JP7142600B2 - Water treatment method for removing fluoride ions - Google Patents

Description

The present invention relates to a water treatment method for removing fluoride ions. It is possible to stably remove fluoride ions from water to be treated (raw water) that fluctuates, avoiding the generation of excessive sludge while miniaturizing equipment and reducing the amount of chemicals used. It relates to a water treatment technology with excellent practical value that can realize a reduction of

For example, since the waste water from the flue gas desulfurization equipment in the flue gas desulfurization method implemented at coal-fired power plants and coke plants contains fluoride ions, its treatment becomes a problem when it is discharged. . As a method for removing fluoride ions (soluble fluorine) in the water to be treated, a method of adding calcium ions to wastewater in the neutral pH range and precipitating and removing the fluoride ions as calcium fluoride is common. (see Patent Document 1, etc.).

However, in this method, if magnesium ions or sulfate ions are present in the water to be treated, such as wastewater from flue gas desulfurization equipment using magnesium hydroxide, the fluoride ion removal rate in the above calcium method is reduced. I had a problem with it getting really bad. This is probably because in the case of such wastewater, a large amount of magnesium ions and fluoride ions are dissolved as a complex in the neutral pH range, and this is the reason why calcium fluoride is not produced.

In response to the above problem, it has been proposed to increase the removal rate of fluoride ions by adjusting the pH of the wastewater to 8.0 to 10.0 after adding calcium ions and performing solid-liquid separation by precipitation. (Patent Document 2) has also been implemented. Furthermore, this method has a practical problem of increasing the amount of sludge generated. There have been proposed treatment methods capable of That is, in these methods, first, in the step of adding an alkaline agent to the water to be treated to generate suspended solids (alkali addition step), magnesium ions in the water to be treated are sufficiently precipitated as magnesium hydroxide, and the precipitation Suspended matter in which fluoride ions are incorporated into magnesium hydroxide is actively generated. Then, a step of solid-liquid separation of the suspended matter into which fluoride ions are taken is performed, and a step of adding an acid to the sludge derived from the solid-liquid separated suspended matter is performed. This acid addition step dissolves magnesium hydroxide, which is the main component of the sludge, and reduces the final amount of sludge (volume reduction). Fluoride ions incorporated in the sludge precipitate as magnesium fluoride (MgF ₂ ), and the MgF ₂ remains in the final sludge at a high concentration. Therefore, the final amount of sludge can be reduced (volume reduction).

In Patent Document 3, an alkali agent is added in the alkali addition step to adjust the pH to 9.0 to 10.0, to generate a precipitate in which fluoride ions in the waste water are incorporated, and in the acid addition step , an acid is added to reduce the sludge concentration so that the separated suspended sludge has a pH of 3.0 to 8.5, and the fluorine content in the sludge is adjusted to the fluorine content in the suspended sludge. It is said that it will increase relative to the rate. In addition, in Patent Document 4, by providing an aging process in which the sludge is stirred for a predetermined time after adding acid in the acid addition process, the final amount of sludge can be reduced and higher processing efficiency is realized. I think it will be possible. Further, in Patent Document 5, the addition amount (mg-OH/L) of the alkali agent as OH to the water to be treated in the alkali addition step is the mass ratio (mg-F/L) to the fluorine concentration in the water to be treated ( OH amount/F amount) is 1.0 or more, so that the fluoride ions in the water to be treated can be effectively removed even when the treatment for removing fluoride ions in the water to be treated is performed in a single step. It is possible to reduce the final amount of sludge (volume reduction).

Japanese Patent Publication No. 58-013230 Japanese Patent No. 4330693 JP 2016-87562 A JP 2017-189724 A JP 2017-189725 A

However, although the treatment methods of the prior art described above can reduce the final amount of sludge generated in the water to be treated in any case, according to the studies of the present inventors, there is room for improvement. . That is, in the conventional method, adjustment of the addition amount of the alkali agent in the alkali addition step for generating a precipitate in which fluoride ions in the water to be treated (raw water) are taken in, and acidification of the separated suspended sludge Introduced into the addition tank, acid is added to lower the sludge concentration, and the fluorine content in the sludge is adjusted to increase the fluorine content in the sludge relative to the fluorine content in the suspended sludge. However, it does not take into consideration the fluctuation of fluoride ions in the water to be treated and the amount of water to be treated. When applied to water treatment such as wastewater, there was a problem that stable treatment was difficult. In other words, until now, the optimum operating conditions for dealing with fluctuating water to be treated have not been found. Considering the residual fluorine in the treatment liquid, two-stage treatment is performed, and the current situation is that excessive equipment and excessive chemicals are used.

On the other hand, the technique described in Patent Document 5 mentioned above is aimed at performing a good treatment in one step, and for that purpose, an alkali agent is added to the water to be treated in the alkali addition step. The amount of OH added (mg-OH/L) is set so that the mass ratio (OH amount/F amount) to the fluorine concentration (mg-F/L) in the water to be treated is 1.0 or more, It is disclosed that it is necessary to generate large amounts of suspended solids. The amount of the alkaline agent to be added is preferably determined based on the relationship between the sludge and the fluorine concentration in the supernatant water separated from the sludge in the solid-liquid separation step after the addition of the alkali. In addition, the concentration of the sludge separated in the solid-liquid separation step is adjusted to 1000 mg / L or more, and the concentration of the sludge after acid addition is lower than the above concentration and is 5000 mg / L or more and 50000 mg / L or less. It is preferable to adjust the range.

In addition, in the technique described in Patent Document 5, the final sludge after the addition of acid and the supernatant liquid after solid-liquid separation contain fluorine. If the raw water is returned and treated together with the raw water, the fluorine concentration to be treated increases, so the amount of alkali agent added must be greater than the OH/F amount required for the raw water. In the case of , the amount of alkali agent added is increased in accordance with the increase in the fluorine concentration of the supernatant water.

