Fluorine gas production method

Abstract

There is provided a fluorine gas production method hardly generating mist. A fluorine gas is produced by a method including an electrolysis step of electrolyzing an electrolyte using an anode and a cathode to generate a fluorine gas in an electrolytic cell. The electrolyte contains hydrogen fluoride, metal fluoride, and metal hexafluoronickelate (IV). The anode is a nickel electrode or a carbonaceous electrode, and the cathode is a metal electrode.

Description

TECHNICAL FIELD

[0001] The present invention relates to a fluorine gas production method.BACKGROUND ART

[0002] A fluorine gas can be synthesized by electrolyzing an electrolyte containing hydrogen fluoride and metal fluoride. The electrolysis of an electrolyte generates mist (e.g., mist of the electrolyte) together with a fluorine gas, and therefore a fluorine gas sent out from an electrolytic cell is accompanied by the mist. The mist accompanied by the fluorine gas sometimes becomes powder, so that there is a risk of blocking pipes or valves used to send the fluorine gas. Therefore, an operation of producing a fluorine gas has been sometimes forced to be interrupted or stopped, which has hindered a continuous operation in the fluorine gas production by an electrolysis method.

[0003] To suppress the blockage of pipes or valves by mist, PTL 1 discloses a technology of heating a pipe, through which a fluorine gas accompanying mist or the fluorine gas passes, to be equal to or higher than the melting point of an electrolyte. PTL 2 discloses a gas generating device having a gas diffusion part, which is a space where mist is roughly collected, and a filler storage part storing a filler for adsorbing mist.CITATION LISTPatent LiteraturesPTL 1: JP 5584904 B

[0005] PTL 2: JP 5919824 BSUMMARY OF INVENTIONTechnical Problem

[0006] However, to more effectively suppress the blockage of pipes or valves by mist, a fluorine gas production technology of suppressing the generation itself of mist has been desired.

[0007] It is an object of the present invention to provide a fluorine gas production method hardly generating mist.Solution to Problem

[0008] To solve the above-described problem, one aspect of the present invention is as described in [1] to [8] below.

[0009] [1] A fluorine gas production method including: an electrolysis step of electrolyzing an electrolyte using an anode and a cathode to generate a fluorine gas in an electrolytic cell, in which

[0010] the electrolyte contains hydrogen fluoride, metal fluoride, and metal hexafluoronickelate (IV), and

[0011] the anode is a nickel electrode or a carbonaceous electrode, and the cathode is a metal electrode.

[0012] [2] The fluorine gas production method according to [1], in which the metal hexafluoronickelate (IV) is at least one of potassium hexafluoronickelate (IV), cesium hexafluoronickelate (IV), and rubidium hexafluoronickelate (IV).

[0013] [3] The fluorine gas production method according to [1] or [2], in which the concentration of nickel (IV) ions in the electrolyte is 100 ppm by mass or more and 1000 ppm by mass or less.

[0014] [4] The fluorine gas production method according to any one of [1] to [3], in which the metal fluoride is at least one of potassium fluoride and cesium fluoride.

[0015] [5] The fluorine gas production method according to any one of [1] to [4], in which the ratio of the molar amount of the hydrogen fluoride to the molar amount of the metal fluoride contained in the electrolyte is 1.6 or more and 3.2 or less.

[0016] [6] The fluorine gas production method according to any one of [1] to [5], in which the cathode is an electrode formed of at least one of iron, nickel, copper, and a copper nickel alloy.

[0017] [7] The fluorine gas production method according to any one of [1] to [6], in which the electrolyte is a mixture of the hydrogen fluoride, the metal fluoride, and the metal hexafluoronickelate (IV) having a water content of 0.5% by mass or less.

[0018] [8] The fluorine gas production method according to any one of [1] to [7], in which the concentration of iron ions in the electrolyte is 500 ppm by mass or less, and the concentration of copper ions is 200 ppm by mass or less.Advantageous Effects of Invention

[0019] According to the present invention, mist is hardly generated when a fluorine gas is produced by electrolyzing an electrolyte.BRIEF DESCRIPTION OF DRAWINGS

[0020] FIG. 1 is a graph showing the correlation between the current density in electrolysis and the number of mists contained in a fluorine gas;

[0021] FIG. 2 is a graph showing the correlation between the current density in electrolysis and the total volume of mists contained in the fluorine gas; and

[0022] FIG. 3 is a graph showing the particle size distribution of the mists contained in the fluorine gas.DESCRIPTION OF EMBODIMENTS

[0023] One embodiment of the present invention will now be described. The embodiment is merely one example of the present invention, and the present invention is not limited to the embodiment. Various modifications or improvements can be made in the embodiment, and such modifications and improvements can be encompassed by the present invention.

[0024] The present inventors have conducted extensive studies on mist causing blockage of pipes or valves in electrolytic synthesis of a fluorine gas (F2). The “mist” in the present invention refers to liquid fine particles or solid fine particles generated together with a fluorine gas in an electrolytic cell by electrolysis of an electrolyte. Specifically, the mist refers to fine particles of an electrolyte, solid fine particles resulting from a phase change of fine particles of an electrolyte, and solid fine particles generated by a reaction of members constituting an electrolytic cell (metal constituting an electrolytic cell, a packing for an electrolytic cell, a carbonaceous electrode, and the like) and a fluorine gas.

[0025] The present inventors have studied mist contained in a fluid generated inside an electrolytic cell in electrolysis of an electrolyte, and, as a result, have found that the generation of mist can be suppressed by adding an additive to the electrolyte, and thus have accomplished the present invention. One embodiment of the present invention is described below.

[0026] A fluorine gas production method according to this embodiment includes an electrolysis step of electrolyzing an electrolyte using an anode and a cathode to generate a fluorine gas in an electrolytic cell. The electrolyte contains hydrogen fluoride (HF), metal fluoride, and metal hexafluoronickelate (IV). The anode is a nickel electrode or a carbonaceous electrode, and the cathode is a metal electrode.

[0027] The metal hexafluoronickelate (IV) is added as an additive to the electrolyte, and therefore, when the electrolyte is electrolyzed to produce a fluorine gas, mist is hardly generated. Mist accompanied by the fluorine gas becomes a powder, so that there is a risk of blocking pipes or valves used to send the fluorine gas. However, the amount of the mist generated in the production of the fluorine gas is small, and therefore the blockage of the pipes or valves used to send the fluorine gas can be suppressed without attaching a mist removal device to a fluorine gas production device. As a result, the frequency of interruptions and stoppages in an operation of producing a fluorine gas can be reduced, and thus the cost of producing a fluorine gas can be lowered.

[0028] The fluorine gas production method of this embodiment is described in more detail below.

[0029] There are no particular restrictions on the mode of the electrolytic cell, and any electrolytic cell can be used insofar as a fluorine gas can be generated by electrolyzing an electrolyte containing hydrogen fluoride and metal fluoride.

[0030] In usual, the inside of the electrolytic cell is partitioned by a partition member, such as a diaphragm, into an anode chamber where an anode is arranged and a cathode chamber where a cathode is arranged, so that a fluorine gas generated at the anode and a hydrogen generated at the cathode are not mixed.

[0031] As the anode, a carbonaceous electrode formed of a carbon material, such as diamond, diamond-like carbon, amorphous carbon, graphite, or glassy carbon, can be used, for example. As the anode, in addition to the above-described carbonaceous electrode, a nickel electrode formed of nickel (Ni), a platinum electrode formed of platinum (Pt), a gold electrode formed of gold (Au), and metal electrodes formed of the other types of metals can also be used, for example. As the cathode, metal electrodes formed of metals, such as iron (Fe), nickel, copper (Cu), and a copper nickel alloy (e.g., Monel (trademark)), can be used, for example.

[0032] The electrolyte contains hydrogen fluoride and metal fluoride. The type of the metal fluoride is not particularly limited, and is preferably alkali metal fluoride and more preferably at least one of potassium fluoride (KF) and cesium fluoride (CsF). One type of metal fluoride may be used alone or two or more types of metal fluorides may be used in combination. More specifically, potassium fluoride and cesium fluoride may be used in combination as the metal fluoride.

[0033] The ratio of the molar amount of the hydrogen fluoride to the molar amount of the metal fluoride contained in the electrolyte ([Molar amount of hydrogen fluoride] / [Molar amount of metal fluoride]) is preferably 1.6 or more and 3.2 or less and more preferably 1.9 or more and 3.0 or less.

[0034] As the electrolyte, a mixed molten salt of hydrogen fluoride and potassium fluoride can be used, for example. In the mixed molten salt of hydrogen fluoride and potassium fluoride, the molar ratio between the hydrogen fluoride and the potassium fluoride can be set to Hydrogen fluoride:Potassium fluoride=1.5 to 2.5:1, for example. KF·2HF in the case of Hydrogen fluoride:Potassium fluoride=2:1 is a typical electrolyte, and this mixed molten salt has a melting point of approximately 72° C.

[0035] As an example of the electrolyte, a mixed molten salt of hydrogen fluoride and cesium fluoride can also be used. In the mixed molten salt of hydrogen fluoride and cesium fluoride, the molar ratio between the hydrogen fluoride and the cesium fluoride can be set to Hydrogen fluoride:Cesium fluoride=1.8 to 3.1:1, for example. CsF·2.4HF in the case of Hydrogen fluoride:Cesium fluoride=2.4:1 is a typical electrolyte, and this mixed molten salt has a melting point of approximately 16° C.

