Fluorine recovery method

Abstract

To provide a fluorine recovery method capable of obtaining a fluorine ion-containing liquid that contains as few sulfate ions as possible and more fluorine ions, when an alkali solution is made contact with an adsorbent having absorbed fluorine ions and sulfate ions.SOLUTION: A fluorine recovery method includes: a first step of introducing an alkali solution to an adsorption tower filled with an adsorbent having absorbed fluorine ions and sulfate ions to obtain a discharge liquid from the adsorption tower; and a second step of introducing water or an alkali solution (here, an alkali solution with lower concentration than the alkali solution used in the first step) to the adsorption tower to obtain a discharge liquid from the adsorption tower, and the first step and the second step are repeated. The discharge liquid obtained at the first step and / or the second step after k+1 times (here, k being an integer of 1 or more) of repetition is dispensed as a fluorine ion-containing liquid to recover fluorine.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a fluorine recovery method in which an adsorbent having adsorbed fluoride ions and sulfate ions is brought into contact with an alkaline solution to recover a fluoride ion-containing liquid. [Background technology]

[0002] Conventionally, a method is known in which water to be treated containing fluoride ions and sulfate ions is brought into contact with an adsorbent to adsorb and remove fluoride ions from the water to be treated (e.g., Patent Documents 1 and 2). The adsorbent that has been brought into contact with the water to be treated containing fluoride ions and sulfate ions can be brought into contact with an alkaline solution to desorb fluoride ions from the adsorbent, thereby obtaining a desorbed liquid containing fluoride ions. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2018-130677 A [Patent Document 2] JP 2019-198826 A Summary of the Invention [Problem to be solved by the invention]

[0004] As described above, when the water to be treated, which contains fluoride ions and sulfate ions, is brought into contact with the adsorbent, a part of the sulfate ions is usually adsorbed on the adsorbent together with the fluoride ions. Therefore, when the adsorbent is subsequently brought into contact with an alkaline solution in order to regenerate it, sulfate ions are desorbed from the adsorbent together with the fluoride ions, and the resulting eluate contains a large amount of sulfate ions as well as fluoride ions. Since fluoride ions are regulated as a regulated substance in the uniform wastewater standards set by the Ministry of the Environment and the sewage discharge standards of the Sewerage Act, eluate containing a large amount of fluoride ions cannot be disposed of as is. Therefore, eluate containing a large amount of fluoride ions usually requires further treatment to separate and recover the fluoride ions. However, if the eluate contains a large amount of sulfate ions together with fluoride ions, it becomes difficult to efficiently treat the eluate, which leads to an increase in the treatment cost of the eluate and also hinders the use of the recovered fluorine as a fluoride source. Therefore, it is desirable that the eluate does not contain as many sulfate ions as possible.

[0005] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a fluoride recovery method which can obtain a fluoride ion-containing liquid which contains as little sulfate ions as possible and as much fluoride ions as possible when an adsorbent having adsorbed fluoride ions and sulfate ions is brought into contact with an alkaline solution. [Means for solving the problem]

[0006] The fluorine recovery method of the present invention, which has been able to solve the above problems, comprises a first step of introducing an alkaline solution into an adsorption tower filled with an adsorbent that has adsorbed fluoride ions and sulfate ions to obtain a discharge liquid from the adsorption tower, and a second step of introducing water or an alkaline solution (however, an alkaline solution having a lower concentration than the alkaline solution used in the first step) into the adsorption tower to obtain a discharge liquid from the adsorption tower, in which the first step and the second step are repeatedly performed, and is characterized in that the discharge liquid obtained in the first step and / or the second step from the (k+1)th time (k is an integer of 1 or more) onwards is separated as a fluoride ion-containing liquid to recover fluorine.

[0007] In the fluorine recovery method of the present invention, the pH of the discharged liquid from the adsorption tower can be easily controlled by repeatedly performing the above-mentioned first and second steps, and therefore, the overlap of the desorption peaks of sulfate ions and fluoride ions from the adsorbent can be reduced, and the fluoride ions desorbed from the adsorbent can be preferentially recovered by separating the discharged liquid obtained in the first step and / or the second step from the k+1th time (where k is an integer of 1 or more) onwards as a fluoride ion-containing liquid.

[0008] The discharged liquid obtained at the end of the kth second step is preferably pH 7.0 or less. This suppresses desorption of fluoride ions from the adsorbent until the kth second step is completed, and allows a larger amount of fluoride ions desorbed from the adsorbent to be transferred to the fluoride ion-containing liquid obtained in the k+1th or later first step and / or second step. As a result, the recovery rate of fluoride ions in the fluoride ion-containing liquid can be increased.

[0009] The discharged liquid obtained at the end of the kth second step is preferably pH 4.0 or more. This makes it easy to reduce the amount of sulfate ions desorbed from the adsorbent that transfers to the fluoride ion-containing liquid obtained in the k+1th or later first step and / or second step. As a result, it becomes easy to obtain a fluoride ion-containing liquid with a reduced sulfate ion concentration.

[0010] At least a part of the discharged liquid obtained in the first step and the second step from the (k+1)th time onward is preferably at pH 12.0 or higher, which allows more fluoride ions to be desorbed from the adsorbent and increases the recovery rate of fluoride ions.

[0011] The sum of the amount of alkali in the alkaline solution used in the first step from the (k+1)th time onwards and the amount of alkali in the alkaline solution used in the second step is preferably greater than the sum of the amount of alkali in the alkaline solution used in the first step up to the kth time and the amount of alkali in the alkaline solution used in the second step. By using the alkaline solution in this manner, it becomes easy to minimize the amount of sulfate ions contained in the effluent obtained in the first step and the second step from the (k+1)th time onwards and to allow the effluent to contain more fluoride ions.

