JP2007038049A - Method for treating fluorine in mixed acid - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To convert hardly treatable fluorine generated in a process for producing a mixed acid by mixing hydrofluoric acid and sulfuric acid into a treatable form with a well-known treatment method using a calcium salt or the like. <P>SOLUTION: The mixed acid produced by mixing hydrofluoric acid and sulfuric acid is loaded with water having 10% to 2,500% of the volume of the mixed acid. Next, the mixed acid after the adjustment is loaded with the calcium salt to carry out solid-liquid separation and treated water is obtained. As described above, the concentration of hardly treatable fluorine is diluted, thereby enabling conversion into the treatable form with the well-known treatment method using the calcium salt or the like. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a method for treating fluorine in a mixed solution (mixed acid) of hydrofluoric acid (hydrofluoric acid) and sulfuric acid used in a glass etching process or the like.

  Fluorine is a substance that is used in large quantities in fields such as the semiconductor manufacturing industry and the chemical industry. However, since fluorine is a harmful substance to the human body, ingestion of a large amount of fluorine causes chronic fluorine poisoning such as patchy teeth, osteosclerosis, and osteomalacia. Is 8 mg / L (L: liter, the same shall apply hereinafter). In addition, some local governments have strict standards added to national standards.

  Conventionally, as a method of treating fluorine-containing wastewater, a method of adding a calcium salt such as slaked lime / calcium chloride to fluorine-containing wastewater to separate the solid and liquid as fluorine with poorly soluble calcium fluoride (Patent Document 1), There is known a method (Patent Document 2), in which an aluminum salt such as polyaluminum chloride (PAC) is added, and fluorine is adsorbed to aluminum hydroxide produced by adjusting to near neutrality to separate the solid and liquid.

  The treatment with calcium salt is mainly used to treat high-concentration wastewater, but treatment up to 10 to 20 mg / L is the limit even if a lot of chemicals are added. Therefore, in a subsequent stage, it is generally discharged after being treated with an aluminum salt, or after being mixed with treated water with calcium salt and waste water from other systems not containing fluorine.

By the way, the fluorine which can be processed by the method using the calcium salt and the aluminum salt is, in principle, fluorine in the form of fluoride ions (F ⁻ ). For example, hydrofluoric acid (HF), which is most frequently used industrially among fluorine compounds, dissociates into H ⁺ and F ⁻ in an aqueous solution, and thus can be treated by the above method. Further, even when fluorine binds to metals to form a complex and does not exist as fluoride ions, it can be treated if the binding force is weak. As such an example, drainage of silicofluoride ions (SiF ₆ ²⁻ ) in which fluorine and silicon are bonded can be mentioned.

On the other hand, boron is mentioned as a substance that binds strongly to fluorine and cannot be treated by the above-described method. When boron and fluorine are bonded to each other, borofluoride (BF ₄ ⁻ ) is formed. However, the bonding force is very strong and it cannot be processed by the above-described method unless it is decomposed by some method. As a decomposition method, an aluminum salt is generally added as a decomposition agent, the pH is adjusted to 5 or less suitable for decomposition, and heating is generally performed.
Japanese Patent Laid-Open No. 9-174063 JP 2000-1313 A

  Since hydrofluoric acid has the property of dissolving glass, it is widely used as a surface treatment for plate glass or crystal glass, that is, as an etching treatment agent. It is rare to use hydrofluoric acid alone, and it is used in the form of a mixture of hydrofluoric acid and sulfuric acid (mixed acid) for reasons such as the surface finish. When hydrofluoric acid and sulfuric acid are mixed, the temperature of the mixture increases and white smoke is generated.

  After the etching, a large amount of water is used to wash away the mixed acid adhering to the glass surface. Accordingly, a concentrated mixed acid waste liquid is discharged from the etching process, and cleaning water containing diluted fluorine is discharged from the cleaning process.

  By the way, in the treatment of fluorine in a mixed acid composed of hydrofluoric acid and sulfuric acid, there is no substance that strongly binds to fluorine, and therefore, the wastewater is treated directly with the above-described calcium salt or aluminum salt. Has been done.

