Patent Publication Number: US-2022235471-A1

Title: Method for producing fluorine gas and device for producing fluorine gas

Description:
TECHNICAL FIELD 
     The present invention relates to a method for producing fluorine gas and a device for producing fluorine gas. 
     BACKGROUND ART 
     Fluorine gas can be synthesized (electrolytically synthesized) by electrolyzing an electrolyte containing hydrogen fluoride and a metal fluoride. Electrolyzing an electrolyte generates mist (for example, a mist of the electrolyte) together with fluorine gas, and thus the fluorine gas sent from an electrolytic cell is accompanied with mist. The mist accompanying fluorine gas becomes fine particles, which may clog pipes and valves used to send fluorine gas. This may force a production operation of fluorine gas to discontinue or stop and has interfered with continuous operation to produce fluorine gas by the electrolytic method. 
     To suppress clogging of pipes and valves with mist, PTL 1 discloses technology of heating fluorine gas accompanied with mist or a pipe through which the gas passes, to a temperature equal to or higher than the melting point of an electrolyte. PTL 2 discloses a gas production device including a gas diffusion unit as a space to roughly collect mist and a filler storage unit storing a filler for adsorbing mist. 
     CITATION LIST 
     Patent Literature 
     
         
         PTL 1: JP 5584904 B 
         PTL 2: JP 5919824 B 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     There is still a demand for technology capable of more efficiently suppress clogging of pipes and valves with mist. 
     The present invention is intended to provide a method for producing fluorine gas and a device for producing fluorine gas capable of suppressing clogging of pipes and valves with mist. 
     Solution to Problem 
     To solve the problems, aspects of the present invention are the following [1] to [5]. [1] A method for producing fluorine gas, the fluorine gas being produced by electrolyzing an electrolyte containing hydrogen fluoride and a metal fluoride, the method including electrolyzing the electrolyte in an electrolytic cell, measuring an electric energy accumulated after the electrolyte is placed in the electrolytic cell, and the electrolyzing is started, and sending a fluid generated in an inside of the electrolytic cell in the electrolyzing the electrolyte, from the inside to an outside of the electrolytic cell through a flow path. 
     In the method for producing fluorine gas, in the sending, the flow path in which the fluid flows is switched in accordance with the electric energy measured in the measuring an electric energy, such that the fluid is sent to a first flow path that sends the fluid from the inside of the electrolytic cell to a first outside when the electric energy measured in the measuring an electric energy is not less than a predetermined reference value, or the fluid is sent to a second flow path that sends the fluid from the inside of the electrolytic cell to a second outside when the electric energy is less than the predetermined reference value, and the predetermined reference value is a numerical value of 40 kAh or more relative to 1,000 L of the electrolyte. 
     [2] The method for producing fluorine gas according to the aspect [1], in which the metal fluoride is a fluoride of at least one metal selected from the group consisting of potassium, cesium, rubidium, and lithium. 
     [3] The method for producing fluorine gas according to the aspect [1] or [2], in which an anode used in the electrolyzing is a carbonaceous electrode formed from at least one carbon material selected from the group consisting of diamond, diamond-like carbon, amorphous carbon, graphite, and glassy carbon. 
     [4] The method for producing fluorine gas according to any one of the aspects [1] to [3], in which the electrolytic cell has a structure in which bubbles generated on the anode or a cathode used in the electrolyzing are capable of rising vertically in the electrolyte to reach a surface of the electrolyte. 
     [5] A device for producing fluorine gas, the fluorine gas being produced by electrolysis of an electrolyte containing hydrogen fluoride and a metal fluoride, the device including 
     an electrolytic cell storing the electrolyte and configured to perform the electrolysis, 
     an electric energy measurement unit configured to measure an electric energy accumulated after the electrolyte is placed in the electrolytic cell, and the electrolysis is started, and 
     a flow path configured to send a fluid generated in an inside of the electrolytic cell during the electrolysis of the electrolyte, from the inside to an outside of the electrolytic cell. 
     In the device for producing fluorine gas, the flow path includes a first flow path configured to send the fluid from the inside of the electrolytic cell to a first outside and a second flow path configured to send the fluid from the inside of the electrolytic cell to a second outside and includes a flow path switching unit configured to switch the flow path in which the fluid flows, to the first flow path or the second flow path in accordance with the electric energy measured by the electric energy measurement unit, 
     the flow path switching unit is configured to send the fluid from the inside of the electrolytic cell to the first flow path when the electric energy measured by the electric energy measurement unit is not less than a predetermined reference value, or to send the fluid from the inside of the electrolytic cell to the second flow path when the electric energy is less than the predetermined reference value, and 
     the predetermined reference value is a numerical value of 40 kAh or more relative to 1,000 L of the electrolyte. 
     Advantageous Effects of Invention 
     According to the present invention, clogging of pipes and valves with mist can be suppressed when an electrolyte containing hydrogen fluoride and a metal fluoride is electrolyzed to produce fluorine gas. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a view schematically illustrating an example light scattering detector used as an average particle size measurement unit in a device for producing fluorine gas pertaining to an embodiment of the present invention; 
         FIG. 2  is a schematic view illustrating an example device for producing fluorine gas pertaining to an embodiment of the present invention; 
         FIG. 3  is a view schematically illustrating an example mist remover used as a mist removal unit in the device for producing fluorine gas in  FIG. 2 ; 
         FIG. 4  is a schematic view illustrating a first alternative embodiment of the device for producing fluorine gas in  FIG. 2 ; 
         FIG. 5  is a schematic view illustrating a second alternative embodiment of the device for producing fluorine gas in  FIG. 2 ; 
         FIG. 6  is a schematic view illustrating a third alternative embodiment of the device for producing fluorine gas in  FIG. 2 ; 
         FIG. 7  is a schematic view illustrating a fourth alternative embodiment of the device for producing fluorine gas in  FIG. 2 ; 
         FIG. 8  is a schematic view illustrating a fifth alternative embodiment of the device for producing fluorine gas in  FIG. 2 ; 
         FIG. 9  is a schematic view illustrating a sixth alternative embodiment of the device for producing fluorine gas in  FIG. 2 ; 
         FIG. 10  is a schematic view illustrating a seventh alternative embodiment of the device for producing fluorine gas in  FIG. 2 ; 
         FIG. 11  is a schematic view illustrating an eighth alternative embodiment of the device for producing fluorine gas in  FIG. 2 ; 
         FIG. 12  is a schematic view illustrating a ninth alternative embodiment of the device for producing fluorine gas in  FIG. 2 ; 
         FIG. 13  is a schematic view illustrating a tenth alternative embodiment of the device for producing fluorine gas in  FIG. 2 ; 
         FIG. 14  is a graph illustrating a particle size distribution of a mist contained in a fluid generated on the anode in Reference Example 1; 
         FIG. 15  is a graph illustrating a relation between average particle size of a mist and amount of the mist generated on the anode in Reference Example 1; and 
         FIG. 16  is a graph illustrating a relation between average particle size of a mist and electric energy in Reference Example 1. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiments of the present invention will now be described. The embodiments are merely examples of the present invention, and the present invention is not limited to the embodiments. Various modifications or improvements can be made in the embodiments, and such modifications and improvements can be encompassed by the present invention. 
     The inventors of the present invention have conducted intensive studies on a mist that causes clogging of pipes or valves in electrolytic synthesis of fluorine gas. In the present invention, a “mist” is liquid microparticles or solid microparticles generated together with fluorine gas in an electrolytic cell by electrolysis of an electrolyte. Specifically, a mist is microparticles of an electrolyte, solid microparticles formed by phase change of microparticles of an electrolyte, and solid microparticles generated by reaction of fluorine gas with members included in an electrolytic cell (for example, metals included in an electrolytic cell, gaskets for an electrolytic cell, and a carbon electrode). 
     The inventors of the present invention have measured the average particle size of a mist contained in a fluid generated in an electrolytic cell during electrolysis of an electrolyte and have found that the average particle size of the mist changes with time. As a result of intensive studies, the inventors of the present invention have also found a relation between average particle size of a mist and accumulated electric energy during electrolysis and have further found a relation between average particle size of a mist and likelihood of clogging of pipes and valves that send a fluid. The inventors of the present invention have found that the clogging of pipes and valves can be suppressed by improving a flow path for sending a fluid generated in an electrolytic cell in accordance with the accumulated electric energy during electrolysis, and the frequency of discontinuance or stop of an operation for producing fluorine gas can be reduced and have completed the present invention. Embodiments of the present invention will now be described. 
