Electrolyte production method and fluorine gas production method

Abstract

There is provided an electrolyte production method capable of producing an electrolyte having a low moisture concentration by a simple operation in a short period of time. The electrolyte production method is a method for producing an electrolyte for generating a fluorine gas by electrolysis using a carbonaceous electrode as an anode, and includes: a dehydration step of adding alkali metal hexafluoronickelate (IV) to a mixture containing hydrogen fluoride and inorganic fluoride and having a moisture concentration of more than 100 ppm by mass and 1% by mass or less to set the moisture concentration to 100 ppm by mass or less.

Description

TECHNICAL FIELD

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

[0002] A fluorine gas can be synthesized by electrolyzing an electrolyte containing hydrogen fluoride and metal fluoride. As an anode when the above-described electrolyte is electrolyzed, a carbonaceous electrode is usually used. However, when water is present in the electrolyte, an insulating coating is likely to be formed on the surface of the anode, so that there is a risk of the occurrence of a phenomenon known as an anode effect, in which the electrolysis voltage sharply rises. Therefore, the moisture concentration of the electrolyte is preferably as low as possible.

[0003] PTL 1 discloses a technology of injecting an inert gas into an electrolyte to reduce the moisture concentration of the electrolyte. PTL 2 discloses a technology of electrolyzing an electrolyte using a nickel electrode to generate a fluorine gas, and reacting the generated fluorine gas with water to remove the water. In general, as disclosed in PTL 3, a method is adopted in many cases in which electrolysis is performed at a low current density using a carbonaceous electrode to electrolyze water, and then electrolysis is performed at an increased current density to synthesize a fluorine gas.CITATION LISTPatent LiteraturesPTL 1: JP H07-2515 A

[0005] PTL 2: JP 3089432 B

[0006] PTL 3: JP 5520280 BSUMMARY OF INVENTIONTechnical Problem

[0007] Although the moisture concentration of an electrolyte can be reduced using the technologies disclosed in PTLS 1 to 3, operations require labor and a long time, and therefore a technology capable of reducing the moisture concentration of an electrolyte by a simple operation in a short period of time has been desired.

[0008] It is an object of the present invention to provide an electrolyte production method capable of producing an electrolyte having a low moisture concentration by a simple operation in a short period of time, and a fluorine gas production method capable of economically producing a fluorine gas.Solution to Problem

[0009] To solve the above-described problem, one aspect of the present invention is as described in [1] to [6] below.

[0010] [1] An electrolyte production method for producing an electrolyte for generating a fluorine gas by electrolysis using a carbonaceous electrode as an anode, including:

[0011] a dehydration step of adding alkali metal hexafluoronickelate (IV) to a mixture containing hydrogen fluoride and inorganic fluoride and having a moisture concentration of more than 100 ppm by mass and 1% by mass or less to set the moisture concentration to 100 ppm by mass or less.

[0012] [2] The electrolyte production method described in [1], in which the alkali metal hexafluoronickelate (IV) is at least one of potassium hexafluoronickelate (IV), cesium hexafluoronickelate (IV), and rubidium hexafluoronickelate (IV).

[0013] [3] The electrolyte production method according to [1] or [2], in which the inorganic fluoride is at least one of potassium fluoride, cesium fluoride, rubidium fluoride, and ammonium fluoride.

[0014] [4] The electrolyte production method according to any one of [1] to [3], in which the addition amount of the alkali metal hexafluoronickelate (IV) is 100 ppm by mass or more and 10% by mass or less of the mass of the mixture.

[0015] [5] A fluorine gas production method including: an electrolysis step of electrolyzing an electrolyte produced by the electrolyte production method according to any one of [1] to [4] using an anode and a cathode to generate a fluorine gas in an electrolytic cell.

[0016] [6] The fluorine gas production method according to [5], in which the anode is a carbonaceous electrode having at least one of graphite, glassy carbon, and amorphous carbon.Advantageous Effects of Invention

[0017] The electrolyte production method according to the present invention can produce an electrolyte having a low moisture concentration by a simple operation in a short period of time. The fluorine gas production method according to the present invention can economically produce a fluorine gas.DESCRIPTION OF EMBODIMENTS

[0018] 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.

[0019] In the synthesis of a fluorine gas (F2) by electrolysis of an electrolyte, water (H2O) in the electrolyte is known to cause the anode effect. The present inventors have conducted extensive studies on a method for removing water in an electrolyte, and, as a result, have found that the moisture concentration of the electrolyte can be reduced by adding an additive, and thus have accomplished the present invention.

[0020] More specifically, an electrolyte production method according to this embodiment is a method for producing an electrolyte for generating a fluorine gas by electrolysis using a carbonaceous electrode as an anode, and includes: a dehydration step of adding alkali metal hexafluoronickelate (IV) to a mixture containing hydrogen fluoride (HF) and inorganic fluoride and having a moisture concentration of more than 100 ppm by mass and 1% by mass or less to set the moisture concentration to 100 ppm by mass or less.

