JP2011126772A - Method for producing high purity calcium hydroxide - Google Patents

Abstract

High purity calcium hydroxide with low chlorine content and low impurity metal content is produced.
A) A step of preparing calcium hydroxide slurry by reacting quick lime obtained by calcining limestone with water, and B) a calcium hydroxide slurry having a pH in the range of 9.5 to 11.5. A step of adding nitric acid, C) a step of filtering the solution obtained by adding nitric acid to obtain a filtrate, and D) adding an alkali metal hydroxide to the filtrate to precipitate calcium hydroxide. And E) a step of separating the calcium hydroxide by filtering the precipitated calcium hydroxide.
[Selection figure] None

Description

  The present invention is a method for producing high-purity calcium hydroxide having a low content of impurity metals such as sodium, iron, magnesium and strontium and a low content of chlorine, and a high purity using the calcium hydroxide obtained by the method. The present invention relates to a production method for producing calcium carbonate, high-purity calcium nitrate, and high-purity calcium fluoride.

  Calcium carbonate produced mainly from calcium hydroxide is used as a raw material for electronic materials such as ceramic capacitors. If an impurity metal such as sodium, iron, magnesium, or strontium is mixed in calcium carbonate, there arises a problem that desired electronic characteristics cannot be obtained.

  In addition, calcium fluoride is used in an exposure apparatus lens for manufacturing a semiconductor. However, when an impurity metal is mixed, there arises a problem that light transmittance during exposure is greatly reduced.

  Calcium nitrate is used for lenses of digital cameras and the like. However, when an impurity metal is mixed, there is a problem that the light transmittance is greatly reduced for the same reason.

  Accordingly, when calcium carbonate, calcium fluoride, and calcium nitrate are used as raw materials for electronic components and optical materials, the content of impurity metals needs to be extremely low.

  In addition, if a large amount of chlorine is contained, the passive film of iron causes a local corrosion called pitting corrosion, which causes a serious problem that the electronic parts and the optical material itself deteriorate and corrode.

  As raw materials for calcium carbonate, calcium fluoride, and calcium nitrate, calcium hydroxide is generally used. For the above reasons, high-purity calcium hydroxide with low impurity metal content and low chlorine content is used. It has been demanded.

  In Patent Document 1, in order to produce calcium carbonate having a low strontium content, at least a part of strontium is eluted and removed into the aqueous phase by separating the calcium hydroxide slurry from the aqueous phase. The residual strontium is separated and removed by dissolving calcium hydroxide in an aqueous ammonium salt solution containing at least one of the above and removing insoluble matter generated in this dissolving operation.

  In Patent Document 2, the iron content in calcium carbonate is reduced by adding a chelating agent to an aqueous slurry of calcium carbonate and subjecting it to a heat treatment.

  In Patent Document 3, hydrochloric acid or nitric acid, or an aqueous solution of ammonium chloride or ammonium nitrate is added to an aqueous suspension of calcium hydroxide to dissolve calcium hydroxide, and then aqueous ammonia is added to precipitate calcium hydroxide. At the same time, impurities are precipitated, the precipitated impurities and the calcium salt aqueous solution are separated, and carbon dioxide is blown into the separated calcium salt aqueous solution to precipitate calcium carbonate.

  Patent Document 3 describes that nitric acid is used to dissolve calcium hydroxide, but ammonia water is added in the subsequent steps. For this reason, when nitric acid is used, ammonium nitrate is deposited that is very dangerous to handle because it has explosive properties when ammonia water is added. Therefore, generally, calcium hydroxide is dissolved using hydrochloric acid. However, when hydrochloric acid is used, there arises a problem that the chlorine content in the obtained calcium hydroxide is increased. When the chlorine content becomes high, as described above, the passive film of iron causes local corrosion called pitting corrosion, thereby causing deterioration and corrosion of the electronic component and the optical material itself.

JP-A-62-36021 Japanese National Patent Publication No. 10-509129 JP 2007-161515 A

  An object of the present invention is to provide a method for producing high-purity calcium hydroxide having a low chlorine content and a low content of impurity metals, and a high-purity calcium carbonate, a high-purity calcium carbonate using the calcium hydroxide obtained by the method. The object is to provide a method for producing pure calcium nitrate and high-purity calcium fluoride.

