Feb. 12, 1974 c. w. tobias etal 3,791,945 METHOD OF PRODUCTION OF ALKALI METALS AND THEIR ALLOYS
Filed Feb. 22, 1973 j^ NaCI Fj 5 rich
- NaCI - Na(Hg)+H20 7^-^NaOH
NaOH+l/ H2 No CI lean " (H20) \ r\; \ \ F Hg Recycle
IZl NaCI rich (H20.) United States Patent Office „ , 3'791 945 ^^^ Patented Feb P .k 12, 1974
methods for the preparation of rubidium and cesium 3 791 945 metals are briefly summarized in Gmelins Handbuch der METHOD OF PRODUCTION OF ALKALI METALS Anorganischen Chemie, 8 Auflage, Verlag Chemie, AND THEIR ALLOYS GMBH, Rubidium, System No. 26, 1932 (pp. 23-25) and Charles W. Tobias, Orinda, Calif., and Jacob Jorne, Gmelins Handbuch der Anorganischen Chemie, 8 Auflage, Southfield, Mich'., assignors to the United States of Verlag Chemie, GMBH, Cesium, System No. 27, 1938 America as represented by the United States Atomic (pp. 14-16). The above references were arbitrarily Energy Commission selected from numerous existing works on the subject Filed Feb. 22* 1973, Ser. No. 334,825 of alkali metal production. Int. CI. C22d 1/02,1/06 U.S. CI. 204—59 AM 10 Claims 10 Separation of the pure alkali metal from its amalgam by distillation, is economically unattractive. The electro- lytic purification methods (refining) proposed thus far ABSTRACT OF THE DISCLOSURE suffer from various deficiencies. Processes employing An impure amalgam of an alkali metal or mixture molten salts involve elevated temperatures at which the thereof is produced by electrolysis. The amalgam is then Jg vapor pressure of mercury is sufficiently high to cause it transferred to. an electrolytic cell containing propylene to transfer into the product alkali metal as an undesirable carbonate with an appropriate salt (or salts) of the alkali impurity. Other nonaqueous electrolytes involving inor- - ganic, or organic solvents considered for ambient tem- metal (or metals), e.g. of CIO* , BF4-, PFS % C1-, Br-, I~, with or without added AICI3. .Then, with appropriate perature refining processes of alkali metals have so far not met one or several of the following primary criteria: voltage, current and other conditions,, one or more se- 2o lected alkali metals is anodically dissolved to yield a pure stability with respect to the alkali metal, good solvent metal or selected metal mixture by electrodepositkm. power for alkali metal salts or complexes, sufficient level of conduction, low melting point, low cost, low toxicity. A technologically feasible, and economically attractive ORIGIN 25 electrolytic refining process for the recovery of pure alkali This invention was made in the course of, or under metals from their amalgams demands a solvent system, Contract W-7408-ENG-48 between the United States which possesses many of the favorable qualities of water, Atomic Energy Commission and the University of but one in which alkali metals are stable. California. The carbonate-ester family of solvents, including pro- pylene carbonate, were reported as suitable solvents for BACKGROUND OF THE INVENTION 30 electrolysis in electrochemical studies in cyclic esters by This invention relates to an improved electrolytic W. S. Harris, Report UCRL-8381. We have made this process for recovery of an alkali metal from its amalgam. invention as a result of work on the behavior of alkali More particularly the present invention relates to a low- metal/alkali metal ion couples, and of their amalgams, temperature electrolytic process for the recovery of lithi- gg which has been documented in Lawrence-Berkeley Report um, sodium, potassium, rubidium, cesium and mixtures LBL-1111, dated September 1972. thereof from their amalgams. SUMMARY OF THE INVENTION The amalgam, is prepared using conventional mercury- chlorine cells well known in the art for producing sodium It is an object of this invention to provide an electrolytic and potassium amalgams by electrolyzing aqueous solu- 40 process for the recovery of an alkali metal from its amal- tions of the chlorides of these alkali metals. This method gam thereby producing an alkali metal of enhanced purity. of producing amalgams is described for instance in In accordance with this invention, an electrolytic proc- Gmelins Handbuch der Anorganischen Chemie, 8 Auflage, ess is provided for the separation of alkali metals by Verlqg Chemie, GMBH, Mercury, System No. 34, Teil anodic dissolution of the alkali metal from its amalgam A. Lieferung 2, 1962 (pp. 915-956, 986-100). Properties 45 into propylene carbonate or related solvent, such as 7- and methods of preparation of the amalgams of lithium, butyrolactone accompanied by simultaneous deposition of rubidium and cesium are also well known (see the above the metal in pure form. reference at pp. 899-911, 1010-18, 1023-1027). The In practicing this invention alkali metals first are amal- method used for the commercial production of sodium gamated with mercury by electrolysis from water solutions and potassium amalgams can be adapted without major go of the salts. This may be performed in a conventional changes to the preparation of lithium; rubidium or cesium amalgam cell used for production of chlorine from alkali amalgams as well. Amalgams- of any of the alkali metals metal chlorides. The amalgam is transferred into a cell can also be prepared by direct dissolution of the metal containing propylene carbonate electrolyte and an appro- in mercury, a technique suitable for purifying impure priate salt or salt combination. Among the salts employed alkali metals. 