JP2000015269A - Treating method for fluorine-containing water - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a treating method for fluorine-contg. water whereby the water content of the resultant calcium fluoride cake is reduced; the effective application of this cake is broadened; and the space occupied by a treating apparatus is reduced by conducting solid-liquid separation by a means selected from among filtration, centrifugal separation, concentration by evaporation, and floatation. SOLUTION: A calcium salt 35 (e.g. calcium hydroxide) is added to fluorine-contg. wastewater in a reaction vessel 26 to form calcium fluoride. After the pH of the wastewater is adjusted in a pH-adjusting vessel 27, the wastewater is subjected to solid-liquid separation in a cocentration apparatus 28. As the concentration apparatus 28, one which can carry out the solid-liquid separation of the calcium fluoride-contg. wastewater discharged from the reaction vessel 26 and which is based on filtration, centrifugal separation, concentration by evaporation, or floatation is used. Since a concentration means based on filtration, centrifugal separation, concentration by evaporation, or floatation makes the separation possible without allowing crystals to grow, the addition of a flocculant is not necessary. An example of a solid-liquid separation means based on filtration is a method based on membrane separation; in this case, an ultrafiltration membrane, a micro-filtration membrane, filter cloth, etc., are used as a filtration membrane.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a method for treating fluorine-containing water. In industries such as semiconductor manufacturing, chemical fertilizer manufacturing, ceramics, and the aluminum industry, wastewater containing fluorine is discharged because hydrogen fluoride is used or has a manufacturing process for generating hydrogen fluoride. The present invention establishes an effective technology for treating fluorine-containing water from the viewpoint of strengthening environmental measures and effectively using resources in recent years.

[0002]

2. Description of the Related Art In industries such as semiconductor manufacturing, chemical fertilizer manufacturing, ceramic industry, and aluminum industry, wastewater containing fluorine is discharged because hydrogen fluoride is used or has a manufacturing process for generating hydrogen fluoride. . Such a fluorine-containing wastewater is reacted with a calcium salt such as calcium hydroxide (Ca (OH) ₂ ) to form calcium fluoride (Ca (OH) ₂ ).
In general, a coagulation sedimentation method in which an insoluble matter of F ₂ ) is generated to fix fluorine and solid-liquid separation is performed by gravity.

FIG. 1 shows a conventional example (for example, Japanese Patent Application Laid-Open No. 8- 1970).
No. 70). The fluorine-containing wastewater is stored in the raw water tank 1. In the primary reaction tank 2, a calcium salt 9 such as calcium hydroxide is added to adjust the pH to 6 to 10 to generate calcium fluoride. At this time, an excessive amount of calcium salt is added to reduce the fluorine content in the wastewater, and an inorganic coagulant of a compound such as aluminum or iron is added to promote the formation of insoluble calcium fluoride in the secondary reaction tank 3. Add 10 The pH of the wastewater is adjusted to 6 to 8 by adding an acid or an alkali in the pH adjusting tank 4 to further promote the formation of calcium fluoride, and then the partial hydrolysis of polyacrylamide in the flocculating tank 5 to increase the sedimentation of the product. An organic coagulant 11 such as a substance is added. Next, the treated wastewater is guided to a precipitation tank 6, where solid-liquid separation is performed by gravity.

[0004] In order to promote crystallization in the primary reaction tank 2 and the secondary reaction tank 3, a method in which the settled sludge 24 is returned to the primary reaction tank 2 through the pipe 13 and used as a nucleus for crystallization. Is generally adopted. Part of the sludge separated in the sedimentation tank 6 is returned, while the rest is stored in the sludge storage tank 7.
, And dewatered by the dehydrator 8, and the dewatered cake 9 is treated as industrial waste.

[0005] The supernatant water is transferred to a subsequent treatment facility because it contains insoluble matter of calcium fluoride generated in the preceding stage. By the filtration tower 17 filled with sand and the like from the storage tank 16,
The insoluble matter of calcium fluoride which has not settled in the settling tank 6 is removed. Further, in order to remove residual fluorine in the wastewater, the treatment is performed in an adsorption tower 19 filled with an adsorbent such as a resin or activated alumina to which a metal ion forming a complex compound with fluorine ions such as zirconium is attached. further,
After the pH is adjusted in the pH adjusting tank 20 so as to satisfy the discharge standard, the water is discharged. In FIG. 1, reference numeral 18 denotes a buffer of filtered treated water and a filtered water tank for monitoring water quality, and reference numeral 21 denotes a buffer of treated pH-adjusted treated water and a storage tank for monitoring water quality.

