Method for purifying metallic sodium

Abstract

It is preferable that metallic sodium to be loaded to an engine valve used for an internal combustion engine such as automobile engine have high purity. However, conventionally, an organic solvent remaining in micropores on a surface of the metallic sodium have been hardly attracted attention. Provided is a method for purifying metallic sodium including steps of placing metallic sodium containing organic solvent in the micropores thereof in a melting tank which is sealed, and heating the melting tank under reduced pressure to vaporize and remove the organic solvent coating the metallic sodium.

Claims

1. A method for purifying metallic sodium coated by a first organic solvent, the method comprising steps of: placing the metallic sodium in a melting tank which is sealed; and heating the melting tank under reduced pressure to vaporize and remove the first organic solvent coating the metallic sodium, wherein the first organic solvent which is vaporized is introduced to a solvent trap filled with a second organic solvent and is caught in the second organic solvent.

2. A method for purifying metallic sodium according to claim 1, wherein a sodium oxide layer covering a surface of melted metallic sodium in the melting tank is physically removed, the sodium oxide layer being formed of a sodium oxide formed by oxidation on the metallic sodium.

3. A method for purifying metallic sodium according to claim 2, wherein a cold trap for removing sodium oxide formed by oxidation of the metallic sodium is installed downward of the melting tank.

4. A method for purifying metallic sodium according to claim 1, further comprising a step of breaking at least a part of a sodium oxide layer which is covering the surface of the melted metallic sodium in the melting tank, the sodium oxide layer being formed of a sodium oxide formed as a result of oxidation of the metallic sodium, and the sodium oxide layer being broken by applying force thereto.

5. A method for purifying metallic sodium according to claim 1, wherein a cold trap for removing sodium oxide formed by oxidation of the metallic sodium is installed downward of the melting tank.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is an entire constitution diagram illustrating a system for purifying and loading metallic sodium according to the first embodiment of the present invention.

(2) FIG. 2 is a longitudinal sectional view illustrating a variation of the melting tank shown in the entire constitution diagram of FIG. 1.

(3) FIG. 3 is an entire constitution diagram illustrating the second embodiment of the present invention.

DESCRIPTION OF THE EMBODIMENTS

(4) Embodiments of the present invention will now be described with reference to the accompanying drawings, but are not limited to.

(5) A first and second embodiments are illustrated as a series of a system for purifying metallic sodium and loading a cylinder with the metallic sodium obtained by purification, but the can be used only for purifying metallic sodium. Further, a cold trap stated below mainly aimed to remove metallic sodium oxide. Thus, if it is unnecessary to remove such metallic sodium oxide, the cold tap need not be installed.

(6) As shown in FIG. 1, a system for purifying and loading metallic sodium 10 according to the first embodiment mainly includes a melting tank 12, solvent trap 14, reservoir tank 16, cold trap 18 and loading device 20.

(7) The melting tank 12 is a cylindrical container with a bottom, to which a pressure-reducing suction pipe 22 is connected at the upper side surface thereof, and a purified-metallic-sodium-discharge pipe 24 and valve 70 are connected at the lower side surface thereof. The pressure-reducing suction pipe 22 is connected to the solvent trap 14 filled with an organic solvent 28 such as liquid paraffin and one end thereof reaches in the organic solvent 28. The solvent trap 14 is configured in such a manner as to keep inside thereof under reducing pressure by a decompression pump (not shown). The purified-metallic-sodium-discharge pipe 24 is connected to the reservoir tank 16 via the valve 70.

(8) The melting tank 12 is provided with a heater 30 on entire side surface below the pressure-reducing suction pipe 22 and the bottom surface. The melting tank 12 is sealed by fixing a lid 33 at an upper opening thereof and the lid 33 is connected with an inert-gas supply pipe 32.

(9) The reservoir tank 16 is a closed tank for reserving temporally the metallic sodium which is purified in the melting tank 12 and supplied to the reservoir tank 16 via the purified-metallic-sodium discharge pipe 24. The reservoir tank 16 is connected with a carrier pipe 36 and return pipe 38 of a purified-sodium circulation line 34, in addition to the purified-metallic-sodium discharge pipe 24. The carrier pipe 36 is branched into two at an opposite end to the end connected with a circulation pump 40, one of the two branches configures the other end of the return pipe 38 and is connected to the reservoir tank 16 via a first electromagnetic valve 42 and the cold trap 18.