[0036] This electrolyte is corrosive, and therefore a site that comes into contact with the electrolyte, such as the inner surface of the electrolytic cell, is preferably formed of metal, such as iron, nickel, or Monel (trademark).

[0037] The electrolysis consumes the hydrogen fluoride in the electrolyte, and therefore hydrogen fluoride is preferably continuously or intermittently supplied to the electrolyte during the electrolysis, for example. The hydrogen fluoride may be supplied to the electrolyte on the cathode chamber side of the electrolytic cell or may be supplied to the electrolyte on the anode chamber side. When the concentration of the hydrogen fluoride in a case where the electrolyte is KF·2HF (hydrogen fluoride concentration: 40.4% by mass) or CsF·2.4HF (hydrogen fluoride concentration: 24.0% by mass) is set as the reference concentration, the concentration of the hydrogen fluoride in the electrolyte is preferably controlled to fluctuate in the range of −5% by mass or more and +5% by mass or less of the reference concentration, more preferably controlled to fluctuate in the range of −2.5% by mass or more and +2.5% by mass or less of the reference concentration, and still more preferably controlled to fluctuate in the range of −1.5% by mass or more and +1.5% by mass or less of the reference concentration.

[0038] In the electrolysis of the electrolyte, a direct current is applied to the anode and the cathode, and gas containing a fluorine gas is generated at the anode and gas containing a hydrogen gas is generated at the cathode. The hydrogen fluoride of the electrolyte has vapor pressure, and therefore the gases generated at the anode and the cathode are individually accompanied by the hydrogen fluoride. Further, in the production of the fluorine gas by the electrolysis of the electrolyte, the gases generated by the electrolysis contain mist of the electrolyte. Thus, a gas phase portion of the electrolytic cell contains the gases generated by the electrolysis, the hydrogen fluoride, and the mist of the electrolyte. Therefore, one sent out from the inside of the electrolytic cell to the outside contains the gases generated by the electrolysis, the hydrogen fluoride, the mist of the electrolyte, and is referred to as a “fluid” in the present invention.

[0039] The main reason why the mist generates in the electrolysis of the electrolyte is as follows. The temperature of the electrolyte in the electrolysis is adjusted to 80 to 100° C., for example. The KF·2HF has a melting point of 71.7° C., and therefore the electrolyte is in a liquid state when adjusted to the above-described temperature. Bubbles generated at both the electrodes of the electrolytic cell rise in the electrolyte and burst on the liquid surface of the electrolyte. At this time, part of the electrolyte is released into the gas phase.

[0040] The temperature of the gas phase is lower than the melting point of the electrolyte, and therefore the released electrolyte undergoes a phase change into an extremely fine powder-like state. This powder is considered to be KF·nHF that is a mixture of the potassium fluoride and the hydrogen fluoride. This powder is carried on the flow of the other generated gas to be mist, and forms a fluid to be generated in the electrolytic cell. Such mist is difficult to effectively remove using common countermeasures, such as installing filters, due to its sticky nature.

[0041] Although the generation amount is small, a reaction between the carbonaceous electrode serving as the anode and the fluorine gas generated by the electrolysis sometimes results in the generation of fine powder of an organic compound as mist. In detail, in a current supply portion supplying a current to the carbonaceous electrode, contact resistance is generated in many cases, and the temperature sometimes becomes higher than the temperature of the electrolyte by the Joule heat. Therefore, a reaction between carbon forming the carbonaceous electrode and the fluorine gas sometimes results in the generation of a soot-like organic compound CFx as mist.

[0042] The average particle size of the mist contained in the fluid is not particularly limited, and may be 3 μm or less. The average particle size of the mist contained in the fluid can be measured by a measuring device measuring the average particle size by a light scattering method. The measuring device measuring the average particle size by a light scattering method is preferable because it can measure the average particle size of mist in a fluid flowing through a flow path, such as a pipe, while the fluorine gas production device is continuously operated.

[0043] The type of the measuring device measuring the average particle size by a light scattering method is not particularly limited. Laser light emitted from a light source is emitted to the fluid inside the pipe. At this time, when a light-reflecting substance, such as mist, is present in the fluid, the laser light is reflected and scattered. Some of the scattered light generated when the laser light is scattered by mist is partially detected by a photodetector. At this time, the average particle size of the mist can be found based on information obtained from the scattered light. The average particle size obtained herein is the number average particle size. Examples of the measuring device measuring the average particle size by a light scattering method include an aerosol spectrometer welas (registered trademark) digital 2000 manufactured by PALAS GmbH.

[0044] The amount of the mist generated in the electrolysis of the electrolyte is considered to be affected by the size when the bubbles generated at the electrodes by the electrolysis are burst on the liquid surface of the electrolyte, the viscosity of the electrolyte, or the like. The present inventors have studied whether the generation amount of the mist can be reduced by adding an additive to the electrolyte, and, as a result, have found that the generation amount of the mist can be reduced by adding metal hexafluoronickelate (IV), and thus have accomplished the present invention.

[0045] More specifically, the electrolyte contains hydrogen fluoride, metal fluoride, and metal hexafluoronickelate (IV), and can be obtained by mixing the hydrogen fluoride, the metal fluoride, and the metal hexafluoronickelate (IV). The type of the metal hexafluoronickelate (IV) is not particularly limited, and includes, for example, potassium hexafluoronickelate (IV) (K2NiF6), cesium hexafluoronickelate (IV) (Cs2NiF6), and rubidium hexafluoronickelate (IV) (Rb2NiF6). One type of metal hexafluoronickelate (IV) may be used alone or two or more types of metal hexafluoronickelates (IV) may be used in combination.

[0046] As the metal hexafluoronickelate (IV), commercially available substances may be used or prepared substances may be used. The metal hexafluoronickelate (IV) can be prepared by mixing potassium fluoride, cesium fluoride, or rubidium fluoride (RbF) and nickel fluoride (NiF2) in a molar ratio of 2:1, and treating the mixture at a temperature of 250° C. or more and 450° C. or less under a fluorine gas atmosphere.

[0047] The following description is given taking a case where potassium hexafluoronickelate (IV) is used as the additive of the electrolyte as an example. It is a matter of course that the same applies when cesium hexafluoronickelate (IV) or rubidium hexafluoronickelate (IV) is used as the additive of the electrolyte.

[0048] Since the potassium hexafluoronickelate (IV) is a solid exhibiting an oxidizing property, and therefore reacts with water to change into potassium fluoride of trivalent nickel (K2NiF5) or potassium fluoride of divalent nickel (K2NiF4). The solubility in electrolyte is lower in potassium pentafluoronickelate (III) (K2NiF5) and potassium tetrafluoronickelate (II) (K2NiF4) than the potassium hexafluoronickelate (IV). Therefore, to prevent a decrease in the effect of reducing the generation amount of the mist, the potassium hexafluoronickelate (IV) having a low water content is preferably used as the additive. The content of water contained in the potassium hexafluoronickelate (IV) is preferably 0.5% by mass or less and more preferably 0.3% by mass or less.

[0049] When the content of water contained in the potassium hexafluoronickelate (IV) is high, the effect of reducing the generation amount of the mist decreases, and in addition thereto, the content of water in the electrolyte increases, and therefore there is a risk of increasing the generation amount of oxygen gas (O2), carbon dioxide (CO2), carbon tetrafluoride (CF4), and the like which are electrolysis by-products.

[0050] A method for adding the potassium hexafluoronickelate (IV) to the electrolyte is not particularly limited. The electrolyte may be prepared by mixing metal fluoride with liquid hydrogen fluoride in which the potassium hexafluoronickelate (IV) has been dissolved or the electrolyte may be prepared by mixing solid potassium hexafluoronickelate (IV) with a mixture of hydrogen fluoride and metal fluoride. A specified amount of the potassium hexafluoronickelate (IV) may be added at once or may be added while being divided into several portions.

[0051] The addition of the potassium hexafluoronickelate (IV) results in the presence of nickel (IV) ions in the electrolyte. The concentration of the nickel (IV) ions in the electrolyte is preferably 100 ppm by mass or more and 1000 ppm by mass or less and more preferably 200 ppm by mass or more and 800 ppm by mass or less. When the concentration of the nickel (IV) ions in the electrolyte is within the numerical range above, the effect of reducing the generation amount of the mist is excellent.

[0052] The concentration of the nickel (IV) ions in the electrolyte can be measured by inductively coupled plasma (ICP) atomic emission spectrometry. By the electrolysis, the nickel (IV) ions in the electrolyte are oxidized and reduced at the anode and the cathode or adsorbed to the anode formed of a carbon material, and therefore the concentration of the nickel ions (IV) in the electrolyte sometimes decreases. Therefore, it is preferable to periodically measure the concentration of the nickel (IV) ions in the electrolyte and control the concentration of the nickel (IV) ions in the electrolyte within a predetermined amount range.

[0053] The inside of the electrolytic cell is partitioned into the anode chamber where the anode is arranged and the cathode chamber whether the cathode is arranged. In some cases, the electrolyte stored in the anode chamber and the electrolyte stored in the cathode chamber are not separated, allowing the electrolytes stored in both the chambers to freely mix. In the other cases, the electrolytes stored in both the chambers are separated by a diaphragm, making it difficult for the electrolytes to mix.

