JPH11277072A - Method and apparatus for treating waste water containing fluorine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable efficient treatment by a method in which the total amount of a calcium compound to be added into the total amount of waste water containing fluorine is added into part of the waste water to be reacted, the rest of the waste water is added stepwise into the obtained reaction liquid, and after the reaction, solid-liquid separation is done. SOLUTION: Part of waste water containing fluorine is introduced into the first reactor. The total amount of a calcium compound to be added into the total drainage is added, as required together with a pH adjusting agent, into the first reactor, agitation is done, and a CaF2 deposition reaction is carried out. The reaction liquid to be obtained is supplied to the second reactor, part of the rest of the waste water is introduced into the second reactor, agitation is done, and the CaF2 deposition reaction is carried out. The reaction liquid to be obtained is supplied to the third reactor, the rest of the waste water is introduced into the third reactor, agitation is done, and the CaF2 deposition reaction is carried out similarly. The reaction liquid to be obtained is sent to a solid-liquid separator to be separated into treated water and sludge by a similar method.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and an apparatus for treating fluorine-containing wastewater discharged in the electronics industry and the like, and more particularly, to the removal of fluorine from the fluorine-containing wastewater by utilizing a calcium fluoride precipitation reaction. Fluorine-containing wastewater treatment method and apparatus.

[0002]

2. Description of the Related Art In the fields of manufacturing semiconductors and liquid crystal substrates and related fields, such as the electronics industry and the metal surface processing industry,
Large amounts of etchant are used. As this etching agent, an etching agent mainly containing hydrogen fluoride or hydrogen fluoride and ammonium fluoride is mainly used. The etching agent containing hydrogen fluoride as a main component contains fluorine in an amount of, for example, about 0.9% as HF and is used in a large amount. On the other hand, the etching agent (buffered hydrofluoric acid) containing hydrogen fluoride and ammonium fluoride as main components contains fluorine, for example, in an amount of about 7% as HF although the amount used is small. In many cases, these etching agents flow out of the etching process almost as they are, resulting in high-concentration fluorine-containing wastewater (treatment process wastewater). On the other hand, during the etching or at the end of the etching, since the material treated with these etching agents is washed with a large amount of washing water, a large amount of low-concentration fluorine-containing wastewater (washing process wastewater) is discharged from this washing step. Thus, for example,
In a semiconductor or liquid crystal substrate manufacturing process, a small flow of high-concentration fluorine-containing wastewater having a fluorine concentration of 500 to 10,000 mg / L (liter) and a large flow of low-concentration fluorine-containing wastewater having a fluorine concentration of 10 to 50 mg / L are discharged. Is common. Most commonly, these two wastewaters are mixed and the fluorine concentration is 100
It is usually about 200 mg / L of fluorine-containing wastewater, but since the concentration of fluorine varies considerably at each plant,
It is not limited to the above fluorine concentration.

As described above, high-concentration fluorine-containing wastewater or low-concentration fluorine-containing wastewater discharged in the electronic industry or the like, or fluorine-containing wastewater that is a mixed wastewater thereof, contains hydrofluoric acid or the like used as an etching agent. Contains fluorine in the form, but the standard for releasing fluorine is 15 mg / L or less.
Many municipalities have stricter additional standards, and many municipalities have standard values of 1 to 8 mg / L or less.

As a conventional method for treating such fluorine-containing wastewater, a substance which generates calcium ions such as calcium chloride and slaked lime [Ca (OH) ₂ ] (hereinafter referred to as “calcium compound”) is used as the fluorine-containing wastewater. Crystallization as calcium fluoride, followed by solid-liquid separation to remove fluorine. The reaction formula of calcium fluoride crystallization is as follows. Ca ²⁺ + 2F ⁻ = CaF ₂ ↓

[0005]