(10) The other of the two branches configures a loading-device supply pipe 46. The loading-device supply pipe 46 is connected to a quantitative supply device 49 via a second electromagnetic valve 48. In the illustrated example, the lower surface of a top plate 50 of the quantitative supply device 49 is electrically connected with five liquid-level detection sensors S.sub.1 to S.sub.5, each having different length. Differences in the lengths in the vertical direction between each pair of adjacent sensors are the same length of d. The quantitative supply device 49 is electrically connected with a supply pipe 53 having a quantitative supply valve 52 at the bottom plate 51. The supply pipe 53 extends to the loading device 20 and is equipped with a sodium dripping nozzle 54 at one end thereof. The loading device 20 is mounted inside thereof with a doughnuts-shaped support 55 to come into contact with the inner circumferential surface of the loading device 20. The loading device 20 is equipped inside thereof with a cylinder 57 having a cylindrical shape and a flange 58, on the lower end of which a cap 56 having a disk shape is detachably attached in such a way that the flange 58 is engaged with a center opening of the support located directly under the sodium dripping nozzle 54.

(11) Next, a function of the system for purifying and loading metallic sodium according to this embodiment, which has the configuration as mentioned above, will be described.

(12) In FIG. 1, a suitable amount of liquid paraffin is put in the solvent trap 14 and the lid 33 of the melting tank 12 is taken off. A bulk body of unpurified metallic sodium that has been immersed and stored in the liquid paraffin is put in the melting tank 12 after wiping off the liquid paraffin with a cloth from the bulk body, and then the lid 33 is attached again. After that, by supplying an inert gas such as argon or nitrogen from the inert-gas supply pipe 32, inside of the melting tank 12 is made under inert gas atmosphere so as to be sufficiently blocked from water and oxygen.

(13) Then, by activating the decompression pump (not shown), insides of the solvent trap 14 and the melting tank 12 are made under reduced pressure. Heating the bulk body of the metallic sodium in the melting tank 12 by energizing the heater 30 allows the liquid paraffin coating the bulk body of the metallic sodium to vaporize to be introduced into the solvent trap 14. The liquid paraffin is absorbed into the liquid paraffin 28 in the solvent trap 14, and thus a purification of the metallic sodium is completed.

(14) It cannot be avoided that commercially available metallic sodium contacts a small amount of water and oxygen to be oxidized at the surface to form sodium oxide, even though the commercially available metallic sodium is stored in an organic solvent such as liquid paraffin or the like. Likewise, a formation of sodium oxide by oxidization of the surface of the metallic sodium in this embodiment cannot be avoided even though a purification operation according to this embodiment is performed under inert gas atmosphere substantially including no water and no oxygen. The sodium oxide formed on the surface has a bulk specific gravity less than that of metallic sodium due to the porosity of the sodium oxide. Accordingly, as shown in FIG. 1, the sodium oxide floats on the surface of the melted metallic sodium 60 to form a sodium oxide layer 62 when the metallic sodium in the melting tank is melted completely.

(15) Due to the existence of the sodium oxide layer 62, the melted metallic sodium 60 cannot come into contact with the atmosphere in the melting tank 12. Even if the liquid paraffin in the melted metallic sodium 60 tries to vaporize, it cannot escape from the melted metallic sodium 60 so that purification of the metallic sodium cannot proceed. In order to avoid such situation, the sodium oxide layer 62 on the surface of the melted metallic sodium 60 can be scooped manually or mechanically with the lid 33 taken off, or, for example as shown in FIG. 2, at least a part of the sodium oxide layer 62 can be broken by generating a forcible flow with a stirring element.

(16) FIG. 2 is a longitudinal sectional view illustrating a variation of the melting tank shown in the entire constitution diagram of FIG. 1. The same components as FIG. 1 are indicated by the same numeral references and a detailed description thereof is omitted. In short, as shown in FIG. 2, a motor 64 is disposed to come into contact with a heater 30 in a lower part of the melting tank 12 and a stirring element 66 is set in the melting tank 12. By energizing a motor 64 during heating and reducing pressure, the stirring element 66 rotates in the melted metallic sodium 60 to generate a spiral flow 68 in the melted metallic sodium 60. The spiral flow 68 breaks at least a part of the sodium oxide layer 62 covering the entire surface of the melted metallic sodium 60, so as to make the melted metallic sodium 60 come into contact with the atmosphere inside the melting tank 12. Thus, removal of the liquid paraffin by vaporization can be achieved regardless of the presence or absence of the sodium oxide layer 62.