  The method for producing high-purity calcium hydroxide of the present invention comprises: A) a step of reacting water with quick lime obtained by calcining limestone to prepare a calcium hydroxide slurry, and B) a pH of 9. A step of adding nitric acid so as to be within a range of 5 to 11.5, C) a step of filtering the solution obtained by adding nitric acid to obtain a filtrate, and D) an alkali metal hydroxide in the filtrate And a step of separating the calcium hydroxide by filtering the precipitated calcium hydroxide.

  In the present invention, nitric acid is added to the calcium hydroxide slurry so that the pH is in the range of 9.5 to 11.5. Since nitric acid is used without using hydrochloric acid, according to the present invention, calcium hydroxide having a low chlorine content can be produced.

  Moreover, since nitric acid is added so as to be within the range of pH 9.5 to 11.5, calcium hydroxide with a small content of impurity metals can be produced with high yield. When the pH is less than 9.5, the content of impurities such as iron (Fe), magnesium (Mg), silicon (Si), and aluminum (Al) increases. If the pH exceeds 11.5, calcium hydroxide cannot be sufficiently dissolved, and the yield of the final high-purity calcium hydroxide decreases.

  Moreover, in the process D of this invention, the alkali metal hydroxide is added to the solution which dissolved calcium hydroxide with nitric acid, and calcium hydroxide is deposited. The alkali metal hydroxide may be potassium hydroxide, lithium hydroxide, or the like, but sodium hydroxide is preferable because it is highly reactive and inexpensive. Even if ammonia water is added to calcium nitrate, the reactivity is poor, and even if the reaction occurs, ammonium nitrate which is a dangerous substance is generated, which is not preferable. In addition, when sodium hydroxide is used as the alkali metal hydroxide, sodium nitrate produced by the reaction of calcium nitrate and sodium hydroxide contained in the filtrate is a compound with high solubility in water. By doing so, it can be easily removed, and high-purity calcium hydroxide with a low sodium content can be produced.

  In step E of the present invention, calcium hydroxide is separated by filtering the precipitated calcium hydroxide. In the present invention, it is preferable to heat the solution in which calcium hydroxide is precipitated to 60 ° C. or higher before filtering. The solubility of calcium hydroxide decreases with increasing temperature. On the other hand, since the solubility of metal compounds such as strontium (Sr) increases as the temperature rises, the metal compounds such as Sr can be further dissolved by heating to 50 ° C. or higher. Can be removed inside.

  In the present invention, the calcium hydroxide obtained in step E is dispersed again in water to obtain a calcium hydroxide dispersion, and the dispersion is heated to 50 ° C. or higher and filtered at least once. It is preferable. Thereby, metal compounds such as strontium can be further removed.

  It is preferable that the heating temperature in the said process after the process E and the process E is 50 degreeC or more, More preferably, it is 60 degreeC or more, and it is preferable that it is 80 degreeC or less.

  In the present invention, ultrafiltration may be used as the filtration in Step C. Impurity metals such as Si and Al can be further reduced by using ultrafiltration. In this case, since the pH of the solution to be filtered is high, it is preferable to use, for example, a material made of PS (polysulfone) as the material for the ultrafiltration membrane.

  In the present invention, ultrafiltration may be used as the filtration in step E. By using ultrafiltration, the impurity concentration can be further reduced, and higher-purity calcium hydroxide can be produced.

The alkali metal hydroxide added in the step D of the present invention is preferably added as an aqueous solution. The concentration of the alkali metal hydroxide aqueous solution is not particularly limited, but is preferably 0.5 mol / liter or less, and more preferably 0.25 mol / liter or less. Producing calcium hydroxide having a low impurity metal content and a BET specific surface area of less than 4.0 m ² / g by adjusting the concentration of the aqueous solution of alkali metal hydroxide to 0.5 mol / liter or less. Can do. By producing calcium carbonate or the like using calcium hydroxide having a BET specific surface area of less than 4.0 m ² / g, high-purity calcium carbonate or the like having a low impurity metal content can be produced with a high yield.

  The high purity calcium hydroxide of the present invention is characterized by being produced by the production method of the present invention.

