Method for producing fluorine gas

Abstract

To provide a method for producing fluorine gas in which hydrogen fluoride produced as a by-product when a raw material compound is reacted with fluorine gas and fluorinated to produce a fluoride of a target component can be reused as a raw material for producing fluorine gas by an electrolytic method.SOLUTION: A method for producing fluorine gas includes: a fluorination step of obtaining a reaction mixture comprising a main fluorinated compound as a target component produced by fluorination of a raw material compound and by-product hydrogen fluoride produced as a by-product; a separation step of separating the reaction mixture to obtain a main product component comprising the main fluorinated compound and a by-product component comprising the by-product hydrogen fluoride; a purification step of purifying the by-product component to obtain a recovered hydrogen fluoride component in which a concentration of an organic substance contained in the by-product component is reduced and a concentration of the by-product hydrogen fluoride is increased; an electrolysis step of performing electrolysis using the recovered hydrogen fluoride component as at least a part of an electrolytic solution to produce fluorine gas; and an introduction step of introducing the fluorine gas obtained in the electrolysis step into a fluorination reaction field in the fluorination step.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for producing fluorine gas. [Background technology]

[0002] Fluorine gas (F2) and fluorine-containing organic compounds are widely used in the nuclear power industry, semiconductor industry, pharmaceutical and agrochemical industries, and consumer sectors. Fluorine gas is a particularly highly reactive substance, so the reaction used to produce it must be an endothermic reaction. Therefore, fluorine gas cannot be produced by any method other than electrolysis, which easily induces an endothermic reaction, and most fluorine gas is produced by electrolysis. Many fluorine-containing organic compounds are synthesized by a direct fluorination reaction, in which an organic compound is reacted with fluorine gas to replace hydrogen atoms in the organic compound with fluorine atoms to produce a fluorine-containing organic compound, and at the same time, hydrogen fluoride (HF) is produced as a by-product from the hydrogen atoms that have been replaced by fluorine atoms and eliminated from the organic compound.

[0003] This direct fluorination reaction is an exothermic reaction that generates a large amount of reaction heat, and therefore the temperature of the reaction site is likely to rise. As a result, by-product fluorine-containing organic compounds other than the target fluorine-containing organic compound are likely to be produced, and therefore by-products containing by-product hydrogen fluoride and by-product fluorine-containing organic compounds remaining after separation of the target fluorine-containing organic compound have often been discarded. If the by-product hydrogen fluoride that has been discarded could be reused as a raw material for producing fluorine gas by electrolysis, the production cost of the target fluorine-containing organic compound could be reduced, which would be economical. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 5621024 [Patent Document 2] Patent No. 4717083 [Patent Document 3] Patent No. 3416066 Summary of the Invention [Problem to be solved by the invention]

[0005] However, since the by-product hydrogen fluoride contains components that inhibit electrolysis, it has been difficult to use it as a raw material for producing fluorine gas by electrolysis. For example, Patent Document 1 describes that the presence of moisture in the electrolyte increases the anode voltage, and therefore it is desirable that the electrolyte not contain moisture. It is clear that it is also desirable that hydrogen fluoride, which is a raw material for producing fluorine gas by electrolysis, not contain moisture. An object of the present invention is to provide a method for producing fluorine gas in which hydrogen fluoride, which is by-produced when a raw material compound is reacted with fluorine gas to fluorinate it and produce a fluoride compound of interest, can be reused as a raw material for producing fluorine gas by electrolysis. [Means for solving the problem]

[0006] In order to solve the above problems, one aspect of the present invention is as follows [1] to [8]. [1] a fluorination step of reacting a raw material compound with fluorine gas to fluorinate the compound, thereby obtaining a reaction mixture containing a main fluorinated product of the target component produced by the fluorination of the raw material compound, a sub-fluorinated product of the non-target component produced as a by-product by the fluorination of the raw material compound, and hydrogen fluoride produced as a by-product in the fluorination; a separation step of separating the reaction mixture to obtain a main component containing the main fluorinated compound as the majority component and a by-component containing the by-product hydrogen fluoride, which is a component other than the main component; a purification step of purifying the by-product components to obtain a recovered hydrogen fluoride component in which the concentration of organic matter contained in the by-product components is reduced and the concentration of the by-product hydrogen fluoride is increased; an electrolysis step of performing electrolysis using the recovered hydrogen fluoride component as at least a part of an electrolytic solution to produce fluorine gas; an introduction step of introducing the fluorine gas obtained in the electrolysis step into a reaction field of the fluorination step so that the fluorine gas obtained in the electrolysis step serves as at least a part of the fluorine gas used in the fluorination step; A method for producing fluorine gas comprising the steps of:

[0007] [2] The method for producing fluorine gas according to [1], wherein the organic substance contained in the by-product is at least one of the unreacted raw material compound, the by-fluoride, a solvent that is made to coexist with the raw material compound and the fluorine gas in the fluorination reaction field, a fluoride of the solvent, a dilution gas that is made to coexist with the raw material compound and the fluorine gas in the fluorination reaction field, a fluoride of the dilution gas, an organic compound that is used in a reaction apparatus in which the fluorination is performed and that may be present in the reaction field, and a fluoride of the organic compound.