The inventors of the present invention have found the following facts as a result of a detailed comparison and examination of the processability of many industries that emit fluorine.
(1) The fluoridation property of factory effluents using mixed acid of hydrofluoric acid and sulfuric acid, represented by the glass processing industry, is very poor compared to other industries.
(2) The reason why the processability is poor is that in the process of producing a mixed acid by mixing hydrofluoric acid and sulfuric acid, some chemical reaction occurs, so that a part of fluorine is changed to a form other than fluoride ions. It is thought that.
(3) This difficult-to-process fluorine is a form in which hydrofluoric acid and sulfuric acid are strongly combined to change their form, or sulfuric acid has some effect on hydrofluoric acid, and its form has changed by itself. It is unclear which one it is. However, it is more stable than borofluoride, which is positioned as the most difficult category in the treatment of fluorine-containing wastewater.

  It is an object of the present invention to form a difficult-to-process fluorine that has not been known in the past, that is, a difficult-to-process fluorine produced in the process of producing a mixed acid by mixing hydrofluoric acid and sulfuric acid with a calcium salt or an aluminum salt. It is to provide a method of converting into a form that can be processed by the above.

As a result of diligent studies on a mixed acid of hydrofluoric acid and sulfuric acid, the present inventors have found the following three points.
(1) When water for adjusting the concentration of 10% to 2500% of the mixed acid volume is added to the concentrated mixed acid waste liquid that is directly discharged from the etching process, the difficult-to-process fluorine is quickly converted into a form that can be processed.
(2) Taking the rich mixed acid directly discharged from the etching process into the tank, and leaving the mixed acid in the open state, the difficult-to-process fluorine is converted into a form that can be treated slowly. Follow it. However, on the other hand, when the tank is held in a sealed state, fluorine remains difficult to process.
(3) On the other hand, adding concentration-adjusting water to the concentrated mixed acid directly discharged from the etching process to adjust the fluorine concentration to 5% by weight or less will result in difficult-to-treat fluorine for a long time regardless of whether it is sealed or open. By storing it, it will be converted into a form that can be processed gently.

  The above (1) means that by changing the concentration range to an optimum concentration range by adding concentration adjustment water to the concentrated mixed acid as the waste liquid, it changes to fluorine (fluorine ion form fluorine) that can be processed quickly. ing. In addition, if too much water for concentration adjustment is added, fluorine does not change with difficulty in processing, which means that an optimum concentration range exists.

  The above (2) and (3) can be converted into a form of fluoride (fluoride ion form) that can be treated by leaving the mixed acid as a waste liquid for a long time, and a method that can be converted according to the fluorine concentration. Means different. When the fluorine concentration is high, it will change to a fluorine form (fluoride ion form) that can be treated slowly only when the mixed acid as waste liquid is left in a released state, and the fluorine concentration is 5% by weight or less. In the dilute state, it means that the mixed acid is allowed to stand regardless of whether it is in an open state or a sealed state, and then changes into a fluorine form (fluoride ion form) that can be processed slowly.

  The present invention has been made on the basis of the above findings, the fluorine-containing wastewater treatment method according to the present invention is a method of treating fluorine in a mixed acid comprising at least hydrofluoric acid and sulfuric acid, It includes a first step of adding 10% to 2500% of the mixed acid volume of water and a second step of solid-liquid separation by adding calcium salt to the mixed acid obtained through the first step. In addition, after the mixed acid having the adjusted fluorine concentration is allowed to stand, used washing water may be added to the left standing mixed acid.

  The method for treating fluorine-containing wastewater according to the present invention is a method for treating fluorine in a mixed acid comprising at least hydrofluoric acid and sulfuric acid, taking the mixed acid into a tank, and maintaining the tank in an open state. It is good also as a structure including the 2nd process of adding a calcium salt to the mixed acid which passed through the said 1st process, and solid-liquid-separating the said 1st process which leaves the said mixed acid.

  Further, the method for treating fluorine-containing wastewater according to the present invention is a method for treating fluorine in a mixed acid comprising at least hydrofluoric acid and sulfuric acid, and the concentration of fluorine is reduced to less than 5% by adding concentration adjusting water to the mixed acid. It is good also as a structure including the 1st process to adjust and the 2nd process of adding a calcium salt to the mixed acid which passed through the said 1st process, and carrying out solid-liquid separation.

  According to the method for treating fluorine-containing wastewater of the present invention, a conventionally known treatment using calcium salt or aluminum salt, which has not been conventionally known, is difficult-to-treat fluorine produced by mixing hydrofluoric acid and sulfuric acid. There is an advantage that it can be converted into a form that can be processed by the method.

  In addition, according to the method for treating fluorine-containing wastewater of the present invention, it becomes possible to treat difficult-to-process fluorine that could not be treated in the past, and therefore, after dilution with wastewater from other systems, it must be discharged. Even in business establishments, there is an advantage that the amount of diluted water can be significantly reduced or treated to below the wastewater standard without requiring diluted water.

  Hereinafter, embodiments of the present invention will be described in more detail with reference to the drawings.

(Embodiment 1) FIG. 1 is an apparatus configuration diagram for carrying out a method for treating fluorine-containing wastewater according to Embodiment 1 of the present invention.

  As shown in FIG. 1, sulfuric acid and hydrofluoric acid are respectively stored in dedicated tanks 1 and 2, and appropriate amounts of sulfuric acid and hydrofluoric acid are transferred from the tanks 1 and 2 to the mixed acid preparation tank 3 to prepare mixed acid. Mixing in the tank 3 produces mixed acid. The factory 4 is supplied with mixed acid from the mixed acid preparation tank 3 and with cleaning water.

  The mixed acid which is a concentrated liquid from the factory 4 is recovered in the fluorine conversion tank 5 and the washing water is recovered in the reaction tank 6. A coagulation tank 7 and a precipitation tank 8 are connected to the reaction tank 6 in this order.

  First, the first step is performed in the fluorine form conversion tank 5 in which the concentrated mixed acid is recovered. In the first step, the fluorine concentration is adjusted by adding 10% to 2500% of the mixed acid volume to the rich mixed acid as the waste liquid discharged directly from the etching step of the glass processing factory 4 or the like. By the dissociation treatment with respect to the rich mixed acid, the hardly-treatable fluorine is dissociated as fluorine in a form that can be treated by a known treatment method using a calcium salt and an aluminum salt, at least a fluoride ion form. The diluted mixed acid is transferred to the subsequent reaction tank 6.

  Next, the second step is performed using the reaction tank 6 to the precipitation tank 8. More specifically, first, a calcium salt is charged into the reaction tank 6 into which the diluted mixed acid has been injected. By the introduction of the calcium salt, calcium and fluorine react in the reaction tank 6 to generate calcium fluoride particles (fluoride ion form).

  Next, the particles generated in the reaction vessel 6 are transferred to the subsequent agglomeration vessel 7, and an aggregating agent is added to the particles in the agglomeration vessel 7. The particles to which the flocculant is added agglomerate to form coarse flocs and have sedimentation properties.

  Next, the particles aggregated in the coagulation tank 7 are transferred to the subsequent precipitation tank 8, and in the precipitation tank 8, coarse particles of particles are precipitated, and the clear supernatant water is obtained as treated water.

  Since the washing water discharged from the factory 4 as waste liquid contains a small amount of fluorine, it is desirable to collect this in the reaction tank 6 and treat it with the diluted mixed acid from the fluorine form conversion tank 5. In this case, after the mixed acid diluted in the fluorine conversion tank 5 is allowed to stand in the reaction tank 6, it is desirable to add wash water as waste liquid discharged from the factory 4 to the diluted mixed acid in the reaction tank 6. . A part of the washing water discharged from the factory 4 can also be used as concentration adjusting water when performing the dilution treatment performed in the fluorine form conversion tank 5.

(Embodiment 2) FIG. 2 is an apparatus configuration diagram for carrying out a method for treating fluorine-containing wastewater according to Embodiment 2 of the present invention. The apparatus configuration of FIG. 2 is the same as that of FIG.

  In FIG. 2, the mixed acid (waste liquid), which is a concentrated liquid discharged from the factory 4, is collected in the fluorine form conversion tank 5, and the washing water as the waste liquid is collected in the reaction tank 6 as in the first embodiment. .

  In Embodiment 2, the concentrated mixed acid directly discharged from the etching process or the like of the glass processing factory 4 is collected in the fluorine form conversion tank 5, the fluorine form conversion tank 5 is opened without being sealed, and the collected rich is collected. The mixed acid is allowed to stand in the fluorine conversion tank 5 for a long time (first step). Refractory fluorine contained in a concentrated mixed acid can be treated by a known treatment method using a calcium salt and an aluminum salt (fluoride ions) by leaving the fluorine form conversion tank 5 to stand for a long time. Form). Thus, the mixed acid that has been allowed to stand in the fluorine form conversion tank 5 for a long time is transferred from the fluorine form conversion tank 5 to the reaction tank 6 after the set time has elapsed. The time for keeping the rich mixed acid in the fluorine form conversion tank 5 is preferably 1 hour or more.