     A method for producing fluorine gas in an embodiment is a method for producing fluorine gas by electrolyzing an electrolyte containing hydrogen fluoride and a metal fluoride. The method includes electrolyzing the electrolyte in an electrolytic cell, measuring an electric energy accumulated after the electrolyte is placed in the electrolytic cell, and the electrolyzing is started, and sending a fluid generated in the inside of the electrolytic cell in the electrolyzing the electrolyte, from the inside to the outside of the electrolytic cell through a flow path. 
     In the sending, the flow path in which the fluid flows is switched in accordance with the electric energy measured in the measuring an electric energy. In other words, the fluid is sent to a first flow path that sends the fluid from the inside of the electrolytic cell to a first outside when the electric energy measured in the measuring an electric energy is not less than a predetermined reference value, or the fluid is sent to a second flow path that sends the fluid from the inside of the electrolytic cell to a second outside when the electric energy is less than the predetermined reference value. The predetermined reference value is a numerical value of 40 kAh or more relative to 1,000 L of the electrolyte. 
     A device for producing fluorine gas in an embodiment is a device for producing fluorine gas by electrolysis of an electrolyte containing hydrogen fluoride and a metal fluoride. The device includes an electrolytic cell storing the electrolyte and configured to perform the electrolysis, an electric energy measurement unit configured to measure the electric energy accumulated after the electrolyte is placed in the electrolytic cell, and the electrolysis is started, and a flow path configured to send a fluid generated in the inside of the electrolytic cell during the electrolysis of the electrolyte, from the inside to the outside of the electrolytic cell. 
     The flow path includes a first flow path configured to send the fluid from the inside of the electrolytic cell to a first outside and a second flow path configured to send the fluid from the inside of the electrolytic cell to a second outside. The flow path also includes a flow path switching unit configured to switch the flow path in which the fluid flows, to the first flow path or the second flow path in accordance with the electric energy measured by the electric energy measurement unit. 
     The flow path switching unit is configured to send the fluid from the inside of the electrolytic cell to the first flow path when the electric energy measured by the electric energy measurement unit is not less than a predetermined reference value, or to send the fluid from the inside of the electrolytic cell to the second flow path when the electric energy is less than the predetermined reference value. The predetermined reference value is a numerical value of 40 kAh or more relative to 1,000 L of the electrolyte. 
     In the method for producing fluorine gas and the device for producing fluorine gas of the embodiments, the flow path in which the fluid flows is switched to the first flow path or the second flow path in accordance with the electric energy accumulated after the electrolyte is placed in the electrolytic cell, and the electrolysis is started (hereinafter also simply called “electric energy” or “accumulated electric energy”). In other words, the flow path is switched to the first flow path or the second flow path in accordance with the average particle size of a mist, and thus the mist is unlikely to cause clogging of the flow paths. Hence, the method for producing fluorine gas and the device for producing fluorine gas of the embodiments can suppress the clogging of pipes and valves with mist when an electrolyte containing hydrogen fluoride and a metal fluoride is electrolyzed to produce fluorine gas. This can reduce the frequency of discontinuance or stop of an operation for producing fluorine gas and facilitates continuous operation. As a result, fluorine gas can be economically produced. 
     In the method for producing fluorine gas and the device for producing fluorine gas of the embodiments, “the electric energy accumulated after an electrolyte is placed in an electrolytic cell, and electrolysis is started” means “the electric energy accumulated after the start of electrolysis when only a fresh electrolyte that has not been used for electrolysis is placed in an electrolytic cell, and then electrolysis is started”. The first flow path differs from the second flow path, but the first outside and the second outside may be different sections or the same section. 
     Examples of the method for producing fluorine gas and the device for producing fluorine gas of the embodiments will be described. The first flow path is a flow path through which a fluid is sent from the inside of the electrolytic cell through a mist removal unit for removing a mist from the fluid to a fluorine gas selection unit for selectively collecting fluorine gas from the fluid. The second flow path is a flow path through which a fluid is sent from the inside of the electrolytic cell to the fluorine gas selection unit but not through the mist removal unit. In other words, a fluid is sent to the mist removal unit on the first flow path when the electric energy is not less than a predetermined reference value, and a fluid is not sent to the mist removal unit when the electric energy is less than the predetermined reference value. In the present example, the fluorine gas selection unit corresponds to the first outside and the second outside, and the first outside and the second outside are the same section, but the first outside and the second outside may be different sections. 
     The second flow path has a clogging suppression mechanism that suppresses the clogging of the second flow path with mist. The clogging suppression mechanism may be any mechanism that can suppress the clogging of the second flow path with mist, and examples include the following mechanisms. In other words, examples include a pipe having a large diameter, an inclined pipe, a rotary screw, and an airflow generator, and these members may be used in combination. 
     In particular, when the second flow path at least partially includes a pipe having a larger diameter than the first flow path, the clogging of the second flow path with mist can be suppressed. Alternatively, when the second flow path at least partially includes a pipe that is inclined relative to the horizontal direction and extends downward from the upstream side to the downstream side, the clogging of the second flow path with mist can be suppressed. 
     When a rotary screw for sending a mist deposited in the second flow path to the upstream side or the downstream side is placed in the second flow path, the clogging of the second flow path with mist can be suppressed. When the second flow path has an airflow generator for sending airflow to increase the flow rate of a fluid flowing in the second flow path, the clogging of the second flow path with mist can be suppressed. Another mist removal unit different from the mist removal unit on the first flow path may be provided on the second flow path as the clogging suppression mechanism. 
     The first flow path is unlikely to be clogged with mist because the mist removal unit removes a mist from the fluid, and the second flow path is unlikely to be clogged with mist because the clogging suppression mechanism is provided. Hence, the method for producing fluorine gas and the device for producing fluorine gas of the embodiments can suppress the clogging of pipes and valves with mist when an electrolyte containing hydrogen fluoride and a metal fluoride is electrolyzed to produce fluorine gas. Even without the mist removal unit or the clogging suppression mechanism, simply switching the flow path in which a fluid flows, to another flow path (a first flow path or a second flow path) can achieve the effect of suppressing clogging of pipes and valves with mist, but the effect is highly achieved when the mist removal unit or the clogging suppression mechanism is provided. 
     The method for producing fluorine gas and the device for producing fluorine gas of the embodiments will next be described in further detail. 
     [Electrolytic Cell] 
     The electrolytic cell may be any cell that can electrolyze an electrolyte containing hydrogen fluoride and a metal fluoride to generate fluorine gas. 
     Typically, the inside of the electrolytic cell is sectioned by a partition member such as a partition wall into an anode chamber having an anode and a cathode chamber having a cathode, and this structure prevents the fluorine gas generated on the anode from mixing with the hydrogen gas generated on the cathode. 
     As the anode, for example, a carbonaceous electrode formed from a carbon material such as diamond, diamond-like carbon, amorphous carbon, graphite, glassy carbon, and indefinite carbon can be used. As the anode, a metal electrode formed from a metal such as nickel and Monel (trademark) can also be used in addition to the carbon material. As the cathode, for example, a metal electrode formed from a metal such as iron, copper, nickel, and Monel (trademark) can be used. 
     The electrolyte contains hydrogen fluoride and a metal fluoride. The metal fluoride may be any type and is preferably a fluoride of at least one metal selected from the group consisting of potassium, cesium, rubidium, and lithium. When containing cesium or rubidium, the electrolyte has a larger specific gravity and thus suppresses the amount of a mist generated during electrolysis. 
     As the electrolyte, for example, a mixed molten salt of hydrogen fluoride (HF) and potassium fluoride (KF) can be used. In the mixed molten salt of hydrogen fluoride and potassium fluoride, the molar ratio of hydrogen fluoride to potassium fluoride can be, for example, hydrogen fluoride:potassium fluoride=1.5 to 2.5:1. A typical electrolyte is KF.2HF where the ratio of hydrogen fluoride to potassium fluoride is 2:1, and the mixed molten salt has a melting point of about 72° C. The electrolyte has corrosivity, and thus a portion to come into contact with the electrolyte, such as the inner face of the electrolytic cell, is preferably formed from a metal such as iron, nickel, and Monel (trademark). 