[0021] A fluorine gas production method according to this embodiment includes an electrolysis step of electrolyzing an electrolyte produced by the above-described electrolyte production method according to this embodiment using an anode and a cathode to generate a fluorine gas in an electrolytic cell.

[0022] The alkali metal hexafluoronickelate (IV) is easily soluble in a mixture containing hydrogen fluoride and inorganic fluoride and has oxidizing power, and therefore the alkali metal hexafluoronickelate (IV) reacts with water in an electrolyte, so that the water is decomposed and the moisture concentration of the electrolyte decreases. Therefore, when a fluorine gas is produced by electrolyzing an electrolyte added with the alkali metal hexafluoronickelate (IV) hardly causes the anode effect.

[0023] The addition of the alkali metal hexafluoronickelate (IV) can reduce the moisture concentration of the electrolyte, and therefore the dehydration step in the electrolyte production method according to this embodiment is an extremely simple operation and the dehydration can be carried out in a short period of time.

[0024] The dehydration of the electrolyte can be performed by a simple operation in a short period of time, and therefore the fluorine gas production method according to the present invention can economically produce a fluorine gas.

[0025] The electrolyte production method according to this embodiment and the fluorine gas production method according to this embodiment are described in more detail below.[Electrolyte]

[0026] An electrolyte to be electrolyzed by the fluorine gas production method according to this embodiment contains hydrogen fluoride and inorganic fluoride and has a moisture concentration of 100 ppm by mass or less. This electrolyte can be produced by the electrolyte production method according to this embodiment. More specifically, the electrolyte can be produced by adding alkali metal hexafluoronickelate (IV) to a mixture containing hydrogen fluoride and inorganic fluoride and having a moisture concentration of more than 100 ppm by mass and 1% by mass or less for dehydration and setting the moisture concentration to 100 ppm by mass or less.

[0027] The type of the inorganic fluoride is not particularly limited, and is preferably metal fluoride and more preferably at least one of potassium fluoride (KF), cesium fluoride (CsF), and rubidium fluoride (RbF). As the inorganic fluoride, ammonium fluoride (NH4F) can be used. When ammonium fluoride is used as the inorganic fluoride, nitrogen trifluoride (NF3) is generated by electrolysis of the electrolyte. One type of inorganic fluoride may be used alone or two or more types of inorganic fluorides may be used in combination.

[0028] When nitrogen trifluoride is produced together with a fluorine gas, an electrolyte is used which contains at least one of potassium fluoride, cesium fluoride, and rubidium fluoride, ammonium fluoride, and hydrogen fluoride.

[0029] The ratio of the molar amount of the hydrogen fluoride to the molar amount of the inorganic fluoride contained in the electrolyte ([Molar amount of hydrogen fluoride] / [Molar amount of inorganic fluoride]) is preferably 1.6 or more and 3.2 or less and more preferably 1.9 or more and 3.0 or less.

[0030] 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.

[0031] 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.

[0032] The electrolysis consumes the hydrogen fluoride in the electrolyte, and therefore hydrogen fluoride may be 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.[Moisture Concentration of Electrolyte]

[0033] In usual, solid KF·HF, KF, and CsF contain several hundred ppm by mass of water, and hydrogen fluoride contains 100 to 200 ppm by mass of water. Water adheres also to the inside of a preparation facility preparing an electrolyte or the inside of an electrolytic cell electrolyzing an electrolyte. Further, in a process of operating an electrolyte, the atmosphere and the electrolyte sometimes come into contact with each other, and therefore the electrolyte sometimes absorbs water in the atmosphere. From these reasons, the electrolyte produced by mixing hydrogen fluoride and inorganic fluoride contains water, and the moisture concentration is usually more than 100 ppm by mass and 1% by mass or less.

[0034] Thus, in this embodiment, an electrolyte having a low moisture concentration is produced by adding alkali metal hexafluoronickelate (IV) to a mixture containing hydrogen fluoride and inorganic fluoride and having a moisture concentration of more than 100 ppm by mass and 1% by mass or less for dehydration and setting the moisture concentration to 100 ppm by mass or less as described above.

[0035] For example, when the electrolyte is KF·2HF, an electrolyte having a low moisture concentration can be produced by mixing liquid or gaseous hydrogen fluoride with solid KF·HF or solid KF to prepare the above-described mixture, and then adding alkali metal hexafluoronickelate (IV) to the mixture for dehydration.

[0036] When the electrolyte is CsF·2.4HF, an electrolyte having a low moisture concentration can be produced by mixing liquid or gaseous hydrogen fluoride with solid CsF to prepare the above-described mixture, and then adding alkali metal hexafluoronickelate (IV) to the mixture for dehydration.

