JP2003206133A - Gypsum wherein eluted fluorine is reduced - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide gypsum wherein eluted fluorine is reduced. <P>SOLUTION: The gypsum wherein eluted fluorine is reduced can be obtained by adding an alkali to gypsum slurry to increase its pH up to ≥9 and then adding phosphoric acids and/or an acid to the resultant gypsum slurry to adjust its pH so that the pH is decreased by ≥1 and up to ≥6. Suppression of eluted fluorine is further promoted by adding calcium phosphate to the gypsum slurry. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention provides gypsum with reduced elution of fluorine.

[0002]

2. Description of the Related Art Gypsum is widely used as a building material such as gypsum board and plaster. In particular, gypsum board is inexpensive and has excellent fire resistance, and is therefore widely used as an interior base material for buildings. It is said that the annual production amount will be about 4.7 million tons and the amount of waste materials will be 15.4 million tons. Although some of these waste materials are recovered and used as raw materials for gypsum board, most of them are inevitably buried and disposed together with other industrial waste. Therefore, when the gypsum contains a substance that dissolves in water and pollutes the soil, it becomes a problem of environmental pollution.

However, since most of the by-product gypsum, which accounts for 40% of the gypsum board raw material, contains fluorine, there is a problem that part of the fluorine begins to melt when it is produced by the conventional method. In order to solve this problem, a technique has been performed in which a calcium compound such as slaked lime is added to gypsum slurry to fix fluorine as calcium fluoride. However, the solubility of calcium fluoride as fluorine is 8 pp
m is about 8 and the emission standard of fluorine is from 15 mg / l to 8
Considering that the amount was changed to mg / l, the technique was not sufficient. In addition, there is a movement in the industry to voluntarily strengthen the elution value of fluorine from gypsum board, and there has been a demand for the development of a technique for further reducing the elution of fluorine.

[0004]

As a result of intensive investigations by the present inventors to solve the above problems, the present invention has been completed. That is, the pH is raised to 9 or higher by adding an alkali to the gypsum slurry, and then phosphoric acid and / or
By adding acid, the range of pH decrease is 1 or more, and the lowered pH is 6
The present invention relates to gypsum having a reduced fluorine content, which is adjusted as described above.

[0005]

BEST MODE FOR CARRYING OUT THE INVENTION Next, the present invention will be described in more detail with reference to embodiments. The gypsum slurry of the present invention may have any concentration as long as the pH of the gypsum can be adjusted, a so-called slurry state in which the gypsum concentration is several tens%, or a dispersed state of several% or less. Although water is usually used as the dispersion medium, it may contain by-products or contaminants generated in the gypsum production process, or may contain a solvent such as an organic substance.

The gypsum of the gypsum slurry may be natural or artificially synthesized. There are various types of synthesized gypsum, for example, phosphate gypsum produced as a by-product during phosphoric acid production, flue gas desulfurization gypsum derived from flue gas desulfurization from power plants, etc., and titanium gypsum produced as a by-product during titanium production. There is. Since the present invention reduces the amount of fluorine eluted from gypsum, it is generally used for by-product gypsum containing fluorine. However, even natural gypsum can be applied as the gypsum of the present invention as long as it contains fluorine.
The gypsum includes dihydrate gypsum, hemihydrate gypsum, and anhydrous gypsum depending on its crystal form, but the present invention is applicable to any gypsum containing fluorine.

The alkali of the present invention may be any alkali that can supply a hydroxyl group in water. As those capable of providing a hydroxyl group in water, hydroxides of alkali metals such as sodium hydroxide and potassium hydroxide, hydroxides of alkaline earth metals such as calcium hydroxide and magnesium hydroxide,
It is possible to use a metal oxide such as aluminum hydroxide, or a compound such as ammonia that releases a hydroxyl group by reaction with water. It is speculated that the supply of calcium ions is necessary to obtain the effects of the present invention. Although calcium ions are supplied from gypsum, the present invention can be more effectively carried out by adding a calcium source. As a calcium source, a water-soluble calcium salt such as calcium chloride may be added, but if calcium hydroxide or calcium carbonate is used as the alkali, the alkali of the present invention and calcium ions can be supplied at the same time, which is economical. Target.

