Patented Dec. 10, 1940 2,224,814

UNITED‘ STATES PATENT OFFICE 2,224,814 ELECTROLYTIC PRODUCTION OF METALS ' Harvey N. Gilbert, Niagara Falls, N. Y., assignor to E. I. du Pont de Nemours & Company, Wil mington, Del., a corporation of Delaware No Drawing. Application August 25, 1938, Serial No. 226,747 .4 Claims. (01. 204-105) I substantially pure metals, including the alkali This invention relates to the production of metals, by a process which involves the ‘elec metals from metal salts and more particularly to trolysis ‘of aqueous solutions with a mercury a method of producing substantially pure metals cathode. A further object is to provide an im _ by methods which involve electrolysis of aqueous proved method for recovering substantially pure .5 5 salt solutions using mercury as a cathode and re alkali metal from dilute alkali metal amalgams covering metals from the resulting amalgams. obtained by electrolysis. Another object is to This is a continuation in part of my copending provide an improved method for separating mer application Serial No. 109,129, ?led November cury from a dilute alkali metal amalgam to ob 4, 1936. tain a concentrated amalgam. Other objects will 10 10 The art of electrolyzing alkali metal salt solu be apparent from the following description of my ‘ tions, using mercury as a cathode to obtain alkali invention. , metal amalgam is a comparatively old one. The above objects are obtained in accordance 1' Heretofore this method of electrolysis has been with the present invention by providing a dilute used only in the production of caustic alkalies l5 and similar processes, wherein the amalgam is liquidabout alkali1% of metalthe amalgamated amalgam, containing metal, cooling less than the 15 reacted to produce alkali metal compounds. dilute amalgam to a temperature sufficiently low Various methods have been proposed whereby to cause precipitation of a solid, concentrated » alkali metal amalgams thus obtained may be amalgam in liquid mercury and separating the treated to recover uncombined alkali metals precipitated solid concentrated amalgam from 20 2o therefrom but heretofore none of these have at the liquid mercury. The dilute amalgam may tained commercial importance. be obtained by electrolysis of an aqueous metal The dilute alkali metal amalgams obtained by salt solution, using mercury or liquid amalgam as using mercury as a cathode for electrolysis of the cathode. - _ alkali metal salt solutions usually contain not The concentrated amalgam separated from the 25 25 more than about 0.2% of alkali metal, since at liquid amalgam by the cooling step may be higher concentrations the amalgam is not easily treated in various ways. I prefer to treat the maintained in a ?owable state at temperatures concentrated amalgam to recover the metal at which it is convenient to operate the elec therefrom in substantially pure state. One trolysis cell. Various methods have been pro method of recovering metal from the concen- .30 30 posed for recovering alkali metal from these di trated amalgam in my process consists in heating lute amalgams, some of which comprise heating the amalgam to decompose it and vaporize off the to distill oil the mercury and others of which mercury. Such distillation or vaporization op- _ comprise utilizing the amalgam as an anode in eration may be carried out to any extent desired, a second cell with a suitable electrolyte, for ex depending. upon whether pure metal or metal ‘35 35 ample, ‘fused alkali metal hydroxide._ Insuch containing mercury is desired. The distillation methods it is dlf?cult to obtain alkali metal which ‘ maybe carried out in a single stage or in a plu does notcontain appreciable amounts of mer rality of stages or some method of recti?cation cury. In order satisfactorily to recover the alkali may be employed. I prefer to distill the amalgam metal from the amalgam in a substantially’pure with a recti?cation column at such temperature .40 ‘9 state by distillation methods it is necessary to that substantially pure metal or metal contain use some method of fractional distillation, which ing only traces of mercury is produced as residue entails the employment of large and costly which may be drawn o? from the base of the still. equipment. In electrolytic methods for recover Under such conditions, the vapors from the still ing alkali metal, the vapor pressure of mercury may be pure 'mercury or may be mercury con- ,45 is such that some mercury tends to pass .through taminated with-more or ‘less of the amalgamated the electrolyte and contaminate the cathodic de metal, depending upon the design and e?lciency' posit of alkali meta . of the