[0054] When the inside of the electrolytic cell is structured such that the electrolytes stored in both the electrode chambers can be freely mixed, the potassium hexafluoronickelate (IV) may be contained in either the electrolyte stored in the anode chamber or the electrolyte stored in the cathode chamber. On the other hand, when the inside of the electrolytic cell is structured such that the electrolytes stored in both the electrode chambers are separated by a diaphragm and are difficult to mix, the potassium hexafluoronickelate (IV) is preferably contained in only the electrolyte stored in the anode chamber.

[0055] When the concentration of the nickel (IV) ions in the electrolyte becomes excessively low, there is a risk that the effect of reducing the generation amount of the mist decreases. To suppress a decrease in the concentration of the nickel (IV) ions in the electrolyte, the concentrations of iron ions and copper ions present in the electrolyte are preferably as low as possible. For example, the concentration of iron ions in the electrolyte is preferably 500 ppm by mass or less and more preferably 300 ppm by mass or less. The concentration of copper ions in the electrolyte is preferably 200 ppm by mass or less and more preferably 100 ppm by mass or less. The type of the iron ions includes divalent iron ions and trivalent iron ions. The type of the copper ions includes monovalent copper ions and divalent copper ions.

[0056] When members constituting the electrolytic cell and immersed in the electrolyte contain iron serving as the generation source of iron ions or copper serving as the generation source of copper ions, the members are dissolved in the electrolyte unless the members are subjected to cathodic protection. Therefore, it is preferable to use no materials containing iron or copper for the members constituting the electrolytic cell and immersed in the electrolyte. When the cathodic protection is applied, there is no particular problem.EXAMPLES

[0057] Hereinafter, the present invention is more specifically described with reference to Examples and Comparative Examples.Comparative Example A

[0058] The inside of an electrolytic cell is partitioned by a diaphragm into an anode chamber where an anode is arranged and a cathode chamber where a cathode is arranged. As the anode, 16 amorphous carbon electrodes (30 cm wide, 45 cm long, 5 cm thick) manufactured by SGL Carbon were installed. As the cathode, a nickel punching plate was installed. The electrolytic cell is electrically connected to the cathode for cathodic protection, and therefore is formed of mild steel. The diaphragm for partitioning the anode chamber and the cathode chamber is formed of nickel.

[0059] Into the electrolytic cell described above, 560 L of an electrolyte (molten salt) prepared by mixing 434 kg of hydrogen fluoride and 630 kg of potassium fluoride was charged. Then, the anode and the cathode were immersed in the electrolyte.

[0060] The electrolysis temperature was controlled to 85 to 95° C. First, the temperature of the electrolyte was set to 85° C., a 1000 A direct current was applied at a current density of 0.036 A / cm2 to the anode and the cathode to start electrolysis, and then a pre-conditioning operation was performed until the integrated energizing quantity reached 100 kAh. The pre-conditioning operation refers to an operation of performing the electrolysis at a relatively low current density to remove the moisture in the electrolyte and make the surface of the carbonaceous electrode compatible with the electrolyte.

[0061] After the pre-conditioning operation was completed, the electrolysis was performed while the current density was changed from 0.006 A / cm2 to 0.079 A / cm2, and a fluorine gas generated from the anode and a hydrogen gas generated from the cathode were separated, obtaining the fluorine gas. The amounts (number and volume) and the particle size of mists in the fluorine gas were measured using an aerosol spectrometer welas (registered trademark) digital 2000 manufactured by PALAS GmbH.

[0062] The results are shown in Table 1. Table 1 shows the number (number per unit volume) of the mists contained in the fluorine gas and the total volume (volume per unit volume) of the mists contained in the fluorine gas. The results shown in Table 1 are graphed and illustrated in FIGS. 1, 2. The total volume of the mists was calculated utilizing the particle size distribution of the mists obtained by this measurement. The total volume of the mists was calculated assuming that the mists were spherical. The particle size distribution of the mists generated when the current density was 0.06 to 0.07 A / cm2 is shown in the graph in FIG. 3.

[0063] After the measurement of the mists was completed, 100 g of the molten electrolyte was sampled from the electrolytic cell. The molten electrolyte was transparent. When the electrolyte was solidified, the electrolyte was turned into a white solid. The concentration of nickel (IV) ions in this electrolyte was 30 ppm by mass as measured by inductively coupled plasma atomic emission spectrometry. It was estimated that the dissolved nickel (IV) ions were formed by dissolution of nickel forming the diaphragm in the electrolyte due to corrosion.TABLE 1Current densityA / cm20.0060.0110.0180.0330.0640.079Number of mistsNumber / cm3227412452735745351444336848802Total volume of mistsμm3 / cm3324922162057328849566089Example A

[0064] A fluorine gas was obtained by performing electrolysis in the same manner as in Comparative Example A, except for adding 3.0 kg of potassium hexafluoronickelate (IV) to the electrolyte and performing the electrolysis while the current density was changed from 0.027 A / cm2 to 0.119 A / cm2 after the same pre-conditioning operation as that of Comparative Example A was performed. Then, the amounts (number and volume) and the particle size of mists in the fluorine gas were measured in the same manner as in Comparative Example A. The results are shown in Table 2. The results shown in Table 2 are graphed and illustrated in FIGS. 1, 2. The particle size distribution of the mists generated when the current density was 0.06 to 0.07 A / cm2 is shown in the graph in FIG. 3.TABLE 2Current densityA / cm20.0270.0480.0690.0690.0690.0690.0900.0900.0900.0900.048Number of mistsNumber / cm3241512506129341368713652128401287214441142931484412828Total volume of mistsμm3 / cm3131310275317342406273357481494375Current densityA / cm20.1010.1010.1010.1010.1110.1110.1190.1190.1190.119Number of mistsNumber / cm320999213752193322576222442739623949253542506923620Total volume of mistsμm3 / cm3443487497440376537563463504430

[0065] To the electrolyte in a dissolved state having a temperature maintained at 85° C., 3.0 kg of potassium hexafluoronickelate (IV) powder was added and allowed to stand for half a day for dissolution.

[0066] After the potassium hexafluoronickelate (IV) was allowed to stand for half a day for dissolution, 100 g of the molten electrolyte was sampled from the electrolytic cell. The molten electrolyte was transparent with a slight yellowish tint. When the electrolyte was solidified, the electrolyte exhibited a pale red color. The concentration of nickel (IV) ions in this electrolyte was 650 ppm by mass as measured by inductively coupled plasma atomic emission spectrometry.

[0067] When the pre-conditioning operation was completed, 100 g of the molten electrolyte was sampled from the electrolytic cell. The concentration of the nickel (IV) ions in the electrolyte was 400 ppm by mass as measured by inductively coupled plasma atomic emission spectrometry. Although the concentration of the nickel (IV) ions decreased, the electrolysis was continued without additionally adding potassium hexafluoronickelate (IV).

[0068] As shown in the graph in FIG. 1, the number of the mists increases depending on the current density, and therefore it has been found that the larger the generation amount of the fluorine gas, the larger the generation amount of the mist. It has also been found that, when the metal hexafluoronickelate (IV) is present, the number of the mists is equal to or less than half of the number of the mists when the metal hexafluoronickelate (IV) is absent. Further, when the differences in the total volume of the mists between when the metal hexafluoronickelate (IV) is present and when the metal hexafluoronickelate (IV) is absent were compared, the total volume of the mists when the metal hexafluoronickelate (IV) is present was 1 / 10 or less of that when the metal hexafluoronickelate (IV) is absent as illustrated in the graph in FIG. 2. This is because the particle size of the mist to be generated becomes smaller due to the presence of the metal hexafluoronickelate (IV).

[0069] Further, it has been found from the particle size distribution illustrated in FIG. 3 that the most frequently occurring particle size is about 0.25 μm regardless of the presence or absence of the metal hexafluoronickelate (IV), but the number of the mists is larger when the metal hexafluoronickelate (IV) is absent than when the metal hexafluoronickelate (IV) is present.

[0070] It has also been found from the particle size distribution illustrated in FIG. 3 that, when the metal hexafluoronickelate (IV) is present, mists having a particle size larger than 1.3 μm are hardly detected, while when the metal hexafluoronickelate (IV) is absent, mists having a particle size larger than 1.3 μm are detected.

[0071] In FIG. 3, the particle size is shown only up to 2 μm, and, when the metal hexafluoronickelate (IV) was absent, the detected maximum mist particle size was 10 μm. Even when the number of mists having a large particle size is small, the amount thereof is large in terms of volume. Therefore, a case where the total volume of mists to be generated is small has a greater influence on the effect of suppressing the blockage of pipes or valves used to send a fluorine gas than a case where the number of mists to be generated is small. When electrolysis is performed by adding the metal hexafluoronickelate (IV), a fluorine gas can be produced in which the total volume of the generated mists (volume calculated assuming that the mist is spherical) is 1000 μm3 / cm3 or less as is understood from the graph in FIG. 2.

[0072] Although the mechanism by which the presence of the metal hexafluoronickelate (IV) reduces the generation amount of the mist is not sufficiently elucidated, it is considered that, when bubbles of a fluorine gas to be generated becomes smaller and the bubbles are burst on the liquid surface of the electrolyte, the bubbles are not burst in such a manner as to generate liquid droplets in the space. Therefore, the additive is required to act on an electrode reaction in which bubbles are generated at the anode and act on the properties of the electrolyte related to the burst of the bubbles. The metal hexafluoronickelate (IV) is considered to satisfy both the requirements.Comparative Example 1

[0073] A fluorine gas was obtained in the same manner as in Comparative Example A, except for performing the electrolysis while hydrogen fluoride was intermittently supplied to the electrolyte such that the hydrogen fluoride concentration in the electrolyte was 39 to 42% by mass, and performing the electrolysis after the same pre-conditioning operation as that of Comparative Example A by applying a 1780 A direct current at a current density of 0.064 A / cm2 to the anode and the cathode until the integrated energizing quantity reached 20 kAh. Then, the amounts (number and volume) and the particle size of mists in the fluorine gas were measured in the same manner as in Comparative Example A. The measured particle size range is a range of 0.18 to 10 μm.

