TW201336788A - Fluorine recovery device, fluorine recovery system, and fluorine recovery method - Google Patents

Abstract

A fluorine recovery device as an embodiment of the present invention includes; a precipitation tank in which calcium carbonate is reacted with fluorine ions contained in water being treated, wherein calcium carbonate is introduced in excess relative to the amount of fluorine ions contained in water being treated, thereby precipitating calcium fluoride and obtaining primary treated water; and a solid/liquid separator in which the primary treated water containing calcium fluoride and calcium carbonate is filtered to form, on the filter, a mixed cake layer having accumulated calcium fluoride and calcium carbonate.

Description

Fluorine recovery unit, fluorine recovery system and fluorine recovery method [reference to relevant application]

The present application is based on the priority benefit of the prior art Japanese Patent Application No. 2012-055945, filed on March 21, 2014, and the benefit of the priority, the entire contents of which are incorporated herein by reference. in.

Embodiments of the present invention relate to a fluorine recovery device, a fluorine recovery system, and a fluorine recovery method.

Recently, the efficient use of water resources has become a demand due to industrial development and population growth. Therefore, the reuse of drainage such as industrial drainage becomes very important. In order to make the drainage reusable, it is necessary to purify the water, that is, to separate other substances from the water. For the method of separating other substances from the liquid, for example, membrane separation, centrifugation, activated carbon adsorption, ozone treatment, and a method of removing suspended matter by agglutination are known.

According to the above method, it is possible to remove chemical substances such as phosphorus or nitrogen which are contained in water, which are environmentally-sensitive, or to remove oils, clays, and the like which are dispersed in water. Among the above methods, membrane separation is one of the most common methods for removing insoluble materials from water. From the viewpoint of membrane protection and from the viewpoint of improving the water passing rate of water containing substances which are difficult to dehydrate, the so-called pre-coating filter filtration method and additive filtration method are often used for membrane separation. A method of using a filter aid.

On the other hand, a method for removing fluoride ions from water is a method of precipitating fluoride ions with calcium fluoride, or a method of adsorbing fluoride ions by polyaluminum chloride, or agglomerating calcium fluoride by using a polymer aggregating agent. This method of recovering calcium fluoride.

For example, Patent Document 1 discloses a technique in which fluorine in water is precipitated as calcium fluoride, and then a flocculating agent is added to recover calcium fluoride. Further, Patent Document 2 discloses a technique in which a part of calcium fluoride produced by a crystallization reaction tank is returned to a crystallization reaction tank to recrystallize calcium fluoride. Further, Patent Document 3 discloses a technique of reacting calcium and an aluminum salt, and then adding a polymer flocculant to recover fluorine or/and phosphorus.

[first technical offer] [Patent Document]

[Patent Document 1] Japanese Patent Laid-Open Publication No. 2010-207755

[Patent Document 2] Japanese Patent Laid-Open Publication No. 2010-110688

[Patent Document 3] Japanese Patent Laid-Open Publication No. 2005-296837

The technique described in Patent Document 1 recovers calcium fluoride by mixing the polymer flocculating agent and causing the precipitated calcium fluoride to become a large floc. In this method, since the purity of the recovered calcium fluoride is lowered, it is difficult to recover calcium fluoride as a valuable substance. Further, in this method, since the polymer aggregating agent is mixed, the amount of sludge increases.

Further, in the technique described in Patent Document 2, a part of the precipitated calcium fluoride is returned to the precipitation tank as a crystal nucleus, and the calcium fluoride after the crystal growth is recovered into large particles. Although the method does not use a polymer flocculant, it is necessary to return a part of the calcium fluoride to the devitrification reaction tank, so that the treatment efficiency is lowered, and it takes time to allow the crystal to grow sufficiently large.

Further, the technique described in Patent Document 3 discloses a method of recovering fluoride ions in water using an aluminum salt (polyaluminum chloride or the like). However, the aluminum salt is very difficult to separate from water, and a polymer flocculant must be used in combination.

Based on the above, the fluorine treatment in water is a troublesome process requiring a plurality of operations. The reason for this is that the generated calcium fluoride particles have a small diameter so that separation of calcium fluoride is difficult; and removal of the aluminum salt added in water is difficult.

An embodiment of the present invention provides a fluorine recovery device, a fluorine recovery system, and a fluorine recovery method capable of efficiently separating calcium fluoride having a small particle diameter and efficiently removing fluorine without using a chemical.

