JP2994405B2 - Purification method of alkali hydroxide - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for purifying an alkali hydroxide. More specifically, the present invention provides a high-quality hydroxide by removing impurities contained in an alkali hydroxide by co-precipitation by adding an aqueous solution of hydrogen peroxide and an inorganic acid salt of a rare earth element to an aqueous alkali hydroxide solution. This is a method for obtaining alkali. Furthermore, the present invention relates to the removal of trace amounts of alkaline earth metals and iron contained below the solubility in an alkali hydroxide solution.

(Background technology)

In recent years, analysis sensitivity has been improved due to the development and progress of analytical instruments, and it has become possible to detect the presence of extremely small amounts of elements. In the field of materials such as electronic components, magnetic materials, catalysts, and semiconductors, particularly, new functions are expected to be exhibited, and high purity of these materials has been actively attempted. Accordingly, a trace amount of impurities has been analyzed. In this way, as the amount of the target element present becomes smaller and the detection amount becomes smaller as the material becomes more purified, the blank amount of the reagent used in the analysis cannot be ignored. Removal of impurities, that is, purification, has become one of the important technologies. In analytical chemistry, alkali hydroxide is used for various purposes such as dissolution of various materials, neutralization of acidified sample, pH adjustment during separation and concentration such as chelation resin and solvent extraction, and alkali melting. Although it is a reagent and widely used, it is known that alkali hydroxide produced from the soda industry contains many metals such as barium, calcium, magnesium, strontium, iron and aluminum. On the other hand, although this alkali hydroxide is difficult to purify and requires a high-purity reagent, there is still no reagent that can satisfy the demand.

On the other hand, also in the semiconductor industry, alkali hydroxide is used in a wet etching process. Even in this case, quality improvement is an important issue, and as a matter of course, with the progress of miniaturization, increasingly strict quality standards are required. That is, the fine particles, metal impurities and microorganisms in the reagent are said to cause a decrease in the yield of the product and the reliability of the device characteristics, and high-purity particles containing no metal impurities are required.

As a conventional technique, a co-precipitation method is generally used in which an impurity contained in a very small amount (μg or less) in a solution is co-precipitated with an additive, and only a small amount of an impurity contained in the solution is removed as a precipitate. However, in this method, the additive always remains in the solution. That is, it can be said that the removal of the precipitant is impossible even in view of the solubility of the precipitant itself. Therefore, contamination based on the precipitant itself is inevitable. For example, an example of a method for purifying caustic soda using a coprecipitation method will be described. The method for quantifying calcium in aluminum specified by the Japan Institute of Light Metals shows a method for purifying a 6M sodium hydroxide solution used in the quantification method. .

6M-caustic soda solution as described there,
Heat and stir, during which time a concentrated solution of iron salt is added dropwise to ripen the precipitate of iron hydroxide. This was filtered off with paper to obtain a calcium-free 6M sodium hydroxide solution. However, in this case, the iron salt added as a coprecipitant will remain. As a method for removing nickel contained at a ppm level in a caustic alkali solution, there is a method described in JP-A-56-40086. In this method, nickel is made into a higher-order compound by adding hypochlorous acid to a caustic alkali solution, and the nickel is removed and purified by reducing its solubility. Even in this method, contamination by hypochlorous acid used in a large amount remains. As described above, it has been found that the conventionally known method is not suitable as a purification method for obtaining high-purity caustic alkali as a result of the added coprecipitant and oxidizing agent remaining in the alkali.

[Disclosure of the Invention]

The present inventors have found that the inorganic acid salt of the rare earth element reduces the solubility of the rare earth itself by adding a small amount of aqueous hydrogen peroxide under alkaline conditions, and at the same time, removes impurities in the alkali from the rare earth water. It was found that it was adsorbed on the surface of the oxide and removed. The present invention has been made based on such findings. That is, the present invention
An aqueous solution of hydrogen peroxide and an inorganic acid salt of a rare earth element are added to an aqueous alkali hydroxide solution containing alkaline earth metals and iron as impurities, and the mixture is stirred, and a coprecipitate is formed under cooling. And a method for purifying an alkali hydroxide.