[0074] As a result of the measurement, the number (number per unit volume) of mists contained in the fluorine gas was 43400 mists / cm3. The total volume of the mists contained in the fluorine gas (volume per unit volume) was 5000 μm3 / cm3.

[0075] After the measurement of the mists was completed, 100 g of the molten electrolyte was sampled from the electrolytic cell. The molten electrolyte was transparent. When the electrolyte was solidified, the electrolyte was turned into a white solid. The concentration of nickel (IV) ions in this electrolyte was 30 ppm by mass as measured by inductively coupled plasma atomic emission spectrometry. It was estimated that the dissolved nickel (IV) ions were formed by dissolution of nickel forming the diaphragm in the electrolyte due to corrosion.Comparative Example 2

[0076] A fluorine gas was obtained by performing electrolysis in the same manner as in Comparative Example 1, except for adding 5 kg of lithium fluoride (LiF) to the electrolyte.

[0077] Then, the amounts (number and volume) and the particle size of mists in the fluorine gas were measured in the same manner as in Comparative Example 1. The measured particle size range is a range of 0.18 to 10 μm. The addition amount of the lithium fluoride is equivalent to 0.5% by mass to the electrolyte. The lithium fluoride is a substance used as an additive for stabilizing the electrolysis voltage in the production of the fluorine gas by the electrolysis of the electrolyte.

[0078] As a result of the measurement, the number (number per unit volume) of the mists contained in the fluorine gas was 38300 mists / cm3. The total volume (volume per unit volume) of the mists contained in the fluorine gas was 5190 μm3 / cm3. The results of Comparative Examples 1, 2 show that the addition of the lithium fluoride does not reduce the total volume of the mists contained in the fluorine gas.Comparative Example 3

[0079] A fluorine gas was obtained by performing electrolysis in the same manner as in Comparative Example 2, except for adding 175 g of nickel fluoride in place of lithium fluoride. The addition amount of the nickel fluoride is such an amount that the concentration of nickel in the electrolyte is 100 ppm by mass when the entire amount of the added nickel fluoride dissolved to become nickel (II) ions.

[0080] As a result of the measurement, the number (number per unit volume) of the mists contained in the fluorine gas was 41100 mists / cm3. The total volume (volume per unit volume) of the mists contained in the fluorine gas was 4870 μm3 / cm3. The results of Comparative Examples 1, 3 show that the addition of the nickel fluoride does not reduce the total volume of the mists contained in the fluorine gas.

[0081] After the measurement of the mists was completed, 100 g of the molten electrolyte was sampled from the electrolytic cell. The molten electrolyte was transparent. When the electrolyte was solidified, the electrolyte was turned into a white solid. Therefore, tetravalent nickel ions are considered to be absent. The concentration of nickel (IV) ions in this electrolyte was 95 ppm by mass as measured by inductively coupled plasma atomic emission spectrometry.Example 1

[0082] A fluorine gas was obtained by performing electrolysis by adding 3.0 kg of potassium hexafluoronickelate (IV) to the electrolyte in the same manner as in Example A, except for performing the electrolysis after the same pre-conditioning operation as that of Comparative Example A by applying a direct current at a current density of 0.069 A / cm2 to the anode and the cathode. Then, the electrolysis was performed until the integrated energizing quantity reached 20 kAh, and the amounts (number and volume) and the particle size of mists in the fluorine gas were measured in the same manner as in Comparative Example 1. The measured particle size range is a range of 0.18 to 10 μm.

[0083] Thereafter, the electrolysis was continued for four consecutive days, and the mists were measured once a day. As a result of the measurement, the number (number per unit volume) of the mists contained in the fluorine gas was 12800 to 13700 mists / cm3 (average value: 13300 mists / cm3). The total volume (volume per unit volume) of the mists contained in the fluorine gas was 380 to 540 μm3 / cm3 (average value: 460 μm3 / cm3). As compared with Comparative Example 1, the number of the mists decreased to ⅓ or less and the total volume of the mists decreased to 1 / 10 or less.Example 2

[0084] A fluorine gas was obtained by performing electrolysis in the same manner as in Example 1, except for adding 6.0 kg of potassium hexafluoronickelate (IV) (This addition amount is an addition amount equal to or higher than the solubility in the electrolyte.) to the electrolyte. Then, the electrolysis was performed until the integrated energizing quantity reached 20 kAh, and the amounts (number and volume) and the particle size of mists in the fluorine gas were measured in the same manner as in Comparative Example 1. The measured particle size range is a range of 0.18 to 10 μm.

[0085] As a result of the measurement, the number (number per unit volume) of the mists contained in the fluorine gas was 13000 mists / cm3. The total volume (volume per unit volume) of the mists contained in the fluorine gas was 480 μm3 / cm3. As compared with Comparative Example 1, both the number of the mists and the total volume of the mists significantly decreased.

[0086] The concentration of nickel (IV) ions in the electrolyte in each of the following stages was measured by inductively coupled plasma atomic emission spectrometry. First, potassium hexafluoronickelate (IV) was dissolved in the electrolyte and allowed to stand at 85° C. for half a day, and then 100 g of the molten electrolyte was sampled from the electrolytic cell. The molten electrolyte was transparent with a slight yellowish tint. When the electrolyte was solidified, the electrolyte exhibited a pale red color. The concentration of the nickel (IV) ions in this electrolyte was 670 ppm by mass as measured by inductively coupled plasma atomic emission spectrometry. The concentration of the nickel (IV) ions in the electrolyte in which the potassium hexafluoronickelate (IV) was saturated and dissolved was predicted to be 650 to 700 ppm by mass.

[0087] After the pre-conditioning operation was completed, 100 g of the molten electrolyte was sampled from the electrolytic cell. The concentration of the nickel (IV) ions was 650 ppm by mass as measured by inductively coupled plasma atomic emission spectrometry. It can be said from the result that the pre-conditioning operation does not reduce the concentration of the nickel (IV) ions.

Examples

examples

[0057]Hereinafter, the present invention is more specifically described with reference to Examples and Comparative Examples.

example a

[0064]A fluorine gas was obtained by performing electrolysis in the same manner as in Comparative Example A, except for adding 3.0 kg of potassium hexafluoronickelate (IV) to the electrolyte and performing the electrolysis while the current density was changed from 0.027 A / cm2 to 0.119 A / cm2 after the same pre-conditioning operation as that of Comparative Example A was performed. Then, the amounts (number and volume) and the particle size of mists in the fluorine gas were measured in the same manner as in Comparative Example A. The results are shown in Table 2. The results shown in Table 2 are graphed and illustrated in FIGS. 1, 2. The particle size distribution of the mists generated when the current density was 0.06 to 0.07 A / cm2 is shown in the graph in FIG. 3.

TABLE 2Current densityA / cm20.0270.0480.0690.0690.0690.0690.0900.0900.0900.0900.048Number of mistsNumber / cm3241512506129341368713652128401287214441142931484412828Total volume of mistsμm3 / cm3131310275317342406273357481494375C...

example 1

[0082]A fluorine gas was obtained by performing electrolysis by adding 3.0 kg of potassium hexafluoronickelate (IV) to the electrolyte in the same manner as in Example A, except for performing the electrolysis after the same pre-conditioning operation as that of Comparative Example A by applying a direct current at a current density of 0.069 A / cm2 to the anode and the cathode. Then, the electrolysis was performed until the integrated energizing quantity reached 20 kAh, and the amounts (number and volume) and the particle size of mists in the fluorine gas were measured in the same manner as in Comparative Example 1. The measured particle size range is a range of 0.18 to 10 μm.

[0083]Thereafter, the electrolysis was continued for four consecutive days, and the mists were measured once a day. As a result of the measurement, the number (number per unit volume) of the mists contained in the fluorine gas was 12800 to 13700 mists / cm3 (average value: 13300 mists / cm3). The total volume (volum...

TECHNICAL FIELD

[0001] The present invention relates to a fluorine gas production method.BACKGROUND ART

[0002] A fluorine gas can be synthesized by electrolyzing an electrolyte containing hydrogen fluoride and metal fluoride. The electrolysis of an electrolyte generates mist (e.g., mist of the electrolyte) together with a fluorine gas, and therefore a fluorine gas sent out from an electrolytic cell is accompanied by the mist. The mist accompanied by the fluorine gas sometimes becomes powder, so that there is a risk of blocking pipes or valves used to send the fluorine gas. Therefore, an operation of producing a fluorine gas has been sometimes forced to be interrupted or stopped, which has hindered a continuous operation in the fluorine gas production by an electrolysis method.

[0003] To suppress the blockage of pipes or valves by mist, PTL 1 discloses a technology of heating a pipe, through which a fluorine gas accompanying mist or the fluorine gas passes, to be equal to or higher than the melting point of an electrolyte. PTL 2 discloses a gas generating device having a gas diffusion part, which is a space where mist is roughly collected, and a filler storage part storing a filler for adsorbing mist.CITATION LISTPatent LiteraturesPTL 1: JP 5584904 B

[0005] PTL 2: JP 5919824 BSUMMARY OF INVENTIONTechnical Problem

[0006] However, to more effectively suppress the blockage of pipes or valves by mist, a fluorine gas production technology of suppressing the generation itself of mist has been desired.

