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 desorption step of introducing an alkali solution or sequentially introducing an alkali solution and water to an adsorption tower filled with an adsorbent having absorbed fluorine ions and sulfate ions to obtain a discharge liquid from the adsorption tower. A discharge liquid with pH of a specified value A (here, A is a value within a range of 4.5 to 7.0) or higher 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, is characterized in that it comprises a desorption step of introducing an alkaline solution or an alkaline solution and water successively into an adsorption tower filled with an adsorbent having adsorbed fluoride ions and sulfate ions to obtain a discharge liquid from the adsorption tower, and separates the discharge liquid having a pH of a predetermined value A (where A is any value in the range of 4.5 to 7.0) or more as a fluoride ion-containing liquid to recover fluorine. According to the fluoride recovery method of the present invention, by separating the discharge liquid having a pH of a predetermined value A (where A is any value in the range of 4.5 to 7.0) or more as a fluoride ion-containing liquid, the content of sulfate ions is reduced, and a fluoride ion-containing liquid containing a larger amount of fluoride ions can be obtained.

[0007] In the desorption step, it is preferable that the pH of the discharged liquid is less than 4.5 when the ratio of the alkaline solution or the alkaline solution and water passing through the adsorption tower is in the range of 2.5 vol / vol or less. By making the pH of the discharged liquid less than 4.5 in the early stage of the desorption step, sulfate ions can be preferentially desorbed from the adsorbent in the early stage of the desorption step. As a result, fluoride ions can be more selectively recovered by separating the discharged liquid as a fluoride ion-containing liquid after the pH of the discharged liquid reaches a predetermined value A or more in the range of 4.5 to 7.0.

[0008] In the desorption step, it is preferable to introduce the alkaline solution or the alkaline solution and water into the adsorption tower until the pH of the discharged liquid reaches 12.0 or more. By introducing the alkaline solution or the alkaline solution and water into the adsorption tower in this manner, more fluoride ions are desorbed from the adsorbent, and the recovery rate of fluoride ions can be increased.

[0009] The ratio of the alkaline solution or the alkaline solution and water passing through the adsorption tower when the pH of the discharged liquid is 12.0 is preferably 4.0 vol / vol or more and 20.0 vol / vol or less, which can reduce the amount of sulfate ions contained in the fluoride ion-containing liquid and can further increase the fluoride ion concentration in the fluoride ion-containing liquid.

[0010] In the desorption step, an alkaline solution is introduced into the adsorption tower at a liquid passing rate of 2.5 vol / vol or more, and it is preferable that the pH of the discharged liquid is less than 4.5 in the range where the liquid passing rate of the alkaline solution into the adsorption tower is 2.5 vol / vol or less. By introducing a certain amount of the alkaline solution into the adsorption tower at the initial stage of the desorption step in this way, sulfate ions can be rapidly desorbed from the adsorbent. Therefore, the time required for the desorption step can be shortened. Also, sulfate ions can be preferentially desorbed from the adsorbent at the initial stage of the desorption step.

[0011] In the desorption step, it is preferable to sequentially introduce an alkaline solution and water into the adsorption tower, or to sequentially introduce an alkaline solution with a concentration C1 and an alkaline solution with a concentration C2 (where C2 < C1) into the adsorption tower. Thereby, the total amount of alkali used in the desorption step can be reduced. Also, it becomes easy to arbitrarily adjust the pH of the discharged liquid. For example, it becomes easy to maintain the pH of the discharged liquid within a predetermined range in a certain liquid passing rate range.

[0012] In the desorption step, it is preferable that the liquid passing rate of the water introduced into the adsorption tower is greater than the liquid passing rate of the alkaline solution, or that the liquid passing rate of the alkaline solution with a concentration C2 introduced into the adsorption tower is greater than the liquid passing rate of the alkaline solution with a concentration C1. Thereby, the desorption of sulfate ions or fluoride ions from the adsorbent is promoted, and the time required for the desorption step can be shortened.