The low-concentration fluorine-containing wastewater, of course, has a low fluorine concentration for the above-described reasons. However, the high-concentration fluorine-containing wastewater normally discharged from a general electronic industry factory is used. Wastewater mixed with low-concentration fluorine-containing wastewater often has a fluorine concentration close to low-concentration fluorine-containing wastewater. Therefore, the efficiency of removing fluorine from such fluorine-containing wastewater is deteriorated. The solubility product Ksp of CaF ₂ at 20 ° C. is Ksp = [Ca ²⁺ ] [F ⁻ ] ² = 3.
It is represented by the formula of 45 × 10 ⁻¹¹ . However, in the actual wastewater, the product of [Ca ²⁺ ] and [F ⁻ ] ² reaches the solubility product due to the addition of the calcium compound, and even if the product slightly exceeds this, CaF ₂ does not precipitate, and Present in solution as a metastable supersaturated state. Therefore, an excessive amount of calcium ions (calcium compound) is added to CaF
_{In addition} to destroying the supersaturated state of _2, the water quality of the treated water is secured by adding a large excess of an inorganic coagulant such as polyaluminum chloride (PAC) and adsorbing and coprecipitating fluorine, It is difficult to treat well, and there are problems such as waste of chemicals, increase in sludge treatment amount, high concentration of calcium ions remaining in treated water, and increase in equipment investment. Also, as can be seen from the above solubility product equation, the lower the F ^- ion concentration, the more CaF ₂
The amount of Ca ²⁺ ions supplied to destroy the supersaturated state of the gas increases in a square relation, and in order to remove low-concentration F ⁻ ions (for example, 20 to 30 mgF ⁻ / L), a reaction tank is required. Wastewater in the very high Ca ²⁺ ion concentration (for example,
If not more than 1000 mg Ca ²⁺ / L), F ⁻ ions cannot be removed as CaF ₂ precipitates,
In this way, the treated water obtained by treating the fluorine-containing wastewater with a high Ca ²⁺ ion concentration involves a problem of scale generation.

[0006] In order to solve such a problem, the applicant of the present invention first added the entire amount of calcium ions (calcium compound) to be added to the entire wastewater to a part of the fluorine-containing wastewater as raw water divided ( injection), and the resulting reaction solution was mixed into the fluorine-containing waste water remaining majority, with crystallized substances CaF ₂ produced in the reaction mixture as seed crystals, to remove the fluorine content from the total water discharge so-called A "raw water injection method" was proposed (Japanese Patent Laid-Open No. 6-312190). With this method, it has become possible to cope with a large amount of fluorine-containing wastewater.

As described above, the solubility product equation Ksp = [Ca
^2+] [F ^-] As can be seen from ^2, [F ^-] in order to contribute with the square, but is much greater effect on the crystallization reaction of CaF ₂ as compared with [Ca ^2+], in reality the fluoride ion concentration ([F ^-]) can not be changed, seed Reducing raw water (fluorine-containing waste water) water of the divided portion to produce a seed as raw water Note how [Ca ²⁺ ] in the production stage can be increased, and a seed crystal can be reliably formed. Problem, in order to ensure the seed formation, the correspondingly therein Reducing the raw water of said portion F ^-
The absolute amount of ions also decreases, and the absolute amount of seed crystals formed decreases. As shown in FIG. 3, if the amount of the seed crystal is insufficient, it may not be possible to efficiently treat the residual fluorine-containing wastewater. FIG. 3 shows the results of an experiment conducted using a simulated wastewater containing NaF as a fluorine component in order to examine the required amount of seed crystals. Seed crystals (calcium ions were added to fluoride ion-containing water in advance and water was added). FIG. 3 is a graph showing a relationship between the concentration of the obtained CaF ₂ precipitate (using the obtained CaF ₂ precipitate) and the F ⁻ ion concentration of treated water. If clogging occurs and the fluorine concentration in the fluorine-containing wastewater is extremely low, or if interfering substances such as various interfering ions coexist, a part of the fluorine-containing wastewater is added to the entire wastewater according to the raw water injection method as described above. Even if the entire amount of calcium ions (calcium compound) to be added is added, if the amount of drainage is increased to increase the absolute amount of the seed crystal, no seed crystal is formed or the seed crystal is formed. In addition, if the amount of the raw water is partially reduced, the number of generated seed crystals may be insufficient, which may make it difficult to treat the fluorine-containing wastewater.

