US3481844A - Electrolytic method of manufacturing alkali metals - Google Patents

Description

United States Patent 3,481,844 ELECTROLYTIC METHOD OF MANUFACTURING ALKALI METALS Takashi Oushiba, Tokyo, Japan, assignor to Showa Denko Kabushiki Kaisha, Tokyo, Japan, a corporation of Japan No Drawing. Filed Sept. 7, 1965, Ser. No. 485,530 Claims priority, application Japan, Sept. 16, 1964, 39/ 52,779; July 8, 1965, 40/ 40,557 Int. Cl. C22d 7/02, N06

US. Cl. 204-59 4 Claims ABSTRACT OF THE DISCLOSURE The present invention relates to an improvement of manufacturing alkali metals from their amalgams, in general, and to an improved method of obtaining alkali metals in which alkali metals are deposited at the cathode by electrolysis utilizing the alkali metal amalgams as the anode and a liquid ammonia solution of the alkali metal salts as the electrolyte, in particular.

It, is one object of the present invention to manufacture alkali metals from their amalgams by the use of the cathode materials which, already known, do not catalyse the reaction between the alkali metals and the ammonia forming the alkali amides as the reaction products, or the use of the liquid ammonia solutions of alkali metals as the cathode which is found. Also, the present invention includes the electrolytic characteristics in which the electrolyte or its part surrounding anode alkali metal amalgam is kept neutral or acidic by adding ammonium saltor by the subsidiary or the separate electrolysis.

As already known, T. Ewan (US. Patent 1,538,389 (19 25); US. Patent 1,538,390 (1925)) disclosed processes relating to a process of obtaining an alkali metal by passing a current from an amalgam of the metal as anode to a cathode through a solution of an inert alkali metal salt in liquid anhydrous ammonia, such solution being of such a concentration that the supply of alkali metal thereto causes the formation of two liquid layers, one of which consists of a solution of alkali metal with little or no alkali metal salt, and removing one of the layers and evaporating the solution of alkali metal to remove the solvent and obtaining the alkali metal, while avoiding the formation of metal amides by preventing a substantial catalysis of the reaction between the alkali metal and ammonia.

The present process has been studied under the laboratory scale using a two-litre autoclave, and it was found that the voltage seldom exceeded 1.7 volts. It was also found that when the electrolysis is carried out using an iron cathode, substantial amounts of sodamide are deposited on the cathode as a firmly adhering crust which is very ditficult to remove, and that the hydrogen formed contains over 20% nitrogen. Moreover, small amounts of a highly explosive substance, presumed to be mercury azide, are formed on the anode.

3,481,844 Patented Dec. 2, 1969 ice These operational difiiculties may serve to explain why this interesting process does not appear to have attained any success on a commercial scale.

Mellors Comprehensive Treatise on Inorganic and Theoretical Chemistry, volume II, supplement II, Li-Na, page 318, Chloro-Fako Discussion Report Gersthofen In this connection, I have found that the drawback was attributed to the formation and the accumulation of sodamide during the electrolysis. That is, for example, by electrolysis of the liquid ammonia solution of pure sodium bromide gram per liquid anhydrous ammonia gram using the 0.2 weight percent sodium amalgam as the anode, and the aluminum plate as the cathode at ordinary temperature, I could obtain the pure sodium which did not contain any mercury, and the electrolyte did not contain any mercury compounds or insoluble mercury azide or nitride on the anode at least in the beginning of the electrolysis. Nevertheless, in the successive electrolysis or in the electrolysis which at the beginning undertook in the presence of sodamide, not only did the sodium obtained contain several percent of mercury, but the mercuric nitride was found on the anode sodium amalgam.

From these facts, I saw that the presence of sodamide was the reason which the mercury as well as the sodium dissolved or reacted with ammonia during the electrolysis of the sodium amalgam anode. As the sodamide is known to be the base in the liquid ammonia solution, it will be more clear than I have described above, if the potential of the mercury in the sodium amalgam is possible to approximate of that of the pure mercury.

