US3234113A - Electrolytic separation of sodium - Google Patents

Description

United States Patent 3,234,113 ELECTROLYTIC SEPARATION 0F SODEUM Karl Ziegler, Kaiser Wilhelm Platz I, Herbert Lehmltuhl,

and Wolfram Grimrne, all of Mulheirn (Ruhr), Germany; said Lchmkuhl and said Grirnrne assignors to said Ziegler No Drawing. Filed May 7, 1962, Ser. No. 192,954 Claims priority, application Germany, May 9, 1961,

8,733 14 Claims. (Cl. 20468) This invention relates to the electrolytic separation of sodium as is described, for example, in our co-pending application Serial No. 299,689, filed July 31, 1963, and which is a continuation of application Serial No. 27,219, filed May 5, 1960 and now abandoned.

Within the scope and details of said application, Serial No. 299,689, which is herein relied upon, there is set forth a process for the cathodic separation of sodium by the electrolysis of sodium-containing organic aluminum compounds in an inert atmosphere, which is characterized by the fact that a melt of complex compounds of the general formula MeAlR R is electrolyzed, wherein Me signifies sodium or a mixture of sodium and potassium, R stands for alkyl residues and R stands for hydrogen, an alkyl residue, an alkoxy residue and/ or an aroxy residue, the last being sometimes modified by substitution. In this process a reaction between anodically formed, non-complex bonded metal alkyl (especially aluminum trialkyl developed in small amounts by side-reactions in the course of a long, continuous process) and the cathodically separated sodium is prevented by the use of a diaphragm, by electrolysis in a vacuum, or particularly by the addition of compounds of the general formula MeAlR OR", Me and R having the same meaning, and R signifying an alkyl, cycloalkyl, phenyl or alkyl-substituted phenyl residue.

In this process of said application, cathodes made of indifferent metals, e.g., copper, brass, iron or nickel, or other indifferent metals or alloys are used, and the sodium is obtained at the cathode.

In the process of said application it may be preferred to use electrolytes made of mixtures of sodium and potassium complex compounds of the general formula MeAlR R. During the electrolysis, the electrolyte grows poor in sodium complex compound, and therefore the latter is added to the electrolytic bath continuously or periodically.

In the process of said application the use of a sodiumcontaining anode is preferred, such as molten crude sodium. It is quite especially expedient, however, to electrolyze with a sodium amalgam anode. Sodium amalgarns are today produced on a large scale in big industries, and the process of the principal patent as well as that of the present application affords an improved method for the electrolytic recovery of sodium from such sodium amalgams. In this case, then, it is important to interrupt the electrolysis while there is still sufficient sodium in the anode material to prevent the formation of mercury alkyls, for example. It is possible therefore to electrolyze until the sodium content is reduced below 1%, say to 0.2 to 0.5% sodium. The current densities up to about 80 amperes per square decimeter are limited during the electrolysis according to the speed of diffusion of the sodium in the anode material, in such a manner that sufficient sodium is available at the anode surface in each case for the formation of sodium alkyl. It is especially preferred to operate at current densities ranging from to 50 amp./dm.

In one special embodiment of the process of the said application the electrolysis is performed with sodium amalgam as the bottom layer, the molten electrolyte as 3,234,113 Patented Feb. 8, 1966 the middle layer and the cathodically separated sodium as the top layer. This is achieved by the fact that the cathodically separated sodium is kept in suspension in the upper part of the electrolyte by a mesh of insulating material such as glass fibers and/ or cellulose fiber fabric. Preferably, this mesh is brought as closely as possible to the amalgam anode, so that only a few mm. of space full of electrolyte fluid are left between the anode and the cathodically separated sodium.

It is quite especially preferred to perform the process at temperatures above the melting temperature of sodium, since in this manner the drawing off of the sodium from the electrolysis apparatus is greatly facilitated. In the process of the said application, even in a preliminary electrolysis a sodium is produced which has a degree of purity of at least 99.97%. If this sodium is again subjected to a corresponding electrolysis, a maximum of 10 micrograms of foreign metal per 10 grams of sodium will exist in the sodium thus produced. The process therefore makes it possible to achieve especially good results as regards the purity of the cathodically separated sodium.