(17) Thus purified metallic sodium is supplied to the reservoir tank 16 from the melting tank 12 in FIG. 1 via a purified-metallic-sodium discharge pipe 24 by opening a valve 70 and is temporally reserved in the reservoir tank 16. The purified metallic sodium in the reservoir tank 16 is supplied to a circulation line 34 via a carrier pipe 36. Under an ordinary state, a first electromagnetic valve 42 is opened and a second electromagnetic valve 48 is closed. In this state, the melted metallic sodium supplied to the circulation line 34 is supplied to a cold trap 18 through the first electromagnetic valve 42. Impurities mainly composed of a metal oxide of sodium and the like contained at a small amount in the melted metallic sodium is isolated by filtration with the cold trap 18 and the melted metallic sodium is returned to the reservoir tank 16 via a return pipe 38. The purity of the melted metallic sodium in the reservoir tank 16 is further improved by the melted metallic sodium circulating through the circulation line 34 for one or more times.

(18) When it is required to load the cylinder 57 with the purified metallic sodium in the reservoir tank 16, the first electromagnetic valve 42 is closed and the second electromagnetic valve 48 is opened. This enables to supply the purified metallic sodium in a melting state from the carrier pipe 36 to the quantitative supply device 49 via the loading-device supply pipe 46. While the purified metallic sodium is supplied to the quantitative supply device 49, a liquid level of the purified metallic sodium rises gradually. When the liquid surface of the melted metallic sodium comes into contact with the lower end of a first liquid-level detection sensor S.sub.1 having the shortest vertical length, a detection signal is transmitted to the quantitative supply valve 52 and the second electromagnetic valve 48, so as to open the quantitative supply valve 52 and close the second electromagnetic valve 48. Thereby, supply of the purified metallic sodium in the melting state to the quantitative supply device 49 is stopped and the purified metallic sodium in the melting state in the quantitative supply device 49 is supplied to the loading device 20, so as to be loaded into the cylinder 57 via the sodium dripping nozzle 54 preferably with a drop condition. This operation usually can be performed by self-weight of the melted metallic sodium, but it may be performed by applying a little positive pressure in the quantitative supply device 49 or applying a little negative pressure in the loading device 20.

(19) When the liquid level of the purified metallic sodium in the melting state in the quantitative supply device 49 lowers to reach to the lower end of the second liquid-level detection sensor S.sub.2, this is detected by the second liquid-level detection sensor S.sub.2 and the quantitative supply valve 52 is closed to stop supplying the purified metallic sodium. Thereby, the cylinder 57 is loaded with a predetermined amount of the purified metallic sodium, corresponding to the vertical length of d of the quantitative supply device 49. At that time, by properly determining a dripping speed, a temperature of the metallic sodium in the sodium dripping nozzle 54, an inner diameter of the cylinder 57, and the amount of the purified metallic sodium to be supplied to the cylinder 57 (a diameter and length of a columnar body of the metallic sodium formed in the cylinder), a molded body of the purified metallic sodium without a microscopic air gap in a unified body can be provided, thanks to a directional solidification in the direction from bottom to the top.

(20) Then, the cylinder 57 loaded with the predetermined amount of the purified metallic sodium is removed from the quantitative supply device 49 and replaced with a second cylinder ready to be loaded with the purified metallic sodium next. The purified metallic sodium in the melting state in the quantitative supply device 49 is supplied to the second cylinder by opening the quantitative supply valve 52 again. When the liquid level of the metallic sodium coming in contact with the lower end of a third liquid-level detection sensor S.sub.3 is detected, the quantitative supply valve 52 is closed again. Thereby, the second cylinder is loaded with the predetermined amount of the purified metallic sodium, corresponding to the vertical length of d of the quantitative supply device 49, in a similar manner to the above first loading. By repeating such operations by a predetermined time, a constant amount of the metallic sodium can be loaded to a predetermined number of the cylinder.

(21) In the first embodiment, the melting tank is intended for removal of the organic solvent such as liquid paraffin or the like and the cold trap 18 is intended for removal of impurities contained in the metallic sodium at a small amount and mainly composed of the metallic sodium oxides, etc. Therefore, the cold trap 18 and equipment accompanied therewith are not necessary if it is intended to remove only an organic solvent and not needed to remove the metallic sodium oxides, etc. An example for this is illustrated in FIG. 3 as the second embodiment.