     During electrolysis of the electrolyte, a direct current is applied to the anode and the cathode. Accordingly, a gas containing fluorine gas is generated on the anode, whereas a gas containing hydrogen gas is generated on the cathode. The hydrogen fluoride in the electrolyte has a vapor pressure, and thus gases generated on the anode and the cathode are accompanied with hydrogen fluoride. In the production of fluorine gas by electrolysis of an electrolyte, a gas generated by the electrolysis also contains a mist of the electrolyte. Accordingly, the gas phase in the electrolytic cell contains a gas generated by electrolysis, hydrogen fluoride, and a mist of the electrolyte. Hence, the substance sent from the inside to the outside of the electrolytic cell contains a gas generated by electrolysis, hydrogen fluoride, and a mist of the electrolyte and is called a “fluid” in the present invention. 
     As electrolysis proceeds, the hydrogen fluoride in the electrolyte is consumed, and thus a pipe for continuously or intermittently feeding and resupplying hydrogen fluoride into the electrolytic cell may be connected to the electrolytic cell. Hydrogen fluoride may be fed to either the cathode chamber or the anode chamber of the electrolytic cell. 
     A mist is generated during electrolysis of an electrolyte mainly due to the following reason. The temperature of an electrolyte during electrolysis is adjusted, for example, at 80 to 100° C. KF.2HF has a melting point of 71.7° C., and thus the electrolyte is in the liquid state when the temperature is adjusted as above. Bubbles of the gas generated on both the electrodes in the electrolytic cell rise in the electrolyte and burst on the surface of the electrolyte. On the bursting, the electrolyte is partially discharged into the gas phase. 
     The gas phase has a temperature lower than the melting point of the electrolyte, and thus the discharged electrolyte changes in phase into such a state as microscopic particles. The fine particles are supposedly a mixture of potassium fluoride and hydrogen fluoride, KF.nHF. The fine particles float on a separately generated gas and become a mist, forming a fluid generated in the electrolytic cell. Such a mist has tackiness and the like and thus is difficult to efficiently remove by conventional countermeasures such as installation of filters. 
     A carbonaceous electrode as the anode may be reacted with fluorine gas generated by electrolysis to generate impalpable particles of an organic compound as a mist in a small amount. Specifically, an electric current supply portion to the carbonaceous electrode has a contact resistance in many cases and may have a temperature higher than the temperature of the electrolyte due to Joule heat. Hence, the carbon included in the carbonaceous electrode may be reacted with fluorine gas to generate a soot-like organic compound, CFx, as a mist. 
     The electrolytic cell preferably has a structure in which bubbles generated on the anode or the cathode used in the electrolysis can vertically rise in the electrolyte to reach the surface of the electrolyte. In an electrolytic cell having a structure in which bubbles are unlikely to vertically rise but rise in a direction inclined relative to the vertical direction, a plurality of bubbles are likely to gather to form large bubbles. The resulting large bubbles reach the surface of the electrolyte and burst, and the amount of a mist is likely to increase. When an electrolytic cell has a structure in which bubbles can vertically rise in an electrolyte to reach the surface of the electrolyte, small bubbles reach the surface of the electrolyte and burst, and thus the amount of a mist is likely to decrease. 
     [Average Particle Size Measurement Unit] 
     The device for producing fluorine gas of the embodiment may have an average particle size measurement unit for measuring the average particle size of a mist contained in a fluid. The average particle size measurement unit may include a light scattering detector for measuring the average particle size by light scattering. The light scattering detector can measure the average particle size of a mist in a fluid flowing in a flow path while the device for producing fluorine gas is continuously operated and thus is preferred as the average particle size measurement unit. 
     An example light scattering detector will be described with reference to  FIG. 1 . The light scattering detector in  FIG. 1  is a light scattering detector usable as the average particle size measurement unit in the device for producing fluorine gas of the embodiment (for example, the devices for producing fluorine gas in  FIG. 2  and  FIGS. 4 to 13  described later). In other words, the light scattering detector measures the average particle size of a mist contained in a fluid generated in the electrolytic cell when an electrolyte containing hydrogen fluoride and a metal fluoride is electrolyzed in the electrolytic cell of the device for producing fluorine gas to produce fluorine gas. 
     The light scattering detector may be connected to the device for producing fluorine gas, and the average particle size of a mist may be measured while a fluid is sent from the inside of the electrolytic cell to the light scattering detector. Alternatively, the light scattering detector may not be connected to the device for producing fluorine gas and may measure the average particle size of a mist while a fluid is sampled from the inside of the electrolytic cell and is introduced to the light scattering detector. 
     The light scattering detector in  FIG. 1  includes a sample chamber  1  for receiving a fluid F, a light source  2  for applying light for light scattering measurement L to the fluid F in the sample chamber  1 , a scattered light detection unit  3  for detecting scattered light S generated when the light for light scattering measurement L is scattered by a mist M in the fluid F, a transparent window  4 A that is placed in the sample chamber  1  and is in contact with the fluid F and through which the light for light scattering measurement L passes, and a transparent window  4 B that is placed in the sample chamber  1  and is in contact with the fluid F and through which the scattered light S passes. The transparent windows  4 A,  4 B are formed from at least one selected from the group consisting of diamond, calcium fluoride (CaF 2 ), potassium fluoride (KF), silver fluoride (AgF), barium fluoride (BaF 2 ), and potassium bromide (KBr). 
     The light for light scattering measurement L (for example, a laser beam) emitted from the light source  2  passes through a converging lens  6  and the transparent window  4 A of the sample chamber  1 , enters the sample chamber  1 , and is applied to the fluid F received in the sample chamber  1 . At the application, when the fluid F contains a light reflective substance such as a mist M, the light for light scattering measurement L is reflected and scattered. The scattered light S generated when the light for light scattering measurement L is scattered by the mist M partially passes through the transparent window  4 B of the sample chamber  1 , is retrieved from the sample chamber  1  to the outside, and enters the scattered light detection unit  3  through a condensing lens  7  and a throttle  8 . From the information of the scattered light S, the average particle size of the mist M can be determined. The average particle size determined by the detector is a number average particle size. As the scattered light detection unit  3 , for example, an aerosol spectrometer, Welas (registered trademark) digital 2000 manufactured by PALAS can be used. 
     The transparent windows  4 A,  4 B are in contact with the fluid F. The fluid F contains highly reactive fluorine gas, and thus the transparent windows  4 A,  4 B are required to be formed from a material that is unlikely to be corroded by fluorine gas. The material for forming the transparent windows  4 A,  4 B is, for example, at least one selected from the group consisting of diamond, calcium fluoride, potassium fluoride, silver fluoride, barium fluoride, and potassium bromide. When the transparent windows  4 A,  4 B are formed from such a material as above, the deterioration by contact with the fluid F can be suppressed. 
     A glass such as quartz having a surface coated with a film formed of such a material as above can also be used as the transparent windows  4 A,  4 B. The portion to come into contact with the fluid F is coated with a film formed of such a material as above, and thus the deterioration by contact with the fluid F can be suppressed while the cost is reduced. Each transparent window  4 A,  4 B may be a laminate in which a face to come into contact with the fluid F is formed of such a material as above, and the other portions are formed of a common glass such as quartz. 
     The members of the light scattering detector except the transparent windows  4 A,  4 B may be made from any material having corrosion resistance against fluorine gas, and, for example, a metal material such as Monel (trademark) that is a copper-nickel alloy, hastelloy (trademark), and stainless steel is preferably used. 
     [Average Particle Size of Mist and Electric Energy] 
     The inventors of the present invention measured the average particle size of a mist generated during production of fluorine gas by electrolysis of an electrolyte, by using the light scattering detector. An example of the result will be described. After the anode of a device for producing fluorine gas is exchanged for a new anode or an electrolytic cell is filled with a fresh electrolyte, electrolysis is started, and the average particle size of a mist in a fluid generated on the anode was measured for a certain period of time from just after the start of electrolysis. As a result, the mist had an average particle size of 0.5 to 2.0 μm. After a sufficient time period of continuous electrolysis, the electrolysis is becoming stable. During the stable electrolysis, the mist in the fluid had an average particle size of about 0.2 μm. 
     As described above, a mist having a relatively large particle size is generated from just after the start of electrolysis to the stable electrolysis. If the fluid containing a mist having a large size just after the start of electrolysis flows in pipes and valves, the mist is likely to adsorb onto the inner face of the pipes and valves, causing clogging of the pipes and valves. 