The process is performed preferably at voltages ranging from 0.5 to 5 volts.

All of the possibilities of the process of said application also exist for the present process of the invention. The process of the invention expands the process of the said application in that new electrolyte compounds can be used, in mixture, if desired, with the organic aluminum compounds described in the said application.

The subject of the invention includes a modification of the process for the cathodic separation of sodium by the electrolysis of aluminum complex compounds of the general formula MeAlR R' in an inert atmosphere according to said application which is characterized by the fact that, in addition to or instead of the aluminum complex compound, complex boron compounds of the formula MeBR R are used, Me in this formula representing sodium and/ or potassium, R representing alkyl residue and R standing for hydrogen, an alkyl residue and/ or an alkoxy residue. The characteristic of the invention thus lies in the fact that the electrolytes contain organic boron compounds. Otherwise, the data of the said application apply.

The use of organic boron compounds can signify important advantages over the use of organic aluminum compounds. As it is known, in the electrolysis using organic aluminum electrolytes, free aluminum trialkyl develops as a decomposition product of the electrolyte. Accordingly, in the electrolysis of the invention, in which complex boron compounds are used, free boron trialkyl develops. Now, whereas the complex aluminum trialkyl reacts in an undesired manner with cathodically separated sodium, forming aluminum metal, the free boron tr-ialkyl that develops according to the invention is stable in relation to sodium. The danger of undesired re-decompositions between the decomposition products of the electrolyte and the cathodically separated sodium therefore does not exist when a boron compound is used as the electrolyte. It is evident that this can produce a considerable simplification of the process.

It is especially preferred according to the invention to use as boron compounds those of the general formula MeBR R, in which R stands for low alkyl residues, especially with up to 6 and preferably with l to 3 atoms of carbon. It is furthermore preferred to use boron compounds in which the residue R is also an alkyl residue. The preferred boron complex compounds of the invention, therefore, are sodium boron tetraalkyls or mixtures of sodium boron tetraalkyls with potassium boron tetraalkyls.

As in the process of the said application, the concomitant use of organic aluminum complex compounds of'the general formula MeAlR OR" may be expedient,

Me standing in this general formula for sodium and/5or potassium, R for alkyl residues and R for alkyl or cycloalkyl residues, whichmay also be modified by substitution. The corresponding complex aluminum alkoxy compounds are especially preferred in this case, because not only do they convert any free aluminum trialkyl that may develop into the complex aluminum tetraalkyl compounds, but. they can also enter into reaction with the free boron trialkyl that develops according to the invention. In German patent application Z 8,721, Process for the Manufacture of Complex Alkali Boron Tetraalkyl Compounds, it is stated that a mixture of free boron trialkyl with the said complex aluminum allroxyv compounds reacts as follows:

In the case of the concomitant use of the complex aluminum alkoxy compound, the boron compound always remains in the electrolyte as a constantly renewed boron complex compound in a stationary phase. The simultaneously developing free dialkyl aluminum alkoxy compound can be converted back to the complex aluminum alkoxy compound in a very simple manner, e.g., by the process described in French Patent 1,223,643, and it can then be re-used in the electrolysis. I

Instead of the aluminum alkoxy complex compound, it is possible according to the invention also to use the corresponding alkali boron alkoxy complex compound of the general formula MeBR ORf. Analogously to the. corresponding aluminum compounds, the following reaction then takes place between the free boron trialkylf and the :complex compound:

BR -l-Me.[BR OR"]- MeBR +BR OR- In these general formulas, the residues R and R" correspond in significance to the corespo-nding residues of the:

.circuit between electrolysis and regeneration.

