JPS5952580A - Treatment of phosphate-contg. water - Google Patents

Abstract

PURPOSE:To curtail the amount of lime to be used, while enhancing the dissolution efficiency of slaked lime under a condition suited to precipitation, by sealing a lime-dissolving device with gas which does not contain carbonic acid one. CONSTITUTION:In supplying calcium or hydroxyl ion to a reaction system, precipitation is performed by adding liquid slaked lime dissolved in a lime-dissolving device sealed with gas decarbonated by bringing air into contact with soda lime, liquid alkali or zeolite, or gas, e.g. nitrogen one, which does not practically contain carbonic acid one. Since the slaked lime dissolved under the sealed condition does not cause the formation of calcium carbonate, the dissolution efficiency of slaked lime is enhanced. Therefore, by bringing said slaked lime into contact with crystal seeds containing calcium phosphate in the presence of calcium ion under the condition of a pH above 6, calcium phosphate formed by Formula I is precipitated on the surfaces of the crystal seeds. Hence, phosphoric ion in water is efficiently removed.

Description

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

As a method for removing phosphates from water, a method has been proposed in which water containing phosphates is brought into contact with a crystal species containing calcium phosphate, such as phosphate rock, in the presence of calcium ions. This method removes phosphate ions contained in water by converting them into calcium phosphate such as hydroxyapatite and crystallizing them into crystal seeds, and the operating method is simpler than the conventional flocculation method. Not only this, but the processing efficiency will also be significantly improved.

In this method, crystallization is generally performed in the presence of calcyrano ions and under conditions of pH 6 or higher, so crystallization is performed by adding slaked lime solution for the purpose of supplying calcium ions or hydroxide ions. Analysis is being carried out. Slaked lime liquid is prepared by dissolving powdered lime in a lime dissolving device, but in normal usage, it is dissolved in contact with air, so calcium carbonate crystals precipitate on the surface of the lime block, causing the surface to deteriorate. As a result, the dissolution efficiency of slaked lime under conditions suitable for crystallization is poor, and it is necessary to add more lime than the theoretical amount, and the undissolved portion moves directly into the liquid and crystallizes as fine SS. Cleaning the seed packed bed (
There was a problem that the frequency of backwashing was increased and the phosphorus concentration of the treated water was also increased.

This invention is intended to improve the problems of the conventional devices as described above. By sealing the lime dissolving device with a gas that does not contain carbon dioxide gas, the efficiency of dissolving lime is maintained at a high level. It is an object of the present invention to provide a method for treating water containing phosphate, which can reduce the amount of lime used, reduce the frequency of washing, and prevent deterioration of the quality of treated water.

This invention provides a method for removing phosphates by bringing water containing phosphates into contact with crystal seeds containing calcium phosphate in the presence of calcium ions and under conditions of pH 6 or higher. This is a method for treating water containing phosphate, which is characterized by performing crystallization by adding dissolved slaked lime in a lime dissolving device sealed with a lime-free gas.

As mentioned above, an investigation into the cause of the low lime dissolution efficiency revealed that calcium carbonate, which forms on the surface of lime due to the absorption of carbon dioxide gas, hardly dissolves under water conditions suitable for crystallizing calcium phosphate. It was found that the dissolution efficiency of lime was reduced. Calcium carbonate, which is generally produced by absorption of carbon dioxide gas, often dissolves when added to the target system, but the conditions in the metastable region described below where the calcium ion concentration and pH conditions are suitable for crystallization. At the bottom, almost no dissolution occurs, and the liquid migrates directly into the liquid. To avoid this, in the present invention, by sealing the lime dissolving device with a gas that does not contain carbon dioxide gas, absorption of carbon dioxide gas is prevented, which in turn prevents the formation of calcium carbonate.

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: 5(4"r+ 3HPO4 ” + 40H-→Ca,
(OH) (PO,) 3 + 3H20”' (1)
(As seen from Equation 11, in order to increase the removal rate of ricylate, it is necessary to allow the reaction to proceed to the right, and at the same time, it is necessary to increase the activity of the crystal seeds.

