JP3112613B2 - Treatment of wastewater containing fluorine and phosphorus - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for removing fluoride ions and phosphate ions contained in various industrial wastewaters such as those in the electronics industry and power plants using a calcium compound.

[0002]

2. Description of the Related Art Fluorine ions of various industrial wastewaters (hereinafter referred to as "fluorine ions")
As a method for removing fluorine and phosphorus from wastewater (fluorine / phosphorus-containing wastewater) containing phosphate ions (hereinafter sometimes simply referred to as “phosphorus”), a fluorine removal technology using a calcium compound and phosphorus Removal techniques are more widely used than before.

[0003] Removal of fluorine by a calcium compound
As shown in the following formula (1), the reaction is performed by generating hardly soluble calcium fluoride by the reaction of calcium ions and fluorine ions, and then performing solid-liquid separation. In this case, the solubility of the generated calcium fluoride is pH 4-11.
, And does not change significantly in this range. _{^{Ca 2+ + 2F - → CaF 2}} ↓ ... (1)

As shown in the following formulas (2) and (3), the removal of phosphorus by a calcium compound is performed by generating hardly soluble calcium phosphate by the reaction of calcium ions and phosphate ions, followed by solid-liquid separation. This is done by:
In this case, the solubility of calcium phosphate is high when the pH is 8.5 or less, and rapidly decreases when the pH exceeds 8.5.
(2) and (3) Formulas for the Formation of Calcium Phosphate
Effectively proceeds in a high pH region exceeding 8.5. _{^{3Ca 2+ + 2PO 4 3- → Ca}} 3 (PO 4) 2 ↓ ... (2) 5Ca 2+ + OH - + 3PO 4 3- → Ca 5 OH (PO 4) 3 ↓ ... (3)

Conventionally, fluorine and phosphorus in waste water have been removed by the method shown in FIG. 2 utilizing the above-described reaction. That is, first, in the first reaction step, the wastewater is pH
By adding a regulator and a calcium compound, and performing the reactions of Formulas (1) to (3) in a high pH region exceeding pH 8.5, fluorine and phosphorus in the wastewater are removed. In this case, the reaction in the first reaction step is performed at a high pH exceeding pH 8.5.
It carried out in the region, to be mainly used the alkaline Ca (OH) ₂ as a calcium compound to be added is why such that the solubility of CaF ₂ does not change significantly in the range of PH4～11.

Next, as a second reaction step, a pH adjuster is added to water after the completion of the first reaction step to make the pH neutral. Further, in the agglutination reaction step, a large amount of an aluminum compound such as PAC (polyaluminum chloride) or a sulfate band (aluminum sulfate) is added to water after the completion of the second reaction step as a coagulant to cause the first reaction. Fluorine that cannot be removed in the reaction step is adsorbed and removed by aluminum hydroxide generated by the agglutination reaction, and then solid-liquid separation is performed. The reason why the pH of the waste water is made neutral in the second reaction step is that if the pH is not about neutral, aluminum hydroxide will not be sufficiently generated, and thus the adsorption reaction of fluorine to aluminum hydroxide will be sufficiently performed. This is because they will not.

Further, the water after the completion of the agglutination reaction step is treated by known solid-liquid separation means such as gravity sedimentation, pressure flotation, sand filtration, or membrane filtration using an ultrafiltration membrane or a microfiltration membrane. Get water.

[0008]

However, the conventional method for treating fluorine- and phosphorus-containing wastewater in which the first reaction step is carried out in a high pH range has the following two problems.

The first problem is that when the fluorine- and phosphorus-containing wastewater is treated by a conventional method, fluorine cannot be sufficiently removed in the first reaction step. That is, the first
In the reaction step, even if an excessive amount of the calcium compound corresponding to the amount consumed by the reaction with phosphorus is added to the phosphorus / fluorine-containing wastewater in addition to the theoretical amount of the calcium compound required for the reaction with fluorine, This means that the concentration of residual fluorine in water after the completion of the first reaction step becomes unexpectedly high.