55 best results were obtaned from Cl~, Br-, I~, ClO^-, For obtaining the pure alkali metal, the amalgam must BF4-, PFg-, with or without the addition of A1C13. The be separated^ into its components: the alkali metal and system undergoes electrolysis and the alkali metal then is mercury; Among; the various procedures proposed to deposited in pure form from the amalgam in a manner accomplish this separation,, distillation, medium tempera- similar to electrorefining. ture electrolysis (refining) using molten alkali metal salt 60 The process set forth can also be employed to produce mixtures, and low ^temperature electrolysis using inorganic alloys of the alkali metals. This may be accomplished by or organic solvent media. deserve mention. These methods carrying out the reaction with an amalgam containing two are described for lithium amalgam in Gmelins Handbuch or more alkali metals and a selected electrolyte composi- der Anorganischen Chemie, 8 Auflage, Vcrlag Chemie, tion in such a manner that two or more alkali metals co- GMBH, Lithium, Erganzungsbans, System No. 20, 1960; gg deposit. This technique is for instance suitable for the (i>p; .1236 and 207) for sOdium amalgam in Ginelins production of Na-K alloys. Handbuch der Anorganischen Chemie,;-8 Auflage, Verlag Chemie,: GMBH. Sodiuni, Eerganzungsband, Lieferung: 1, DETAILED DESCRIP TION OF THE INVENTION System No. 21,1964 (pp. 8-14, 49-75) and for potassium ;A schematic diagram of the proposed process is shown amalgam' in Gmelins' Handbuch der Anorganischen y0 in: the drawings. FIG. 1 is a schematic representation of Chemie, 8 Auflage, Verlag Chemie,nGMBH, Potassium. the well-known mercury-chlorine cell where, as an ex- System No. 22, 1938 (pp. 69-71, 207-209). Various ample, chlorine and sodium hydroxide are the two main 3,791,945 products. FIG. 2 represents the modified process. The The actual deposition and dissolution potentials appli- first stage is identical to the first stage of FIG. 1, however, cable to an individual alkali metal as well as to its amal- the second stage is a non-aqueous propylene carbonate gam is determined by its reversible (thermodynamic) po- cell. The main products of the modified process are there- tential and the overpotential required for the anodic or fore chlorine (as before) and metallic sodium (instead of 5 cathodic reaction to proceed at a predetermined rate (cur- aqueous NaOH solution). rent density). In the MC1-1 m A1C13-PC electrolyte cell In the system depicted the lean amalgam is continuous- systems it has been found that the overpotential as related ly recycled to the aqueous cell where it picks up by cath- to the charge transfer reaction is lowest for lithium, and odic deposition fresh alakli metal. Unlike in the cell con- highest for potassium. figuration illustrated in FIG. 1, the second stage (refining io The new cell, stage II, can be operated at ambient tem- cell) will be typically not contiguous with the first stage perature, although elevated temperature lowers cell po- (amalgam-chlorine cell), to allow greater flexibility in tential because of improved conductance, and lower re- mechanical design and operation. Each cell is to provide versible and overpotentials. However, increased reactivity for the proper flow of amalgam, which is to form a planar of the product metal with the electrolyte requires tempera- cathode surface in stage I, and an anode in stage II. Be- 15 tures to be kept below approximately 100° C. fore transfer of the amalgam from stage I to II, appro- priate measures have to be taken for eliminating any en- EXAMPLE 1 trainment of the aqueous phase into stage II. Similarly, A potassium amalgam solution containing 0.22 weight provisions have to be made to prevent entrainment of the percentage of potassium, taken from the cathode section organic phase upon recirculation of the lean amalgam 20 of a conventional mercury-chlorine cell is introduced from stage II to stage I. into the anodic part of an electrolytic cell of a design In the new cell, stage II, the alkali metal is dissolved corresponding to that shown in stage II of FIG. 2. The anodically from the amalgam stream entering from stage electrolyte in the cell is composed of pure propylene car- I, and the pure alkali metal is simultaneously deposited bonate, to which is added both A1C13 and KC1 in the in equivalent amount on the cathode, a sheet or screen 25 amount of 1 gram molecular weight each per 1000 grams fabricated from a low cost alloy having suitable mechani- of propylene carbonate. A steel screen serves as the cal properties. As the amalgam passes through stage II, its cathode. Liquid droplets of the potassium rise to the alkali metal content gradually diminishes; this decrease electrolyte surface, from where the product potassium in concentration being determined by the rate at which the metal is withdrawn by skimming, or overflow. The gap amalgam passes through, and by the current applied 30 between the steel screen and the cathode is 1 cm. through the cell. The product alkali metal may be re- The cell is operated at 65° C., slightly above the melting moved periodically by changing the cathode, or con- point of potassium. Mercury flows across the bottom tinuously, by moving belt cathode surface. When the of the cell in turbulent flow, and the screen is attached product metal is obtained in the liquid state (such as to a vibrator to assure good convective ionic transport potassium above 62.3° C.), by virtue of its low density 35 2 3 to the cathode. Current density is 0.05 a./cm. The amal- (0.83 g./cm. ) it rises to the electrolyte surface, where it gam flow through the cell is so regulated that the efflu- can be skimmed off periodically, or collected continuously ent stream contains not less than 0.02 weight percent by a suitable overflow arrangement. In the case of the (corresponding to 0.01 mol percent) potassium. heavier alkali metals, Rb and Cs, which are obtained in 40 The cell voltage is about 5.0 volts. Hence the power the liquid state already at 38.5° and 28.5° C., respectively, required per pound of potassium, assuming 100% cur- the product metal would tend to sink onto the amalgam rent efficiency, is: surface. However, collection of the product metal in these cases can be achieved by a suitably designed cathode. The electrolyte consists of an alkali metal salt, such as perchlorate, borofluoride, hexafluorophosphate, chloride, 45 Power/lb. 2