Crystal seeds containing calcium phosphate include hydroxyapatite [Cas(o)()(PO4)3), fluoroanotite CCa5(F)(PO4)3:]
Alternatively, crystal seeds containing calcium phosphate such as tricalcium phosphate (Ca3(PO4)2:) can be used, and natural phosphate rock or bone char, which contain calcium phosphate as a main component, are suitable as crystal seeds. Furthermore, crystal seeds in which calcium phosphate is precipitated on the surface of a furnace 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 that produces hydroxyapatite, the use of hydroxyapatite facilitates the precipitation of new crystals, efficiently removes phosphate, and increases the removal rate. □ The crystallization conditions are the same as those of the conventional method, and when brought into contact with a crystal seed containing calcium phosphate in the presence of calcium ions and under conditions of 2146 or higher, the calcium phosphate produced by the above formula (1) crystallizes. Crystals precipitate on the seed surface and grow, removing phosphorous ions in the water.

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 of addition should be in excess of the reaction equivalent, but if it is added slowly and in too large a quantity, fine precipitates or deposits may occur in places other than the crystal seeds, and impurities such as calcium carbonate may form -S. Therefore, it should be set within a range where these do not occur. That is, the amounts of calcium ions and hydroxyl ions are set to be higher than the solubility of the hydroxyapatite produced in equation (1), but lower than the supersolubility, that is, the conditions are such that hydroxyapatite is produced at 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. In other words, at a concentration higher than the supersolubility, new crystals precipitate where no crystal seeds exist, forming fine precipitates and clogging the P bed, but in the metastable region where the supersolubility is lower, crystal seeds do not exist. New crystals are precipitated on top, and no precipitate is formed, only the crystals grow.Also, in systems with low dissolution and retention, crystals do not precipitate.

The amount of hydroxyapatite produced is controlled by the phosphorus exposure ion concentration, calcium ion concentration, and pH of the reaction system. The amount of calcium ions and pH value that bring the amount of produced hydroxyapatite within the metastable range can be determined experimentally by changing these values for each reaction system. The approximate □ range is phosphate ion 5[ff~/ in the following cases, calcium ion ゛1 D ~
100m? /l, pH is about 6 to 12, but varies depending on each condition.

In the present invention, in order to maintain the reaction system within the above range, when supplying calcium ions or hydroxide ions, a slaked lime solution dissolved in a lime dissolving device sealed with a gas not containing carbon dioxide gas is added to perform crystallization. conduct. There are no particular restrictions on the lime dissolving device, and any device that can dissolve lime in water and prepare slaked lime under a sealed state with carbon dioxide-free gas is sufficient, but in general, lime powder storage tanks and lime A dissolving tank is used, and a diluting tank can also be used. The sealing method is to make each of these structures airtight, purge them with gas that does not contain carbon dioxide, and seal them to prevent contact with the atmosphere.

As a gas that does not contain carbon dioxide, soda lime °゛□
A gas substantially free of carbon dioxide gas, such as a gas decarboxylated by contacting air with an alkali solution (sodium hydroxide solution, potassium hydroxide solution, slaked lime solution, etc.) or zeolite, or nitrogen gas, can be used. The lime to be dissolved may be either slaked lime or quicklime. The water for dissolving lime may be raw water, treated water, industrial water, or other water. In this case, it is desirable to seal the dissolved water so that carbon dioxide gas does not come into contact with it, and decarboxylation can be performed depending on the case.

Lime is dissolved in a sealed state by supplying lime powder from a lime powder storage tank to dissolved water in a lime dissolving tank and dissolving it by stirring. The slaked lime solution thus produced is diluted in a dilution tank,
It may be a predetermined concentration. Although the concentration of the slaked lime solution added to the reaction system is not limited, it is practically preferably about 0.01 to 0.3% by weight. The slaked lime solution dissolved in a sealed state does not generate calcium carbonate, so the slaked lime dissolution efficiency is high and the undissolved portion is small.

When the reaction system is maintained in the metastable region by supplying slaked lime solution to the raw water, dihydroxyapatite is produced according to the formula (1), so it is brought into contact with a crystal seed containing calcium phosphate and crystallized.

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. The smaller the size of the crystal seeds, the larger the surface area, and the easier it is for new crystals to precipitate. However, if the dispersion is too small, it is difficult to contact or separate the crystal seeds from water.

If the particle size is too large, the specific surface area per unit filling amount will be small, so a particle size of about 9 to 500 mesh is 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 65 mesh are packed, and water is passed upwardly or downwardly at a flow rate of SV1 to 5 hr' to precipitate calcium phosphate crystals.

If the crystal seed surface becomes contaminated or clogged during water flow, periodically clean the crystal seed bed with upward flow to spread out and clean the crystal seed bed to remove impurities attached to the surface. It is desirable to peel it off. The water flow conditions during cleaning are as follows: the flow rate is 2
The washing speed is about 0 to 80 m/hr, and the cleaning time is about 5 to 60 minutes. By cleaning such crystal seeds, the SS components are removed, making it possible to open the town with water.

