JP2001038368A - Treatment of water containing fluorine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce high concentration sludge containing CaF2 sludge particles having a larger particle size by adding sulfate ions to fluorine-containing water in the process of treating the fluorine-containing water by bringing a part of the produced calcium fluoride sludge into contact with a calcium compound and then adding the resultant product to the fluorine-containing water. SOLUTION: The raw water (fluorine-containing water) discharged from a semiconductor factory or the like is reserved in a raw water tank 1, to which a sulfate ion-containing liquid is added, and then the water is introduced into a neutralization tank 3 to produce CaF2 sludge by the reaction with a reformed sludge added from a reaction tank 2. The neutralized reaction liquid is sent to a flocculation tank 4, to which a polymer is added from a polymer reservoir 7 to flocculate the liquid. Then the liquid is subjected to solid-liquid separation in a sedimentation tank 5. A part of the separated sludge drained from the sedimentation tank 5 is sent back to the reaction tank 2, however, the increment of the sludge is drained from the system so as to keep the sludge amount in the system constant. In the reaction tank 2, Ca(OH)2 from a Ca(OH)2 reservoir 6 and the returned sludge are mixed and the obtained reformed sludge is supplied to the neutralization tank 3.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a fluorine-containing wastewater discharged from a fluorine compound production plant, a steel industry, a fertilizer production plant, or a semiconductor plant, which is treated with a calcium (Ca) salt to convert the fluorine contained. Calcium (CaF ₂ )
The present invention relates to a method for separating and removing sludge as sludge, and particularly to a method for obtaining high-concentration sludge containing CaF ₂ sludge particles having a large particle diameter in this method.

[0002]

2. Description of the Related Art Conventionally, as a method of treating fluorine-containing water,
A method is known in which a Ca salt is added to fluorine-containing water in a reaction tank to separate and remove fluorine contained as CaF ₂ . In such a method of treating water containing fluorine, a method of obtaining high-concentration sludge by returning separated CaF ₂ sludge to a reaction tank, more specifically, converting CaF ₂ sludge to C
A method has been proposed in which sludge is reformed by mixing it with a salt and adding it to a reaction tank (JP-A-5-293474 and JP-A-10-479; hereinafter, this method is referred to as "sludge return method"). Called.)

In the treatment of fluorine-containing water, high-concentration sludge can be obtained by adopting the above-mentioned sludge return method, and the high-concentration sludge generated has a high dehydration efficiency in the subsequent dehydration step and a high water content. Since a low dewatered cake can be obtained, it is very effective in reducing the amount of industrial waste generated and the like, and in recent years, the number of cases in which the sludge return method is adopted has been increasing.

[0004] Recently, hydrogen fluoride (HF) has been recovered from CaF ₂ sludge obtained by such treatment of fluorine-containing water and reused. In this case, HF is recovered by drying a dehydrated cake obtained by dehydrating CaF ₂ sludge and then adding sulfuric acid (H ₂ S
O ₄ ) and add CaF ₂ + H ₂ SO ₄ → CaSO ₄ + 2H
This is performed by generating HF gas by the reaction of F.

In the case of recovering HF as described above, in order to dry the dehydrated cake, it is necessary to obtain a dehydrated cake having a lower moisture content from the viewpoint of the drying efficiency. In the treatment of wastewater, it was considered optimal to adopt a sludge return method that can obtain a dewatered cake having a low water content.

[0006]

When the dehydrated cake is dried, not only the dehydrated cake has a low water content but also CaF ₂ contained in the dehydrated cake in order to carry out efficient drying.
An important requirement is that the particle size of the sludge particles is large.

That is, when the particle size of the CaF ₂ sludge particles is small, the dewatered cake becomes sticky, and it is difficult to sufficiently dry the inside of the dewatered cake. In addition, drying of the dehydrated cake is usually performed by blowing hot air,
At this time, fine CaF ₂ sludge particles are scattered by hot air,
Since the bag filter at the outlet of the dryer is closed at an early stage, the frequency of maintenance increases and the work efficiency becomes poor.

