Electrolytic enrichment method for heavy water

Abstract

An electrolytic enrichment method for heavy water includes enriching heavy water by electrolysis using an alkaline water electrolysis cell including an anode chamber that holds an anode, a cathode chamber that holds a cathode, and a diaphragm. In the method, an electrolyte prepared by adding high-concentration alkaline water to raw material water containing heavy water is circularly supplied to the anode chamber and the cathode chamber from a circulation tank; an anode-side gas-liquid separator and an anode-side water-seal device are connected to the anode chamber, and a cathode-side gas-liquid separator and a cathode-side water-seal device are connected to the cathode chamber; and electrolysis is continued while the alkali concentration in the electrolyte supplied to both electrolysis chambers is maintained at a constant concentration by circularly supplying, to the circulation tank, the electrolyte from which the gas generated from the anode-side gas-liquid separator and the cathode-side gas-liquid separator is separated.

Claims

1. An electrolytic enrichment method for heavy water, the method comprising: enriching heavy water by electrolysis using an alkaline water electrolysis cell consisting of an anode chamber that holds an anode, a cathode chamber that holds a cathode, and a diaphragm that divides between the anode chamber and the cathode chamber, wherein an electrolyte prepared by adding high-concentration alkaline water to raw material water consisting of heavy water containing tritium is circularly supplied to both electrolysis chambers including the anode chamber and the cathode chamber from a circulation tank containing the electrolyte; an anode-side gas-liquid separator and an anode-side water-seal device are connected to the anode chamber, and a cathode-side gas-liquid separator and a cathode-side water-seal device are connected to the cathode chamber; and continuing electrolysis while the alkali concentration in the electrolyte supplied to the both electrolysis chambers is maintained at a constant concentration by circularly supplying, to the circulation tank, the electrolyte from which the gas generated from each of the anode-side gas-liquid separator and the cathode-side gas-liquid separator is separated, so that heavy water in the electrolyte is enriched and, at the same time, the hydrogen gas is recovered or discharged from the cathode-side gas-liquid separator and the oxygen gas is recovered or discharged from the anode-side gas-liquid separator.

2. The electrolytic enrichment method for heavy water according to claim 1, wherein electrolysis is continued while the alkali concentration of the electrolyte is maintained at the initial concentration by supplying raw material water in the amount corresponding to the water disappearing by the electrolysis to the circulation tank.

3. The electrolytic enrichment method for heavy water according to claim 1, wherein the alkali concentration of the electrolyte is 1.5% to 40% by mass.

4. The electrolytic enrichment method for heavy water according to claim 1, wherein the alkali concentration of the electrolyte is 20% to 30% by mass.

5. The electrolytic enrichment method for heavy water according to claim 1, wherein electrolysis is continued while the alkali concentration of the electrolyte is maintained at a constant concentration by supplying the raw material water to the circulation tank so that the alkali concentration of the electrolyte does not exceed 40% by mass.

6. The electrolytic enrichment method for heavy water according to claim 5, wherein electrolysis is continued while the alkali concentration of the electrolyte is maintained at a constant concentration by supplying the raw material water to the circulation tank so that the alkali concentration of the electrolyte does not exceed 30% by mass.

7. The electrolytic enrichment method for heavy water according to claim 1, wherein the pressure in the cathode chamber and the pressure in the anode chamber are adjusted by adjusting the height of a water surface in the cathode-side water-seal device and the height of a water surface in the anode-side water-seal device, respectively, in order to control the ratio of the oxygen gas generated in the anode chamber mixed into the hydrogen gas generated in the cathode chamber and/or the ratio of the hydrogen gas generated in the cathode chamber mixed into the oxygen gas generated in the anode chamber.

8. The electrolytic enrichment method for heavy water according to claim 1, wherein the diaphragm is a neutral diaphragm.

9. The electrolytic enrichment method for heavy water according to claim 1, wherein the diaphragm is a cation-exchange membrane.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) FIGURE is a flow diagram illustrating an electrolytic enrichment method for heavy water according to an embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

(2) An embodiment of the present invention is described below with reference to the drawing.