[0012] In the second step, it is preferable to introduce water into the adsorption tower, or to introduce into the adsorption tower an alkaline solution having a concentration 1 / 10 or less of the alkaline solution used in the first step, which effectively slows down the rate of increase in pH of the liquid discharged from the adsorption tower, making it easier to control the pH of the liquid discharged.

[0013] The flow rate of water or alkaline solution introduced into the adsorption tower in the second step is preferably higher than the flow rate of alkaline solution introduced into the adsorption tower in the first step, which promotes desorption of sulfate ions or fluoride ions from the adsorbent and shortens the time required for the second step.

[0014] The fluorine ion adsorption amount of the adsorbent used in the first step for the first time is preferably 0.1 to 10 times the sulfate ion adsorption amount. This makes it possible to more effectively achieve the effect of the present invention of preferentially recovering fluorine ions from the adsorbent that has adsorbed fluorine ions and sulfate ions. In addition, the fluorine ion adsorption amount per unit dry mass of the adsorbent used in the first step for the first time is preferably 5 g-F / kg to 50 g-F / kg, and the sulfate ion adsorption amount is preferably 5 g-SO4 / kg to 50 g-SO4 / kg. The concentration of the alkaline solution used in the first step is preferably 0.05 N to 0.5 N. In addition, it is preferable to use a cerium-based adsorbent as the adsorbent. Effect of the Invention

[0015] According to the fluorine recovery method of the present invention, the first step of introducing an alkaline solution and the second step of introducing water or an alkaline solution (however, the alkaline solution has a lower concentration than that used in the first step) are repeatedly performed, thereby making it easy to control the pH of the discharge liquid from the adsorption tower. Therefore, it is possible to reduce the overlap of the desorption peaks of sulfate ions and fluoride ions from the adsorbent, and by separating the discharge liquid obtained in the first step and / or the second step after the k+1th time (k is an integer of 1 or more) as a fluoride ion-containing liquid, it is possible to preferentially recover the fluoride ions desorbed from the adsorbent. [Brief description of the drawings]

[0016] [Figure 1] 4 shows the results of changes in pH of the discharged liquid, the fluoride ion desorption rate, and the sulfate ion desorption rate in treatment No. 1 of the embodiment. [Diagram 2] 4 shows the results of changes in pH of the discharged liquid, the fluoride ion desorption rate, and the sulfate ion desorption rate in treatment No. 2 of the embodiment. [Diagram 3] 1 shows the results of changes in pH of the discharged liquid, the fluoride ion desorption rate, and the sulfate ion desorption rate in treatment No. 3 of the embodiment. [Figure 4] 1 shows the results of changes in pH of the discharged liquid, the fluoride ion desorption rate, and the sulfate ion desorption rate in treatment No. 4 of the embodiment. [Diagram 5] 1 shows the results of changes in pH of the discharged liquid, the fluoride ion desorption rate, and the sulfate ion desorption rate in treatment No. 5 of the embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] The fluorine recovery method of the present invention involves introducing an alkaline solution into an adsorption tower packed with an adsorbent having adsorbed fluoride ions and sulfate ions, thereby obtaining a discharge liquid from the adsorption tower. When the adsorbent having adsorbed fluoride ions and sulfate ions is brought into contact with an alkaline solution, the fluoride ions and sulfate ions are desorbed from the adsorbent, and the adsorbent is regenerated. Therefore, when an alkaline solution is introduced into an adsorption tower packed with an adsorbent having adsorbed fluoride ions and sulfate ions, a discharge liquid containing fluoride ions and sulfate ions is obtained from the adsorption tower. The fluorine recovery method of the present invention recovers fluorine from the discharge liquid thus obtained from the adsorption tower as a fluoride ion-containing liquid that contains as little sulfate ions as possible and as much fluoride ions as possible.

[0018] In the present invention, in order to obtain a fluoride ion-containing liquid that contains as little sulfate ion as possible and more fluoride ion, an alkaline solution and water, or a concentrated alkaline solution and a dilute alkaline solution, are repeatedly introduced in sequence into an adsorption tower filled with an adsorbent that has adsorbed fluoride ions and sulfate ions. In this case, the discharged liquid from the adsorption tower is not recovered from the beginning, but the discharged liquid discharged from the adsorption tower when the alkaline solution and water, or the concentrated alkaline solution and the dilute alkaline solution are introduced in sequence from the k+1th time (where k is an integer of 1 or more) onwards is recovered as a fluoride ion-containing liquid. In this way, by separating the discharged liquid from the adsorption tower as a fluoride ion-containing liquid, the fluoride ions desorbed from the adsorbent can be preferentially recovered. That is, the fluorine recovery method of the present invention comprises a first step of introducing an alkaline solution into an adsorption tower filled with an adsorbent having adsorbed fluoride ions and sulfate ions to obtain an effluent from the adsorption tower, and a second step of introducing water or an alkaline solution (however, an alkaline solution having a lower concentration than the alkaline solution used in the first step) into the adsorption tower to obtain an effluent from the adsorption tower, in which the first step and the second step are repeatedly performed, and the effluent obtained in the first step and / or the second step from the k+1th time (k is an integer of 1 or more) onwards is separated as a fluoride ion-containing liquid to recover fluorine. Note that in the present invention, the whole process of repeatedly performing the first step and the second step is referred to as a "desorption step".