The fluorine recovery apparatus according to the embodiment of the present invention includes a precipitation tank and a solid-liquid separation device. In the precipitation tank, calcium fluoride and fluoride ions are reacted in a state in which an excessive amount of calcium carbonate is contained in the water to be treated, thereby precipitating calcium fluoride to form primary treated water. Further, the solid-liquid separation device is configured to filter the cake with the primary treated water containing calcium fluoride and calcium carbonate, thereby forming a mixed cake layer in which calcium fluoride and calcium carbonate are deposited on the filter.

[Embodiment of the Invention]

The fluorine recovery device includes a precipitation tank and a solid-liquid separation device. In the precipitation tank, calcium carbonate and fluoride ions are reacted with respect to the amount of fluorine ions contained in the water to be treated containing the fluoride ions and the water to be treated, and calcium fluoride and fluoride ions are reacted to precipitate calcium fluoride to form primary treated water. The solid-liquid separation device has a filter that filters a primary treated water containing calcium fluoride and calcium carbonate into a filter cake to form a mixed cake layer in which calcium fluoride and calcium carbonate are deposited on the filter.

Next, an embodiment of the present invention will be described with reference to the drawings. Fig. 1 is a schematic block diagram showing a fluorine removal system of the present embodiment.

The fluorine removal system 10 includes a precipitation tank 11, a solid-liquid separation device 15, a slurry recovery tank 16, and a dewatering tank 17. In the precipitation tank 11, the treated water INF containing the fluoride ions and the calcium carbonate CC are reacted to precipitate calcium fluoride (CaF ₂ ) to form primary treated water. The solid-liquid separation device 15 has a filter 13 and a cleaning mechanism 14. The filter 13 performs solid-liquid separation by filtering the primary treated water INF1 to form a cake layer CK on the filter 13. The cleaning mechanism 14 disperses (sprays) the washing water CW to the cake layer CK, thereby obtaining a concentrated slurry SL by mixing the cake layer CK and the cleaning liquid CW, and then taking the concentrated mud SL to the solid-liquid separation device 15 outer. The mud recovery tank 16 recovers the concentrated mud SL taken out from the solid-liquid separation device 15. The dewatering tank 17 dehydrates the concentrated mud SL recovered in the mud recovery tank 16.

In the solid-liquid separation device 15, the treated water PW after passing through the filter 13 A part of it is used as a washing water CW after the washing mechanism 14. As a result, the unreacted fluorine ions contained in the treated water PW react with the unreacted calcium carbonate contained in the filter cake tank, whereby calcium fluoride can be formed and the increase in the effective treated water can be suppressed.

Further, the other portion of the treated water PW is supplied to the mud recovery tank 16 as the concentration adjustment water DW of the concentrated mud SL. Further, since the concentration of fluorine ions in the treated water PW is already very low, the treated water PW can be directly discharged as drainage.

First, the solid-liquid separation device 15 will be described.

In the present embodiment, the solid-liquid separation device 15 is a filter that is horizontally arranged using the filter 13. In this way, the cake layer CK can be stabilized so that the water quality of the treated water is stabilized.

The solid-liquid separation device 15 is provided with the cleaning mechanism 14 as described above. The cleaning mechanism 14 is provided with a pump 21 and a washing nozzle unit 22. The washing nozzle unit 22 disperses the washing water CW supplied from the permeate pump 21 on the cake layer CK, and causes the cake layer CK to be the concentrated mud SL to be discharged to the mud through the mud discharge port 15A of the solid-liquid separating device 15. Slot 16.

Next, the configuration of the filter 13 will be described.

The filter 13 has a filter cloth which can be selected according to the water quality requirements of the treated water. In the present embodiment, a filter cloth having a gas permeability of 30 to 1500 cc/cm ² ‧ min is used. Here, the permeability is based on the Frazier method (Translator's Note: Frazier method Shape method) was determined. Specifically, when the filter is made of a woven fabric, the Fraser type gas permeability tester manufactured by Yasuda Seiki Co., Ltd. can be used. The air permeability tester) was measured.

As the filter 13, for example, a filter cloth for a dehydrator can be used. In other words, a filter cloth made of a material such as polypropylene-based stellite (trademark registration), polyester-based texa (trademark registration), or polyamide-based nylon (trademark registration) can be used as the filter 13. Further, the filter cloth is preferably a material having a good peeling property of the filter cake layer GK. For example, polypropylene is not only inexpensive, but the differential pressure after recovery of the cake layer CK is also relatively stable and less deteriorated, so that it is easy to use in water treatment. Based on this, it is preferred to use a polypropylene fiber for the filter cloth.