[0008] [3] The method for producing fluorine gas according to [1] or [2], wherein the raw material compound is a compound having 2 to 18 carbon atoms and one or more hydrogen atoms. [4] The method for producing fluorine gas according to [3], wherein the compound having 2 to 18 carbon atoms and one or more hydrogen atoms is an alkane or a halogenated alkane, and the halogenated alkane is an alkane in which some or all of the hydrogen atoms have been substituted with halogen atoms other than fluorine. [5] The method for producing fluorine gas according to any one of [1] to [4], wherein the method for purifying the by-product in the purification step uses at least one of a distillation operation, an adsorption operation, and a liquid-liquid separation operation.

[0009] [6] The method for producing fluorine gas according to [5], wherein the distillation operation is performed by distilling an aqueous hydrogen fluoride solution obtained by absorbing the by-product hydrogen fluoride in the by-product components into water. [7] The method for producing fluorine gas according to [5], wherein the adsorption operation is performed by adsorbing the organic matter in the by-products onto an adsorbent having at least one of activated carbon and alumina. [8] The method for producing fluorine gas according to any one of [1] to [7], wherein in the purification step, the by-product components are purified so that the total concentration of the organic substances in the recovered hydrogen fluoride component is 200 ppm by mass or less. [Effects of the Invention]

[0010] The method for producing fluorine gas according to the present invention allows hydrogen fluoride, which is by-produced when a raw material compound is reacted with fluorine gas to fluorinate it and produce a fluoride compound of the target component, to be reused as a raw material for producing fluorine gas by electrolysis, and is therefore economical, and allows stable electrolysis to be carried out with little deterioration in the performance of the electrodes used in the electrolysis. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is a process diagram illustrating one embodiment of a method for producing fluorine gas according to the present invention. FIG. [Figure 2] 1 is a graph showing the relationship between the concentration of HFTCB in an electrolytic solution and the current value (relative value) in electrolysis. DETAILED DESCRIPTION OF THE INVENTION

[0012] An embodiment of the present invention will be described below. Note that this embodiment is merely an example of the present invention, and the present invention is not limited to this embodiment. Furthermore, various modifications and improvements can be made to this embodiment, and such modifications and improvements can also be included in the present invention.

[0013] The method for producing fluorine gas according to this embodiment includes a fluorination step, a separation step, a purification step, an electrolysis step, and an introduction step (see FIG. 1). The fluorination step is a step in which a raw material compound is reacted with fluorine gas to fluorinate it, and a reaction mixture is obtained that contains a main fluoride of the target component produced by the fluorination of the raw material compound, a by-fluoride of the non-target component produced as a by-product by the fluorination of the raw material compound, and by-product hydrogen fluoride produced in the fluorination.

[0014] The separation step is a step in which the reaction mixture obtained in the fluorination step is separated to obtain a main component containing the main fluorinated product as the majority component, and a by-product component which is a component other than the main component and contains by-product hydrogen fluoride. The purification step is a step of purifying the by-product components obtained by separation in the separation step to reduce the concentration of organic matter contained in the by-product components and obtain a recovered hydrogen fluoride component having an increased concentration of by-product hydrogen fluoride. The purification step provides a recovered hydrogen fluoride component containing a high concentration of hydrogen fluoride.

[0015] The electrolysis step is a step in which electrolysis is carried out using the recovered hydrogen fluoride component obtained in the refining step as at least a part of an electrolytic solution to produce fluorine gas. The introduction step is a step of introducing the fluorine gas obtained in the electrolysis step into a fluorination reaction field in the fluorination step so that the fluorine gas obtained in the electrolysis step becomes at least a part of the fluorine gas used in the fluorination step.

[0016] The by-product contains components other than the main component, and may contain various organic substances together with by-product hydrogen fluoride. For example, the by-product may contain unreacted raw material compounds and by-fluorinated products. In addition, in the fluorination step, the reaction may be carried out in the presence of a solvent or a diluent gas in the fluorination reaction field. When the solvent or diluent gas is organic, the solvent, fluorinated products of the solvent, diluent gas, and fluorinated products of the diluent gas may be contained as organic substances in the by-product. Furthermore, an organic compound such as oil may be used in the reaction apparatus in which fluorination is carried out, and the reaction may be carried out in the presence of the organic compound such as oil in the reaction field. In such cases, the organic compound and fluorinated products of the organic compound may be contained as organic substances in the by-product. Note that it is preferable for the by-product to contain by-product hydrogen fluoride as the majority component from the viewpoint of ease of purification of the by-product.