[0007] It is an object of the present invention to provide a fluorine gas production method hardly generating mist.Solution to Problem

[0008] To solve the above-described problem, one aspect of the present invention is as described in [1] to [8] below.

[0009] [1] A fluorine gas production method including: an electrolysis step of electrolyzing an electrolyte using an anode and a cathode to generate a fluorine gas in an electrolytic cell, in which

[0010] the electrolyte contains hydrogen fluoride, metal fluoride, and metal hexafluoronickelate (IV), and

[0011] the anode is a nickel electrode or a carbonaceous electrode, and the cathode is a metal electrode.

[0012] [2] The fluorine gas production method according to [1], in which the metal hexafluoronickelate (IV) is at least one of potassium hexafluoronickelate (IV), cesium hexafluoronickelate (IV), and rubidium hexafluoronickelate (IV).

[0013] [3] The fluorine gas production method according to [1] or [2], in which the concentration of nickel (IV) ions in the electrolyte is 100 ppm by mass or more and 1000 ppm by mass or less.

[0014] [4] The fluorine gas production method according to any one of [1] to [3], in which the metal fluoride is at least one of potassium fluoride and cesium fluoride.

[0015] [5] The fluorine gas production method according to any one of [1] to [4], in which the ratio of the molar amount of the hydrogen fluoride to the molar amount of the metal fluoride contained in the electrolyte is 1.6 or more and 3.2 or less.

[0016] [6] The fluorine gas production method according to any one of [1] to [5], in which the cathode is an electrode formed of at least one of iron, nickel, copper, and a copper nickel alloy.

[0017] [7] The fluorine gas production method according to any one of [1] to [6], in which the electrolyte is a mixture of the hydrogen fluoride, the metal fluoride, and the metal hexafluoronickelate (IV) having a water content of 0.5% by mass or less.

[0018] [8] The fluorine gas production method according to any one of [1] to [7], in which the concentration of iron ions in the electrolyte is 500 ppm by mass or less, and the concentration of copper ions is 200 ppm by mass or less.Advantageous Effects of Invention

[0019] According to the present invention, mist is hardly generated when a fluorine gas is produced by electrolyzing an electrolyte.BRIEF DESCRIPTION OF DRAWINGS

[0020] FIG. 1 is a graph showing the correlation between the current density in electrolysis and the number of mists contained in a fluorine gas;

[0021] FIG. 2 is a graph showing the correlation between the current density in electrolysis and the total volume of mists contained in the fluorine gas; and

[0022] FIG. 3 is a graph showing the particle size distribution of the mists contained in the fluorine gas.DESCRIPTION OF EMBODIMENTS

[0023] One embodiment of the present invention will now be described. The embodiment is merely one example of the present invention, and the present invention is not limited to the embodiment. Various modifications or improvements can be made in the embodiment, and such modifications and improvements can be encompassed by the present invention.

[0024] The present inventors have conducted extensive studies on mist causing blockage of pipes or valves in electrolytic synthesis of a fluorine gas (F2). The “mist” in the present invention refers to liquid fine particles or solid fine particles generated together with a fluorine gas in an electrolytic cell by electrolysis of an electrolyte. Specifically, the mist refers to fine particles of an electrolyte, solid fine particles resulting from a phase change of fine particles of an electrolyte, and solid fine particles generated by a reaction of members constituting an electrolytic cell (metal constituting an electrolytic cell, a packing for an electrolytic cell, a carbonaceous electrode, and the like) and a fluorine gas.

[0025] The present inventors have studied mist contained in a fluid generated inside an electrolytic cell in electrolysis of an electrolyte, and, as a result, have found that the generation of mist can be suppressed by adding an additive to the electrolyte, and thus have accomplished the present invention. One embodiment of the present invention is described below.

[0026] A fluorine gas production method according to this embodiment includes an electrolysis step of electrolyzing an electrolyte using an anode and a cathode to generate a fluorine gas in an electrolytic cell. The electrolyte contains hydrogen fluoride (HF), metal fluoride, and metal hexafluoronickelate (IV). The anode is a nickel electrode or a carbonaceous electrode, and the cathode is a metal electrode.

[0027] The metal hexafluoronickelate (IV) is added as an additive to the electrolyte, and therefore, when the electrolyte is electrolyzed to produce a fluorine gas, mist is hardly generated. Mist accompanied by the fluorine gas becomes a powder, so that there is a risk of blocking pipes or valves used to send the fluorine gas. However, the amount of the mist generated in the production of the fluorine gas is small, and therefore the blockage of the pipes or valves used to send the fluorine gas can be suppressed without attaching a mist removal device to a fluorine gas production device. As a result, the frequency of interruptions and stoppages in an operation of producing a fluorine gas can be reduced, and thus the cost of producing a fluorine gas can be lowered.

[0028] The fluorine gas production method of this embodiment is described in more detail below.

[0029] There are no particular restrictions on the mode of the electrolytic cell, and any electrolytic cell can be used insofar as a fluorine gas can be generated by electrolyzing an electrolyte containing hydrogen fluoride and metal fluoride.

[0030] In usual, the inside of the electrolytic cell is partitioned by a partition member, such as a diaphragm, into an anode chamber where an anode is arranged and a cathode chamber where a cathode is arranged, so that a fluorine gas generated at the anode and a hydrogen generated at the cathode are not mixed.

[0031] As the anode, a carbonaceous electrode formed of a carbon material, such as diamond, diamond-like carbon, amorphous carbon, graphite, or glassy carbon, can be used, for example. As the anode, in addition to the above-described carbonaceous electrode, a nickel electrode formed of nickel (Ni), a platinum electrode formed of platinum (Pt), a gold electrode formed of gold (Au), and metal electrodes formed of the other types of metals can also be used, for example. As the cathode, metal electrodes formed of metals, such as iron (Fe), nickel, copper (Cu), and a copper nickel alloy (e.g., Monel (trademark)), can be used, for example.

[0032] The electrolyte contains hydrogen fluoride and metal fluoride. The type of the metal fluoride is not particularly limited, and is preferably alkali metal fluoride and more preferably at least one of potassium fluoride (KF) and cesium fluoride (CsF). One type of metal fluoride may be used alone or two or more types of metal fluorides may be used in combination. More specifically, potassium fluoride and cesium fluoride may be used in combination as the metal fluoride.

[0033] The ratio of the molar amount of the hydrogen fluoride to the molar amount of the metal fluoride contained in the electrolyte ([Molar amount of hydrogen fluoride] / [Molar amount of metal fluoride]) is preferably 1.6 or more and 3.2 or less and more preferably 1.9 or more and 3.0 or less.

[0034] As the electrolyte, a mixed molten salt of hydrogen fluoride and potassium fluoride can be used, for example. In the mixed molten salt of hydrogen fluoride and potassium fluoride, the molar ratio between the hydrogen fluoride and the potassium fluoride can be set to Hydrogen fluoride:Potassium fluoride=1.5 to 2.5:1, for example. KF·2HF in the case of Hydrogen fluoride:Potassium fluoride=2:1 is a typical electrolyte, and this mixed molten salt has a melting point of approximately 72° C.

[0035] As an example of the electrolyte, a mixed molten salt of hydrogen fluoride and cesium fluoride can also be used. In the mixed molten salt of hydrogen fluoride and cesium fluoride, the molar ratio between the hydrogen fluoride and the cesium fluoride can be set to Hydrogen fluoride:Cesium fluoride=1.8 to 3.1:1, for example. CsF·2.4HF in the case of Hydrogen fluoride:Cesium fluoride=2.4:1 is a typical electrolyte, and this mixed molten salt has a melting point of approximately 16° C.

[0036] This electrolyte is corrosive, and therefore a site that comes into contact with the electrolyte, such as the inner surface of the electrolytic cell, is preferably formed of metal, such as iron, nickel, or Monel (trademark).

[0037] The electrolysis consumes the hydrogen fluoride in the electrolyte, and therefore hydrogen fluoride is preferably continuously or intermittently supplied to the electrolyte during the electrolysis, for example. The hydrogen fluoride may be supplied to the electrolyte on the cathode chamber side of the electrolytic cell or may be supplied to the electrolyte on the anode chamber side. When the concentration of the hydrogen fluoride in a case where the electrolyte is KF·2HF (hydrogen fluoride concentration: 40.4% by mass) or CsF·2.4HF (hydrogen fluoride concentration: 24.0% by mass) is set as the reference concentration, the concentration of the hydrogen fluoride in the electrolyte is preferably controlled to fluctuate in the range of −5% by mass or more and +5% by mass or less of the reference concentration, more preferably controlled to fluctuate in the range of −2.5% by mass or more and +2.5% by mass or less of the reference concentration, and still more preferably controlled to fluctuate in the range of −1.5% by mass or more and +1.5% by mass or less of the reference concentration.