Therefore, conventionally, for the purpose of improving the quality of treated water, an agglutination reaction step is provided after the first reaction step,
In the subsequent agglutination reaction step, a large amount of an aluminum compound is added as an emergency measure, and the resulting aluminum hydroxide is adsorbed and removed by fluorine. However, the use of a large amount of the aluminum compound in such a coagulation reaction step leads to a large increase in the amount of chemicals used, and most of the aluminum compound added in excess becomes aluminum hydroxide sludge, and the amount of generated sludge increases. Therefore, it was not economically and environmentally favorable. Note that the first problem described above also occurs when a solid-liquid separation step is provided between the first reaction step and the second reaction step.

Second, in the treatment of wastewater containing fluorine and phosphorus by a conventional method, if a solid-liquid separation step is not provided between the first reaction step and the second reaction step, the pH is increased in the second reaction step. First to adjust to 6-7 neutrality
There is a problem that calcium phosphate generated in the reaction step is re-eluted and the concentration of phosphorus in water increases.

Therefore, in order to improve the quality of the treated water, the amount of aluminum compound to be added in the subsequent agglutination reaction step is greatly increased as an urgent countermeasure for removing phosphorus that has been re-eluted, thereby reducing phosphorus. As aluminum acid,
Alternatively, aluminum hydroxide is adsorbed and removed, but the use of a large amount of aluminum compound in such a coagulation reaction step causes a large increase in the amount of chemicals used and a large amount of aluminum hydroxide sludge as described above. It was connected, and it was unfavorable both economically and environmentally.

The present invention has been made in view of the above circumstances, and has achieved efficient removal of fluorine and prevention of re-elution of calcium phosphate. Thus, fluorine and phosphorus can be used without using a large amount of aluminum compound in a subsequent agglutination reaction step. An object of the present invention is to provide a wastewater treatment method capable of removing fluorine and phosphorus in contained wastewater.

[0014]

Means and Action for Solving the Problems The present inventors have
In order to achieve the above object, first, a study was made on a fluorine removal reaction by a calcium compound, and the following findings were obtained.

The formation reaction of CaF ₂ shown in the equation (1) is composed of two steps. The first step is a reaction for forming a seed crystal of CaF ₂ , and the second step is a reaction for precipitating CaF _{2 on} the seed crystal. There is.

The first stage of the seed crystal formation reaction is the product of the calcium ion concentration in water and the square of the fluoride ion concentration in water, that is, [Ca] · [F] ² ([Ca] and [F]
Indicate the ion concentrations of Ca and F, respectively). This means that, for wastewater having a predetermined fluorine concentration, the larger the amount of the calcium compound added during the reaction, the faster the first-stage seed crystal formation reaction proceeds.

The second-stage precipitation reaction is more likely to occur as the concentration of calcium ions in the water is higher, and the deposition rate is also faster. This means that the more calcium ions are present in the water, the lower and better the fluorine concentration after the completion of the reaction.

Comparing the formation reaction of calcium fluoride and the formation reaction of calcium phosphate, the formation reaction of calcium phosphate occurs preferentially and quickly in a high pH region exceeding pH 8.5. In addition, pH4 ~
In the pH range of 8.5, the formation of calcium phosphate by the reaction between calcium ions and phosphate ions does not substantially occur.

As a result of further studies based on the above findings, the present inventors found that p
In order to react in a high pH range exceeding H8.5, the added calcium ions first react with phosphate ions,
As a result, the concentration of calcium ions in the water decreases, and as a result, the concentration of calcium ions in the water decreases. It was conceived that the reaction did not proceed sufficiently, and that the residual fluorine concentration after the reaction was increased. As a result of considering the characteristics of the Ca-F-based reaction and the Ca-P-based reaction, the present inventors have made the following invention as an optimum treatment method for fluorine- and phosphorus-containing wastewater.

That is, the present invention provides a first reaction step in which a calcium compound is added to wastewater containing fluorine ions and phosphate ions to cause a reaction in a pH range of pH 4 to 8.5, and a step of ending the first reaction step. And a second reaction step in which the subsequent water is brought to a pH range exceeding pH 8.5 to carry out the reaction.
And in the first reaction step,
(4), (5) [When the phosphorus concentration is 600 mg PO ₄ / l or less] (F × 1.053) + 400 ≦ C ≦ (F × 1.053) +800 (4) [The phosphorus concentration is 600 mg PO ₄ / 1 or more] (F × 1.053) + (P × 0.63) ≦ C ≦ (F × 1.053) + (P × 0.63) +400 (5) where F: Fluorine concentration in wastewater (mgF / l) C: Calcium compound amount added to 1 liter of wastewater mgCa) P: Fluorine / phosphorus content characterized by adding a calcium compound to wastewater so as to have a phosphorus concentration (mgPO ₄ / l) in the wastewater Provide a method for treating wastewater.