[0017] Because the organic matter described above may inhibit electrolysis, hydrogen fluoride containing organic matter may not be usable as a raw material for producing fluorine gas by electrolysis. However, the method for producing fluorine gas according to this embodiment includes a purification step in which the by-product is purified to reduce the concentration of organic matter contained in the by-product and to obtain a recovered hydrogen fluoride component in which the concentration of by-product hydrogen fluoride is increased. Therefore, the recovered hydrogen fluoride component has a low concentration of organic matter, and can be reused as at least a part of the electrolyte for producing fluorine gas by electrolysis.

[0018] Furthermore, according to the method for producing fluorine gas of this embodiment, by-product hydrogen fluoride can be effectively reused, making it possible to produce fluorine gas economically and contributing to the economical production of the main fluoride of the target component. Furthermore, according to the method for producing fluorine gas of this embodiment, the performance of the electrodes used in electrolysis is less likely to deteriorate, and electrolysis can be carried out stably for a long period of time.

[0019] The method for producing fluorine gas according to this embodiment will be described in further detail below. The electrolytic cell used in the electrolysis step is not particularly limited in form; for example, any electrolytic cell capable of generating fluorine gas by electrolyzing a molten salt electrolyte containing hydrogen fluoride can be used in the method for producing fluorine gas according to this embodiment. Carbonaceous electrodes such as diamond electrodes, graphite electrodes, and amorphous carbon electrodes can be used as the anode of the electrolytic cell, and metal electrodes such as iron, copper, nickel, and Monel can be used as the cathode. The electrolyte can be, for example, KF·2HF (melting point: approximately 72°C) molten salt containing hydrogen fluoride. The body of the electrolytic cell can be made of metals such as iron, nickel, and Monel; however, because the electrolyte is corrosive, corrosion-resistant alloys such as Monel are preferred.

[0020] In the electrolysis step, a direct current is applied between the anode and the cathode, generating fluorine gas from the anode and hydrogen gas from the cathode. Because hydrogen fluoride in the electrolyte has vapor pressure, it flows out of the electrolytic cell together with the fluorine gas and hydrogen gas. Furthermore, hydrogen fluoride in the electrolyte is consumed as the electrolysis proceeds. Therefore, it is preferable to perform electrolysis while continuously or intermittently supplying hydrogen fluoride to the electrolytic cell for replenishment.

[0021] The supply of hydrogen fluoride to the electrolytic cell may be carried out by transferring raw hydrogen fluoride contained in a raw hydrogen fluoride tank, by transferring the recovered hydrogen fluoride component obtained in the purification step, or by both of these, as shown in Figure 1. The raw hydrogen fluoride and the recovered hydrogen fluoride component may be mixed in advance and then supplied to the electrolytic cell, or they may be supplied separately to the electrolytic cell.

[0022] The fluorine gas containing hydrogen fluoride discharged from the electrolytic cell is sent to the fluorination step via the introduction step, and is used for fluorination by reacting the fluorine gas with a raw material compound. The temperature and pressure of the fluorination reaction in the fluorination step, as well as the concentrations of the fluorine gas and the raw material compound, depend on the types of the raw material compounds and the reaction form and cannot be uniquely determined, but the reaction is carried out while controlling each of these to appropriate predetermined values.

[0023] The type of fluorination reaction is not particularly limited, and any of a gas-phase direct fluorination reaction in which fluorine gas is reacted with a gas-phase raw material compound, a liquid-phase direct fluorination reaction in which fluorine gas is reacted with a liquid-phase raw material compound, and a solid-phase direct fluorination reaction in which fluorine gas is reacted with a solid raw material compound may be employed.

[0024] The type of raw material compound is not particularly limited, and examples thereof include hydrocarbons, halogenated hydrocarbons, alcohols, ethers, esters, ketones, carboxylic acids, etc., but compounds having 2 to 18 carbon atoms and one or more hydrogen atoms are preferred. Compounds having 2 to 18 carbon atoms and one or more hydrogen atoms are less likely to undergo side reactions in which carbon-carbon bonds are broken due to the reaction heat of the fluorination reaction.

[0025] From this viewpoint, the compound having 2 to 18 carbon atoms and one or more hydrogen atoms is preferably an alkane or a halogenated alkane, in which some or all of the hydrogen atoms have been substituted with halogen atoms other than fluorine. For example, when 1,2,3,4-tetrachlorobutane is used as the raw material compound, 1,1,2,3,4,4-hexafluoro-1,2,3,4-tetrachlorobutane is produced as the main fluorinated product, and dichlorotetrafluoroethane, 1,1-dichloro-2,2,2-trifluoroethane, etc. are produced as secondary fluorinated products.