     In contrast, during stable electrolysis, the generated mist has a relatively small particle size. Such a small mist is unlikely to settle or deposit in a fluid and thus can flow stably in pipes and valves. Hence, during stable electrolysis, a fluid consisting of a mist and a gas generated on an electrode has a relatively low possibility of causing clogging of pipes and valves. The time from the start of electrolysis to the stable electrolysis is typically 25 hours or more and 200 hours or less. From the start of electrolysis to the stable electrolysis, an electric energy of about 40 kAh or more is required to be applied for 1,000 L of an electrolyte. 
     The inventors of the present invention have found a close relation between the average particle size of a mist and the electric energy. Typically, when only a fresh electrolyte that has not been used for electrolysis is placed in an electrolytic cell, and then electrolysis is started, the mist has an average particle size of more than 0.4 μm at the time of the start of electrolysis (i.e., when the electric energy accumulated from the start of electrolysis is small). As the electrolysis is continued (i.e., as the electric energy accumulated from the start of electrolysis increases), the mist has a smaller average particle size. When the electric energy, for example, exceeds 60 kAh relative to 1,000 L of the electrolyte, the mist has an average particle size of 0.4 μm or less. 
     As described above, the average particle size of a mist has a relation to the electric energy. Hence, the electric energy can be measured during electrolysis in place of the average particle size of a mist, and the measurement result can be used to switch a flow path. In other words, when the electric energy accumulated from the start of electrolysis is constantly measured, and the measurement result of the electric energy is used at certain timing during the electrolysis, the flow path in which a fluid generated by the electrolysis flows can be appropriately switched at the certain timing in accordance with the measurement result. 
     Based on such knowledge, the inventors of the present invention have invented the method for producing fluorine gas and the device for producing fluorine gas, having a structure in which a flow path in which a fluid flows can be switched in accordance with the electric energy during electrolysis. The device for producing fluorine gas of the embodiment has a first flow path and a second flow path, and a flow path switching unit (for example, a switching valve) may be used to select, from the two flow paths, a flow path used to convey a fluid. 
     Alternatively, the device for producing fluorine gas of the embodiment may have two flow paths and a transfer and replacement mechanism for transferring and replacing an electrolytic cell. From the two flow paths, a flow path used to convey a fluid may be selected, and an electrolytic cell may be transferred near the flow path and be connected to the flow path. This can switch the flow path. 
     The device has the first flow path and the second flow path as described above. Hence, even while one flow path is blocked and cleaned, the other flow path can be opened, and the device for producing fluorine gas can be continuously operated. 
     From studies by the inventors of the present invention, a mist having a relatively large average particle size is generated from the start of electrolysis to the stable electrolysis, and thus a fluid can be sent to the second flow path having a clogging suppression mechanism. When the electrolysis becomes stable as time passes, a mist having a relatively small average particle size is generated, and thus the flow path can be switched such that the fluid is sent to the first flow path having a mist removal unit. 
     Such switching the flow path is performed in accordance with the electric energy measured during electrolysis, and the flow path is switched on the basis of a predetermined reference value. The appropriate reference value of the average particle size of a mist generated on an anode varies with devices and is, for example, 0.1 μm or more and 1.0 μm or less, preferably 0.2 μm or more and 0.8 μm or less, and more preferably 0.4 μm. 
     From the relation between the average particle size of a mist and the electric energy, the lower limit of the appropriate reference value of the electric energy is 40 kAh or more, preferably 50 kAh or more, relative to 1,000 L of the electrolyte. The upper limit of the reference value is preferably 100 kAh or less and more preferably 80 kAh or less. 
     The most appropriate reference value of the electric energy is 60 kAh. When the electric energy is less than the reference value, the fluid can be sent to the second flow path, and when the electric energy is not less than the reference value, the fluid can be sent to the first flow path. 
     The electric energy is the product of a current value and time, and thus the accumulated electric energy during electrolysis can be measured, for example, by using an ammeter, a timer, and a calculating device. In other words, the current supplied to electrodes for electrolysis is measured with the ammeter, and the total electrolysis time from the start of electrolysis is measured with the timer such as a clock. The measured values can be multiplied with the calculating device such as a computer to give an accumulated electric energy during electrolysis. The accumulated electric energy during electrolysis can also be measured with a coulombmeter. 
     A fluid (mainly containing hydrogen gas) generated on the cathode, for example, contains 20 to 50 μg of fine particles (calculated assuming that a mist has a specific gravity of 1.0 g/mL) per unit volume (1 liter), and the fine particles have an average particle size of about 0.1 μm with a distribution of ±0.05 μm. 
     In the fluid generated on the cathode, a large difference in particle size distribution of the generated fine particles was not observed even when the electric energy varied. The mist contained in the fluid generated on the cathode has a smaller average particle size than the mist contained in the fluid generated on the anode and thus is unlikely to cause clogging of pipes and valves as compared with the mist contained in the fluid generated on the anode. Hence, the mist contained in the fluid generated on the cathode can be removed from the fluid by using an appropriate removal method. 
     An example of the device for producing fluorine gas of the embodiment will be described in detail with reference to  FIG. 2 . The device for producing fluorine gas in  FIG. 2  is an example including two electrolytic cells, but a single electrolytic cell may be included, or three or more, for example, 10 to 15 electrolytic cells may be included. 
     The device for producing fluorine gas illustrated in  FIG. 2  includes electrolytic cells  11 ,  11  in which an electrolyte  10  is stored and electrolysis is performed, an anode  13  placed in each electrolytic cell  11  and immersed in the electrolyte  10 , and a cathode  15  placed in each electrolytic cell  11 , immersed in the electrolyte  10 , and facing the anode  13 . 
     The inside of each electrolytic cell  11  is sectioned into an anode chamber  22  and a cathode chamber  24  by a partition wall  17  extending from a ceiling face in the electrolytic cell  11  downward in the vertical direction and having a lower end immersed in the electrolyte  10 . In the anode chamber  22 , the anode  13  is placed, and in the cathode chamber  24 , the cathode  15  is placed. The space above the surface of the electrolyte  10  is separated by the partition wall  17  into a space in the anode chamber  22  and a space in the cathode chamber  24 , and a portion of the electrolyte  10  above the lower end of the partition wall  17  is separated by the partition wall  17 , but a portion of the electrolyte  10  below the lower end of the partition wall  17  is not directly separated by the partition wall  17  but continues. 
     The device for producing fluorine gas illustrated in  FIG. 2  includes a first average particle size measurement unit  31  that measures the average particle size of a mist contained in a fluid generated in each electrolytic cell  11  during electrolysis of the electrolyte  10 , a first mist removal unit  32  that removes a mist from a fluid, a fluorine gas selection unit (not illustrated) that selectively collects fluorine gas from a fluid, and a flow path configured to send a fluid from the inside of each electrolytic cell  11  to the fluorine gas selection unit. 
     The device for producing fluorine gas illustrated in  FIG. 2  further includes an ammeter (not illustrated) for measuring the current supplied to the anode  13  and the cathode  15  for electrolysis, a timer (not illustrated) for measuring the total electrolysis time from the start of electrolysis, and a calculating device (not illustrated) for calculating the accumulated electric energy during electrolysis by multiplying the current value measured with the ammeter and the total electrolysis time measured with the timer. The ammeter, the timer, and the calculating device constitute the electric energy measurement unit as a constituent element of the present invention. 
     The flow path includes a first flow path that sends a fluid from the inside of each electrolytic cell  11  through the first mist removal unit  32  to the fluorine gas selection unit and a second flow path that sends the fluid from the inside of each electrolytic cell  11  to the fluorine gas selection unit but not through the first mist removal unit  32 . The flow path also includes a flow path switching unit configured to switch the flow path in which a fluid flows, to the first flow path or the second flow path in accordance with the electric energy measured by the electric energy measurement unit. In other words, at an intermediate point of the flow path extending from the electrolytic cell  11 , the flow path switching unit is provided, and the flow path switching unit can alter the flow path in which a fluid flows. 
     The flow path switching unit sends a fluid from the inside of each electrolytic cell  11  to the first flow path when the electric energy measured by the electric energy measurement unit is not less than a predetermined reference value or sends a fluid from the inside of each electrolytic cell  11  to the second flow path when the electric energy is less than the predetermined reference value. The second flow path has a clogging suppression mechanism that suppresses the clogging of the second flow path with mist. 