distilling‘ column and the volatility of the Likewise, it has long been known that dilute metal. amalgams of various other metals can be made. by In practicing my invention for the'production .50 5o.v aqueous electrolysis with mercury cathode but of alkali metals, e. g. sodium, lithium or potas heretofore, so far as I am aware, no satisfactory sium, I may carry out the electrolysis of an aque- ' process for recovering metals from such amal ous solution of an alkali metal~ salt, e. g. a halide gams has been proposed. ' or- carbonate, with a mercury cathode under such An object of the present invention is to pro- I conditions that a liquid amalgam is obtained’. . vide an improved method for the production of 2 2,224,814 Preferably the electrolysis is carried out at an ration also may be obtained if the amalgam is elevated temperature, e. g. 60 to 80° (3., so that an cooled several hours later, provided it is not sub amalgam liquid at such temperature may be ob jected to excessive agitation in the interim. tained, containing the maximum amount of The degree of cooling required will vary con amalgam which will not be too viscous to ?ow siderably, depending, for example, on the alkali from the cell. The dilute liquid amalgam ?ow metal content of the amalgam, the method of ing from the cell is cooled su?iciently to pre separation to be employed, and the extent of cipitate a concentrated, solid alkali metal amal separation desired. In general, it is preferable gam. If the amalgam at 60 to 80° C. has a suf to cool the amalgam to a temperature at least 10 ?ciently high alkali metal content, usually it is 20° C. lower than the temperature at which it is 10 not necessary to cool below 10 to 30° C. to obtain formed in the electrolytic cell. For example, an the desired precipitate. For more dilute amal amalgam coming from the cell at a temperature gam, it may be necessary to cool to still lower of 60 to 80° C. and containing 0.2 to 0.5% by temperatures, which are above the freezing point 15 weight of sodium may be cooled to 10 to 30° C., of the mercury, e. g., to 0 to —10° C‘. The pre while an amalgam of the same sodium concen cipitated amalgam then may be separated by me tration initially at a temperature of 20—30° C. chanical operations, such as ?ltering or pressing. may advantageously be cooled to —10 to 0° C. It has been found that alkali metal amalgam The invention, however, is not restricted to a produced by electrolysis with mercury cathode is particular cooling range, as the effect of decreas 20 not in a homogeneous condition as it comes from ing the solubility of the solid phase in the liquid 20 the electrolytic cell. During the electrolysis, a portion is relative and directly proportional to layer of more concentrated amalgam tends to the extent to which the temperature is lowered. form at the surface of the mercury, this layer The liquid mercury separated from the con having a lower speci?c gravity than that of mer 25 centrated amalgam may be returned to the elec cury. This more concentrated amalgam tends to trolysis cell to be reused as cathode. The sepa dissolve in the more dilute amalgam only very rated solid amalgam may be treated to recover slowly. As a result, the material issuing from the alkali metal either by distilling off the mercury cell usually consists of a dilute liquid amalgam or by a chemical method of separating mercury having a ?oating upper layer of particles of more 30 as more fully explained below. In separating the concentrated solid amalgam. The proportionate mercury from the concentrated amalgam by dis concentrations of alkali metal in the two layers tilling off the mercury, I prefer to employ a dis will vary, depending on the conditions of the tillation method which will leave a residue of sub electrolysis operation. For example, the main stantially pure alkali metal or alkali metal con portion of the amalgam, may contain around 0.2 35 taining not more than about 5% by weight of to 0.5% by weight of alkali metal, the ?oating mercury. material containing 0.7 to 1.0% by weight of If the distillation step is carried out under alkali metal, while the ratio of the two may be such conditions that an alkali metal product is such that the mean alkali metal concentration in obtained which contains relatively small amounts the total amalgam is 0.3 to 0.7% by weight. I of mercury; e. g. 1—5% or less, I may remove sub 4.0 have discovered that such non-homogeneous stantially all of the residual mercury by treating 40 alkali metal amalgams may be circulated through the alkali metal in substantially liquid state with pipes or allowed to stand for considerable periods a substance which reacts with the mercury to of time, e. g., for several hours, without becom form a soid substance but which does not react ing homogeneous. In order to quickly obtain a with the alkali metal to any appreciable extent. 