[0026] As another example, when 1,1,2,2-tetrafluoroethane is used as the raw material, perfluoroethane is produced as the main fluorinated product, and nonafluorobutane and other fluorinated minor products are produced. As another example, when 1-chloro-1,1,2,3,3-pentafluoropropane is used as the raw material, 1-chloroheptafluoropropane is produced as the main fluorinated product, and perfluoroethane and perfluoromethane and other fluorinated minor products are produced.

[0027] In the case of a gas-phase direct fluorination reaction, in order to reduce the amount of heat of reaction, the fluorine gas and the raw material compound may be diluted with a diluent gas that is less reactive with fluorine gas and then subjected to fluorination, and the diluent gas may be organic.In the case of a liquid-phase direct fluorination reaction, in order to reduce the amount of heat of reaction, the liquid-phase raw material compound may be diluted with a solvent (liquid solvent) that is less reactive with fluorine gas and then subjected to fluorination, and the solvent may be organic.

[0028] Furthermore, in the case of a gas-phase direct fluorination reaction, a blower for circulating the gas in the reaction field (reactor) may be installed as part of the reaction equipment where fluorination is carried out, and in the case of a liquid-phase direct fluorination reaction, a stirrer for stirring the liquid in the reaction field (reactor) and a pump for sending the liquid in the reaction field (reactor) to the next step may be installed as part of the reaction equipment where fluorination is carried out. Since organic compounds such as hydraulic oil may be used in these devices such as blowers, there is a risk that the organic compounds such as hydraulic oil may leak into the region of the reaction equipment that comes into contact with hydrogen fluoride and become present in the reaction field.

[0029] Therefore, the by-products (such as by-product hydrogen fluoride) obtained by the separation step may contain organic substances in addition to the by-fluorinated compounds. That is, the by-products (such as by-product hydrogen fluoride) obtained by the separation step may contain the above-mentioned diluent gas, solvent, organic compound, and fluorinated compounds thereof.

[0030] Examples of dilution gases include inert gases such as nitrogen gas, helium, and argon, as well as organic gases that do not easily react with fluorine gas, such as tetrafluoromethane (CF4), hexafluoroethane (C2F6), chloropentafluoroethane (C2ClF5), and perfluoropropane (C3F8). Examples of solvents include acetonitrile (CH3CN), methanol, trichlorofluoromethane, carbon tetrachloride, trichlorotrifluoroethane, hexafluorotetrachlorobutane, trichloroheptafluorobutane, and the like.

[0031] Examples of organic compounds in hydraulic oils include chlorotrifluoroethylene polymers (e.g., Daifloil (trade name)) and perfluoropolyethers (e.g., Fomblin (trade name)). Chlorotrifluoroethylene polymers and perfluoropolyethers are sometimes used as sealing materials for operating parts, and have sometimes leaked into areas of the reactor that come into contact with hydrogen fluoride. As a result, these organic compounds have sometimes been contained in the recovered hydrogen fluoride components.

[0032] As described above, these organic substances contained in the by-product obtained in the separation step such as gas-liquid separation may inhibit the electrolysis of hydrogen fluoride, so these organic substances are removed from the by-product in the purification step. The method for purifying the by-products in the purification step is not particularly limited, but examples thereof include a method using at least one of a distillation operation, an adsorption operation, and a liquid-liquid separation operation.

[0033] The distillation operation can be performed using a distillation column of a tray type, a packed type, etc. The by-product component in the gas phase or liquid phase removed from the reactor is directly supplied to the distillation column, and purified by distilling off components having a boiling point lower than that of hydrogen fluoride and components having a boiling point higher than that of hydrogen fluoride, thereby obtaining a recovered hydrogen fluoride component in which the concentration of organic matter has been reduced and the concentration of by-product hydrogen fluoride has been increased.

[0034] Organic substances that are difficult to remove even by distillation are removed by adsorption using a packed column filled with at least one of activated carbon and alumina, or, if the organic substances are not soluble in hydrogen fluoride, by liquid-liquid separation. It is more preferable to pass the by-products through an activated carbon adsorption tower after carrying out the distillation operation, since this can effectively reduce the amount of organic matter.

[0035] Since organic substances that are poorly soluble in hydrogen fluoride are often also poorly soluble in water, when the gaseous or liquid by-product components removed from the reactor are mixed with water, the concentration of the poorly water-soluble organic substances in the water decreases, and an aqueous hydrogen fluoride solution containing a large amount of water-soluble hydrogen fluoride is obtained. By distilling this aqueous hydrogen fluoride solution to remove the water, hydrogen fluoride with a low organic substance content can be obtained. The total concentration of organic substances in the recovered hydrogen fluoride component obtained by purifying by-products is preferably 200 ppm by mass or less, more preferably 100 ppm by mass or less, even more preferably 50 ppm by mass or less, and most preferably 10 ppm by mass or less.