The above-described method is useful because it can remove fluorine in one step, but in the case of return treatment, it becomes more difficult to determine operating conditions. Specifically, the concentration of fluorine (concentration of soluble fluorine) in the supernatant obtained by solid-liquid separation after the addition of the alkaline agent is Therefore, in the treatment system in which the supernatant of the final sludge concentration step is returned to the alkali addition step, it is necessary to add a larger amount of alkali agent in consideration of the fluorine concentration in the supernatant. , it becomes more difficult to determine the optimum operating conditions for efficient processing.

For this reason, when applying the above-described conventional technology for returning fluorine to actual wastewater (raw water) in which the content of fluorine fluctuates, the fluorine content in the water to be treated (raw water) must be It is necessary to determine the amount of alkali agent to be added in the alkali addition process, considering the fluctuation of the concentration of compound ions and the increase in fluorine load due to the fluoride ions in the supernatant liquid to be returned. As a result, there is a tendency to increase the amount of the alkali agent added. When the amount of alkali agent added increases, the amount of suspended matter (sludge) generated in the alkali addition process increases, and then the amount of oxidizing agent added in the acid addition process for reforming the solid-liquid separated sludge increases. will also increase. In addition, at that time, it is necessary to take a sufficient amount of acid to be added in the acid addition step and the time required for reforming so that the sludge is sufficiently reformed. It is difficult to say that the above-described conventional techniques, such as the need to provide an aging process such as stirring the sludge for a predetermined period of time, are established treatment methods that enable stable, economical and efficient treatment.

Therefore, the object of the present invention is to solve the above-described problems, for example, waste water from a flue gas desulfurization apparatus that treats exhaust gas using magnesium hydroxide, and at least magnesium ions in addition to fluoride ions. In the water treatment method for removing fluoride ions from the water to be treated that contains It is possible to realize a water treatment method that removes fluoride ions from water to be treated that contains fluoride ions and magnesium ions, which is more economical, performs stable and good treatment, and has excellent practicality. to do. Specifically, the object of the present invention is to eliminate excess sludge as in the prior art by merely appropriately determining the operating conditions, although the basic configuration is the same as that of the prior art. ) can be obtained, and if desired, it is possible to reduce the size of processing equipment and reduce the amount of chemicals used compared to the conventional technology, which is more economical and more efficient An object of the present invention is to realize a highly practical and useful water treatment method for removing fluoride ions from water to be treated (raw water).

The above objects are achieved by the present invention described below. That is, the present invention provides the following water treatment method.
[1] A water treatment method for removing fluoride ions in water to be treated (raw water) containing at least fluoride ions and magnesium ions,
an alkali addition step of adding at least one alkali agent selected from the group consisting of alkali metal hydroxides and alkaline earth metal hydroxides to the water to be treated;
a solid-liquid separation step of solid-liquid separation of the suspended matter containing the fluoride ions generated in the alkali addition step;
an acid addition step of adding an acid in an acid addition tank to the sludge derived from the suspended matter that has been solid-liquid separated in the solid-liquid separation step to modify the sludge;
When the modified sludge after the acid addition step is subjected to solid-liquid separation to remove the sludge, and the supernatant liquid is returned to the alkali addition step and treated again together with the water to be treated,
The alkali agent addition amount C _OH (mg/L) as OH in the alkali addition step is less than 2000 mg/L, and the amount of the alkali agent with respect to the fluoride ion concentration C _F 1 (mg/L) in the raw water The mass ratio M (OH amount/F amount) of the added amount C _OH (mg/L) is more than 1 and less than 25, and the sedimentation tank withdrawal sludge concentration C _SS 3 (mg/L ) above 10000 mg/L and below 110000 mg/L, and
The raw water flow rate Q1 (m ³ /h), the fluoride ion concentration C _F 1 (mg/L) in the raw water, the mass ratio M (OH amount/F amount), and the drawn sludge concentration C _SS 3 (mg/L), the fluoride ion concentration C _F 5 (mg/L) of the supernatant of the modified sludge after the acid addition step, and the withdrawal amount Q5 (m ³ /h) of the supernatant. A water treatment method characterized in that the water treatment method is operated under the condition that the residence time t of the sludge in the acid addition tank is 30 minutes or more and 600 minutes or less, which is obtained by the following formula from (1) to (3).
t = e ^ ((Q1 + Q5) x C _OH ÷ (Q5 x M x A) - (Q1 x C _F 1 ÷ Q5 ÷ A) - (B ÷ A))
(In the formula, A and B are the concentration of sludge drawn from the sedimentation tank C _SS 3 (mg/L) in the solid-liquid separation process, the retention time t of the sludge in the acid addition tank, and the acid addition process, which are obtained from the test results. It is a coefficient obtained by the following formula showing the correlation with the fluoride ion concentration C _F 5 (mg/L) in the supernatant liquid of the sludge after reforming.)
C _F 5=A×In(t)+B

Preferred embodiments of the present invention include the following.
[2] The water treatment method according to [1] above, wherein the alkaline agent is calcium hydroxide.
[3] The water treatment method according to [1] or [2], further comprising a concentration step of concentrating the reformed sludge after the acid addition step.
[4] Any one of the above [1] to [3], wherein the water to be treated is discharged from a flue gas desulfurization unit that carries out a flue gas desulfurization method implemented in a coal-fired power plant or a coke plant The water treatment method described in .

According to the present invention, for example, in addition to fluoride ions, at least magnesium ions are included, such as waste water from a flue gas desulfurization apparatus that treats exhaust gas using magnesium hydroxide, and fluoride in the water to be treated Even if the concentration of ions and the amount of treated water to be treated fluctuate, the operating conditions are appropriately designed even though the basic configuration is the same as the treatment method of the prior art. It is possible to provide a more economical and industrially useful water treatment method for removing fluoride ions from water to be treated. Specifically, according to the present invention, by appropriately designing the operating conditions, for example, stable removal treatment can be performed even for fluctuating raw water. It is possible to reduce the scale of the treatment tank in the acid addition process for reforming (sludge), and furthermore, it is possible to reduce the amount of alkali agent added in the alkali addition process that generates sludge in the previous process. , it is possible to provide a water treatment method of high practical value that can stably treat in a desired efficient state and can reduce the final amount of sludge generated as in the case of treatment with conventional technology. be possible.