45 homogeneous amalgam I have found that it is This chemical method of purifying alkali metal necessary to stir the material vigorously and to separate mercury therefrom is described and preferably at elevated temperatures. claimed in U. S. Patent 2,124,564, entitled “Metal In the preferred embodiment of my invention, puri?cation.” As reagent for this chemical I utilize the above-described non-homogeneous method of removing mercury from an alkali 50 character of liquid amalgams obtained by elec metal, I prefer to use calcium or other alkaline. trolysis. I have discovered that by cooling the earth metal. In place of distillation to remove non-homogeneous amalgam, the tendency for the mercury from a concentrated alkali metal the solid amalgam present to go into solution is amalgam obtained by cooling the dilute amalgam markedly decreased. Hence, by cooling the from the electrolysis cell, I may treat the con 55 amalgam, I may place it in a condition for fur centrated amalgam by the above mentioned 55 ther mechanical handling for separation of the chemical method to remove part or substantially solid phase from the liquid phase, for example, by all of the mercury therefrom. ' such methods as ?ltration, centrifuging, pressing or skimming. The cooling operation thus in For‘ the purpose of illustrating my invention 60 sures the recovery of substantially all of the solid I shall describe one method of utilizing it to pro material initially ?oating on the amalgam ?ow duce substantially pure sodium. An alkali metal 60 ing from the electrolytic cell. salt, e. g., sodium chloride or sodium carbonate, In conducting the amalgam from the place of is dissolved in water and the aqueous solution is electrolysis to the cooling point, I avoid stirring electrolyzed by known means with a mercury 65 or otherwise agitating the amalgam coming from cathode. The electrolysis preferably is carried the electrolytic cell, insofar as possible, prior to out at a temperature of 60-80° C. and, I prefer cooling the amalgam. Preferably, the amalgam to use the method and apparatus described and is directly flowed from the electrolyte cell into a claimed in my copending application Serial No. cooling zone. I may cool the amalgam by ?ow 68,274. Preferably, the mercury or dilute amal 70 ing it through a pipe surrounded by a refrigerat gam cathode is continuously circulated through ing medium and continuously pass the cooled the cell, the rate of circulation being adjusted so as to produce an amalgam of maximum concen amalgam to a ?ltering device or I may cool it in tration which still will be su?iciently liquid to a reservoir. The best results are obtained by ?ow from the cell. The dilute amalgam leav cooling the amalgam immediately after it leaves ing the cell is cooled to a temperature of 10-30" 75 the electrolytic cell. However, satisfactory sepa- ' C. and then ?ltered. The ?ltrate, which is sub 75 2,224,814 a 3 stantially liquid mercury, is recirculated through mercury compound and drive o? mercury vapors the cell. _The ?lter residue consists of the con which may be condensed to provide liquid mer centrated solid amalgam, wet with more or less cury for the electrolysis cell cathode. The resi liquid mercury. If desired, this residue may be due from such distillation will consist of a mix subjected to a pressing operation, e. g. by means ture of alkali metal and the recovered reagent; 5 of a suitable hydraulic press in order to remove e. g. alkaline earth metal, which may or may further amounts of liquid mercury and leave a not be substantially free from mercury. This ’ more concentrated press cake. metallic residue which contains alkaline earth The concentrated sodium amalgam thus ob metal in ?nely divided form is suitable for use 10 tained then may be heated in a still preferably as reagent for treating a further quantity of 10 provided with a fractionating column, to decom alkali metal and mercury metal to remove mer pose the amalgam and distill oiT mercury. cury therefrom by the treating method described Molten sodium is withdrawn from the lower above. In this manner the alkaline earth metal portion of the still, the purity of the sodium with or other metal reagent used may be used over 15 drawn from the still depending on the ef?ciency and over without any substantial losses. 