[0037] Even when the moisture concentration of the mixture containing hydrogen fluoride and inorganic fluoride exceeds 1% by mass, the moisture concentration can be reduced by adding alkali metal hexafluoronickelate (IV). However, when the moisture concentration of the mixture is 1% by mass or less, the amount of nickel fluoride (NiF2) produced as a by-product by the reaction of alkali metal hexafluoronickelate (IV) with water is small, and therefore the accumulation of the nickel fluoride on the bottom surface of the electrolytic cell is less likely to occur. Considering the accuracy of a method for measuring the moisture concentration of the electrolyte, there is a risk that the moisture concentration of less than 100 ppm by mass cannot be accurately measured.

[0038] Herein, a method for measuring the moisture concentration of an electrolyte is described taking a case where the electrolyte is KF·2HF as an example. The moisture concentration of an electrolyte can be measured using a moisture analyzer having the following configuration, for example. The moisture analyzer includes a nickel cylindrical heating part, a gas supply pipe supplying an inert gas, such as a nitrogen gas, into the cylindrical heating part, a heater, such as an electric heater, heating the cylindrical heating part, a thermometer, such as a thermocouple, measuring the temperature of the inside of the cylindrical heating part, and an analyzer, such as an FT-IR analyzer, analyzing a gas discharged from the cylindrical heating part. The moisture analyzer is installed inside a glove box kept at low humidity.

[0039] A sample of the electrolyte whose mass has been measured is placed in a platinum boat-shaped tray, the boat-shaped tray is installed inside the cylindrical heating part, and an inert gas is supplied into the cylindrical heating part from the gas supply pipe. The cylindrical heating part is subjected to pressure storage with the inert gas, so that airtightness is held. Then, the cylindrical heating part is heated by a heater while an inert gas of a constant flow rate is supplied from the gas supply pipe to the cylindrical heating part. The cylindrical heating part is heated such that the temperature increases stepwise from room temperature to 350° C.

[0040] A gas evaporated from the sample by the heating is discharged from the cylindrical heating part together with the inert gas and sent to the FT-IR analyzer. Then, continuous analysis is performed using the FT-IR analyzer, the moisture concentration is quantified from the peak intensity in the absorption wavenumber range of 1400 cm−1 to 1899 cm−1, and the total amount of evaporated water is determined from the integrated value in a period from when the water peak is detected to when the water peak is not detected. From the total amount of the evaporated water and the mass of the charged sample, the moisture concentration of the electrolyte can be determined.

[0041] The moisture concentration of the electrolyte (the above-described mixture) measured using the above-described measurement method varies depending on an electrolyte preparation method (a method for mixing hydrogen fluoride and inorganic fluoride), the atmosphere in the preparation of the electrolyte (atmosphere when hydrogen fluoride and inorganic fluoride are mixed), and the electrolyte preparation scale (prepared amount of the above-described mixture). For example, in a case where the prepared amount of the electrolyte is about several liters, the moisture concentration of the electrolyte can be set to more than 100 ppm by mass and 500 ppm by mass or less when the electrolyte is prepared in a dry box. When the prepared amount of the electrolyte is an industrial-scale amount of 500 L or more and 1000 L or less, the moisture concentration of the electrolyte before pre-electrolysis is 0.2% by mass or more and 1.0% by mass or less.[Pre-Electrolysis]

[0042] Although a description is given in the section of Examples described later, the present inventors have studied a condition under which pre-electrolysis to be applied to an electrolyte as a prestage of main electrolysis, which is electrolysis for producing a fluorine gas, is carried out to enable stable main electrolysis hardly causing an increase in the electrolysis voltage.

[0043] The inside of an electrolytic cell where electrolysis is performed 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 are installed. As the cathode, a nickel (Ni) punching plate is 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.

[0044] 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. The electrolysis temperature was controlled to 85° C. to 95° C. Hydrogen fluoride can be intermittently supplied to the electrolyte of the electrolytic cell such that the hydrogen fluoride concentration of the electrolyte was 39 to 42% by mass. Before the start of the electrolysis, the electrolyte was sampled from the electrolytic cell. The moisture concentration of the electrolyte was 0.8% by mass as measured by the above-described method.

[0045] The temperature of the electrolyte was set to 85° C., and a constant current was applied to two amorphous carbon electrodes among the 16 amorphous carbon electrodes to gradually increase the current. Then, when the current increased to 250 A (current density of 0.072 A / cm2), the electrolysis voltage exceeded 12 V, and therefore the electrolysis was discontinued. The energizing quantity at this time was 1.3 kAh. It was able to be confirmed from this result that, when the moisture concentration of the electrolyte is 0.8% by mass, a result that stable electrolysis cannot be carried out at a high current density is obtained, and the electrolyte is required to be dehydrated before the main electrolysis.