BRIEF DESCRIPTION OF THE DRAWINGS It is a schematic flow diagram of an example of the water-treatment method of this invention. In the acid addition step, shows the correlation between the retention time (t) of the sludge in the acid addition tank and the fluoride ion concentration C _F 5 (mg/L) in the supernatant of the modified sludge after the acid addition step. It is a logarithmic graph. When the fluoride ion concentration C _F 5 (mg/L) of the supernatant of the modified sludge after the acid addition step is approximated by A × In (t) + B, A and B are the sedimentation tank withdrawal sludge concentrations It is a graph which shows that there exists a correlation with _CSS3 . 4 is a graph showing the correlation between the amount of alkaline agent added and the amount of suspended solids (SS) produced. 4 is a graph showing the correlation between the retention time (t) of sludge in an acid addition tank and the sludge concentration C _SS 4 (mg/L) of modified sludge after the acid addition step in the acid addition step.

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in further detail below with reference to preferred embodiments. The present inventors have found that when treating water to be treated containing at least magnesium ions in addition to fluoride ions, such as wastewater from flue gas desulfurization equipment, an alkali agent is added in the alkali addition step, Suspended matter in which fluoride ions are incorporated in the water to be treated is generated, and in the subsequent acid addition step, an acid is added to the sludge containing the solid-liquid separated suspended matter to determine the fluorine content in the suspended sludge. In order to solve the above-mentioned technical problems, the conventional technology of the method of reforming the suspended sludge (sludge) that relatively increases the gone. Specifically, in consideration of the safety of treatment of fluctuating water to be treated, in actual wastewater treatment, two-stage treatment is performed in consideration of fluorine remaining in the treatment liquid that cannot be completely removed. Although it is possible to perform a one-stage treatment, it is necessary to provide an acid addition step, and it is difficult to determine efficient operating conditions. If it becomes possible to use it for practical purposes, we will consider improving the conventional technology to a technology with more practical value. In this case, the concentration of fluoride ions in the water to be treated and the amount of treated wastewater can be changed in a stable manner, and furthermore, for example, the treatment equipment can be made smaller than in the conventional technology. Furthermore, the inventors considered that it would be extremely useful if an economical treatment method could be found that would enable the amount of chemicals to be used to be reduced.

The present inventors have investigated the basic treatment flow shown in FIG. The properties of sludge containing suspended solids and the state of existence of fluorine components were examined in detail. Then, when the suspended matter containing fluoride ions in the water to be treated, which is generated in the alkali addition step, is solid-liquid separated and treated in the next acid addition step, the suspension extracted by solid-liquid separation We investigated in detail what kind of difference occurs in treatment due to the difference in the concentration of sludge containing substances (concentration of drawn sludge).

As a result, surprisingly, just by increasing the drawing sludge concentration, compared with the case where the drawing sludge concentration is low, the modified sludge after the acid addition step is recycled to the alkali addition step. New knowledge was obtained that the amount of fluorine in the clear liquid (the amount of soluble fluorine) can be significantly reduced. Then, in the treatment system defined in the present invention, when the sludge that has been modified after the acid addition step and the supernatant liquid separated is returned to the alkali addition step and treated again together with the water to be treated (raw water), withdrawal It was confirmed that the amount of alkaline agent to be added can be significantly reduced compared to when the sludge concentration is low. In addition, solid-liquid separation is performed to increase the concentration of the drawn sludge to be treated in the next acid addition process. It has been found that industrially extremely useful effects can be realized.

Regarding the above points, the fluoride ion concentration C _F 1 (mg/L) in the water to be treated (raw water) to be treated is 140 mg/L, and the soluble fluoride ion concentration C _F in the final treated water 2 (mg/L) to 10 mg/L will be described below by taking as an example a case where the processing flow shown in FIG. First, in the alkali addition step, an alkali agent is added and the fluorine in the raw water is incorporated into the generated suspended matter (SS) C _SS 1 . If the amount Q1 of the water to be treated is 100 m ³ /d and the amount of SS generated C _SS 1 is 2000 mg/L, the amount of SS generated per day is C _SS 1×Q1=2000 mg/L×100 m ³ /d=200 kg/d. In the next solid-liquid separation step, this 200 kg/d SS is separated. At this time, the sedimentation tank drawing sludge concentration C _SS 3 (mg / L) after solid-liquid separation is, for example, 10,000 mg / L, and 5 times higher, 50000 mg / L. think. Incidentally, the adjustment of the drawn sludge concentration can be easily carried out with a pump or the like.

Here, the form (solid/dissolved) of fluorine in the sludge withdrawn for one day is as follows.
・The amount of solid fluorine in the sludge is as follows, since the final treated water has a fluoride ion concentration C _F 2 (mg/L) of 10 mg/d for any of the above-mentioned extraction sludge concentrations. .
≈(C _F 1−C _F 2)×Q1=(140 mg/L−10 mg/L)×100 m ³ /d=13 kg/d
・When the drawing sludge concentration C _SS 3 (mg/L) after solid-liquid separation is 10000 mg/L, the soluble fluorine content in the drawing sludge is C _F 2 ≒ C _F 3, so it is as follows. Become.
≈C _F 3×Q3=10 mg/L×20 m ³ /d=0.2 kg/d
Since this amount does not change in the subsequent treatment, the amount of soluble fluorine returned to the alkali addition step is 0.2 kg/d, so the drawn sludge concentration C _SS after solid-liquid separation is 3 (mg/L ) is 10000 mg/L, the total amount of fluorine treated per day is 13.2 kg/d.