[0019] In the first step, an alkaline solution is introduced into an adsorption tower filled with an adsorbent that has adsorbed fluoride ions and sulfate ions, and a discharge liquid from the adsorption tower is obtained. In the adsorbent that has adsorbed fluoride ions and sulfate ions, fluoride ions and sulfate ions are electrostatically captured at the adsorption sites of the adsorbent. When an alkaline solution is brought into contact with this adsorbent, an ion exchange reaction occurs between the hydroxide ions of the alkaline solution and the fluoride ions or sulfate ions at the adsorption sites of the adsorbent, and the fluoride ions or sulfate ions are desorbed from the adsorbent. Therefore, when an alkaline solution is introduced into an adsorption tower filled with an adsorbent that has adsorbed fluoride ions and sulfate ions, the pH of the discharge liquid discharged from the adsorption tower decreases. The pH of the discharge liquid from the adsorption tower is lowest at the beginning and tends to gradually increase thereafter. As a result of ion exchange of more hydroxide ions in the first step, the pH of the discharge liquid is usually at its lowest, less than 4.0.

[0020] The alkaline solution is not particularly limited as long as it is a solution that exhibits alkalinity, but it is preferable to use a solution of an alkali metal hydroxide. As the alkali metal hydroxide, lithium hydroxide, potassium hydroxide, sodium hydroxide, rubidium hydroxide, cesium hydroxide, etc. can be used, and these may be used alone or in combination of two or more. In addition, it is preferable to use sodium hydroxide as the alkali metal hydroxide from the viewpoint of cost.

[0021] The alkaline solution used in the first step preferably has a relatively high hydroxide ion concentration, since the higher the hydroxide ion concentration, the faster the ion exchange reaction between the hydroxide ions and the fluorine ions or sulfate ions occurs, and an effluent containing a high concentration of fluorine ions is obtained. The concentration (hydroxide ion concentration) of the alkaline solution used in the first step is preferably, for example, 0.05N or more, more preferably 0.1N or more. On the other hand, if the concentration of the alkaline solution is too high, the ability to separate fluorine ions and sulfate ions may decrease, so the concentration of the alkaline solution is preferably 0.5N or less, more preferably 0.4N or less, and even more preferably 0.3N or less.

[0022] In the second step, after the first step, water or an alkaline solution is introduced into the adsorption tower filled with the adsorbent that has adsorbed fluoride ions and sulfate ions, and a discharge liquid from the adsorption tower is obtained. When an alkaline solution is used in the second step, it is preferable that the alkaline solution is the same as the alkaline solution used in the first step. However, the alkaline solution used in the second step has a lower concentration than the alkaline solution used in the first step. Therefore, in this case, a concentrated alkaline solution is introduced in the first step, and a dilute alkaline solution is introduced in the second step. The concentration of the alkaline solution here means the hydroxide ion concentration. In the second step, the pH of the discharge liquid usually tends to gradually increase due to the influence of the alkaline solution remaining in the adsorption tower. However, compared to the case where the first step is continued without performing the second step, the rate of increase in the pH of the discharge liquid from the adsorption tower can be slowed.

[0023] In the second step, it is preferable to introduce water into the adsorption tower, or to introduce an alkaline solution having a concentration of 1 / 10 or less of the alkaline solution used in the first step into the adsorption tower. This makes it possible to more effectively slow down the rate of increase in pH of the discharged liquid from the adsorption tower. More preferably, water is introduced into the adsorption tower in the second step.

[0024] The amount of water or alkaline solution introduced into the adsorption tower (or the liquid passing ratio) in the second step is preferably larger than the amount of alkaline solution introduced into the adsorption tower (or the liquid passing ratio) in the immediately preceding first step, which makes it possible to more effectively slow down the rate of increase in pH of the liquid discharged from the adsorption tower.

[0025] In the desorption step, the above-mentioned first and second steps are repeated. By repeating the first and second steps, the pH of the discharged liquid from the adsorption tower gradually increases overall, while the degree of increase in the pH of the discharged liquid can be controlled. This reduces the overlap of the desorption peaks of sulfate ions and fluoride ions from the adsorbent, making it possible to preferentially recover fluoride ions. When an adsorbent that has adsorbed fluoride ions and sulfate ions is contacted with an alkaline solution, sulfate ions tend to desorb from the adsorbent before fluoride ions, but by repeating the first and second steps to control the increase in the pH of the discharged liquid, it becomes possible to significantly shift the desorption peaks of sulfate ions from the desorption peaks of fluoride ions.

[0026] In the desorption step, the discharged liquid obtained in the first step and / or the second step from the k+1th time (k is an integer of 1 or more) onwards is separated as a fluoride ion-containing liquid to recover fluorine. This makes it possible to obtain a discharged liquid as a fluoride ion-containing liquid that contains as little sulfate ion as possible and more fluoride ions. The fluoride ion-containing liquid may be separated from the middle of the first step or the second step from the k+1th time onwards. Preferably, the entire amount of the discharged liquid obtained in the first step and the second step from the k+1th time onwards is separated as a fluoride ion-containing liquid to recover fluorine.

[0027] On the other hand, the effluent obtained in the first step and / or the second step up to the kth time can be separated as a sulfate ion-containing liquid. This allows the effluent to be obtained as a sulfate ion-containing liquid that contains as little fluoride ion as possible and more sulfate ions. In this case, it is preferable to separate the effluent obtained in the first step and the second step up to the kth time as a sulfate ion-containing liquid.

[0028] The value of k can be set according to the treatment conditions of the first and second steps from the first to the kth time, basically the amount of alkali (amount of hydroxide ions) introduced into the adsorption tower. For example, if a predetermined amount of alkali capable of desorbing sulfate ions from the adsorbent is introduced at once, the value of k can be set to 1, and if the predetermined amount of alkali is introduced in portions, the value of k is 2 or more. From the viewpoint of simple treatment, it is preferable that the value of k is 1. The amount of alkali can be calculated from the product of the hydroxide ion concentration of the alkaline solution and the amount of the alkaline solution.