[0046] The concentration of carbon dioxide in an anode gas (fluorine gas) generated by this electrolysis was 3800 ppm by volume as measured by the FT-IR analyzer. When the water in the electrolyte is electrolyzed, an oxygen gas (02) is generated. Therefore, the concentration of the oxygen gas may be tracked. However, it is difficult to accurately analyze the oxygen gas in the fluorine gas, and therefore the concentration of carbon dioxide (CO2) in place of the oxygen gas was tracked.

[0047] The temperature of the electrolyte was set to 85° C., and a 875 A constant current (current density of 0.036 A / cm2) was applied to the remaining 14 amorphous carbon electrodes, which had not been energized, for 34 hours, thereby performing 30 kAh energization.

[0048] The concentration of carbon dioxide in an anode gas generated by this electrolysis was 130 ppm by volume as measured by the FT-IR analyzer. The sampled electrolyte had a moisture concentration of 400 ppm by mass.

[0049] Continuously using these 14 amorphous carbon electrodes, a current was gradually increased and 2625 A (current density of 0.108 A / cm2) was applied, the temperature of the electrolyte was set to 85° C., and electrolysis was performed. The electrolysis voltage at this time was 9 V. Then, the electrolysis was continued, and 300 hours later the electrolysis voltage started to gradually increase and reached 12 V, and therefore the electrolysis was stopped.

[0050] The electrolyte and the electrodes of the electrolytic cell were replaced with new ones. In the electrolyte sampled from the electrolytic cell, the moisture concentration before the start of the electrolysis was 0.75% by mass. The temperature of the electrolyte was set to 85° C., a 1000 A constant current (current density of 0.036 A / cm2) was applied to the 16 amorphous carbon electrodes for 100 hours, and 100 kAh pre-electrolysis was performed. The concentration of carbon dioxide in an anode gas generated by this pre-electrolysis was 50 ppm by volume or less as measured by the FT-IR analyzer. The moisture concentration of the electrolyte sampled after the pre-electrolysis decreased up to 100 ppm by mass or less.

[0051] Subsequent to the pre-electrolysis, the process shifted to the main electrolysis, the constant current to be applied was gradually increased to 3000 A, and the current density was set to 0.108 A / cm2. The electrolysis voltage at this time was 9 V. Then, the electrolysis was continued, and 1000 hours later the electrolysis voltage started to gradually increase and reached 12 V, and therefore the electrolysis was stopped.

[0052] This voltage increase was expected to be caused by the use of the same amorphous carbon electrodes used in dehydration electrolysis (pre-electrolysis), in which the electrolysis is performed at a low current density, also for the main electrolysis. It is considered that, when an electrolyte having a high moisture concentration is electrolyzed, an oxide coating is formed on the carbon surface of the amorphous carbon electrode, and the coating affects the main electrolysis.

[0053] Therefore, the electrolyte and the amorphous carbon electrodes of the above-described electrolytic cell were replaced with new ones. In the electrolyte sampled from the electrolytic cell, the moisture concentration before the start of the electrolysis was 0.72% by mass. Two amorphous carbon electrodes among the 16 amorphous carbon electrodes were used for the pre-electrolysis. The temperature of the electrolyte was set to 85° C., and a 125 A constant current (current density of 0.036 A / cm2) was applied to the two amorphous carbon electrodes for 800 hours, and 100 kAh pre-electrolysis was performed. The concentration of carbon dioxide in an anode gas generated by this pre-electrolysis was 50 ppm by volume or less as measured by the FT-IR analyzer. The moisture concentration of the electrolyte sampled after the pre-electrolysis decreased up to 100 ppm by mass or less.

[0054] Subsequent to the pre-electrolysis, the process shifted to the main electrolysis, the constant current to be applied was gradually increased, using the 14 amorphous carbon electrodes which had not been used for the pre-electrolysis, to 3000 A, and the current density was set to 0.108 A / cm2. The temperature of the electrolyte is 85° C. The electrolysis voltage at the start of the electrolysis was 9 V. Then, the electrolysis was continued, and 1000 hours later the electrolysis voltage was 10.5 V, and thus the electrolysis voltage did not increase to 12 V.

[0055] When the behaviors of the concentration of carbon dioxide in the anode gas and the moisture concentration of the electrolyte during the dehydration electrolysis were analyzed, the logarithmic value of the carbon dioxide concentration and the logarithmic value of the moisture concentration had a good linear relation to the logarithmic value of the energizing quantity.

[0056] In a case where the type of the electrolyte is KF·2HF and the amount is 560 L, and the electrolyte has a moisture concentration before electrolysis of 0.7 to 0.8% by mass, when 100 kAh electrolysis is carried out, the concentration of carbon dioxide in an anode gas in the electrolysis at a current density of 0.036 A / cm2 becomes 50 ppm by volume or less, and the moisture concentration of the electrolyte decreases to 100 ppm by mass or less.