  Next, the second step is performed using the reaction tank 6 to the precipitation tank 8. More specifically, first, a calcium salt is charged into the reaction tank 6 into which the diluted mixed acid has been injected. By the introduction of the calcium salt, calcium and fluorine react in the reaction tank 6 to generate calcium fluoride particles (fluoride ion form).

  Next, the particles generated in the reaction tank 6 are transferred to the next aggregation tank 7, and an aggregating agent is added to the particles in the aggregation tank 7. The particles to which the flocculant is added agglomerate to form coarse flocs and have sedimentation properties.

  Next, the particles aggregated in the coagulation tank 7 are transferred to the subsequent precipitation tank 8, and in the precipitation tank 8, coarse particles of the flocs are precipitated, and the clear supernatant water is obtained as treated water. .

  Since the washing water (waste liquid) discharged from the factory 4 contains a small amount of fluorine, it is desirable to collect it in the reaction tank 6 and treat it with the concentrated mixed acid from the fluorine form conversion tank 5.

(Embodiment 3) FIG. 3 is an apparatus configuration diagram for carrying out a method for treating fluorine-containing wastewater according to Embodiment 3 of the present invention. The apparatus configuration of FIG. 3 is the same as that of FIG.

  In FIG. 3, the mixed acid (waste liquid), which is a concentrated liquid discharged from the factory 4, is collected in the fluorine form conversion tank 5, and the washing water as the waste liquid is collected in the reaction tank 6. A coagulation tank 7 and a precipitation tank 8 are connected to the reaction tank 6 in this order.

  First, the first step is performed in the fluorine form conversion tank 5 in which the concentrated mixed acid is recovered. In said 1st process, water is added to the rich mixed acid (waste liquid) directly discharged | emitted from the etching process of the glass processing factory 4, etc., and the mixed acid content rate is adjusted to 5 weight% or less. By the dilution treatment of the rich mixed acid, the hardly-treatable fluorine changes into a form (fluoride ion form) that can be treated by a known treatment method using a calcium salt and an aluminum salt. If the content of the mixed acid is adjusted to 5% by weight or less, even if the fluorine conversion tank 5 is sealed or released, the difficult-to-process fluorine is a known processing method using a calcium salt and an aluminum salt. To a form that can be treated with (fluoride ion form). The diluted mixed acid is transferred to the subsequent reaction tank 6.

  Next, the second step is performed using the reaction tank 6 to the precipitation tank 8. More specifically, a calcium salt is introduced into the reaction tank 6 into which the diluted mixed acid has been injected. By the introduction of the calcium salt, calcium and fluorine react in the reaction tank 6 to generate calcium fluoride particles (fluoride ion form).

  Next, the particles generated in the reaction vessel 6 are transferred to the subsequent agglomeration vessel 7, and an aggregating agent is added to the particles in the agglomeration vessel 7. The particles to which the flocculant is added agglomerate to form coarse flocs and have sedimentation properties.

  Next, the particles aggregated in the coagulation tank 7 are transferred to the subsequent precipitation tank 8, and in the precipitation tank 8, coarse particles of particles are precipitated, and the clear supernatant water is obtained as treated water.

  Since the washing water (waste liquid) discharged from the factory 4 contains a small amount of fluorine, it is desirable to collect it in the reaction tank 6 and treat it with the diluted mixed acid from the fluorine form conversion tank 5. A part of the washing water discharged from the factory 4 can also be used as concentration adjusting water when performing the dilution treatment performed in the fluorine form conversion tank 5.

  The type of calcium salt used in the embodiment of the present invention shown in FIGS. 1, 2 and 3 is not particularly limited, and calcium hydroxide (slaked lime), calcium chloride, calcium oxide (quick lime), calcium carbonate, calcium sulfate, etc. Can be used as a calcium salt. From the viewpoint of safety and price, it is optimal to use slaked lime or calcium chloride as the calcium salt.

  In the embodiment, the solid-liquid separation apparatus used for the treatment performed using the reaction tank 6, the coagulation tank 7 and the precipitation tank 8 can use an apparatus used for general waste water treatment without limitation, and the precipitation separation apparatus. In addition, filter dehydration devices such as filter presses and belt presses, membrane separation devices, and the like can be used.