As described above, in order to treat the fluorine-containing wastewater by the coagulation sedimentation method as in the prior art, it is necessary to add an excessive calcium salt and a large amount of a coagulant. Costs increase. In the method of returning a part of the sludge settled in the sedimentation tank to the primary reaction tank in order to improve the coagulation property and using the sludge as a nucleus for crystallization, the coagulant becomes a hindrance factor for crystallization. This is because calcium fluoride carried from the primary reaction tank reacts with an aluminum compound or the like added as an inorganic coagulant to the secondary reaction tank, causing gelation, and crystallization does not proceed. Therefore, although treatment with calcium fluoride alone containing no flocculant is more effective, calcium fluoride does not settle in the sedimentation tank, so that a flocculant needs to be added.

In the method in which the insoluble matter of calcium fluoride is separated into solid and liquid by gravity sedimentation, the outflow of insoluble matter of calcium fluoride into the supernatant water (usually 20 to 50 mg / day).
L) was unavoidable and required a subsequent filtration facility. Sand filtration or the like is used in the subsequent filtration equipment, but periodic cleaning work is required. In addition, in a process of adding a calcium salt and a flocculant to a fluorine-containing wastewater and gravity separating an insoluble salt of calcium fluoride, the wastewater has a large amount of residual fluorine, and thus requires treatment in an adsorption tower.

Further, the sludge discharged from the conventional treatment equipment has a poor dewatering property due to the addition of a flocculant, and a great deal of cost is required to treat the dewatered cake. Also, if the purity of calcium fluoride in the sludge is high, it can be effectively used in high value-added applications such as raw materials for the production of hydrogen fluoride, but mixing of flocculant components into the sludge,
95% at most due to contamination of impurities such as SiO _{2 in} wastewater
%, It was only used with low added value, such as an extender for cement, even if it could be used effectively.

In the precipitation step, since solid-liquid separation is performed by gravity sedimentation, a volume for sedimentation is required, and a large exclusive space is required.

[0010]

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, in the present invention, calcium fluoride generated by the reaction between fluorine and calcium salt in fluorine-containing water is filtered and centrifuged without gravity sedimentation. Solid-liquid separation is performed by separation, evaporation concentration, flotation separation, etc., and the concentrated sludge is partially returned to the reaction tank to be used as nuclei for crystallization, thereby reducing the fluorine concentration in the fluorine-containing water.

Therefore, the present invention provides a method for treating fluorine-containing water at a low running cost without using an organic or inorganic coagulant, and reducing the water content of the obtained calcium fluoride cake and improving its effectiveness by purifying the cake. The purpose is to expand the use of the use and reduce the exclusive space.

[0012]

In order to solve the above-mentioned problems, the present invention comprises contacting fluorine-containing water with a calcium salt in a reaction tank, and reacting fluorine in the fluorine-containing water with the calcium salt to form calcium fluoride. After fixing, the solid-liquid separation is performed to discharge the treated water, and a method for treating fluorine-containing water, which includes returning a part of the sludge concentrated by the solid-liquid separation to the reaction tank, By a means selected from the group consisting of filtration, centrifugation, evaporation and concentration, and flotation.

According to the present invention, it is not necessary to use an organic or inorganic coagulant in treating the fluorine-containing water.

[0014]

Preferred embodiments of the present invention will be described below with reference to the drawings. In the following, a case where fluorine-containing wastewater is treated as fluorine-containing water will be described as an example. FIG. 2 is a system diagram schematically showing the method of the present invention. The fluorine-containing wastewater is stored in the raw water tank 25. In the reaction tank 26, a calcium salt 35 such as calcium hydroxide is added to adjust the pH of the fluorine-containing wastewater to 4 to 10 to generate calcium fluoride. The pH is adjusted to a pH range in which calcium fluoride is easily crystallized in the pH adjusting tank 27, the wastewater is subjected to solid-liquid separation in the concentration equipment 28, and the concentrated sludge is transferred to the sludge storage tank 30, and the dewatering machine 31 Dehydrated. Part of the concentrated sludge is supplied to the reaction tank 2 by the pipe 33.
6 and used as nuclei for crystallization of calcium fluoride in the reaction tank 26. At this time, the return sludge 34 and the fluorine-containing wastewater 41 are stored in the storage tank 39, and calcium salts 40 are added to grow calcium fluoride crystals which are nuclei for crystallization, and the crystals are returned to the reaction tank 26. This is effective to promote the
This is particularly effective when the concentration of fluorine ions in the inside is low. The amount of the calcium salt 40 to be added is slightly larger than the equivalent to the amount of fluorine ions in the fluorine-containing wastewater 41, for example, preferably about 1.2 times equivalent.
Is preferably about 10 to 20% of the total amount of the fluorine-containing wastewater 38. Generally, it is preferable that the amount of returned sludge is about 10 to 40% of the total amount of generated sludge.

