroxide of an alkali metal. 8. The method for producing high-purity calcium fluoride according to claim 2, wherein in step 4, the alkali is a hydroxide of an alkali metal. 9. The method for producing high-purity calcium fluoride according to claim 7, wherein the alkali metal hydroxide is sodium hydroxide and / or potassium hydroxide. 10. The high acid according to claim 1, wherein the acid added to the calcium hydroxide obtained through the step 1 or the solution or suspension of the calcium hydroxide in the step 2 is hydrochloric acid and / or nitric acid. Method for producing pure calcium fluoride. 11. The high-purity calcium fluoride according to claim 1, wherein in the step 2, the oxalic acid, the oxalate salt and the oxalic acid compound are purified by bringing them into contact with a solid substance having an ion exchange property. Manufacturing method. 12. The method for producing high-purity calcium fluoride according to claim 11, wherein the solid substance having an ion-exchange property is an ion-exchange resin, a chelate resin, zeolite and / or activated carbon. 13. The method according to claim 1, wherein in the step 2, the oxalic acid compound is methyl oxalate, and the method includes a step (step 6) of heating the solution to 40 to 100 ° C. after adding the methyl oxalate. A method for producing high-purity calcium fluoride. 14. The concentration of the solution of oxalic acid, oxalate or oxalate compound added in the step 2,
It is 0.1% or more and 10% or less in terms of oxalic acid.
The method for producing high-purity calcium fluoride according to 1. 15. The method includes a step (step 7) of recovering oxalic acid from a solution from which calcium fluoride has been separated and performing purification after performing the step 3, wherein the purified oxalic acid is added to the step 2.
The method for producing high-purity calcium fluoride according to claim 1, which is reused in step 1. 16. High-purity calcium fluoride, characterized in that the content of strontium contained as an impurity is 0.1 mass ppm or less. 17. The high-purity calcium fluoride according to claim 16, wherein the contents of potassium, sodium, magnesium, barium and iron contained as impurities are each 0.1 mass ppm or less. 18. The content of oxygen element contained as an impurity is 100 mass ppm or less.
High-purity calcium fluoride described in. 19. Calcium fluoride having a content of strontium, potassium, sodium, magnesium, barium and iron contained as impurities of 0.1 mass ppm or less and an oxygen element content of 100 mass ppm or less. A calcium fluoride single crystal for steppers. 20. Calcium fluoride having a content of strontium, potassium, sodium, magnesium, barium and iron contained as impurities of 0.1 mass ppm or less and an oxygen element content of 100 mass ppm or less. Calcium fluoride lens for steppers. 21. An alkali is added to a calcium compound or a solution or suspension of a calcium compound to adjust the pH, and a precipitate of calcium hydroxide is produced in a region of the pH of 12 to 13.5 to obtain a solid-liquid solution. A method for producing high-purity calcium hydroxide, which comprises a step (step 1) of separating calcium hydroxide to obtain calcium hydroxide having a reduced content of impurity metals. 22. In the step 1, the pH is adjusted by adding an alkali to the calcium compound or a solution or suspension of the calcium compound, and the calcium compound is contained as an impurity in the region of the pH of 10 to 12.5. The step (step 4) of forming a precipitate of a metal hydroxide and separating the solid-liquid to obtain a solution of a calcium compound.
1. The method for producing high-purity calcium hydroxide according to 1. 23. The method for producing high-purity calcium hydroxide according to claim 21, wherein in step 1, the alkali is an organic alkali. 24. The method for producing high-purity calcium hydroxide according to claim 22, wherein in step 4, the alkali is an organic alkali. 25. The method for producing high-purity calcium hydroxide according to claim 23, wherein the organic alkali is tetramethylammonium hydroxide. 26. The method for producing high-purity calcium hydroxide according to claim 21, wherein the alkali in step 1 is an alkali metal hydroxide. 27. The method for producing high-purity calcium hydroxide according to claim 22, wherein in step 4, the alkali is an alkali metal hydroxide. 28. The alkali metal hydroxide is sodium hydroxide and / or potassium hydroxide.
Or the method for producing high-purity calcium hydroxide according to 27. 29. An alkali is added to a calcium compound or a solution or suspension of a calcium compound to adjust the pH, and a precipitate of calcium hydroxide is produced in a region where the pH is between 12 and 13.5. To obtain calcium hydroxide having a reduced content of impurity metals (step 1), and adding an acid to the calcium hydroxide or the solution or suspension of the calcium hydroxide obtained through the step 1 Then p
H is adjusted to 1 to 7 and at least one compound selected from the group consisting of oxalic acid, oxalate and oxalic acid compounds is added to form a precipitate of calcium oxalate,
A method for producing high-purity calcium oxalate, which comprises a step (step 2) of separating solid-liquid to obtain calcium oxalate having a reduced content of impurity metals. 30. In the step 1, the pH is adjusted by adding an alkali to a calcium compound or a solution or suspension of the calcium compound, and the calcium compound is contained as an impurity in the region of the pH of 10 to 12.5. The step (step 4) of forming a precipitate of a metal hydroxide and separating the solid-liquid to obtain a solution of a calcium compound.
9. The method for producing high-purity calcium oxalate according to item 9. 31. The method for producing high-purity calcium oxalate according to claim 29 or 30, wherein the alkali in the step 1 is an organic alkali. 32. The method for producing high-purity calcium oxalate according to claim 30, wherein the alkali in the step 4 is an organic alkali. 33. The method for producing high-purity calcium oxalate according to claim 31, wherein the organic alkali is tetramethylammonium hydroxide. 34. The method for producing high-purity calcium oxalate according to claim 29 or 30, wherein the alkali in the step 1 is a hydroxide of an alkali metal. 35. The method for producing high-purity calcium oxalate according to claim 32, wherein in step 4, the alkali is a hydroxide of an alkali metal. 36. The alkali metal hydroxide is sodium hydroxide and / or potassium hydroxide.
Or the method for producing high-purity calcium oxalate according to 35. 37. The high acid according to claim 29, wherein the acid added to the calcium hydroxide obtained through the step 1 or the solution or suspension of the calcium hydroxide in the step 2 is hydrochloric acid and / or nitric acid. Method for producing high purity calcium oxalate. 38. The oxalic acid, the oxalate salt, and the oxalic acid compound in step 2, which are purified by contacting with a solid substance having an ion exchange property.
The method for producing high-purity calcium oxalate according to 1. 39. The method for producing high-purity calcium oxalate according to claim 38, wherein the solid substance having an ion exchange property is an ion exchange resin, a chelate resin, zeolite and / or activated carbon. 40. The method according to claim 29, wherein in step 2, the oxalic acid compound is methyl oxalate, and after the addition of methyl oxalate, the step of heating the solution to 40 to 100 ° C. (step 6) is included. A method for producing high-purity calcium oxalate. 41. In the step 2, the concentration of the solution of oxalic acid, oxalate or oxalic acid compound added is
It is 0.1% or more and 10% or less in terms of oxalic acid.
9. The method for producing high-purity calcium oxalate according to item 9.