- When the concentration of drawn sludge after solid-liquid separation is 50,000 mg/L, the amount of soluble fluorine in the sludge is 1/5 of that when the concentration of drawn sludge is 10,000 mg/L.
≈C _F 2×Q3=10 mg/L×4 m ³ /d=0.04 kg/d
Therefore, when the drawn sludge concentration C _SS 3 (mg/L) after solid-liquid separation is 50000 mg/L, the total amount of fluorine treated per day is 13.04 kg/d. That is, the above shows that the treatment with a higher drawing sludge concentration C _SS 3 (mg/L) after solid-liquid separation is more effective than the treatment with a lower total fluorine amount per day in the alkali addition step. can be reduced.

In the above description, the reason why the amount of soluble fluorine returned to the alkali addition step is 0.2 kg/d when the concentration of drawn sludge after solid-liquid separation is 10000 mg/L will be explained. Since the amount of suspended solids (SS) generated in the water to be treated (raw water) is 2000 mg/L, when the sludge is concentrated to a concentration of 10000 mg/L, the concentration is 10000/2000=5 times. Therefore, the flow rate of sludge is 1/5, so 100 m ³ /d×(1/5)=20 m ³ /d, and the amount of soluble fluorine to be returned is 0.2 kg/d. When the drawn sludge concentration after solid-liquid separation is concentrated to 50,000 mg/L ^, the concentration is 50,000÷2,000=25 times. , calculated as above. That is, under the same conditions, by increasing the drawing sludge concentration C _SS 3 after solid-liquid separation, the flow rate of the sludge to be treated in the acid addition process performed to reform the drawing sludge and reduce the amount of sludge is markedly reduced.

The present inventors have recognized that the following effects can be obtained by obtaining the new knowledge described above. First, increasing the drawn sludge concentration C _SS 3 after solid-liquid separation means that the flow rate of sludge treated in the acid addition step can be reduced. An extremely useful industrial effect can be obtained in that the tank can be made smaller. Furthermore, as described above, by increasing the drawing sludge concentration C _SS 3 after solid-liquid separation, the amount of soluble fluorine in the supernatant liquid returned to the alkali addition step is reduced. Fluorine content is reduced as compared with the conventional treatment method, and the effect that the amount of alkali agent to be added can be reduced can be obtained. Therefore, in the present invention, based on the above-described configuration, further detailed studies were conducted to find out more stable operating conditions under which the above-described effects can be reliably obtained, resulting in the present invention.

The present invention contains at least magnesium ions in addition to fluoride ions, and relatively removes fluorine from water to be treated (raw water) in which the concentration of fluoride ions in the water to be treated and the amount of water to be treated vary. The water treatment method for removing fluoride ions by solid-liquid separation of the sludge taken in at a high concentration in the water treatment method basically has the same configuration as the conventional water treatment method described above. In the treatment method of the present invention, in the alkali addition step, suspended solids (SS) incorporating fluorine are generated, and in the next acid addition step, acid is added to each of the solid-liquid separated drawn sludge, and the acid addition tank The sludge is allowed to stay inside for a desired period of time to reform the sludge and reduce the volume of the sludge. At that time, in the case of treatment with any drawing sludge concentration C _SS 3, the addition of acid dissolves the sludge and the amount of SS is greatly reduced. Specifically, for example, when calcium hydroxide is used as the alkali agent, the amount of sludge in the acid treatment tank is about 3/5, and in the above example, the amount of SS generated in the alkali addition step is 200 kg/d becomes about 120 kg/d. In addition, as described above, by adding an acid, the magnesium hydroxide in the sludge is dissolved, and the solid fluorine that has been taken into the sludge is precipitated as magnesium fluoride (MgF ₂ ), and the sludge is reformed. be.

According to the study of the present inventors, the amount of soluble fluorine in the water to be treated after the acid addition process varies depending on the difference in the sludge concentration C _SS 3 withdrawn from the sedimentation tank, as described above. Because it will be very different, it will fluctuate greatly. First, the amount of soluble fluorine in the water to be treated after the acid addition step is the same as the fluorine amount in the supernatant liquid returned to the alkali addition step. Therefore, the amount of fluorine in the supernatant to be returned to the alkali addition step is the soluble fluoride ion concentration of the supernatant×flow rate. Although the details will be described later, the fluoride ion concentration C _F 5 of the supernatant in the above example can be calculated as follows. That is, according to the present invention, the sedimentation tank drawing sludge concentration C _SS 3 is 10000 mg/L and 50000 mg/L, and the coefficients A and B in each case are calculated from FIG. Inputting the coefficient and the residence time t of the sludge in the acid addition tank of 5 hours (300 minutes) into C _F 5=A×ln(t)+B, C _F 5 is 180 mg/L and 310 mg/L, respectively. Calculated. Therefore, in the above example, the amount of fluorine in the supernatant liquid returned to the alkali addition step is 180 mg/L×20 m ³ /d=3 in a system in which the sedimentation tank drawing sludge concentration C _SS 3 is 10000 mg/L. .6 kg/d and the sedimentation tank drawing sludge concentration C _SS 3 is 50000 mg/L, 310 mg/L × 4 m ³ /d = 1.2 kg/d, and the sedimentation tank drawing sludge concentration C _SS 3 is By increasing the concentration, the amount of fluorine in the returned supernatant is greatly reduced.

As described later, the concentration of soluble fluoride ions in the supernatant was tested using raw water to be treated and stepwise changing the concentration of sludge drawn after solid-liquid separation C _SS 3. , the amount of sludge (SS amount) at each point in the residence time t of the sludge in the acid addition tank is measured, the correlation between these values is obtained, and by using the obtained correlation, the withdrawal after solid-liquid separation It was found that it can be obtained from an approximate expression with the sludge concentration C _SS 3 as a variable. As a result, it was found that by performing the above-described treatment test in advance and obtaining the above-mentioned correlation, it is possible to appropriately determine the treatment conditions (operating conditions) that can be expected to have the desired effect for the raw water to be treated. Then, by processing under the determined processing conditions, it is possible to obtain effects such as downsizing of equipment and reduction of the amount of chemicals used according to the desired degree even for fluctuating raw water. It was confirmed that it is possible to perform stable processing with a high The details of the test method performed in advance to determine the processing conditions (operating conditions) in the actual processing will be described later.