[0036] In some cases, the recovered hydrogen fluoride component alone cannot provide the entire amount of hydrogen fluoride used as an electrolyte in the electrolysis process. If the amount of recovered hydrogen fluoride component is insufficient, the missing hydrogen fluoride is supplied from the raw hydrogen fluoride tank. Therefore, the concentration of organic matter in the recovered hydrogen fluoride component is diluted by the hydrogen fluoride supplied from the raw hydrogen fluoride tank, and this diluted concentration becomes the concentration of organic matter in the hydrogen fluoride used as an electrolyte in the electrolysis process. Therefore, this diluted concentration (the concentration of organic matter in the hydrogen fluoride used as an electrolyte in the electrolysis process) may be 100 mass ppm or less.

[0037] Next, the influence of organic substances contained in hydrogen fluoride on electrolysis will be described in detail below. Patent Document 2 describes the behavior of the anode when water is supplied to an electrolytic cell. It states that when water is electrolyzed, an oxide film is formed on the surface of the carbon electrode serving as the anode, and this film changes to a fluoride film, which reduces wettability with the electrolyte and causes an increase in the electrolysis voltage.

[0038] Examples of organic substances that may be contained in the recovered hydrogen fluoride component recovered from the fluorination reaction of raw material compounds include chlorofluorocarbons, fluorocarbons, hydrochlorofluorocarbons, hydrofluorocarbons, etc. These organic substances are organic substances produced by the fluorination reaction of raw material compounds with fluorine gas, and are therefore organic substances that have been converted into a form that is less likely to react with fluorine gas even when they come into contact with fluorine gas. Therefore, these organic substances do not ionize in the electrolyte but exist as molecules, and are suspended in a dissolved state or in a liquid phase, and can be considered to be unrelated substances that do not participate in the electrode reaction like water.

[0039] When these organic substances are present in an electrolyte and placed in an electric field, they are adsorbed to the carbon electrode due to the polarity of the molecules, and the surface of the carbon electrode to which these organic substances are adsorbed is no longer in contact with the electrolyte, preventing the electrode reaction from proceeding. In constant-voltage electrolysis, the current drops significantly, and in some cases, if the amount of organic substances present in the electrolyte becomes too large, the current may stop flowing altogether. In constant-current electrolysis, the voltage at the anode rises dramatically.

[0040] In the electrolysis process, nickel can be used as the anode in addition to carbonaceous anodes. However, nickel anodes may not be usable for long-term electrolysis due to the consumption of the electrode due to the occurrence of nickel dissolution reactions. Therefore, carbonaceous anodes are generally used in the electrolysis process.

[0041] To compare the behavior with that of carbonaceous materials, constant-voltage electrolysis was performed using nickel as the anode. Although the presence of organic matter slightly hindered the flow of current, the amount of current decrease was almost constant and did not depend on the concentration of organic matter, and the behavior of the carbon electrode where current ceased to flow was unlikely to occur even when the amount of organic matter present increased. This also shows that the organic matter contained in the hydrogen fluoride recovered from the fluorination process is more likely to inhibit the electrode reaction on the surface of the carbon electrode than on the nickel electrode.

[0042] For example, when 1,1,2,3,4,4-hexafluoro-1,2,3,4-tetrachlorobutane (hereinafter sometimes referred to as "HFTCB") is added to a KF·2HF electrolyte (electrolyte temperature: 85°C), the current value decreases by about 10% compared to when no HFTCB is added when the HFTCB concentration is 200 ppm by mass (Log200 = 2.3), and when the HFTCB concentration is 10,000 ppm by mass, almost no current flows (see Figure 2).

[0043] The solubility of HFTCB in the electrolyte is not very high, and when added in amounts greater than 500 ppm by mass, HFTCB separated into two layers on the surface of the electrolyte. Despite this separation, the effect on the electrode reaction is concentration-dependent, so it is thought that electrode degradation occurs through a mechanism in which HFTCB dissolved in the electrolyte adsorbs onto the electrode, reducing the HFTCB in the electrolyte, causing the HFTCB to dissolve into the electrolyte. Additionally, electrolysis generates fluorine gas bubbles that agitate the electrolyte, causing liquid particles to float in the electrolyte and adsorb to the electrode, possibly resulting in electrode degradation. Although the mechanism that causes electrode deterioration is not clear, this phenomenon of a decrease in current value occurs similarly even when the organic matter contained in the hydrogen fluoride recovered from the fluorination step is other organic matter.

[0044] Furthermore, analysis of the fluorine gas generated during electrolysis revealed no organic matter present in the fluorine gas. For example, if fluorine gas generated during electrolysis reacts with HFTCB, HFTCB is not discharged from the electrolyte. However, if HFTCB is present in hydrogen fluoride, HFTCB gradually accumulates in the electrolyte, and when the total amount accumulated reaches 200 ppm by mass relative to the electrolyte, a current decrease of approximately 10% occurs. Therefore, it is desirable to keep the concentration of organic matter in hydrogen fluoride at 200 ppm by mass or less. The lower the concentration of organic matter in hydrogen fluoride, the less the above-mentioned effects become. Therefore, the concentration of organic matter in hydrogen fluoride is more preferably 100 ppm by mass or less, even more preferably 50 ppm by mass or less, and most preferably 10 ppm by mass or less.