[0057] When the moisture concentration of the electrolyte decreases up to 100 ppm by mass or less, the increase in the electrolysis voltage is suppressed even when a new amorphous carbon electrode is used, and therefore the pre-electrolysis can be defined as electrolysis that reduces the moisture concentration of the electrolyte to 100 ppm by mass or less. The energizing quantity of the pre-electrolysis varies depending on the amount of water contained in the initial electrolyte and the electrolyte amount.[Alkali Metal Hexafluoronickelate (IV)]

[0058] Alkali metal hexafluoronickelate (IV) reacts with water, and therefore, when the alkali metal hexafluoronickelate (IV) is added to an electrolyte, water in the electrolyte is decomposed, so that the moisture concentration of the electrolyte decreases.

[0059] Products generated by the reaction between the alkali metal hexafluoronickelate (IV) and water are an oxygen gas and nickel fluoride, which are less likely to inhibit the electrolysis of the electrolyte. Therefore, the fluorine gas production method according to this embodiment can efficiently produce a fluorine gas.

[0060] Further, the reaction between the alkali metal hexafluoronickelate (IV) and water does not generate hydrogen chloride (HCl), hydrogen bromide (HBr), hydrogen iodide (HI), and the like, and therefore the electrolysis is less likely to generate chlorine molecules (Cl2), bromine molecules (Br2), iodine molecules (I2), and the like. Therefore, a fluorine gas produced by the fluorine gas production method according to this embodiment has few impurities and is of high purity.

[0061] Examples of the type of the alkali metal hexafluoronickelate (IV) include, but are not particularly limited to, potassium hexafluoronickelate (IV) (K2NiF6), cesium hexafluoronickelate (IV) (Cs2NiF6), and rubidium hexafluoronickelate (IV) (Rb2NiF6). One alkali metal hexafluoronickelate (IV) may be used alone or two or more types of alkali metal hexafluoronickelates (IV) may be used in combination.

[0062] The potassium hexafluoronickelate (IV) quickly dissolves in KF·2HF, and has solubility as high as about 3000 ppm by mass. Therefore, the removal of water in an electrolyte can be performed in a short period of time. The cesium hexafluoronickelate (IV) and the rubidium hexafluoronickelate (IV) have relatively large molecular weights, and therefore the addition amounts to an electrolyte are relatively large, while the potassium hexafluoronickelate (IV) has a relatively small molecular weight, and therefore the addition amount to an electrolyte can be kept relatively small.

[0063] Water in an electrolyte can be decomposed using metal fluoride complexes of metals other than nickel in place of the alkali metal hexafluoronickelate (IV). For example, alkali metal hexafluoromanganate (IV), such as potassium hexafluoromanganate (IV) (K2MnF6), and alkali metal hexafluorocobaltate (IV), such as potassium hexafluorocobaltate (IV) (K2COF6), can also be used.

[0064] Further, even when metal fluorides, such as antimony trifluoride (SbF3), silver difluoride (AgF2), and cobalt trifluoride (CoF3), are used in place of the alkali metal hexafluoronickelate (IV), water in an electrolyte can be decomposed as in the case of using the alkali metal hexafluoronickelate (IV).

[0065] The addition amount of the alkali metal hexafluoronickelate (IV) to be added to a mixture containing hydrogen fluoride and inorganic fluoride varies depending on the amount of water contained in the above-described mixture, and the alkali metal hexafluoronickelate (IV) in which the number of moles is 1 times or more and 1.2 times or less the number of moles of water contained in the above-described mixture is preferably added.

[0066] In usual cases, the addition amount of the alkali metal hexafluoronickelate (IV) to be added to a mixture containing hydrogen fluoride and inorganic fluoride is preferably 100 ppm by mass or more and 10% by mass or less, more preferably 300 ppm by mass or more and 9% by mass or less, and still more preferably 600 ppm by mass or more and 8% by mass or less of the mass of the above-described mixture.

[0067] The timing of adding the alkali metal hexafluoronickelate (IV) to a mixture containing hydrogen fluoride and inorganic fluoride is not particularly limited. It is preferable that, after the above-described mixture is charged into the electrolytic cell and the moisture concentration is measured, the alkali metal hexafluoronickelate (IV) in an amount according to the measurement result is added for dehydration, and the electrolysis is started.