The phosphoric acid used in the present invention may be a phosphoric acid salt formed by the reaction of neutralizing phosphoric acid and an alkali, or phosphoric acid itself. For example, ammonium phosphate, sodium phosphate, hypophosphorous acid, phosphorous acid, sodium hypophosphite, sodium pyrophosphate, sodium acid pyrophosphate, sodium acid metaphosphate, sodium tripolyphosphate, sodium hexametaphosphate, potassium pyrophosphate. , Phosphate, calcium hypophosphite, calcium phosphate, salts formed from other metals and phosphoric acid, and as calcium phosphate, only general calcium phosphates such as tricalcium phosphate, dicalcium phosphate, octacalcium phosphate, and tetracalcium phosphite. Not only that, but also includes apatite composed of phosphorus typified by hydroxyapatite. Phosphate ore, which is a naturally occurring phosphate compound, is no exception. As the phosphates of the present invention, it is necessary to supply phosphate ions in the liquid, and those having high water solubility are desirable, but when the water solubility is low, the reaction is carried out over time, or they are decomposed with an acid or the like in advance. It can also be used.

The acid referred to in the present invention may be any compound as long as it can supply a cation to oxo in water. For example, it may be a mineral acid such as phosphoric acid, sulfuric acid or hydrochloric acid, or an organic acid such as acetic acid or formic acid. Further, salts produced by the neutralization reaction of an acid and an alkali and containing hydrogen in the molecule, such as monosodium phosphate, disodium phosphate, etc., can also be used as the acid of the present invention.

In the present invention, when the pH is raised, the alkali may be added as it is, or may be diluted with water or the like and added. The pH to be reached may be any as long as it is 9 or more, but when the amount of fluorine eluted from the gypsum before treatment of the present invention is large, it is possible to suppress the elution of fluorine more effectively by increasing the pH. .

After the pH of the gypsum slurry has been raised to 9 or higher, the phosphoric acid and / or the acid may be added as it is, like the alkali, or may be diluted with water or the like and then added. good. Although the phosphoric acid and the acid may be added together, the effect of the present invention can be obtained by adding only the phosphoric acid as long as the phosphoric acid exhibits acidity such as phosphoric acid.

The pH reduction range when the phosphoric acid and / or acid of the present invention is added may be 1 or more, but the higher the reduction range is, the more effectively the elution of fluorine can be suppressed. The lowered pH needs to be 6 or more. This is because it is expected that once the pH is lower than 6, the fluorine once fixed will be dissolved again.

The calcium phosphate referred to in the present invention includes
Not only common ones such as tricalcium phosphate, dicalcium phosphate, monocalcium phosphate, octacalcium phosphate and tetracalcium phosphate are included, but also apatite composed of phosphorus represented by hydroxyapatite is included. Phosphate ore, which is a naturally occurring phosphate compound, is no exception. Further, a compound having an apatite structure composed of phosphoric acid and other metals is also included. The molar ratio (Ca / P) of (Ca) and (P) of the calcium phosphate of the present invention is 1.2 to
It is preferably 2.3. Further, the calcium phosphate of the present invention preferably has an apatite structure. The component having an apatite structure may be the main component in calcium phosphate or a part of the composition, and the effect of the present invention can be obtained.

The calcium phosphate of the present invention may be added to the gypsum slurry in advance, or may be added before or simultaneously with the addition of phosphoric acid and / or acid.
As a method for adding calcium phosphate, it may be added as it is, or may be added after being dispersed in water or the like.

According to the present invention, the amount of fluorine eluted from gypsum can be suppressed by changing the fluorine in the gypsum slurry to the calcium ion supplied from gypsum or the added alkali or calcium salt, and the added phosphoric acid. I guess it can be fixed as. Further, it is speculated that the calcium phosphate of the present invention has an effect as a seed crystal at the time of producing fluoroapatite and can promote immobilization of fluorine. Further, by the treatment of the present invention, not only fluorine but also heavy metal compounds such as lead, zinc and cadmium, and further elution from gypsum such as arsenic compounds can be reduced.

[0016]

The present invention will be described in detail below with reference to examples. As the gypsum slurry, a dihydrate gypsum slurry produced as a by-product during the production of phosphoric acid was used. Table 1 shows the analytical values of the gypsum slurry.

