JPS622878B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a dephosphorization method for removing phosphates from water containing phosphates by crystallization.

[Prior art]

As a method for removing phosphates contained in tap water, sewage, industrial water, factory wastewater, boiler water, etc.
A dephosphorization method has been proposed in which water containing phosphate is brought into contact with a crystalline 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. The processing efficiency is also significantly improved.

In this type of dephosphorization method, the activity of the crystal seeds decreases while the treatment continues and the dephosphorization performance decreases, so it is necessary to perform reactivation treatment intermittently to recover the dephosphorization performance. is being carried out. Conventional reactivation methods include treating crystal seeds with chemicals such as chlorine agents, acids, and alkaline agents.

However, such a method has problems such as complicated operations due to the use of chemicals, high costs, and the need for waste liquid treatment.

[Purpose of the invention]

This invention is intended to solve the above-mentioned conventional problems. By peeling off the surface layer of crystal seeds and reactivating them, reactivation can be achieved with a simple operation without the use of chemicals. The purpose of the present invention is to provide a dephosphorization method that can efficiently perform dephosphorization.

[Structure of the invention]

This invention allows water containing phosphate to contact crystal seeds containing a calcium phosphate compound at a pH of 6 or higher and in the presence of calcium ions to crystallize calcium phosphate without producing calcium carbonate. In the dephosphorization method that reactivates the crystal seeds after depositing them on the surface of the seeds, calcium phosphate on the surface of the precipitated crystal seeds is removed so that the crystalline apatite layer deposited on the surface of the crystal seeds is exposed.
This dephosphorization method is characterized by removing 50% or more of the material, reactivating it, and then bringing it into contact with water containing phosphate again to perform crystallization.

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⁺ +3HPO ^2- +4OH ^- →Ca ₅ (OH) (PO ₄ ) ₃ +3H ₂ O……(1) Fluoroapatite is produced when fluorine is included in the reaction system, but the formula (1) The reaction is similar to . As can be seen from equation (1), in order to increase the phosphate removal rate, it is necessary to allow the reaction to proceed to the right, and at the same time, it is necessary to keep the surface of the crystal seeds clean and maintain high activity. .

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 seeds containing calcium phosphate, such as . Furthermore, crystal seeds in which calcium phosphate is precipitated, supported, or captured on the surface of granular materials such as sand or calcium carbonate 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 raw water to be crystallized by contacting with the crystal seeds containing calcium phosphate as described above is water containing phosphate, and includes secondary treated water such as sewage, human waste, and industrial wastewater.

The crystallization conditions are the same as those of the conventional method, and when brought into contact with crystal seeds containing calcium phosphate in the presence of calcium ions and at a pH of 6 or higher, the crystallization occurs by the above formula (1) or a reaction similar to this. Calcium phosphate such as hydroxyapatite that is generated is deposited on the surface of the crystal seed, crystals grow, and phosphate ions in the water are 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 should be in excess of the reaction equivalent, but if too much is added, fine precipitates may precipitate in places other than the crystal seeds, and impurities such as calcium carbonate may be generated, so these should not be generated. should be within the range. That is, the amounts of calcium ions and hydroxyl ions are higher than the solubility of hydroxyapatite, etc., produced in equation (1), but lower than the supersolubility, that is, the conditions are such that hydroxyapatite, etc., 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 there are no crystal seeds, forming fine precipitates and clogging the bed, but in a metastable region lower than the supersolubility, new crystals precipitate on top of the crystal seeds. No precipitate is formed, only crystals precipitate and grow. In addition, crystals do not precipitate in systems lower than the solubility.

The amount of hydroxyapatite etc. produced is mainly controlled by the phosphate ion concentration, calcium ion concentration and pH of the reaction system. The amount of calcium ions and PH value to bring hydroxyapatite etc. into the metastable range can be determined experimentally by changing these values for each reaction system. The approximate range is phosphate ion 50mg/or less, calcium ion 10-100mg/, PH 6
~12, but varies depending on each condition. When targeting secondary treated sewage water with a phosphorus concentration of 2 to 5 mg, calcium ion concentration of 40 to 40
It is practical to treat at 80mg/with a pH of about 8.5 to 9.

The method of contacting the raw water containing phosphate with the crystal seeds containing calcium phosphate may be a fixed bed type or a fluidized bed type. 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 upward or downward at a flow rate of SV1 to 5 hr ^-1 to precipitate calcium phosphate crystals.