(3) FIGURE is a flow diagram illustrating an electrolytic enrichment method for heavy water according to an embodiment of the present invention. In FIGURE, reference numeral 1 denotes an alkaline water electrolysis cell, and the alkaline water electrolysis cell 1 includes an anode chamber 2 that holds an anode, a cathode chamber 3 that holds a cathode, and a diaphragm 4 that divides between the anode chamber 2 and the cathode chamber 3. Reference numeral 5 denotes a circulation tank; reference numeral 6, an alkaline water tank that stores high concentration of alkaline water 7 generally required only for adjusting the initial alkaline electrolyte; reference numeral 8, a feed pump that supplies the alkaline water 7 in the alkaline water tank 6 to the circulation tank 5; reference numeral 9, a raw material tank that stores raw material water 10; and reference numeral 11, a feed pump that supplies the raw material water 10 in the raw material tank 9 to the circulation tank 5. The alkaline water 7 and the raw material water 10 are mixed in the circulation tank 5 to produce an electrolyte 16 adjusted to alkaline water at a predetermined concentration.

(4) The electrolyte controlled to a predetermined concentration by mixing in the circulation tank 5 is supplied to the anode chamber 2 of the alkaline water electrolysis cell 1 through a circulation pump 12a and a heat exchanger 13a and is supplied to the cathode chamber 3 of the alkaline water electrolysis cell 1 through a circulation pump 12b and a heat exchanger 13b.

(5) The electrolyte controlled to a predetermined concentration of alkaline water is electrolyzed in the anode chamber 2 and enriched by electrolysis to produce an enriched electrolyte, and oxygen gas is generated in the anode chamber 2. The generated oxygen gas and electrolyte are separated into gas and liquid by a gas-liquid separator 14a, and the separated electrolyte is circulated to the circulation tank 5. The oxygen gas separated by the anode-side gas-liquid separator 14a is exhausted through an anode-side water-seal device 15a.

(6) At the same time, hydrogen gas is generated in the cathode chamber 3. The generated hydrogen gas and electrolyte are separated into gas and liquid by a cathode-side gas-liquid separator 14b, and the separated electrolyte is circulated to the circulation tank 5. The hydrogen gas separated by the anode-side gas-liquid separator 14b is exhausted through a cathode-side water seal device 15b. In addition, water is supplied as raw material water to be supplied from the raw material tank 9 by supplying the raw material water in an amount corresponding to the water disappearing by electrolysis in order to continue electrolysis while maintaining the electrolysis conditions constant and to control the alkali concentrations in both electrolysis chambers.

(7) The electrolyte at the alkali concentration initially adjusted can be maintained by continuously supplying the raw material water in an amount corresponding to the water disappearing by electrolysis. On the other hand, an intermittent operation of volume reduction of the electrolyte (raw material water to be treated) can also be carried out by continuing intermittent alkaline water electrolysis circulation without continuous supply of the raw material water.

(8) (Condition for Alkaline Water Electrolysis)

(9) In alkaline water electrolysis according to the present invention, an electrolyte prepared by adding high-concentration alkaline water to the raw material water composed of heavy water containing tritium so that a predetermined alkali concentration is obtained is used as the electrolyte. The electrolyte is preferably a caustic alkali such as caustic potassium, caustic sodium, or the like, and the concentration thereof is preferably 1.5% to 40% by mass. In particular, in view of suppressing the power consumption, a concentration of 15% to 40% by mass within a region with high electric conductivity is preferred. However, in view of electrolysis cost, corrosion resistance, viscosity, and operationality, the concentration is more preferably 20% to 30% by mass.

(10) The concentration of the high-concentration alkaline water added to the raw material water is preferably 10% to 30% by mass.

(11) A method for operation at a constant alkali concentration includes, for example, continuously supplying the raw material water in an amount corresponding to the amount of water consumed after controlling the initial alkali concentration. When an intermittent operation is desired for reducing the volume of the raw material water to be treated, a method of checking a reduction in amount of the electrolyte initially adjusted may be used. In this case, the initially adjusted alkali concentration is increased in proportion to the amount of water reduced.