The temperature of the electrolysis bath is limited upwardly by the nature ofthe boron compounds used. In practice, the upper limit lies at a maximum of about 200 C. Preferred are temperatures in the range from about 145 to about 200 C. Since the boron compounds generally melt at a somewhat higher temperature than the corresponding aluminum compounds, it is possible, therefore,according to the invention, to operate .at somewhat higher temperatures than in the said US. application. However, if operation at lower temperaturesis desired, it is possible to achieve this without difficulty: it is necessary to add to the electrolyte bath only small amounts of auxiliary substances which act as melting point depressants. Indifierent polar organic compounds are suitable for this purpose, particularly ether and tertiary amines. Especially preferred are cyclic ethers, especially on the order of tetrahydrofuran. If such auxiliary substances are added to the boron-containing electrolyte baths, even only in slight amounts, the melting point of the entire electrolyte drops significantly. In-

a further embodiment of the invention, solutions of the boron compounds in these solvents can also be used.: Surprisingly, even solutions containing, for example, 50% and more solvent are outstanding conductors which are extraordinarily well suited for the performance of the electrolysis. Anotherv possibility for depressing the melting point of the electrolyte is the simultaneous use of various boron compounds or a mixture of various boron;

and aluminum compounds. Organic complex compounds of the said type are known for the fact that they can form lower-melting eutectics in mixture ,with one another. Decomposition of the electrolyte can also be prevented by-leading through. the electrolyte the olefin which was split oiffrom the boron compound during the decomposition of the latter. I

For information not given; herein in detail on the practical performance of the process,--reference is made to the corresponding information in the said U.S. application. As already mentioned,such'inforrnation alsoappliesto the process of the invention. Attention is directed particularly to the performance of the process by the three-layer. electrolysis method. In the process of the invention, it is. also expedient to perform the electrolysis by the three-layer method, the cathodically separated metallic sodium againbeing kept in suspension by a mesh of insulating material at a distance of a fewmillimeters above the anode material. It has. developed that this mesh isoutstandingly suited in general for theperformance of the electrolytic process of the type used in the invention and in the said US. application.

Example 1 In an electrolysis cell in the form of a closed vat of enameled sheet steel there is in the middle. a cathode of sheetcopper Whichextendsas far as thebottom of the vat, and onboth sides, at a distance of, about 1 to 2 cm; therefrom, there are anodes,- also of sheetcopper, which are at least 10 to 20 cm. shorter than the cathodes- A molten mixture of;50% sodium boron tetraethyl'and 50% potassium borontetraethyl is pouredinto the vat under be drawn off from there. ,At the anode develops a 1:21

mixture of ethane: and ethylene. The boron triethyl released by the electrolytic decomposition'of the sodium boron tetraethyl distills out of the reaction vessel and is collected in the refrigerated receiver.

The boron triethyl can easily be converted back to sodium boron tetraethyl with vNaH and ethylene in a knownmanner, and it can then-be fedback to the electrolysis cell. If a crude sodium'is used for the production of 'NaH, the: process constitutes a refining process for sodium,since the sodium produced at the cathode is at least 99.99% pure. The impurities, from the .sodium are a left behind, when the NaH is added to the boron triethyl, asinsoluble, removable impurities.

Example 2 The procedure is the same as in Example 1, usingthe same electrolyte mixture and ,the same current'density.

But to replace the consumed electrolyte, sodium 'butoxyaluminum triethyl is added in the amount of 7.87 grams per ampere-hour. With this manner of procedure, no free boron triethyl develops-in other words, the'instaL lation of a descending condenser. and receiver can be avoided; instead, a second liquid-phase collects on top of the electrolyte, consistingof purebutoxyaluminum diethyl, which can be drawn of: from time to time or con- 1 tinuously andcan be converted in a known manner, with NaH and ethylene, back into Na(C H' AlOC4H which is then fed backto the electrolysis cell. To prevent any free boron triethyl frombeing produced and from being lost with the escaping ethane-ethylene mixture, care must be taken in the electrolysis to see that a certain amount,

of sodium butoxyaluminum triethyl always remains in the electrolyte. A very slight, steady content will suffice. However, since the proportion cannot always be adjusted to be precisely equivalent at all times to the current flow, due to occasional current fluctuations, it is expedient to operate with a content of a few percent of the alkoxy compound in the electrolyte.