In the first reaction step of the present invention, a sufficient amount of a calcium compound for removing fluorine and phosphorus in the waste water containing fluorine and phosphorus is added to the waste water and the reaction is carried out at a pH of 4 to 8.5. is there. In this case, at a pH of 4 to 8.5, the formation of calcium phosphate by the reaction between calcium ions and phosphate ions does not substantially occur, so that calcium ions react preferentially with fluoride ions to generate calcium fluoride, and At the stage, calcium ions necessary for removing phosphorus also remain in the reaction system,
The calcium ion concentration in the water is higher by that amount,
Therefore, fluorine is sufficiently removed.

In the second reaction step of the present invention, the pH of water after the completion of the first reaction step is raised to a high pH region exceeding pH 8.5 suitable for removing phosphorus, and the reaction is carried out. At this time, since the fluorine has already been removed in the first reaction step, the residual calcium ions in the wastewater react with the phosphate ions to generate calcium phosphate, and the phosphorus is removed. Further, even if the pH is increased in the second reaction step, the solubility of calcium fluoride does not increase, so that re-elution of calcium fluoride does not occur. Furthermore, since fluorine and phosphorus in the wastewater have been sufficiently removed at the end of the second reaction step, the PAC
It is not necessary to add a large amount of an aluminum compound such as a sulfuric acid band or a sulfuric acid band and to carry out a coagulation reaction as in the prior art in which the pH of water is made neutral to about 6 to 7, so that re-elution of calcium phosphate can be prevented.

Hereinafter, the present invention will be described in more detail with reference to FIG. In the first reaction step of the present invention, a calcium compound is added to the waste water to carry out the reaction at a pH of 4 to 8.5, preferably at a pH of 4 to 8.0. If the pH is less than 4, the removal of fluorine ions becomes insufficient and p
If H exceeds 8.5, calcium ions react with phosphate ions, so that fluorine cannot be sufficiently removed. The pH can be adjusted by adding a pH adjuster or the like. In this case, the addition of the pH adjuster may be performed before the addition of the calcium compound, or may be performed substantially simultaneously with the addition of the calcium compound.

As the calcium compound to be added, any calcium compound that releases calcium ions in wastewater can be used. Examples of such calcium compounds include calcium chloride (CaCl ₂ ), calcium hydroxide (Ca (OH) ₂ ),
Examples thereof include calcium sulfate (CaSO ₄ ).

The amount of the calcium compound added to the wastewater in the first reaction step varies depending on the concentrations of fluorine and phosphorus in the wastewater and the target quality of treated water, and is not necessarily limited. 10m
When it is desired to be not more than gF / l, it is preferable to add such that the calcium concentration in the wastewater falls within the range of the following formulas (4) and (5). [When the phosphorus concentration is 600 mg PO ₄ / l or less] (F × 1.053) + 400 ≦ C ≦ (F × 1.053) +800 (4) [When the phosphorus concentration is 600 mg PO ₄ / l or more] (F × 1.053) + ( P × 0.63) ≦ C ≦ (F × 1.053) + (P × 0.63) +400 (5) where F: Fluorine concentration in wastewater (mgF / l) C: Calcium compound added to 1 liter of wastewater Amount (mgCa) P: Phosphorus concentration in wastewater (mgPO ₄ / l)

That is, the theoretical amount of Ca required to generate CaF ₂ and remove F is approximately F × 1.053 (mg Ca / l) by calculation based on the molecular weight.
The theoretical amount of Ca required to generate ₃ (PO ₄ ) ₂ and Ca ₅ OH (PO ₄ ) ₃ and remove P is calculated by the same calculation as P
× 0.63 (mgCa / l). Equations (4) and (5) indicate that, in the first reaction step, the total amount of the calcium compound necessary for removing fluorine and phosphorus is added, even when the phosphorus concentration in the wastewater is not so high. At least 400 mg Ca / l, preferably 400-800 m2, in addition to the amount of Ca required to remove F
This shows that it is advantageous to add Ca (g Ca / l) in excess (equation 4).

