US1882525A - Process for the electrolytic production of metals of the alkalis or alkaline earths - Google Patents

Description

Oct. 11, 1932. w. SIECKE 1,882,525

PROCESS FOR THE ELECTROLYTIC PRODUCTION "F METALS OF THE ALKALIS OR ALKALINE EARTHS Filed Nov. 15, 1929 aka (M4436.

Patented Oct. 11 '1932 UNITED STATES PATENT OFFICE WOLFHART srnoxn, or ,IRANKFORT-ON-THE-MAIN, GERMANY PROCESS FOR THE ELECTROLYTIC PRODUCTION OF METALS OF THE ALKALIS OR ALKALINE EAB'IHS I application filed November 13, 1929, Serial No. 407,003, and in Austria November 18, 1928.

This invention relates to the production of alloys of the alkaline earth metals or mixtures of the same, by the electrolysis of the corresponding halides with the aid of molten cathodes, especially lead cathodes The known processes, which are especially intended for the production of alloys of lead and alkaline earth metals, are attended with the drawback that, aside from other difficulties, they require an extremely large consumption of current.

I have found that these difiiculties and drawbacks can be overcome in various ways.

In the course of the electrolysis of molten halides of alkaline earth metals, such as alkaline earth chlorides or saline mixturescontaining the'same, in contact'with a cathode of molten lead the liberated metals form alloys with the molten lead, such as, for example, the compound Pb ca, which cover the surface of the lead bath as a solid crust which prevents thelead from taking up any further amounts of the alkaline earth metal, for example calcium, deposited at the cathode so that the calcium which has not been taken up rises to the surface of the electrolyte and burns there or forms calcium carbide with the anode which consi"ts of carbon or graphite. To prevent this inconvenience, it has constituent, it has been found necessary, in

general, to maintain the bath at temperatures i above 630 (3., and preferably above 650 (1.,

for example between 650 and 700 C.

\Vhen treating a mixture of, for example, calcium chloride and sodium chloride or potassium chloride, with a fused lead cathode,

mixing of the molten cathode metal, for

it was observed that there was formed during the progress of the electrolysis, the compound Pb Ca, the melting point of which lies at about 630 G. Since this compound segregates out directly both in the solid as well as in the fused state, the danger arises when working at temperatures below 650 C., that the surface of the lead cathode may on occasion solidify to a solid crust, so that the calcium separating out can no longer be taken up by the lead, but rises to the surface where it burns, or it reacts with the carbon to form calcium carbide. The current yield then decreases; and the impurities set up an anode effect. These drawbacks are avoided in accordance with the invention by keeping the fused cathode at suitably high temperatures.

I have moreover foundit necessary to carry out the process while thoroughly mixing the fused cathode in order to avoid disturbing separations such as for example the segrega tion of Pb ca even in the fused state when working at sufliciently high temperatures, for example above 650 C. I have found that the example the molten lead, can be effected in a particularly favorable manner by the use of vertically reciprocating stirring members The stirrer may for example consist of a perforated sheet of metal which is moved up and down in thefused cathode. This method of mixing-ofi'ers the advantage that the sur face of the fused cathode metal remains horizontal, whereas if it is agitated or if other types of stirring means are used, disturb- 85 ances may arise, for example, the centrifugal action of rotating stirrers causes the surfaces of the molten cathode-metal to assume a fun-' nel-like shape, the electrolysis being thereby 9o adversely affected and, in particular, the danger of short circuits with the anode arises.

The accompanying drawing is adiagram- 'matic representation of one form of appara-v tus suitable for carrying out my invention.

In the drawing, 1 is the containerffmade of metal, for example iron, 2 is the electrolyte,

3 is the molten lead cathode and 4 and 5 are anode plates. The agitating member 6 is provided with apertures 6' and is given a re- 10o ciprocating vertical motion in the molten cathode bymeans of shaft 7.

Furthermore, mixtures of halides, of the alkaline-earth metals with other halides of a metal-other than alkaline earth metal which are ada tedto prevent the saline mixture from ta ing up and, retaining water at the temperature of the electrolysis such as for example, mixtures of chlorides of alkaline earth metals with halides, forexample chloridesof sodium or potassium, for example in an amount of at least 20% of the total saline mixture, have been found specially suitable for the electro'deposition of alkaline earth metals, such as for example, calcium.-

The water introduced with the electrolyte exertsa disturbing actionrin two-ways; on the one hand because the yield per unit of current is reduced by reason of the accompanying electrolysis of the water; and on the,

other hand because any light metal formed or already present reacts with the water to form reaction products, such as CaOwhich promote the occurrence of the anode effect. :It.