     In other words, when the electric energy measured by the electric energy measurement unit is not less than a reference value, the electrolytic cell  11  is connected to a fluorine gas selection unit, and a fluid is sent to the first flow path with the first mist removal unit  32 . When the electric energy measured by the electric energy measurement unit is less than the reference value, the electrolytic cell  11  is connected to a fluorine gas selection unit, and a fluid is sent to the second flow path with the clogging suppression mechanism. 
     As the first mist removal unit  32 , for example, a mist remover capable of removing a mist having an average particle size of 0.4 μm or less from a fluid is used. The type of mist remover, or the system of removing a mist is not specifically limited, but a mist has a small average particle size, and thus, for example, an electric dust collector, a venturi scrubber, or a filter can be used as the mist remover. 
     Of the above mist removers, the mist remover illustrated in  FIG. 3  is preferably used. The mist remover illustrated in  FIG. 3  is a scrubber type mist remover using a liquid hydrogen fluoride as a circulating liquid. The mist remover illustrated in  FIG. 3  can efficiently remove a mist having an average particle size of 0.4 μm or less from a fluid. The mist remover uses a liquid hydrogen fluoride as a circulating liquid. The circulating liquid is preferably cooled in order to reduce the concentration of hydrogen fluoride in a fluorine gas, and thus the concentration of hydrogen fluoride in a fluorine gas can be controlled by adjusting the cooling temperature. 
     The device for producing fluorine gas illustrated in  FIG. 2  will be described in further detail. A first pipe  41  that sends a fluid generated in the anode chamber  22  in each electrolytic cell  11  (hereinafter also called “anode gas”) to the outside connects the electrolytic cell  11  to a fourth pipe  44 , and the anode gases sent from the two electrolytic cells  11 ,  11  are sent through the first pipes  41  to the fourth pipe  44  and are mixed. The main component of the anode gas is fluorine gas, and accessory components are mist, hydrogen fluoride, carbon tetrafluoride, oxygen gas, and water. 
     The fourth pipe  44  is connected to the first mist removal unit  32 , and the anode gas is sent through the fourth pipe  44  to the first mist removal unit  32 . The first mist removal unit  32  removes mist and hydrogen fluoride in the anode gas from the anode gas. The anode gas from which the mist and hydrogen fluoride have been removed is sent from the first mist removal unit  32  through a sixth pipe  46  connected to the first mist removal unit  32  to a fluorine gas selection unit (not illustrated). The fluorine gas selection unit then selectively collects fluorine gas from the anode gas. 
     The first mist removal unit  32  is connected to an eighth pipe  48 , and a liquid hydrogen fluoride as the circulating liquid is supplied through the eighth pipe  48  to the first mist removal unit  32 . The first mist removal unit  32  is further connected to a ninth pipe  49 . The ninth pipe  49  is connected through third pipes  43  to the electrolytic cells  11 ,  11 , and a circulating liquid (liquid hydrogen fluoride) containing a mist and having used to remove a mist in the first mist removal unit  32  is returned from the first mist removal unit  32  to the electrolytic cells  11 ,  11 . 
     The cathode chamber  24  in each electrolytic cell  11  is substantially the same as the anode chamber  22 . In other words, a second pipe  42  that sends a fluid generated in the cathode chamber  24  in each electrolytic cell  11  (hereinafter also called “cathode gas”) to the outside connects the electrolytic cell  11  to a fifth pipe  45 , and the cathode gases sent from the two electrolytic cells  11 ,  11  are sent through the second pipes  42  to the fifth pipe  45  and are mixed. The main component of the cathode gas is hydrogen gas, and accessory components are mist, hydrogen fluoride, and water. 
     The cathode gas contains a fine mist and 5 to 10% by volume of hydrogen fluoride, and thus it is unfavorable to directly discharge the cathode gas to the atmosphere. To address this, the fifth pipe  45  is connected to a second mist removal unit  33 , and the cathode gas is sent through the fifth pipe  45  to the second mist removal unit  33 . The second mist removal unit  33  removes mist and hydrogen fluoride in the cathode gas from the cathode gas. The cathode gas from which the mist and hydrogen fluoride have been removed is discharged from the second mist removal unit  33  through a seventh pipe  47  connected to the second mist removal unit  33  to the atmosphere. The type of second mist removal unit  33 , or the system of removing a mist is not specifically limited, and a scrubber type mist remover using an aqueous alkali solution as the circulating liquid can be used. 
     The pipe diameters and the installation directions (i.e., a pipe extending direction, for example, the vertical direction, the horizontal direction) of the first pipe  41 , the second pipe  42 , the fourth pipe  44 , and the fifth pipe  45  are not specifically limited. The first pipe  41  and the second pipe  42  are preferably installed so as to extend from the electrolytic cell  11  in the vertical direction and preferably have a pipe diameter such that fluids flowing in the first pipe  41  and the second pipe  42  have a flow rate of 30 cm/sec or less in a normal state. In such conditions, even when a mist contained in a fluid falls under its own weight, the mist settles in the electrolytic cell  11 , and thus the clogging in the first pipe  41  and the second pipe  42  with fine particles is unlikely to be caused. 
     The fourth pipe  44  and the fifth pipe  45  are preferably installed so as to extend in the horizontal direction and preferably have a pipe diameter such that fluids flowing in the fourth pipe  44  and the fifth pipe  45  have a flow rate about 1 to 10 times more than that in the first pipe  41  and the second pipe  42 . 
     A second bypass pipe  52  for sending the anode gas to the outside of the electrolytic cell  11  is further provided separately from the first pipe  41 . In other words, the second bypass pipe  52  connects each electrolytic cell  11  to a first bypass pipe  51 , and the anode gases sent from the two electrolytic cells  11 ,  11  are sent through the second bypass pipes  52  to the first bypass pipe  51  and are mixed. Through the first bypass pipe  51 , the anode gas is sent to a fluorine gas selection unit (not illustrated). The fluorine gas selection unit selectively collects fluorine gas from the anode gas. The fluorine gas selection unit connected to the first bypass pipe  51  may be the same as or different from the fluorine gas selection unit connected to the sixth pipe  46 . 
     The pipe diameter and the installation direction of the second bypass pipe  52  are not specifically limited, and the second bypass pipe  52  is preferably installed so as to extend from the electrolytic cell  11  in the vertical direction and preferably has a pipe diameter such that a fluid flowing in the second bypass pipe  52  has a flow rate of 30 cm/sec or less in a normal state. 
     The first bypass pipe  51  is installed so as to extend in the horizontal direction. The first bypass pipe  51  has a larger pipe diameter than the fourth pipe  44 , and the pipe diameter of the first bypass pipe  51  is such a size as to be unlikely to cause clogging of the first bypass pipe  51  with depositing fine particles. The first bypass pipe  51  has a larger pipe diameter than the fourth pipe  44 , and this functions as the clogging suppression mechanism. 
     The pipe diameter of the first bypass pipe  51  is preferably more than 1.0 time and not more than 3.2 times that of the fourth pipe  44  and more preferably not less than 1.05 times and not more than 1.5 times. In other words, the first bypass pipe  51  preferably has a flow path cross-sectional area not more than 10 times that of the fourth pipe  44 . 
     As apparent from the above description, the first pipes  41  and the fourth pipe  44  constitute the above first flow path, and the first bypass pipe  51  and the second bypass pipes  52  constitute the above second flow path. The first bypass pipe  51  included in the second flow path has the clogging suppression mechanism. 
     Next, the flow path switching unit will be described. Each first pipe  41  has a first pipe valve  61 . By switching the first pipe valve  61  to an open state or a closed state, whether the anode gas is sent from the electrolytic cell  11  to the first mist removal unit  32  can be controlled. Each second bypass pipe  52  has a bypass valve  62 . By switching the bypass valve  62  to an open state or a closed state, whether the anode gas is sent from the electrolytic cell  11  to the first bypass pipe  51  can be controlled. 
     Between the electrolytic cells  11  and the first mist removal unit  32 , specifically, at an intermediate point of the fourth pipe  44  and at the downstream side of the junctions to the first pipes  41 , a first average particle size measurement unit  31  is provided. The first average particle size measurement unit  31  measures the average particle size of a mist contained in the anode gas flowing in the fourth pipe  44 . By analyzing fluorine gas and nitrogen gas contained in the anode gas after measuring the average particle size of a mist, the current efficiency in the production of fluorine gas can be determined. 
     At an intermediate point of the first bypass pipe  51  and at the downstream side of the junctions to the second bypass pipes  52 , a second average particle size measurement unit  34  is also provided, and the second average particle size measurement unit  34  measures the average particle size of a mist contained in the anode gas flowing in the first bypass pipe  51 . The device for producing fluorine gas illustrated in  FIG. 2  may not include the first average particle size measurement unit  31  or the second average particle size measurement unit  34 . 
     The device for producing fluorine gas illustrated in  FIG. 2  includes the electric energy measurement unit as described above. The electric energy measurement unit may be installed at any position and may be installed on the electrolytic cell  11 , for example. The electric energy measurement unit may be installed at any position of the device for producing fluorine gas where the current supplied to the anode  13  and the cathode  15  for electrolysis and the total electrolysis time from the start of electrolysis can be measured, and the accumulated electric energy during electrolysis can be calculated. The ammeter, the timer, and the calculating device constituting the electric energy measurement unit may be an integrated device or separate devices. 