(12) As the region of the alkali concentration, a region in which liquid resistance is increased is undesired. For example, when the alkali concentration exceeds 40% by mass, generated gases tend to become difficult to remove from the electrolyte (due to increase in liquid viscosity), and thus the cell voltage is increased, resulting in an increase in cell temperature due to the generation of Joule heat and the need for an excessive operation management such as the need for cooling the electrolyte or the like.

(13) Therefore, since the alkali concentration is increased by enrichment of the raw material water, it is preferred that the alkali concentration is kept constant by adding the raw material water so that the alkali concentration does not exceed 40% by mass or 30% by mass.

(14) In the present invention, in view of economy, heavy water is enriched about 10 times by electrolytic volume reduction, and when the initial concentration of heavy water in the raw material water is 2.5% by mass, the final concentration is 25% by mass because water is released by electrolysis.

(15) (Water-Seal System)

(16) Further, in the present invention, the electrolyte from which the generated gas is separated by each of the gas-liquid separators 14a and 14b is circularly supplied to each of the electrolysis chambers including the anode chamber 2 and the cathode chamber 3, thereby controlling the alkali concentrations in both electrolysis chambers. At the same time, the raw material water in an amount corresponding to the amount of water disappearing by electrolysis is supplied to both electrolysis chambers including the anode chamber 2 and the cathode chamber 3 from the raw material tank 9 through the circulation tank 5. Therefore, heavy water in the raw material water is enriched by continuing electrolysis while maintaining the electrolysis conditions constant.

(17) In order to control the concentration constant, the raw material water 10 in an amount corresponding to the water consumed is continuously supplied to the circulation tank 5.

(18) On the other hand, even when the alkali concentration is allowed to gradually increase to a high concentration up to a concentration limit of the alkaline water electrolysis of 40% by mass, volume reduction of the electrolyte can be confirmed. Also, under these conditions, the final concentration of 40% by mass can be then maintained by starting the supply of the raw material water.

(19) Therefore, the circulation system proposed in the present invention can be operated by any of the methods and thus has flexibility.

(20) Further, in the present invention, the pressure in the cathode chamber 3 and the pressure in the anode chamber 2 are controlled by controlling the height of the water surface in the cathode-side water-seal device 15b and the anode-side water-seal device 15a, respectively, in order to control a ratio of the oxygen gas generated in the anode chamber 2 and mixed with the hydrogen gas generated in the cathode chamber 3.

(21) The anode gas (oxygen gas) and the cathode gas (hydrogen gas) are separated by the anode-side gas-liquid separator 14a and the cathode-side gas-liquid separator 14b, water-sealed in the anode-side water-seal device 15a and the cathode-side water-seal device 15b, respectively, and then exhausted. In this case, the height of the water surface in the cathode-side water-seal device 15b is controlled to be higher than that in the anode-side water-seal device 15a so that the gas pressure on the cathode side is higher than the gas pressure on the anode side. This can decrease the transfer of the oxygen gas generated in the anode chamber 2 to the cathode chamber 3, thereby improving the purity of hydrogen gas. Conversely, when the purity of oxygen gas is desired to be improved, the height of the water surface in the anode-side water-seal device 15a is controlled to be higher than that in the cathode-side water-seal device 15b so that the gas pressure on the anode side is higher than the gas pressure on the cathode side. This can decrease the transfer of the hydrogen gas generated in the cathode chamber 3 to the anode chamber 2, thereby improving the purity of oxygen gas.