High purity calcium hydroxide according to another aspect of the present invention is characterized by an impurity metal content of less than 200 ppm and a BET specific surface area of less than 4.0 m ² / g. By using such high-purity calcium hydroxide, high-purity calcium carbonate or the like having a low impurity metal content can be obtained in a high yield. More preferably, the high-purity calcium hydroxide of the present invention has an Na content of less than 20 ppm and an Sr content of less than 10 ppm. By using such high-purity calcium hydroxide, higher-purity calcium carbonate or the like can be produced.

  The method for producing high-purity calcium carbonate of the present invention is characterized in that calcium carbonate is produced using the calcium hydroxide produced by the method of the present invention.

  According to the method for producing high-purity calcium carbonate of the present invention, since the calcium hydroxide of the present invention is used, it is possible to produce high-purity calcium carbonate having a low chlorine content and a low impurity metal content. it can.

  The method for producing high-purity calcium nitrate according to the present invention is characterized in that calcium nitrate is produced using the calcium hydroxide produced by the above-described method according to the present invention.

  According to the method for producing high-purity calcium nitrate of the present invention, since the calcium hydroxide of the present invention is used, it is possible to produce high-purity calcium nitrate with low chlorine content and low impurity metal content. it can.

  The method for producing high-purity calcium fluoride of the present invention is characterized in that calcium fluoride is produced using the calcium hydroxide produced by the above-described method of the present invention.

  According to the method for producing high-purity calcium fluoride of the present invention, since the calcium hydroxide of the present invention is used, high-purity calcium fluoride having a low chlorine content and a low impurity metal content is produced. be able to.

  According to the present invention, high-purity calcium hydroxide having a low chlorine content and a low impurity metal content can be produced.

  Further, according to the method for producing high-purity calcium carbonate, high-purity calcium nitrate, and high-purity calcium fluoride of the present invention, high-purity calcium carbonate and high-purity nitric acid having a low chlorine content and a low impurity metal content. Calcium and high purity calcium fluoride can be produced.

By using the high purity calcium hydroxide of the present invention having a BET specific surface area of less than 4.0 m ² / g, higher purity calcium carbonate and the like can be produced in a high yield.

  In the method for producing high-purity calcium hydroxide of the present invention, in step A, calcium hydroxide slurry is prepared by reacting quick lime (calcium oxide) obtained by firing limestone with water. For example, a calcium hydroxide slurry can be prepared by firing limestone in a kiln at about 1000 ° C. to produce quick lime, adding about 10 times the amount of hot water to the quick lime, and stirring for 30 minutes. it can. The above reaction is represented by the chemical reaction formula as follows.

CaCO ₃ → CaO + CO ₂
CaO + H ₂ O → Ca (OH) ₂

  Next, nitric acid is added to the obtained calcium hydroxide slurry so that the pH is in the range of 9.5 to 11.5. As a result, calcium hydroxide and nitric acid react to form calcium nitrate and dissolve in the aqueous solution. The chemical reaction formula is as follows.

Ca (OH) ₂ + 2HNO ₃ → Ca (NO ₃ ) ₂ + 2H ₂ O
Since calcium nitrate is readily soluble in water, it dissolves in an aqueous solution. However, since the pH is within the range of 9.5 to 11.5, impurity metals such as iron (Fe) and magnesium (Mg) are water. It maintains an oxide state and is hardly soluble in water. Accordingly, by filtering the solution obtained by adding nitric acid in the next step C, it is possible to remove impurity metals such as iron (Fe), magnesium (Mg), silicon (Si), and aluminum (Al). it can.

  In Step C, by using ultrafiltration as filtration, impurity metals such as Si and Al can be further removed with high accuracy.

  In step D, sodium hydroxide is added to the filtrate to precipitate calcium hydroxide. The chemical reaction formula is as follows. Sodium hydroxide is preferably added until the pH reaches 12.0 or more, and more preferably added until the pH is in the range of 12.5 to 13.0.

Ca (NO ₃ ) ₂ + 2NaOH → Ca (OH) ₂ + 2NaNO ₃
Next, in step E, the calcium hydroxide precipitated solution is heated to 50 ° C. or higher and filtered to separate the calcium hydroxide. The solubility of calcium hydroxide decreases with increasing temperature. On the other hand, the solubility of metal compounds such as strontium (Sr) increases as the temperature increases. In addition, sodium nitrate as a by-product also increases in solubility as the temperature increases. For this reason, in step E, the solubility of calcium hydroxide was lowered by heating the solution in which calcium hydroxide was precipitated to 50 ° C. or higher, while the solubility of impurity metal compounds such as strontium and sodium nitrate was increased. In this state, impurities can be efficiently removed.