As described above, the difference in the drawing sludge concentration C _SS 3 _causes a difference in the amount of soluble fluorine in the supernatant liquid to be returned. quantity is reduced. Here, since the total amount of fluorine does not change before and after acid addition, in the above example, the forms (solid/dissolved) of fluorine after acid addition are considered to be as follows. In a system where the concentration of drawn sludge C _SS 3 after solid-liquid separation is 10000 mg/L, the amount of soluble fluorine is 3.6 kg/d. d=9.6 kg/d. On the other hand, in the system in which the drawn sludge concentration C _SS 3 after solid-liquid separation was 50000 mg/L, the amount of soluble fluorine was 1.2 kg/d, so the solid fluorine was 13.04 kg/d-1. 2 kg/d=11.8 kg/d. The above shows that, compared with a treatment system with a drawing sludge concentration C _SS 3 after solid-liquid separation of 10,000 mg/L, a treatment system with a drawing sludge concentration C _SS 3 after solid-liquid separation of 50,000 mg/L has a solid content of It shows that the amount of fluorine increases and the amount of soluble fluorine in the water to be treated can be reduced. Since this soluble fluorine corresponds to the amount of fluoride ions returned to the alkali addition step as the supernatant water of the modified sludge after the acid addition step, the drawn sludge concentration C _SS 3 after solid-liquid separation is 50000 mg/L. By performing the treatment in the system of (1), the fluorine load in the alkali addition step can be reduced as compared with the treatment in the system in which the drawn sludge concentration after solid-liquid separation is 10000 mg/L. And this means that the amount of alkali agent used in the alkali addition step can be reduced.

As described above, as a result of detailed studies by the present inventors, it was found that only an extremely simple method of treating in the alkali addition step and increasing the drawing sludge concentration C _SS 3 after solid-liquid separation was effective for reforming after the acid addition step. It was found that the amount of soluble fluorine in the supernatant water of the purified sludge can be remarkably reduced. As a result, in the treatment method of the present invention, which has a basic configuration in which the supernatant water is returned to the alkali addition step and treated together with the raw water, the fluorine load applied to the overall treatment can be reduced, so the amount of alkali agent used can be reduced. It is possible to obtain a useful effect of practical value. Further, after the alkali addition step, the generated sludge is solid-liquid separated, the drawn sludge concentration C _SS 3 is increased, and the sludge is reformed in the acid addition step, so that the flow rate of the sludge to the acid addition step Q3 can be remarkably reduced, the size of the acid addition tank to be used can be reduced, which is also a great industrial advantage.

Specifically, according to the treatment method of the present invention, the amount of alkali agent added in the alkali addition step can be reduced to less than 2000 mg/L, and further to 1500 mg/L or less. Furthermore, by setting the drawing sludge concentration C _SS 3 in the solid-liquid separation process to be more than 10000 mg/L and less than 110000 mg/L, the above-described stable and effective treatment can be performed. By setting the drawing sludge concentration C _SS 3 in the solid-liquid separation process as described above, the size of the acid addition tank used in the acid addition process for reforming the drawing sludge can be remarkably reduced. For example, as described above with reference to an example, when the concentration of drawn sludge is concentrated by a factor of 5, the flow rate of sludge is reduced to 1/5, so the capacity of the acid addition tank can be reduced to 1/5. In addition, in the water treatment method of the present invention, in order to perform more stable and effective treatment, the mass ratio (OH amount/F amount) M of the addition amount of the alkali agent to the fluoride ion concentration of the raw water to be treated is , adjusted to be greater than 1 and less than 25.

From the knowledge obtained, the present inventors recognized that it is important in processing to consider the form of fluorine (solid/dissolved) after the acid addition step and the processing flow in terms of overall balance, and have developed the present invention. completed. In the series of treatment flows defined in the present invention, in which the final sludge supernatant is returned to the alkali addition step and treated again, all of the above-described operational requirements are taken into consideration and the present invention is applied. By treating water under treatment conditions designed to satisfy all the requirements specified in , while avoiding the generation of excessive sludge, the flow rate of the water to be treated (raw water) and the amount in the water to be treated (raw water) Even if the amount of fluoride ions fluctuates, it is possible to remove fluoride ions in a more stable and favorable state, and furthermore, it is possible to reduce equipment and running costs, which is industrially excellent and has high practical value. It becomes possible to provide a water treatment method.

In the following, it will be explained that all the processing conditions (operating conditions) listed above are required to obtain the remarkable effects of the present invention. First, in the prior art, the alkali agent is added in the alkali addition step by adjusting the pH in the reaction tank within a specific range. However, the balance of fluorine in the entire treatment system is (inflow fluorine load) + (return fluorine load) = (total fluorine load). It is necessary to consider the Specifically, since the balance of fluorine in the entire treatment system is as shown in the following formula, an increase in the fluorine load in the alkali addition step by optimizing the treatment balance and returning the supernatant for treatment can be avoided. should be as low as possible. For that purpose, at least Q5 (amount returned to the alkali addition step) of the supernatant liquid of the modified sludge after the acid addition step and in the supernatant liquid of the modified sludge after the acid addition step and the fluoride ion concentration C _F 5 of . The return fluorine load can be, for example, on the order of 10 g/min to 50 g/min in the examples previously described.

