JPS5943238B2 - How to treat water containing phosphates - 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, marshes, and inner bays has become a serious problem.

Phosphate present in water has been highlighted as a cause of eutrophication, and its removal has been raised as an urgent issue.

Phosphates that cause eutrophication are contained in tap water, sewage, industrial water, industrial wastewater, boiler water, etc., and include inorganic phosphates such as orthophosphates and condensed phosphates, and organic It exists in the form of phosphate.

As a method for removing such phosphates, a method has been proposed in which water containing phosphates is brought into contact with crystal species containing calcium phosphate, such as phosphate rock, in the presence of calcium ions (Dissertation Abstracts I
internationalVo I, 33.412
, Part I, page 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. It has attracted particular attention in recent years because it has significantly improved processing efficiency.

However, this method has a drawback in that the presence of magnesium ions in the target water deteriorates the phosphate removal efficiency.

This invention is intended to eliminate the above-mentioned drawbacks of conventional methods. By bringing crystal seeds into contact with an activating solution and activating them, the phosphoric acid removal rate can be maintained at a high level. The purpose is to provide a method for treating water containing salt.

This invention relates to a method in which water containing magnesium ions and phosphate is brought into contact with crystal seeds containing calcium phosphate in the presence of calcium ions and under conditions of pH 7 or higher. A method for treating water containing phosphate, characterized in that the crystal seeds are activated by contacting the crystal seeds with an activating liquid containing calcium ions at a calcium ion concentration higher than that at the time of crystallization. It is.

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.

However, when magnesium ions are present in water, (
The reaction of formula 1) becomes difficult to proceed and the removal rate of phosphate becomes poor.

Although the detailed mechanism of the magnesium ion inhibition reaction is not necessarily clear, crystals such as Ca4H(P04)s, which have higher solubility than hydroxyapatite, precipitate and inhibit the crystal formation of hydroxyapatite.
It is thought that the phosphate removal activity as a seed crystal decreased.

When such crystal seeds with reduced activity are brought into contact with an activating solution having a pH higher than the pH at the time of crystallization and containing calcium ions higher than the calcium ion concentration at the time of crystallization, the crystal seeds will be reactivated. The activity is maintained for a while afterward, even when raw water containing magnesium is treated.

Therefore, if the treatment is continued while being activated intermittently, phosphates can be removed at a high removal rate.

Such a tendency becomes remarkable when the activation liquid further contains fluoride ions.

The reason why the crystal seeds are activated in this way is not clear, but the Ca 4 H (P 04 ) a crystallized on the surface of the crystal seeds is converted into stable Ca 5 (
This is because OHX PO4)3, or hydroxyapatite, is converted into hydroxyapatite, and the presence of fluoride ions accelerates this reaction, and some of it also converts into stable Ca=, F (PO4)a, which has a lower solubility. It is assumed that this is because it changes to apatite.

Such hydroxyapatite, fluoroapatite, etc. have extremely high activity, and when crystallization is performed using crystal seeds activated in this way, crystallization can be performed efficiently even in a system in which magnesium ions are shared.

The activation liquid has a pH of 1 or higher than the pH at the time of crystallization, that is, pH 7.
This is the above, and the higher the pH, the better.

If the pH is lower than that at the time of crystallization, good activation can be achieved.

The upper limit is also determined by economic efficiency. The concentration of calcium ions in the activation liquid is greater than the concentration of calcium ions in the water to be treated during crystallization.

Further, the absolute amount thereof is desirably 30% by weight or more of the phosphorus crystallized in the coexistence of magnesium ions before the activation treatment.

This amount can be practically reduced to about 1/3 when fluoride ions are present, with calcium ions being 10% by weight or more and fluoride ions being 1-20/l.

It is desirable that the activation liquid does not contain magnesium ions.

When using water containing magnesium ions, for example treated water, adding slaked lime at a high concentration will cause the magnesium ions to coagulate and be removed, so it is better to use it in that state.

The contacting method is to pass the activating liquid through the crystal seed packed bed, circulate the outflowing activating liquid, and practically allow the contact to take place for about 5 to 48 hours.

The method of passing the liquid may be either a fluidized bed type or a fixed bed type, and may be either an upward flow or a downward flow.

By contacting with the activating liquid in this manner, the crystal seeds are reactivated.