  Further, the calcium hydroxide obtained in step E is dispersed again in water to obtain a calcium hydroxide dispersion, and the dispersion is heated to 50 ° C. or higher and filtered at least once more, Impurity content can be reduced.

  Further, by using ultrafiltration as the filtration in the step E, the filtration can be performed more efficiently, and the content of impurities can be further reduced.

  According to the present invention, the content of impurity metals such as Na, Mg, Fe, Sr, Si, and Al can be made 10 ppm or less, for example. Or in this invention, since hydrochloric acid is not used, chlorine content can be made into 10 ppm or less, for example.

Moreover, according to this invention, the high purity calcium hydroxide whose oxalic acid reactivity ( _TpH7 ) is 5 or less can be manufactured. Oxalic acid reactivity (T _pH7 ) is an index indicating the activity of calcium hydroxide when oxalic acid is added. Specifically, a calcium hydroxide slurry adjusted to 5.0% by weight with pure water was kept at 25 ± 1 ° C., and similarly 40 g of 0.5 mol / liter oxalic acid aqueous solution kept at 25 ± 1 ° C. Is added to the calcium hydroxide slurry at once, and the time (minutes) from when the aqueous oxalic acid solution is added to when the pH becomes 7.0 is measured, and this is _defined as T _pH7 . When an aqueous oxalic acid solution is added, the pH is once lowered and gradually increased. The time when the pH reaches 7.0 is measured.

The high-purity calcium hydroxide that can be produced according to the present invention preferably has an oxalic acid reactivity (T _{pH 7} ) of 5 or less, more preferably 3 or less, and even more preferably 1 or less.

  Hereinafter, the present invention will be described with reference to specific examples, but the present invention is not limited to the following examples.

<Experiment 1>
Example 1
Using an ultrafast heating furnace, limestone was calcined at 1000 ° C. for 3 hours to produce quick lime. To 150 g of the quicklime, 1500 g of pure water at 80 ° C. was added and hydrated with stirring for 30 minutes to prepare a calcium hydroxide slurry. About the calcium hydroxide in a calcium hydroxide slurry, content of the impurity metal was measured using the ICP-AES analysis method (the Shimadzu Corporation make, ICPS-8100). Further, the chlorine content was measured by an ion chromatography method. The measurement results are shown in Table 1.

  To a calcium hydroxide slurry containing 100 g of calcium hydroxide, 60% nitric acid was added at room temperature with stirring until the pH reached 9.5.

  The obtained solution was filtered with a filter paper to obtain a filtrate. As the filter paper, quantitative filter paper 5C (manufactured by ADVANTEC, diameter 110 mm) was used.

  The obtained filtrate was ultrafiltered using a UF module (membrane material: polysulfone, membrane inner diameter 1.4 mm, fractional molecular weight 10,000) to obtain a filtrate.

  To the obtained filtrate, an aqueous sodium hydroxide solution having a concentration of 1 mol / liter was added until the pH reached 12.5 to precipitate calcium hydroxide.

  Next, the solution in which calcium hydroxide was precipitated was heated to 80 ° C. and then filtered using the same filter paper as described above. After filtration, the calcium hydroxide remaining on the filter paper was dried to collect high-purity calcium hydroxide.

(Example 2)
High purity calcium hydroxide was produced in the same manner as in Example 1 except that nitric acid was added to the calcium hydroxide slurry until the pH reached 10.0.

(Example 3)
High purity calcium hydroxide was produced in the same manner as in Example 1 except that nitric acid was added to the calcium hydroxide slurry until the pH reached 11.0.

Example 4
High purity calcium hydroxide was produced in the same manner as in Example 1 except that nitric acid was added to the calcium hydroxide slurry until the pH reached 11.5.

(Example 5)
In the operation of Example 2, the filtered calcium hydroxide was again dispersed in ion-exchanged water so as to be 6.0% by weight to obtain a dispersion, and this dispersion was heated to 80 ° C. and filtered. After repeating this process three times, the calcium hydroxide remaining on the filter paper by filtration was dried to collect high purity calcium hydroxide.