When the sludge concentration or the fluorine ion concentration in the returned sludge is low, a fluorine-containing wastewater 38 may be added to a storage tank 39. As the concentration device 28, a device having a function of solid-liquid separation of waste water containing calcium fluoride discharged from the reaction tank 26 is used.
By centrifugation, evaporation concentration or flotation.
When solid-liquid separation is performed by gravity sedimentation, it is necessary to increase the specific gravity of calcium fluoride crystals, so the addition of a flocculant is essential.However, filtration, centrifugation, evaporation concentration or flotation separation means According to this, the separation can be performed without growing the crystal, and therefore, the addition of the coagulant is unnecessary.

As a solid-liquid separation means by filtration, for example, a method by membrane separation can be mentioned. As the membrane, an ultrafiltration (UF) membrane, a microfiltration (MF) membrane, a filter cloth, or the like is employed.

[0017]

The present invention will be further described below with reference to examples. Reference Example 1 In order to verify the effects of the present invention, Reference Example 1 was performed as follows. 200 mg / L, 100 mg of fluoride ion
/ L, 50 mg / L, and 20 mg / L, simulated wastewater was prepared, calcium hydroxide (Ca (OH) ₂ ) was added, and the concentration of fluorine ions remaining in the wastewater was measured. The pH at this time was adjusted to a range of 6 to 8 with sodium hydroxide (NaOH). The results are shown in FIG.

FIG. 3 shows that in wastewater with a high fluoride ion concentration (200 mg / L, 100 mg / L), the fluorine ion concentration immediately decreases even when a small amount of calcium salt is added. It can be seen that it reacts efficiently with calcium ions generated from the added calcium salt to precipitate insoluble matter of calcium fluoride (CaF ₂ ). On the other hand, wastewater with a low fluorine ion concentration (50
(mg / L, 20 mg / L), it can be seen that the fluorine ion concentration in the wastewater cannot be reduced unless an excess calcium salt is added.

The details of such a mechanism are not clear, but can be guessed as follows. That is, when the fluoride ion concentration is high, insoluble matter of calcium fluoride precipitates with a small amount of calcium ions, and the precipitated calcium fluoride serves as a nucleus for crystallization, and is crystallized to a place where the remaining fluorine ion concentration is low. Is performed. On the other hand, when the fluorine ion concentration is low, the start of crystallization is delayed, and it is considered that fluorine ions remain in the wastewater even if calcium salts are added excessively.

Reference Example 2 The inventors carried out Reference Example 2 in order to verify the above fact. In a simulated wastewater having a fluorine ion concentration of 20 mg / L and a calcium ion concentration of 320 mg / L, a fluorine concentration of 50 mg / L was added.
A solution containing calcium fluoride prepared at 0 mg / L and a calcium concentration of 580 mg / L was gradually added. At this time
The pH was adjusted to between 6 and 8 with sodium hydroxide (NaOH). FIG. 4 shows the results.

According to FIG. 4, the fluorine ion concentration is 20 mg.
In the case of / L, the fluoride ion concentration in the wastewater was not reduced under the conditions shown in Reference Example 1. However, by adding a small amount of calcium fluoride, which is a nucleus for crystallization, the fluorine ions were insoluble in calcium fluoride. It can be seen that the reduction of the residual fluorine ions in the wastewater is realized. In addition, by comparing FIG. 3 with FIG. 4, the conventional method requires a residual calcium ion concentration of 100 mg / L to reduce the fluorine ion concentration to 5 mg / L (FIG. 3).
According to the method of the present invention, the residual calcium ion concentration was 20 mg / L (FIG. 4), indicating that the amount of the calcium salt used in the present invention can be reduced.

Here, the above-mentioned Reference Examples 1 and 2
Is only for verification of the principle of the present invention, and does not limit the design conditions and operating conditions of the equipment employing the present invention. Example 1 An apparatus as shown in FIG. 2 was used. This embodiment is an embodiment of the method for treating fluorine-containing water of the present invention, and the present invention is not limited to the illustrated method.

In this example, solid-liquid separation was performed by filtration using a filter cloth. As the filter cloth, use fibers that have chemical resistance such as polyester, polyvinyl chloride, polyvinyl alcohol, and polyacrylonitrile, have heat resistance to withstand use temperature, and have mechanical strength to withstand use. I do. A woven fabric (twisted woven fabric) is used as the filter cloth, and the aperture and the diameter of the twisted yarn are determined by the fluorine ion concentration of the waste water, other ion concentrations, and the properties of other contained substances.