 Hereinafter, the present invention will be described in detail.

The aqueous alkali hydroxide solution is usually provided as a 0.5 to 5 M aqueous solution, and this is cooled, and a 30% strength aqueous hydrogen peroxide solution is dropped and stirred. At this time, impurities such as iron become higher-order compounds and the solubility itself decreases. An aqueous solution of an inorganic acid salt of a rare earth element is further dropped into this aqueous alkali hydroxide solution while stirring. When left to cool for a certain period of time, amorphous precipitates such as rare earth oxides, hydroxides or hydrates are formed, and the above impurities are adsorbed on the surface and coprecipitate. By passing this through a filter, barium, calcium, magnesium, strontium, iron, etc., which are present at the ppm level or lower in the alkali hydroxide solution, are removed to obtain a high-purity alkali hydroxide solution. Can be. That is, in the method of the present invention, a small amount of aqueous hydrogen peroxide is added to an aqueous solution of an alkali hydroxide, and the mixture is stirred under cooling, and further, an inorganic acid salt of a rare earth element, for example, a chloride, bromide, or fluoride of a rare earth element,
A concentrated solution of phosphate, sulfate or nitrate (usually
(1000 mg / ml) is dropped, stirred, cooled, and allowed to stand to form and precipitate amorphous hydroxide of rare earth element.
At this time, alkaline earth metals and heavy metals such as iron, cobalt, cadmium, nickel and chromium contained in a trace amount in the aqueous alkali hydroxide solution are adsorbed and precipitated on the surface of the rare earth hydroxide. By passing the alkaline solution using a filter, a high-purity alkaline hydroxide solution can be obtained.

The present inventors conducted various studies on the concentration of alkali hydroxide, the amount of hydrogen peroxide to be added, the type of rare earth element, and the treatment temperature with respect to the method for purifying alkali hydroxide according to the present invention. Obtained.

The maximum concentration that can be used for purification of alkali hydroxide solution is sodium hydroxide or potassium hydroxide.
7M, preferably 6M or less. In the case of lithium hydroxide, the maximum concentration is 3M. At higher concentrations, the added precipitant mixes into the alkali hydroxide.

The addition amount of the 30% hydrogen peroxide solution to be added to the alkali hydroxide solution 1 is preferably 0.1 to 5 ml, particularly preferably 0.1 to 5 ml.
5 to 2 ml. If the amount is less than 0.1 ml, the removal rate of impurities decreases. If it is 5 ml or more, no further removal effect can be obtained.

The rare earth elements used include scandium, yttrium, lanthanum, cerium, praseodymium, neodymium, promesium, samarium, europium, gadolinium, terbium, dysprosium, holmium,
Erbium, thulium, ytterbium or lutetium are mentioned, and their inorganic acid salts are used. Particularly effective are the inorganic acid salts of yttrium, neodymium, europium or thulium.

The treatment temperature is preferably in the range of 10 ° C to -10 ° C, particularly preferably 5 ° C to -5 ° C. Above 10 ° C., the dissolution of impurities increases and the removal effect decreases. Temperatures below -10 ° C have no further removal effect and are not economical.

Hereinafter, the present invention will be described in more detail with reference to examples of the present invention.

Example 1 120 g of commercially available special grade sodium hydroxide is dissolved in water, and water is added to make 1 to prepare a 3M sodium hydroxide solution. Using a commercially available metal standard solution (1 mg / ml), 1 ml of each of barium, calcium, magnesium and strontium solutions was added to this solution. When barium, calcium, magnesium and strontium were quantified in the obtained solution by high frequency inductively coupled plasma emission analysis, it was found to contain about 1 ppm of barium, calcium, magnesium and strontium. At a temperature of 0 ° C., 1.0 ml of a 30% concentration of hydrogen peroxide solution
Add and stir. In addition, yttrium nitrate solution (10mgY / m
l) was added and stirred for 1 hour. A white amorphous precipitate forms upon standing at 0 ° C. for 1 day. 0.2 μ of the precipitate
Use a PTFE filter with a pressure of m. This 3M-
Barium, calcium, magnesium, strontium and yttrium in sodium hydroxide solution were quantified by high frequency inductively coupled plasma emission spectrometry. The removal rates of these alkaline earth metals and yttrium at this time are shown below.