[0068] As the alkali metal hexafluoronickelate (IV), commercially available substances may be used or prepared substances may be used. The alkali metal hexafluoronickelate (IV) can be prepared by mixing potassium fluoride, cesium fluoride, or rubidium fluoride with 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.[Electrolytic Cell]

[0069] There are no particular restrictions on the mode of the electrolytic cell to be used in the electrolyte production method according to this embodiment, and any electrolytic cell can be used insofar as a fluorine gas or nitrogen trifluoride (hereinafter referred to as a “fluorine gas and the like”) can be generated by electrolyzing an electrolyte. The 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 (Fe), nickel, or Monel (trademark).

[0070] When members constituting the electrolytic cell and immersed in the electrolyte contain iron serving as the generation source of iron ions or copper (Cu) 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.

[0071] 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 the fluorine gas and the like generated at the anode and a hydrogen gas (H2) generated at the cathode are not mixed.

[0072] 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 and metal electrodes formed of a copper nickel alloy (e.g., Monel (trademark)) and the other types of metals can also be used, for example. As the cathode, metal electrodes formed of metals, such as iron, nickel, copper, a copper nickel alloy (e.g., Monel (trademark)), can be used, for example.EXAMPLES

[0073] Hereinafter, the present invention is more specifically described with reference to Examples and Comparative Examples.Comparative Example 1

[0074] 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.

[0075] Into the electrolytic cell described above, 560 L of an electrolyte (molten salt) prepared by mixing 217 kg of hydrogen fluoride and 847 kg of acidic potassium fluoride (KF·HF) was charged. Then, the anode and the cathode were immersed in the electrolyte.

[0076] The electrolysis temperature was controlled to 85° C. to 95° C. Before the start of the electrolysis, the electrolyte was sampled from the electrolytic cell. The moisture concentration of the electrolyte was 0.8% by mass as measured by the above-described method. The electrolysis was performed while hydrogen fluoride was intermittently supplied to the electrolyte of the electrolytic cell such that the hydrogen fluoride concentration of the electrolyte during the electrolysis was 39% by mass or more and 42% by mass or less.

[0077] First, the temperature of the electrolyte was set to 85° C., a 1000 A constant current was applied at a current density of 0.036 A / cm2 to all of the 16 amorphous carbon electrodes and the cathode to start the electrolysis, and then the pre-electrolysis (dehydration electrolysis) was performed for 100 hours until the integrated energizing quantity reached 100 kAh. By the pre-electrolysis, the concentration of carbon dioxide in an anode gas became 50 ppm by volume or less, and the moisture concentration of the electrolyte decreased up to 100 ppm by mass or less.

[0078] Next, subsequent to the pre-electrolysis, the process shifted to the main electrolysis (production of a fluorine gas). More specifically, the constant current to be applied was gradually increased to 3000 A, and the current density was set to 0.108 A / cm2. The electrolysis voltage at this time was 9 V. Then, the electrolysis was continued, and 1000 hours later the electrolysis voltage started to gradually increase and reached 12 V, and therefore the electrolysis was stopped.Comparative Example 2

[0079] The same electrolytic cell and electrolyte as those of Comparative Example 1 were prepared. The moisture concentration of the electrolyte before the start of the pre-electrolysis was 0.75% by mass.

[0080] The temperature of the electrolyte was set to 85° C., a 125 A constant current was applied at a current density of 0.036 A / cm2 to two amorphous carbon electrodes among the 16 amorphous carbon electrodes and the cathode to start the electrolysis, and then the pre-electrolysis (dehydration electrolysis) was performed for 800 hours until the integrated energizing quantity reached 100 kAh. By the pre-electrolysis, the concentration of carbon dioxide in an anode gas became 50 ppm by volume or less, and the moisture concentration of the electrolyte decreased up to 100 ppm by mass or less.

[0081] Next, subsequent to the pre-electrolysis, the process shifted to the main electrolysis (production of a fluorine gas). More specifically, the temperature of the electrolyte was set to 85° C., a current was applied to the 14 amorphous carbon electrodes which had not been used for the pre-electrolysis and the cathode, the constant current to be applied was gradually increased to 2625 A, and the current density was set to 0.108 A / cm2. The electrolysis voltage at this time was 9 V. Then, the electrolysis was continued, and 1000 hours later the electrolysis voltage was 10.5 V.Comparative Example 3

[0082] The same electrolytic cell and electrolyte as those of Comparative Example 1 were prepared. The moisture concentration of the electrolyte before the start of the pre-electrolysis was 0.7% by mass.

[0083] The temperature of the electrolyte was set to 85° C., a 125 A constant current was applied at a current density of 0.036 A / cm2 to two amorphous carbon electrodes among the 16 amorphous carbon electrodes and the cathode to start the electrolysis, and then the pre-electrolysis (dehydration electrolysis) was performed for 240 hours until the integrated energizing quantity reached 30 kAh. By the pre-electrolysis, the concentration of carbon dioxide in an anode gas became 140 ppm by volume, and the moisture concentration of the electrolyte decreased up to 430 ppm by mass.