[0029] The number of times that the first and second steps are performed after the k+1th time can be one time if a predetermined amount of alkali capable of desorbing fluoride ions from the adsorbent is introduced at once (i.e., the first and second steps are performed once each), and two or more times can be performed if a predetermined amount of alkali is introduced in portions. From the viewpoint of simplifying the treatment, it is preferable that the first and second steps are performed once after the k+1th time. Therefore, in the present invention, it is particularly preferable to separate the effluent obtained in the second first and / or second steps as a fluoride ion-containing liquid.

[0030] In the desorption process, when the first and second steps are each repeated a total of n times (where n is an integer of 2 or more, and k is an integer of 1 to n-1), the treatment conditions for the first step from the 1st to nth steps may be the same or different, and the treatment conditions for the second step from the 1st to nth steps may be the same or different. Usually, the first step from the 1st to kth steps and the first step from the k+1th to nth steps are each optimized and have different treatment conditions. Also, the second step from the 1st to kth steps and the second step from the k+1th to nth steps are each optimized and have different treatment conditions.

[0031] The timing of collecting the discharged liquid from the adsorption tower as a fluoride ion-containing liquid can be determined based on the pH of the discharged liquid discharged from the adsorption tower as an index. For example, the discharged liquid obtained at the end of the kth second step preferably has a pH of 7.0 or less, more preferably 6.8 or less, and even more preferably 6.6 or less. This prevents fluoride ions from being desorbed from the adsorbent until the kth second step is completed, and more fluoride ions desorbed from the adsorbent can be transferred to the fluoride ion-containing liquid obtained in the k+1th or later first step and / or second step. As a result, the fluoride ion recovery rate in the fluoride ion-containing liquid can be increased. From the same viewpoint, it is preferable to start collecting the discharged liquid from the adsorption tower as a fluoride ion-containing liquid when the pH is 7.0 or less, more preferably 6.8 or less, and even more preferably 6.6 or less.

[0032] The pH of the discharged liquid obtained at the end of the k-th second step is preferably 4.0 or more, more preferably 4.5 or more, and even more preferably 5.0 or more. This makes it easy to reduce the amount of sulfate ions desorbed from the adsorbent that transfers to the fluoride ion-containing liquid obtained in the k+1th or later first step and / or second step. As a result, it becomes easy to obtain a fluoride ion-containing liquid with a reduced sulfate ion concentration. From the same viewpoint, it is preferable to start separating the discharged liquid from the adsorption tower as a fluoride ion-containing liquid when the pH of the discharged liquid from the adsorption tower is 4.0 or more, and the pH is more preferably 4.5 or less, and even more preferably 5.0 or less. In this case, the pH of a part of the discharged liquid discharged from the adsorption tower becomes less than 4.0 by the end of the k-th second step.

[0033] The amount of alkali introduced into the adsorption tower by the end of the kth second step, i.e., the amount of alkali introduced into the adsorption tower in the first and second steps from the 1st to the kth steps, is preferably 1.0 times or more by mol of the sulfate ion adsorption amount of the adsorbent, more preferably 1.2 times or more by mol. This allows more sulfate ions to be desorbed from the adsorbent by the end of the kth second step, and makes it easier to reduce the sulfate ion content of the fluoride ion-containing liquid obtained in the first and / or second steps from the k+1th step onwards. On the other hand, the amount of alkali introduced into the adsorption tower by the end of the kth second step is preferably 3.0 times or less by mol of the sulfate ion adsorption amount of the adsorbent, more preferably 2.5 times or less by mol. This prevents fluoride ions from being desorbed from the adsorbent until the end of the kth second step, and makes it easier to increase the fluoride ion content of the fluoride ion-containing liquid obtained in the first and / or second steps from the k+1th step onwards. The sulfate ion adsorption amount of the adsorbent described here means the amount of sulfate ions adsorbed onto the adsorbent, more specifically, the amount of sulfate ions adsorbed onto the adsorbent used in the first step for the first time.

[0034] In the desorption step, the pH of the discharge liquid coming out of the adsorption tower is measured, and the first and second steps of the k+1th time are determined based on the pH value, and the discharge liquid can be separated as a fluoride ion-containing liquid. If the relationship between the treatment conditions of the desorption step and the fluctuation of the pH value is known in advance from an experiment or the like, the pH of the discharge liquid coming out of the adsorption tower can be measured, but the desorption step can be routinely set to treatment conditions that allow the discharge liquid obtained in the first and / or second steps of the k+1th time and thereafter to be separated as a fluoride ion-containing liquid, and the discharge liquid obtained in the first and / or second steps of the k+1th time can be separated as a fluoride ion-containing liquid.

[0035] It is preferable that at least a part of the discharged liquid obtained in the first step and the second step after the k+1th time has a pH of 12.0 or more. This allows more fluoride ions to be desorbed from the adsorbent, and the recovery rate of fluoride ions can be increased. The pH of the discharged liquid may once be 12.0 or more in the first step, the second step, or both. It is preferable to continue introducing the alkaline solution or water into the adsorption tower even after the pH of the discharged liquid once becomes 12.0 or more. In this case, the pH of the discharged liquid thereafter may be maintained at 12.0 or more, or may fall below 12.0. It is more preferable that the pH of the discharged liquid once becomes 12.5 or more.

[0036] In order to make the pH of the discharge liquid once 12.0 or more, it is preferable to appropriately set the concentration and amount of the alkaline solution introduced into the adsorption tower so that the amount of alkali introduced into the adsorption tower is a predetermined value or more. The amount of alkali introduced into the adsorption tower can be set based on the fluorine ion adsorption amount and sulfate ion adsorption amount of the adsorbent. For example, the total amount of alkali introduced into the adsorption tower in the first and second steps up to the kth time and the total amount of alkali introduced into the adsorption tower in the first and second steps from the k+1th time onwards is preferably 1.0 times or more by mole, more preferably 1.2 times or more by mole, of the total amount of fluorine ion adsorption amount and sulfate ion adsorption amount of the adsorbent. Note that the fluorine ion adsorption amount and sulfate ion adsorption amount of the adsorbent described here mean the amount of fluorine ion and the amount of sulfate ion adsorbed by the adsorbent, and more specifically, the amount of fluorine ion and the amount of sulfate ion adsorbed by the adsorbent provided for the first step of the first time.