     The accumulated electric energy during electrolysis is measured by the electric energy measurement unit. When the measurement result is less than a predetermined reference value, the bypass valve  62  is switched to an open state to send the anode gas from the electrolytic cell  11  to the first bypass pipe  51 , and the first pipe valve  61  is switched to a closed state not to send the anode gas to the fourth pipe  44  and the first mist removal unit  32 . In other words, the anode gas is sent to the second flow path. 
     In contrast, when the measurement result is not less than a predetermined reference value, the first pipe valve  61  is switched to an open state to send the anode gas to the fourth pipe  44  and the first mist removal unit  32 , and the bypass valve  62  is switched to a closed state not to send the anode gas from the electrolytic cell  11  to the first bypass pipe  51 . In other words, the anode gas is sent to the first flow path. 
     As apparent from the above description, the first pipe valve  61  and the bypass valve  62  constitute the above flow path switching unit. 
     As described above, by operating the device for producing fluorine gas while the flow path is switched in accordance with the accumulated electric energy during electrolysis, continuous operation can be smoothly performed while clogging of pipes and valves with mist is suppressed. By using the device for producing fluorine gas illustrated in  FIG. 2 , fluorine gas can be economically produced. 
     For example, as the mist removal unit, a plurality of pipes with filters may be prepared, and electrolysis may be performed while the pipes are appropriately switched to exchange the filters. 
     A time period when frequent exchange of filters is needed and a time period when frequent exchange of filters is not needed can be determined by measuring the accumulated electric energy during electrolysis. By appropriately controlling the switching frequency of pipes in which a fluid flows on the basis of the above determination, the device for producing fluorine gas can be efficiently, continuously operated. 
     Alternative embodiments of the device for producing fluorine gas illustrated in  FIG. 2  will next be described. 
     First Alternative Embodiment 
     A first alternative embodiment will be described with reference to  FIG. 4 . In the device for producing fluorine gas illustrated in  FIG. 2 , the second bypass pipes  52  connect the electrolytic cells  11  to the first bypass pipe  51 . In contrast, in a device for producing fluorine gas in the first alternative embodiment illustrated in  FIG. 4 , second bypass pipes  52  connect first pipes  41  to a first bypass pipe  51 . The device for producing fluorine gas in the first alternative embodiment has substantially the same constitution as the device for producing fluorine gas in  FIG. 2  except the above structure, and thus similar structures are not described. 
     Second Alternative Embodiment 
     A second alternative embodiment will be described with reference to  FIG. 5 . A device for producing fluorine gas in the second alternative embodiment illustrated in  FIG. 5  includes a single electrolytic cell  11 . A first average particle size measurement unit  31  is not provided on a fourth pipe  44  but on a first pipe  41  and is provided at the upstream side of a first pipe valve  61 . The device includes no second bypass pipe  52 , and a first bypass pipe  51  is directly connected to an electrolytic cell  11  but not through a second bypass pipe  52 . 
     The first bypass pipe  51  has a larger diameter than the fourth pipe  44  and thus functions as the clogging suppression mechanism. A mist pool space is further provided, for example, at the downstream end of the first bypass pipe  51 , and this can further improve the clogging suppression effect. 
     Examples of the mist pool space include a space formed from the downstream end portion of the first bypass pipe  51  and having a larger pipe diameter than the center portion in the installation direction (for example, a pipe diameter not less than 4 times that at the center portion in the installation direction) and a space formed from the downstream end portion of the first bypass pipe  51  and having a container shape. The mist pool space can suppress clogging of the first bypass pipe  51 . This is aimed at a clogging suppression effect by a large flow path cross-sectional area and a clogging suppression effect using mist free fall by a reduction in linear velocity of a flowing gas. 
     In addition, a bypass valve  62  is provided on a third bypass pipe  53  that connects the first bypass pipe  51  to a fluorine gas selection unit (not illustrated). The device for producing fluorine gas in the second alternative embodiment has substantially the same constitution as the device for producing fluorine gas in  FIG. 2  except the above structure, and thus similar structures are not described. 
     Third Alternative Embodiment 
     A third alternative embodiment will be described with reference to  FIG. 6 . In a device for producing fluorine gas in the third alternative embodiment, a first average particle size measurement unit  31  is provided on an electrolytic cell  11 , and the average particle size of a mist is measured by introducing the anode gas in the electrolytic cell  11  directly into the first average particle size measurement unit  31 . The device for producing fluorine gas in the third alternative embodiment has no second average particle size measurement unit  34 . The device for producing fluorine gas in the third alternative embodiment has substantially the same constitution as the device for producing fluorine gas in the second alternative embodiment except the above structure, and thus similar structures are not described. 
     Fourth Alternative Embodiment 
     A fourth alternative embodiment will be described with reference to  FIG. 7 . A device for producing fluorine gas in the fourth alternative embodiment differs from that in the second alternative embodiment illustrated in  FIG. 5  in the clogging suppression mechanism. In the device for producing fluorine gas in the second alternative embodiment, the first bypass pipe  51  is provided so as to extend in the horizontal direction. In the device for producing fluorine gas in the fourth alternative embodiment, a first bypass pipe  51  is inclined relative to the horizontal direction and extends downward from the upstream side to the downstream side. This inclination prevents fine particles from depositing in the first bypass pipe  51 . As the inclination is larger, the effect of suppressing fine particle deposition is larger. 
     The inclination angle of the first bypass pipe  51  is preferably 30 degrees or more and more preferably 40 degrees or more and 60 degrees or less where the depression angle from the horizontal plane is less than 90 degrees. When the first bypass pipe  51  is about to be clogged, hammering the inclined first bypass pipe  51  facilitates moving the deposit in the first bypass pipe  51 , and thus clogging can be prevented. 
     The device for producing fluorine gas in the fourth alternative embodiment has substantially the same constitution as the device for producing fluorine gas in the second alternative embodiment except the above structure, and thus similar structures are not described. 
     Fifth Alternative Embodiment 
     A fifth alternative embodiment will be described with reference to  FIG. 8 . A device for producing fluorine gas in the fifth alternative embodiment differs from that in the third alternative embodiment illustrated in  FIG. 6  in the clogging suppression mechanism. In the device for producing fluorine gas in the third alternative embodiment, the first bypass pipe  51  is provided so as to extend in the horizontal direction. In the device for producing fluorine gas in the fifth alternative embodiment, a first bypass pipe  51  is inclined relative to the horizontal direction and extends downward from the upstream side to the downstream side. This inclination prevents fine particles from depositing in the first bypass pipe  51 . The inclination angle of the first bypass pipe  51  is preferably substantially the same as in the fourth alternative embodiment. The device for producing fluorine gas in the fifth alternative embodiment has substantially the same constitution as the device for producing fluorine gas in the third alternative embodiment except the above structure, and thus similar structures are not described. 
     Sixth Alternative Embodiment 
     A sixth alternative embodiment will be described with reference to  FIG. 9 . A device for producing fluorine gas in the sixth alternative embodiment differs from that in the second alternative embodiment illustrated in  FIG. 5  in the structure of an electrolytic cell  11 . The electrolytic cell  11  has one anode  13  and two cathodes  15 ,  15  and is sectioned into one anode chamber  22  and one cathode chamber  24  by a cylindrical partition wall  17  surrounding the one anode  13 . The anode chamber  22  is formed to extend above the top face of the electrolytic cell  11 , and a first bypass pipe  51  is connected to the top section of the anode chamber  22  of the electrolytic cell  11 . The device for producing fluorine gas in the sixth alternative embodiment has substantially the same constitution as the device for producing fluorine gas in the second alternative embodiment except the above structure, and thus similar structures are not described. 
     Seventh Alternative Embodiment 
     A seventh alternative embodiment will be described with reference to  FIG. 10 . A device for producing fluorine gas in the seventh alternative embodiment differs from that in the sixth alternative embodiment illustrated in  FIG. 9  in the structure of a first bypass pipe  51 . In other words, in the device for producing fluorine gas in the seventh alternative embodiment, a first bypass pipe  51  is inclined relative to the horizontal direction and extends downward from the upstream side to the downstream side as with the fourth alternative embodiment and the fifth alternative embodiment. The inclination angle of the first bypass pipe  51  is preferably substantially the same as in the fourth alternative embodiment. The device for producing fluorine gas in the seventh alternative embodiment has substantially the same constitution as the device for producing fluorine gas in the sixth alternative embodiment except the above structure, and thus similar structures are not described. 
