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PATENT OFFICE 2,224,814 ELECTROLYTIC PRODUCTION of METALS ' Harvey N

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PATENT OFFICE 2,224,814 ELECTROLYTIC PRODUCTION of METALS ' Harvey N 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.
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