If the crystal seed surface becomes contaminated or clogged during water flow, periodically perform upward flow cleaning (backwashing) to expand and clean the crystal seed bed and remove impurities attached to the surface. It is desirable to peel off the
Water flow conditions during backwashing include a flow rate of about 20 to 80 m/hr and a washing time of about 5 to 60 minutes.

If the crystallization operation as described above is continued, the activity of the crystal seeds decreases and the dephosphorization performance decreases. Such a tendency is remarkable when raw water contains organic substances, magnesium ions, etc.

According to electron micrographs and X-ray diffraction results of the crystal seeds, it was found that the deep layer of calcium phosphate precipitated on the surface of the crystal seeds showed highly crystalline hydroxyapatite, and the surface layer showed an amorphous shape.
Under normal crystallization conditions, apatite crystals are formed in about 14 days from the start of crystallization, and the crystals crystallize from the inner layer close to the surface when crystallization starts, and the new surface layer is amorphous. will increase.

The drawing is an X-ray diffraction diagram of the crystal seeds used in the Examples described below.
B is the diffraction diagram of the second layer of 51-80% when 80% of the precipitate is further exfoliated, C is the diffraction diagram of the second layer of 51% to 80% when 80% of the precipitate is further exfoliated. Diffraction diagram of 81-95% third layer when peeled to D
is a diffraction pattern of an unused crystal seed.

As is clear from the results in the drawings, the precipitates on the surface layer of the crystal seeds have a broad diffraction pattern, indicating that they are nearly amorphous precipitates. Furthermore, in the diffraction pattern, the peak indicating hydroxyapatite becomes sharper as one goes to the inner layer, indicating that the degree of crystallinity progresses and an apatite structure is formed. Therefore, by peeling off the surface layer of the precipitate to expose the crystal seed apatite layer deposited on the surface of the crystal seeds, the crystal seeds whose performance has deteriorated can be reactivated.

As a method for peeling off the surface layer of the precipitate, the crystal seeds are mechanically or thermally treated in a dry or wet state and then peeled off. Mechanical methods include methods of stirring with a ball mill or stirring blade, methods of bringing crystal seeds into contact with each other or colliding with a wall surface by impact with water or air flow, and methods of applying mechanical vibration or ultrasonic vibration. Since the amorphous calcium phosphate that has precipitated does not come off when the crystal seeds come into contact with each other through development cleaning (backwashing) or fluidized bed contact performed during crystallization, it is necessary to apply a stronger impact than these. . The thermal method includes a method of expanding and contracting by heating and cooling.

The degree of peeling is such that an apatite layer (crystal) formed on the surface of the crystal seed is exposed. Generally, it is desirable to peel off at least 50%, preferably at least 70%, of the calcium phosphate in the surface layer of the precipitate to expose the crystal planes near the original outer surface. Theoretically, it is possible to remove 100% of the deposited part, but
In actual operation, the original crystal seeds are destroyed and the grain size becomes smaller, so in the present invention, the peeling is suppressed to less than 100% so that the newly generated apatite layer is exposed. Therefore, if the initial peeling is less than 100%, then 100% of the new precipitate may be peeled off. Of course, less than 100% of the new precipitate may be removed at all times.

The time to perform peeling may be when the activity of the crystal seeds decreases and the dephosphorization performance decreases, but it may also be performed before the activity completely decreases. Calcium phosphate precipitated on crystal seeds usually crystallizes as apatite if crystallization continues for 14 days or more, so it is desirable to expose the surface where crystallization has continued for 14 days or more. For this reason, it is preferable to perform peeling at intervals of 14 days or more, but if the peeling rate is less than 100%, the remaining portion will gradually crystallize, so if this layer is exposed in the next peeling operation, may be less than 14 days. Generally, it is desirable to perform peeling at intervals of 10 to 30 days or more, preferably 14 days or more.

The above-mentioned stripping operation may be carried out by periodic development cleaning, or if crystallization is performed using a fluidized bed method under conditions where crystal seeds come into contact with each other, as a separate stripping operation. Although it is necessary to perform the contacting and peeling operations at predetermined intervals, the contacting and peeling operations may be performed at the same time using spreading cleaning or a fluidized bed method.

By performing the above peeling operation, a crystalline apatite layer is exposed on the surface of the crystal seeds, which is reactivated and the dephosphorization performance is restored.
The volume of crystal seeds enlarged by crystallization can be reduced, the increase in the amount of filling can be reduced, and in some cases, the amount of filling can be made constant. The surface layer of the separated precipitate is separated by gravity sedimentation, filtration, etc., and discharged from the system.