[0045] Patent Document 3 describes the following three methods for organic electrolytic fluorination reactions, but these methods are not related to the effects of the present invention. (1) The Simmons process involves dissolving organic matter in anhydrous hydrogen fluoride and fluorinating it using a nickel anode. (2) The Phillips process uses KF·2HF as the electrolyte, blows gaseous organic matter into an electrolytic cell, and fluorinates it using a carbon anode. (3) A method in which a fluorine source and an organic substance are dissolved in an aprotic solvent and fluorination is carried out using a diamond electrode.

[0046] The organic substances used in these methods are compounds that are susceptible to fluorination reactions, and because the fluorine atoms generated at the electrode by electrolysis fluorinate the hydrogen atoms in the organic substances, the organic substances near the electrode undergo fluorination reactions with the fluorine atoms on the surface of the electrode before being adsorbed onto the electrode. Therefore, the organic substances do not accumulate on the surface of the electrode, but diffuse from near the electrode into the electrolyte.

[0047] In contrast, the organic matter used in this invention is generated during the fluorination process and is therefore a compound that is unlikely to undergo the fluorination reaction. Therefore, the organic matter near the electrode rarely interacts with the fluorine atoms on the electrode, resulting in its adsorption to the electrode. For example, the waveform of a cyclic voltammogram obtained by scanning the cell voltage from 0 to 10 V is identical whether or not HFTCB is added to the KF·2HF electrolyte. This indicates that no current corresponding to the oxidation of HFTCB during the fluorination reaction is observed, indicating that HFTCB does not contribute to the fluorination reaction. [Example]

[0048] The present invention will be described in more detail below with reference to examples and comparative examples. Example 1 As a fluorination reaction, fluorine gas was reacted with the raw material compound 1,2,3,4-tetrachlorobutane (hereinafter referred to as "TCB") to synthesize the main fluorinated product 1,1,2,3,4,4-hexafluoro-1,2,3,4-tetrachlorobutane (boiling point 134°C).

[0049] Volume 4m 3 A reaction solution prepared by adding TCB to an HFTCB solvent to a concentration of 10 mass % was placed in a stainless steel reactor. The reaction solution was then poured into the reactor. The reaction solution was then poured into a stainless steel ... 3 ) was placed in the flask, and while stirring at a rotation speed of 70 rpm with a stirrer equipped with stirring blades (six flat turbines), a fluorine gas-containing gas diluted with nitrogen gas to a fluorine gas concentration of 30% by volume was supplied, and the TCB was fluorinated at a reaction temperature of 70°C.

[0050] The fluorine gas-containing gas was supplied into the reaction liquid from a fluorine gas supply port provided below the stirrer in the reactor, and the supply rate of the fluorine gas-containing gas was 277 L / min (0° C., 0 MPaG equivalent). An exhaust gas pipe for discharging gas from the reactor was connected to the reactor's gas phase portion, and a control valve for adjusting the pressure inside the reactor was attached to the exhaust gas pipe. The reaction was carried out while adjusting the pressure of the gas phase in the reactor to 0.05 MPaG in gauge pressure using the control valve. The reactor's liquid phase portion was connected to an extraction pipe for extracting the reaction liquid from the reactor.

[0051] As a result of the above fluorination reaction, the hydrogen atoms of TCB were replaced with fluorine atoms to produce HFTCB, and hydrogen fluoride was simultaneously produced as a by-product. The liquid phase in the reactor was the main product, with HFTCB being the predominant component, and was separated and transported as a liquid in the separation process. Because hydrogen fluoride has a boiling point of 19.5°C, the by-product, with by-product hydrogen fluoride as the predominant component, was extracted from the gas phase in the reactor via an exhaust gas pipe to the outside of the reactor. A portion of the exhaust gas extracted from the reactor was cooled to -70°C, and the uncondensed nitrogen gas was discarded. 50 kg of by-products, including liquefied by-product hydrogen fluoride, were collected. The collected by-product (hereinafter referred to as "Liquid A") contained various organic substances (e.g., organic substances by-produced in the fluorination reaction).

[0052] Analysis of Solution A revealed that it contained a total of approximately 1% by mass of organic matter relative to hydrogen fluoride (250 ppm by mass of dichlorotetrafluoroethane, 530 ppm by mass of 1,1-dichloro-2,2,2-trifluoroethane, 7400 ppm by mass of HFTCB, and 1800 ppm by mass of other organic matter).