[0013] The fluoride ion adsorption amount of the adsorbent used in the desorption step is preferably 0.1 times or more and 10 times or less the sulfate ion adsorption amount. Thereby, the effect of the present invention of preferentially recovering fluoride ions from the adsorbent that has adsorbed fluoride ions and sulfate ions can be more effectively achieved. Also, the fluoride ion adsorption amount per unit dry mass of the adsorbent used in the desorption 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 and 50 g-SO 4 / kg or less. The concentration of the alkaline solution introduced into the adsorption tower is preferably 0.05 N or more and 0.5 N or less. In addition, it is preferable to use a cerium-based adsorbent as the adsorbent. Effect of the Invention

[0014] According to the fluoride recovery method of the present invention, by separating the discharged liquid having a pH equal to or higher than a predetermined value A (where A is any value within the range of 4.5 to 7.0) as a fluoride ion-containing liquid, the content of sulfate ions in the fluoride ion-containing liquid can be reduced, and a fluoride ion-containing liquid containing a larger amount of fluoride ions can be obtained. [Brief description of the drawings]

[0015] [Figure 1] 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. [Diagram 2] 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. 6 of the embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] The fluorine recovery method of the present invention includes a desorption step of introducing an alkaline solution or an alkaline solution and water successively into an adsorption tower filled with an adsorbent having adsorbed fluoride ions and sulfate ions to obtain a discharge liquid from the adsorption tower. In the desorption step, the adsorbent having adsorbed fluoride ions and sulfate ions is brought into contact with an alkaline solution, whereby fluoride ions and sulfate ions are desorbed from the adsorbent. Therefore, when an alkaline solution is introduced into an adsorption tower filled 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 containing as little sulfate ions as possible and as much fluoride ions as possible.

[0017] In the desorption step, in order to obtain a fluoride ion-containing liquid that contains as little sulfate ions as possible and more fluoride ions, the discharged liquid discharged from the adsorption tower and having a pH of a predetermined value A (where A is any value in the range of 4.5 to 7.0) or more is separated as a fluoride ion-containing liquid and fluorine is recovered. In the adsorbent that has adsorbed fluoride ions and sulfate ions, the fluoride ions and sulfate ions are electrostatically captured at the adsorption sites of the adsorbent. When an alkaline solution is brought into contact with the 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 discharged liquid discharged from the adsorption tower initially has more hydroxide ions ion-exchanged, and the pH drops, usually below 4.5. When an alkaline solution is further introduced into the adsorption tower, or when water is introduced following the alkaline solution, the pH of the discharged liquid from the adsorption tower tends to gradually increase.

[0018] In the desorption step, while the pH of the discharged liquid changes in this way, the discharged liquid having a pH of a predetermined value A (where A is any value in the range of 4.5 to 7.0) or more is separated as a fluoride ion-containing liquid. That is, after the pH of the discharged liquid becomes a predetermined value A or more in the range of 4.5 to 7.0, the discharged liquid having a pH of the predetermined value A or more is separated as a fluoride ion-containing liquid. Sulfate ions are desorbed from the adsorbent even when the pH of the discharged liquid is less than 4.5, but the desorption rate of fluoride ions from the adsorbent is low when the pH of the discharged liquid is less than 4.5, and most of the fluoride ions are desorbed from the adsorbent after the pH of the discharged liquid becomes 4.5 or more. On the other hand, from the viewpoint of ensuring the recovery amount of fluoride ions as much as possible, it is preferable to start separating the discharged liquid as a fluoride ion-containing liquid when the pH of the discharged liquid is 7.0 or less. In this way, by separating the discharged liquid having a pH of a predetermined value A (where A is any value in the range of 4.5 to 7.0) or more as a fluoride ion-containing liquid, the content of sulfate ions is reduced, and a fluoride ion-containing liquid containing more fluoride ions can be obtained.

[0019] The predetermined value A is not limited to any value in the range of 4.5 to 7.0, but is preferably 5.0 or more, and more preferably 5.3 or more. This allows the sulfate ion content in the discharged liquid separated as the fluoride ion-containing liquid to be reduced. On the other hand, the predetermined value A is preferably 6.6 or less, and more preferably 6.3 or less, which allows the amount of fluoride recovered in the fluoride ion-containing liquid to be increased.

[0020] In the desorption step, the pH of the discharged liquid coming out of the adsorption tower is measured, and when the pH value reaches or exceeds a predetermined value A, the discharged liquid is separated as a fluoride ion-containing liquid. When the desorption step is repeatedly performed under the same conditions, it is possible to separate the discharged liquid as a fluoride ion-containing liquid based on the flow rate, for example, by once grasping the relationship between the pH of the discharged liquid and the flow rate. In this case as well, the discharged liquid having a pH of the predetermined value A or more is separated as a fluoride ion-containing liquid.

[0021] In the desorption step, only an alkaline solution may be introduced into the adsorption tower as a desorption solvent from the adsorbent, or an alkaline solution and water may be introduced sequentially. When an alkaline solution is introduced, an alkaline solution of a single concentration may be introduced, or a concentrated alkaline solution (an alkaline solution of concentration C1) and a dilute alkaline solution (an alkaline solution of concentration C2 lower than concentration C1) may be introduced sequentially. When a concentrated alkaline solution and a dilute alkaline solution are used, it is preferable to use the same type of alkaline solution with different concentrations. When an alkaline solution and water are introduced sequentially, the sequential introduction of the alkaline solution and water may be repeated. When a concentrated alkaline solution and a dilute alkaline solution are introduced sequentially, the sequential introduction of the concentrated alkaline solution and the dilute alkaline solution may be repeated. Note that even when water is introduced into the adsorption tower, the pH of the discharged liquid usually tends to gradually increase due to the influence of the alkaline solution remaining in the adsorption tower.

[0022] 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.

[0023] The alkaline solution used in the desorption step is preferably one having a relatively high hydroxide ion concentration, since the higher the hydroxide ion concentration, the faster the ion exchange reaction between the hydroxide ion and the fluorine ion or sulfate ion occurs, and the more fluorine ion-containing discharge liquid is obtained. The concentration of the alkaline solution (i.e., hydroxide ion concentration) is preferably 0.05N or more, and more preferably 0.1N or more. On the other hand, if the concentration of the alkaline solution is too high, the overlap of the desorption peaks of the sulfate ion and the fluorine ion increases, and the separation ability of the fluorine ion and the sulfate ion may decrease, so the concentration of the alkaline solution is preferably 0.5N or less, and more preferably 0.4N or less. When a concentrated alkaline solution and a dilute alkaline solution are used as the alkaline solution, it is preferable that the concentration of the concentrated alkaline solution is in such a range.

[0024] At the beginning of the desorption step, the pH of the discharge liquid is preferably less than 4.5. For example, the pH of the discharge liquid is preferably less than 4.5 when the flow rate of the alkaline solution or the alkaline solution and water to the adsorption tower is at least 2.5 vol / vol or less. By maintaining the pH of the discharge liquid less than 4.5 at the beginning of the desorption step in this way, sulfate ions can be preferentially desorbed from the adsorbent at the beginning of the desorption step. As a result, fluoride ions can be more selectively recovered by separating the discharge liquid as a fluoride ion-containing liquid after the pH of the discharge liquid becomes equal to or greater than a predetermined value A in the range of 4.5 to 7.0. In the desorption step, the pH of the discharge liquid is more preferably less than 4.5 when the flow rate is 3.0 vol / vol or less. The flow rate can be calculated by dividing the volume of the alkaline solution and water introduced into the adsorption tower by the apparent volume of the adsorbent filled in the adsorption tower.

[0025] In the initial stage of the desorption step, as described above, it is preferable to appropriately set the treatment conditions so that the pH of the discharged liquid is maintained below 4.5 with the flow rate of the alkaline solution or the alkaline solution and water through the adsorption tower being in the range of 2.5 vol / vol or less. To achieve this, in the desorption step, it is preferable to prevent, for example, the concentration of the alkaline solution introduced into the adsorption tower from becoming excessively high, and to prevent the flow rate of the alkaline solution through the adsorption tower from becoming excessively high.

[0026] At the beginning of the desorption step, it is preferable that the alkaline solution is introduced into the adsorption tower at a rate of 2.5 vol / vol or more, and the pH of the discharged liquid is less than 4.5 while the rate of the alkaline solution passing through the adsorption tower is at least 2.5 vol / vol or less. By introducing a certain amount of alkaline solution into the adsorption tower at the beginning of the desorption step in this way, sulfate ions can be quickly desorbed from the adsorbent. Therefore, the time required for the desorption step can be shortened. More preferably, the alkaline solution is introduced into the adsorption tower at a rate of 3.0 vol / vol or more, and the pH of the discharged liquid is less than 4.5 while the rate of the alkaline solution passing through the adsorption tower is 3.0 vol / vol or less.

[0027] In the desorption step, it is preferable to introduce the alkaline solution or the alkaline solution and water into the adsorption tower until the pH of the discharged liquid becomes 12.0 or more. By introducing the alkaline solution or the alkaline solution and water into the adsorption tower in this way, more fluoride ions are desorbed from the adsorbent, and the recovery rate of fluoride ions can be increased. The pH of the discharged liquid may become 12.0 or more when the alkaline solution is being introduced into the adsorption tower, or when water is being introduced. It is preferable to continue introducing the alkaline solution or water into the adsorption tower even after the pH of the discharged liquid becomes 12.0 or more. In this case, the pH of the discharged liquid may be maintained at 12.0 or more thereafter, or may fall below 12.0. It is more preferable that the pH of the discharged liquid becomes 12.5 or more once.

[0028] In order to temporarily make the pH of the discharge liquid 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 (i.e., the amount of hydroxide ions) 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 fluoride ion adsorption amount and sulfate ion adsorption amount of the adsorbent. For example, in the desorption step, it is preferable to introduce, as the alkaline solution, hydroxide ions in an amount of 1.0 times or more the sum of the fluoride ion adsorption amount and the sulfate ion adsorption amount of the adsorbent, more preferably 1.2 times or more. 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] When the pH of the discharged liquid becomes 12.0 once, that is, when the pH of the discharged liquid becomes 12.0 from less than 12.0 for the first time in the desorption step, the liquid passing ratio of the alkaline solution or the alkaline solution and water to the adsorption tower is preferably 4.0 vol / vol or more and 20.0 vol / vol or less. This makes it possible to reduce the amount of sulfate ions contained in the fluoride ion-containing liquid and to further increase the fluoride ion concentration contained in the fluoride ion-containing liquid. Specifically, if the liquid passing ratio when the pH of the discharged liquid becomes 12.0 is 4.0 vol / vol or more, the overlap of the desorption peaks of sulfate ions and fluoride ions from the adsorbent can be reduced, and fluoride ions can be more selectively recovered. If the liquid passing ratio when the pH of the discharged liquid becomes 12.0 is 20.0 vol / vol or less, the amount of the solvent of the fluoride ion-containing liquid, that is, the amount of the alkaline solution and water contained in the fluoride ion-containing liquid can be reduced, and the fluoride ion concentration of the fluoride ion-containing liquid can be increased, enabling more efficient fluoride recovery. The flow rate when the pH of the discharged liquid becomes 12.0 is preferably 5.0 vol / vol or more, more preferably 5.5 vol / vol or more, even more preferably 6.0 vol / vol or more, and is preferably 18.0 vol / vol or less, more preferably 16.0 vol / vol or less, and even more preferably 15.0 vol / vol or less.

[0030] In the desorption step, as described above, it is preferable to appropriately set the treatment conditions so that the liquid passing ratio of the alkaline solution or the alkaline solution and water to the adsorption tower when the pH of the discharged liquid becomes 12.0 is 4.0 vol / vol or more and 20.0 vol / vol or less. For example, by appropriately setting the concentration of the alkaline solution introduced into the adsorption tower and the liquid passing speed of the alkaline solution through the adsorption tower, the liquid passing ratio when the pH of the discharged liquid becomes 12.0 can be adjusted. In addition, the liquid passing ratio when the pH of the discharged liquid becomes 12.0 can also be adjusted by sequentially introducing the alkaline solution and water into the adsorption tower and appropriately setting the amount of the alkaline solution introduced and the amount of water introduced at that time. In this case, it is easy to adjust the liquid passing ratio when the pH of the discharged liquid becomes 12.0 to be relatively high.

[0031] In the desorption process, 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, and the desorption process may be terminated at the same timing. Alternatively, the electrical conductivity (EC) of the discharged liquid may be measured as an index correlated with the fluoride ion concentration of the discharged liquid from the adsorption tower, and the timing for stopping the separation of the discharged liquid as a fluoride ion-containing liquid may be determined when the electrical conductivity of the discharged liquid becomes sufficiently low. Usually, the introduction of an alkaline solution or 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.

[0032] In the desorption step, it is preferable to sequentially introduce an alkaline solution and water into the adsorption tower, or to sequentially introduce a concentrated alkaline solution (an alkaline solution with a concentration C1) and a dilute alkaline solution (an alkaline solution with a concentration C2 lower than the concentration C1) into the adsorption tower. The concentrations C1 and C2 of the alkaline solutions refer to the concentrations of hydroxide ions, and it is preferable that the concentration C2 is 1 / 10 or less of the concentration C1. By using the desorption solvent in this way, the total amount of alkali used in the desorption step can be reduced. In addition, it becomes easy to arbitrarily adjust the pH of the discharged liquid, and for example, it becomes easy to maintain the pH of the discharged liquid within a predetermined range within a certain range of the liquid passing ratio. As a result, it is possible to reduce the overlap of the desorption peaks of sulfate ions and fluoride ions from the adsorbent, and it becomes possible to more selectively recover fluoride ions. In this case, it is preferable that the amount of water introduced into the adsorption tower (or the liquid passing ratio) is greater than that of the alkaline solution, and it is preferable that the dilute alkaline solution is greater than that of the concentrated alkaline solution.

[0033] In the desorption step, it is also preferable to repeatedly introduce an alkaline solution and water into the adsorption tower in sequence, or to repeatedly introduce a concentrated alkaline solution and a dilute alkaline solution into the adsorption tower in sequence. This can further reduce the overlap of the desorption peaks of sulfate ions and fluoride ions from the adsorbent, making it possible to more selectively recover fluoride ions. In this case, it is preferable to separate the discharged liquid obtained when the alkaline solution and water or the concentrated alkaline solution and the dilute alkaline solution are introduced in sequence for the second or subsequent times as a fluoride ion-containing liquid.

[0034] The desorption solvent may be passed through the adsorption tower in an upflow manner or a downflow manner. In this case, the passing speed 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 desorption solvent is set to 40 h -1 Less than 30 hours is preferable. -1 The following is more preferred:

[0035] When an alkaline solution and water are introduced into the adsorption tower in the desorption step, it is preferable that the flow rate of the water introduced into the adsorption tower is greater than the flow rate of the alkaline solution. By introducing the alkaline solution into the adsorption tower at a relatively low flow rate, the ion exchange reaction between hydroxide ions and sulfate ions or fluoride ions at the adsorption sites of the adsorbent is promoted. On the other hand, the flow rate of water does not have a significant effect on the properties of the resulting discharge liquid. Therefore, by increasing the flow rate of water, the time required for the desorption step can be shortened. Similarly, when a concentrated alkaline solution and a dilute alkaline solution are introduced into the adsorption tower in the desorption step, it is preferable that the flow rate of the dilute alkaline solution introduced into the adsorption tower is greater than the flow rate of the concentrated alkaline solution.

[0036] In the above case, the flow rate of the alkaline solution or concentrated alkaline solution is 1 hr -1 More than 2 hours 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 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, the flow rate of alkaline solution or concentrated alkaline solution.

[0037] Any adsorbent can be used without particular limitations as long as it can adsorb both fluorine ions and sulfate ions. 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 are usually capable of adsorbing sulfate ions as well. Examples of such adsorbents include alumina-based adsorbents, ferrite iron-based adsorbents, zirconium-based adsorbents, and cerium-based adsorbents. Of these, it is preferable to use a cerium-based adsorbent as an adsorbent capable of highly adsorbing fluorine ions. As a cerium-based adsorbent, cerium oxide (CeO 2 ), especially hydrous cerium oxide (CeO 2 nH 2 The adsorbent may contain a resin, and the cerium oxide or hydrous cerium oxide may be fixed or reinforced by the resin.

[0038] The fluorine ion adsorption amount of the adsorbent to be subjected to the desorption 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 desorption solvent 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 fluorine ions and sulfate ions have been desorbed is dried at 110°C.

[0039] The amount of sulfate ions adsorbed by the adsorbent used in the desorption step is not particularly limited. In order to more effectively utilize the effect of the present invention of preferentially recovering fluoride ions from the adsorbent that has adsorbed fluoride ions and sulfate ions, the amount of sulfate ions adsorbed by the adsorbent used in the desorption step is set to 5 g-SO 3 as the amount of sulfate ions adsorbed per unit dry mass of the adsorbent. 4 / kg-adsorbent more than 50g-SO 4 The sulfate ion adsorption amount of the adsorbent used in the desorption step is preferably 8 g-SO 4 / kg-adsorbent may be more than 10g-SO 4 / kg-adsorbent or more, and 40g-SO 4 / kg-adsorbent or less, or 35g-SO 4 The amount of sulfate ions adsorbed by the adsorbent used in the desorption step described here means the amount of sulfate ions adsorbed to the adsorbent used in the desorption step.

[0040] 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.

[0041] The fluoride ion-containing liquid obtained in the desorption step, i.e., the discharged liquid separated after the pH reaches a predetermined value A or more, 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 separating the calcium fluoride from the liquid. 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) produced is suppressed and the amount of calcium fluoride contained is increased. In addition, since the production of calcium sulfate is suppressed, the amount of calcium compound added can be reduced.

[0042] 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.

[0043] 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.

[0044] 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.

[0045] 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 when the pH is less than the predetermined value A, 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 when the pH is less than the predetermined value A 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 with a pH less than the predetermined value A to recover sulfate ions as calcium sulfate.

[0046] 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.

[0047] 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.

[0048] 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.

[0049] 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.

[0050] 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.

[0051] 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.

[0052] 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 -1Less than 30 hours is preferable. -1 Less than 25 hours is preferable. -1 The following is even more preferred:

[0053] 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.

[0054] 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.

[0055] 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.

[0056] 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

[0057] 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.

[0058] (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 this was used as the adsorption tower before the desorption process. Sodium hydroxide solution and water were introduced into the adsorption tower in a downward flow in sequence under the conditions shown in Table 1 as the desorption solvent, 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 5, sodium hydroxide solution and water were introduced into the adsorption tower in sequence once each, and the conditions for the fluoride ion adsorption amount and sulfate ion adsorption amount of the adsorbent used and the alkaline liquid passing conditions were changed. In treatment No. 6, the treatment was performed by repeating the sequential introduction of sodium hydroxide solution and water into the adsorption tower twice. 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 to examine the fluoride ion recovery rate and sulfate ion recovery rate. 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.

[0059] Table 1 shows the treatment conditions and the measurement results of pH 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. In Table 1, the alkali liquid passing conditions and water liquid passing conditions for treatment No. 6 are shown as the first liquid passing conditions and the second liquid passing conditions, respectively. In addition, Fig. 1 shows the results of the pH of the discharged liquid, the fluoride ion desorption rate, and the sulfate ion desorption rate in treatment No. 3, and Fig. 2 shows the results of the pH of the discharged liquid, the fluoride ion desorption rate, and the sulfate ion desorption rate in treatment No. 6.

[0060] [Table 1]

[0061] [Table 2]

[0062] (2) Experimental results As can be seen from the results shown in Table 2, in all the treatments, when the entire discharged liquid was collected from the beginning, the fluoride ion recovery rate and sulfate ion recovery rate were 100%, but after the pH of the discharged liquid reached a predetermined value A in the range of 4.5 to 7.0, the discharged liquid thereafter, that is, the discharged liquid with a pH of the predetermined value A or more, was separated as a fluoride ion-containing liquid, so that the sulfate ion recovery rate could be suppressed while maintaining the fluoride ion recovery rate as high as possible. Specifically, the sulfate ion recovery rate could be reduced to 50% or less while the fluoride ion recovery rate was 95% or more, or the sulfate ion recovery rate could be reduced to 40% or less or 30% or less while the fluoride ion recovery rate was 90% or more. As a result, fluoride ions could be preferentially recovered from the adsorbent that adsorbed fluoride ions and sulfate ions. In particular, in treatment No. 6, in which the treatment was performed by repeating the introduction of sodium hydroxide solution and water in sequence twice into the adsorption tower, the fluoride ion recovery rate was 99% and the sulfate ion recovery rate was 3%, and fluoride ions were recovered with high selectivity. [Industrial Applicability]

[0063] 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 desorption step of introducing an alkaline solution or an alkaline solution and water successively into an adsorption tower filled with a cerium-based adsorbent having adsorbed fluoride ions and sulfate ions, to obtain a discharge liquid from the adsorption tower; A method for recovering fluorine, comprising the steps of: an initial pH value of an effluent in a desorption step is less than 4.5; and, when the pH value of the effluent reaches or exceeds a predetermined value A (where A is any value in the range of 4.5 to 7.0), separating the effluent having a pH of the predetermined value A or more as a fluoride ion-containing liquid to recover fluorine.

2. 2. The method for recovering fluorine according to claim 1, wherein the ratio of the alkaline solution or the alkaline solution and water fed to the adsorption tower is not more than 2.5 vol / vol and the pH of the discharged liquid is less than 4.

5.

3. 3. The method for recovering fluorine according to claim 1, wherein the alkaline solution or the alkaline solution and water are introduced into the adsorption tower until the pH of the discharged liquid reaches 12.0 or more.

4. 4. The method for recovering fluorine according to claim 3, wherein the ratio of the alkaline solution or the alkaline solution and water passing through the adsorption tower when the pH of the discharged liquid reaches 12.0 is 4.0 vol / vol or more and 20.0 vol / vol or less.

5. An alkaline solution is introduced into the adsorption tower at a flow rate of 2.5 vol / vol or more, 5. The method for recovering fluorine according to claim 1, wherein the alkaline solution is passed through the adsorption tower at a rate of not more than 2.5 vol / vol and the pH of the discharged solution is less than 4.

5.

6. Sequentially introducing an alkaline solution and water into the adsorption tower; or 6. The method for recovering fluorine according to claim 1, wherein an alkaline solution having a concentration C1 and an alkaline solution having a concentration C2 (wherein C2<C1) are sequentially introduced into the adsorption tower.

7. The flow rate of the water introduced into the adsorption tower is greater than the flow rate of the alkaline solution, or 7. The method for recovering fluorine according to claim 6, wherein the rate at which the alkaline solution having the concentration C2 is passed through the adsorption tower is higher than the rate at which the alkaline solution having the concentration C1 is passed through the adsorption tower.

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

9. The fluoride ion adsorption amount per unit dry mass of the adsorbent used in the desorption 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 9. The method for recovering fluorine according to claim 1, wherein the fluorine content is 1 / kg or less.

10. 10. The method for recovering fluorine according to claim 1, wherein the concentration of the alkaline solution introduced into the adsorption tower is 0.05N or more and 0.5N or less.

Images