As described above, in the first reaction step, the total amount of the calcium compound required for removing fluorine and phosphorus is added, and at least 400 m is required for the amount of Ca required for removing F.
By the presence of excess gCa / l Ca, the first
The concentration of calcium in the waste water in the reaction step can be sufficiently increased to ensure that the removal of fluorine can proceed, and as a result, the concentration of fluorine in the resulting treated water can be reduced to 10 mgF.
/ L or less. If the amount of Ca is less than the range of the formulas (4) and (5), the fluorine may not be sufficiently removed. There is a possibility that a large amount of Ca in the reaction remains and troubles such as precipitation of calcium scale in the subsequent processing system occur.

Next, in the present invention, in the second reaction step, the pH of the water after the completion of the first reaction step is raised to a pH range exceeding pH 8.5, preferably pH 9 to 11, to cause the reaction. When the pH is 8.5 or less, the reaction between calcium ions and phosphate ions does not proceed sufficiently. The pH can be adjusted, for example, by adding a pH adjuster composed of an alkaline agent such as sodium hydroxide.

In the present invention, the first reaction step and the second reaction step may be performed continuously using two continuous reaction tanks, or may be performed batchwise using the same reaction tank. In the present invention, since the calcium fluoride is not re-eluted in the second reaction step, it is not necessary to provide a solid-liquid separation step between the first reaction step and the second reaction step, but it may be provided in some cases. . In the present invention, in the first and second reaction steps, fluorine and phosphorus in the wastewater can be effectively removed only by the action of the calcium compound. Alternatively, it may be processed by solid-liquid separation means such as pressure flotation, sand filtration, or membrane filtration using an ultrafiltration membrane or a microfiltration membrane to obtain clear treated water. However, since calcium fluoride and calcium phosphate present in the water after the second reaction step are usually very fine particles, a polymer coagulant alone is added to the water after the second reaction step, Alternatively, performing a flocculation reaction by adding a small amount of PAC or a sulfate band in addition to a polymer flocculant is effective in that solid-liquid separation is efficiently performed. Even if an aluminum compound such as PAC is added, it is not intended to remove residual fluorine as in the conventional agglutination reaction step. There is no need to adjust the pH of the system to near neutral.

[0030]

EXAMPLES Next, the present invention will be specifically described by way of examples, but the present invention is not limited to the following examples. The following Examples and Comparative Examples are examples in which wastewater treatment was performed using synthetic wastewater containing fluorine ions and phosphate ions. Wastewater treatment was performed by a beaker test. In addition,
In the following Examples and Comparative Examples, NaOH was used for pH adjustment.
Aqueous and HCl aqueous solutions were used.

Example 1 100 mg of NaF in pure water
F / l, using synthetic wastewater a KH ₂ PO ₄ was dissolved 400mgPO ₄ / l. 500 ml of synthetic wastewater was collected in a beaker, and calcium chloride was placed in the beaker at 600 mgCa / l
(≒ (F × 1.053) + 495mgCa / l)
Was adjusted to 7.0 and the reaction was run for 10 minutes. Then p
H was adjusted to 9.5 and the reaction was carried out for 10 minutes, and the water after the reaction was filtered with No. 2 filter paper and evaluated as treated water.
Table 1 shows the concentrations of fluorine and phosphorus in the treated water.

Comparative Example 1 500 ml of the same synthetic waste water as used in Example 1 was collected in a beaker, and calcium chloride was added thereto in an amount of 600 mg Ca / l (≒ (F × 1.053) +495).
(mgCa / l), the pH was adjusted to 9.5, and the reaction was carried out for 10 minutes. Thereafter, the pH was adjusted to 7 and the reaction was carried out for 10 minutes, and the water after the reaction was filtered with No. 2 filter paper and evaluated as treated water. Table 1 shows the concentrations of fluorine and phosphorus in the treated water.

Example 2 100 mg of NaF in pure water
F / l, using synthetic wastewater a KH ₂ PO ₄ was dissolved 800mgPO ₄ / l. 500 ml of synthetic wastewater was collected in a beaker, and calcium chloride was added at 810 mgCa / l (≒ (F ×
1.053) + (P × 0.63) +200 mgCa / l)
The pH was adjusted to 7.0 and the reaction was run for 10 minutes. Thereafter, the pH was adjusted to 9.5, the reaction was carried out for 10 minutes, and the water after the reaction was filtered with No. 2 filter paper to evaluate as treated water. Table 1 shows the concentrations of fluorine and phosphorus in the treated water.