15 of the distillation process. By utilizing a prop I claim: erly designed distilling column or by carrying out 1. A process for the production of an alkali successive distillations (in other words, by uti metal which comprises electrolyzing an aqueous lizing the well-known principles of fractional dis solution of an alkali metal compound with a 20 tillation), it is possible to obtain substantially mercury cathode to obtain a dilute, non-homo- ~20 pure sodium. If the distillation conditions are geneous, substantially liquid alkali metal amal such that the resulting sodium contains a small gam, ?owing said amalgam from the zone of elec amount of mercury, I then may purify the sodium trolysis with minimum agitation, cooling said by treatment with calcium in the following man liquid amalgam before mixing can occur, suffi ner. Finely divided calcium ?rst is prepared by ciently to precipitate a solid amalgam therein, 25 dissolving metal calcium in liquid sodium at a and separating said solid amalgam from the temperature of 500 to 700° C. and cooling the cooled liquid amalgam and recovering alkali melt to precipitate metal calcium, e. g. to about metal from said solid amalgam. 100 to 120° C. The cooled melt then is ?ltered 2. A process for the production of an alkali to obtain a ?lter residue consisting of calcium metal which comprises electrolyzing an aqueous 30 crystals mixed with molten sodium, which ?lter solution of an alkali metal compound with a residue I term “calcium sludge.” The sodium to mercury cathode to obtain a dilute, non-homo be puri?ed is melted and heated to 200 to 300° C. geneous, substantially liquid alkali metal amal and an amount of the calcium sludge containing gam, ?owing said amalgam from the zone of 35 calcium equal to 3 to 5 times the weight of the ' electrolysis with minimum agitation, cooling said 35 mercury present is added to the molten sodium liquid amalgam before mixing can occur to at . and mixed thoroughly therewith. After the melt least 20° C. below its formation temperature ' has been maintained at a temperature of 200 to thereby precipitating solid amalgam and sepa 300° C. for a period of 1 to 10 minutes. the melt rating said precipitated solid amalgam from the 40 is cooled to about 100 to 120° C. and ?ltered. cooled liquid amalgam and recovering alkali ‘9 The resulting ?ltrate will consist of sodium sub metal from said solid amalgam. stantially free from mercury. 3. A process for the production of sodium In an alternative method of thus producing which comprises electnolyzing an aqueous solu pure sodium I may add the concentrated amal tion of a sodium salt witha mercury cathode to gam obtained by the cooling .step to a quantity obtain a dilute, non-homogeneous, substantially ‘5 of molten alkali metal in such proportions that liquid alkali metal amalgam, ?owing said amal the resulting mixture contains 50 to 75% . of gam from the zone of electrolysis with minimum alkali metal. To this mixture, heated to a tem agitation, cooling said liquid amalgam before perature of 200 to 300° C., I then may add a. mixing can occur to at least 20° C. below its 50 suitable quantity of calcium sludge prepared as formation temperature thereby precipitating 50 described above so that the amount of calcium solid amalgam and ?ltering said precipitated added is equal to 3 to 5 times the weight of‘the solid amalgam from the cooled liquid amalgam mercury present. The mixture then is main and recovering sodium from said solid amalgam. tained with thorough agitation at a temperature 4. A process for the production of sodium 55 of 200-300°, C. until the reaction between the which comprises electrolyzing an aqueous solu- 55 calcium and the mercury is completed which tion of a sodium salt at a temperature of 60 usually requires 1-10 minutes. The melt then to 80° C. with a mercury cathode to obtain a is ?ltered at about 110° C. whereby a ?ltrate of dilute, non-homogeneous, substantially liquid al substantially pure sodium is obtained. kali metal amalgam, ?owing said amalgam from 00 The residue obtained from this ?nal ?ltration the zone of electrolysis with minimum agitation, after utilizing the above mentioned chemical cooling said liquid amalgam before mixing can method of separating the mercury will consist occur to a temperature of 10 to 30° C. thereby of a sludgy mixture of the reagent; e. g., cal precipitating solid amalgam and ?ltering said cium or other alkaline earth metal, mercury precipitated solid amalgam from the cooled 65 and alkali metal; probably the alkaline earth liquid amalgam and recovering sodium from said , . metal and mercury are present in the form of solid amalgam,v a solid amalgam. This mixture may be heated HARVEYN. GILBERT. in a still or retort to decompose the amalgam or