An object of the present invention is to provide a method and an apparatus for efficiently treating fluorine-containing wastewater which can cope with such a case.

[0009]

As a result of various studies to achieve the above object, the present inventor has found that certainty of formation of seed crystals and absolute absoluteness of seed crystals particularly in fluorine-containing wastewater such as low-concentration fluorine-containing wastewater. It has been found that the above mentioned discrepancies in quantity can be resolved in the following way. Clogging, a part of the fluorine-containing wastewater is added with the entire amount of the calcium compound to be added to the total amount of the wastewater and reacted to produce CaF ₂ crystals as seed crystals. In this case, the amount of the fluorine-containing wastewater is partially Reduce to an amount where crystals can be reliably formed. Although the amount of seed crystals formed may be insufficient when mixed with all of the remaining fluorine-containing wastewater, it has been found that this problem can be solved by further dividing the remaining fluorine-containing wastewater. That is, when a part of the residual waste water having an amount sufficient to generate the seed crystal is mixed with the reaction solution containing the seed crystal and reacted, Ca remaining in the reaction solution is obtained.
^{Since the} seed crystal promotes a precipitation reaction between ²⁺ ions and F ⁻ ion mainly contained in the part of the residual waste water, the amount of the seed crystal contained in the obtained reaction solution is larger than the amount of the initial seed crystal. It turned out to be more. Therefore, this reaction solution is mixed with another portion of the residual waste water in an amount sufficient to contain the seed crystals contained in the reaction solution and reacted. When seed crystals are generated in a stepwise manner and reacted with residual wastewater, the amount of wastewater that can be added increases as the amount of seed crystals increases, and eventually all of the remaining wastewater is effectively treated. Can be.

That is, according to the present invention, when a calcium compound is added to a fluorine-containing wastewater to remove the fluorine content in the wastewater as calcium fluoride, the total amount of the calcium compound to be added to the total fluorine-containing wastewater is the same as the fluorine-containing wastewater. Of the fluorine-containing wastewater, characterized in that solid-liquid separation is performed after stepwise adding and reacting the remaining most of the fluorine-containing wastewater to the obtained reaction solution and reacting. To provide.

That is, the reaction solution containing freshly produced CaF ₂ (sol-like CaF ₂ ) is supplied to the remaining fluorine-containing wastewater sequentially and stepwise as it is, so that the remaining Ca ²⁺ remains on the surface of the sol-like CaF _2. F in the ion and residual fluorine-containing waste water ^- the CaF ₂ crystallization reaction between ions proceed efficiently, it is performed a process of stable fluorine-containing waste water.

The method of the present invention can be carried out batchwise, or can be carried out continuously by providing a series of reaction tanks as shown in FIGS. In the continuous method, for example, as shown in FIG. 1, the same number of reactors as the divided portion of the fluorine-containing wastewater are installed, and a small amount of the fluorine-containing wastewater and all the calcium compounds are added to the first reaction tank, mixed, and obtained. The reaction solution thus obtained is sequentially sent to each reaction tank, and in each reaction tank, it can be mixed and reacted with the fluorine compound-containing wastewater distributed to each reaction tank. However, in such a method, the apparatus becomes complicated and a large space is required. In addition, the second and subsequent reaction tanks contain CaF ₂ contained in the reaction solution from the previous reaction tank. Is used as a seed crystal, and it is not necessary to completely isolate each reaction tank. Therefore, as shown in FIG. 2, the inside of one fluorine-containing wastewater treatment apparatus is divided by a partition plate, and each partitioned space is separated. Is preferably used as a single reaction tank for treating fluorine-containing wastewater, so that a very simple and compact fluorine-containing wastewater treatment apparatus can be manufactured.