When added with 1.0 ml of hydrogen peroxide solution, barium becomes 0.0
3ppm, 0.01ppm for calcium, 0.05ppm for magnesium,
0.01 ppm strontium and 0.006 ppm yttrium
The removal rate of each is 97% barium, calcium
99%, 95% of magnesium, 99% of strontium, and 99.998% of yttrium showed that the removal effect was good.

Example 2 A commercially available special grade, 120 g of sodium hydroxide was dissolved in water, and water was added to make 1. Prepare a 3M sodium hydroxide solution. Further, 0.1 ml of a commercially available iron standard solution (1 mg Fe / ml) was added, iron was quantified by high frequency inductively coupled plasma emission spectrometry, and a 3M sodium hydroxide solution containing 0.132 ppm of iron was added.
Is prepared. At a temperature of 0 ° C,
2 ml of 30% aqueous hydrogen peroxide is added and stirred. Further, 40 ml of an yttrium nitrate solution (10 mgY / ml) was added and the mixture was stirred for 1 hour.
Upon standing at 0 ° C. for 3 days, a white amorphous precipitate forms. The precipitate is vacuumed using a 0.2 μm PTFE filter. This 3M sodium hydroxide solution was quantified by high frequency inductively coupled plasma emission spectrometry. The removal rates of iron and yttrium at this time are shown below.

Treatment with 0.132 ppm iron gives 0.006 ppm iron and a removal rate of 93%. In addition, yttrium is 0.002 ppm, and the removal rate is 99.999%.

As seen above, 0.132 ppm of iron showed a removal rate of 93%, indicating that the removal effect was good. Also,
Almost 100% of the added yttrium can be removed.

Example 3 120 g of commercially available special grade sodium hydroxide is dissolved in water, and water is further added to make 1 to prepare a 3M sodium hydroxide solution. Further, 0.1 ml of a standard solution of 1 mgFe / ml was added, and iron was quantified by high-frequency inductively coupled plasma emission spectrometry.
A 3M sodium hydroxide solution 1 containing 0.130 ppm is prepared. At a temperature of 0 ° C., 2 ml of a 30% strength hydrogen peroxide solution is added to the solution and stirred. Further, 40 ml of a neodymium chloride solution (10 mgNd / ml) was added and stirred for 1 hour. On standing at 0 ° C. for one day, an amorphous precipitate forms. The precipitate
Reduce the pressure using a 0.2 μm PTFE filter. This 3
As a result of quantifying iron in the aqueous solution of M-sodium hydroxide by high frequency inductively coupled plasma emission spectrometry, the iron concentration was 0.00
It was 7 ppm and the removal rate was found to be 95%.

Example 4 120 g of commercially available special grade sodium hydroxide is dissolved in water, and water is further added to make 1 to prepare a 3M sodium hydroxide solution. Next, 0.1 ml of a 1 mg Fe / ml standard solution was added, and iron was quantified by high-frequency inductively coupled plasma emission spectrometry.
A 3M sodium hydroxide solution 1 containing 0.150 ppm is prepared. This solution was heated at 5 ° C at a concentration of 30% hydrogen peroxide 2m
Add l and stir. Further, an yttrium nitrate solution (10 mg Y
/ ml) and stirred for 1 hour. An amorphous precipitate forms when left overnight at 5 ° C. The precipitate was washed with 0.2 μm
Reduce pressure using an E-filter. As a result of quantifying iron and yttrium in the 3M-sodium hydroxide solution by high frequency inductively coupled plasma emission spectrometry, the iron concentration was 0.02.
The removal rate was 6 ppm, and it was confirmed that the removal rate was 83%.