Further, the weaving method of the filter cloth is various weaves such as a plain weave, a twill weave, and a satin weave. Any weave can be used for the filter cloth. In other words, when the filter cloth is selected, the gas permeability and the weaving method of the desired filter cloth may be appropriately selected in view of the production cost and the like.

The material of the filter cloth is particularly preferably polypropylene, and the weave of the filter cloth is preferably a plain weave. The filter cloth can also be subjected to calender processing as required.

In the present embodiment, the excess amount of calcium carbonate charged in the deposition tank 11 is formed on the filter (filter cloth) 13 by the cake layer CK, and the cake layer CK is used as a filter. That is, in the embodiment, so-called filter cake filtration is performed. Since the primary treatment solution contains calcium carbonate, the calcium carbonate acts as a filter aid and increases the filtration flow rate.

As described above, in the present embodiment, calcium carbonate is used for the calcium source when the fluoride ion is reacted with calcium. Calcium carbonate, because it is not easily soluble in water, reacts on the surface to form calcium fluoride on the surface of calcium carbonate, so that calcium fluoride is dispersed in water in the form of flaking. When calcium fluoride is peeled off and dispersed in water, calcium fluoride is a very fine particle dispersed in water.

Based on this, the precipitated calcium fluoride is very difficult to separate from solid and liquid. Therefore, the prior art has been to form a floc by adding a cationic polymer or the like, thereby separating the entire amount of calcium fluoride formed. However, in the present embodiment, the filter 13 is selected based on the treated water concentration, whereby the precipitate can be directly dehydrated. Therefore, the present embodiment does not require an operation other than this.

In the present embodiment, the particle diameter of the calcium carbonate CC to be introduced into the deposition tank 11 is in the range of 5 μm to 200 μm. When the particle diameter of the calcium carbonate CC is less than 5 μm, the reactivity is improved, but the workability (flow rate) is lowered, and the efficiency of solid-liquid separation, that is, the filtration efficiency is lowered. In addition, as a result of the clogging of the filter cloth, the filtration life may be lowered. Further, when the particle diameter of the calcium carbonate CC exceeds 200 μm, although the gas permeability (flow rate) is sufficient, the precipitation reaction of calcium fluoride is only reacted on the surface of the calcium carbonate, so that the effective reaction area per unit mass is lowered. A larger amount of calcium carbonate is required relative to the same amount of fluoride ion.

Further, in the present embodiment, calcium carbonate after the particle diameter range of 5 μm to 200 μm is mainly used, but calcium carbonate having a particle diameter other than the above is not excluded.

Next, the outline operation of this embodiment will be described.

First, the water to be treated INF containing fluorine ions is supplied to the deposition tank 11, and an excess amount of calcium carbonate CC is added to the amount of fluorine ions to be mixed. After the lapse of the lapse of a predetermined period of time, the primary treated water INF1 (turbid liquid) in which the calcium carbonate CC is turbid in the water to be treated INF is filtered under pressure in the filter 13 to perform water filtration. Thereby using calcium fluoride in the primary treated water INF1 Excess calcium carbonate CC is separately filtered off, and calcium fluoride and calcium carbonate CC remain on the filter 13 to form a cake layer CK. In the cake layer CK, calcium carbonate and calcium fluoride are mixed.

Here, the cake layer CK is formed and held by the action of an external force. For example, the filter 13 is disposed so as to seal the container opening 15C of the designated container 15B for constituting the solid-liquid separation device 15, and calcium carbonate and calcium fluoride remain and are deposited on the filter 13 in the down state. In this form, the filter cake layer CK can be formed and held on the filter 13 by an external force (gravity) directed downward from the wall surface of the container 15B and the weight of the calcium fluoride and calcium carbonate located above. In addition, the thickness of the cake layer CK may vary depending on the amount of liquid to be treated, but it is about 0.1 to 20 mm.

The filter cake layer CK containing the fluorine compound separated from the primary treated water INF1 by the above process is washed by the cleaning mechanism 14. In the present embodiment, the cleaning of the cake layer CK in the container 15B provided with the filter 13 is performed by the cleaning mechanism 14. Washing can also be carried out in other containers. When the filter cake layer CK is to be washed in another container, the filter cake layer CK is decomposed into a slurry shape by a cleaning mechanism provided separately, and then the filter cake layer CK which is decomposed into a sediment is discharged. . In the present embodiment, the material is washed with water, but it may be washed with a surfactant or an organic solvent.