[0038] In the electrolysis of the electrolyte, a direct current is applied to the anode and the cathode, and gas containing a fluorine gas is generated at the anode and gas containing a hydrogen gas is generated at the cathode. The hydrogen fluoride of the electrolyte has vapor pressure, and therefore the gases generated at the anode and the cathode are individually accompanied by the hydrogen fluoride. Further, in the production of the fluorine gas by the electrolysis of the electrolyte, the gases generated by the electrolysis contain mist of the electrolyte. Thus, a gas phase portion of the electrolytic cell contains the gases generated by the electrolysis, the hydrogen fluoride, and the mist of the electrolyte. Therefore, one sent out from the inside of the electrolytic cell to the outside contains the gases generated by the electrolysis, the hydrogen fluoride, the mist of the electrolyte, and is referred to as a “fluid” in the present invention.

[0039] The main reason why the mist generates in the electrolysis of the electrolyte is as follows. The temperature of the electrolyte in the electrolysis is adjusted to 80 to 100° C., for example. The KF·2HF has a melting point of 71.7° C., and therefore the electrolyte is in a liquid state when adjusted to the above-described temperature. Bubbles generated at both the electrodes of the electrolytic cell rise in the electrolyte and burst on the liquid surface of the electrolyte. At this time, part of the electrolyte is released into the gas phase.

[0040] The temperature of the gas phase is lower than the melting point of the electrolyte, and therefore the released electrolyte undergoes a phase change into an extremely fine powder-like state. This powder is considered to be KF·nHF that is a mixture of the potassium fluoride and the hydrogen fluoride. This powder is carried on the flow of the other generated gas to be mist, and forms a fluid to be generated in the electrolytic cell. Such mist is difficult to effectively remove using common countermeasures, such as installing filters, due to its sticky nature.

[0041] Although the generation amount is small, a reaction between the carbonaceous electrode serving as the anode and the fluorine gas generated by the electrolysis sometimes results in the generation of fine powder of an organic compound as mist. In detail, in a current supply portion supplying a current to the carbonaceous electrode, contact resistance is generated in many cases, and the temperature sometimes becomes higher than the temperature of the electrolyte by the Joule heat. Therefore, a reaction between carbon forming the carbonaceous electrode and the fluorine gas sometimes results in the generation of a soot-like organic compound CFx as mist.

[0042] The average particle size of the mist contained in the fluid is not particularly limited, and may be 3 μm or less. The average particle size of the mist contained in the fluid can be measured by a measuring device measuring the average particle size by a light scattering method. The measuring device measuring the average particle size by a light scattering method is preferable because it can measure the average particle size of mist in a fluid flowing through a flow path, such as a pipe, while the fluorine gas production device is continuously operated.

[0043] The type of the measuring device measuring the average particle size by a light scattering method is not particularly limited. Laser light emitted from a light source is emitted to the fluid inside the pipe. At this time, when a light-reflecting substance, such as mist, is present in the fluid, the laser light is reflected and scattered. Some of the scattered light generated when the laser light is scattered by mist is partially detected by a photodetector. At this time, the average particle size of the mist can be found based on information obtained from the scattered light. The average particle size obtained herein is the number average particle size. Examples of the measuring device measuring the average particle size by a light scattering method include an aerosol spectrometer welas (registered trademark) digital 2000 manufactured by PALAS GmbH.

[0044] The amount of the mist generated in the electrolysis of the electrolyte is considered to be affected by the size when the bubbles generated at the electrodes by the electrolysis are burst on the liquid surface of the electrolyte, the viscosity of the electrolyte, or the like. The present inventors have studied whether the generation amount of the mist can be reduced by adding an additive to the electrolyte, and, as a result, have found that the generation amount of the mist can be reduced by adding metal hexafluoronickelate (IV), and thus have accomplished the present invention.

[0045] More specifically, the electrolyte contains hydrogen fluoride, metal fluoride, and metal hexafluoronickelate (IV), and can be obtained by mixing the hydrogen fluoride, the metal fluoride, and the metal hexafluoronickelate (IV). The type of the metal hexafluoronickelate (IV) is not particularly limited, and includes, for example, potassium hexafluoronickelate (IV) (K2NiF6), cesium hexafluoronickelate (IV) (Cs2NiF6), and rubidium hexafluoronickelate (IV) (Rb2NiF6). One type of metal hexafluoronickelate (IV) may be used alone or two or more types of metal hexafluoronickelates (IV) may be used in combination.

[0046] As the metal hexafluoronickelate (IV), commercially available substances may be used or prepared substances may be used. The metal hexafluoronickelate (IV) can be prepared by mixing potassium fluoride, cesium fluoride, or rubidium fluoride (RbF) and nickel fluoride (NiF2) in a molar ratio of 2:1, and treating the mixture at a temperature of 250° C. or more and 450° C. or less under a fluorine gas atmosphere.

[0047] The following description is given taking a case where potassium hexafluoronickelate (IV) is used as the additive of the electrolyte as an example. It is a matter of course that the same applies when cesium hexafluoronickelate (IV) or rubidium hexafluoronickelate (IV) is used as the additive of the electrolyte.

[0048] Since the potassium hexafluoronickelate (IV) is a solid exhibiting an oxidizing property, and therefore reacts with water to change into potassium fluoride of trivalent nickel (K2NiF5) or potassium fluoride of divalent nickel (K2NiF4). The solubility in electrolyte is lower in potassium pentafluoronickelate (III) (K2NiF5) and potassium tetrafluoronickelate (II) (K2NiF4) than the potassium hexafluoronickelate (IV). Therefore, to prevent a decrease in the effect of reducing the generation amount of the mist, the potassium hexafluoronickelate (IV) having a low water content is preferably used as the additive. The content of water contained in the potassium hexafluoronickelate (IV) is preferably 0.5% by mass or less and more preferably 0.3% by mass or less.

[0049] When the content of water contained in the potassium hexafluoronickelate (IV) is high, the effect of reducing the generation amount of the mist decreases, and in addition thereto, the content of water in the electrolyte increases, and therefore there is a risk of increasing the generation amount of oxygen gas (O2), carbon dioxide (CO2), carbon tetrafluoride (CF4), and the like which are electrolysis by-products.

[0050] A method for adding the potassium hexafluoronickelate (IV) to the electrolyte is not particularly limited. The electrolyte may be prepared by mixing metal fluoride with liquid hydrogen fluoride in which the potassium hexafluoronickelate (IV) has been dissolved or the electrolyte may be prepared by mixing solid potassium hexafluoronickelate (IV) with a mixture of hydrogen fluoride and metal fluoride. A specified amount of the potassium hexafluoronickelate (IV) may be added at once or may be added while being divided into several portions.

[0051] The addition of the potassium hexafluoronickelate (IV) results in the presence of nickel (IV) ions in the electrolyte. The concentration of the nickel (IV) ions in the electrolyte is preferably 100 ppm by mass or more and 1000 ppm by mass or less and more preferably 200 ppm by mass or more and 800 ppm by mass or less. When the concentration of the nickel (IV) ions in the electrolyte is within the numerical range above, the effect of reducing the generation amount of the mist is excellent.

[0052] The concentration of the nickel (IV) ions in the electrolyte can be measured by inductively coupled plasma (ICP) atomic emission spectrometry. By the electrolysis, the nickel (IV) ions in the electrolyte are oxidized and reduced at the anode and the cathode or adsorbed to the anode formed of a carbon material, and therefore the concentration of the nickel ions (IV) in the electrolyte sometimes decreases. Therefore, it is preferable to periodically measure the concentration of the nickel (IV) ions in the electrolyte and control the concentration of the nickel (IV) ions in the electrolyte within a predetermined amount range.

[0053] The inside of the electrolytic cell is partitioned into the anode chamber where the anode is arranged and the cathode chamber whether the cathode is arranged. In some cases, the electrolyte stored in the anode chamber and the electrolyte stored in the cathode chamber are not separated, allowing the electrolytes stored in both the chambers to freely mix. In the other cases, the electrolytes stored in both the chambers are separated by a diaphragm, making it difficult for the electrolytes to mix.

[0054] When the inside of the electrolytic cell is structured such that the electrolytes stored in both the electrode chambers can be freely mixed, the potassium hexafluoronickelate (IV) may be contained in either the electrolyte stored in the anode chamber or the electrolyte stored in the cathode chamber. On the other hand, when the inside of the electrolytic cell is structured such that the electrolytes stored in both the electrode chambers are separated by a diaphragm and are difficult to mix, the potassium hexafluoronickelate (IV) is preferably contained in only the electrolyte stored in the anode chamber.

[0055] When the concentration of the nickel (IV) ions in the electrolyte becomes excessively low, there is a risk that the effect of reducing the generation amount of the mist decreases. To suppress a decrease in the concentration of the nickel (IV) ions in the electrolyte, the concentrations of iron ions and copper ions present in the electrolyte are preferably as low as possible. For example, the concentration of iron ions in the electrolyte is preferably 500 ppm by mass or less and more preferably 300 ppm by mass or less. The concentration of copper ions in the electrolyte is preferably 200 ppm by mass or less and more preferably 100 ppm by mass or less. The type of the iron ions includes divalent iron ions and trivalent iron ions. The type of the copper ions includes monovalent copper ions and divalent copper ions.

[0056] When members constituting the electrolytic cell and immersed in the electrolyte contain iron serving as the generation source of iron ions or copper serving as the generation source of copper ions, the members are dissolved in the electrolyte unless the members are subjected to cathodic protection. Therefore, it is preferable to use no materials containing iron or copper for the members constituting the electrolytic cell and immersed in the electrolyte. When the cathodic protection is applied, there is no particular problem.EXAMPLES

[0057] Hereinafter, the present invention is more specifically described with reference to Examples and Comparative Examples.Comparative Example A

[0058] The inside of an electrolytic cell is partitioned by a diaphragm into an anode chamber where an anode is arranged and a cathode chamber where a cathode is arranged. As the anode, 16 amorphous carbon electrodes (30 cm wide, 45 cm long, 5 cm thick) manufactured by SGL Carbon were installed. As the cathode, a nickel punching plate was installed. The electrolytic cell is electrically connected to the cathode for cathodic protection, and therefore is formed of mild steel. The diaphragm for partitioning the anode chamber and the cathode chamber is formed of nickel.

