JPS6139876B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for treating water containing phosphate to remove phosphate.

In recent years, the progress of eutrophication in closed water bodies such as lakes and inner bays has been remarkable and has become a problem. Phosphate present in water has been highlighted as a cause of eutrophication, and its removal has been raised as an urgent issue. Phosphates, which cause eutrophication, are contained in tap water, sewage, industrial water, factory wastewater, boiler water, etc.; It exists in the form of phosphate.

As a method to remove such phosphates, water containing phosphates is treated in the presence of calcium ions,
A method has been proposed in which contact with crystal species containing calcium phosphate, such as phosphate rock (Dissertation
Abstracts International, Vol.30, No.12, Part
, p. 5878-B, etc.). This method removes phosphate ions contained in water by converting them into calcium phosphate, such as hydroxyapatite, and crystallizing them into crystal seeds. , processing efficiency will also be significantly improved. However, with this method,
Depending on the reaction conditions, fine crystals such as calcium phosphate and calcium carbonate may precipitate in areas other than the crystal seeds, causing clogging, or impurities may precipitate on the surface of the crystal seeds, hindering subsequent crystallization. As a result, there was a problem in that the ability to remove phosphates was significantly reduced.

This invention solves these problems with conventional methods and provides a method for treating water containing phosphates that can stably remove phosphates over a long period of time without depositing impurities or precipitates. It is intended to.

This invention is characterized in that water containing phosphate is brought into contact with crystal seeds containing calcium phosphate in the presence of one or several substances selected from polycarboxylic acids, phosphonic acids, and salts thereof. This is a method for treating water containing acid salts.

The reaction that occurs when water containing phosphate is brought into contact with crystal seeds containing calcium phosphate in the presence of calcium ions varies depending on the reaction conditions, but is usually expressed by the following formula.

5Ca ²⁺ +7OH ^- +3H ₂ PO ₄ ^- →Ca ₅ (OH) (PO ₄ ) ₃ +6H ₂ O...(1) As can be seen from equation (1), in order to increase the phosphate removal rate, the reaction It is necessary to make the reaction proceed to the right, and for this purpose it is necessary to increase the calcium ion concentration and pH during the reaction. However, if these concentrations are too high, the reaction will proceed rapidly, causing calcium phosphates such as hydroxyapatite to precipitate as fine precipitates in areas other than the crystal seeds, and excessively added calcium ions and alkalis. As a result of the reaction, impurities (such as calcium carbonate and magnesium hydroxide) are deposited on the surface of the crystal seeds and other parts. Such a phenomenon occurs even when the amount of calcium ions or alkali added is not increased, due to fluctuations in the phosphate concentration in the raw water.

In other words, when removing phosphates from water by crystallization without causing precipitation as in the present application, operation is performed in a metastable region determined by the phosphate concentration in water, the calcium ion concentration in water, and PH. The key is to do so. Here, the metastable region is a region higher than the solubility of the generated hydroxyapatite and lower than the supersolubility, and the supersolubility is the concentration at which crystals begin to precipitate when no crystal seeds exist in the reaction system. When the balance of pH and calcium concentration exceeds the metastable region, it becomes an unstable region where precipitation of calcium phosphate, calcium carbonate, etc. occurs.

In the present invention, by allowing polycarboxylic acids, phosphonic acids, or their salts to exist under conditions that are originally unstable, the formation of precipitates and crystal seeds of impurities can be prevented even when the pH and calcium concentration increases. It promotes precipitation of hydroxyapatite crystal seeds while preventing precipitation on the surface, and can stably remove phosphates in water at a high removal rate over a long period of time. That is, according to the present invention, the metastable region is expanded, and the rate of analytical crystallization can be increased when the pH and calcium concentration are increased.

Polycarboxylic acid is a carboxylic acid monomer or a polymer of this with other polymerizable monomers,
Examples include homopolymers or copolymers of acrylic acid, methacrylic acid, maleic acid or its anhydride, fumaric acid, itaconic acid, and crotonic acid. Homopolymers include polyacrylic acid, polymethacrylic acid, polyitaconic acid, etc.; copolymers include maleic acid or its anhydride and isobutene, fyalic acid and acrylic acid, acrylic acid and hydroxyethyl methacrylic acid, and maleic acid or its anhydride. Examples include copolymers of monomer and acrylic acid, maleic acid and butene, acrylic acid and itaconic acid, acrylic acid and methacrylic acid, and maleic acid or its anhydride and styrene sulfonic acid. In the case of copolymers,
When both monomers to be polymerized are carboxylic acids, the molar ratio of carboxylic acids should be 30% or more, especially 50%.
% or more is desirable. The molecular weight of these polymers is 500 to 100,000, for example, 500 to 40,000 in the case of polyacrylic acid, and 5,000 to 100,000 in the case of a copolymer of isobutylene and maleic anhydride, usually at a molar ratio of 1:1.