When such a filter cloth is used, when calcium fluoride having an insoluble particle having a certain particle size is passed through a woven cloth, coarse particles are mainly affected by inertial impact, and fine particles are mainly diffused and blocked. To form a particle bridge between the yarns.
The thus formed primary adhesion layer has a large number of bent pores, and has a porosity larger than that of a new filter cloth. Collection of fine particles is performed by this primary adhesion layer. Also, by filtering particles larger than the particle size of the insoluble matter of calcium fluoride normally generated in the stage of forming the primary adhesion layer, it is possible to improve the porosity and increase the flux passing through the filter cloth. confirmed. As the large particles used herein, calcium carbonate, SiO ₂ particles, or the like, which do not adversely affect the washing described below, are used.

As described above, according to the present invention, the removal of calcium fluoride can be performed extremely efficiently, so that the subsequent filtration equipment as shown in FIG. 1 becomes unnecessary. In addition, in the method of the present invention, a high-purity calcium fluoride can be obtained as a treated sludge without using a flocculant. It can be reused for the purpose.

As the structure of the filtration tower constituting the concentrating equipment 28, a filter cloth having a cylindrical shape or a plate shape can be used. In order to verify the filtration performance, as shown in FIG.
A cylinder 42 having a diameter of 2 mm (FIG. 5 (A)) was loaded into a PVC cylinder 43 having an inner diameter of about 35 mm (FIG. 5 (B)). A filtration device 44 (FIG. 5 (C)) )It was used.

For the treatment, a fluorine-containing wastewater containing 400 mg / L of fluorine ions was provided. 100 g of calcium hydroxide was added to 60 L of the drainage water, and a circulation operation was performed. FIG. 6 shows the change of the flux at this time and the degree of recovery of the flux by washing.
Shown in As can be seen from FIG. 6, the circulating operation tends to reduce the flux. This is presumably because calcium fluoride insolubles accumulated on the surface of the filter cloth and the pressure loss increased. Further, it can be confirmed that the flux is recovered by removing the calcium fluoride deposited on the surface of the filter cloth by washing. In this example, the cleaning was performed using hydrochloric acid. However, the method is not limited to hydrochloric acid cleaning as long as the method is a method for removing calcium fluoride.

Embodiment 2 FIGS. 7 and 8 show a filtration device constituting the concentration equipment 28.
Example 2 was performed using the apparatus shown in FIG. 0.1μ pore size
This is an example in which an MF membrane having a size of m to 100 μm is used for a filtration device. The flat plates 47 in which the MF film 45 is fixed to the support 46 are arranged at regular intervals, and the treated wastewater is passed through and bubbling is performed from below to prevent excessive calcium fluoride deposition on the film surface. If the flux is reduced, the flux can be restored by chemical cleaning or physically removing the deposits.

The same results as described above can be obtained by using well-known centrifugal separators, evaporative concentrators, and flotation devices having various configurations as the concentration equipment 28.

[0030]

As described in detail above, according to the method for treating fluorine-containing water of the present invention, the amount of treating agent, the amount of sludge, the equipment, the installation space, and the quality of calcium fluoride are reduced. The effective use of fluorine, etc., by the recovery of fluorine is realized.

[Brief description of the drawings]

FIG. 1 is a schematic system diagram showing a conventional processing method.

FIG. 2 is a schematic system diagram showing a processing method of the present invention.

FIG. 3 is a graph showing the results of Reference Example 1.

FIG. 4 is a graph showing the results of Reference Example 2.

FIG. 5 is a schematic diagram showing a filtration device used in Example 1.

FIG. 6 is a graph showing the results of Example 1.

FIG. 7 is a schematic view showing a filtration device used in Example 2.

FIG. 8 is a partially enlarged view of the device of FIG. 7;

[Explanation of symbols]

 1,25 ... raw water tank 2,3,26 ... reaction tank 4,27 ... pH adjustment tank 6 ... precipitation tank 28 ... concentration equipment 42 ... filter cloth cylinder 43 ... PVC cylinder 44 ... filtration device 45 ... MF membrane 46 ... support Body 47 ... flat plate

Claims (4)

[Claims] 1. Fluorine-containing water is brought into contact with a calcium salt in a reaction tank, and fluorine in the fluorine-containing water is reacted with the calcium salt to be fixed as calcium fluoride. Then, solid-liquid separation is performed to discharge treated water. A method for treating fluorine-containing water, comprising returning part of the sludge concentrated by the solid-liquid separation to the reaction tank, wherein the solid-liquid separation is selected from filtration, centrifugation, evaporation concentration and flotation separation. A method for treating fluorine-containing water. 2. The method according to claim 1, wherein the fluorine-containing water is contacted with the calcium salt in the reaction vessel without the presence of an organic or inorganic flocculant. 3. The method according to claim 1, wherein the pH is adjusted after the reaction and before the solid-liquid separation. 4. The method according to claim 1, wherein a calcium salt is added to the sludge returned to the reaction tank during the return.