[0084] Next, subsequent to the pre-electrolysis, the process shifted to the main electrolysis (production of a fluorine gas). More specifically, the temperature of the electrolyte was set to 85° C., a current was applied to the 14 amorphous carbon electrodes which had not been used for the pre-electrolysis and the cathode, the constant current to be applied was gradually increased to 2625 A, and the current density was set to 0.108 A / cm2. The electrolysis voltage at this time was 9 V. Then, the electrolysis was continued, and 340 hours later the electrolysis voltage reached 12 V, and therefore the electrolysis was stopped.Comparative Example 4

[0085] 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, an amorphous carbon electrode (1 cm wide, 1 cm long) manufactured by SGL Carbon was installed. As the cathode, a nickel punching plate (5 cm wide, 5 cm long) was installed. The electrolytic cell is formed of Teflon (registered trademark). The diaphragm for partitioning the anode chamber and the cathode chamber is also formed of Teflon (registered trademark).

[0086] In a glove box filled with a nitrogen gas having a dew point of −80° C. or less, 434 g of hydrogen fluoride and 630 g of potassium fluoride were mixed to prepare 0.56 L of an electrolyte. Then, the prepared electrolyte was charged into the above-described electrolytic cell installed in the glove box, and the anode and the cathode were immersed in the electrolyte.

[0087] The electrolysis temperature was controlled to 85° C. to 95° C. Before the start of the electrolysis, the electrolyte was sampled from the electrolytic cell. The moisture concentration of the electrolyte was 200 ppm by mass as measured by the above-described method. The electrolysis was performed while hydrogen fluoride was intermittently supplied to the electrolyte of the electrolytic cell such that the hydrogen fluoride concentration of the electrolyte during the electrolysis was 39% by mass or more and 42% by mass or less.

[0088] The temperature of the electrolyte was set to 85° C., a constant current was applied at a current density of 0.025 A / cm2 to the amorphous carbon electrodes and the cathode to start the electrolysis, and the current density of the current to be applied was increased by 0.025 A / cm2 every 15 minutes. Then, the electrolysis voltage sharply increased at a current density of 0.225 A / cm2.Example 1

[0089] The same electrolytic cell and electrolyte as those of Comparative Example 1 were prepared. The moisture concentration of the electrolyte before the start of the electrolysis was 0.7% by mass. The amount of water contained in the electrolyte is 413 mol, and therefore an equimolar amount of potassium hexafluoronickelate (IV), i.e., 104 kg of potassium hexafluoronickelate (IV), was charged into the electrolytic cell. The addition amount of the potassium hexafluoronickelate (IV) is 9.7% by mass of the mass of the mixture of hydrogen fluoride and potassium fluoride.

[0090] The electrolyte thus prepared was allowed to stand for 24 hours, and then the moisture concentration of the electrolyte was measured, so that the moisture concentration of the electrolyte decreased to 100 ppm by mass or less.

[0091] The temperature of the electrolyte was set to 85° C., and a 3000 A constant current was applied at a current density of 0.108 A / cm2 to all of the 16 amorphous carbon electrodes and the cathode to perform electrolysis. The electrolysis voltage at this time was 9 V. Then, the electrolysis was continued, and 1000 hours later the electrolysis voltage was 10 V.Example 2

[0092] The same electrolytic cell and electrolyte as those of Comparative Example 1 were prepared. The moisture concentration of the electrolyte before the start of the pre-electrolysis was 0.75% by mass.

[0093] The temperature of the electrolyte was set to 85° C., a 125 A constant current was applied at a current density of 0.036 A / cm2 to two amorphous carbon electrodes among the 16 amorphous carbon electrodes and the cathode to start the electrolysis, and then the pre-electrolysis (dehydration electrolysis) was performed for 240 hours until the integrated energizing quantity reached 30 kAh. By the pre-electrolysis, the concentration of carbon dioxide in an anode gas became 120 ppm by volume, and the moisture concentration of the electrolyte decreased up to 390 ppm by mass.

[0094] The amount of water contained in the electrolyte after the pre-electrolysis was 23 mol, and therefore an equimolar amount of potassium hexafluoronickelate (IV), i.e., 5.8 kg of potassium hexafluoronickelate (IV), was charged into the electrolytic cell. The addition amount of the potassium hexafluoronickelate (IV) is 0.54% by mass of the mass of the mixture of hydrogen fluoride and potassium fluoride.

[0095] The electrolyte thus prepared was allowed to stand for 24 hours, and then the moisture concentration of the electrolyte was measured, so that the moisture concentration of the electrolyte decreased to 100 ppm by mass or less.