The sustainability of the treatment effect after activation depends on the phosphate concentration in the raw water and the treatment conditions, but normally water containing magnesium ions can be stably treated for about a month, and high-quality water can be treated. Treated water is obtained.

For crystallization, a calcium agent and/or an alkaline agent is added to the water to be treated, and the water is brought into contact with crystal seeds under conditions of pH 7 or higher.
The reaction is carried out according to the above formula (1).

In this case, the generated calcium phosphate crystals are crystallized in a stable form because the surface of the crystal seeds is activated, and the phosphate concentration in the treated water is reduced.

The water to be treated in this invention is water containing magnesium salts and phosphates, and these may be contained in the form of magnesium phosphate.

Examples of such water include sewage water diluted with seawater and human waste treatment plant discharge water systems.

The amount of calcium agent or alkaline agent to be added to raw water should be in excess of the reaction equivalent expressed by formula (1), but if too large a amount is added, fine precipitates may precipitate in places other than crystal seeds. Furthermore, since impurities such as calcium carbonate may be generated, the temperature should be within a range where these are not generated.

That is, the amounts of calcium ions and hydroxide ions are
The conditions are such that hydroxyapatite is produced at a concentration higher than the solubility of hydroxyapatite produced in formula (1) and lower than supersolubility, that is, a concentration in the metastable range.

Here, supersolubility is the concentration at which crystals begin to precipitate when no crystal seeds are present in the reaction system.

The calcium and pH values that keep the amount of hydroxyapatite within the metastable range can be determined experimentally by changing these values for each reaction system, but the approximate range is when the phosphate ion is 50 m9/l or less. , calcium ion is about 10 to 100 m9/V, and pH is about 7 to 12.

Calcium agents used in this invention include calcium hydroxide and calcium chloride, and alkaline agents include sodium hydroxide, potassium hydroxide, calcium hydroxide, and the like.

Crystal seeds containing calcium phosphate include hydroxyapatite (Ca5(OH)(P04)3),
Crystal species containing calcium phosphate such as fluoroapatite (Ca, (F) (PO4)3) or tricalcium phosphate [Ca3(PO4)2] can be used, and natural phosphate rocks have these calcium phosphates as their main components. Suitable as a crystal seed.

Furthermore, crystal seeds in which calcium phosphate is precipitated on the surface of a filter medium 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 that produces hydroxyapatite, the use of hydroxyapatite facilitates the precipitation of new crystals, efficiently removes phosphates, and increases the removal rate.

The method of contacting the water containing phosphate with the crystal seeds may be a fixed bed method or a fluidized bed method.

Usually, the smaller the size of the crystal seeds, the larger the specific surface area, so new crystals can easily precipitate, but if they are 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 will be small for the unit filling amount, so particles of about 9 to 300 mesh are usually used.

Among these, 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 having a particle size of 9 to 35 meshes are packed, and water is passed upwardly or downwardly at a flow rate of S1 to i o h r -1 to precipitate calcium phosphate crystals.

When water is passed in an upward flow, suspended matter can be captured in the large particle size part of the lower layer, and crystallization can be performed in the small particle size part of the upper layer with high activity.

Similarly, when passing water in a downward flow, in order to avoid adhesion of suspended matter to the crystal seed surface, a filter medium with a smaller specific gravity and larger particle size than the crystal seeds is layered on top of the crystal seed fixed layer. Preferably, suspended matter is removed by a filter medium.

If the surface of the crystal seeds becomes contaminated or clogged during water flow, it is recommended to periodically backwash the crystal seeds using an upward flow to develop and clean the crystal seeds and remove impurities attached to the surface. desirable.

Water flow conditions during backwashing include a flow rate of 20 to 80 m/hr.
The backwashing time is about 5 to 60 minutes.

By the crystallization operation as described above, phosphates in the raw water are removed, and the phosphate concentration in the treated water is reduced.

When the activity of the crystal seeds decreases and the quality of the treated water deteriorates by continuing such operations, the treatment can be carried out at a high removal rate by performing the activation operation again.

As described above, according to the present invention, the p
By treating with an activating solution containing H and containing calcium ions at a concentration higher than the calcium ion concentration at the time of crystallization, it is possible to reactivate crystal seeds whose phosphate removal effect has decreased, and thereby magnesium ions are removed. It is possible to remove phosphates at a high removal rate even from raw water containing phosphates, and to obtain treated water with a low phosphate concentration.