has been found that the dehydrating action of added substances such as chlorides of .alkali metals, upon the halides of the alkaline earth metals possessing a great aflinity for water, such as for example, calcium chloride generally increases as the amount of the added substances increases. Relatively large amounts of sodiu however, are separated out in addition to he alkaline earth metal, such as calcium, from mixtures of halogen salts,

containing comparatively large amounts. of sodium in addition to calcium salts, for example. This is not the case when employing potassium chloride even in considerable amounts. The employment for elec- J trolysis of such mixtures rich in potassium chloride, which for example, may contain about 20 to of calcium chloride and about 35 to of. potassium chloride, not only enables the salt serving as the fitst charge for the bath to be dehydrated with ease,'but above all, enables the halides consumed during the electrolysis, which may on occasion be halides which can only with difiiculty-be dehydrated, such as calcium chloride, to be introduced into the electrolytic bath in a water-containing form, for example, in the form of ordinary calcium chloride containingnabout 7 to 10% of water,whilst maintaining the temperatures, of for example, upwards of 630? C. At the increased temperatures of the each piece of the salt introduced is immedi ately surrounded after the manner of Leidenfrost drops with a pellicle of steam and is therefore prevented from coming directly into contact with the melt until the completion of the rapid evaporation of the water, where-. upon the now completely dehydrated salt is readily taken up by the melt. In this manner, it is possible, by employing the above mentioned higher temperatures, to avoid the very disagreeable explosive phenomena which are otherwise unavoidable if attempts are made to introduce incompletely dehydrated salts, such as for example, the calcium chloride hereinbefore set forth, at less elevated temperatures into the melt, since in this case the entire amount of the water presviolenc By virtue of the possibility, even when employing halides having an extraordinary aflinity for water, such as calcium chloride ot being able to avoid a preliminary del1y-' dration of the amounts of these salts to be introduced during the electrolysis a substantial cheapening and simplification of the process is possible, inasmuchas it is now no longer necessary to completely dehydrate these salts with a large expenditure of fuel and crucible material and only then to introout evaporates all at once with explosive duce them into the electrolysis. Moreover, 1

there is also the further advantage that the saline mixture required for the initial charging of the electrolytic cell can be made in a much more simple manner than if the constituents have first to be dehydrated separately or if saline mixtures are employed which could not be as easily dehydrated.

Similar advantages as are-obtainable according to the foregoing, in the production of calcium by the addition of potassium chloride, preferably in relatively large amounts, can naturally also be obtained in the electrolysis of other metals of 'the alkaline earths by the employment of deliquescent halides of the metals to be deposited in combination with other halides of the same or of another metal which are adapted to form with the salt in question, mixtures which are easily dehydratable and which can be easily kept free from water in a fused state or which form mixtures which are not adapted to take up water on thesubsequent addition of the water-contain- "ing-salt tobe electrolyzed. The. deposition at the molten cathode of cations of the salt or salt mixture, employed as addition, besides the principal metal, may be avoided when the process is to be carried out under definite operating conditions, such as temperature, voltage and so forth, by so choosing the salt or salt mixture, or when the salt or salt mixture to be'added is fixed, by so choosing the remaining operating conditions that the undesired cathodic depositions are avoided.

For practicalworking it has been found advisable" to heat the bath externally as well as by the electric current. Although it is I an easy matter to heat the salt melt to 700 C. while continually stirring the molten cathand over, since in electrical heating the electrolyte acts as resistance, it has been'found in practice that the underlying molten cathode metal, which offers practically no resistance to'the current, may remain up to 200 C. cooler than the supernatent heated salt layer. For the reasons given above it is, however, important to maintain the cathode metal, for example lead, at the temperature therein specified during the electrolysis. To this end use is advantageously made of electrolyzing vessels, the lower portions of which are freely accessible, in order that the cathode metal contained therein can be externally heated, while the upper portions are insulated, for example with fire-clay. It is essential that this fire clay layer should extend downwards through the saline melt and as far as the surface of the lead, in order to assure complete electrical insulation between the anode and the cathode. That is to say, the metallic calcium must be deposited inthe lead exclusively and not on any uninsulated iron parts above the lead.

It has also been found that, in carrying out the electrolytic process, the so-ealled anode effect, i. e., the occurrence of thin insulating layers which are broken through by the electric current with formation of small arcs, is a notable, source of disturbance. It has now been ascertained that the anode effeet is promoted by high voltage, while, on the other hand, this inconvenience can be counteracted by operating with comparatively low current densities. In general it has been found advantageous to employ current densities which do not substantially exceed 4 amps. per sq. cm. and which are preferably below that figure.

The invention is applicable to the treatment of halides of the alkaline earth metals, such as salts of calcium, magnesium and the like, and particularly to mixtures of various originating materials. In addition to chlorides, it can be applied to the treatment of fluorides, for example, and also to mixtures of chlorides and fluorides, such as, for example, calcium fluoride and sodium chloride. The lead may be replaced by other metals or me tallic alloys, which are molten at the temperatures coming under consideration, to serve as cathode material, such as aluminium and the like.