When water flow resumes, the quality of treated water will temporarily deteriorate, but
This is because phosphorus that is not completely fixed leaks out due to disturbance in the crystal seed packed bed, and by continuing water flow, the quality of the treated water will be restored and treated water with a low phosphorus concentration will be obtained. become.

After repeating crystallization and washing, if the dephosphorization efficiency does not recover even after washing, a treatment for reactivating the crystal seeds can be performed. This treatment method includes, for example, contacting with a solution containing a chlorine agent or calcium compound during or after cleaning the crystal seeds, or draining and drying after cleaning. dephosphorization efficiency increases.

Incidentally, prior to the above-mentioned crystallization, there is no problem in performing pre-treatment such as SS removal from the raw water, or performing post-treatment such as sterilization. Furthermore, the sealing with a gas that does not contain carbon dioxide gas may be applied not only to the lime dissolving device but also to the entire crystallization reaction device.

In summary, according to the present invention, the lime dissolving device is configured to be sealed with a gas that does not contain carbon dioxide gas, thereby preventing the formation of calcium carbonate and maintaining a high lime dissolving efficiency. This has the effect of reducing the amount of phosphorus used, reducing the frequency of washing, preventing deterioration of treated water quality, and stably obtaining treated water with a low phosphorus concentration.

Next, examples and comparative examples of the present invention will be described.

Example: Secondary treated sewage water whose pH was adjusted by adding slaked lime to a fixed bed dephosphorization tower (cross-sectional area: 0.63 m) filled with phosphate rock with a particle size of 0.5 to 1.0 mm. (phosphorus concentration 1-3m
y/1SS81-20+++g/l) was allowed to flow upward at a flow rate of 5V2hr- to perform crystallization. The slaked lime solution is produced by dissolving and diluting the slaked lime in a sealed state using treated water as dissolution water in a lime dissolving device consisting of a slaked lime powder storage tank, a lime dissolving tank, and a dilution tank that are sealed using a method of continuously purging with nitrogen gas. The concentration is 0.15N,
This slaked lime liquid was injected into the raw water in a sealed state at the inlet of the dephosphorization tower, and the amount added was controlled so that the ptls was 8 to 90.

As a result, the phosphorus concentration in the treated water was below 0.5 my / , and stable treatment effects were maintained. Also pH8,8
The amount of slaked lime added to make it ~90 is 20~40
y+y//L, the cleaning frequency of the dephosphorization tower was once/6 days.

Comparative Example Crystallization was carried out under the same conditions as in the example except that the lime dissolving apparatus consisting of a slaked lime powder storage tank, a lime dissolving tank and a dilution tank was operated in an open atmosphere condition. The average amount of slaked lime added was 65 rrq/
It became l. In addition, the rate of increase in pressure drop in the dephosphorization tower tends to be higher than that in the example, and the cleaning frequency is 1/1.
It's been 1 day. Furthermore, the phosphorus concentration of the treated water showed a higher value than in the example, 0.5-1. Onrg/l-
Met.

As a result of the above, by sealing carbon dioxide gas with a gas containing carbon dioxide, the amount of slaked lime added and the frequency of cleaning are reduced.
It can be seen that the average phosphorus concentration of the treated water is lower, indicating that stable treatment can be achieved.

Agent Patent Attorney Sei Yanagi Hara

Claims (5)

[Claims] (1) A method for removing phosphate by bringing water containing phosphate into contact with crystal seeds containing calcium phosphate in the presence of calcium ions and under conditions of p1]6 or more. , a method for treating water containing phosphates, characterized in that crystallization is carried out by adding dissolved slaked lime in a lime dissolving device sealed with a gas that does not contain carbon dioxide. (2) The method for treating water containing phosphates according to claim 1, wherein the lime dissolving device comprises a lime powder storage tank and a lime dissolving tank. (3) The gas that does not contain carbon dioxide is a desired gas or a gas obtained by contacting air with soda lime, alkaline liquid, or zeolite.Claim 1 or 2
A method for treating water containing phosphates as described in Section 1. (4) The phosphate according to any one of claims 1 to 3, wherein the crystal species containing calcium phosphate are one or more selected from hydroxyapatite, fluoroapatite, and tricalcium phosphate. water treatment methods including; (5) The method for treating water containing phosphate according to any one of claims 1 to 4, wherein the crystal seed containing calcium phosphate is phosphate rock or bone char.