(22) (Alkaline Water Electrolysis Cell)

(23) A two-chamber electrolysis cell including an anode and a cathode provided on both sides of the diaphragm 4 is used as the alkaline water electrolysis cell 1. Also, a zero-gap electrolysis cell including an anode and a cathode which adhere to the diaphragm 4, a finite electrolysis cell including an anode and a cathode which are provided slightly apart from the diaphragm 4, or a spaced-type electrolysis cell including an anode and a cathode which are provided apart from the diaphragm 4 can be used. In order to prevent variation in position and oscillation of the membrane and to prevent damage to the membrane diaphragm 4 during the operation, an operating differential pressure is preferably provided between the anode chamber and the cathode camber, depending on the operation electric current density. For example, a differential pressure of 50 to 500 mmH.sub.2O can be provided, and this differential pressure permits further control of the ratio of the oxygen gas generated in the anode chamber 2 and mixed in the hydrogen gas generated in the cathode chamber 3.

(24) In addition, when a neutral diaphragm is used as the diaphragm, the pore size of the diaphragm used is decreased or the diaphragm with a specially-treated surface is used so that transfer of the oxygen gas generated in the anode chamber to the cathode chamber or transfer of the hydrogen gas generated in the cathode chamber to the anode chamber can be decreased.

(25) (Diaphragm)

(26) A neutral diaphragm, a fluorine-type or hydrocarbon-type cation exchange membrane for brine electrolysis, and a cation exchange membrane for fuel cells can be used as the diaphragm 4. When a cation exchange membrane is used, a hydrogen concentration in oxygen is about 0.13% at an oxygen concentration in hydrogen of 0.07%.

(27) On the other hand, when a neutral diaphragm specially treated is used as the diaphragm 4, a hydrogen concentration in oxygen is 0.05% to 0.08% at an oxygen concentration in hydrogen of 0.06% to 0.09%.

(28) (Anode and Cathode)

(29) The anode and the cathode are selected to be made of a material which can resist alkaline water electrolysis and to have low anode overvoltage and cathode overvoltage, respectively. In general, the anode composed of iron or Ni-plated iron is used, and the cathode composed of a Ni base material or a Ni base material coated with an active cathode material is used. A nickel expanded mesh, a porous nickel expanded mesh, a metal electrode including an iron base having a surface coated with a noble metal or an oxide thereof, or the like can be used as each of the anode and the cathode.

EXAMPLES

(30) Next, examples of the present invention are described, but the present invention is not limited to these examples.

Example 1

(31) A test was conducted with an electrolysis cell having an electrolysis area of 1.0 dm.sup.2. Both an anode chamber (volume 400 ml) and a cathode chamber (volume 400 ml) were composed of Ni, and the anode included an expanded mesh (thickness 0.8 mm?short width (SW) 3.7 mm?long width (LW) 8.0 mm) with an active anode coating. The cathode included a fine mesh (thickness 0.15 mm?SW 2.0 mm?LW 1.0 mm) with a noble metal-based active cathode coating.

(32) A polypropylene-based film of 100 ?m was used as a diaphragm, held between both electrodes, and assembled with a zero gap.

(33) A test process is as illustrated in FIGURE, in which an electrolysis temperature is controlled with a heater provided at the bottom of an electrolysis cell. An electrolyte is circulated by a method in which the electrolyte is supplied with circulation pumps 12a and 12b at a flow rate of 40 to 60 ml/min to the anode chamber 2 and the cathode chamber 3 through electrolyte supply nozzles from the circulation tank 5 (electrolyte volume: 2.5 L) provided below the alkaline water electrolysis cell 1. The liquids in gas-liquid fluids discharged from upper nozzles of the electrolysis cell 1 are returned to the circulation tank 5 through the gas-liquid separators 14a and 14b, and gases are discharged to the outside. The operation conditions include 40 A/dm.sup.2, 10% by mass KOH, an electrolysis temperature of 75? C. to 85? C., and pressure in the cell system which is determined by water-sealing the oxygen gas and hydrogen gas discharged from the anode chamber and the cathode chamber, respectively. In order to prevent vibration of the diaphragm during the operation, a differential pressure between the anode chamber and the cathode chamber is kept at 50 to 100 mmH.sub.2O.

(34) On the other hand, the liquid height in each of the water-seal systems can be controlled depending on which of the produced hydrogen gas and oxygen gas is expected to have desired purity. In this example, in order to increase hydrogen purity, a differential pressure was 50 mmH.sub.2O with pressure applied to the cathode.