According to the oxydation potential of some species in the liquid ammonia solutions at 25 C. by W. L. Jolly (Journal of Chemical Education, volume 33, No. 10, pages 5l25l7 October 1956),

Acid solutions: E volts Na=Na++e- 1.89 Hg=Hg+++2e- 0.67

Basic solution: E, volts Na+NH -=NaNH +e 2.02 1.1

and, according to Lewis, G.N., and Randall, M. (Thermodynamics McGr-aw-Hill, New York, 1st ed., page 194 (1923)) the potential of the following half-cell, Na (solid), Na (amalgam 0.206%); E=0.8452 volt at 25 C. Thus we can calculate the following potential: Na (amalgam 0.206%)=Na++e E=l.0448 volts at 25 C.

Accordingly, in the basic solution of the liquid ammo'nia the anode of the above mentioned sodium amalgam possesses approximately the same potential for both sodium and mercury. To the contrary, in the acidic solution of the liquid ammonia for the anode of the sodium amalgam, the potential of the sodium is fairly positive than that of the mercury. From these caluculated potentials, the theoretical estimation will be possible for my invention.

According to E. C. Franklin (Journal of the American Chemical Society 27, 820 (1905)), mercuric nitride was formed by the reaction between the liquid ammonia solution of mercuric halides and the sodamide in excess.

and the liquid ammonia solution of the ammonium salt seems to convert the mercuiic nitride to the soluble mercuric compounds.

Analogous properties will be estimated for my invention though the former has no direct relation in the above mentioned electrolytes. Thus, in my invention, instead of the simple liquid ammonia solution of mercuric compounds the liquid ammonia solution containing relatively large quantities of electrolytes is dealt, so that the equilibriums for the above chemical equations will not be applied to the latter. Accordingly, so long as the sodamide in the electrolytes is not too excessive, mercuric halides will be soluble in the basic solution of the liquid ammonia which contains comparatively large amounts of the alkali metal salts, and, therefore, sodium deposited at the cathode is contaminated with mercury in the electrolysis of the basic solution of the liquid ammonia where the anode is sodium amalgam.

As the alkali metal deposited at the cathode contacts with the liquid ammonia or dissolves in the liquid ammonia during the electrolysis, the alkali metal converts naturally to the alkali amide by the following amidation reaction where M is alkali metal.

As it is seen natural to exclude any catalytic substances for the above amidation in the electrolysis, and also as it is known to use the materials as the cathode which does not catalyse the amidation, i.e., Al, Mg, Zn, Monel, Cu, etc., I have discovered that beside the above cathode materials, the liquid ammonia solutions of the alkali metals themselves are proper as the cathode, when this solution separates from the original solution of the electrolyte, the former floating on the latter. The use of the liquid ammonia solution alkali metal is especially fitted when the deposited alkali metal has a marked tendency to solidify at the cathode using a comparatively high cathode current density or the relatively concentrated electrolyte.

Notwithstanding the use of such a cathode, yet the liquid ammonia solution of alkali metal has essentially the amidation properties forming alkali amide. So during the electrolysis the alkali amide successively forms, making the liquid ammonia solution of the electrolyte, basic which in turn, accelerates the amidation reaction, and alkali metal deposited at the cathode is contaminated by the mercury and finally converts the mercury to mercuric nitride.

In order to avoid these defects I have discovered the method of electrolysis in which the electrolytes or the part of the electrolytes surrounding the anode alkali metal amalgam is always neutral or acidic by the addition of ammonium salts. That is, in the liquid ammonia solution, analogous to the aqueous solutions the neutralization occurs,

MNH +NH X=MX+2NH where M is an alkali metal, and X is a negative ion, Cl-, Br, 1-, CN-, etc.

Accordingly, if the electrolytes are always kept neutral or acidic at the above neutralization by adding any proper ammonia salt to the alkali metal amides the accumulation of alkali metal amide will be avoided.