(Example 6)
In the operation of Example 2, the solution in which calcium hydroxide was precipitated was heated to 80 ° C., and then ultrafiltered using a UF module (membrane material: polysulfone, membrane inner diameter 1.4 mm, molecular weight cut off 10,000). The calcium hydroxide in the UF module was dried to collect high purity calcium hydroxide.

(Comparative Example 1)
High-purity calcium hydroxide was produced in the same manner as in Example 1 except that nitric acid was added to the calcium hydroxide slurry until the pH reached 9.0.

(Comparative Example 2)
High purity calcium hydroxide was produced in the same manner as in Example 1 except that nitric acid was added to the calcium hydroxide slurry until the pH reached 12.0.

(Comparative Example 3)
High purity calcium hydroxide was produced in the same manner as in Example 1 except that hydrochloric acid was added to the calcium hydroxide slurry until the pH reached 9.0.

(Comparative Example 4)
High-purity calcium hydroxide was produced in the same manner as in Example 1 except that hydrochloric acid was added to the calcium hydroxide slurry until the pH reached 10.0.

[Measurement of impurity content of high purity calcium hydroxide]
The impurity content in the high purity calcium hydroxide obtained in Examples 1 to 6 and Comparative Examples 1 to 4 was measured. Na, Mg, Fe, Sr, Si, and Al were measured by the same ICP-AES analysis method as described above. Cl was measured by ion chromatography. The measurement results are shown in Table 2.

〔yield〕
In Examples 1 to 6 and Comparative Examples 1 to 4, the yield of the finally obtained high-purity calcium hydroxide was calculated with respect to 100 g of calcium hydroxide in the calcium hydroxide slurry, and was collected in Table 2. Shown as a rate.

[BET specific surface area]
The BET specific surface area of the high-purity calcium hydroxide obtained in Examples 1 to 6 and Comparative Examples 1 to 4 was measured using a flow-type specific surface area automatic measuring device (manufactured by Shimadzu Corporation, Flow Soap II2300 type), 2 shows the BET specific surface area.

[Oxalic acid reactivity test]
The high purity calcium hydroxide slurry obtained in Examples 1 to 6 and Comparative Examples 1 to 4 is adjusted to a concentration of 5.0% by weight with pure water. The calcium hydroxide slurry whose concentration was adjusted was kept at 25 ± 1 ° C., and 40 g of 0.5 mol / liter oxalic acid aqueous solution kept at 25 ± 1 ° C. was added to the calcium hydroxide slurry at once. The time (minutes) from when the aqueous oxalic acid solution was added until the pH reached 7.0 was measured and is shown in Table 2 as oxalic acid reactivity (T _pH7 ).

As apparent from the comparison between Examples 1 to 4 and Comparative Examples 1 and 2 shown in Table 2, impurities were added to the calcium hydroxide slurry by adding nitric acid so that the pH was in the range of 9.5 to 11.5. It can be seen that calcium hydroxide with low content and excellent oxalic acid reactivity (T _pH7 ) can be obtained in high yield.

Further, as is clear from the comparison between Examples 1 to 4 and Comparative Examples 3 to 4, by using nitric acid, the chlorine content can be reduced, the impurity metal content is low, and the oxalic acid reactivity is reduced. It turns out that the high purity calcium hydroxide excellent in (T _pH7 ) can be obtained with a high yield.

Further, as is clear from the comparison between Example 2 and Example 5 or 6, the chlorine content and the impurity metal content can be further increased by repeating the warming filtration process or using the ultrafiltration process. It can be seen that high-purity calcium hydroxide that is more excellent in oxalic acid reactivity (T _{pH 7} ) can be obtained.

(Example 7)
In this example, high-purity calcium carbonate was produced using the calcium hydroxide obtained in Example 2. Pure water was added without drying the calcium hydroxide remaining on the filter paper in Example 2 to prepare a calcium hydroxide slurry having a concentration of 6.0% by weight. Carbon dioxide gas was blown into the calcium hydroxide slurry until the pH reached 7.0 to precipitate calcium carbonate. The precipitated calcium carbonate was filtered and then dried to produce high purity calcium carbonate.