That is, according to the method of the present invention, the inside of the fluorine-containing wastewater treatment apparatus is divided by a partition plate, and the fluorine-containing wastewater is divided into the respective stages so that the amount of the fluorine-containing wastewater supplied to the partition space of each stage increases sequentially. Step (1) of supplying the total amount of the calcium compound to be added to the total amount of the wastewater to the fluorine-containing wastewater in the first-stage partition space and reacting the resultant reaction solution with the fluorine in the next-stage partition space. Step (2) of adding and reacting to the wastewater containing water, and step (3) of adding and reacting the obtained reaction solution to the wastewater containing the fluorine compound in the partition space of the next stage. It is preferable to perform the solid-liquid separation after repeating the same step as the step (3) in the partition space of the above step. It is needless to say that the fluorine-containing wastewater may be evenly supplied to the partition space of each stage.

Further, according to the present invention, the interior of the fluorine-containing wastewater treatment apparatus is divided into at least three stages by a partition plate to form a partition space for each stage as each reaction tank, and the reaction solution from the preceding reaction tank is supplied to the next stage. The first tank is provided with calcium compound adding means for adding the total amount of calcium compound to be added to the total amount of fluorine-containing wastewater in the reaction tank in the first partition space, and the fluorine-containing wastewater is supplied to each reaction tank. Another object of the present invention is to provide an apparatus for treating fluorine-containing wastewater, which is provided with wastewater supply means for dividing and supplying wastewater.

The ^- ([F]) [0015] Drainage distribution ratio of the reaction vessel of the first stage to the total fluorine-containing waste water is varies also depending on the presence or absence of obstructions, principle of fluorine-containing wastewater fluoride ion concentration The lower the fluoride ion concentration in the wastewater, the more the fluorine-containing wastewater distribution rate needs to be reduced. For example, when the fluoride ion concentration is 50 mg
/ L (liter) or more, it is desirable to make the drainage distribution ratio in the first-stage reaction tank about 1/25 to about 1/5, and when the fluoride ion concentration is less than 50 mg / L, the first-stage reaction tank It is desirable that the drainage distribution ratio in the tank be about 1/200 to about 1/20. Further, as can be seen from FIG. 3, the amount of the CaF ₂ seed crystal has a very large effect on the treatment effect of the remaining fluorine-containing wastewater. In case of simulated drainage, 10m as seed crystal
g / L or more of CaF ₂ was necessary, but in the case of actual fluorine-containing wastewater, it varies from case to case due to the influence of obstacles and the like, but generally from about 10 mg / L to about 200 m
An amount of CaF ₂ in the range of g / L is required. Thus, as the added amount of CaF ₂ as a seed in each reaction vessel subsequent second stage is equal to or greater than a predetermined value in a range of 10 to 200 mg / L on a case-by-case, the distribution ratio of the fluorine-containing waste water Should be considered.

In some cases, it is effective to increase the number of CaF ₂ seed crystals by adding a suitable fluorine compound as an auxiliary for forming CaF ₂ seed crystals, particularly at the stage of adding the calcium compound. In many cases, the method of the present invention does not require the addition of a fluorine compound. As such a fluorine compound, sodium fluoride, ammonium fluoride, calcium fluoride and the like are preferable, and silicate is not preferably used because it may cause an adverse effect of hindering precipitation. The addition of the fluorine compound may be performed continuously, but may be performed intermittently or in accordance with the amount of the CaF ₂ seed crystal (which can be calculated from the flow rate of the fluorine-containing wastewater and the fluorine concentration) or the treated water. May be controlled by monitoring the fluorine concentration of the compound.

As the calcium compound, calcium chloride [CaCl ₂ ], slaked lime [Ca (OH) ₂ ], calcium carbonate [CaCO ₃ ] and the like can be used. The addition amount of the calcium compound may be feed-forward control based on the fluorine concentration and the flow rate of the waste water, but it is more preferable to perform feedback control based on the calcium ion concentration in the treated waste water using a calcium ion monitor.