[0059] Into the electrolytic cell described above, 560 L of an electrolyte (molten salt) prepared by mixing 434 kg of hydrogen fluoride and 630 kg of potassium fluoride was charged. Then, the anode and the cathode were immersed in the electrolyte.

[0060] The electrolysis temperature was controlled to 85 to 95° C. First, the temperature of the electrolyte was set to 85° C., a 1000 A direct current was applied at a current density of 0.036 A / cm2 to the anode and the cathode to start electrolysis, and then a pre-conditioning operation was performed until the integrated energizing quantity reached 100 kAh. The pre-conditioning operation refers to an operation of performing the electrolysis at a relatively low current density to remove the moisture in the electrolyte and make the surface of the carbonaceous electrode compatible with the electrolyte.

[0061] After the pre-conditioning operation was completed, the electrolysis was performed while the current density was changed from 0.006 A / cm2 to 0.079 A / cm2, and a fluorine gas generated from the anode and a hydrogen gas generated from the cathode were separated, obtaining the fluorine gas. The amounts (number and volume) and the particle size of mists in the fluorine gas were measured using an aerosol spectrometer welas (registered trademark) digital 2000 manufactured by PALAS GmbH.

[0062] The results are shown in Table 1. Table 1 shows the number (number per unit volume) of the mists contained in the fluorine gas and the total volume (volume per unit volume) of the mists contained in the fluorine gas. The results shown in Table 1 are graphed and illustrated in FIGS. 1, 2. The total volume of the mists was calculated utilizing the particle size distribution of the mists obtained by this measurement. The total volume of the mists was calculated assuming that the mists were spherical. The particle size distribution of the mists generated when the current density was 0.06 to 0.07 A / cm2 is shown in the graph in FIG. 3.

[0063] After the measurement of the mists was completed, 100 g of the molten electrolyte was sampled from the electrolytic cell. The molten electrolyte was transparent. When the electrolyte was solidified, the electrolyte was turned into a white solid. The concentration of nickel (IV) ions in this electrolyte was 30 ppm by mass as measured by inductively coupled plasma atomic emission spectrometry. It was estimated that the dissolved nickel (IV) ions were formed by dissolution of nickel forming the diaphragm in the electrolyte due to corrosion.TABLE 1Current densityA / cm20.0060.0110.0180.0330.0640.079Number of mistsNumber / cm3227412452735745351444336848802Total volume of mistsμm3 / cm3324922162057328849566089Example A

[0064] A fluorine gas was obtained by performing electrolysis in the same manner as in Comparative Example A, except for adding 3.0 kg of potassium hexafluoronickelate (IV) to the electrolyte and performing the electrolysis while the current density was changed from 0.027 A / cm2 to 0.119 A / cm2 after the same pre-conditioning operation as that of Comparative Example A was performed. Then, the amounts (number and volume) and the particle size of mists in the fluorine gas were measured in the same manner as in Comparative Example A. The results are shown in Table 2. The results shown in Table 2 are graphed and illustrated in FIGS. 1, 2. The particle size distribution of the mists generated when the current density was 0.06 to 0.07 A / cm2 is shown in the graph in FIG. 3.TABLE 2Current densityA / cm20.0270.0480.0690.0690.0690.0690.0900.0900.0900.0900.048Number of mistsNumber / cm3241512506129341368713652128401287214441142931484412828Total volume of mistsμm3 / cm3131310275317342406273357481494375Current densityA / cm20.1010.1010.1010.1010.1110.1110.1190.1190.1190.119Number of mistsNumber / cm320999213752193322576222442739623949253542506923620Total volume of mistsμm3 / cm3443487497440376537563463504430

[0065] To the electrolyte in a dissolved state having a temperature maintained at 85° C., 3.0 kg of potassium hexafluoronickelate (IV) powder was added and allowed to stand for half a day for dissolution.

[0066] After the potassium hexafluoronickelate (IV) was allowed to stand for half a day for dissolution, 100 g of the molten electrolyte was sampled from the electrolytic cell. The molten electrolyte was transparent with a slight yellowish tint. When the electrolyte was solidified, the electrolyte exhibited a pale red color. The concentration of nickel (IV) ions in this electrolyte was 650 ppm by mass as measured by inductively coupled plasma atomic emission spectrometry.

[0067] When the pre-conditioning operation was completed, 100 g of the molten electrolyte was sampled from the electrolytic cell. The concentration of the nickel (IV) ions in the electrolyte was 400 ppm by mass as measured by inductively coupled plasma atomic emission spectrometry. Although the concentration of the nickel (IV) ions decreased, the electrolysis was continued without additionally adding potassium hexafluoronickelate (IV).

[0068] As shown in the graph in FIG. 1, the number of the mists increases depending on the current density, and therefore it has been found that the larger the generation amount of the fluorine gas, the larger the generation amount of the mist. It has also been found that, when the metal hexafluoronickelate (IV) is present, the number of the mists is equal to or less than half of the number of the mists when the metal hexafluoronickelate (IV) is absent. Further, when the differences in the total volume of the mists between when the metal hexafluoronickelate (IV) is present and when the metal hexafluoronickelate (IV) is absent were compared, the total volume of the mists when the metal hexafluoronickelate (IV) is present was 1 / 10 or less of that when the metal hexafluoronickelate (IV) is absent as illustrated in the graph in FIG. 2. This is because the particle size of the mist to be generated becomes smaller due to the presence of the metal hexafluoronickelate (IV).

[0069] Further, it has been found from the particle size distribution illustrated in FIG. 3 that the most frequently occurring particle size is about 0.25 μm regardless of the presence or absence of the metal hexafluoronickelate (IV), but the number of the mists is larger when the metal hexafluoronickelate (IV) is absent than when the metal hexafluoronickelate (IV) is present.

[0070] It has also been found from the particle size distribution illustrated in FIG. 3 that, when the metal hexafluoronickelate (IV) is present, mists having a particle size larger than 1.3 μm are hardly detected, while when the metal hexafluoronickelate (IV) is absent, mists having a particle size larger than 1.3 μm are detected.

[0071] In FIG. 3, the particle size is shown only up to 2 μm, and, when the metal hexafluoronickelate (IV) was absent, the detected maximum mist particle size was 10 μm. Even when the number of mists having a large particle size is small, the amount thereof is large in terms of volume. Therefore, a case where the total volume of mists to be generated is small has a greater influence on the effect of suppressing the blockage of pipes or valves used to send a fluorine gas than a case where the number of mists to be generated is small. When electrolysis is performed by adding the metal hexafluoronickelate (IV), a fluorine gas can be produced in which the total volume of the generated mists (volume calculated assuming that the mist is spherical) is 1000 μm3 / cm3 or less as is understood from the graph in FIG. 2.

[0072] Although the mechanism by which the presence of the metal hexafluoronickelate (IV) reduces the generation amount of the mist is not sufficiently elucidated, it is considered that, when bubbles of a fluorine gas to be generated becomes smaller and the bubbles are burst on the liquid surface of the electrolyte, the bubbles are not burst in such a manner as to generate liquid droplets in the space. Therefore, the additive is required to act on an electrode reaction in which bubbles are generated at the anode and act on the properties of the electrolyte related to the burst of the bubbles. The metal hexafluoronickelate (IV) is considered to satisfy both the requirements.Comparative Example 1

[0073] A fluorine gas was obtained in the same manner as in Comparative Example A, except for performing the electrolysis while hydrogen fluoride was intermittently supplied to the electrolyte such that the hydrogen fluoride concentration in the electrolyte was 39 to 42% by mass, and performing the electrolysis after the same pre-conditioning operation as that of Comparative Example A by applying a 1780 A direct current at a current density of 0.064 A / cm2 to the anode and the cathode until the integrated energizing quantity reached 20 kAh. Then, the amounts (number and volume) and the particle size of mists in the fluorine gas were measured in the same manner as in Comparative Example A. The measured particle size range is a range of 0.18 to 10 μm.

[0074] As a result of the measurement, the number (number per unit volume) of mists contained in the fluorine gas was 43400 mists / cm3. The total volume of the mists contained in the fluorine gas (volume per unit volume) was 5000 μm3 / cm3.

[0075] After the measurement of the mists was completed, 100 g of the molten electrolyte was sampled from the electrolytic cell. The molten electrolyte was transparent. When the electrolyte was solidified, the electrolyte was turned into a white solid. The concentration of nickel (IV) ions in this electrolyte was 30 ppm by mass as measured by inductively coupled plasma atomic emission spectrometry. It was estimated that the dissolved nickel (IV) ions were formed by dissolution of nickel forming the diaphragm in the electrolyte due to corrosion.Comparative Example 2

[0076] A fluorine gas was obtained by performing electrolysis in the same manner as in Comparative Example 1, except for adding 5 kg of lithium fluoride (LiF) to the electrolyte.