(Example 8)
In this example, high-purity calcium nitrate (tetrahydrate) was produced using the calcium hydroxide of Example 2. Nitric acid (concentration 60% by weight) was added to the calcium hydroxide of Example 2 to prepare a high-purity calcium nitrate aqueous solution (concentration 20% by weight), which was used as a UF module (membrane material: polysulfone, membrane inner diameter 1.4 mm, Ultrafiltration was performed using a molecular weight cut off of 10,000), and the filtrate heated by the ultrafiltration step was cooled to room temperature and crystallized.

  Moreover, also when nitric acid was added to the high purity calcium carbonate produced in Example 7 to produce a high purity calcium nitrate aqueous solution, high purity calcium nitrate could be produced in the same manner.

Example 9
In this example, high-purity calcium fluoride was produced using the calcium hydroxide of Example 2. In a sealed container coated with Teflon (registered trademark) on the inside, a hydrofluoric acid aqueous solution (concentration 50% by weight) was added to the calcium hydroxide of Example 2 and sealed, and then heated to about 110 ° C. for reaction. I let you. Thereafter, the produced high purity calcium fluoride slurry was filtered and dried at room temperature.

<Experiment 2>
(Example 10)
In Example 2, the concentration of the aqueous sodium solution added to the filtrate was changed to 1 mol / liter in the same manner as in Example 2 except that an aqueous sodium solution having a concentration of 0.1 mol / liter was used. High purity calcium hydroxide was produced.

(Example 11)
In Example 2, the concentration of the aqueous sodium solution added to the filtrate was changed to 1 mol / liter in the same manner as in Example 2 except that an aqueous sodium solution having a concentration of 0.25 mol / liter was used. High purity calcium hydroxide was produced.

(Example 12)
In Example 2, the concentration of the aqueous sodium solution added to the filtrate was changed to 1 mol / liter in the same manner as in Example 2 except that an aqueous sodium solution having a concentration of 0.5 mol / liter was used. High purity calcium hydroxide was produced.

(Example 13)
In Example 2, the concentration of the aqueous sodium solution added to the filtrate was changed to 1 mol / liter in the same manner as in Example 2 except that an aqueous sodium solution having a concentration of 2.5 mol / liter was used. High purity calcium hydroxide was produced.

[Measurement of impurity content of high purity calcium hydroxide]
In the same manner as described above, the impurity content in the obtained high-purity calcium hydroxide was measured. Table 3 shows the measurement results.

[BET specific surface area]
In the same manner as described above, the BET specific surface area of the obtained high purity calcium hydroxide was measured. Table 3 shows the measurement results.

[Pure water consumption]
In each of the above examples for producing calcium hydroxide, the precipitated calcium hydroxide is filtered, and then pure water is added to the calcium hydroxide remaining on the filter paper and washed. The pure water is washed until there is no change in the electric conductivity of the liquid after being used for washing. The amount of pure water used for this cleaning is shown in Table 3 as “Pure Water Usage”.

  Table 3 also shows the results of Example 2 using an aqueous sodium hydroxide solution having a concentration of 1 mol / liter.

  As shown in Table 3, as the concentration of the aqueous sodium hydroxide solution added to precipitate calcium hydroxide decreases, the impurity metal content of the obtained calcium hydroxide decreases and the BET specific surface area decreases. I understand. In addition, as shown in Table 3, the amount of pure water used for cleaning the obtained calcium hydroxide is almost the same in any of the examples shown in Table 3.

From the results shown in Table 3, by setting the concentration of the sodium hydroxide aqueous solution used for precipitating calcium hydroxide to 0.5 mol / liter or less, the BET specific surface area of the obtained calcium hydroxide is 4.0 m ^2. It can be seen that the BET specific surface area of the obtained calcium hydroxide can be made less than 1.0 m ² / g by making the amount less than / g, and further by making it 0.25 mol / liter or less.

(Examples 14 to 17: Production of high purity calcium carbonate)
Using the high purity calcium hydroxide obtained in Examples 2 and 10 to 12, high purity calcium carbonate was produced as follows.

  Add 50 g of calcium hydroxide into a 2 liter beaker, add 0.5 liter of pure water, and stir for 30 minutes at a rotation speed of 500 rpm using a stir bar (shaft diameter 8 mm, blade length 100 mm). After passing, pure water was further added to prepare a calcium hydroxide slurry having a concentration of 6.0% by mass.