Examples of phosphonic acids include phosphonocarboxylic acids such as phosphonobutanetricarboxylic acid, hydroxyalkylenephosphonic acids such as hydroxyethylidene diphosphonic acid, aminoalkylphosphonic acids such as aminotri(methylenephosphonic acid), and ethylenediaminetetra(methylenephosphonic acid). Can be mentioned.

These salts include water-soluble salts such as sodium salts, potassium salts, and ammonium salts.

It is also possible to use one substance among polycarboxylic acids, phosphonic acids or their salts, but
A mixture of two or more types can also be used. These agents are added continuously or intermittently to water containing phosphate at a concentration of about 0.01 to 10 ppm, preferably
Presence of 0.1-5ppm.

When water containing phosphate to which a drug has been added is brought into contact with crystal seeds containing calcium phosphate in the presence of calcium ions, the calcium phosphate produced by the above formula (1) precipitates on the surface of the crystal seeds and crystals form. The phosphoric acid in the water is removed. In this case, the addition of the drug prevents precipitation of calcium phosphate and calcium carbonate even if the pH and calcium concentration are high, but since calcium phosphate is retained in the liquid in a supersaturated state, precipitation on the crystal seeds is prevented. promoted. Further, even if the amount of calcium ions or hydroxide ions is increased, the formation of precipitates and precipitation of impurities will not occur, so the reaction of formula (1) can be accelerated by increasing the amount of these ions.

In other words, as shown in the figure based on the experimental results described below, if raw water contains 10 ppm of phosphate ions, for example, in the conventional method, the calcium ion concentration in the metastable range at PH9 is at the limit of about 45 ppm; However, in the method of this invention, by adding only 1 ppm of polycarboxylic acid or/and phosphonic acid, calcium ions can be absorbed under the same conditions.
It becomes possible to add up to 120ppm.

Conversely, under these conditions, calcium ions
At 45ppm, the pH can be raised from 9.0 to 9.6, which means that the metastable region has been greatly expanded. Therefore, the crystallization reaction rate increases accordingly, and phosphates in water can be efficiently removed.

Calcium ions and hydroxide ions to be present in water do not need to be added from the outside if they are present in the raw water from the beginning, but if they are not present in the raw water or are insufficient, they are added from the outside. The amount added is in excess of the reaction equivalent. PH of reaction solution
is preferably 6 to 12.

Crystal species containing calcium phosphate include hydroxyapatite [Ca ₅ (OH) (PO ₄ ) ₃ ], fluoroapatite [Ca ₅ (F) (PO ₄ ) ₃ ], or tricalcium phosphate [Ca ₃ (PO ₄ ) ₂ ] Crystal particles containing calcium phosphate such as these can be used, and natural phosphate rock has these calcium phosphates as a main component and is suitable as a crystal seed. Furthermore, crystal seeds in which calcium phosphate is precipitated on the surface of a material such as sand can also be used. The crystal seed is preferably one whose main component is calcium phosphate of the same type as the calcium phosphate produced by the reaction. For example, in a system where hydroxyapatite is produced, the use of crystal seeds containing hydroxyapatite as a main component promotes the precipitation of new crystals.

The method of contacting the water containing phosphate with the crystal seeds containing calcium phosphate may be a fixed bed method or a fluidized bed method. A fixed bed type is preferable when turbidity removal is also performed at the same time, and a fluidized bed type is suitable when turbidity removal is not required. The smaller the size of the crystal seeds, the larger the surface area, which makes it easier for new crystals to precipitate, but if the size of the seeds is too small, it will be difficult to contact or separate the crystal seeds from water. Furthermore, if the particle size is too large, the specific surface area per unit filling amount will be small, so particles of about 9 to 300 mesh are usually used. The larger ones are suitable for fixed beds, and the smaller ones are suitable for fluidized beds. In the case of a fixed bed, crystal seeds with a particle size of 9 to 35 mesh are packed, and water is passed upwardly or downwardly at a flow rate of SV1 to 2 (1/hr) to precipitate calcium phosphate crystals. When passing water in a downward flow, it is desirable to pre-treat the raw water to remove impurities in order to avoid adhesion of impurities to the surface of the crystal seeds.