Example 5 120 g of commercially available special grade sodium hydroxide is dissolved in water, and water is further added to make 1 to prepare a 3M sodium hydroxide solution. 1mgCd / ml, 1mgCo / ml, 1mgNi / ml and 1mgCr / ml
Of cadmium, cobalt, nickel and chromium are determined by high frequency inductively coupled plasma emission spectrometry to prepare a 3M sodium hydroxide solution containing about 1 ppm of cadmium, cobalt, nickel and chromium. To this solution, at a temperature of 0 ° C., 2 ml of 30% aqueous hydrogen peroxide was added and stirred. Further, 40 ml of an yttrium nitrate solution (10 mgY / ml) is added and stirred. On standing at 0 ° C. for one day, an amorphous precipitate forms. 0.2μ of the precipitate
Reduce the pressure using a PTFE filter. Cadmium, cobalt, nickel and chromium in the 3M sodium hydroxide solution were quantified by high frequency inductively coupled plasma emission spectrometry. The removal rate at this time is shown below.

When processed, cadmium 0.014 ppm, cobalt 0.013 ppm,
Nickel: 0.015 ppm and chromium: 0.3 ppm. The removal rates are 86% for cadmium, 87% for cobalt, and 95% for nickel, respectively.
And 70% chromium.

From the above results, it was confirmed that the effect of removing iron, cadmium, cobalt, nickel and chromium was good.

Example 6 360 g of commercially available special-grade potassium hydroxide is dissolved in water, and water is further added to make 1 to prepare a 6M potassium hydroxide solution. Further mix 1mgCa / ml, 1mgMg / ml, 1mgSr / ml standard solution
1 ml is added, and calcium, magnesium, and strontium are quantified by high frequency inductively coupled plasma emission spectrometry, and 6M-potassium hydroxide solution 1 containing 1.42 ppm of calcium, 1.03 ppm of magnesium, and 1.30 ppm of strontium is prepared. At a temperature of 0 ° C., 1 ml of a 30% strength aqueous hydrogen peroxide solution was added to the potassium hydroxide solution, followed by stirring.
Further, 20 ml of an yttrium nitrate solution (6 mgY / ml) was added and the mixture was stirred for 1 hour. A white amorphous precipitate forms upon standing at 0 ° C. for 1 day. The precipitate is vacuumed using a 0.2 μm PTFE filter. This 6M-potassium hydroxide solution was quantified by high frequency inductively coupled plasma emission spectrometry. The removal rates of calcium, magnesium, strontium and yttrium at this time are shown below.

When the 6M potassium hydroxide solution is treated, the calcium concentration becomes 0.048 ppm, and the removal rate is 97%. The magnesium concentration was 0.036 ppm, and the removal rate was 97%. The strontium concentration is 0.116 ppm, and the removal rate is 91%. The yttrium concentration was 0.004 ppm, and the removal rate was 99.99%, indicating that the removal effect was good.

Example 7 360 g of potassium hydroxide of the first grade is dissolved in water and further added with water to make 1 to prepare a 6M potassium hydroxide solution.
The calcium, magnesium and strontium in this solution are quantified by high frequency inductively coupled plasma emission spectrometry. To this 6M-potassium hydroxide solution, at a temperature of 0 ° C., 1 ml of 30% hydrogen peroxide was added and stirred.
Further, 20 ml of an yttrium nitrate solution (6 mgY / ml) was added and the mixture was stirred for 1 hour. A white amorphous precipitate forms upon standing at 0 ° C. for 1 day. The precipitate is vacuumed using a 0.2 μm PTFE filter. This 6M-potassium hydroxide solution was quantified by high frequency inductively coupled plasma emission spectrometry. The removal rates of calcium, magnesium, strontium and yttrium at this time are shown below.

0.001ppm when processing calcium concentration 0.540ppm
And the removal rate is 99%. Magnesium is 0.019p
The pm becomes 0.005 ppm, and the removal rate is 74%. Further, strontium has a removal rate of 0.001 ppm from 0.015 ppm, and a removal rate of 93%. Yttrium is 0.001 ppm, and the removal rate is 99.998%. These alkaline earth elements, which were present at 0.1-0.01 ppm level, are purified to 0.001 ppm level; ie, ppb level.