[0053] Next, Liquid A was charged into a packed distillation column. This distillation column was made of stainless steel, with a diameter of 20 mm, a packed bed height of 1 m, and stainless steel ring packing. The vessel at the bottom of the column was heated to 20°C, and the top was cooled to 0°C. Liquid A was distilled at normal pressure while the uncondensed fraction was vented to the outside of the distillation column. When the pressure no longer increased, the venting of the uncondensed fraction from the top of the column was stopped, and the gas phase was withdrawn from the top of the column while refluxing, and the condensed liquid (hydrogen fluoride) was collected. This collected liquid will be referred to as "Liquid B" hereinafter.

[0054] Analysis of the organic substances contained in Solution B revealed that the concentrations of dichlorotetrafluoroethane were 27 ppm by mass, 1,1-dichloro-2,2,2-trifluoroethane 12 ppm by mass, HFTCB 3 ppm by mass, and the concentrations of other organic substances were below the detection limit of 1 ppm by mass. In other words, the total amount of organic substances contained in Solution B was 43 ppm by mass or less.

[0055] Next, an adsorption tower containing activated carbon in a 300 mL container was prepared, and Liquid B was purified by passing it through this adsorption tower. The liquid (hydrogen fluoride) purified in the adsorption tower is hereinafter referred to as "Liquid C." Analysis of the organic matter contained in Liquid C revealed that no organic matter was present at a concentration exceeding the detection limit of 1 ppm by mass. The method for quantifying organic matter in hydrogen fluoride is as follows: a gasified sample was mixed with nitrogen gas, and the resulting mixed gas was passed through an alkaline aqueous solution or soda lime to neutralize the hydrogen fluoride, and the neutralized mixed gas was analyzed by gas chromatography.

[0056] Next, an electrolytic cell (made of Monel) filled with 500 mL of KF 2HF was prepared, and the electrolyte was electrolyzed to produce fluorine gas. The anode of the electrolytic cell was a diamond electrode, and the cathode was a nickel electrode. The opposing surfaces of the electrodes each had an area of 1 cm. 2 The electrolytic cell also has a pipe for discharging the generated fluorine gas and hydrogen gas from the electrolytic cell, and a pipe for supplying hydrogen fluoride to the electrolytic cell.

[0057] Constant-current electrolysis was performed at an electrolyte temperature of 85°C and a current of 1.0 A, resulting in a cell voltage of 6.8 V. As electrolysis continued, the hydrogen fluoride in the electrolyte gradually disappeared, so the lost hydrogen fluoride was replenished every 100 hours. The hydrogen fluoride used for replenishment was the aforementioned Solution C, with 39 g replenished every 100 hours. Constant-current electrolysis was performed for 1,000 hours, but the cell voltage remained unchanged at 6.8 V.

[0058] Example 2 Constant-current electrolysis was carried out in the same manner as in Example 1, except that solution B in Example 1 was used as the hydrogen fluoride replenished to the electrolytic cell. Solution B was replenished in 39 g increments every 100 hours after the start of constant-current electrolysis, and a total of 390 g of hydrogen fluoride was replenished over 1000 hours of constant-current electrolysis. However, the only change was that the cell voltage rose to 7.2 V, and there was no change in the amount of fluorine gas generated.

[0059] Comparative Example 1 Constant-current electrolysis was carried out in the same manner as in Example 1, except that Solution A from Example 1 was used as the hydrogen fluoride replenished to the electrolytic cell. 100 hours after the start of constant-current electrolysis, the first replenishment of hydrogen fluoride was carried out. The replenishment amount was 40 g. When constant-current electrolysis was resumed after the replenishment, the cell voltage rose to 8.0 V. Thereafter, 40 g of Solution A was replenished every 100 hours.

[0060] When current was applied after the third replenishment after 300 hours, the cell voltage rose to 9.1 V, when current was applied after the sixth replenishment after 600 hours, the cell voltage rose to 10.5 V, and when current was applied after the ninth replenishment after 900 hours, the cell voltage rose to 11.5 V. In this way, the cell voltage gradually rose without stabilizing and went out of control, making it impossible to continue constant-current electrolysis.

[0061] Example 3 As a fluorination reaction, fluorine gas was reacted with the raw material compound 1,1,2,2-tetrafluoroethane (hereinafter referred to as "TFE") to synthesize the main fluorinated product perfluoroethane (hereinafter referred to as "PFE") in a gas-phase reaction.

[0062] Fluorine gas and TFE were supplied to a cylindrical reactor having an inner diameter of 50 cm, a length of 8 m, and a capacity of 1570 L. The temperature of the reactor was controlled to 350°C, and the reaction was carried out at a reaction pressure of 0.2 MPaG (gauge pressure). The fluorine gas was diluted with PFE to a concentration of 4% by volume and supplied at a rate of 30 Nm 3 The TFE was diluted with PFE to a concentration of 2% by volume and supplied at a supply rate of 15 Nm 3 It was decided.