Instead of adding the ammonium salt to the electrolyte, it is possible to form the necessary ammonium salts by the electrolysis of the liquid ammonium solution of alkali metal salt.

Thus, for example, when the electrolyte designated as MX is electrolysed in liquid ammonia solution using the insoluble anode as graphite or the metal oxide, and using the above mentioned cathode, M is deposited at the cathode, and ammonium salt NH X is formed at the anode following with nitrogen and hydrogen. Ammonium salt thus formed is really utilized in the above neutralization. In this case, the electrolyte is not necessary to be the same with the original electrolyte and the cathode of this subsidiary electrolysis is possible to be common to that of the original electrolysis. Or, the secondary electrolysis to prepare the ammonium salt may be done in a separate electrolytic cell.

where m, n are positive integers; 3(n-m)20; M is alkali metal and X is Cl, Br, I or an organic acid ion.

Whether the liquid ammonia solution is basic or not is easily seen by the indicator utilizing triphenyl methane; that is, the basic solution exhibits a strong red color by transmittedlight. In the neutral or acidic solution the red color disappears while an acidic solution exhibits no colors, but is transparent.

For the realization of the present invention, the electrolyte may previously contain triphenyl-me'thane as the indicator and consequently during the electrolysis the solution in the cell may be controlled to a neutral or acidic colorimetrically by means of the observation or by the use of the photoelectric system indicating the amount of the ammonium salt to be added or the time and electric current for the subsidiary or the second electrolysis respectively. If only the part of the electrolyte surrounding the anode alkali metal amalgam can be controlled to be neutral or acidic, it will be more efficient.

In conclusion, according to my invention, the alkali metal may be obtained efliciently and in a pure state in the sense of commercial manufacturing by electrolyzing the liquid ammonia solution of alkali metal salt which wholly or partially surrounds the anode alkali metal amalgam which is kept neutral or acidic during the electrolysis, the cathode of which being the materials which do not catalyze the amidation of the alkali metal in the liquid ammonia or the alkali metal solution of the liquid ammonia solution itself, thus retarding the accumulation of the alkali amide and the formation of the mercuric nitride.

For the realization of my invention the temperature of the electrolysis may be approximately between -30 and +30 C. The liquid ammonia solution and the alkali amalgam for the electrolysis of my invention should be as anhydrous as possible. The alkali metal salt used for my invention may be not only alkali metal halide, cyanide, and other inorganic substance but alkali metal salt containing organic anion, i.e., sodium triphenyl methyl. The portion of the containing vessel especially the electrolytic cell which comes in contact with the alkali metal solutions should be made of a material such as ebonite, polyethylene, polypropylene, glass, enamelled iron or copper or aluminum which does not catalyze the reaction forming the metal amide.

For the higher current density for the same concentration of alkali metal amalgam, it is essential that the alkali metal amalgam is intentionally stirred or flows in a thin sheet, especially for the last case of the T. Ewan patent as described in the examples referring to the drawings in his patent.

EXAMPLE 1 A glass-lined cell is equipped with an aluminum plate as the cathode conductor. The subsidiary graphite anodes are supported above the anode sodium amalgam film at a distance of 4 mm. and the slightly inclined iron plate as the sodium amalgam film base at the bottom of the cell. The liquid ammonia solution which contains NaBr at the rate of g. NaBr vs. g. liquid ammonia and contains a little of tn'phenyl methane indicator is introduced to the cell with the saturated liquid ammonia solution of sodium; the latter floats above the former. The sodium solution contacts the aluminum plate conductor to form the cathode. Sodium amalgam which initially contains 0.2% sodium by weight is introduced, flows on the iron plate serving also as the anode conductor and exits with the sodium concentration 0.01%. The electrolysis undergoes with the bath voltage of 2.0 volts with the distance of mm. between anode and cathode, an anodic current density 30 a./dm. at C.

During the course of the electrolysis, the indicator exhibits a red color at the upper part of the solution of the electrolyte and the red color diffused towards the lower parts. When the red color layer reached the height of 6 mm. above the sodium amalgam, the subsidiary electrolysis between the graphite anode and the sodium solution cathode starts with the potential difference of 6.0 volts and with the current corresponding to 3% of that of main electrolysis.