[0037] The sum of the amount of alkali in the alkaline solution used in the first step from the k+1th step onwards and the amount of alkali in the alkaline solution used in the second step is preferably greater than the sum of the amount of alkali in the alkaline solution used in the first step up to the kth step and the amount of alkali in the alkaline solution used in the second step. By using the alkaline solution in this manner, it becomes easier to make the discharged liquid obtained in the first and second steps from the k+1th step onwards contain as little sulfate ion as possible and contain more fluorine ions. For example, the sum of the amount of alkali in the alkaline solution used in the first step from the k+1th step onwards and the amount of alkali in the alkaline solution used in the second step is preferably 1.5 times or more, more preferably 2.0 times or more, and even more preferably 2.5 times or more, of the amount of alkali in the alkaline solution used in the first step up to the kth step and the amount of alkali in the alkaline solution used in the second step.

[0038] In the first and second steps from the k+1th time onwards, the timing for stopping the separation of the discharged liquid as a fluoride ion-containing liquid may be appropriately set based on the fluoride ion concentration of the discharged liquid from the adsorption tower. When the fluoride ion concentration of the discharged liquid from the adsorption tower becomes sufficiently low, the separation of the discharged liquid as a fluoride ion-containing liquid may be stopped. The desorption step may be terminated at the same timing. Usually, the introduction of the alkaline solution or the alkaline solution and water into the adsorption tower is continued for a while after the pH of the discharged liquid becomes 12.0 or higher, and the discharged liquid is separated as a fluoride ion-containing liquid.

[0039] In the first and second steps, the alkaline solution or water may be passed through the adsorption tower in either an upflow or downflow manner. The speed of the passage is, for example, 1 hr in terms of space velocity (SV). -1 ~50hr -1 In order to ensure that the ion exchange reaction between hydroxide ions and sulfate ions or fluoride ions is carried out stably in the adsorption tower, the space velocity (SV) of the alkaline solution or water introduced into the adsorption tower is set to 40 hr -1 Less than 30 hours is preferable. -1 The following is more preferred:

[0040] The flow rate of water introduced into the adsorption tower in the second step is preferably faster than the flow rate of the alkaline solution introduced into the adsorption tower in the first step. In the first step, the alkaline solution is introduced into the adsorption tower at a relatively slow flow rate, which can promote the ion exchange reaction between hydroxide ions and sulfate ions or fluoride ions at the adsorption sites of the adsorbent. On the other hand, in the second step, the flow rate of water does not have a significant effect on the properties of the resulting discharge liquid. Therefore, the time required for the second step can be shortened by increasing the flow rate of water. Similarly, when a concentrated alkaline solution is used in the first step and a dilute alkaline solution is used in the second step, the flow rate of the dilute alkaline solution introduced into the adsorption tower in the second step is preferably faster than the flow rate of the dilute alkaline solution introduced into the adsorption tower in the first step.

[0041] In the above case, the flow rate of the alkaline solution or concentrated alkaline solution in the first step is 1 hr -1 More than 2hr is preferable. -1 More than 3 hours is preferable. -1 More than 25 hours is more preferable. -1 Less than 20 hours is preferable. -1 Less than 15 hours is preferable. -1 The flow rate of water or dilute alkaline solution in the second step is preferably 1.2 times or more, more preferably 1.5 times or more, and is preferably 5.0 times or less, more preferably 4.0 times or less, of the flow rate of alkaline solution or concentrated alkaline solution in the first step.

[0042] Any adsorbent that adsorbs both fluorine ions and sulfate ions can be used without any particular limitation. In the present invention, since it is desirable to recover as much fluorine as possible, it is preferable to use a fluorine adsorbent as the adsorbent. Fluorine adsorbents usually also adsorb sulfate ions. Examples of such adsorbents include alumina-based adsorbents, ferrite iron-based adsorbents, zirconium-based adsorbents, and cerium-based adsorbents. Among them, it is preferable to use a cerium-based adsorbent as an adsorbent that can highly adsorb fluorine ions. Examples of cerium-based adsorbents include adsorbents containing cerium oxide (CeO2), particularly cerium oxide hydroxide (CeO2·nH2O). The adsorbent may contain a resin, and cerium oxide or cerium oxide hydroxide may be fixed or reinforced by the resin.

[0043] The fluorine ion adsorption amount of the adsorbent to be subjected to the desorption step, i.e., the fluorine ion adsorption amount of the adsorbent to be subjected to the first step, is preferably 5 g-F / kg-adsorbent or more, more preferably 8 g-F / kg-adsorbent or more, and even more preferably 10 g-F / kg-adsorbent or more, as the fluorine ion adsorption amount per unit dry mass of the adsorbent. If the adsorbent to be contacted with the alkaline solution in the desorption step adsorbs such an amount of fluorine ions, the fluorine ions can be efficiently recovered in the desorption step. On the other hand, the upper limit of the fluorine ion adsorption amount of the adsorbent to be subjected to the desorption step is not particularly limited, but is usually 50 g-F / kg-adsorbent or less, and may be 40 g-F / kg-adsorbent or less, or 35 g-F / kg-adsorbent or less. The fluorine ion adsorption amount of the adsorbent to be subjected to the desorption step described here means the amount of fluorine ions adsorbed on the adsorbent to be subjected to the desorption step. The dry mass of the adsorbent means the mass of the adsorbent after the adsorbent from which the fluoride ions and sulfate ions have been desorbed has been dried at 110°C.