(22) The second embodiment illustrated in FIG. 3 is an improvement of the first embodiment. The same components as the first embodiment are indicated by the same numeral references, and a detailed description thereof is omitted. In the second embodiment, the carrier pipe 36 is directly connected to the quantitative supply device 49 and the circulation line 34, first and second electromagnetic valves 42, 48, cold trap 18 and return pipe 38 are not connected. Supplied to the quantitative supply device 49 by activating the circulation pump 70 is the melted metallic sodium having been purified and reserved in the reservoir tank 16 in the same way as the first embodiment, in order to load it to the cylinder 57. After that, the purified metallic sodium in the melting state can be loaded into the cylinder 57 as a solidified matter having a uniform structure by a fixed quantity.

(23) The system according to the second embodiment is remarkably advantageous in terms of space and cost compared with that according to the first embodiment. It is desirable to use the system of the second embodiment in the case where removal of metal oxides in the metallic sodium material is not needed.

EXAMPLE(S)

(24) Hereinafter the present invention is described on the basis of the examples. However, the present invention is not limited to such examples. For example, the lid is taken off in the following examples in order to visually confirm a removal state of the liquid paraffin, this operation is not usually required.

Example 1

(25) A melting tank for purifying metallic sodium is configured by connecting a cylindrical container with a bottom which is 250 mm in diameter and 375 mm in height with one end of a pressure-reducing suction pipe at the upper side surface and with one end of a purified-sodium take off pipe at the lower side surface. Connected to the other end of the pressure-reducing suction pipe is a solvent trap (paraffin trap) filled with liquid paraffin. Connected to the other end of the purified-sodium take off pipe is a reservoir tank for a purified metallic sodium. Further, the melting tank is provided with a heater on the bottom surface and the side surface below the pressure-reducing suction pipe of the melting tank.

(26) Then, unpurified metallic sodium immersed in liquid paraffin is purchased and taken off from a storage container. After that, the unpurified metallic sodium is put into the melting tank from an upper opening thereof, a lid to which an inert-gas supply pipe is connected is fastened on the upper round opening to seal the melting tank, and argon gas is supplied into the melting tank from the inert-gas supply pipe, so that internal air in the melting tank is substituted by argon.

(27) By activating a pressure-reducing pump connected to the solvent trap, the pressure in the melting tank is reduced to about 50 kPaG and kept this pressure for five minutes. After that, the lid being taken off, a gas looking like steam is observed in the melting tank. The upper opening is closed again with the lid, the pressure in the melting tank is reduced again to about 50 kPaG and kept this pressure for five minutes. After that, the lid being taken off, a gas looking like steam is not observed. The value of the reduced pressure and heating time may vary depending on the dimension of the cylindrical container with the bottom and thus is not fallen under a numerical limitation. In particular, it is needless to say, the lower the reduced pressure is, the more easily the removal of the organic solvent and thus the less a reduced-pressure keeping period.

(28) The melting tank is gradually heated with keeping the degree of the reduced pressure by the heater and continued heating for five minutes. After that, the lid being taken off, a gas looking like a steam same as the above is observed. The upper opening is closed again with the lid, the pressure in the melting tank is reduced to about 50 kPaG again and kept this pressure for five minutes. Then, the lid being taken off, the metallic sodium is thoroughly melted and a gas looking like a steam is not observed.

(29) Subsequently, the melting tank is gradually heated with keeping the degree of the reduced pressure by the heater to be continued heating for five minutes with rotating the stirring element under the condition where the surface of the metallic sodium is kept melting. After that, the lid being taken off, a gas looking like a steam same as the above is observed.

(30) From these experimental results, it is found that the liquid paraffin contained in the metallic sodium cannot be removed sufficiently only by keeping the metallic sodium under the reduced pressure, and substantially all of the liquid paraffin can be removed by keeping the metallic sodium under heating state as well as the reduced pressure further with stirring the metallic sodium using the stirring element.

DESCRIPTION OF REFERENCE NUMERALS

(31) 10 System for purifying and loading metallic sodium 12 Melting tank 14 Solvent trap (paraffin trap) 16 Reservoir tank 18 Cold trap 20 Loading device 22 Pressure-reducing suction pipe 24 Purified-metallic-sodium discharge pipe 28 (Second)organic solvent 30 Heater 34 Purified-sodium circulation line 46 Loading-device supply pipe 49 Quantitative supply device 54 Sodium dripping nozzle 57 Cylinder 60 Melted metallic sodium 62 Sodium oxide layer 64 Motor 66 Stirring element S.sub.1-S.sub.5 Liquid-level detection sensor