The amount of calcium to be added varies depending on the components of raw water (fluorine-containing wastewater) and the required quality of treated water.
It is an excessive amount so as to become 00 mg / L. This is 200
If the amount is less than mg / L, the quality of the treated water tends to deteriorate,
If it exceeds 800 mg / L, it is not preferable from the viewpoint of economy and scale generation.

If the pH of the treated water is 4 or more, Ca
Although there is almost no re-dissolution of F ₂ and basically no problem, the pH of the treated water in the final reaction tank is desirably in the range of 4 to 12, preferably 4 to 8.

The whole reaction solution to be subjected to solid-liquid separation may be directly subjected to solid-liquid separation by a method such as membrane separation, or a pH adjusting agent may be added by using a flocculant and, if necessary, adding a pH adjusting agent. While performing a coagulation reaction of CaF ₂ precipitates in the coagulation reaction tank,
If necessary, the mixture may be settled in a sedimentation tank (the coagulation reaction tank may be used as a sedimentation tank, or the sedimentation tank may be omitted), separated into supernatant treated water and a precipitate slurry, and subjected to solid-liquid separation. Also,
A membrane separation device may be used instead of the settling tank. Therefore, as a means for solid-liquid separation, if necessary, the membrane separation may be performed simply by omitting the aggregation-precipitation treatment, the combination of the aggregation treatment and the membrane separation, or the aggregation treatment. Examples of the membrane separation device include those using a reverse osmosis membrane, an ultrafiltration membrane, a microfiltration membrane, and the like.

As the coagulant, an aluminum coagulant such as polyaluminum chloride (PAC) or a sulfuric acid band, and an iron coagulant such as ferric chloride can be used. In many cases, an organic polymer flocculant is used in combination as a flocculant to achieve effective solid-liquid separation. The use of the flocculant is intended not only to promote solid-liquid separation, but also to further remove fluoride ions (F ⁻ ) by a coprecipitation effect.

It is also possible to directly return a part of the sludge obtained by solid-liquid separation by direct membrane separation to one or more of a series of reaction tanks, because the amount of seed crystals can be increased. It is effective to perform the processing of. When an aluminum-based flocculant such as polyaluminum chloride (PAC) or a sulfuric acid band is used, at least a part of the sludge containing solids after solid-liquid separation is treated with an alkali such as sodium hydroxide or potassium hydroxide. After dissolving the aluminum floc, solid-liquid separation is performed, for example, by standing, and the supernatant liquid is returned to the coagulation reaction tank and reused as a coagulant. Returning to one or more of them is also effective for performing a desired treatment because the amount of seed crystals can be increased. When slaked lime is used as a calcium compound, part or all of the slaked lime is used as an alkali agent for dissolving the above-mentioned sludge, and the residual precipitate slurry obtained after dissolving the aluminum floc is returned to one or more of a series of reaction tanks. Preferably, it is supplied.

The method of the present invention is disclosed in Japanese Patent Application No. 9-4 / 1993.
It can also be carried out in combination with the method for treating fluorine-containing wastewater proposed in No. 6927. That is, for example, the fluorine-containing wastewater is separated into the small-flow high-concentration fluorine-containing wastewater and the large-flow low-concentration fluorine-containing wastewater as described above, and the entire amount of the calcium compound to be added to the entire wastewater is discharged. To a part or the whole of the reaction solution, and the remaining most fluorine-containing wastewater is added stepwise to the obtained reaction solution.
After the reaction, a solid-liquid separation method can be adopted. (When adding the entire amount of calcium compound to "part of the high-concentration fluorine-containing wastewater," use the remaining high-concentration fluorine-containing wastewater first. However, it is naturally preferable because the amount of seed crystals can be easily increased.) According to this method, as can be seen from the solubility product of the above, F ^- ion concentration is higher the high-concentration fluorine-containing waste water of crystallization CaF ₂ as compared to the mixed waste water with high concentration fluorine-containing waste water and the low-concentration fluorine-containing waste water analysis occurs remarkably easy, moreover, was added the total amount of the high concentration of fluorine-containing calcium compound to be added to all the waste water in a part or all of the waste water, since allowed to promote CaF ₂ precipitation reaction also enhanced Ca ²⁺ ion concentration,
Seed crystal formation is more reliable and sufficient.