However, in the conventional sludge return method, although a dewatered cake having a low water content can be obtained, the particle size of the CaF ₂ sludge particles cannot be said to be sufficiently large, and the drying efficiency and the recovery efficiency of HF are low. From the viewpoint of improvement, it has been desired to generate CaF ₂ sludge particles having a larger particle size.

[0009] The present invention has been made in view of the above-mentioned conventional circumstances, and in the treatment of fluorine-containing water by the sludge return method, it is possible to generate high-concentration sludge containing CaF ₂ sludge particles having a larger particle diameter. An object of the present invention is to provide a method for treating fluorine-containing water that can be performed.

[0010]

The method for treating fluorine-containing water according to the present invention comprises adding a calcium compound to fluorine-containing water to form calcium fluoride sludge, and separating the generated calcium fluoride sludge. A method of treating a fluorine-containing water, wherein a part of the generated calcium fluoride sludge is brought into contact with a calcium compound and then added to the fluorine-containing water, wherein sulfate ions are added to the fluorine-containing water. It is characterized by.

According to the present invention, CaF ₂ sludge particles having a large particle size can be obtained by adding a sulfate ion to the fluorine-containing water as raw water and treating it by a sludge return method.

In the present invention, CaF ₂ by sulfate ion is used.
Although the details of the mechanism of the effect of increasing the size of the sludge particles are not clear, it is presumed to be mainly due to the following and / or reaction.

In the presence of sulfate ions, CaSO ₄ crystals are locally and transiently generated, which serve as nuclei for CaF ₂ sludge particles, and larger CaF ₂ sludge particles can be obtained as compared with the case where CaSO ₄ does not coexist. .

Ca _x (SO ₄ ) _{y in} the presence of sulfate ions
Although a mixed crystal of F _z is be generated, the mixed crystal CaF ₂
To grow into crystals having a larger grain size than
F ₂ sludge particles are obtained.

In the present invention, the amount of sulfate ion relative to fluoride ion in the fluorine-containing water is 0.04%.
It is preferable to add so as to be 1 to 0.1, preferably 0.02 to 0.1 equivalent.

In the present invention, the return ratio of CaF ₂ sludge is preferably 15 to 60.

The return ratio of the CaF ₂ sludge is a value R defined by the following equation in the sludge return method shown in FIG. 1 described later.

[0018]

(Equation 1)

[0019] FIG. 2 described later, (sometimes hereinafter referred to as "direct sludge return method".) Sludge return method for returning the CaF ₂ sludge-containing liquid directly without solid-liquid separation CaF ₂ sludge in , A value R defined by the following equation:

[0020]

(Equation 2)

The method of the present invention is particularly effective when the obtained CaF ₂ sludge is dehydrated, dried and reused as a raw material for producing HF.

[0022]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an embodiment of the method for treating fluorine-containing water of the present invention will be described in detail with reference to the drawings.

FIGS. 1 and 2 are system diagrams showing an embodiment of the method for treating fluorine-containing water of the present invention. In FIGS. 1 and 2, members having the same function are denoted by the same reference numerals.

In the method of FIG. 1, raw water (fluorine-containing water) is added to a raw water tank 1 after a sulfate ion-containing liquid is added thereto.
CaF ₂ sludge is generated by a reaction with the later-described modified sludge introduced into the neutralization tank 3 and added from the reaction tank 2. The neutralized reaction solution is then fed to the coagulation tank 4, where the polymer is added from the polymer (polymer coagulant) storage tank 7 and subjected to coagulation treatment, and then solid-liquid separated in the precipitation tank 5. The separated liquid in the precipitation tank 5 is discharged out of the system as treated water. A part of the separated sludge withdrawn from the sedimentation tank 5 is returned to the reaction tank 2, but the increased sludge is discharged out of the system in order to keep the amount of sludge in the system constant.

In the reaction tank 2, Ca (OH) ₂ (slaked lime)
Ca (OH) ₂ from the storage tank 6 and the returned sludge are mixed (hereinafter, this mixed sludge is referred to as “modified sludge”), and the modified sludge is supplied to the neutralization tank 3.