[0044] The amount of sulfate ions adsorbed by the adsorbent used in the desorption step, i.e., the amount of sulfate ions adsorbed by the adsorbent used in the first step, is not particularly limited. In order to more effectively utilize the effect of the present invention of preferentially recovering fluorine ions from the adsorbent that has adsorbed fluorine ions and sulfate ions, the amount of sulfate ions adsorbed by the adsorbent used in the desorption step is preferably 5 g-SO4 / kg-adsorbent or more and 50 g-SO4 / kg-adsorbent or less as the amount of sulfate ions adsorbed per unit dry mass of the adsorbent. The amount of sulfate ions adsorbed by the adsorbent used in the desorption step may be 8 g-SO4 / kg-adsorbent or more, 10 g-SO4 / kg-adsorbent or more, or 40 g-SO4 / kg-adsorbent or less, or 35 g-SO4 / kg-adsorbent or less. The amount of sulfate ions adsorbed by the adsorbent used in the desorption step described here means the amount of sulfate ions adsorbed by the adsorbent used in the desorption step.

[0045] The fluorine ion adsorption amount of the adsorbent used in the desorption step is preferably 0.1 to 10 times the sulfate ion adsorption amount. This makes it possible to more effectively utilize the effect of the present invention of preferentially recovering fluorine ions from the adsorbent having adsorbed fluorine ions and sulfate ions. The fluorine ion adsorption amount of the adsorbent used in the desorption step is preferably 0.3 to 10 times the sulfate ion adsorption amount, more preferably 0.5 to 10 times the sulfate ion adsorption amount.

[0046] The fluoride ion-containing liquid obtained in the desorption step, i.e., the discharged liquid obtained in the first step and / or the second step after the k+1th time, may be recovered as a liquid containing fluoride ions at a high concentration, or the fluoride ions may be converted into a solid and recovered. In the latter case, for example, fluoride ions can be reacted with a calcium compound to easily recover fluoride as calcium fluoride. Since calcium fluoride is a solid containing fluoride at a high concentration, fluoride can be recovered efficiently by converting fluoride ions into calcium fluoride. Therefore, the fluoride recovery method of the present invention may include a recovery step after the desorption step, in which a calcium compound is added to the fluoride ion-containing liquid to generate calcium fluoride, and the fluoride ions are recovered as calcium fluoride by concentrating or performing solid-liquid separation. As described above, the fluoride ion-containing liquid obtained in the desorption step contains as little sulfate ions desorbed from the adsorbent as possible and recovers more fluoride ions, so that when a calcium compound is added, the amount of calcium sulfate (gypsum) generated is suppressed and more calcium fluoride is contained. In addition, since the production of calcium sulfate is suppressed, the amount of calcium compounds added can be reduced.

[0047] As the calcium compound to be added to the fluoride ion-containing liquid in the recovery step, it is preferable to use a calcium salt such as calcium chloride or calcium carbonate. From the viewpoint of minimizing the generation of solids other than calcium fluoride when added to the fluoride ion-containing liquid, it is more preferable to use calcium chloride. The calcium compound may be added to the fluoride ion-containing liquid as a solid (e.g., powder) or as a solution or dispersion.

[0048] By adding a calcium compound to the fluoride ion-containing liquid, the fluoride ions in the fluoride ion-containing liquid are precipitated as calcium fluoride, and a suspension containing calcium fluoride is obtained. In this case, in order to suppress dissolution of calcium fluoride, the pH of the fluoride ion-containing liquid to which the calcium compound has been added is preferably 3 or more, more preferably 4 or more.

[0049] In the recovery step, the suspension containing calcium fluoride is concentrated or solid-liquid separated. When the suspension containing calcium fluoride is concentrated, a concentrated slurry containing calcium fluoride is obtained, and a dilute fluoride liquid with a reduced fluoride ion concentration is obtained. When the suspension containing calcium fluoride is subjected to solid-liquid separation, a cake containing calcium fluoride is obtained, and a dilute fluoride liquid is obtained as a separated liquid. The concentration or solid-liquid separation of the suspension containing calcium fluoride may be performed by a known means such as sedimentation, centrifugation, or filtration. The concentrated slurry or cake of calcium fluoride thus obtained may be discharged outside the system and reused as a fluorine raw material, or may be disposed of as waste.

[0050] On the other hand, the discharged liquid that is not separated as a fluoride ion-containing liquid in the desorption step, i.e., the discharged liquid separated in the first and / or second steps up to the kth time, contains a large amount of sulfate ions and a small amount of fluoride ions, so it can be discharged directly into public water areas or discharged into the sewer. For example, according to the uniform wastewater standards set by the Ministry of the Environment and the sewage discharge standards set by the Sewerage Act, standards are set for fluoride ions, but no standards are set for sulfate ions. Therefore, if the fluoride ion concentration of the discharged liquid separated in the first and / or second steps up to the kth time is sufficiently low, it can be discharged directly into public water areas or discharged into the sewer after confirming that the other items are within the standards. Of course, it is also possible to add a calcium compound to the discharged liquid separated in the first and / or second steps up to the kth time to recover sulfate ions as calcium sulfate.

[0051] The adsorbent to be subjected to the desorption step, i.e., the adsorbent having adsorbed fluoride ions and sulfate ions, can be easily obtained by contacting the water to be treated containing fluoride ions and sulfate ions with the adsorbent. Therefore, the fluoride recovery method of the present invention preferably includes an adsorption step, prior to the desorption step, of introducing the water to be treated containing fluoride ions and sulfate ions into an adsorption tower filled with the adsorbent to obtain treated water from which at least a part of the fluoride ions and sulfate ions in the water to be treated have been removed. By carrying out the adsorption step and the desorption step, fluoride ions can be preferentially recovered from the water to be treated containing fluoride ions and sulfate ions.