Q1× _CF1 +Q5× _CF5 =(Q1+Q5)× _CF
Q1: Flow rate of treated water (m ³ /h)
C _F 1: Fluoride ion concentration in water to be treated (mg/L)
Q5: Withdrawal amount (m ³ /h) of the supernatant liquid of the modified sludge after the acid addition process
C _F 5: Fluoride ion concentration (mg/L) in supernatant of modified sludge after acid addition step

As shown in the treatment flow of FIG. 1, the water treatment method of the present invention has an alkali addition step and an acid addition step defined in the present invention, and furthermore, after the acid addition step, the water is reformed and reduced in volume. The basic configuration is to return the supernatant liquid of the sludge to the alkali addition step and treat it again. As a result of a detailed study of the processing flow shown in FIG. 1, the inventors newly discovered that the following approximation formula holds. Coefficients A and B in the following approximation formula are obtained from test results described later.
_CF4 = _CF5 = _CF6 =A*In(t)+B

That is, the above A and B are coefficients obtained from the test results, and specifically, the drawing sludge concentration C _SS 3 (mg/L) in the solid-liquid separation process and the sludge retention time t in the acid addition tank and the fluoride ion concentration C _F 5 (mg/L) in the supernatant of the modified sludge after the acid addition step. An example is shown below to explain how to obtain the coefficient.

The extraction sludge concentration C _SS 3 in the solid-liquid separation process was set to 53000 mg/L, 82000 mg/L, and 107000 mg/L, and the conditions were the same except that three different concentrations were used, and each treatment was performed according to the treatment flow of FIG. gone. The amount of alkali agent added C _OH was 1500 mg / L, and in the next acid addition step, the sludge retention time (also referred to as sludge retention time) t in the acid addition tank was 15 minutes to 600 minutes. The fluoride ion concentration C _F 5 (mg/L) of the supernatant was measured. The results are shown in Table 1.

As a result of _examining the test results obtained above, the values shown in FIG. , it was found that there is a good correlation with logarithmic approximation. As shown in FIG. 2, in order to reduce the fluoride ion concentration C _F 5 of the supernatant, the residence time t of the sludge in the acid addition tank is set to 30 minutes or more and 600 minutes or less. , 60 minutes or more and 600 minutes or less.

As a result of further investigation, as shown in FIG. 3, the coefficients A and B of C _F 5=A×In(t)+B were best reproduced by linear approximation, so they were obtained in this way. It was found that by adopting A and B, the processing balance in the entire processing flow of FIG. 1 can be adjusted. As shown in FIG. 3, for example, in the test example described above, the values of A and B satisfy the following formula in relation to C _SS 3.
A=−0.0006×C _SS 3−36.689
B = _0.0066 x CSS3 + 357.9

In the treatment method of the present invention, it is essential to return the supernatant liquid and process it again together with the water to be treated (raw water) in the alkali addition step. xC _F 1 + Q5 x C _F 5 = (Q1 + Q5) x C _F. Moreover, C _F 4=C _F 5=C _F 6. Furthermore, since the above-described correlation of C _F 5 = A × In (t) + B was newly found, the following relationship between the retention time t of the sludge in the acid addition tank and these numerical values It was found that the formula holds true, and by treating under operating conditions in which this relationship holds, it is possible to avoid the generation of excessive sludge and reduce the increase in the fluorine load caused by the supernatant liquid returned to the alkali addition process. . That is, if each operating condition is appropriately determined so that the sludge retention time t is in the range of 30 minutes or more and 600 minutes or less, the remarkable effect of the present invention can be obtained.
t = e ^ ((Q1 + Q5) x C _OH ÷ (Q5 x M x A) - (Q1 x C _F 1 ÷ Q5 ÷ A) - (B ÷ A))

In the above formula, Q1 (m ³ /h) is the flow rate of the water to be treated, C _F 1 (mg/L) is the fluoride ion concentration in the raw water, and M is the fluoride ion concentration _CF is the mass ratio (OH amount/F amount) of the added amount C _OH (mg/L) of the alkaline agent to 1 (mg/L), and C _SS 3 (mg/L) is the drawing sludge concentration in the solid-liquid separation process where C _F 5 (mg/L) is the fluoride ion concentration of the supernatant of the modified sludge after the acid addition step, and Q5 (m ³ /h) is the withdrawal amount of the supernatant. is. Since this withdrawn supernatant is returned to the alkali addition step, the fluorine load in the treatment in the alkali addition step increases by the return fluorine load of Q5×C _F 5 . As described above, A and B in the above formula are coefficients that establish the relationship C _F 5 = A × In (t) + B obtained from the test results, and the concentration of the drawn sludge in the solid-liquid separation process It is a value obtained from measurements obtained with C _SS 3 (mg/L) as a variable.

Therefore, according to the treatment method of the present invention, the alkali agent addition amount C _OH (mg/L) as the OH of the alkali agent used in the alkali addition step, the fluoride ion concentration C _F 1 ( mass ratio M (OH amount/F amount) of the added amount C _OH (mg/L) of the alkaline agent to (mg/L), the extraction sludge concentration C _SS 3 (mg/L) in the solid-liquid separation process, the supernatant liquid By appropriately designing the withdrawal amount Q5 (m ³ /h) and operating the system, it is possible to avoid the generation of excessive sludge and reduce the increase in the fluorine load caused by the supernatant liquid returned to the alkali addition step. In addition, excellent effects of practical value such as a reduction in the amount of alkali agent to be added and a large reduction in the capacity of the acid addition tank in the oxidation treatment process are exhibited.