Next, the washed concentrated sludge SL is recovered to the slurry recovery tank 16. Alternatively, the concentrated sludge SL may be passed through a screen and returned to the precipitation tank for liquid that cannot pass through the screen. The addition of this screening operation enables calcium carbonate to be used efficiently.

Next, the concentrated sludge SL recovered in the slurry recovery tank 16 is transferred to the dehydrator 17, and the dewatering machine 17 performs dehydration of the concentrated slurry SL. The method of dehydration is not particularly limited. For example, the dehydrator 17 is a method of directly recovering calcium carbonate and calcium fluoride by a filter press dehydrator, or a method of recovering calcium carbonate and calcium fluoride by adding a small amount of a polymer auxiliary agent, or the dehydrator 17 is recovered by a centrifugal separator. Calcium carbonate and calcium fluoride methods. In addition, the filter press dehydrator is an effective recovery method because it does not use a polymer additive.

Next, the comparative embodiment will be described in more detail.

(for filter cloth)

First, in order to review the optimum filter cloth, the following four kinds of filter cloths FA to FD are prepared.

Filter cloth FA: filter cloth of plain weave fabric (average air permeability 40cc/cm ² ‧ min)

Filter cloth FB: filter cloth of twill weave fabric (average air permeability 500cc/cm ² ‧ min)

Filter cloth FC: filter cloth of plain weave fabric (average air permeability 24cc/cm ² ‧min)

Filter cloth FD: filter cloth made of Baolun satin weave (average air permeability 1800cc/cm ² ‧min)

(for calcium carbonate)

Next, in order to review the optimum calcium carbonate particle diameter, the following five kinds of calcium carbonates CC1 to CC5 are prepared. The prepared calcium carbonate is the average grain Calcium carbonate after crushing and classification of heavy calcium carbonate having a diameter of 2 mm.

Calcium carbonate CC1: Calcium carbonate average particle diameter 5.5μm

Calcium carbonate CC2: Calcium carbonate average particle diameter 20μm

Calcium carbonate CC3: Calcium carbonate average particle diameter 200μm

Calcium carbonate CC4: Calcium carbonate average particle diameter 3μm

Calcium carbonate CC5: Calcium carbonate average particle diameter 300μm

[Example 1]

In Example 1, fluorine removal was performed using the fluorine removal system 10 shown in Fig. 1.

First, the water to be treated INF containing fluorine ions is supplied to the precipitation tank 11. Next, calcium carbonate CC to be a calcium source is introduced into the precipitation tank 11 to be mixed with the water to be treated INF, whereby primary treated water INF1 is produced. After the lapse of the designated time, the primary treated water INF1 is supplied to the solid-liquid separation device 15 under pressure, and the primary treated water INF1 is subjected to solid-liquid separation (filtration).

Here, the treated water PW (filtrate) passing through the solid-liquid separation device 15 is a weakly alkaline treatment liquid from which fluoride ions have been removed, and can be directly drained. Further, as described above, a part of the treated water PW can be used as the washing water CW of the solid-liquid separating device 15, and the other portion of the treated water PW can be used as the concentration adjusting water DW of the concentrated mud SL.

When the solid-liquid separation (filtration) of the primary treated water INF1 is completed, the mixed filter cake layer CK of the precipitated fluorine compound (mainly calcium fluoride) and calcium carbonate remains in the filter 13 in the solid-liquid separation device 15.

In order to wash the mixed cake layer CK, the pump 21 and the cleaning nozzle unit 22 for constituting the cleaning mechanism 14 are disposed, and the washing water SW is scattered (supplied) from the upper side of the filter 13 to collapse the cake layer CK. The cake layer CK is discharged (supplied) to the mud recovery tank 16 via the slurry discharge port 15A of the solid-liquid separation device 15 in the state of the concentrated slurry SL.

As a result, a part of the washing water SW becomes a concentrated mud (fluorine concentrated water) SL containing a high concentration of a fluorine compound (calcium fluoride and calcium carbonate), which is recovered in the slurry recovery tank 16.

The calcium fluoride recovered in the slurry recovery tank 16 is dehydrated into a solid matter recovery by being supplied to a dewatering machine 17 such as a filter press dehydrator together with calcium carbonate.