[0063] In the separation process, the gas discharged from the reactor outlet was cooled to -45°C, and the uncondensed gas was used to refine PFE, and 50 kg of a by-product liquid containing liquefied by-product hydrogen fluoride as the majority component was separated. The separated liquid (hereinafter referred to as "Liquid D") contains various organic substances (for example, organic substances by-produced in the fluorination reaction). When the organic substances contained in Solution D were analyzed by gas chromatography, the concentration of nonafluorobutane (boiling point: 20 to 30°C) produced by coupling of TFE was 150 ppm by mass, and the concentration of unreacted TFE was 220 ppm by mass.

[0064] Liquid D was distilled using the distillation column used in Example 1, and a liquid containing hydrogen fluoride as a main component (hereinafter referred to as "Liquid E") was collected in a container at the top of the column. When the organic matter contained in Liquid E was analyzed by gas chromatography, the concentration of nonafluorobutane was 21 ppm by mass and the concentration of TFE was 18 ppm by mass. Electrolysis was carried out in the same manner as in Example 1, and the lost hydrogen fluoride was replenished by supplying 39 g of Solution E every 100 hours. During 1000 hours of constant-current electrolysis, a total of 390 g of hydrogen fluoride was replenished, but the cell voltage only increased to 7.2 V, and there was no change in the amount of fluorine gas generated.

[0065] Comparative Example 2 Constant-current electrolysis was performed in the same manner as in Example 3, except that Solution D from Example 3 was used as the hydrogen fluoride replenished to the electrolytic cell. 100 hours after the start of constant-current electrolysis, the first replenishment of hydrogen fluoride was performed. The replenishment amount was 40 g. When constant-current electrolysis was restarted after the replenishment, the cell voltage rose to 8.2 V and occasional popping sounds were generated. Solution D was replenished in 40 g increments every 100 hours. After the third replenishment, 300 hours later, the cell voltage rose to 10.9 V. After the sixth replenishment, 600 hours later, the cell voltage exceeded the specified value, making it impossible to continue constant-current electrolysis.

Claims

1. a fluorination step of reacting a raw material compound with fluorine gas to fluorinate the compound, thereby obtaining a reaction mixture containing a main fluoride of the target component produced by the fluorination of the raw material compound, a sub-fluoride of the non-target component produced as a by-product by the fluorination of the raw material compound, and by-product hydrogen fluoride produced in the fluorination; a separation step of separating the reaction mixture to obtain a main component containing the main fluorinated compound as the majority component and a by-component containing the by-product hydrogen fluoride, which is a component other than the main component; a purification step of purifying the by-product components to obtain a recovered hydrogen fluoride component in which the concentration of organic matter contained in the by-product components is reduced and the concentration of the by-product hydrogen fluoride is increased; an electrolysis step of performing electrolysis using the recovered hydrogen fluoride component as at least a part of an electrolytic solution to produce fluorine gas; an introduction step of introducing the fluorine gas obtained in the electrolysis step into a reaction field of the fluorination step so that the fluorine gas obtained in the electrolysis step serves as at least a part of the fluorine gas used in the fluorination step; A method for producing fluorine gas comprising the steps of:

2. 2. The method for producing fluorine gas according to claim 1, wherein the organic substance contained in the by-product is at least one of the unreacted raw material compound, the by-fluoride, a solvent that is made to coexist with the raw material compound and the fluorine gas in the fluorination reaction field, a fluoride of the solvent, a dilution gas that is made to coexist with the raw material compound and the fluorine gas in the fluorination reaction field, a fluoride of the dilution gas, an organic compound that is used in a reaction apparatus in which the fluorination is performed and that can be present in the reaction field, and a fluoride of the organic compound.

3. 3. The method for producing fluorine gas according to claim 1, wherein the raw material compound is a compound having 2 to 18 carbon atoms and one or more hydrogen atoms.

4. 4. The method for producing fluorine gas according to claim 3, wherein the compound having 2 to 18 carbon atoms and one or more hydrogen atoms is an alkane or a halogenated alkane, and the halogenated alkane is an alkane in which some or all of the hydrogen atoms have been substituted with halogen atoms other than fluorine.

5. 5. The method for producing fluorine gas according to claim 1, wherein the method for purifying the by-product in the purification step uses at least one of a distillation operation, an adsorption operation, and a liquid-liquid separation operation.

6. 6. The method for producing fluorine gas according to claim 5, wherein the distillation operation comprises distilling an aqueous hydrogen fluoride solution obtained by absorbing the by-product hydrogen fluoride in the by-product components into water.

7. 6. The method for producing fluorine gas according to claim 5, wherein the adsorption operation comprises adsorbing the organic matter in the by-products onto an adsorbent having at least one of activated carbon and alumina.

8. 8. The method for producing fluorine gas according to claim 1, wherein in the purification step, the by-product components are purified so that the total concentration of the organic substances in the recovered hydrogen fluoride component is 200 ppm by mass or less.

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