In the treatment method of the present invention, the amount of alkali agent added as OH C _OH (mg/L) in the alkali addition step is set to less than 2000 mg/L. One of the purposes of the treatment method of the present invention is to suppress the addition of excess alkali chemicals and to minimize the amount of alkali chemicals to be added. The present inventors have studied the relationship between the amount of alkali agent added and the amount of suspended solids (SS) generated, and found that setting the upper limit of the amount of alkali agent added to less than 2000 mg / L is a well-balanced and stable I confirmed that it is necessary for processing. Specifically, calcium hydroxide was used as an alkaline agent, and the amount of calcium hydroxide added and the amount of suspended solids (SS) generated were investigated. FIG. 4 is a graph showing the correlation between the amount of calcium hydroxide added and the amount of SS generated in simulated wastewater obtained by adding fluoride ions to magnesium sulfate. As shown in FIG. 4, it was confirmed that when the amount of calcium hydroxide added was 2000 mg/L or more, the increasing tendency of the amount of SS generated changed. As shown in FIG. 4, when the addition amount of the alkaline agent C _OH is 2000 mg/L or more, the amount of SS generation increases sharply, but the well-balanced and stable treatment that is the object of the present invention can be achieved. In order to do so, it is necessary to avoid a rapid increase in the amount of SS generated. In particular, the amount of _SS generated in the alkali addition step is an important treatment requirement in the treatment method of the present invention. A rapid increase in the amount of SS generated is not preferable because it also affects the determination of (mg/L).

In the treatment method of the present invention, when determining the addition amount _{C OH} of the alkali agent, as in the prior art, the alkali The mass ratio M (OH amount/F amount) of the additive amount C _OH (mg/L) of the agent is set within a range of more than 1 and less than 25. In the treatment method of the present invention, it is preferable to set it to about 8 to 12.

As described above, the treatment method of the present invention can improve the quality of the sludge after the acid addition step by simply increasing the drawing sludge concentration C _SS 3 (mg/L) compared to when the drawing sludge concentration is low. This was made based on the new knowledge that the amount of fluorine (the amount of soluble fluorine) in the supernatant liquid returned to the alkali addition process in sludge can be significantly reduced. , the volume of the tank, etc., which will affect the treatment equipment, just by appropriately determining the extraction sludge concentration C _SS 3 (mg / L), the above-mentioned remarkable effect of the present invention can be obtained. This was achieved by discovering that driving becomes possible.

Furthermore, the treatment method of the present invention aims to reduce the increase in fluorine loading caused by the supernatant liquid returned to the alkali addition step, while avoiding the generation of excessive sludge. The basic configuration is to reduce the amount of sludge C _SS 4 after acid addition by modifying the . Therefore, the present inventors investigated the relationship between the retention time t (min) of the sludge in the acid addition tank and the concentration C _SS 3 (mg/L) of solid-liquid separated drawing sludge to which acid is added, as described below. did the test. That is, the treatment was performed according to the treatment flow of FIG. 1 under the same conditions except that the extraction sludge concentration C _SS 3 was changed to 53,000 mg/L, 82,000 mg/L, and 107,000 mg/L with three different sludge concentrations. At that time, the amount of alkali agent added C _OH was 1500 mg / L, the sludge retention time t in the next acid addition step was 15 minutes to 600 minutes, and the amount of sludge (SS) after acid addition at each elapsed time C _SS 4 was measured. The results are shown in Table 2 and FIG.

In order to reduce the increase in fluorine load caused by the supernatant liquid returned to the alkali addition step, from Table 1 and FIG. do. In order to avoid excessive sludge (SS) generation, from Table 2 and FIG. 5, it is preferable to set the sludge retention time t in the acid addition tank to 60 minutes or more and 600 minutes or less. In the treatment method of the present invention, the following formula showing the above is used so that the residence time t of the sludge in the acid addition tank is in the range of 30 minutes or more and 600 minutes or less, more preferably 60 minutes or more. Considering the effect desired in the actual treatment, each requirement in the following formula is determined so as to be in the range of 600 minutes or less, the operating conditions are designed, and the operation is performed under these conditions. It has been achieved to stably obtain a remarkable effect.
t = e ^ ((Q1 + Q5) x C _OH ÷ (Q5 x M x A) - (Q1 x C _F 1 ÷ Q5 ÷ A) - (B ÷ A))

As described above, the basic configuration of the processing method of the present invention is the same as the method described in the prior art mentioned above. The alkali agent used in the alkali addition step of the treatment method of the present invention can be selected from the group consisting of alkali metal hydroxides and alkaline earth metal hydroxides. For example, sodium hydroxide, calcium hydroxide, or the like can be used.

As described above, in the treatment method of the present invention, the suspended matter (SS) containing fluoride ions generated in the alkali addition step is solid-liquid separated, and in the next acid addition step, solid-liquid Acid is added to the sludge derived from the separated suspended matter, and when the sludge is reformed by staying in the acid addition tank for a certain period of time, the extraction sludge concentration C _SS 3 (mg / L ) is processed with a high value. According to studies by the present inventors, the effects of the present invention can be obtained by setting the drawing sludge concentration C _SS 3 in the solid-liquid separation process to more than 10000 mg/L and less than 110000 mg/L. Depending on the degree of effect desired, for example, the effect of reducing the capacity of the acid addition bath or reducing the increased fluorine load caused by the supernatant liquid returned to the alkali addition step, which is obtained by the present invention, may be more pronounced. In order to make it remarkable, the drawing sludge concentration C _SS 3 in the solid-liquid separation process should be 50000 mg/L or more, preferably 60000 mg/L or more.

In the treatment method of the present invention, by increasing the drawing sludge concentration C _SS 3, as described in detail above, the fluorine content C _F 5 in the supernatant of the modified sludge (sludge) after the acid addition step is reduced to can be reduced. By increasing the drawing sludge concentration C _SS 3, the capacity V of the acid addition tank used in the acid addition step can be reduced. For example, when the extraction sludge concentration C _SS 3 is 10000 mg/L and when C _SS 3 is 50000 mg/L, the capacity V of the acid addition tank can be reduced to 1/5. Specifically, for example, when the amount of sludge treated per day is about 200 kg/d, if C _SS 3 is 50000 mg/L, the volume V of the acid addition tank should be reduced to about 10 m ³ . can be done.

