JPS5855840B2 - How to treat water containing phosphates - Google Patents

Description

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

A method has been proposed to remove phosphate by bringing water containing phosphate into contact with phosphate rock in the presence of calcium ions (D 1ssertationAbstrac
ts International Vol 33
, Nii 12, Part I, pp. 5878-13).

The Kono method removes phosphate ions contained in water by converting them into calcium phosphate such as hydroxyapatite and crystallizing them on the surface of phosphate rock, and the operation method is simpler than conventional flocculation methods. (The processing efficiency will also be significantly improved.)

However, even with this method, the removal rate of phosphate is small, and there are cases where practically satisfactory results are not obtained (for example, Jour, WPCF Vo146, &
11.2498 pages).

The purpose of the present invention is to improve such drawbacks in conventional methods and to propose a method for treating phosphate-containing water that has an extremely high phosphate removal rate.

This invention relates to a method for removing phosphate by bringing water containing phosphate into contact with phosphate rock in the presence of calcium ions, which is characterized in that a phosphate rock that satisfies the following conditions is used. This is a method for treating water containing salt.

■ The ratio of calcium content to phosphorus content in phosphate rock is 2.2 or more.

■ The ratio of the CO2 content in the phosphate rock to the calcium content excluding calcium phosphate (CO2/Ca) is 0.7 or more.

■ The specific surface area of phosphate rock is 87i/1 or more.

Phosphate is hydroxyapatite ((Ca5(OH)(PO4)s) and/or fluoroapatite CCa5F(po4)a) and/or tricalcium phosphate (Cas(PO4)2).
) are mixed and crystallized in an appropriate ratio and may contain other impurities.

The reaction that occurs when water containing phosphate is brought into contact with the above-mentioned phosphate rock in the presence of calcium phosphate differs depending on the reaction conditions, but is usually expressed by the following formula.

In the reaction of formula (1), the generated hydroxyapatite crystallizes on the surface of the phosphate rock as a crystal seed,
As the crystals grow, the phosphate ends in the water are removed.

For this reason, it was conventionally thought that the higher the purity of phosphate rock as a crystal seed, the higher the phosphate removal activity.

However, in general, high-purity phosphate rock has a small specific surface area due to advanced crystallization of hydroxyapatite and fluoroapatite, making it unsuitable as a crystal seed.

That is, phosphate rock is used as a crystal seed, and the crystal seed originally has the function of breaking the supersaturation of the solution and precipitating crystals, and the function of serving as a growth matrix for the precipitated crystals.

For the former precipitation action, it is desirable for the crystal seeds to have a large specific surface area, and for the latter action as a host, it is desirable for the crystals to have high purity.

However, if the purity of the crystal is high, the specific surface area becomes small as described above, making it unsuitable as a crystal seed.

In the present invention, phosphate rock that satisfies the above conditions is used as a crystal seed, but such phosphate rock contains calcium salts other than calcium phosphate, such as hydroxyapatite, fluoroapatite, and tricalcium phosphate, as impurities.
The specific surface area is large.

The present invention was completed based on the discovery that these impurities, particularly calcium carbonate, have a large effect on the effectiveness of removing phosphates from water.

In other words, the theoretical value of Ca/P of tricalcium phosphate is 1.94,
Ca of hydroxyapatite and fluoroapatite
Since the theoretical value of /P is 2.15, this means that phosphate rock with a ratio of 2.2 or more contains calcium compounds other than tricalcium phosphate, hydroxyapatite, and fluoroapatite.

Such impurities include, for example, calcium carbonate, carbonate apatite (Ca1(1(COa)6(O
H) 2) is mentioned, and it is thought that sala also contains calcium fluoride, etc.

Chemical analysis of phosphate rock yields CO2 measurements, which are presumed to originate from calcium carbonate and carbonate apatite in phosphate rock.

Among calcium compounds as impurities, these calcium carbonate compounds have a C02/Ca ratio of 0.7.
By containing the above, the degree of crystallinity is reduced,
It has a large specific surface area and can be used as a good phosphate rock for removing phosphates.

There is a relationship between the amount of impurities in phosphate rock and the specific surface area, but even phosphate rock with the same chemical analysis value has a large or small specific surface area due to the difference in crystallinity.
■ Use phosphate rock that satisfies all of the conditions.

In addition, examples of chemical analysis methods for phosphate rock include the fertilizer analysis method of the Agricultural Technology Research Institute of the Ministry of Agriculture and Forestry, and examples of specific surface area measurement methods include the BET method.

Since phosphate rock has a large specific surface area as described above, it tends to break the supersaturation of the solution and promote the precipitation of crystals, but since the impurities it contains include Ca and CO3, phosphate ions in the water are mixed with hydroxyapatite. It has a catalytic effect when precipitated as hydroxyapatite, and since the impurity has a crystal structure similar to that of hydroxyapatite to be produced, it promotes crystal growth.

To remove phosphates from water by the reaction of equation (1),
Raw water containing phosphate is brought into contact with the phosphate rock in the presence of calcium ions.

As can be seen from equation (1), in order to increase the removal rate of phosphate, it is necessary to advance the reaction to the right, and at the same time, in order to make it easier to crystallize the calcium phosphate produced, It is necessary to keep it clean and its activity level high.