[0077] Then, the amounts (number and volume) and the particle size of mists in the fluorine gas were measured in the same manner as in Comparative Example 1. The measured particle size range is a range of 0.18 to 10 μm. The addition amount of the lithium fluoride is equivalent to 0.5% by mass to the electrolyte. The lithium fluoride is a substance used as an additive for stabilizing the electrolysis voltage in the production of the fluorine gas by the electrolysis of the electrolyte.

[0078] As a result of the measurement, the number (number per unit volume) of the mists contained in the fluorine gas was 38300 mists / cm3. The total volume (volume per unit volume) of the mists contained in the fluorine gas was 5190 μm3 / cm3. The results of Comparative Examples 1, 2 show that the addition of the lithium fluoride does not reduce the total volume of the mists contained in the fluorine gas.Comparative Example 3

[0079] A fluorine gas was obtained by performing electrolysis in the same manner as in Comparative Example 2, except for adding 175 g of nickel fluoride in place of lithium fluoride. The addition amount of the nickel fluoride is such an amount that the concentration of nickel in the electrolyte is 100 ppm by mass when the entire amount of the added nickel fluoride dissolved to become nickel (II) ions.

[0080] As a result of the measurement, the number (number per unit volume) of the mists contained in the fluorine gas was 41100 mists / cm3. The total volume (volume per unit volume) of the mists contained in the fluorine gas was 4870 μm3 / cm3. The results of Comparative Examples 1, 3 show that the addition of the nickel fluoride does not reduce the total volume of the mists contained in the fluorine gas.

[0081] After the measurement of the mists was completed, 100 g of the molten electrolyte was sampled from the electrolytic cell. The molten electrolyte was transparent. When the electrolyte was solidified, the electrolyte was turned into a white solid. Therefore, tetravalent nickel ions are considered to be absent. The concentration of nickel (IV) ions in this electrolyte was 95 ppm by mass as measured by inductively coupled plasma atomic emission spectrometry.Example 1

[0082] A fluorine gas was obtained by performing electrolysis by adding 3.0 kg of potassium hexafluoronickelate (IV) to the electrolyte in the same manner as in Example A, except for performing the electrolysis after the same pre-conditioning operation as that of Comparative Example A by applying a direct current at a current density of 0.069 A / cm2 to the anode and the cathode. Then, the electrolysis was performed until the integrated energizing quantity reached 20 kAh, and the amounts (number and volume) and the particle size of mists in the fluorine gas were measured in the same manner as in Comparative Example 1. The measured particle size range is a range of 0.18 to 10 μm.

[0083] Thereafter, the electrolysis was continued for four consecutive days, and the mists were measured once a day. As a result of the measurement, the number (number per unit volume) of the mists contained in the fluorine gas was 12800 to 13700 mists / cm3 (average value: 13300 mists / cm3). The total volume (volume per unit volume) of the mists contained in the fluorine gas was 380 to 540 μm3 / cm3 (average value: 460 μm3 / cm3). As compared with Comparative Example 1, the number of the mists decreased to ⅓ or less and the total volume of the mists decreased to 1 / 10 or less.Example 2

[0084] A fluorine gas was obtained by performing electrolysis in the same manner as in Example 1, except for adding 6.0 kg of potassium hexafluoronickelate (IV) (This addition amount is an addition amount equal to or higher than the solubility in the electrolyte.) to the electrolyte. Then, the electrolysis was performed until the integrated energizing quantity reached 20 kAh, and the amounts (number and volume) and the particle size of mists in the fluorine gas were measured in the same manner as in Comparative Example 1. The measured particle size range is a range of 0.18 to 10 μm.

[0085] As a result of the measurement, the number (number per unit volume) of the mists contained in the fluorine gas was 13000 mists / cm3. The total volume (volume per unit volume) of the mists contained in the fluorine gas was 480 μm3 / cm3. As compared with Comparative Example 1, both the number of the mists and the total volume of the mists significantly decreased.

[0086] The concentration of nickel (IV) ions in the electrolyte in each of the following stages was measured by inductively coupled plasma atomic emission spectrometry. First, potassium hexafluoronickelate (IV) was dissolved in the electrolyte and allowed to stand at 85° C. for half a day, and then 100 g of the molten electrolyte was sampled from the electrolytic cell. The molten electrolyte was transparent with a slight yellowish tint. When the electrolyte was solidified, the electrolyte exhibited a pale red color. The concentration of the nickel (IV) ions in this electrolyte was 670 ppm by mass as measured by inductively coupled plasma atomic emission spectrometry. The concentration of the nickel (IV) ions in the electrolyte in which the potassium hexafluoronickelate (IV) was saturated and dissolved was predicted to be 650 to 700 ppm by mass.

[0087] After the pre-conditioning operation was completed, 100 g of the molten electrolyte was sampled from the electrolytic cell. The concentration of the nickel (IV) ions was 650 ppm by mass as measured by inductively coupled plasma atomic emission spectrometry. It can be said from the result that the pre-conditioning operation does not reduce the concentration of the nickel (IV) ions.

Examples

examples

[0057]Hereinafter, the present invention is more specifically described with reference to Examples and Comparative Examples.

example a

[0064]A fluorine gas was obtained by performing electrolysis in the same manner as in Comparative Example A, except for adding 3.0 kg of potassium hexafluoronickelate (IV) to the electrolyte and performing the electrolysis while the current density was changed from 0.027 A / cm2 to 0.119 A / cm2 after the same pre-conditioning operation as that of Comparative Example A was performed. Then, the amounts (number and volume) and the particle size of mists in the fluorine gas were measured in the same manner as in Comparative Example A. The results are shown in Table 2. The results shown in Table 2 are graphed and illustrated in FIGS. 1, 2. The particle size distribution of the mists generated when the current density was 0.06 to 0.07 A / cm2 is shown in the graph in FIG. 3.

TABLE 2Current densityA / cm20.0270.0480.0690.0690.0690.0690.0900.0900.0900.0900.048Number of mistsNumber / cm3241512506129341368713652128401287214441142931484412828Total volume of mistsμm3 / cm3131310275317342406273357481494375C...

example 1

[0082]A fluorine gas was obtained by performing electrolysis by adding 3.0 kg of potassium hexafluoronickelate (IV) to the electrolyte in the same manner as in Example A, except for performing the electrolysis after the same pre-conditioning operation as that of Comparative Example A by applying a direct current at a current density of 0.069 A / cm2 to the anode and the cathode. Then, the electrolysis was performed until the integrated energizing quantity reached 20 kAh, and the amounts (number and volume) and the particle size of mists in the fluorine gas were measured in the same manner as in Comparative Example 1. The measured particle size range is a range of 0.18 to 10 μm.

[0083]Thereafter, the electrolysis was continued for four consecutive days, and the mists were measured once a day. As a result of the measurement, the number (number per unit volume) of the mists contained in the fluorine gas was 12800 to 13700 mists / cm3 (average value: 13300 mists / cm3). The total volume (volum...

Claims

1. A fluorine gas production method comprising: an electrolysis step of electrolyzing an electrolyte using an anode and a cathode to generate a fluorine gas in an electrolytic cell, whereinthe electrolyte contains hydrogen fluoride, metal fluoride, and metal hexafluoronickelate (IV), andthe anode is a nickel electrode or a carbonaceous electrode, and the cathode is a metal electrode.

2. The fluorine gas production method according to claim 1, wherein the metal hexafluoronickelate (IV) is at least one of potassium hexafluoronickelate (IV), cesium hexafluoronickelate (IV), and rubidium hexafluoronickelate (IV).

3. The fluorine gas production method according to claim 1, wherein a concentration of nickel (IV) ions in the electrolyte is 100 ppm by mass or more and 1000 ppm by mass or less.

4. The fluorine gas production method according to claim 1, wherein the metal fluoride is at least one of potassium fluoride and cesium fluoride.

5. The fluorine gas production method according to claim 1, wherein a ratio of a molar amount of the hydrogen fluoride to a molar amount of the metal fluoride contained in the electrolyte is 1.6 or more and 3.2 or less.

6. The fluorine gas production method according to claim 1, wherein the cathode is an electrode formed of at least one of iron, nickel, copper, and a copper nickel alloy.

7. The fluorine gas production method according to claim 1, wherein the electrolyte is a mixture of the hydrogen fluoride, the metal fluoride, and the metal hexafluoronickelate (IV) having a water content of 0.5% by mass or less.

8. The fluorine gas production method according to claim 1, wherein a concentration of iron ions in the electrolyte is 500 ppm by mass or less, and a concentration of copper ions is 200 ppm by mass or less.

9. The fluorine gas production method according to claim 2, wherein a concentration of nickel (IV) ions in the electrolyte is 100 ppm by mass or more and 1000 ppm by mass or less.

10. The fluorine gas production method according to claim 2, wherein the metal fluoride is at least one of potassium fluoride and cesium fluoride.

11. The fluorine gas production method according to claim 2, wherein a ratio of a molar amount of the hydrogen fluoride to a molar amount of the metal fluoride contained in the electrolyte is 1.6 or more and 3.2 or less.

12. The fluorine gas production method according to claim 2, wherein the cathode is an electrode formed of at least one of iron, nickel, copper, and a copper nickel alloy.

13. The fluorine gas production method according to claim 2, wherein the electrolyte is a mixture of the hydrogen fluoride, the metal fluoride, and the metal hexafluoronickelate (IV) having a water content of 0.5% by mass or less.

14. The fluorine gas production method according to claim 2, wherein a concentration of iron ions in the electrolyte is 500 ppm by mass or less, and a concentration of copper ions is 200 ppm by mass or less.

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