The crystal seeds that have been reactivated are brought into contact with water containing phosphate again to perform crystallization. The subsequent operations are the same as in the previous case. By performing crystallization in this manner, dephosphorization can be efficiently performed.

In the present invention, other activation treatment using drugs or the like is not necessary, but it may be used in combination with this.

〔Effect of the invention〕

According to the present invention, the surface layer of the precipitated crystal seeds is peeled off and reactivated so that the apatite layer formed on the surface of the crystal seeds is exposed, so it is possible to easily reactivate the crystal seeds without using chemicals or the like. It can be reactivated by a simple operation and dephosphorization can be carried out efficiently.

[Embodiments of the invention]

Next, examples of the present invention will be described.

Example 1 Using 300 g of phosphate rock with a particle size of 0.25 to 0.42 mm (harmonic mean particle size of 0.31 mm) as a crystal seed, a crystal with a diameter of 3 cm and a height of 1
Packed into an acrylic resin column of P=2 mg/
, M alkalinity = 100mg/, Ca ²⁺ = 45mg/
Add sodium hydroxide to the synthetic water to adjust the pH to 9.
After that, water was passed in an upward flow at LV=10 m/hr to form a fluidized bed while contacting the solution, and crystallization was performed for 60 days. Then, all of the crystal seeds are taken out and dried, stirred gently in a mortar, and the surface layer corresponding to 70 to 80% of the weight increase due to crystallization is peeled off. After peeling, the crystal seeds are refilled and crystallized. The analysis was restarted.
Thereafter, a similar peeling operation was performed every 30 days to peel off the surface layer portion corresponding to 70 to 80% of the weight increase after restarting crystallization, and further crystallization was performed.

As a result, the average phosphorus concentration in the treated water was 1.40 mg/day for 60 to 90 days, 1.32 mg/for 91 to 120 days, and 1.32 mg/for 121 to 120 days.
Stable treatment was possible at 1.41 mg/day for 150 days.

Example 2 After crystallizing for 60 days under the same conditions as in Example 1, peeling and recrystallization were performed, and peeling was repeated every 15 days thereafter.

As a result, the average phosphorus concentration in the treated water was 1.38 mg/day for 60 to 75 days, 1.45 mg/for 76 to 90 days, and 1.45 mg/for 90 to 105 days.
Almost stable treatment was possible with 1.53 mg/day and 1.47 mg/day from 106 to 120 days.

Example 3 After performing crystallization for 60 days under the same conditions as in Example 1, peeling and recrystallization were performed, and peeling was repeated every 10 days thereafter.

As a result, the average phosphorus concentration in the treated water was 1.30 mg/day for 60 to 70 days, 1.49 mg/for 71 to 80 days, 1.65 mg/for 81 to 90 days, 1.63 mg/ for 91 to 100 days, and Although it was reactivated by peeling, the activity was not completely recovered because there was not enough time for crystallization after that.

Comparative Example After crystallizing for 60 days under the same conditions as in Example 1, crystallization was continued for an additional 30 days without peeling, and then 30 to 40% peeling was performed in the same manner as in Example 1. After that, recrystallization was performed, and 30 days later, peeling was performed again under the same conditions.

As a result, the average phosphorus concentration in the treated water was 1.77 mg/day for 60 to 90 days, 1.64 mg/for 91 to 120 days, and 1.64 mg/for 120 to 120 days.
The concentration was 1.71 mg/day for 150 days, indicating that the activity did not recover.

From the above results, it can be seen that by peeling off the surface layer of the precipitate so as to expose the apatite layer formed on the surface of the crystal seeds, the crystal seeds are reactivated and stable processing can be performed.

[Brief explanation of the drawing]

The drawings show the crystal seeds and exfoliated product X in the examples.
It is a line diffraction diagram.

Claims (1)

[Claims] 1. Contacting water containing phosphate and crystal seeds containing a calcium phosphate compound at a pH of 6 or higher and in the presence of calcium ions without producing calcium carbonate, In a dephosphorization method that reactivates the crystal seeds after precipitating calcium phosphate on the surface of the crystal seeds, 50% of the calcium phosphate on the surface of the precipitated crystal seeds is removed so that the crystalline apatite layer precipitated on the surface of the crystal seeds is exposed. A dephosphorization method characterized by peeling off and reactivating more than % of phosphate, and then bringing it into contact with water containing phosphate again to crystallize. 2. The dephosphorization method according to claim 1, wherein the peeling exposes the surface on which crystallization has been performed for 14 days or more.