(35) In an actual process, a large amount of raw material water is simply treated by continuously supplying the raw material water in an amount corresponding to the amount of water hydrolyzed. However, in this example, effectiveness was examined by measuring the enrichment rate of sample water containing tritium, cell voltage, and hydrogen purity without adding the raw material water in an amount corresponding to the amount of water hydrolyzed to the initial prepared electrolyte in the circulation system.

(36) When the operation was continued until an integrated current value was 4800 Ah (continuous operation for 5 days), the total amount of the electrolyte was decreased to 1.7 L from the initial prepared volume of 3.3 L. In view of slight evaporation and the amount of unrecovered water in electrolysis pipes in spite of recovery from the system, the amount of water reduced is a value substantially equivalent to a theoretical value.

(37) As a result, the electrolyte in a volume of 4.125 times the volume of the electrolysis cell was enriched 1.96 times. This represents that raw material water in a volume of 4 times or more the volume of the electrolysis cell can be treated with no trouble, and continuous enrichment can be performed by continuously supplying raw material water. That is, from the viewpoint of volume reduction of raw material water, the volume of raw material water can be reduced in proportion to the integrated current value applied to the system.

(38) The 10 mass % KOH electrolyte initially prepared was finally 19.6 mass % KOH after current supply of 4800 Ah. This represents that the concentration was increased by a value corresponding to the water disappearing. That is, this indicates that the initially prepared alkali (here, caustic alkali KOH) is stayed in the system without being consumed. The same applies to the case where caustic soda NaOH is used as the alkali, and the alkali is not limited to caustic potassium KOH.

(39) On the other hand, the initial adjusted alkali concentration can be kept at the initial value by continuously supplying the raw material water in an amount corresponding to the amount of water hydrolyzed.

(40) When the integrated current was 4800 Ah, the voltage, hydrogen purity, and tritium recovery rate were as follows.

(41) Test results: 1.7 V, hydrogen purity 99.9%, tritium recovery rate 0.6

(42) All of the gas purity, tritium recovery rate, and operation voltage were good.

Example 2

(43) A test was conducted by the same method as in Example 1 except that a PTFE film having a thickness of 70 to 90 ?m and an average pore size of 1 ?m or less was used as a diaphragm. The test results were as follows.

(44) Test results: 1.95 V, hydrogen purity 98.9%, tritium recovery rate 0.6

(45) All of the gas purity, tritium recovery rate, and operation voltage were good.

Example 3

(46) A test was conducted by the same method as in Example 1 except that an ion exchange membrane for brine electrolysis was used as a diaphragm. The test results were as follows.

(47) Cation exchange membrane used: Flemion (trade name of Asahi Glass Co., Ltd.) F8020SP

(48) Test results: 2.1 to 2.4 V, hydrogen purity 99.93%, tritium recovery rate 0.6

(49) The highest gas purity and good tritium recovery rate were obtained, but the operation voltage was high, resulting in the tendency to increase power consumption.

Example 4

(50) A test was conducted by the same method as in Example 1 except that an ion exchange membrane for fuel cells described below was used as a diaphragm. The test results were as follows.

(51) Cation exchange membrane used: Nafion (trade name of DuPont Company) N117

(52) Test results: 2.3 to 2.6 V, hydrogen purity 99.92%, tritium recovery rate 0.6

(53) The good gas purity and good tritium recovery rate were obtained, but the operation voltage was very high, resulting in the tendency to increase power consumption.

(54) According to the present invention, radioactive waste containing a large amount of tritium can be efficiently enriched and fractionated by electrolysis with high-concentration alkaline water, and high-concentration, high-purity hydrogen gas can be efficiently recovered. Also, the alkali concentration in the system can be always kept constant by providing an anode chamber and a cathode chamber on both sides of a diaphragm and circularly supplying a common alkaline electrolyte to both the anode chamber and the cathode chamber from a circulation tank. Therefore, a plant-level operation can be safely performed, thereby expecting wide-ranging utilization.