Next, examples of the present invention will be described.

Example 1 Into an acrylic column with an inner diameter of 3.0 Ca and a length of 50 C1 rL, magnesium ions were added at 40 to 60 μ/l and phosphates were added at 1 to 2 m9/IIcI) (the same applies hereinafter).
16 used for about 6 months to remove phosphates from water containing
~32 meshes of Jordanian phosphate ore for 150 rILl
Filled, phosphate concentration 1 m9/IJ, total alkalinity approx. 1
00 m9/l and a magnesium ion concentration of 50 m9/l, an aqueous solution of calcium chloride and sodium hydroxide was added at the column inlet to adjust the calcium ion concentration to 80-85 ■/l and the pH to 8.8-8. After adjusting to 9.0, water was continuously passed through the phosphate rock packed bed at a flow rate of 300 ml/hr using an upward flow fixed bed water flow system to perform crystallization.

The phosphate concentration of the treated water was 0.197Q immediately after the water flow started.
, #, but gradually increased to 0.3 m after 8 days.
It reached 9/7.

The total amount of phosphate removed by crystallization up to this point was approximately 900.
It was 7n9/d.

Therefore, the continuous crystallization process was interrupted and reactivated.

The reactivation is based on treated water in which magnesium ions have been coagulated and removed with slaked lime, and the calcium ion concentration is adjusted to 100 Da/7 and the fluorine ion concentration is adjusted to 15η/l. The packed column was circulated for 24 hours.

After the circulation treatment, the phosphate rock packed bed was backwashed with tap water and crystallization was restarted under the same conditions as above, and the phosphate concentration in the treated water gradually decreased to 0.12 m9/l after 13 days.
After that, stable processing was possible for about 20 days.

(The reason why crystal seeds used for 6 months were used was to improve the reproducibility of the influence of magnesium ions.

) Furthermore, after 10 days, there was a tendency for the phosphate concentration in the treated water to remain again, so the above reactivation treatment was performed again.

The amount of phosphate removed by crystallization during the 43 days from the start of the above operation was approximately 4001n9/V, and the amount of calcium ions in the reactivation liquid was 30% by weight, except that fluorine ions were not added. A similar operation was performed.

As a result, almost the same phosphate removal effect as above was obtained.

Example 2 Water was passed under the same conditions as in Example 1, and when the phosphate concentration in the treated water reached 0.3 yn9/l, phosphate rock was reactivated.

Add slaked lime to tap water 11 to 0.55.9 (297 as Ca)
m1? ) Added aqueous solution (pH 12,1) to 800wL
The solution was circulated for 7 hours at a flow rate of 1/hr.

After the circulation treatment, the packed bed was allowed to stand for 3 days, and then the packed bed was backwashed with tap water, and then phosphate-containing water was passed through it.

The phosphate concentration in the treated water was 0.11 for 4 days after the start of water flow.
It became less than n9/l and continued to be 0.15η for about a month after that.
A stable treated water quality of around 1/l was obtained.

In addition, the amount of lime added was 1.1g (594■ as Ca).
The same treatment as above was carried out except for the added (p)(12,2), and 0, 1 yn for 10 days after the start of water flow.
g/II or less, and showed a stable processing performance of around 0.111971 even after that.

However, the amount of lime added was reduced to 0.1. F (50m as Ca
9) When added (pH 8.8), the phosphate concentration in the treated water is 0.1 m9/1 until the second day after the start of water flow.
Although it remained around the same level, the residual amount gradually increased to 100.
The day is 0.31ru? / It was around 1.

Claims (1)

[Scope of Claims] 1. A method in which water containing magnesium ions and phosphate is brought into contact with crystal seeds containing calcium phosphate in the presence of calcium ions and under conditions of pH 7 or higher, in which: Water containing phosphate, characterized in that the crystal seeds are activated by contacting the crystal seeds with an activation liquid having a pH higher than the pH value and containing calcium ions higher than the calcium ion concentration at the time of crystallization. processing method. 2. The method for treating water containing phosphate according to claim 1, wherein the activation liquid contains calcium ions in an amount of 30% by weight or more of the crystallized phosphorus.