[0024]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, referring to FIGS.
Embodiments of the present invention will be specifically described, but it goes without saying that the present invention is not limited to these.

FIG. 1 is a flow chart showing an example of a method for treating fluorine-containing wastewater according to the present invention. A part of the fluorine-containing wastewater is introduced into the first-stage reaction tank as raw water. It is preferable to increase the capacity of the reaction tank in a later stage.
Further, a pH adjuster is added to the first-stage reaction tank, if necessary, together with the total amount of the calcium compound to be added to the entire wastewater, and the mixture is stirred to perform a CaF ₂ precipitation reaction. In the first stage reaction tank,
Since the entire amount of calcium compounds to be added to the entire wastewater is added, the Ca2 ⁺ ion concentration in the wastewater becomes extremely high,
Further, CaF ₂ is quickly precipitated because wastewater is surely small amounts, such as CaF ₂ precipitation occurs. In the CaF ₂ generation reaction, if the pH becomes 4 or more, the pH becomes CaF 2
_{(2) Since} there is almost no adverse effect on the precipitation reaction, p
H may be adjusted within the range of 4 to 12 as necessary.
In terms of stability, it is preferable to adjust the pH within the range of 5 to 12. If the pH is lower than 4, the CaF ₂ precipitate may be redissolved, and the CaF ₂ precipitation reaction may not proceed smoothly.

The obtained reaction solution is supplied from the first-stage reaction tank to the second-stage reaction tank, and a part of the remaining fluorine-containing wastewater is caused to flow into the second-stage reaction tank and stirred to carry out the CaF ₂ precipitation reaction. Do. In the second-stage reaction tank, CaF ₂ contained in the reaction solution from the first-stage reaction tank acts as a seed crystal, and the calcium compound remaining in the reaction solution and the fluorine content in the wastewater flowing into the second-stage reaction tank are reduced. As a result, CaF ₂ is newly formed, so that the amount of seed crystals sent to the next third-stage reaction tank increases.

[0027] with the resultant reaction liquid is supplied to the third stage reaction vessel from the second stage reactor, the fluorine-containing wastewater residual flows towards the third stage reactor, stirred, similarly CaF ₂ precipitation reaction Do. Although the treatment apparatus of FIG. 1 has three reaction tanks, four or more reaction tanks may be used, and an appropriate number of reaction tanks may be determined in consideration of the concentration of fluorine and the concentration of interfering substances in the fluorine-containing wastewater. Just decide.

The pH of the waste water in the second and subsequent reaction tanks is adjusted, if necessary, for example, so that the pH of the waste water is in the range of 4 to 12, preferably 4 to 8, and more preferably substantially neutral. Thus, an excessive amount of the pH adjuster may be added to the first-stage reaction tank in advance, or the pH may be adjusted again by adding the pH adjuster to the second and subsequent reaction tanks. Add a pH adjuster to the third-stage reaction tank and add p
The addition of the H adjuster may be omitted, but any method may be used. Specifically, it may be determined appropriately depending on the stability of drainage, problems with equipment, and the chemical used.

Since the reaction for producing CaF ₂ is an ionic reaction,
Essentially, this reaction proceeds instantaneously, but the precipitation reaction is strongly affected by the above-mentioned solubility product and interfering factors. From this viewpoint, it is of course better that the residence time in each stage reaction tank is longer, but in practice, it is sufficient to set the residence time to about 5 to 45 minutes. In particular, the residence time in the second and subsequent reaction tanks is preferably set to be longer from the viewpoint that CaF ₂ is deposited as completely as possible.

The obtained reaction solution is sent from the third-stage reaction tank to a solid-liquid separation device, where it is subjected to solid-liquid separation by the method described above, and is separated into treated water and sludge. Although not shown, sludge or a part of the residual sediment slurry derived from sludge is returned to at least one or more reaction tanks, and CaF ₂ contained in the slurry can be used as a seed crystal.