- cathode having a temperature above 600 C.

ode. I

2. Process of preparing alloys of lead and alkaline earth metals which comprises electrolyzing a molten salt mixture containing at least one halide of the alkaline earth metal to be deposited and at least one other halide of a metal other than an alkaline earth metal in contact with a cathode of molten lead the temperature of which is above 600 C. while continually stirring the molten cathode.

3. Process of preparing alloys of lead and alkaline earth metals which comprises electrolyzing a molten salt mixturecontaining at least one halide of the alkaline earth metal to be deposited and at least one other halide of a metal other than an alkaline earth metal in contact with a cathode of molten lead the temperature of which is between 630 and 700 while continually stirring the molten cathodes l. Process of preparing alloys-oflead and alkaline earth metals which comprises electrolyzing a molten salt mixture containing at least one halide of the alkaline earth metal to be deposited and at least one other halide of a metal otherthan an alkaline earth metal in contact with a cathode of molten lead the temperature of which is at least 630 C. while continually stirring the molten cathode without disturbing the smooth upper surface thereof to any great extent.

5. Process ofpre'paring alloys of laid and alkaline earth metals which comprises electrolyzing a molten salt mixture containing at least one halide of the alkaline earth metal to be deposited and at least one other halide of metal other than an alkaline earth metal in contact with a cathode of molten lead the temperature of which is at least 630 C. while continually stirring the molten cathode in a vertical direction.v

6. Process of preparing alloys of alkaline earth metals which comprises electrolyzing a molten salt mixture containing at least one halide of the alkaline earth metal to be deposited and a suificient amount of at least one other salt, which is adapted to prevent the salt mixture from taking up and retaining water at the temperature of electrolysis, in contact with a cathode of molten lead the temperature of which is at least 630 C. while continually stirring the molten cathode.

7. Process of preparing alloys of alkaline earth metals which comprises electrolyzing a molten salt mixture containing at least one halide of the alkaline earth metal to be deposited and at least onehalide of an alkali metal in contact with a cathode of molten,

halide of calcium and at least one halide of j an alkali metal in contact with a cathode molten lead, the temperature of which is at least 630 C. while continually stirring the molten cathode.

'9. Process of preparing alloys of alkaline earth metals which comprises electrolyzing' a molten salt mixture containing at least one halide of the alkaline earth metal to be deposited and sodium chloride in contact with a cathode of molten lead the temperature of which is at least 630 C. while continually molten salt mixture containing calcium chloride and potassium chloride in an amount of at least 35% of the total saline mixture in contact with a cathode of molten lead the temperatureof which is at least 630C. wglile continually stirring the molten cath- I o e.

12. Process of preparing alloys of lead lead the temperature of which is at least 630 v C. while continually stirring the molten cathode and heating the cathode externally.

16. Process of preparing alloys of lead with calcium whichcomprises electrolyzing a molten salt mixture containing calcium chloride and at least one chloride of an alkali metal in contact with a cathode of molten lead the temperature of which is at least 630 C. while continually stirringthe molten cathode the current density employed not sub-- stantially exceeding 4 .amperes centimeter of anode surface.

17 Process of preparing alloys of alkaline per square I earth metals which compnses electrolyzing' a molten salt mixture containing at least one halide ofthe alkaline earth metal to be .deposited and sodium chloride in an amount of at least 20% of the total -saline mixture in contact with a cathode of molten lead the temperature of which is at least 630 C. while continually stirring the molten cathode.

" In testimony whereof, I affix m signature.

WOLFHART IEGKE- with calcium which comprises ele'ctrolyzing a molten salt mixture containing about 35 to of potassium chloride and about 65 to 20% of calcium chloride in contact with a cathode of molten lead the temperature of which is at least 630 C. while continually stirring the molten cathode.

13. Process of preparing alloys of lead and alkaline earth metals which comprises elec-' trolyzing a molten salt mixture containing at least one halide of the alkaline earth metal to be deposited and at least one other halide of a metal'other than an alkaline earth metal in contact with a cathode of molten. lead the temperature of which is at least 630C while continually stirring the molten cathode and H replacing the alkaline earth halide consumed during electrolysis in a water-containing form.

14. Process of preparing alloys of lead with calcium which comprises electrolyzing a molten salt mixture containin calcium chloride arid at least one chloride 0 an alkali metal'iri'contact with a cathode of molten lead 4 thetemperature of which is at least 630 C..

while continually stirring the molten cathode and adding portions of calcium chloride in a water-containing form to replace the calcium v chloride consumed during the electrolysis. 15. Process of preparing alloys of lead with calcium which comprises electrolyzingr a molten salt mixture containing calcium chloride and at least one chloride of an alkali metal in contact with a cathode of molten

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