The fluorine concentration in gypsum was measured with a fluoride ion electrode after decomposing gypsum with aqua regia and diluting with water. Also,
The dissolution test was performed as follows in accordance with the Environmental Agency Notification No. 13, “Soil Dissolution Test Method”. 80 g of the gypsum obtained by the treatment of the example and 800 ml of pure water were placed in a 1000 ml separating funnel, and the mixture was shaken at room temperature (25 ° C.) and atmospheric pressure (1 atm) to shake about 200 times per minute. Width 4-5c
It shook for 6 hours at m. After that, let stand for 30 minutes, ADV
It was filtered with a glass filter GA-100 manufactured by ANTEC, and the fluorine concentration in the filtrate was measured with a fluorine ion electrode. The fluorine ion electrode was used according to the "ion electrode method" of JIS K0102.

Example 1 500 g of the gypsum slurry shown in Table 1 was added with slaked lime with stirring to raise the pH to 12. Then 75% phosphoric acid was added to lower the pH to 7. The treated slurry was filtered, and the obtained gypsum was dried at 40 ° C. for 24 hours. The concentration of fluorine eluted from the gypsum was measured according to the Environmental Agency Notification No. 13, “Soil Leach Test Method”. The results are shown in Table 2.

Example 2 500 g of the gypsum slurry shown in Table 1 was added with slaked lime with stirring to raise the pH to 10. Next, the pH was lowered to 7 by adding monosodium phosphate. Hereinafter, the concentration of fluorine eluted from gypsum was measured by the same method as in Example 1.
The results are shown in Table 2.

Example 3 500 g of the gypsum slurry shown in Table 1 was added with slaked lime with stirring to raise the pH to 12. Next, after adding 8 g of disodium phosphate, the pH was lowered to 7 with 40% sulfuric acid.
Hereinafter, the concentration of fluorine eluted from gypsum was measured by the same method as in Example 1. The results are shown in Table 2.

Example 4 To 500 g of the gypsum slurry shown in Table 1, 3.5 g of tricalcium phosphate manufactured by Shimonoseki Mitsui Chemicals was added, and slaked lime was added with stirring to raise the pH to 9. The pH was then lowered to 6 after the addition of 75% phosphoric acid. Hereinafter, the concentration of fluorine eluted from gypsum was measured by the same method as in Example 1. The results are shown in Table 2.

Comparative Example 1 500 g of the gypsum slurry shown in Table 1 was filtered, and the obtained gypsum was dried at 40 ° C. for 24 hours. The concentration of fluorine eluted from the gypsum was measured according to the Environmental Agency Notification No. 13, “Soil Leach Test Method”. The results are shown in Table 2.

Comparative Example 2 The same operation as in Example 1 was performed except that the pH after adding calcium hydroxide was adjusted to 8. The results are shown in Table 2.

Comparative Example 3 The same operation as in Example 1 was carried out except that the pH was adjusted to 7 by using sulfuric acid instead of 75% phosphoric acid. The results are shown in Table 2.

[0025]

As described above, the value of fluorine eluted from gypsum by the treatment of the present invention is three times as much as 0.8 mg / l which is the environmental standard of fluorine, that is, 2.4 mg / l.
It can be reduced to In addition, by limiting the conditions, the environmental standard of fluorine is 0.8 mg / l.
It is also possible to lower it to the following.

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Claims (6)

[Claims] 1. An alkali is added to the gypsum slurry to obtain p.
Raise H to 9 or more, and then add phosphoric acid and / or acid to adjust the pH reduction range to 1 or more and the lowered pH to 6 or more. Gypsum. 2. A gypsum with reduced leaching fluorine, which comprises adding calcium phosphate to the gypsum slurry according to claim 1. 3. The gypsum with reduced fluorine elution according to claim 1, wherein the alkali added to the gypsum slurry is a calcium salt. 4. The molar ratio of calcium (Ca) and phosphorus (P) in the calcium phosphate added to the gypsum slurry (C
a / P) is 1.2 to 2.3, and the gypsum with reduced elution of fluorine according to claims 2 to 3. 5. The gypsum with reduced leached fluorine according to claim 2, wherein the calcium phosphate added to the gypsum slurry has an apatite structure. 6. The amount of fluorine eluted from gypsum is 2.4 mg.
It is below 1 / l, The gypsum which reduced the eluted fluorine of Claims 1-5 characterized by the above-mentioned.