FIG. 2 is a flow chart showing another example of the method for treating fluorine-containing wastewater according to the present invention, and is a schematic view of the structure of a fluorine-containing wastewater treatment apparatus according to the present invention. Unlike FIG. 1,
In FIG. 2, the inside of the fluorine-containing wastewater treatment device is divided into five stages by a partition plate, and a partition space of each stage is preferably formed as each reaction tank so that the amount of the supplied fluorine-containing wastewater is sequentially increased. It is characterized in that a calcium compound adding means for adding the whole amount of the calcium compound to be added to the first-stage partition space (reaction tank) is provided. For the flow of liquid between the adjacent partition space (reaction tank), an opening may be provided at an appropriate position of the partition plate, or an overflow method may be used. The embodiment of the method of the present invention using the apparatus for treating fluorine-containing waste water of FIG. 2 is substantially the same as the embodiment of FIG. 1, and thus the description thereof is omitted.

The solid matter (precipitated sludge or residual sediment slurry derived therefrom) discharged by the method of the present invention as described above is subjected to a treatment such as a recovery treatment of CaF ₂ according to a conventional method. The recovered CaF ₂ can be reused as a raw material for producing hydrofluoric acid.

[0033]

EXAMPLES Hereinafter, the present invention will be described in detail with reference to Examples, but it goes without saying that the present invention is not limited to these Examples.

Example 1 1 L (liter) of raw water (simulated waste water) having a fluoride ion concentration of 20 mg / L prepared by dissolving sodium fluoride in pure water
Transfer 10 ml (milliliter) from the mixture into a 1 L beaker, and adjust the calcium ion concentration to about 3
3 ml of a 30% by weight aqueous solution of calcium chloride was added so that the concentration became 00 mg / L, and the mixture was stirred with a magnetic stirrer for 10 minutes. Every 10 minutes, the total volume of the reaction solution is about 30 ml, about 100 ml, about 300 ml, and about 1 ml.
Raw water was added in small portions while stirring with a magnetic stirrer so that the volume became 000 ml. The fluoride ion concentration of the obtained reaction solution was measured by a direct fluorine electrode method without solid-liquid separation. The calcium ion concentration of the reaction solution was measured by a titration method. Table 1 shows the processing results.

Comparative Examples 1 and 2 In addition, for comparison, a test was performed by a so-called raw water injection method in which raw water was divided into two parts. That is, in Comparative Example 1, a fluoride ion concentration of 20 prepared by dissolving sodium fluoride in pure water was used.
10 ml of 1 mg / L of raw water is transferred to a 1 L beaker, and 3 ml of a 30% by weight calcium chloride aqueous solution is added so that the calcium ion concentration becomes about 300 mg / L with respect to the total amount of 1 L of raw water. After stirring with a stirrer, 25 minutes later, the remaining raw water was added, and the mixture was further stirred and reacted for 25 minutes. Table 1 also shows the processing results. In Comparative Example 2, 10 L of the same raw water 1 L was used.
0 ml was transferred to a 1 L beaker, and the same test as in Comparative Example 1 was performed thereafter. Table 1 also shows the processing results. In addition,
Example 1 and Comparative Examples 1 and 2 were performed simultaneously in parallel to make the conditions other than the operating procedure as identical as possible.

[0036]

Table 1 ─────────────────────────────────── F ^- concentration (mg / L) Ca ²⁺ Concentration (mg / L) Dispensing rate ─────────────────────────────────── Example 1 12 290 Comparison Example 1 20.1 306 1/100 Comparative Example 2 19.8 303 1/10 ────

As can be seen from the results shown in Table 1, in comparison with Comparative Examples 1 and 2 according to the conventional raw water injection method in which raw water is divided into two parts, in Example 1 according to the method of the present invention, F ^{− of the} final reaction solution was obtained.
The ion concentration is considerably low, indicating that the crystallization reaction of CaF ₂ has proceeded sufficiently, and it can be seen that the quality of treated water obtained by solid-liquid separation also improves accordingly.