  Carbon dioxide gas was blown into the calcium hydroxide slurry until the pH reached 7.0 to precipitate calcium carbonate. The precipitated calcium carbonate was filtered and then dried to produce high purity calcium carbonate.

[Measurement of impurity content of high purity calcium carbonate]
The impurity content in the high purity calcium carbonate was measured in the same manner as the measurement of the impurity content of the high purity calcium hydroxide. Table 4 shows the measurement results.

[BET specific surface area]
In the same manner as described above, the BET specific surface area of the obtained high purity calcium carbonate was measured. Table 4 shows the measurement results.

〔yield〕
The yield of high purity calcium carbonate was calculated from the following formula. Table 4 shows the yield of high purity calcium carbonate.

  Yield (%) = {Production amount of high purity calcium carbonate (g) / Use amount of high purity calcium hydroxide (g)} × {74 (molecular weight of calcium hydroxide) / 100 (molecular weight of calcium carbonate)}

As is apparent from the results shown in Table 4, the calcium carbonates of Examples 14 to 16 using the raw material calcium hydroxide of Examples 10 to 12 were compared with Example 17 using the raw material calcium hydroxide of Example 2. It can be seen that the impurity metal content is low and the purity is higher. In Examples 14 to 16 using the raw material calcium hydroxide of Examples 10 to 12 having a BET specific surface area of less than 4.0 m ² / g, Example 17 using the raw material calcium hydroxide of Example 2 was used. It can be seen that a higher yield is obtained than in FIG. In particular, in Examples 14 and 15 using the raw material calcium hydroxide of Examples 10 and 11 having a BET specific surface area of less than 1.0 m ² / g, higher yields are obtained.

Claims (15)

A) reacting water with quick lime obtained by firing limestone to prepare a calcium hydroxide slurry;
B) adding nitric acid to the calcium hydroxide slurry so that the pH is in the range of 9.5 to 11.5;
C) filtering the solution obtained by adding nitric acid to obtain a filtrate;
D) adding an alkali metal hydroxide to the filtrate to precipitate calcium hydroxide;
E) A process for separating calcium hydroxide by filtering the precipitated calcium hydroxide, and a method for producing high-purity calcium hydroxide.   The method for producing high-purity calcium hydroxide according to claim 1, wherein the solution in which calcium hydroxide is precipitated is heated to 50 ° C or higher and then filtered.   The calcium hydroxide obtained in step E is again dispersed in water to obtain a calcium hydroxide dispersion, and the dispersion is heated to 50 ° C or higher and filtered at least once. Item 3. A method for producing high-purity calcium hydroxide according to Item 1 or 2.   The method for producing high-purity calcium hydroxide according to any one of claims 1 to 3, wherein ultrafiltration is used as the filtration in the step C.   The method for producing high-purity calcium hydroxide according to any one of claims 1 to 4, wherein ultrafiltration is used as the filtration in the step E.   The method for producing high-purity calcium hydroxide according to any one of claims 1 to 5, wherein the alkali metal hydroxide is sodium hydroxide.   The method for producing high-purity calcium hydroxide according to any one of claims 1 to 6, wherein an alkali metal hydroxide is added as an aqueous solution.   The method for producing high-purity calcium hydroxide according to claim 7, wherein the concentration of the alkali metal hydroxide is 0.5 mol / liter or less.   The method for producing high-purity calcium hydroxide according to claim 7, wherein the concentration of the aqueous solution of the alkali metal hydroxide is 0.25 mol / liter or less.   A high purity calcium hydroxide produced by the method according to any one of claims 1 to 9. High-purity calcium hydroxide characterized by having an impurity metal content of less than 200 ppm and a BET specific surface area of less than 4.0 m ² / g.   The high-purity calcium hydroxide according to claim 11, wherein the Na content is less than 20 ppm and the Sr content is less than 10 ppm.   A method for producing high-purity calcium carbonate, comprising producing calcium carbonate using the calcium hydroxide according to any one of claims 10 to 12.   A method for producing high-purity calcium nitrate, comprising producing calcium nitrate using the calcium hydroxide according to any one of claims 10 to 12.   A method for producing high-purity calcium fluoride, comprising producing calcium fluoride using the calcium hydroxide according to any one of claims 10 to 12.