In such a method, as the sulfate ion-containing liquid to be added to the raw water, a sulfate such as H ₂ SO ₄ or Na ₂ SO ₄ or waste water containing sulfate ions can be used. Is preferably an amount such that the amount of sulfate ion with respect to F in the raw water is 0.01 to 0.1 equivalent, preferably 0.02 to 0.1 equivalent. When the sulfate ion concentration is less than 0.01 equivalent, a sufficient improvement effect by adding the sulfate ion-containing solution cannot be obtained, and when the sulfate ion concentration exceeds 0.1 equivalent, use of a chemical for adjusting the pH of the subsequent neutralization tank is not possible. The amount increases, which is not preferable, and the F concentration of the treated water also increases.

The reaction in the neutralization tank 3 is carried out at pH 5-1.
It is preferable to perform the reaction in the neutralization tank 3 through the reaction tank 2 by opening and closing the valves V ₁ and V ₂ linked to the pH meter 3A provided in the neutralization tank 3 so that the pH is within such a range. The amount of Ca (OH) ₂ supplied is adjusted.

As the Ca compound, Ca (OH)
_In addition to ₂ , CaCl ₂ and other calcium compounds can be used. When a non-alkaline Ca salt is used, the pH may be adjusted by separately adding an alkali such as NaOH. When Ca (OH) ₂ is used, hydrochloric acid or the like may be used in combination to secure necessary calcium ions.

According to this sludge return method, the growth of CaF ₂ sludge particles can be promoted by adding a sulfate ion-containing solution to the raw water, large particles can be obtained, and the returned sludge is Ca (OH) ₂ The sludge is reformed by being mixed with and added to the neutralization tank 3, and high-concentration sludge can be obtained.

In the method shown in FIG. 2, instead of returning the sludge separated in the sedimentation tank 5 to the reaction tank 2, the sludge-containing liquid from the neutralization tank 3 is returned to the reaction tank 2 without coagulation and separation. 1 is different from the method shown in FIG. This direct sludge return method is particularly effective in treating concentrated raw water having a high F concentration. In this method as well, the addition of a sulfate ion-containing solution increases the particle size of CaF ₂ sludge and reduces the F return sludge. Ca (OH) ₂
And the mixture is added to the neutralization tank 3 to obtain high-concentration sludge.

In the method for treating fluorine-containing water of the present invention, the sludge return ratio R is 15 to 60.
It is preferred that If the return ratio R is less than 15, the effect of returning sludge cannot be sufficiently improved, and high-concentration sludge and high-quality treated water cannot be obtained. Even if the return ratio R exceeds 60, the sludge concentration tends to decrease and the quality of the treated water tends to decrease. Therefore, the return ratio R is preferably set to 15 to 60.

The method shown in FIGS. 1 and 2 is an example of the embodiment of the method for treating fluorine-containing water of the present invention, and the present invention is not limited to the illustrated method unless it exceeds the gist. Not something.

For example, the addition point of the sulfate ion-containing liquid is as follows:
It is sufficient that the addition point is such that sulfate ions are present in the system in which the CaF ₂ sludge is generated. As shown in FIGS. 1 and 2, the addition to the raw water tank 1 and the addition of raw water from the raw water tank 1 to the neutralization tank 3 are performed. May be added to the transfer pipe, the neutralization tank 3 or the reaction tank 2, but is preferably added to the raw water tank. Further, the flocculation tank 4 is provided for improving the sedimentation and separability of the sludge, but is not always necessary, and a method of adding a polymer to the pipes to perform flocculation may be employed.

[0034]

The present invention will be described more specifically below with reference to examples and comparative examples.

Examples 1 to 4 CaF ₂ sludge was recovered by the method shown in FIG.

As raw water, semiconductor factory waste liquid (pH: 7.
3, F: 20, 200 mg / L, NH ₄ —N: 15, 7
(00 mg / L) at a flow rate of 40 L / hr.
In the raw water, H was added to the raw water tank together with 1.2 equivalents of hydrochloric acid.
₂ SO ₄ was added in an amount shown in Table 1 with respect to fluorine. Further, as the Ca compound, 25% Ca (OH) ₂
Using an emulsion, the treatment pH was adjusted to 6.5. The treatment was carried out by the sludge return method at a sludge return ratio R of 30. In the flocculation tank, 5 mg / L of polyacrylamide partial hydrolyzate was added as a polymer.