[0052] The water to be treated is not particularly limited as long as it contains fluorine ions and sulfate ions, and the concentration of each of these ions is also not particularly limited. For example, flue gas desulfurization effluent from a coal-fired power plant, a coke plant, a steelworks, etc., can be used as the water to be treated that contains relatively high concentrations of fluorine ions and sulfate ions. In coal-fired power plants, coke plants, steelworks, etc., exhaust gas containing sulfur and fluorine is discharged by burning coal and coke, and when the exhaust gas is desulfurized by a flue gas desulfurization device, flue gas desulfurization effluent containing high concentrations of sulfate ions and fluorine ions is generated. Therefore, in the adsorption step, flue gas desulfurization effluent discharged from a flue gas desulfurization facility may be introduced into the adsorption tower as the water to be treated that contains fluorine ions and sulfate ions.

[0053] The fluorine ion concentration of the water to be treated may be 10 mg / L or more, 20 mg / L or more, or 30 mg / L or more. When flue gas desulfurization wastewater is used as the water to be treated, the fluorine ion concentration of the water to be treated may be 50 mg / L or more, 70 mg / L or more, or 90 mg / L or more. On the other hand, the upper limit of the fluorine ion concentration of the water to be treated is preferably 200 mg / L or less, more preferably 150 mg / L or less, even more preferably 135 mg / L or less, and even more preferably 120 mg / L or less, in order to stably obtain treated water with a reduced fluorine ion concentration. The fluorine ion concentration can be determined by ion chromatography or the like.

[0054] The sulfate ion concentration of the water to be treated may be 30,000 mg / L or more, 35,000 mg / L or more, 40,000 mg / L or more, or 50,000 mg / L or more. The upper limit of the sulfate ion concentration of the water to be treated is not particularly limited, but may be, for example, 150,000 mg / L or less, 100,000 mg / L or less, or 80,000 mg / L or less. The sulfate ion concentration can be determined by ion chromatography or the like.

[0055] The water to be treated may contain magnesium ions, sodium ions, etc. When the water to be treated contains flue gas desulfurization wastewater from a coal-fired power plant, a coke plant, a steel plant, or the like, magnesium ions and sodium ions may be contained in the water to be treated in addition to sulfate ions.

[0056] In the adsorption step, it is preferable that as many fluoride ions as possible are adsorbed and removed, and thus treated water with a reduced fluoride ion concentration can be obtained. Therefore, after confirming that the other items are within the standards, the treated water can be discharged into public water areas or removed as a sewer. In addition, by adsorbing and removing as many fluoride ions as possible in the adsorption step, it is possible to recover more fluorine in the subsequent desorption step. Therefore, in order to allow the adsorption and removal of fluoride ions by the adsorbent to be performed favorably, the pH of the water to be treated is preferably 2.0 or more, more preferably 2.1 or more, even more preferably 2.2 or more, and preferably 3.5 or less, more preferably 3.3 or less, and even more preferably 3.1 or less. When the pH of the water to be treated is high, the pH of the water to be treated can be adjusted by adding an acid, and it is preferable to use hydrochloric acid or sulfuric acid as the acid. Conversely, when the pH of the water to be treated is low, the pH of the water to be treated can be adjusted by adding an alkali, and it is preferable to use an alkali metal hydroxide as the alkali, and it is more preferable to use sodium hydroxide as the alkali.

[0057] In the adsorption step, the water to be treated may be passed through the adsorption tower in either an upward or downward flow manner. The flow rate in this case is, for example, 1 hr in terms of space velocity (SV).-1 ~50hr -1 In order to prevent the adsorption tower equipment from becoming too large, the space velocity (SV) should be set to 3 hr -1 More than 5 hours is preferable. -1 More than 8 hours is preferable. -1 More preferably, 12 hours or more -1 More preferably, the space velocity (SV) is 40 hr -1 Less than 30 hours is preferable. -1 Less than 25 hours is preferable. -1 The following is even more preferred:

[0058] The adsorption treatment using the adsorption tower may be performed in one stage or in multiple stages (two or more stages). In particular, when the fluorine ion concentration of the water to be treated is high, it is preferable to treat the water to be treated using adsorption towers connected in multiple stages, or to treat the water to be treated by combining treatment using one adsorption tower and treatment using adsorption towers connected in multiple stages. This makes it possible to stably reduce the fluorine ion concentration of the treated water. The number of adsorption towers connected in multiple stages may be two or more, but if the number of adsorption towers connected in series (i.e., the number of adsorption towers through which the water to be treated passes) is too large, management of the adsorption towers becomes complicated, so the number is preferably 5 or less, more preferably 4 or less, and even more preferably 3 or less.

[0059] In the fluorine recovery method of the present invention, it is preferable to carry out an acid treatment step of introducing an acid solution into the adsorption tower after the desorption step. The adsorbent from which fluoride ions and sulfate ions have been desorbed in the desorption step can be brought into contact with an acid solution in the next acid treatment step, so that it can be used again as an adsorbent that adsorbs fluoride ions and sulfate ions.

[0060] In the acid treatment step, it is preferable to use hydrochloric acid or sulfuric acid as the acid. The hydrogen ion concentration of the acid solution is, for example, preferably 0.01 mol / L or more, more preferably 0.03 mol / L or more, and preferably 2.0 mol / L or less, more preferably 1.0 mol / L or less. Before the acid treatment step, the adsorbent from which the fluoride ions and sulfate ions have been desorbed in the desorption step may be washed with water. Also, after the acid treatment step, the adsorbent may be washed with water.