     Eighth Alternative Embodiment 
     An eighth alternative embodiment will be described with reference to  FIG. 11 . A device for producing fluorine gas in the eighth alternative embodiment differs from that in the second alternative embodiment illustrated in  FIG. 5  in the clogging suppression mechanism. In the device for producing fluorine gas in the eighth alternative embodiment, a rotary screw  71  constituting the clogging suppression mechanism is provided in a first bypass pipe  51 . The rotary screw  71  has a rotating shaft that is parallel to the longitudinal direction of the first bypass pipe  51 . 
     The rotary screw  71  is rotated by a motor  72 , and accordingly a mist deposited in the first bypass pipe  51  can be sent to the upstream side or the downstream side. This structure prevents fine particles from depositing in the first bypass pipe  51 . The device for producing fluorine gas in the eighth alternative embodiment has substantially the same constitution as the device for producing fluorine gas in the second alternative embodiment except the above structure, and thus similar structures are not described. 
     Ninth Alternative Embodiment 
     A ninth alternative embodiment will be described with reference to  FIG. 12 . A device for producing fluorine gas in the ninth alternative embodiment differs from that in the second alternative embodiment illustrated in  FIG. 5  in the clogging suppression mechanism. In the device for producing fluorine gas in the ninth alternative embodiment, an airflow generator  73  constituting the clogging suppression mechanism is provided on a first bypass pipe  51 . The airflow generator  73  sends an airflow (for example, a nitrogen gas stream) from the upstream side toward the downstream side in the first bypass pipe  51  and increases the flow rate of an anode gas flowing in the first bypass pipe  51 . This structure prevents fine particles from depositing in the first bypass pipe  51 . 
     In the embodiment, the flow rate of an anode gas flowing in the first bypass pipe  51  is preferably 1 m/sec or more and 10 m/sec or less. The flow rate can be increased to more than 10 m/sec, but in such a case, the pipe resistance in the first bypass pipe  51  increases the pressure loss, and the pressure in an anode chamber  22  of an electrolytic cell  11  increases. The pressure in the anode chamber  22  and the pressure in a cathode chamber  24  are preferably substantially the same. If the difference between the pressure in the anode chamber  22  and the pressure in the cathode chamber  24  were excessively large, an anode gas could go over a partition wall  17  and flow into the cathode chamber  24 , and fluorine gas could be reacted with hydrogen gas to impair fluorine gas generation. 
     The device for producing fluorine gas in the ninth alternative embodiment has substantially the same constitution as the device for producing fluorine gas in the second alternative embodiment except the above structure, and thus similar structures are not described. 
     Tenth Alternative Embodiment 
     A tenth alternative embodiment will be described with reference to  FIG. 13 . In a device for producing fluorine gas in the tenth alternative embodiment, a first average particle size measurement unit  31  is provided on an electrolytic cell  11 , and the average particle size of a mist is measured by introducing the anode gas in the electrolytic cell  11  directly into the first average particle size measurement unit  31 . The device for producing fluorine gas in the tenth alternative embodiment has no second average particle size measurement unit  34 . The device for producing fluorine gas in the tenth alternative embodiment has substantially the same constitution as the device for producing fluorine gas in the ninth alternative embodiment illustrated in  FIG. 12  except the above structure, and thus similar structures are not described. 
     EXAMPLES 
     The present invention will next be described more specifically with reference to examples and comparative examples. 
     Reference Example 1 
     An electrolyte was electrolyzed to produce fluorine gas. As the electrolyte, a mixed molten salt (560 L) of 434 kg of hydrogen fluoride and 630 kg of potassium fluoride was used. As the anode, 16 amorphous carbon electrodes manufactured by SGL Carbon (30 cm in width, 45 cm in length, and 7 cm in thickness) were placed in an electrolytic cell. As the cathode, punching plates formed from Monel (trademark) were placed in the electrolytic cell. One anode faced two cathodes, and portions of one anode facing the cathodes had a total area of 1,736 cm. 
     The electrolysis temperature was controlled at 85 to 95° C. First, the temperature of the electrolyte was set at 85° C., and a direct current of 1,000 A was applied at a current density of 0.036 A/cm 2  to start electrolysis. At the start, the electrolyte had a water concentration of 1.01% by mass. The water concentration was measured by Karl Fischer analysis method. 
     Electrolysis was started in the above conditions, and small explosive sound was observed near the anodes in the anode chamber until the accumulated electric energy reached 10 kAh after the start of the electrolysis. The explosive sound is supposed to be caused by reaction of fluorine gas generated and water in the electrolyte. 
     The fluid generated on the anodes at this stage was collected when sent out from the anode chamber of the electrolytic cell to the outside, and the mist contained in the fluid was analyzed. As a result, 1 L of the fluid generated on the anodes contained 5.0 to 9.0 mg of fine particles (calculated assuming that the mist has a specific gravity of 1.0 g/mL, hereinafter the same is applied), and the fine particles had an average particle size of 1.0 to 2.0 μm. The fine particles were observed under an optical microscope, and particles having a hollow spherical shape were mainly observed. At this stage, the current efficiency of fluorine gas production was 0 to 15%. 
     The electrolysis continued until the accumulated electric energy reached 30 kAh, and the frequency of explosive sound in the anode chamber was reduced. At this stage, the electrolyte had a water concentration of 0.7% by mass. The fluid generated on the anodes at this stage was collected when sent out from the anode chamber of the electrolytic cell to the outside, and the mist contained in the fluid was analyzed. As a result, 1 L of the fluid generated on the anodes contained 0.4 to 1.0 mg of a mist, and the mist had an average particle size of 0.5 to 0.7 μm. At this stage, the current efficiency of fluorine gas production was 15 to 55%. The step of electrolysis from the start of electrolysis to this stage is regarded as “step (1)”. 
     Following the step (1), the electrolyte was continuously electrolyzed. Accordingly, hydrogen fluoride was consumed, and the level of the electrolyte was reduced. Hence, hydrogen fluoride was appropriately resupplied from a hydrogen fluoride tank into the electrolytic cell. The hydrogen fluoride to be resupplied had a water concentration of 500 ppm by mass or less. 
     When the electrolysis was continued, and the accumulated electric energy exceeded 60 kAh, the mist contained in the fluid generated on the anodes had an average particle size of 0.36 μm (i.e., 0.4 μm or less). At this stage, absolutely no explosive sound was observed in the anode chamber. At this stage, the electrolyte had a water concentration of 0.2% by mass (i.e., 0.3% by mass or less). At this stage, the current efficiency of fluorine gas production was 65%. The step of electrolysis from the end of the step (1) to this stage is regarded as “step (2)”. 
     Following the step (2), the current was increased to 3,500 A to increase the current density to 0.126 A/cm 2 , and the electrolyte was continuously electrolyzed. The fluid generated on the anodes at this stage was collected when sent out from the anode chamber of the electrolytic cell to the outside, and the mist contained in the fluid was analyzed. As a result, 1 L of the fluid generated on the anodes contained 0.03 to 0.06 mg of fine particles, and the fine particles had an average particle size of about 0.2 μm (0.15 to 0.25 μm) with a particle size distribution of about 0.1 to 0.5 μm.  FIG. 14  illustrates the measurement result of particle size distribution of the fine particles. At this stage, the current efficiency of fluorine gas production was 94%. The step of electrolysis from the end of the step (2) to this stage is regarded as “stable step”. 
     Details of the electrolysis performed as above in Reference Example 1 are summarized in Table 1. Table 1 illustrates electric current, electrolysis time, electric energy, the water concentration in an electrolyte, the mass of a mist contained in 1 L of a fluid generated on the anodes (“anode gas” in Table 1), the average particle size of a mist, current efficiency, the amount of a fluid (containing fluorine gas, oxygen gas, and a mist) generated on the anodes, the amount of a mist generated on the anodes, the intensity of explosive sound, and the water concentration in a fluid formed on the cathodes (the water concentration in a cathode gas” in Table 1). 
     A graph representing the relation between average particle size of a mist and amount of the mist generated on the anodes is illustrated in  FIG. 15 . The graph in  FIG. 15  reveals that the average particle size of a mist has a relation to the amount of the mist generated on the anodes. As the amount of a mist generated increases, the clogging of pipes and valves is more frequently caused. When a mist having an average particle size of more than 0.4 μm is generated, the amount of a mist generated increases, and the mist is settled by gravity. The relation represented by the graph in  FIG. 15  therefore illustrates a relation between the average particle size of a mist and likelihood of clogging of pipes and valves. 
     A graph representing the relation between average particle size of a mist and accumulated electric energy is illustrated in  FIG. 16 . As the average particle size of a mist increases, the clogging of pipes and valves is more frequently caused. The relation represented by the graph in  FIG. 16  therefore illustrates a relation between accumulated electric energy and likelihood of clogging of pipes and valves. 
     