In the acid addition step constituting the treatment method of the present invention, as described above, when calcium hydroxide is used, 3/5 of the suspended matter generated in the alkali addition step is dissolved, so excessive sludge is not generated. can be avoided. In the present invention, the suspended matter remaining undissolved may be separated in the acid addition tank used in the acid addition step, but as shown in FIG. It is preferable to condense the suspended matter remaining without dehydration, and to return the supernatant to the alkali addition step. As described above, by increasing the drawing sludge concentration C _SS 3 in the solid-liquid separation step, as a result, the return fluorine load (C _F 5×Q5) of the supernatant liquid to be returned can be reduced. As a result, it is also possible to reduce the amount of alkali agent added in the alkali addition step in which the water to be treated (raw water) and the supernatant water are treated together.

EXAMPLES Next, the present invention will be described more specifically with reference to examples and comparative examples. As water to be treated (raw water), waste water from a flue gas desulfurization unit using magnesium hydroxide was used.
[Examples 1 and 2, Comparative Examples 1 and 2]
In Examples 1 and 2, raw water having a fluorine concentration C _F 1 of 140 mg/L was treated at a flow rate Q1=1 m ³ /min under the treatment conditions shown in Table 3 based on the flow shown in FIG. . As a result, in Examples 1 and 2 in which the treatment was performed within the range of the conditions specified in the present invention, compared with the treatment in Comparative Examples 1 and 2 in which the treatment was performed without satisfying the conditions specified in the present invention. , as shown in Table 3, while the flow rate Q3 into the acid addition tank decreases, the sludge residence time t decreases, resulting in the acid addition tank volume determined as the product of the flow rate Q3 and the residence time t It was confirmed that V could be remarkably reduced, and furthermore, the returning fluorine load could be remarkably reduced. Also, from a comparison between Example 1 and Example 2, for example, the volume V of the acid addition tank is 10 m ³ . It was confirmed that _SS 3 should be increased from 50000 mg/L in Example 2 to 65000 mg/L. This means that, according to the treatment method of the present invention, it is possible to obtain an industrially extremely useful effect that the equipment can be reduced simply by changing the operating conditions by simply increasing the drawing sludge concentration C _SS 3. showing. On the other hand, from the results of Comparative Examples 1 and 2, it was confirmed that the effects of the present invention cannot be obtained when even one of the requirements defined by the treatment method of the present invention is not satisfied.

[Examples 3 and 4, Comparative Example 3]
In Examples 3 and 4, raw water having a fluorine concentration C _F 1 of 70 mg/L was treated at a flow rate Q1 of 1 m ³ /min under the treatment conditions shown in Table 4 based on the flow shown in FIG. As a result, in Examples 3 and 4 in which the treatment was performed within the range of the conditions specified in the present invention, Table 4 compared with the treatment in Comparative Example 3 in which the treatment was performed under conditions that did not satisfy the conditions specified in the present invention. As shown in , it was confirmed that the volume V of the acid addition tank could be significantly reduced, and the returned fluorine load could be reduced. Further, from the results of Comparative Example 3, it was confirmed that, as with Comparative Examples 1 and 2, the excellent effects of the present invention cannot be obtained if even one of the requirements defined by the treatment method of the present invention is not satisfied. .

Claims (4)

A water treatment method for removing fluoride ions in water to be treated (raw water) containing at least fluoride ions and magnesium ions,
an alkali addition step of adding at least one alkali agent selected from the group consisting of alkali metal hydroxides and alkaline earth metal hydroxides to the water to be treated;
a solid-liquid separation step of solid-liquid separation of the suspended matter containing the fluoride ions generated in the alkali addition step;
an acid addition step of adding an acid in an acid addition tank to the sludge derived from the suspended matter that has been solid-liquid separated in the solid-liquid separation step to modify the sludge;
When the modified sludge after the acid addition step is subjected to solid-liquid separation to remove the sludge, and the supernatant liquid is returned to the alkali addition step and treated again together with the water to be treated,
The alkali agent addition amount C _OH (mg/L) as OH in the alkali addition step is less than 2000 mg/L, and the amount of the alkali agent with respect to the fluoride ion concentration C _F 1 (mg/L) in the raw water The mass ratio M (OH amount/F amount) of the added amount C _OH (mg/L) is more than 1 and less than 25, and the sedimentation tank withdrawal sludge concentration C _SS 3 (mg/L ) above 10000 mg/L and below 110000 mg/L, and
The raw water flow rate Q1 (m ³ /h), the fluoride ion concentration C _F 1 (mg/L) in the raw water, the mass ratio M (OH amount/F amount), and the drawn sludge concentration C _SS 3 (mg/L), the fluoride ion concentration C _F 5 (mg/L) of the supernatant of the modified sludge after the acid addition step, and the withdrawal amount Q5 (m ³ /h) of the supernatant. A water treatment method characterized in that the water treatment method is operated under the condition that the residence time t of the sludge in the acid addition tank is 30 minutes or more and 600 minutes or less, which is obtained by the following formula from (1) to (3).
t = e ^ ((Q1 + Q5) x C _OH ÷ (Q5 x M x A) - (Q1 x C _F 1 ÷ Q5 ÷ A) - (B ÷ A))
(In the formula, A and B are the concentration of sludge drawn from the sedimentation tank C _SS 3 (mg/L) in the solid-liquid separation process, the retention time t of the sludge in the acid addition tank, and the acid addition process, which are obtained from the test results. It is a coefficient obtained by the following formula showing the correlation with the fluoride ion concentration C _F 5 (mg/L) in the supernatant liquid of the sludge after reforming.)
C _F 5=A×In(t)+B The water treatment method according to claim 1, wherein the alkaline agent is calcium hydroxide. 3. The water treatment method according to claim 1, further comprising a concentration step of concentrating the reformed sludge after the acid addition step. The water according to any one of claims 1 to 3, wherein the water to be treated is discharged from a flue gas desulfurization apparatus that performs a flue gas desulfurization method implemented in a coal-fired power plant or a coke plant. Processing method.

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