If the crystal seed surface becomes contaminated or clogged during water flow, backwashing is performed periodically using an upward flow to spread out the crystal seed bed and clean it to remove impurities attached to the surface. It is desirable to peel off the

Experimental Example Water containing 10 ppm of phosphate ions and 100 ppm of M alkalinity was taken into multiple beakers, and the pH was adjusted to 6, 7, 8, 9, and 10, respectively. Next, for those with PH6, calcium chloride is used as calcium ions at 40, 60, 80, and 100 ppm.
After stirring at room temperature for one hour, the solution was filtered through a 0.2 micron Millipore filter, and the concentration of calcium ions in the solution was measured.
This work was performed sequentially for PH7, 8, 9, and 10. The point at which the amount of calcium ions in the solution decreased compared to the amount added (supersolubility) was determined and graphed.

Next, the same operation as above was repeated except that 1 ppm of phosphonobutanetricarboxylic acid was added to the water, and an expanded supersolubility line in the presence of phosphonic acid was determined. On the other hand, for the solubility, the theoretical value of 0.5 ppm of phosphate ion is plotted as a graph. The results are shown in the drawing. In the figure, a is the solubility line, b is the supersolubility line, c is the enlarged supersolubility line, E is the stable region where no precipitate is formed, F is the metastable region where crystallization occurs if crystals are present, and G is the enlarged supersolubility line. H indicates the metastable region, and H indicates the unstable region where precipitation occurs.

Example 1 1 ppm of sodium polyacrylate (molecular weight 5000) was added to artificial wastewater containing 10 ppm of phosphate ions, then an aqueous calcium chloride solution was added to give a calcium concentration of 40 ppm, and an aqueous sodium hydroxide solution was further added to adjust the pH to 9. .5 (this condition is originally an unstable region where precipitation occurs),
SV2hr ^-1 (1400 mL
ml/hr) in a fluidized bed. In this case, the development rate was 15%, and the phosphate ion concentration in the treated water could be stably reduced to 1 ppm or less for about 3 months.

On the other hand, as a comparative example, as a result of continuous treatment under the same conditions without adding sodium polyacrylate, impurities containing calcium phosphate and calcium carbonate were generated in the crystal seeds at and near the column inlet, and the liquid could not be passed after about 40 days. .

Example 2 1 ppm of phosphonobutanetricarboxylic acid was added to artificial wastewater containing 10 ppm of phosphate ions, and then an aqueous solution of calcium chloride was added as calcium.
The pH was adjusted to 80 ppm, and an aqueous sodium hydroxide solution was added to adjust the pH to 9 (this condition is normally an unstable region where precipitation occurs), and a particle size of 32 to 60 mesh was added as calcium phosphate crystal seeds. Inner diameter 30mm filled with 300ml of Florida phosphate rock
The liquid was passed through the cylindrical cylinder in an upward flow at SV2hr ^-1 . As a result, the phosphate ion concentration in treated water could be stably maintained at 1 ppm or less for about 3 months.

On the other hand, as a comparative example, as a result of continuous liquid flow treatment under the same conditions without adding phosphonic acid, a fine white precipitate containing calcium phosphate and calcium carbonate was generated at the inlet of the crystal seed packed bed, increasing the pressure loss of the packed bed and approx. After 20 days, water was no longer available.

As described above, according to the present invention, as a result of expanding the metastable region, it is possible to prevent the formation of precipitates and the precipitation of impurities on the surface of the crystal seeds, while promoting the precipitation of calcium phosphate on the crystal seeds. can be stably removed over a long period of time with a high removal rate. Therefore, even if the phosphate concentration in the raw water fluctuates, stable treatment is possible, and the concentration of calcium ions and pH can be increased to speed up the reaction.

[Brief explanation of the drawing]

The figure is a graph showing the expansion of the metastable region due to drug addition in experimental examples, where a is the solubility line;
b is a supersolubility line, c is an expanded supersolubility line, E is a stable region, F is a metastable region, G is an expanded metastable region, and H is an unstable region.

Claims (1)

[Claims] 1. In the presence of one or several substances selected from polycarboxylic acids, phosphonic acids, and salts thereof,
A method for treating water containing phosphate, comprising bringing water containing phosphate into contact with crystal seeds containing calcium phosphate. 2. The polycarboxylic acid includes a phosphate according to claim 1, which is a homopolymer or copolymer of acrylic acid, methacrylic acid, maleic acid or its anhydride, fumaric acid, itaconic acid, or crotonic acid. How to treat water. 3. The phosphonic acid includes the phosphate salt according to claim 1 or 2, wherein the phosphonic acid is phosphonobutanetricarboxylic acid, hydroxyethylidene diphosphonic acid, aminotri(methylenephosphonic acid) or ethylenediaminetetra(methylenephosphonic acid). How to treat water. 4. The method for treating water containing phosphate according to any one of claims 1 to 3, wherein the crystal seeds containing calcium phosphate contain hydroxyapatite, fluoroapatite, or tricalcium phosphate. 5. The method for treating water containing phosphate according to any one of claims 1 to 3, wherein the crystal seed containing calcium phosphate is phosphate rock.