Example 8 120 g of commercially available special grade lithium hydroxide-hydrate is dissolved in water, and water is further added to make 1 to prepare a 3 M lithium hydroxide solution. The calcium, magnesium and strontium in this solution are quantified by high frequency inductively coupled plasma emission spectrometry. To this solution, at a temperature of 0 ° C., 1 ml of a 30% strength aqueous hydrogen peroxide solution is added and stirred. Further, 40 ml of an yttrium nitrate solution (6 mgY / ml) was added and the mixture was stirred for 1 hour. On standing at 0 ° C. for one day, an amorphous precipitate forms. The precipitate is vacuumed using a 0.2 μm PTFE filter. Calcium, magnesium and strontium in the approximately 3M lithium hydroxide solution were quantified by high-frequency inductively coupled plasma emission spectrometry. As a result, the calcium concentration was changed from 8.7 ppm to 0.47 ppm, and the removal rate was found to be 95%. Magnesium is reduced from 0.058 ppm to 0.025 ppm, the removal rate is 57%, and strontium is 0.17 ppm
From 2 ppm to 0.015 ppm, the removal rate was found to be 91%. Thus, a favorable refining effect was also observed for lithium hydroxide.

Example 9 240 g of commercially available special grade sodium hydroxide is dissolved in water, and water is further added to make 1 to purify a sodium hydroxide solution.
A commercially available metal standard solution (1 mg Ca / ml), (1 mg Mg /
(ml) and (1 mgSr / ml) were added, and the obtained sodium hydroxide solution was quantified for calcium, magnesium and strontium by high frequency inductively coupled plasma emission spectrometry. Containing sodium hydroxide solution. To this solution, 1 ml of 30% strength hydrogen peroxide solution is added at 0 ° C. and stirred. Further, 20 ml of an yttrium nitrate solution (6 mgY / ml) was added and the mixture was stirred for 1 hour. A white amorphous precipitate forms upon standing at 0 ° C. for 1 day. The precipitate is vacuumed using a 0.2 μm PTFE filter. Calcium, magnesium, strontium and yttrium in the sodium hydroxide solution were quantified by high frequency inductively coupled plasma emission spectrometry. Table 1 shows the analysis values of these alkaline earth metals and yttrium at this time.

Comparative Example 240 g of commercially available special grade sodium hydroxide was dissolved in water, and water was added to make 1 to prepare a 6M sodium hydroxide solution.
1 ml each of a commercially available metal standard solution, 1 mg Ca / ml, 1 mg Mg / ml and 1 mg Sr / ml was added to this solution, and the obtained sodium hydroxide solution was quantified for calcium, magnesium and strontium by high frequency inductively coupled plasma emission spectrometry. As a result, it was a sodium hydroxide solution containing about 1 ppm of calcium, magnesium and strontium. To this solution, 20 ml of an yttrium nitrate solution (6 mgY / ml) was added at 0 ° C., and the mixture was stirred for 1 hour. A white amorphous precipitate forms upon standing at 0 ° C. for 1 day. The precipitate
Reduce the pressure using a 0.2 μm PTFE filter. Calcium and magnesium in this sodium hydroxide solution,
Strontium and yttrium were quantified by high frequency inductively coupled plasma emission spectrometry. Table 1 shows the analysis values and removal rates of these alkaline earth metals and yttrium at this time.

As can be seen from the results shown in this table, there is no significant difference in the effect of purifying calcium, magnesium and strontium in Examples and Comparative Examples, but a large difference appears in Yttrium. That is, the added yttrium concentration is not detected in the examples (the detection limit is 0.005 ppm), but in the comparative example, the added concentration is 120 ppm and the removal rate is 98%, but the remaining yttrium concentration is
2.450 ppm, indicating that a large amount of the precipitant yttrium remains in the sodium hydroxide solution.

Claims (2)

(57) [Claims] 1. An aqueous solution of hydrogen peroxide and an inorganic acid salt of a rare earth element are added to an aqueous solution of an alkali hydroxide containing an alkaline earth metal and iron as impurities, and the mixture is stirred and cooled to form a coprecipitate. A method for purifying an alkali hydroxide, comprising removing a coprecipitate by filtration. 2. The method according to claim 1, wherein the inorganic acid salt of the rare earth element is scandium, yttrium, lanthanum, cerium, praseodymium, neodymium, promesium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium or The method for purifying an alkali hydroxide according to claim 1, which is a lutetium chloride or nitrate.