In order to review the recovery efficiency of the fluorine ion having the above configuration, a hydrogen fluoride aqueous solution having a fluoride ion concentration of 500 mg/L is prepared as the water to be treated INF. In addition, the fluoride ion concentration of the primary treated water INF1 after the fluorine removal treatment which is the criterion for the completion of the treatment is set to 10 mg/L or less.

The calcium carbonate CC1 is added with calcium carbonate in the treated water INF in a state where the solid content is 1000 mg/L, and the primary treated water INF1 is generated after stirring in the precipitation tank 11 for 10 minutes (= designated time). As a result, the white precipitate precipitated, and the fluoride ion concentration in the primary treated water INF1 was 8 mg/L or less of the reference fluoride ion concentration (10 mg/L) or less.

Then, the primary treatment water INF1, which is a turbid liquid containing a white precipitate, is supplied from the deposition tank 11 to the solid-liquid separation device 15, and the primary treated water INF1 is filtered. As a result, it was confirmed that the treated water PW (filtered water) can recover 98% or more of the fluorine compound (reactant of calcium carbonate and fluoride ion) in the primary treated water INF1.

After the filtration treatment, the washing water CW is supplied from the upper portion of the filter 13 from the washing mechanism 14 using the solid-liquid separating device 15, so that the filter cake layer mainly composed of calcium fluoride and calcium carbonate formed on the filter 13 is a filter cake. The layer CK collapses, whereby the cake layer CK is discharged to the slurry recovery tank 14 in the state of the concentrated slurry SL.

Then, the concentrated mud SL in the slurry recovery tank 14 is supplied to the dehydrator 17 to perform dehydration treatment of the concentrated slurry SL. As a result, calcium fluoride and calcium carbonate can be recovered as a solid matter without any problem.

[Example 2]

The same apparatus as in Example 1 was used, and the same test as in Example 1 was carried out except that the filter cloth FB was used instead of the filter cloth FA. As a result, the recovery rate of the fluorine compound was about 80%, and the fluorine concentration in the treated water PW was 80 to 100 mg/L. In the second embodiment, the liquid-water separation speed of the solid-liquid separation device 15 of the second embodiment is substantially twice as high as that of the first embodiment, so that the apparatus can be operated without any problem.

[Example 3]

The same apparatus as in Example 1 was used, and the same test as in Example 1 was carried out except that calcium carbonate CC2 was used instead of calcium carbonate CC1. As a result, the recovery rate of the fluorine compound was about 90%, and the fluorine concentration in the treated water PW was 20 to 50 mg/L. In the third embodiment, the solid-water separation device 15 of the third embodiment has a water passing speed of about 1.1 times as compared with the first embodiment, so that the device can be operated without any problem.

[Example 4]

The same apparatus as in Example 1 was used, and the same test as in Example 1 was carried out except that calcium carbonate CC3 was used instead of calcium carbonate CC1. As a result, the recovery rate of the fluorine compound was about 80%, and the fluorine concentration in the treated water PW was 80 to 100 mg/L. In the fourth embodiment, the liquid-water separation speed of the solid-liquid separation device 15 of the fourth embodiment is substantially twice as high as that of the first embodiment, so that the apparatus can be operated without any problem.

(Comparative Example 1)

The same apparatus as in Example 1 was used, and the test was carried out in the same manner as in Example 1 except that the filter cloth FC was used instead of the filter cloth FA. As a result of recovering fluorine (fluoride ion), water filtration cannot be performed during filtration.

(Comparative Example 2)

The same apparatus as in Example 1 was used, and the same test as in Example 1 was carried out except that the filter cloth FD was used instead of the filter cloth FA. As a result of recovery of fluorine (fluorine ion), almost all of the fluorine compound flows out into the treated water PW.

(Comparative Example 3)

The same apparatus as in Example 1 was used, and the same test as in Example 1 was carried out except that calcium carbonate CC4 was used instead of calcium carbonate CC1. As a result of recovering fluorine (fluoride ion), water filtration cannot be performed during filtration.

(Comparative Example 4)

The same apparatus as in Example 1 was used, and the same test as in Example 1 was carried out except that calcium carbonate CC5 was used instead of calcium carbonate CC1. As a result, no reaction was formed in the precipitation tank 11, and almost all of the fluorine ions flowed out into the treated water PW.