Comparative Example 2 500 ml of the same synthetic waste water used in Example 2 was collected in a beaker, and calcium chloride was added at 810 mg Ca / l (≒ (F × 1.053) + (P × 0.63)
After adding (+200 mgCa / l), the pH was adjusted to 9.5 and the reaction was carried out for 10 minutes. Thereafter, the pH was adjusted to 7 and the reaction was carried out for 10 minutes, and the water after the reaction was filtered with No. 2 filter paper and evaluated as treated water. Table 1 shows the concentrations of fluorine and phosphorus in the treated water.

[0035]

[Table 1]

From the above Examples and Comparative Examples, it can be seen that pH 4 to 4 in which the formation of calcium phosphate does not substantially occur in the first reaction step.
The calcium compound is reacted in the range of 8.5 and contained in the wastewater by the method of the present invention in which the pH of water after the completion of the first reaction step in the second reaction step is raised to a high pH region exceeding 8.5. Fluoride ions and phosphate ions can be efficiently removed, and as a result, the concentration of fluorine ions and phosphate ions in the water (treated water) after the completion of the second reaction step is lower than that of the comparative example which is the conventional method. It can be seen that it can be significantly reduced.

[0037]

As described above, according to the treatment method of the present invention, the fluorine concentration in water after the completion of the first reaction step can be reliably reduced, and phosphorus can be removed in the second reaction step. And the re-elution of calcium fluoride and calcium phosphate does not occur in the second reaction step, so that fluorine and phosphorus in the fluorine / phosphorus-containing wastewater can be satisfactorily removed to obtain treated water with improved water quality. be able to.

In the treatment method of the present invention, fluorine and phosphorus can be removed only by the action of a calcium compound. Therefore, a conventional agglutination reaction using a large amount of an aluminum compound such as PAC at the subsequent stage of the second reaction step. Since there is no need to provide a process, the amount of generated sludge can be reduced as compared with the conventional method, so that it is economically and environmentally superior.

In the treatment method of the present invention, a slight excess of the calcium compound may be added in addition to the theoretical amount of the calcium compound consumed for the reaction with fluorine and phosphorus in the wastewater. The use amount of the calcium compound can be reduced, which is economically advantageous.

Furthermore, in the treatment method of the present invention, the calcium ions remaining in the water after the first reaction step in the second reaction step are consumed by the reaction with phosphorus, and the residual calcium concentration in the treatment water decreases. The trouble of the precipitation of the calcium compound scale in the subsequent processing system can be avoided.

[Brief description of the drawings]

FIG. 1 is a flow chart showing a method for treating a wastewater containing fluorine and phosphorus according to the present invention.

FIG. 2 is a flow chart showing a conventional method for treating wastewater containing fluorine and phosphorus.

────────────────────────────────────────────────── (5) References JP-A-52-67152 (JP, A) JP-A-52-28165 (JP, A) JP-A-52-28164 (JP, A) (58) Field (Int.Cl. ⁷ , DB name) C02F 1/58

Claims (1)

(57) [Claims] 1. A first reaction step in which a calcium compound is added to waste water containing fluorine ions and phosphate ions to cause a reaction in a pH range of pH 4 to 8.5, and water after the first reaction step is removed. a second reaction step in which the reaction is carried out in a pH range exceeding pH 8.5 , and wherein the first reaction is carried out.
In the process, the calcium concentration in the wastewater
(4), (5) [When phosphorus concentration is 600 mg PO ₄ / l or less] (F × 1.053) + 400 ≦ C ≦ (F × 1.053) +800 (4) [When phosphorus concentration is 600 mg PO ₄ / l or more] (F × 1.053) + (P × 0.63) ≦ C ≦ (F × 1.053) + (P × 0.63) +400 (5) where F: Fluorine concentration in wastewater (mgF / l) C: 1 liter of wastewater calcium compound amount MgCa) P is added to: phosphorus concentration (mgPO ₄ / l) processing method of a fluorine-phosphorus-containing waste water, which comprises adding a calcium compound to waste water to be in the range of in the waste water.