[0061] As described above, by carrying out the acid treatment step following the desorption step, the adsorbent is regenerated and the adsorption step can be carried out again, allowing the adsorbent to be reused repeatedly and enabling efficient recovery of fluoride ions from the treated water containing fluoride ions and sulfate ions. EXAMPLES

[0062] The present invention will be described in more detail below by showing examples, but the scope of the present invention is not limited to these examples.

[0063] (1) Experimental method A cerium-based adsorbent was packed in a column with a diameter of 45 mm so that the apparent volume was 150 mL, and the adsorbent was allowed to adsorb fluoride ions and sulfate ions, and the adsorption tower was used before the adsorption step. Sodium hydroxide solution and water were introduced into the adsorption tower in a downward flow in sequence under the conditions shown in Table 1, and the pH, fluoride ion concentration, and sulfate ion concentration of the discharged liquid discharged from the adsorption tower were measured. In treatments No. 1 to 4, the treatment was carried out by repeating the sequential introduction of sodium hydroxide solution and water into the adsorption tower twice, and the liquid passing conditions of the sodium hydroxide solution in the first step of the first treatment were changed. In treatment No. 5, the treatment was carried out by sequentially introducing sodium hydroxide solution and water once each into the adsorption tower. The concentrations of fluoride ions and sulfate ions were measured by ion chromatography. In each treatment, the timing of collecting the discharged liquid as a fluoride ion-containing liquid was changed, and the fluoride ion recovery rate and sulfate ion recovery rate were examined. The change in fluoride ion concentration versus the flow rate was subjected to curve regression using Gaussian approximation, and the change in sulfate ion concentration versus the flow rate was subjected to curve regression using Lorentz approximation. The coefficient of determination (correlation coefficient) was 0.97 or more.

[0064] Table 1 shows the treatment conditions for each treatment, and Table 2 shows the results of the fluoride ion recovery rate and sulfate ion recovery rate when the timing of starting collection of the fluoride ion-containing liquid in each treatment is changed. Table 2 shows the results of collecting the discharged liquid from the adsorption tower as a fluoride ion-containing liquid from the start of the first step of the second run (the first run in Treatment No. 5) in each treatment, and the results of collecting the discharged liquid from the adsorption tower as a fluoride ion-containing liquid from the middle of the first step of the second run (the first run in Treatment No. 5) (the point at which the fluoride ion recovery rate becomes 99% and 97%). Also, Figs. 1 to 5 show the results of the transition of the pH of the discharged liquid, the fluoride ion desorption rate, and the sulfate ion desorption rate in Treatment Nos. 1 to 5.

[0065] [Table 1]

[0066] [Table 2]

[0067] (2) Experimental results As can be seen from the results shown in Table 2 and Figures 1 to 5, Processes Nos. 1 to 4, in which the first step of introducing an alkaline solution into the adsorption tower and the second step of introducing water were repeated twice, were able to recover fluoride ions with higher selectivity than Process No. 5, in which the first step and the second step were each performed once. In Processes Nos. 1 to 4, by separating the discharged liquid from the adsorption tower midway through the second first step, the obtained fluoride ion-containing liquid had a fluoride ion recovery rate of 99% and a sulfate ion recovery rate of 5% or less. [Industrial Applicability]

[0068] The present invention can be used for treating water containing fluoride ions and sulfate ions with an adsorbent, and can be used, for example, for treating flue gas desulfurization effluent from coal-fired power plants, coke plants, steel plants, etc.

Claims

1. a first step of introducing an alkaline solution into an adsorption tower filled with a cerium-based adsorbent having adsorbed fluoride ions and sulfate ions, thereby obtaining a discharge liquid from the adsorption tower; a second step of introducing water or an alkaline solution (wherein the alkaline solution has a lower concentration than the alkaline solution used in the first step) into the adsorption tower to obtain a discharge liquid from the adsorption tower; A fluorine recovery method comprising repeatedly carrying out a first step and a second step, collecting the discharged liquid obtained in the first step and / or the second step from the k+1th time (k is an integer of 1 or more) onward as a fluoride ion-containing liquid to recover fluorine; A method for recovering fluorine, characterized in that the pH of the effluent obtained at the end of the kth second step is 4.0 or more and 7.0 or less.

2. 2. The method for recovering fluorine according to claim 1, wherein at least a part of the discharged liquids obtained in the first step and the second step in the (k+1)th or later times has a pH of 12.0 or higher.

3. 3. The method for recovering fluorine according to claim 1, wherein a sum of an amount of alkali in the alkaline solution used in the first step in the (k+1)th or later time and an amount of alkali in the alkaline solution used in the second step is greater than a sum of an amount of alkali in the alkaline solution used in the first step up to the kth time and an amount of alkali in the alkaline solution used in the second step.

4. 4. The method for recovering fluorine according to claim 1, wherein in the second step, water is introduced into the adsorption tower, or an alkaline solution having a concentration 1 / 10 or less of the alkaline solution used in the first step is introduced into the adsorption tower.

5. 5. The method for recovering fluorine according to claim 1, wherein a flow rate of the water or the alkaline solution introduced into the adsorption tower in the second step is higher than a flow rate of the alkaline solution introduced into the adsorption tower in the first step.

6. 6. The method for recovering fluorine according to claim 1, wherein the amount of fluoride ions adsorbed on the adsorbent used in the first step for the first time is 0.1 to 10 times the amount of sulfate ions adsorbed on the adsorbent.

7. The fluoride ion adsorption amount per unit dry mass of the adsorbent used in the first step is 5 g-F / kg or more and 50 g-F / kg or less, and the sulfate ion adsorption amount is 5 g-SO 4 / kg or more 50g-SO 4 7. The method for recovering fluorine according to claim 1, wherein the fluorine content is 1 / kg or less.

8. 8. The method for recovering fluorine according to claim 1, wherein the concentration of the alkaline solution used in the first step is 0.05N or more and 0.5N or less.

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