According to the above-described examples and comparative examples, it is found that at least the form of the filter cloth using the plain weave or the twill weave of the resin can be constructed to have an average air permeability of 40 cc/cm ² ‧ min to an average air permeability of 500 cc. /cm ² ‧ min and a very practical fluorine removal system with an average particle diameter of 5 to 200 μm.

As described above, according to the fluorine removal system of the present embodiment, it is possible to remove fluorine ions in the water to be treated INF with a sufficient filtration efficiency.

The embodiments of the present invention have been described above, but are not intended to limit the scope of the present invention. The present invention can be embodied in a variety of other forms, and various omissions, substitutions and changes can be made without departing from the scope of the invention. The invention and its modifications are intended to be included within the scope and spirit of the invention and are intended to

10‧‧‧Fluorine removal system

11‧‧‧Precipitation slot

13‧‧‧ filter

14‧‧‧Cleaning agency

15‧‧‧Solid-liquid separation device

15A‧‧‧Mud discharge

15B‧‧‧Specified container

15C‧‧‧ container mouth

16‧‧‧ mud recovery tank

17‧‧‧Dehydrator (dewatering tank)

CC‧‧‧calcium carbonate

CK‧‧ filter cake layer

CW‧‧·washing water

DW‧‧‧Concentration water adjustment

EW‧‧‧Drainage

PW‧‧‧Processing water

SL‧‧‧Concentrated mud

INF‧‧‧ treated water

INF1‧‧‧ primary treated water

Fig. 1 is a schematic block diagram showing a fluorine removal system according to an embodiment of the present invention.

10‧‧‧Fluorine removal system

11‧‧‧Precipitation slot

13‧‧‧ filter

14‧‧‧Cleaning agency

15‧‧‧Solid-liquid separation device

15A‧‧‧Mud discharge

15B‧‧‧Specified container

15C‧‧‧ container mouth

16‧‧‧ mud recovery tank

17‧‧‧Dehydrator (dewatering tank)

CC‧‧‧calcium carbonate

CK‧‧ filter cake layer

CW‧‧·washing water

DW‧‧‧Concentration water adjustment

EW‧‧‧Drainage

PW‧‧‧ treated water

SL‧‧‧Concentrated mud

INF‧‧‧ treated water

INF1‧‧‧ primary treated water

Claims (6)

A fluorine recovery device comprising: an excess amount of calcium carbonate added to the amount of the fluoride ion contained in the water to be treated containing the fluoride ion and the water to be treated, and the calcium carbonate and the fluoride ion are reacted And precipitating the calcium fluoride to form a precipitation tank for the primary treated water; and filtering the filter cake by filtering the primary treated water containing the calcium fluoride and the calcium carbonate to form a mixture of the calcium fluoride and the calcium carbonate deposit on the filter. A solid-liquid separation device of the filter cake layer having the above filter. The fluorine recovery device according to the first aspect of the invention, wherein the calcium carbonate charged in the precipitation tank has a particle diameter of 5 to 200 μm. The fluorine recovery device according to claim 1 or 2, further comprising a cleaning device for removing the mixed cake layer from the filter by using washing water. A fluorine recovery system comprising: adding an excessive amount of calcium carbonate to the water to be treated containing fluorine ions and the amount of the fluorine ions contained in the water to be treated, and reacting the calcium carbonate with the fluoride ions The precipitated tank for precipitating calcium fluoride to form primary treated water; and the filter cake for filtering the primary treated water containing the calcium fluoride and the calcium carbonate to form a mixed filter cake on which the calcium fluoride and the calcium carbonate are stacked a solid-liquid separation device having the above-mentioned filter; a cleaning device for removing the mixed filter cake layer from the filter by using washing water; and A dehydrator for dehydrating the concentrated slurry containing the above-mentioned mixed cake cake layer and the above-mentioned washing water. The fluorine recovery system according to the fourth aspect of the invention, wherein the calcium carbonate charged in the precipitation tank has a particle diameter of 5 to 200 μm. A method of recovering fluorine, wherein the calcium carbonate and the fluorine are added to an amount of the fluoride ion contained in the water to be treated and the fluoride ion contained in the water to be treated in the deposition tank. The ion formation reaction thereby precipitating calcium fluoride to form a precipitation process of the primary treated water; and filtering the cake on the primary treated water containing the calcium fluoride and the calcium carbonate obtained by the precipitation process to form the fluorine on the filter A solid-liquid separation process of a mixed filter cake layer of calcium and the above calcium carbonate.