       
         
           
               
               
               
               
               
               
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                   
                   
                 Water 
                   
                   
                   
                 Water 
               
               
                   
                 Electrolysis 
                 concentration in 
                 Mist in anode gas 
                 Current 
                 Intensity of 
                 concentration in 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 Electric 
                 Elapsed 
                 Electric energy 
                 electrolyte, 
                   
                 Average particle size 
                 efficiency 
                 explosive 
                 cathode gas (% 
               
               
                 Step 
                 current (A) 
                 time (h) 
                 (kAh) 
                 (% by mass) 
                 Amount (mg/L) 
                 (μm) 
                 (%) 
                 sound (dB) 
                 by volume) 
               
               
                   
               
               
                 Step (1) 
                 1000 
                 0-30 
                 0-30 
                 1.0 
                 5.0-9.0 
                 1.0-2.0 
                  0-15 
                 50-70 
                 0.10 
               
               
                 Step (1) 
                 1000 
                 30 
                 30   
                 0.7 
                 0.4-1.0 
                 0.5-0.7 
                 15-55 
                 25-35 
                 0.07 
               
               
                 Step (2) 
                 1000 
                 60 
                 60   
                 0.2 
                 not measured 
                 0.36 
                 65 
                 15-30 
                 0.02 
               
               
                 Stable step 
                 3500 
                 65 
                 77.5 
                 less than 0.2 
                 0.03-0.06 
                 0.15-0.25 
                 94 
                 2-5 
                 not measured 
               
               
                   
               
            
           
         
       
     
     Example 1 
     Electrolysis was performed in the same manner as in Reference Example 1 using the device for producing fluorine gas illustrated in  FIG. 2 . In the electrolysis in the step (1), the fluid generated on the anodes was allowed to flow through the second bypass pipes, the bypass valves, and the first bypass pipe. After the completion of electrolysis in the step (1), the electrolysis was once stopped, and the inside of the device for producing fluorine gas was inspected. As a result, a mist deposited in the first bypass pipe, but the first bypass pipe had a large pipe diameter, and thus the pipe was not clogged. 
     The electrolysis reached the step (2) where the mist had an average particle size of 0.4 μm or less (the accumulated electric energy reached the reference value, 60 kAh), and thus the fluid generated on the anodes was allowed to flow through the first pipes, the first pipe valves, the fourth pipe, and the first mist removal unit. Neither mist deposition nor clogging was caused in the first pipes, the first pipe valves, or the fourth pipe, but the fluid generated on the anodes was fed to the first mist removal unit, and the mist was removed by the first mist removal unit. The first mist removal unit was a scrubber type mist remover that sprayed liquid hydrogen fluoride to remove microparticles such as a mist and had a mist removal rate of 98% or more. 
     Comparative Example 1 
     Electrolysis was performed in the same manner as in Example 1 except that the fluid generated on the anodes in the electrolysis in the step (1) was allowed to flow through the first pipes, the first pipe valves, the fourth pipe, and the first mist removal unit. 
     During the electrolysis in the step (1), of pressure gauges attached to the anode side and the cathode side of the electrolytic cell, the measured value of the pressure gauge at the anode side gradually increased, and the differential pressure from the pressure at the cathode side reached 90 mmH 2 O. The electrolysis was thus stopped. The reason for the stop is described below. Of the partition wall in the electrolytic cell, a portion immersed in the electrolyte had a vertical length (immersion depth) of 5 cm. If the pressure at the anode side were higher than the pressure at the cathode side by about 100 mmH 2 O, the surface of the electrolyte at the anode side would be below the lower end of the partition wall. As a result, fluorine gas could flow over the partition wall and be mixed with hydrogen gas at the cathode side to suddenly cause dangerous reaction between fluorine gas and hydrogen gas. 
     After the system was purged with nitrogen gas or the like, the insides of the first pipes, the first pipe valves, and the fourth pipe were inspected. The first pipes were not clogged because the pipes extended in the vertical direction. Deposition of a small amount of fine particles was observed in the first pipe valves, and the inlet portions to the downstream pipe of the first pipe valves, or to the fourth pipe, were clogged with fine particles. Deposition of fine particles was also observed in the fourth pipe, but the deposition was such a small amount as not to clog the pipe. 
     REFERENCE SIGNS LIST 
     
         
           1  sample chamber 
           2  light source 
           3  scattered light detection unit 
           4 A,  4 B transparent window 
           10  electrolyte 
           11  electrolytic cell 
           13  anode 
           15  cathode 
           22  anode chamber 
           24  cathode chamber 
           31  first average particle size measurement unit 
           32  first mist removal unit 
           33  second mist removal unit 
           34  second average particle size measurement unit 
           41  first pipe 
           42  second pipe 
           43  third pipe 
           44  fourth pipe 
           45  fifth pipe 
           46  sixth pipe 
           47  seventh pipe 
           48  eighth pipe 
           49  ninth pipe 
           51  first bypass pipe 
           52  second bypass pipe 
           61  first pipe valve 
           62  bypass valve 
         F fluid 
         L light for light scattering measurement 
         M mist 
         S scattered light