[0038]

According to the present invention, first, the whole amount of the calcium compound to be added to the total amount of the wastewater is added to a part of the fluorine-containing wastewater whose seed crystal formation is a certain amount. By doing so, [Ca ²⁺ ] × greatly exceeds the solubility product of CaF ₂
A CaF ₂ seed crystal can be reliably formed at the value of [F ⁻ ] ² . By adding a part of the remaining fluorine-containing wastewater to the obtained reaction solution and reacting by using the seed crystal previously generated, the amount of CaF ₂ seed crystal can be surely increased. Further, by adding the whole or a part of the remaining fluorine-containing wastewater to the obtained reaction solution and reacting using the increased seed crystals, CaF ₂ can be more reliably precipitated. In the method of the present invention, the reaction operation between the reaction solution and the remaining fluorine-containing wastewater is repeated as many times as necessary, but as the number of seed crystals increases, the amount of fluorine-containing wastewater to be supplied to the next-stage reaction is increased. be able to. The treated water obtained by solid-liquid separation of the final reaction solution thus obtained has better water quality.

If the fluorine-containing wastewater treatment apparatus of the present invention as shown in FIG. 2 is used as an apparatus for carrying out the method of the present invention, the apparatus itself is simple and does not require a large space. That is, the second and subsequent reaction vessels use CaF ₂ contained in the reaction solution from the preceding reaction vessel as a seed crystal, and it is not necessary to completely isolate each reaction vessel. As shown in FIG. 2, a fluorine-containing wastewater treatment apparatus is configured so that the inside of one fluorine-containing wastewater treatment apparatus is divided by a partition plate, and each partitioned space is treated as one reaction tank to treat the fluorine-containing wastewater. Is convenient.

[Brief description of the drawings]

FIG. 1 is a flow chart showing an example of a method for treating fluorine-containing wastewater according to the present invention.

FIG. 2 is a flow chart showing another example of the method for treating fluorine-containing wastewater according to the present invention, and is a schematic diagram of the configuration of a fluorine-containing wastewater treatment apparatus of the present invention.

FIG. 3 shows the results of an experiment conducted using simulated drainage to check the required amount of seed crystals, and is a graph showing the relationship between the concentration of seed crystals and the F ^- ion concentration of treated water. FIG.

Claims (3)

[Claims] When a calcium compound is added to a fluorine-containing waste water to remove fluorine content in the waste water as calcium fluoride, the whole amount of the calcium compound to be added to the whole fluorine-containing waste water is included in a part of the fluorine-containing waste water. Add, react,
A method for treating fluorine-containing wastewater, which comprises adding a large part of the remaining fluorine-containing wastewater to the obtained reaction solution in a stepwise manner, and then performing solid-liquid separation. 2. A fluorine-containing wastewater treatment device is divided by a partition plate, and fluorine-containing wastewater is supplied to each stage partition space so that the amount of fluorine-containing wastewater supplied to each stage partition space increases sequentially. Step (1) of adding and reacting the entire amount of the calcium compound to be added to the total amount of the wastewater to the fluorine-containing wastewater in the first-stage partition space, and adding the resulting reaction solution to the fluorine-containing wastewater in the next-stage partition space; (2), adding the resulting reaction solution to the fluorine-containing wastewater in the next-stage partition space, and reacting (3), if necessary, in the next-stage partition space as many times as necessary. The method for treating a fluorine-containing wastewater according to claim 1, wherein the solid-liquid separation is performed after repeating the same step as the step (3). 3. The fluorine-containing wastewater treatment apparatus is divided into at least three stages by a partition plate to form a partition space for each stage as each reaction tank, and the reaction solution from the preceding reaction tank to the next reaction tank. And a means for adding the total amount of calcium compound to be added to the total amount of fluorine-containing wastewater is provided in the reaction tank in the first partition space, and the fluorine-containing wastewater is divided and supplied to each reaction tank. A fluorine-containing wastewater treatment apparatus, comprising: