JP6438770B2 - How to recover iodine - Google Patents

Description

  The present invention relates to a method for recovering iodine. More specifically, the present invention relates to an iodine recovery method capable of recovering iodine from various iodine-containing waste liquids with high yield.

  Iodine is widely used as a raw material for drugs such as gargles and radiation prevention drugs, X-ray contrast agents, resin stabilizers, antibacterial agents, catalysts, polarizing films used in liquid crystal displays, etc. R & D and production.

  On the other hand, iodine is contained in seawater, seaweed, brackish water, minerals (nitrite), etc. in the state of iodide ions and iodine compounds. limited. In this way, iodine is a valuable natural resource, so it is important to collect and recycle it from the viewpoint of its effective use. At present, iodine is collected and recycled from used waste liquids such as X-ray contrast media and polarizing film manufacturing baths, but the recycled iodine is only a small part of the amount used.

As a conventional iodine recovery technique, a blow-out method in which iodide ions are oxidized by using an oxidizing agent such as chlorine gas, converted into simple iodine (I ₂ ) that easily volatilizes, and recovered by gasification (Patent Document) 1) Combustion method in which an alkali metal compound and a solvent are mixed with an iodine-containing material, the resulting mixture is introduced into a combustion furnace, and the iodine compound contained in the heat treatment gas is absorbed into an alkaline aqueous solution (Patent Document 2). An incineration continuous oxidation method is known in which iodine-containing materials are continuously supplied to a combustion furnace to generate free iodine, and this iodine gas is absorbed by an aqueous sodium thiosulfate solution or the like.

Japanese Patent Laid-Open No. 51-106695 Japanese Patent Laid-Open No. 06-157005

  In general, iodine-containing wastewater may contain various wastewater regulating substances such as heavy metals, boron, and phosphorus in addition to organic matter (BOD). However, in the conventional technology as described above, organic matter (BOD) is included in wastewater except for the combustion method, and therefore it is necessary to provide a process for lowering BOD, and it takes time and effort to drain the river. was there. In addition, when the wastewater contains boron or phosphorus, the furnace is damaged in the combustion process, so that there is a problem that the treatment method is limited such that pretreatment is required. Furthermore, since volatile organic substances cannot be separated by the blow-out method, the recovery of iodine from waste water containing organic substances (BOD) is basically based on the combustion method. On the other hand, the combustion method has a problem that it cannot be applied to wastewater containing components that corrode the combustion furnace as described above. In this way, each method eventually has both advantages and disadvantages, so as long as a suitable iodine recovery method is applied according to the composition of the wastewater to be treated, the iodine is changed according to the change in the composition of the wastewater. The collection method is also forced to change, and there is a problem that it is not efficient in terms of equipment and work.

  Then, this invention is made | formed in view of the said situation, and it aims at providing the versatile iodine collection | recovery method which can collect | recover iodine with a high yield from various iodine containing waste liquid.

  The present inventor has intensively studied to solve the above problems. As a result, the present inventors have found that the above problem can be solved by oxidizing iodide ions contained in waste water into iodine, extracting with an organic solvent, reducing this, and then extracting (back extracting) with an aqueous solvent. It came to complete.

  That is, the present invention oxidizes the iodide ions to iodine by adding an oxidizing agent to a solution containing iodide ions, transfers the iodine to an organic solvent, and the organic phase containing the iodine (in the present specification) , Also referred to as “first organic phase”), and by adding an aqueous reducing agent solution to the first organic phase, the iodine is reduced to iodide ions, and an aqueous phase containing the iodide ions is obtained. It is the collection | recovery method of the iodine characterized by including isolation | separation.

  ADVANTAGE OF THE INVENTION According to this invention, the versatile iodine collection | recovery method which can collect | recover iodine with a high yield from various iodine containing waste liquid is provided.

It is a flowchart of the collection | recovery method of the iodine which concerns on one Embodiment of this invention.

The present invention oxidizes the iodide ion to iodine by adding an oxidizing agent to a solution containing iodide ion, and extracts the iodine with an organic solvent (hereinafter also referred to as “first organic phase”), The first organic phase containing iodine is separated, and an aqueous reducing agent solution is added to the first organic phase to reduce the iodine to iodide ions, thereby separating the aqueous phase containing iodide ions. This is a method for recovering iodine. In the present invention, the iodide ion means general ions containing iodine such as I ⁻ , I ₃ ⁻ and IO ₃ ⁻ unless otherwise specified.

  With such a configuration, a general-purpose iodine recovery method capable of recovering iodine from various iodine-containing waste liquids with high yield is provided.

  The reason why iodine can be recovered in high yield from various iodine-containing waste liquids according to the iodine recovery method of the present invention is unknown, but is considered to be as follows.

  According to the conventional blowout method or the like, it is necessary to wash the precipitated iodine using a large amount of water. In such a method, a large amount of waste water is produced, and the treatment costs a lot.

  On the other hand, in the iodine recovery method according to the present invention, iodide ions contained in waste water are oxidized into iodine soluble in an organic solvent, and this is extracted with an organic solvent to selectively extract only iodine. be able to. For this reason, the amount of waste water can be greatly reduced without requiring a washing step with a large amount of water. Further, according to the method of extracting only iodine as described above, boron or phosphorus that damages the furnace can be removed in advance in the first oxidation step of iodine even if the combustion step is performed later. Furthermore, since water-soluble heavy metals, wastewater-regulating substances such as boron and phosphorus move to the aqueous phase when iodine is extracted with an organic solvent, they can be separated from iodine in this step. In addition, since the organic matter (BOD) moves to the organic phase in the back extraction step of iodide ions in which iodine is reduced, it is also separated from iodine in this step. Thus, according to the iodine recovery method of the present invention, a general-purpose iodine recovery method independent of the type and amount of wastewater control substances contained in wastewater is provided by a simple technique. Therefore, it is possible to cope with the treatment of waste liquids from the manufacturing process of products containing boron and phosphorus, and it is possible to recover iodine from various iodine-containing waste liquids with high yield.

  In addition, said mechanism is based on presumption and this invention is not limited to the said mechanism at all.

  Hereinafter, preferred embodiments of the present invention will be described. In addition, this invention is not limited only to the following embodiment.

<Iodine recovery method>
The iodine recovery method of the present invention includes three methods: oxidation of iodide ions to iodine, separation of the first organic phase containing iodine, and separation of an aqueous phase containing iodide ions obtained by reducing iodine. Process. You may provide another process between each process as needed.

[Oxidation of iodide ion to iodine]
An oxidant is added to the solution containing iodide ions to oxidize iodide ions to iodine. Oxidized iodine is difficult to dissolve in water (0.029 g / 100 mL, 20 ° C.), and thus precipitates from water. Iodine precipitated from water precipitates in the waste liquid when this step is carried out in advance without an organic solvent. When this step is performed in the presence of an organic solvent that dissolves iodine, the produced iodine quickly dissolves in the phase of the organic solvent (that is, the first organic phase). Therefore, iodine can be selectively extracted and separated from the aqueous solution containing iodide ions.

(Solution containing iodide ion)
The waste water (solution containing iodide ions) that is the target of the iodine recovery method according to the present invention may be a solution prepared in a form suitable for treatment, such as making an iodine-containing material an aqueous solution. Examples of iodine-containing materials include liquids and solids containing iodine, hydrogen iodide, iodine oxide, iodic acid, iodate, periodic acid, periodate, iodine halides, iodine salts, organic iodine compounds, etc. There may be.

  When the iodine-containing material is a liquid, it may be an acidic aqueous solution, a basic aqueous solution, or a solution using an organic solvent as a solvent. Each property may be a highly viscous liquid or solid. If the iodine-containing substance itself is in a liquid state, the iodide ion may be oxidized after the aqueous solution is diluted by appropriately adding a solvent such as water, and in the case of a solution having a low iodide ion concentration, Oxidation of fluoride ions may be performed.

  When the iodine-containing material is solid, it is dissolved in water or an appropriate solvent containing water to obtain an aqueous solution containing iodide ions. When the iodine-containing material is an organic iodide such as methyl iodide, ethyl iodide, iodobenzene, iodotoluene, 2,4,6-triiodo-1,3-benzenedicarboxylic acid derivative, it is decomposed by a known method. Thus, an aqueous solution containing iodide ions can be obtained. Examples of the decomposition method of the organic iodine compound include a method of decomposing the organic iodine compound using copper as a catalyst under alkaline conditions.

  Examples of acidic iodine-containing solutions include iodine, hydrogen iodide, sodium iodide, potassium iodide, ammonium iodide, iodine monochloride, iodic acid, water, as well as inorganic acids such as hydrochloric acid and sulfuric acid, formic acid, What melt | dissolved in the acidic aqueous solution which organic acids, such as an acetic acid, melt | dissolved is mentioned.

  Examples of the basic iodine-containing solution include iodine, sodium iodate, potassium iodide, sodium iodide and the like, as well as water, bases such as sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, and ammonia, diethylamine And those dissolved in a basic aqueous solution in which an amine such as triethylamine is dissolved. In addition, the iodine recovery method according to the present invention can also be applied to waste liquids containing boron that are produced during the production of polarizing films.

  The concentration of iodide ions contained in the solution containing iodide ions is preferably 1% by mass or more, more preferably 3% by mass or more, and further preferably 5% by mass or more. Further, the upper limit of the concentration of the aqueous solution containing iodide ions is preferably 60% by mass or less, more preferably 40% by mass or less, and further preferably 20% by mass or less. If it is such a range, it can prevent that the recovery rate of iodine is impaired.

(Oxidant)
In this step, iodide ions contained in the above waste liquid (solution containing iodide ions) are converted to iodine by oxidizing with an oxidizing agent. The oxidizing agent used for oxidation of iodide ions is not particularly limited as long as it can oxidize iodide ions to iodine. For example, oxygen or ozone, or permanganic acids, dichromates, chromium Examples thereof include acids, or molecular halogen, or halogen acids or salts thereof. As the permanganic acid, potassium permanganate is used. As the dichromic acid, dichromic acid or a salt thereof is used. As the chromic acid, chromic acid or a pyridine salt thereof is used. As the halogen acids, chlorine, bromine, iodine hypohalous acid, halous acid, halogen acid or perhalogen acid is used. As these salts, alkali metal salts such as lithium, potassium and sodium, and alkaline earth metal salts such as calcium and magnesium are used. As an oxidizing agent, molecular halogen such as chlorine gas or bromine gas may be used. In some cases, halogen acids or salts thereof and molecular halogen can be used in combination. Among these oxidants, hypohalous acid or a salt thereof is preferable from the viewpoint of having an appropriate oxidizing power and easily controlling the oxidation of iodide ions by adding as an aqueous solution. Is hypochlorite, particularly preferably sodium hypochlorite.

  In chlorine generally used as an oxidizing agent, a part of an organic solvent such as xylene is also oxidized, so that the organic solvent is mixed into the aqueous phase. As a result, some iodine may remain in the aqueous phase, leading to a reduction in the recovery rate.

  On the other hand, when iodide ions are oxidized using hypochlorite, iodide ions can be selectively oxidized without oxidizing the organic solvent even in the presence of the organic solvent. For this reason, oxidation of iodide ions and extraction of iodine with an organic solvent can be performed continuously in one step.

  Moreover, it is preferable to oxidize the iodide ion with hypochlorite under acidic conditions. This is because the oxidation of iodide ions by hypochlorite proceeds as represented by the following reaction formula, so that the iodine recovery rate can be improved by increasing the proton concentration.

On the other hand, when iodide ions are oxidized using an oxidizing agent such as hypochlorite, hydroxide ions are generated by the following reaction, acid is consumed, and iodide ions (I ⁻ ) are further converted. It became clear that the iodate ion (IO ₃ ⁻ ) was oxidized. When oxidized to iodate ions, iodine to be recovered remains in the aqueous phase, resulting in a decrease in recovery efficiency.

  For this reason, it is preferable to suppress the production | generation of an iodate ion by neutralizing a hydroxide ion rapidly by oxidizing an iodide ion, maintaining acidic conditions. The acidic condition is less than pH 7, and preferably less than pH 4. Further, when iodide ions are oxidized under acidic conditions as described above, when carbonate is contained in the iodine-containing solution, the carbonate is converted into carbon dioxide (carbon dioxide) gas and discharged out of the system. (Decarbonation treatment). Therefore, such a decarboxylation treatment can be particularly suitably employed when iodine is recovered from an iodine-containing solution containing a carbonate (for example, a waste liquid from a polarizing film manufacturing process).

In a preferred embodiment of the present invention, an aqueous oxidizing agent solution in which a predetermined proton acid is contained in an aqueous oxidizing agent solution such as hypohalous acid or a salt thereof can be used. When such an oxidizing agent aqueous solution is used, even if hydroxide ions are generated by the reaction of Chemical Formula 2-1, they are quickly neutralized by protons, and the reaction of Chemical Formula 2-2 is suppressed and the increase in pH is suppressed. can do. As a result, it is possible to prevent the hydroxide ion (I ⁻ ) from being oxidized to the iodate ion (IO ₃ ⁻ ). As a result, it is possible to extract almost all of the iodide ions dissolved in the aqueous solution containing iodide ions as iodine with an organic solvent. However, the addition form of the oxidizing agent is not limited to the mixed addition with the acid as described above, and it is of course possible to adopt a form in which the oxidizing agent is added separately after the acid is added to the iodine-containing solution in advance to make the acid condition. Good.

  The proton acid contained in the oxidizing agent is not particularly limited, but is preferably a water-soluble proton acid, more preferably acetic acid, succinic acid, citric acid, tartaric acid, diacetyltartaric acid, malic acid, adipic acid, glutaric acid. , Maleic acid, fumaric acid, hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid and polyacid, more preferably inorganic acid, and still more preferably hydrochloric acid, sulfuric acid, nitric acid and phosphoric acid. As the acid added to the aqueous solution of the oxidizing agent, it is preferable not to form a salt that is hardly soluble in water after neutralization, and a particularly preferred acid is hydrochloric acid.

  By using hydrochloric acid, the concentration of the aqueous solution containing iodide ions can be increased to the solubility of sodium chloride (NaCl). Specifically, since the solubility of NaCl is 26.4% by mass at 20 ° C., if 36% hydrochloric acid is added to 12% sodium hypochlorite aqueous solution to oxidize iodide ions, NaI It becomes possible to recover iodine from an aqueous solution containing iodide ions at a saturation concentration of about 64.1% by mass (20 ° C.).

  The concentration of the oxidizer aqueous solution can be appropriately adjusted according to the amount of iodide ions, but is preferably 3% by mass or more, more preferably 5% by mass or more, and further preferably 10% by mass or more. It is. Moreover, it is preferable that the density | concentration of oxidizing agent aqueous solution is 50 mass% or less, More preferably, it is 30 mass% or less, More preferably, it is 20 mass% or less. Such a concentration range is preferable from the viewpoints of handleability and processing efficiency.

  The addition amount of the protonic acid is preferably 1 equivalent or more, more preferably 1.2 equivalents or more with respect to the iodide ion. Moreover, it is preferable that the addition amount of a protonic acid is 5 equivalent or less, More preferably, it is 3 equivalent or less. If it is such a range, it is low-cost and generation | occurrence | production of an iodic acid can fully be suppressed.

  The degree of oxidation of iodide ions to iodine can be monitored by referring to the redox potential (ORP). In the iodine recovery method according to the present invention, it is possible to oxidize iodide ions to iodine by performing oxidation of iodide ions to iodine until the oxidation-reduction potential reaches 680 to 730 mV.

  In a preferred embodiment of the present invention, oxidation of iodide ions is performed using an acidic aqueous solution of hypochlorous acid while referring to the ORP. By carrying out oxidation of iodide ions by such a method, it is possible to suppress the oxidation from proceeding excessively to produce iodate ions and lowering the iodine recovery rate. Moreover, since the first organic phase is not oxidized, iodine can be recovered continuously.

  In a more preferred embodiment of the present invention, with reference to the ORP, the iodide ion is oxidized using a hypochlorous acid aqueous solution containing 1.2 equivalents or more of hydrochloric acid with respect to the iodide ion. is there. By using hydrochloric acid as the acid, a salt insoluble in water is not generated, and an aqueous solution containing a higher concentration of iodide ions can be oxidized.

[Separation of organic phase containing iodine (first organic phase)]
After the iodide ion is oxidized to iodine in the above step, the iodine is transferred to the organic solvent (first organic phase). This first organic phase is then separated from the aqueous phase after extraction of iodine. The aqueous phase is an aqueous solution as a portion other than the first organic phase, which remains after the iodine and components dissolved in the organic solvent are transferred to the organic solvent from the aqueous solution (waste liquid) containing the iodide ions. .

(Organic solvent)
As an organic solvent for extracting iodine produced in the step of oxidizing iodide ions into iodine, it is preferable that iodine is highly soluble, has low toxicity, is easy to handle, and has low solubility in water. Such an organic solvent is not particularly limited, and examples thereof include aromatic hydrocarbons having 6 to 10 carbon atoms, halogenated aromatic hydrocarbons having 6 to 10 carbon atoms and 1 to 3 halogen atoms, and 3 to 10 carbon atoms. And esters having 2 to 10 carbon atoms.

  The aromatic hydrocarbon having 6 to 10 carbon atoms is not particularly limited. For example, in addition to o-xylene, m-xylene, p-xylene, or a mixture thereof (mixed xylene), toluene, mesylylene, tetralin, and the like can be used. Can be mentioned.

  Examples of the halogenated aromatic hydrocarbon having 6 to 10 carbon atoms and 1 to 3 halogen atoms include chlorobenzene, iodobenzene, dichlorobenzene, chloroiodobenzene, fluoroiodobenzene, iodobenzofluoride, iodotoluene, fluoroiodotoluene, iodo Anisole, dichloroiodobenzene, difluoroiodobenzene, dimethyliodobenzene, ethyliodobenzene, 3,5-bis (trifluoromethyl) iodobenzene, n-butyliodobenzene, tert-butyliodobenzene, iodonaphthalene, trichloroiodobenzene, Tetramethyliodobenzene and the like can be mentioned.

  Examples of the ester having 3 to 10 carbon atoms include methyl acetate, ethyl acetate, and butyl acetate.

  Examples of the ether having 2 to 10 carbon atoms include dimethyl ether, diethyl ether, diisopropyl ether, benzyl methyl ether, propylene glycol propyl ether, bis (2-chloroethyl) ether, bis- (2-chloroisopropyl) ether, bis (2- ( N, N-dimethylamino) ethyl) ether, 1-methoxypropane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether and the like.

  The organic solvent is preferably toluene, mixed xylene, p-xylene, or butyl acetate from the viewpoint of iodine solubility, and more preferably toluene, mixed xylene, or p-xylene. Among these, mixed xylene is particularly preferable from the viewpoint of cost.

  The mass of the organic solvent used when transferring iodine to the organic solvent is preferably 3 times or more, more preferably 5 times or more, and further preferably 8 times or more with respect to the mass of iodine. . Moreover, it is preferable that the mass of an organic solvent is 20 times or less with respect to the mass of iodine, More preferably, it is 15 times or less, More preferably, it is 12 times or less. If it is such a range, the produced | generated iodine can be efficiently collect | recovered with an organic solvent.

  The organic solvent for extracting iodine (first organic phase) may coexist with an aqueous solution containing iodide ions during the oxidation of iodide ions, and only after the oxidation of iodide ions is completed. It may be mixed with the aqueous solution after the oxide ion oxidation. The addition of the organic solvent and the separation of the first organic phase and the aqueous phase may be performed a plurality of times.

(Separation)
The solution before separating the first organic phase in this step is composed of two phases of an aqueous phase and a first organic phase due to the low solubility of the organic solvent in water. The aqueous phase and the first organic phase may be separated and extracted by opening and closing a drain pipe provided in the container, and the upper phase of the aqueous phase or the organic phase is separated by decantation. It can also be separated by other known methods.

  The aqueous phase after separation can be delivered to an external treatment after performing known purification treatments such as concentration, filtration, centrifugation, decantation as necessary.

[Separation of aqueous phase containing iodide ion obtained by reducing iodine (back extraction)]
Iodine contained in the first organic phase separated from the aqueous phase is extracted (back extracted) into the aqueous phase as iodide ions by adding an aqueous reducing agent solution to the first organic phase. By performing this operation, iodine can be separated from the first organic phase that may contain impurities soluble in the organic solvent. For example, when a solution containing an organic substance together with iodide ions is treated by the iodine recovery method according to the present invention, impurities soluble in an organic solvent and iodine can be separated. In this specification, the residual organic phase after the iodide ion extracted (back-extracted) in the aqueous phase is separated from the first organic phase is also referred to as “second organic phase”.

(Reducing agent)
The reducing agent that can be used for the preparation of the reducing agent aqueous solution is not particularly limited as long as it can reduce iodine to iodide ions. For example, an organic complex containing iron (II) such as hexacyanoiron (II) or iron ( Inorganic salts containing II), sodium borohydride, tin (II) ions, sulfites, bisulfites, thiosulfates, alkali metal amalgams, hydrazine, formic acid, oxalic acid, malonic acid and the like. Of these, sulfites and hydrogen sulfites are preferable from the viewpoint of good handleability, low cost, and use as an aqueous solution, more preferably bisulfites, and particularly preferably sodium hydrogen sulfite. .

  The lower limit of the concentration of the reducing agent aqueous solution is preferably 3% by mass or more, more preferably 5% by mass or more, and further preferably 7% by mass or more. Moreover, as an upper limit of the density | concentration of reducing agent aqueous solution, Preferably it is 35 mass% or less, More preferably, it is 25 mass% or less, More preferably, it is 20 mass% or less. Within such a range, iodide ions contained in the first organic phase can be efficiently reduced.

  The addition amount of a reducing agent can be suitably adjusted according to the density | concentration of the aqueous solution containing the iodide ion to which the iodine collection | recovery method concerning this invention is applied.

  In addition, when purifying a salt that is hardly soluble in water, it can be dissolved by adding water as appropriate. The water that dissolves the reducing agent can be selected according to the purity required for the iodine to be recovered, and examples thereof include tap water, ion-exchanged water, pure water, and ultrapure water.

  The degree of reduction of iodine to iodide ions can be monitored by referring to the redox potential (ORP). In the iodine recovery method according to the present invention, the reduction of iodine can be completed by reducing iodine to iodide ions until the oxidation-reduction potential becomes 130 mV or less.

  In a preferred embodiment of the present invention, iodine is reduced by adding an aqueous reducing agent solution such as an aqueous sodium bisulfite solution while referring to the ORP. By carrying out reduction of iodine by such a method, it can be confirmed that iodine has been completely reduced to iodide ions. Therefore, it is possible to reduce iodine recovery loss in this step.

(Separation)
In the stage before separating the second organic phase from the aqueous phase, the aqueous phase and the second organic phase are phase-separated because the solubility of the organic solvent in water is low. These aqueous phase and second organic phase may be separated and extracted by opening and closing a drain pipe provided in the container, and the upper phase of the aqueous phase or organic phase is separated by decantation. Alternatively, it may be separated by other known methods.

  The second organic phase, which is the organic phase after the iodine is extracted by this step, can be reused by mixing with the first organic phase. By reusing the second organic phase, the amount of the organic solvent used can be reduced, and the cost and environmental load can be reduced.

  You may perform a refinement | purification process as needed, before mixing a 2nd organic phase with a 1st organic phase. For example, when impurities are contained in the second organic phase, these impurities are removed by a known purification process such as washing with water, filtration and distillation, and then the second organic phase is mixed with the first organic phase. May be. For example, when processing the solution after decomposing | disassembling an organic iodine compound in presence of a copper catalyst, organic compounds other than an organic solvent may melt | dissolve in a 2nd organic phase as an impurity. It is preferable to reuse such impurities after removing them from the second organic phase.

[Purification step after back extraction]
In the iodine recovery method according to the present invention, the aqueous phase containing iodide ions obtained by back extraction may be subjected to an iodine product production step (oxidation → iodine melting → product production) and further purified. It may be processed.

  High purity iodine can be obtained by subjecting the aqueous phase containing iodide ions obtained by back extraction to a further purification step. Such purification treatment is not particularly limited, and examples thereof include roasting treatment, blowout treatment, activated carbon treatment, and the like.

(Roasting process (R-ID))
The roasting process refers to supplying an aqueous phase containing iodide ions to a roasting furnace, burning the combustible material by heat treatment, oxidizing and decomposing organic matter and nitrogenous organic matter into CO ₂ , H ₂ O, N ₂ , iodine This is a purification method in which a component is converted into a stable salt and absorbed in water or an aqueous solution. It can be applied to various iodine-containing solutions containing iodine and iodine compounds, and purified iodine can be recovered safely in an economically high yield. For details, the patent 5134951 gazette etc. can be referred.

(Blowout process)
An oxidant is added to the aqueous phase containing iodide ions to release iodine (I ₂ ), and then contacted with air. The iodine is expelled (blowout) with the air, and absorbed, crystallized, and purified to produce iodine. It is a method to purify. By applying to the aqueous phase containing iodide ions after back extraction obtained by the iodine recovery method according to the present invention, high purity iodine can be recovered. For details, Japanese Patent No. 2732642 can be referred to.

(Activated carbon treatment process)
When the aqueous phase containing iodide ions is sent to the activated carbon tank, and various organic substances that inhibit iodine precipitation, in particular, polymer substances, are removed in advance, so that iodine is precipitated by oxidation of iodide ions according to the present invention. Furthermore, iodine can be easily precipitated. For details, JP-A-2006-218475 can be referred to.

  The effects of the present invention will be described using the following examples and comparative examples. However, the technical scope of the present invention is not limited only to the following examples.

[Analysis method]
(Silver nitrate titration: I ^- content measurement)
The content of I ⁻ in each step of the iodine recovery method according to the present invention was measured by silver nitrate titration.

  The sample was added to a 200 ml beaker while weighing. After weighing, 0.5 ml of 33% aqueous hypophosphorous acid solution was added to confirm that the solution was colorless, and then diluted to 100 ml with pure water, and an appropriate amount of NaCl was added. After the automatic potentiometric titration apparatus was activated and the settings were confirmed, a beaker containing the above prepared solution was installed and titrated with a 0.1 mol / l silver nitrate aqueous solution.

  The standard of the amount of the sample is as follows.

(Gas chromatography analysis: Measurement of the amount of organic solvent contained in the aqueous phase)
After oxidation of iodide ions in the presence of the first organic phase, the amount of organic solvent contained in the aqueous phase was analyzed by gas chromatography. The analysis conditions are as follows.

Equipment used: GC SHIMADZU GC-14B
Column: G-205
Temperature rise condition: 100 ° C. (5 minutes hold) → Temperature rise (10 ° C./minute)→200° C. (15 minute hold)
Injection volume: 0.5ml + air 0.5ml
Detector: FID
Injection part temperature: 200 ° C
Detector temperature: 200 ° C
Carrier gas: He
Flow rate: 20 ml / min Chromatopack C-R8A
WIDTH 5 SLOPE 30
DRIFT 0 MIN. ARIA 100
T.A. DBL 0 STOP. T 30
ATTEN 3 SPEED 2.5
METHOD 1
[Selection of organic solvent used for iodine extraction]
(Experimental example 1)
A 300 ml eggplant flask was charged with 200 ml of the organic solvent described in Table 2, and stirred using a magnetic stirrer. 80 g of iodine was added thereto, and the mixture was stirred for 1 hour in a water bath adjusted to a solubility measurement temperature. After 1 hour of stirring, sampling analysis was performed by the following method.

First, 10 ml of 10% Na ₂ SO ₃ aqueous solution was charged into a 200 ml beaker using a micropipette and diluted to 100 ml with pure water.

  Next, 1 ml of the solution portion of the iodine slurry solution stirred for 1 hour in a water bath adjusted to the dissolution measurement temperature was sampled using a micropipette and added to the 200 ml beaker.

After stirring until the iodine color disappeared, the amount of I ⁻ was measured with a 0.1 mol / l silver nitrate (AgNO ₃ ) aqueous solution. The amount of I ⁻ was calculated by the following formula. In the formula, f means the titer of a 0.1 mol / l silver nitrate aqueous solution.

  Since the solubility of toluene, mixed xylene, and p-xylene was almost the same, it was considered most preferable to use cheaper mixed xylene.

[Iodine recovery rate confirmation, preparation amount optimization and standard setting in each process]
Example 1
-Oxidation of iodide ion to iodine An aqueous solution of sodium iodide (NaI) was used as a model solution of an aqueous solution (waste liquid) containing iodide ion to which the iodine recovery method according to the present invention was applied. A sodium iodide aqueous solution containing 8% by mass of iodide ion (I ⁻ ) was prepared by adding 18.9 g of NaI to a 200 ml beaker and dissolving it in 181 g of tap water.

  In a 500 ml conical beaker, 200 g of the sodium iodide aqueous solution prepared above was charged, and chlorine gas (oxidant) was blown into the 500 ml conical beaker while stirring with a stirrer. Iodine precipitated as the chlorine was supplied, and the supply of chlorine was stopped when the ORP reached about 700 mV.

-Separation of first organic phase containing iodine After completion of oxidation, 175 g of mixed xylene was added as an organic solvent and stirred for 10 minutes. After stirring, the entire amount was transferred to a 500 ml separatory funnel. After standing for 30 minutes, the solution was divided into two layers with reddish purple mixed xylene dissolved with iodine formed by oxidation of iodide ions as the upper layer (first organic phase) and orange aqueous phase as the lower layer. separated. The cock was opened and the lower aqueous phase was drained. The extraction with mixed xylene was performed twice in total.

  The mixed xylene phase (first organic phase) was transferred to a 300 ml conical beaker and titrated with a 10N aqueous silver nitrate solution to calculate the iodine recovery rate. Oxidation of iodide ions to iodine and the recovery rate of iodine when extracting iodine into the first organic phase was 99%. Extraction with mixed xylene after oxidation was performed twice, but iodide ions remained in the aqueous phase at 0.05% for the first time and 0.02% for the second time, and extraction with mixed xylene after oxidation was performed once. Was enough.

-Separation of aqueous phase containing iodide ions obtained by reducing iodine Transfer mixed xylene phase (first organic phase) dissolved in iodine to 300 ml conical beaker, 35% sodium bisulfite solution (reducing agent aqueous solution) 100.4 g of an aqueous solution diluted 5 times with tap water was added and stirred for 10 minutes using a magnetic stirrer. After completion of the stirring, the whole amount was transferred to a 300-ml separatory funnel and allowed to stand for 30 minutes. As a result, the solution was mixed with xylene, which was colorless and transparent as iodide ions produced by reduction of iodine were transferred to the aqueous phase. The organic phase was separated from the mixed xylene and the aqueous phase turned yellow was separated into two layers. The ORP value of the aqueous phase was 60 mV. The cock was opened and the lower layer was taken out.

  The iodine recovery rate when the iodine contained in the first organic phase was reduced to iodide ions and extracted into the aqueous phase was 98%. The total recovery rate was 97%.

(Example 2: Examination of the effect of increasing iodide ion concentration on oxidation of iodide ions)
In Example 1, when oxidizing iodide ions to iodine, Example 1 was used except that a sodium iodide aqueous solution containing 16% by mass of iodide ions (I ⁻ ) was used as the aqueous solution (waste liquid) containing iodide ions. In the same manner, iodine was recovered. Total recovery was 98.2%. Thus, a reduction in the recovery rate due to an increase in the iodide ion (I ⁻ ) concentration in the aqueous solution containing iodide ions was not observed.

(Example 3: Examination of iodide ion oxidation in the presence of an organic solvent)
The iodine was collected in the same manner as in Example 1 except that chlorine was bubbled in a state where mixed xylene was added in advance to a 200 ml beaker to oxidize iodide ions. The total recovery rate was 96.2%.

(Example 4: Examination of influence of temperature during extraction of mixed xylene (5 ° C))
The temperature at the time of extracting iodine with mixed xylene was 5 ° C., and the amount of the aqueous solution containing iodide ions was increased to 300 g to recover iodine. The total recovery was 96.9%. Thus, even if the temperature at the time of extraction was lowered, there was no problem in the recovery rate of iodine. If 8.7 g of xylene was mixed with 1 g of iodine, iodine could be sufficiently extracted even at a low temperature of 5 ° C. Furthermore, it was confirmed that the ORP during iodine reduction was 130 mV or less and the xylene phase became colorless.

(Example 5: Examination of influence of temperature during extraction of mixed xylene (40 ° C.))
The iodine was recovered in the same manner except that the temperature when extracting iodine with mixed xylene was 40 ° C. The yield decreased by about 2% due to generation of iodine vapor during xylene extraction, but the total recovery was 95.1%.

(Example 6: Examination of concentration of reducing agent aqueous solution (2-fold dilution))
Extraction of iodine with mixed xylene was performed at room temperature, and iodine was collected in the same manner as in Example 4 except that a 35% aqueous sodium hydrogen sulfite solution was used as a reducing agent after being diluted twice with tap water. A solid content of sodium hydrogen sulfate was produced in the aqueous phase, but the total recovery rate was 97.8%.

(Example 7: Examination of concentration of reducing agent aqueous solution (4-fold dilution))
At the time of reduction, iodine was collected in the same manner as in Example 6 except that an equal volume of tap water was added to a 35% aqueous solution of sodium bisulfite diluted twice with tap water. A solid content of sodium hydrogen sulfate was not generated, and the total recovery rate was 98.5%.

(Example 8: Examination of concentration of reducing agent aqueous solution (3-fold dilution))
At the time of reduction, iodine was recovered in the same manner as in Example 6 except that a half volume of tap water was added to a 35% aqueous sodium hydrogen sulfite solution diluted twice with tap water. A solid content of sodium hydrogen sulfate was not generated, and the total recovery rate was 97.3%.

[Examination of the amount of acid to be added]
In Example 1, the amount of xylene was 136 g, and the oxidizing agent was changed to a hypochlorous acid aqueous solution to oxidize iodide ions.

(Example 9: 1.0 equivalent to iodide ion)
A 500 ml conical beaker was charged with 200 g of a sodium iodide aqueous solution whose concentration was adjusted, and 1.0 equivalent of sulfuric acid was added to iodide ions while stirring in an ice bath using a magnetic stirrer. Subsequently, 12 mass% sodium hypochlorite aqueous solution was dripped by making ORP700mV into a standard. The iodine loss after iodide ion oxidation was 6.39% by mass.

  After completion of the oxidation of iodide ions, a prescribed amount of mixed xylene was added and stirred for 10 minutes. After completion of the stirring, the whole amount was transferred to a 500 ml separatory funnel and allowed to stand for 30 minutes. After that, the lower aqueous phase was discharged, and the mixed xylene phase and the aqueous phase were separated.

  The mixed xylene phase was transferred to a 300 ml conical beaker, charged with tap water and an aqueous sodium hydrogen sulfite solution, and stirred for 10 minutes using a magnetic stirrer. After the stirring was completed, the entire amount was transferred to a 300 ml separatory funnel, and allowed to stand for 30 minutes. The total iodine recovery was 89%.

(Example 10)
Iodine ions were oxidized and iodine was recovered in the same manner as in Example 9, except that the amount of sulfuric acid added to the aqueous sodium iodide solution was changed to 2 equivalents. The iodine loss after iodide ion oxidation was 0.32% by mass, and the total recovery rate was 97.3%.

(Example 11)
Iodine ions were oxidized and iodine was recovered in the same manner as in Example 9, except that the amount of sulfuric acid added to the aqueous sodium iodide solution was changed to 1.5 equivalents. The iodine loss after iodide ion oxidation was 0.34% by mass, and the total recovery rate was 97.8%.

(Example 12)
Iodine ions were oxidized and iodine was recovered in the same manner as in Example 9 except that the amount of sulfuric acid added to the aqueous sodium iodide solution was changed to 1.2 equivalents. The iodine loss after iodide ion oxidation was 0.34% by mass, and the total recovery rate was 98.1% by mass.

  When oxidizing iodide ion using 12 mass% hypochlorous acid aqueous solution, it turned out that the quantity of an acid should just be 1.2 equivalent with respect to the preparation amount of iodide ion.

(Example 13)
Iodine ions were oxidized and iodine was collected in the same manner as in Example 9 except that the concentration of the aqueous solution containing iodide ions was changed to 16% by mass. Although the total recovery rate was 98.7%, an insoluble matter presumed to be Na ₂ SO ₄ was produced during oxidation.

(Example 14)
Iodine ions were oxidized and iodine was recovered in the same manner as in Example 12 except that iodide ions were oxidized in the presence of xylene. The iodine loss after iodide ion oxidation was 0.03% by mass, and the total recovery rate was 97.8% by mass.

  In this example, no loss of mixed xylene due to oxidation of the mixed xylene and transition to the aqueous phase was not observed.

[Examination of the amount of hydrochloric acid to be added]
In the above example, when sulfuric acid was used as the acid added to the aqueous sodium iodide solution, precipitation of solids seen as Na ₂ SO ₄ was observed when the aqueous solution containing 16% iodide ions was oxidized. . Therefore, hydrochloric acid was used as a protonic acid that hardly generates a solid content.

(Example 15)
I as an acid to be added to the sodium iodide solution ^- except using 36% hydrochloric acid 1.2 equivalents relative to the was recovered iodine perform the oxidation of iodide ions in the same manner as in Example 13. The iodine loss after iodide ion oxidation was 0.55% by mass, and the total recovery rate was 97.9% by mass.

[Examination of mixed xylene recycling]
Oxidation of iodide ions in the presence of mixed xylene, and back extraction of iodine from the first organic phase, the second organic phase (mixed xylene phase) is mixed again with the first organic phase to recover iodine. We examined to do.

(Example 16)
In the same manner as in Example 15, iodide ions were oxidized to recover iodine. The iodine loss after iodide ion oxidation was 0.58% by mass, and the total recovery rate was 97.3% by mass.

(Example 17: First xylene recycling)
When extracting iodine, iodide ions were oxidized and iodine was recovered in the same manner as in Example 16 except that the mixed xylene recovered as the second organic phase in Example 16 was used. The iodine loss after extraction with mixed xylene was 1.14% by mass, and the total recovery rate was 97.8% by mass.

(Example 18: Second mixed xylene recycling)
When extracting iodine, the iodide ion was oxidized and iodine was recovered in the same manner as in Example 17 except that the mixed xylene recovered as the second organic phase in Example 17 was used. The iodine loss after extraction with mixed xylene was 0.72% by mass, and the total recovery rate was 97.5% by mass.

  The iodine loss after the first and second iodide ion oxidations of xylene recycling was slightly larger. Since the pH of the aqueous phase was 0.5, it was confirmed that the lack of acid was not the cause. Since it changed from brown to yellow at the beginning of the addition of NaClO, it was assumed that this was caused by the presence of a reducing agent in the recycled mixed xylene.

(Example 19: Removal of trace reducing agent in recycled mixed xylene)
After removing the trace reducing agent by washing the recycled mixed xylene with tap water, iodide ions were oxidized and iodine was recovered in the same manner as in Example 17 except that it was used for extraction of iodine. The iodine loss after extraction with mixed xylene was 0.53% by mass, and the loss to the aqueous phase was improved. The total recovery rate was 97.6% by mass.

[Evaluation of impurity (boron) removal]
(Example 20)
The boron concentration was quantified by using an ICP emission analyzer by a standard addition method, and the iodide ion concentration was titrated by a 0.1 N aqueous silver nitrate solution.

  While stirring 200.0 g of waste water containing 614 ppm of boron and 8.11% by mass of iodide ions, chlorine was blown until the ORP (redox potential) value exceeded 700 mV. When the ORP value of the drainage was stabilized at 718 mV, the blowing of chlorine was stopped. Iodine precipitated in the waste water to form an iodine slurry.

  After adding 173.9 g (200 ml) of xylene to the iodine slurry, the mixture was vigorously stirred for 5 minutes and then allowed to stand for 90 minutes. The aqueous phase in the lower layer was separated. The boron concentration in 186.4 g of the aqueous phase was 605 ppm, and the iodide ion concentration was 0.12% by mass.

  To the remaining organic phase, 100.2 g of a 7% by mass aqueous sodium hydrogen sulfite solution was added and stirred vigorously for 5 minutes. The ORP value of the aqueous phase was −468 mV. After standing for 60 minutes, the lower aqueous phase was separated. The boron concentration in 115.5 g of the aqueous phase was 5 ppm, and the iodide ion concentration was 13.57% by mass. The iodide ion recovery rate was 96.6%, and the boron removal rate was 99.5%.

Claims (5)

Oxidizing the iodide ion to iodine by adding an oxidizing agent to a solution containing iodide ion (excluding those containing iron (II) ion) ,
Extracting the iodine with an organic solvent, separating the first organic phase containing the iodine,
A method for recovering iodine, comprising: adding an aqueous reducing agent solution to the first organic phase to reduce the iodine to iodide ions and separating the aqueous phase containing the iodide ions. The amount of the oxidizing agent added is iodine derived from the iodide ion (I ₂ The method for recovering iodine according to claim 1, wherein the amount is 0.93 to 1.24 moles per mole. The method for recovering iodine according to claim 1 or 2 , wherein the pH of the solution when oxidizing the iodide ion to iodine is less than 7. When oxidizing the iodide ion to iodine, the oxidant is added in the form of an aqueous oxidizer solution. At this time, the aqueous oxidizer solution is 1.2 to 5 equivalents of protons with respect to the iodide ion. The method for recovering iodine according to any one of claims 1 to 3 , further comprising an acid. Reusing the organic solvent by mixing the second organic phase obtained by separating the iodide ions from the first organic phase to the first organic phase, claim 1-4 1 The method for recovering iodine according to item.