When the red color disappears in certain ranges of the solution of electrolyte, the current of the subsidiary electrolysis is cut off. Thus, the part of the liquid ammonia solution of sodium bromide surrounding anode sodium amalgam is kept always neutral or acidic. During the electrolysis, ammonia is supplied with the amount as to dissolve the sodium formed by the electrolysis. The solution separated from the liquid ammonia solution of sodium bromide is vaporized, evacuated and melted. The sodium obtained contains practically a negligible amount of mercury. The current efiiciency for sodium including the subsidiary 1.7% current is about 95%.

EXAMPLE 2 The air tight electrolytic cell is glass-lined. The cathode is the aluminum plate and the anode is the potassium amalgam at the bottom of the cell. Both the electrolyte and the potassium amalgam are stirred mechanically. The liquid ammonia solution of potassium iodide (0.7 g. KI per 1 cc. solution) is used as the electrolyte and 10 kg. of the potassium amalgam of which potassium content is 0.1% by weight is introduced. The temperature of the electrolysis, is about C., the distance between cathode and anode 5 mm., and the current density ampere per dm.

When the electrolyte exhibits a red color, the liquid ammonia solution of ammonium iodide is added until the red color disappears. Metallic potassium of 99.8% purity is obtained by 93% current efliciency, after the evaporation of the solvent.

What I claim is:

1. A method of manufacturing alkaline metal from an amalgam of alkaline metal by electrolysis, comprising the steps of using an amalgam of alkaline metal as an anode,

supplying as an electrolyte a liquid ammonia solution of alkaline metal salt,

maintaining the region of contact between the electrolyte and the amalgam in a neutral or acid state by adding to said region a liquid ammonia solution of an ammonium salt to the electrolyte,

using a metal which does not promote the formation of amides as the cathode, and

performing electrolysis with the above substances by passing a current from the anode to the cathode through the electrolyte.

2. A method of manufacturing alkaline metal from an amalgam of alkaline metal by electrolysis, comprising the steps of using an amalgam of alkaline metal as an anode,

supplying as an electrolyte a liquid ammonia solution of alkaline metal salt,

maintaining the region of contact between the electrolyte and the amalgam in a neutral or acid state by adding to said region a liquid ammonia solution 1 of an ammonium salt to the electrolyte,

using a liquid ammonia solution of alkaline metal as the cathode, and

performing electrolysis with the above substances by passing a current from the anode to the cathode I through the electrolyte.

3. A method of manufacturing alkaline metal from an amalgam of alkaline metal by electrolysis, comprising the steps of using an amalgam of alkaline metal as an anode,

supplying as an electrolyte a liquid ammonia solution of alkaline metal salt, maintaining the region of contact between the electrolyte and the amalgam in a neutral or acid state by inserting therebetween an auxiliary insoluble anode for performing a secondary electrolysis using .said auxiliary insoluble anode,

using a metal as the cathode which does not promote the formation of amides, and

performing electrolysis with the above substances by passing a current from the anode to the cathode through the electrolyte.

4. A method of manufacturing alkaline metal from an amalgam of alkaline metal by electrolysis, comprising the steps of using an amalgam of alkaline metal as an anode,

supplying as an electrolyte a liquid ammonia solution of alkaline metal salt,

maintaining the region of contact between the electrolyte and the amalgam in a neutral or acid state by inserting therebetween an auxiliary insoluble anode for performing a secondary electrolysis using said auxiliary insoluble anode,

using a liquid ammonia solution of alkaline metal as the cathode, and

performing electrolysis with the above substances by L passing a current from the anode to the cathode through the electrolyte.

References Cited UNITED STATES PATENTS 1,538,389 5/1925 Ewan 204-59 FOREIGN PATENTS 441,753 1/1936 Great Britain.

HOWARD S. WILLIAMS, Primary Examiner