Example 3 In the apparatus described in Example 1, the electrolysis is performed with sodium boron triethylhydride, which is liquid at room temperature. The conductivity of this complex com-pound amounts at 70 to 0.2X 9- (2111. at 100 to 0.57Xl0' Q- cmr and at 130 to 1.16 10 t2- cm? The electrolysis temperature is 140. With 5 to 6 volts at the terminals, a current of amperes is established. 12.8 grams of sodium (=100% of the theoretical) are separated per hour, and 6.3 liters of hydrogen gas form at the anode; the boron triethyl that also forms distills away in the distillation receiver at the temperature of the electrolysis, and can be converted with NaH back into NaB(C H I-I, in a known manner, and the latter is fed back into the electrolysis cell.

Example 4 A cylindrical, internally enameled steel kettle is used as the electrolysis cell, on the floor of which the crude sodium is placed in molten form. In the kettle there is suspended .a cylinder of somewhat smaller diameter, open at both ends and made of enameled sheet steel, the bottom opening being covered with a coarse mesh net made of glass fiber fabric, stretched horizontally over said opening, with a mesh spacing of 1 to 3 mm. The net is located at a distance of 3 to 5 mm. above the surface of the liquid crude sodium. Close to the upper side of the net there is a screen of copper or iron wire as the cathode. Electrolysis temperature 150. Sodium boron tetraethyl is used as the electrolyte (conductivity at 150:

2.5 X 10 9 (JUL-1) The electrolyte melt must stand above the upper margin of the suspended cylinder, so that the sodium that is cathodically separated will be surrounded above and below with the electrolyte. An electrode current density of amperes per square decimeter can be maintained with a voltage at the terminals of 4.0 volts. The cathodically separated sodium collects above the glass fiber net and can be let out of this area from time to time. By feeding crude sodium in during the electrolysis it is possible to provide so that the spacing of anode and cathode remains the same.

The sodium yield amounts to 23 grams per 26.8 ampere hours, 23 grams of sodium were dissolved by the same amount of current, the yield is 100%.

Example 5 One proceeds as in Example 4, with N-aB(C H H as the electrolyte. The voltage at the terminals, at the same current density, is approximately twice as high as in Example 4.

Example 6 One proceeds as described in Example 4, with an electrolyte mixture of equal par-ts of NaB(C H and NEB (C2H5)3 O5CH3 The voltage at the terminals is about 2030% higher than in Experiment 4. The sodium yield is 100%.

Example 7 One proceeds as described in Example 4, but uses as the electrolyte a mixture of 50% sodium boron tetraethyl and 50% potassium boron tetraethyl, and replaces the liquid cr-ude sodium with an equal volume of 1% sodium amalgam. The electrolysis is performed at 150 with a current density of 30 ampere-s per square decimeter and an input voltage of about 4 volts. The cathodically separated sodium collects as a cohesive liquid layer above the glass fiber fabric net. To carry away the heat produced by the current and to keep the electrolyte thoroughly mixed, a large electrolyte storage tank is used, and the electrolyte is pumped from it into the space between the anode and the cathode at a rate per unit of time that is equal to the rate at which it is pumped back into the storage tank from the electrolysis vessel. After 185 ampere-hours, 1 60 grams of sodium have been separated cathodically, which are allowed to flow out of the upper part of the electrolysis cell.

The sodium thus produced still contains about 0.3% mercury. The mercury content falls still lower if a tempcrature of 130 is maintained during the electrolysis. It is then easy to achieve 0.01% Hg and less. This is the same mercury content which is achieved in a process wherein sodium is produced from sodium amalgam using an electrolyte of molten sodium bromide, iodide or hydroxide, after additional treatment with metallic calcium. The primary sodium content in this old process is on the order of 1%. The final traces of quicksilver can be removed by repeating the electrolysis as in Example 5.

The original 1% sodium amalgam is reduced to about 0.2% Na in the mercury. It is expedient to refill it from the cell and, after it has again been concentrated to 1%, it can be used for another electrolysis.

The sodium yield amounts to 23 grams of sodium (=100% of the theoretical) for 26.8 ampere-hours.