  There are no particular restrictions on the type of aluminum salt used when the fluorine component treated in the fluorine form conversion tank 5 is treated by a known treatment method. Aluminum sulfate (sulfate band), polyaluminum chloride (PAC), chloride Aluminum, aluminum hydroxide, sodium aluminate (sodium aluminate) or the like can be used as the aluminum salt.

  Next, examples of the present invention will be shown and described in detail. The reagents used in the examples were all special grade reagents, and pure water or tap water was used as the water used for adjusting the concentration of the mixed acid. Moreover, the measurement of the fluorine concentration of mixed acid, raw water, and treated water was performed based on the factory drainage test method defined in JISK0102 34.1.

Example 1
First, an experiment was conducted to confirm that difficult-to-process fluorine was produced by mixing hydrofluoric acid and sulfuric acid. A mixed acid was prepared by mixing 98% by weight as sulfuric acid and 46% by weight of a special grade reagent as hydrofluoric acid and mixing at a volume ratio of 10: 8. The fluorine concentration in the mixed acid was 14% by weight, and the sulfate ion concentration was 67% by weight.

  Then, assuming dilution of the mixed acid with the washing water in the glass processing step, pure water 50 times (volume ratio) of the mixed acid was added to the mixed acid to obtain raw water before treatment. The raw water fluorine concentration was 4000 mg / L, and the sulfate ion concentration was 19500 mg / L.

  Slaked lime was added to the raw water until the pH reached 12 or more, and after stirring for 1 hour, a polymer flocculant was added and agglomerated to obtain clear supernatant water (treated water). The fluorine concentration of the treated water was 310 mg / L.

(Comparative Example 1)
A solution having a fluorine concentration and a sulfate ion concentration equivalent to the raw water of Example 1 was prepared by dissolving a special grade reagent of sodium fluoride and sodium sulfate. When the same amount of slaked lime was added to the fluorine / sulfate ion mixed solution in the same manner as in Example 1, the fluorine concentration of the treated water was 12 mg / L.

  In Example 1 and Comparative Example 1, the fluorine concentration of the treated water is greatly different even though the solutions having the same fluorine concentration and sulfate ion concentration are treated by the same method. Therefore, in the process of mixing hydrofluoric acid and sulfuric acid, a part of the fluorine is changed to a form that is difficult to treat.

(Example 2)
A processing experiment based on the device configuration diagram shown in FIG. 1 was conducted. For the preparation of the mixed acid, 96% sulfuric acid and 46% hydrofluoric acid were used, and the mixture ratio was kept constant at 9: 1. The fluorine concentration after mixing is 3.1% by weight.

  Next, tap water was added to the adjusted mixed acid as the first step in FIG. The amount of tap water was in the range of 10% to 3000% of the mixed acid capacity. Then, after leaving still for 1 hour, the tap water was added and diluted assuming the washing water in a glass etching process, and what made the final dilution rate constant by 50 times was used as raw water. Thereafter, slaked lime was added to the raw water until the pH reached 12 or more, and after stirring for 1 hour, a polymer flocculant was added and aggregated to obtain clear supernatant water (treated water). The result of the treated fluorine concentration was as shown in FIG.

  As shown in FIG. 4, when the treatment was performed after adding 50 times the washing water to the mixed acid without passing through the first step (No. 1 in FIG. 4), the treatment was performed only up to 96 mg / L. On the other hand, in the dilution treatment step, tap water having the same volume of mixed acid is added, the mixture is allowed to stand for 1 hour, and then treated with washing water (No. 4 in FIG. 4). It turns out that it improves significantly with 1 mg / L. In addition, the results (No. 2 to No. 9) in FIG. 4 also show that there is an optimum range for the amount of tap water added in the dissociation process, and conversely when adding more than 30 times the volume of the mixed acid. Since it deteriorates, it turns out that the optimal addition amount of water is 10 to 2500% of the mixed acid volume, and preferably 50 to 500%.

(Example 3)
A processing experiment based on the device configuration diagram shown in FIG. 2 was performed. For the preparation of the mixed acid, 96% sulfuric acid and 46% hydrofluoric acid were used, and the mixture ratio was kept constant at 10: 8. The fluorine concentration after mixing was 14% by weight.