[0096] Next, subsequent to the pre-electrolysis, the process shifted to the main electrolysis (production of a fluorine gas). More specifically, the temperature of the electrolyte was set to 85° C., a current was applied to the 14 amorphous carbon electrodes which had not been used for the pre-electrolysis and the cathode, the constant current to be applied was gradually increased to 2625 A, and the current density was set to 0.108 A / cm2. The electrolysis voltage at this time was 9 V. Then, the electrolysis was continued, and 1000 hours later the electrolysis voltage was 10.2 V.Example 3

[0097] The same electrolytic cell and electrolyte as those of Comparative Example 4 were prepared. The moisture concentration of the electrolyte before the start of the electrolysis was 200 ppm by mass.

[0098] The amount of water contained in the electrolyte is 0.0118 mol, and therefore an equimolar amount of potassium hexafluoronickelate (IV), i.e., 3.0 g of potassium hexafluoronickelate (IV), was charged into the electrolytic cell. The addition amount of the potassium hexafluoronickelate (IV) is 0.28% by mass of the mass of the mixture of hydrogen fluoride and potassium fluoride.

[0099] The electrolyte thus prepared was allowed to stand for 24 hours, and then the moisture concentration of the electrolyte was measured, so that the moisture concentration of the electrolyte decreased to 100 ppm by mass or less.

[0100] The temperature of the electrolyte was set to 85° C., and a 6250 A constant current was applied at a current density of 0.225 A / cm2 to the amorphous carbon electrode and the cathode to perform electrolysis. The electrolysis was performed while hydrogen fluoride was intermittently supplied to the electrolyte of the electrolytic cell such that the hydrogen fluoride concentration of the electrolyte during the electrolysis was 39% by mass or more and 42% by mass or less. The electrolysis voltage at this time was 9.3 V. Although the electrolysis was continued for 100 hours, the electrolysis voltage did not increase.Example 4

[0101] Into 1140 g of cesium fluoride contained in a 1-L capacity Teflon (registered trademark) container, 360 g of gaseous hydrogen fluoride was injected to set the total weight to 1500 g. The moisture concentration of the mixture thus obtained was measured to be 350 ppm by mass.

[0102] The amount of water contained in the mixture is 0.0291 mol, and therefore an equimolar amount of cesium hexafluoronickelate (IV), i.e., 12.8 g of cesium hexafluoronickelate (IV), was charged into the container. The addition amount of the cesium hexafluoronickelate (IV) is 0.85% by mass of the mass of the mixture of hydrogen fluoride and cesium fluoride.

[0103] The electrolyte thus prepared was allowed to stand for 24 hours, and then the moisture concentration of the electrolyte was measured, so that the moisture concentration of the electrolyte decreased to 100 ppm by mass or less.

Claims

1. An electrolyte production method for producing an electrolyte for generating a fluorine gas by electrolysis using a carbonaceous electrode as an anode, comprising:a dehydration step of adding alkali metal hexafluoronickelate (IV) to a mixture containing hydrogen fluoride and inorganic fluoride and having a moisture concentration of more than 100 ppm by mass and 1% by mass or less to set the moisture concentration to 100 ppm by mass or less.

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

3. The electrolyte production method according to claim 1, wherein the inorganic fluoride is at least one of potassium fluoride, cesium fluoride, rubidium fluoride, and ammonium fluoride.

4. The electrolyte production method according to claim 1, wherein an addition amount of the alkali metal hexafluoronickelate (IV) is 100 ppm by mass or more and 10% by mass or less of a mass of the mixture.

5. A fluorine gas production method comprising:an electrolysis step of electrolyzing an electrolyte produced by the electrolyte production method according to claim 1 using an anode and a cathode to generate a fluorine gas in an electrolytic cell.

6. The fluorine gas production method according to claim 5, wherein the anode is a carbonaceous electrode having at least one of graphite, glassy carbon, and amorphous carbon.

7. A fluorine gas production method comprising:an electrolysis step of electrolyzing an electrolyte produced by the electrolyte production method according to claim 2 using an anode and a cathode to generate a fluorine gas in an electrolytic cell.

8. The fluorine gas production method according to claim 7, wherein the anode is a carbonaceous electrode having at least one of graphite, glassy carbon, and amorphous carbon.

9. A fluorine gas production method comprising:an electrolysis step of electrolyzing an electrolyte produced by the electrolyte production method according to claim 3 using an anode and a cathode to generate a fluorine gas in an electrolytic cell.

10. The fluorine gas production method according to claim 9, wherein the anode is a carbonaceous electrode having at least one of graphite, glassy carbon, and amorphous carbon.

11. A fluorine gas production method comprising:an electrolysis step of electrolyzing an electrolyte produced by the electrolyte production method according to claim 4 using an anode and a cathode to generate a fluorine gas in an electrolytic cell.

12. The fluorine gas production method according to claim 11, wherein the anode is a carbonaceous electrode having at least one of graphite, glassy carbon, and amorphous carbon.