The capacity of each tank is as follows.

Raw water tank: 100 L Reaction tank: 3 L Neutralization tank: 10 L Coagulation tank: 10 L Precipitation tank: 150 L In this treatment, the concentration of the sludge obtained after the treatment condition is stabilized (the concentration of separated sludge and returned sludge) is as follows: As shown in Table 1, the particle size (maximum peak value in the particle size distribution) of the CaF ₂ sludge particles was as shown in Table 1.

The particle size distribution of the CaF ₂ sludge particles in Example 3 was as shown in FIG.

Comparative Example 1 In Example 1, without adding H ₂ SO ₄ , 1.2 equivalents of hydrochloric acid with respect to the fluorine in the raw water was added to secure Ca (OH) ₂ necessary for producing CaF ₂ . Except for this, processing was performed in the same manner under the same apparatus and under the same conditions.

The sludge concentration (concentration of separated sludge and returned sludge) obtained in this treatment is as shown in Table 1, and C
The particle size (peak value in the particle size distribution) of the aF ₂ sludge particles was as shown in Table 1. The particle size distribution of the CaF ₂ sludge particles in Comparative Example 1 was as shown in FIG.

[0042]

[Table 1]

From Table 1 and FIGS. 3 and 4, according to the present invention,
It can be seen that CaF ₂ sludge particles having a large particle size can be obtained.

The same applies to the case where the same treatment is performed by the direct sludge return method shown in FIG. 2 (however, in the method shown in FIG. 2, the capacity of the reaction tank and the neutralization tank was 100 L). Was obtained.

[0045]

As described in detail above, according to the method for treating fluorine-containing water of the present invention, in the treatment of fluorine-containing water by the sludge return method, high-concentration sludge containing CaF ₂ sludge particles having a larger particle size is removed. Can be generated. For this reason,
The CaF ₂ sludge obtained by the method of the present invention dehydrates it,
Upon drying to obtain a raw material for the recovery of HF, a dehydrated cake having a low moisture content is obtained, and the dehydrated cake is efficiently dried with a small amount of heat and good operability to efficiently convert HF. Can be recovered.

[Brief description of the drawings]

FIG. 1 is a system diagram showing an embodiment of a method for treating fluorine-containing water of the present invention.

FIG. 2 is a system diagram showing another embodiment of the method for treating fluorine-containing water of the present invention.

FIG. 3 is a graph showing the particle size distribution of CaF ₂ sludge obtained in Example 3.

FIG. 4 is a graph showing the particle size distribution of CaF ₂ sludge obtained in Comparative Example 1.

[Explanation of symbols]

 1 Raw water tank 2 Reaction tank 3 Neutralization tank 4 Coagulation tank 5 Sedimentation tank

Continued on the front page F term (reference) 4D038 AA08 AB41 BA02 BA04 BB13 BB18 4D059 AA30 BD00 BE00 BE53 BE56 BF12 BF13 BF14 DA03 DA05 DA32 DA33 DA35 DB24 EB11

Claims (4)

[Claims] 1. A method of treating a fluorine-containing water, wherein a calcium compound is added to the fluorine-containing water to generate calcium fluoride sludge, and the generated calcium fluoride sludge is separated. A method for treating fluorine-containing water, which comprises adding a sulfuric acid ion to the fluorine-containing water after contacting the part with a calcium compound and then adding the fluorine-containing water to the fluorine-containing water. 2. The method for treating fluorine-containing water according to claim 1, wherein the sulfate ions are added so that the amount of sulfate ions with respect to the fluoride ions in the fluorine-containing water is 0.01 to 0.1 equivalent. . 3. The return ratio of calcium fluoride sludge is 15 to
The method for treating fluorine-containing water according to claim 1 or 2, wherein the number is 60. 4. A method for treating fluorine-containing water, comprising dehydrating and drying calcium fluoride sludge and reusing it as a raw material for producing hydrogen fluoride.