  Next, two types of mixed acid containing 50 mL each in a 100 mL capacity polybin were prepared, and the mixed acid was allowed to stand in a state where one of the polybins was opened and the other one was sealed. The 1st process shown in FIG. 2 has shown that the tank 5 was made into the open state and mixed acid (waste liquid) was left still.

  Thereafter, the mixed acid was collected after a certain period of time, and tap water (washing water) was added 50 times the volume of the mixed acid to obtain raw water. Slaked lime was added to the raw water until the pH reached 12 or more, and after stirring for 1 hour, a polymer flocculant was added and agglomerated to obtain clear supernatant water (treated water). The treated water fluorine concentration is as shown in FIG. 5, and it can be seen that the treatment performance of only the mixed acid left in the open state of the polybin is improved, and the treatment performance is slowly improved over several tens of hours. . It was found that the treatment time was 24 hours or more, and when the treatment time exceeded about 120 hours, there was not much change in the processability.

Example 4
A processing experiment based on the device configuration diagram shown in FIG. 3 was performed. For the preparation of the mixed acid, 96% sulfuric acid and 46% hydrofluoric acid were used, and the mixture ratio was kept constant at 10: 8. The fluorine concentration after mixing is 14% by weight. To this mixed acid, tap water (washing water) is added 50 times the volume of the mixed acid, then two types of 50 mL each in a 100 mL volume plastic bottle are prepared. One is open and the other is sealed and static. I put it. In addition, the fluorine concentration of the mixed acid after adding tap water was 4300 mg / L (less than 5% by weight).

  Thereafter, the raw water was obtained by collecting the mixed acid after a certain period of time. Slaked lime was added to the raw water until the pH reached 12 or more, and after stirring for 1 hour, a polymer flocculant was added and agglomerated to obtain clear supernatant water (treated water). The treated water fluorine concentration is as shown in FIG. 6, and it can be seen that the processability is slowly improved over several days regardless of sealed storage or open storage.

  As described above, according to the method for treating fluorine-containing wastewater of the present invention, a hardly-treatable fluorine produced by mixing hydrofluoric acid and sulfuric acid, which has not been conventionally known, is converted into calcium salt or aluminum salt. It can convert into the form which can be processed with the well-known processing method used.

FIG. 1 is an apparatus configuration diagram for carrying out a method for treating fluorine-containing wastewater according to Embodiment 1 of the present invention. FIG. 2 is an apparatus configuration diagram for implementing the fluorine-containing wastewater treatment method according to Embodiment 2 of the present invention. FIG. 3 is an apparatus configuration diagram for carrying out the fluorine-containing wastewater treatment method according to Embodiment 3 of the present invention. FIG. 4 is a table showing the results of processing experiments in Example 2 of the present invention. FIG. 5 is a characteristic diagram showing the relationship between the standing time and the processability when left in an open state and left in a sealed state before processing the mixed acid in Example 3 of the present invention. It is. FIG. 6 is a characteristic diagram showing the relationship between the standing time from the addition of water to a mixed acid to adjust the fluorine concentration to 5% by weight until the treatment is started and the processability in Example 4 of the present invention. .

Claims (4)

A method for treating fluorine in a mixed acid comprising at least hydrofluoric acid and sulfuric acid,
A first step of adding 10% to 2500% of water of the mixed acid volume to the mixed acid;
A method for treating fluorine-containing wastewater, comprising a second step in which a calcium salt is added to the mixed acid that has undergone the first step to perform solid-liquid separation.   The method for treating fluorine-containing wastewater according to claim 1, wherein after the mixed acid having the adjusted fluorine concentration is allowed to stand, used washing water is added to the left standing mixed acid. A method for treating fluorine in a mixed acid comprising at least hydrofluoric acid and sulfuric acid,
Taking the mixed acid into the tank, holding the tank in an open state, and allowing the mixed acid to stand; and
A method for treating fluorine-containing wastewater, comprising a second step in which a calcium salt is added to the mixed acid that has undergone the first step to perform solid-liquid separation. A method for treating fluorine in a mixed acid comprising at least hydrofluoric acid and sulfuric acid,
A first step of adjusting the fluorine concentration to less than 5% by weight by adding concentration adjusting water to the mixed acid;
A method for treating fluorine-containing wastewater, comprising a second step in which a calcium salt is added to the mixed acid that has undergone the first step to perform solid-liquid separation.

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