It is alsoto be noted that, due to the air and water sensitivity of the electrolytes, all operations have to be performed with all moisture excluded and in an inert gas atmosphere, such as nitrogen or argon.

Example 8 In the electrolysis apparatus described in Example 4, a mixture of equimolar quantities of sodium boron tetraethyl and dry and air-free tetrahydrofuran, melting at about is used. The electrolysis is performed with an approximately 0.5% sodium amalgam as the anode, at 105. The conductivity of the electrolyte at 105 amounts to 9.7 10- n cmr An electrode current density of 20 amperes per sq. dm. can be maintained with an input voltage of 9.3 voltes.

The sodium yield amounts to 23 grams for 27 amperehours of the theoretical).

It is expedient to let the sodium amalgam fall to about 0.1 to 0.05% and restore it continuously or from time to time with an equal volume of an 0.5% sodium amalgam.

We claim:

1. Process for electrolytic production of sodium which comprises: passing an electrolysis current between an anode and a cathode through an electrolyte comprising a complex compound of the general formula MeBR R in which Me is alkali metal selected from the group con sisting of sodium and a mixture of sodium and potassium, R is an alkyl radical and R is selected from the group consisting of hydrogen, alkyl, alkoxy, said anode containing sodium, to electrolyze sodium from the anode and deposit sodium at the cathode, the sodium electrolyzed at the anode replacing in the electrolyte sodium deposited at the cathode.

2. Process according to claim 1, characterized by the fact that the electrolysis is performed with boron complex compounds in which R signifies low alkyl residues with up to 6 carbon atoms.

3. Process according to claim 2, wherein R is lower alkyl containing up to 3 carbon atoms.

4. Process according to claim 1, characterized by the fact that the electrolyte is sodium boron tetraalkyl complex.

5.. Process-according to claim' 4, theelec-trolyte con- 1 taining potassium boron tetraalkyl.

' 6. Process vaccording to claim 1, characterized by the fact that the electrolyte contains a polar, inert organic compound.

7.Process according to claim 6, the said polar compound being selected from the group consisting of others and tertiary amines.

8. Process according to claim 7, the said polar compound- 'being a cyclic ether.

9." Process according to claim 8, the said polar compound being tetra'hyd'rofuran.

10. Process according to claim 1, characterized by the fact that the anode contains sodium.

11. Process according to claim 10, the anode contain 1 ing raw sodium.-

12. Process according to claim 1, characterizedby the fact that the electrolysis is performed-"in the presence of a compound selected from the group consisting, of,

MeAlR OR" and MeB R ORV, wherein Me and R have 8 E' the, meaning stated above, and- R, 4isse1ected from-the group consisting 0E allcyl'and cycloalleyl.

13. :Process. according to claim 1:,icharacterized by-the;

fact. that the temperature for the electrolysisv is in the; range from 145 to 209C;

14.;Process; according, to; claim 1,, the anode being,

sodium amalgam.

References Cited hy; the Examiner JOHN" Hit MACK, Primary Examiner.

JOHN R. :SPECK, Examiner.

Claims (1)

1. PROCESS FOR ELECTROLYTIC PRODUCTION OF SODIUM WHICH COMPRISES: PASSING AN ELECTROLYSIS CURRENT BETWEEN AN ANODE AND A CATHODE THROUGH AN ELECTROLYTE COMPRISING A COMPLEX COMPOUND OF THE GENERAL FORMULA MEBR3R'' IN WHICH ME IS ALKALI METAL SELECTED FROM THE GROUP CONSISTING OF SODIUM AND A MIXTURE OFSODIUM AND POTASSIUM, R IS AN ALKYL RADICAL AND R'' IS SELECTED FROMTHE GROUP CONSISTING OF HYDROGNE, ALKYL, ALKOXY, SAID ANODE CONTAINING SODIUM, TO ELECTROLYZE SODIUM FROM THE ANODE AND DEPOSIT SODIUM AT THE CATHODE, THE SODIUM ELECTROLYZED AT THE ANODE REPLACING IN THE ELECTROYTE SODIUM DEPOSITED AT THE CATHODE.