When removing phosphates by such reaction operations, the residual phosphate concentration in the treated water is mainly controlled by the pH and calcium ion concentration at the time of the reaction, and the treatment can be carried out by increasing the pH or increasing the calcium ion concentration. The residual phosphate concentration in water can be lowered.

In order to advance the reaction of formula (1), it is desirable to increase the degree of supersaturation by increasing the amount of calcium ions and hydroxide ions present in the reaction system, but if the degree of supersaturation is increased, precipitation will occur in places other than phosphate rock. may precipitate and reduce the activity of phosphate rock, so the concentration of calcium phosphate produced is higher than the solubility and lower than the supersolubility (the concentration at which crystals begin to precipitate when no crystal seeds are present in the reaction system). Crystallization takes place in the region of calcium ions and pH, ie, the metastable region.

That is, in an unstable region higher than the supersolubility, a precipitate is formed, but in a metastable region lower than the supersolubility, new crystals are crystallized on the phosphate rock and the crystals grow, but no precipitate is formed.

The supersolubility of hydroxyapatite is controlled by the phosphate ion concentration, calcium ion concentration, and pH of the reaction system, and the amount of calcium ion and pH value that keep the amount of hydroxyapatite produced within the metastable range are determined by these values for each reaction system. It can be determined experimentally by changing the value.

The approximate range is 99m when phosphate ion is below 50, calcium ion is 10 to 100 ppm, pH
is about 6 to 12, but it varies depending on each condition.

If the raw water is already in such a metastable range, there is no need to further add calcium or alkaline agents, but if the metastable range has not been reached, calcium and/or alkaline agents may be added as necessary. do.

Calcium agents such as calcium chloride and calcium hydroxide can be used, and alkaline agents such as sodium hydroxide and calcium hydroxide can be used.

The method of contacting the water containing phosphate with the phosphate rock may be either 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 may be used when turbidity removal is not required.

The smaller the size of phosphate rock, the larger the specific surface area, which makes it easier for new crystals to precipitate, but if the size is too small, it will be difficult to contact or separate the phosphate rock and 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.

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, it is filled with phosphate rock having a thread diameter of 9 to 35 meshes, and water is passed upwardly or downwardly at a flow rate SV of 1 to 10 hr' to precipitate calcium phosphate crystals.

It is preferable to carry out upward water flow because suspended matter can be captured in the large particle size portion of the lower layer and crystallization can be performed in the small particle size portion of the upper layer with high activity.

When passing water in a downward flow, in order to avoid impurities from adhering to the phosphate rock surface, the raw water should be pretreated to remove impurities, or a filter medium with a smaller specific gravity and larger particle size than the phosphate rock should be used. Laminated on a fixed layer of phosphate rock,
It is desirable to remove impurities using a filter medium.

If the surface of the phosphate rock becomes contaminated or clogged during water flow, it is recommended to periodically perform backwashing using an upward flow to expand and clean the phosphate rock and remove impurities attached to the surface. desirable.

Next, examples of the present invention will be described.

Example: An acrylic resin column with an inner diameter of 3 CrrL and a height of 100 cm was filled with 300 TL of various phosphate rocks with particle sizes adjusted to 16 to 32 mesh. Synthetic wastewater with a phosphorus concentration of 3 ■/L was charged with calcium chloride at the column inlet. The solution and sodium hydroxide solution were continuously injected to adjust the calcium concentration to about 40 μ/l and the pH to about 9 while adding 600 ml.
Water was passed in an upward direction at a flow rate of /hr.

The packed bed was backwashed with treated water once a day.

Table 1 shows the composition, physical properties, and phosphorus removal performance of various phosphate rocks.

In Table 1, the specific surface area was measured by the BET method, and the chemical analysis values were analyzed by the fertilizer analysis method of the Agricultural Technology Research Institute of the Ministry of Agriculture and Forestry.

In addition, Ca/P is the ratio of calcium content to phosphorus content in phosphate rock, CO2/Ca is the ratio of the CO2 value obtained by chemically analyzing phosphate rock to the content of all calcium compounds other than calcium phosphate in phosphate rock, The treated water quality has a phosphorus concentration of 0.1ml! /l: The water flow rate (Bed Volume) when maintained below, and the treated water P indicates the phosphorus concentration of the treated water in a steady state.

From the results in Table 1, it can be seen that the phosphorus removal rates of Examples A to E are extremely better than those of Comparative Examples.

As described above, according to the present invention, phosphates can be removed at a high removal rate, and a large amount of water can be treated to obtain treated water of high quality.

Claims (1)

[Claims] 1. A method for removing phosphate by bringing water containing phosphate into contact with phosphate rock in the presence of calcium ions, characterized by using phosphate rock that satisfies the following conditions: A method for treating water containing phosphates. ■ The ratio of calcium content to phosphorus content in phosphate rock is 2.2 or more. ■ The ratio of CO2 content in phosphate rock to calcium content excluding calcium phosphate (CO2/Ca: 0.
7 or more. ■ The specific surface area of phosphate rock is 8 m'/11 or more. 2 Phosphate is hydroxyapatite and/or
The method for treating water containing phosphate according to claim 1, wherein fluoroapatite and/or tricalcium phosphate are crystallized. 3. Claim 1 or 2, wherein the method of contacting phosphate-containing water with phosphate rock is a fixed method or a fluidized bed method.
A method for treating water containing phosphates as described in Section 1.