US1816304A - Electrolytic refining of metallic aluminum - Google Patents

Description

I Patented July 28 1931 UNITED STATES WILLIAM D. TREADWELL, OF ZURICH, SWITZERLAND,-ASSIGNOR TO ALUMINUM IN- PATENT OFFICE I DUS'IRIE AKTIENGESELLSCHAFT, F NEUEAUSEN, SWITZERLAND, A JOINT-STOCK COMPANY OF SWITZERLAND ELECTROLYTIC REFINING OF METALLIC ALUMINUM No Drawing. Application filed November 30, 1929, Serial No. 410,878, and in Germany November 30, 1928.

This invention relates to the electrolytic refining of metallic aluminum, and consists in a novel process for producing highly pure metallic aluminum from less pure metal.

- The invention is directed particularly to the extraction of refined aluminum from pig or ingot aluminum, aluminum alloys and the like. It constitutes an improvement more particularly in that refining processes wherein the electrodes, used in a solid form, are im- 'mersed in a molten electrolyte, for example containing halogen salts, which electrolyte has a melting point lower than that of either electrode and is maintained in a molten con dition below the melting point of either electrode] Such a process has been described in and made the subject of patent of Weber and Hauser, No. 1,7 09,759,'granted April 16, 1929, and of, application of Weber and Zeerleder, Serial No. 195,861, filed June 1, 1927, which may be referred to for details and features not, herein fully described.

The general object of the present invention is to render more efficient the refining process, and to enlarge the rate of output or produc tion. A particular object .is to provide a treatment or step for purifying the electrodes upon their removal from the electrolytic bath by freeing them from the salts ofv the bath which tend to adhere thereto. .A further object is to increase the practical economy of the process by recovering and rendering available for further use substantially the entire quantity of electrolyte material so ad.- hering to and removed with the electrodes, and without impairment or change in the chemical composition thereof. Other objects and advantages of the invention will be explained in the hereinafter following description of an illustrative embodiment thereof or will be understood to those conversant with the subject. To the attainment of such objects and advantages the present invention consists in the noval electrolytic refinin process and the novel steps, operations and agents herein described.

The present improved process can best be explained by first pointing out certain conditions which pertain to the process as already known and the consideration in vo ved in further perfecting the same. In practicing the described refining process I have observed that on lifting the electrodes out of the electrolytic bath they are in an undesirable condition due to carrying or being covered with substantial quantities of the salts constituting the electrolytic bath. The amount of such electrolyte material is substantially larger in the case of the anodes from which the aluminum has been exhausted than in the case of cathodes. This is due to the fact thatthe electrolyte not only adheres to the surface of the electrodes, but tends also to penetrate into the narrow canals and pores left in the anode by the extractionof aluminum, which latter may in some cases be replaced by an aproximately equal volume of salts ofthe electrolyte.

In the case of the anode, and taking the.

specific gravity of the electrolyte as 1.7 then the exhausted anode will contain for every kilogram of refined aluminum removed from it 680 grams of electrolyte material, in'addition to the amount thereof which adheres ,to the surface. We have found that the total amount of the electrolyte which cannot readily-be separated by ;mechanical means from the anode will amount in general to 700 to 800 grams for every kilogram of refined aluminum, when the specific gravity of the electrolyte is 1.7.

This substantial loss of electrolyte mate.

rial impaired the economy and efiiciency of the process, and constituted a practical problem in the perfecting of the process. dditionally, the subsequent working and utilization of the electrodes is rendered much more difiicult by reason of the presence of the electrolyte adhering to them, especially because the readily volatile aluminum chloride compels special precautions to be taken in the melting of the electrodes. Furthermore the strongly hydroscopic character of the electrolyte renders it impracticable to storethe anodes in uncleaned condition after removal fromthe bath, since the electrolyte material del iquesces under the influence of atmospheric moisture and tends to produce crumbling of the electrodes.

It is entirely insufficient and impractical x to wash the removed electrodes with water, for several reasons. Thus, the electrolyte can not be easily recovered from the aqueous solution owing to the decomposition of the "aluminum salts. Further, suchan aqueous manner, leaving the electrodes in suitable condition, and conserving the salts for repeated use.

' readily recovered by the ill The present improved process is based upon the determination that the salt mixture employed as an electrolyte in the previously described process of refining metallic aluminum is readily soluble in anhydrous sulfur dioxide. This solvent is further advantageous in that no decomposition of the salts forming the electrolyte take place, and moreover the entire amount of such salts are quite mere distillin oil of the solvent. This capability of anhyd rous sulfur dioxide to dissolve in substantial quantities such a salt mixture was not to be expected, since the respective salts which enter the composition of the electrolyte, if treated separately, are dissolved only in very small amounts, insufiicient for practical purposes. Thus for example, 100 parts of anhydrous sulfur dioxide will dissolve at most five parts of the chlorides of aluminum or sodium or potassium respectively if brought individually into contact with the solvent.

e same quantity of the solvent will however dissolve a far greater proportion, in some cases up to 170 parts, of an electrolyte such as that described in the prior application of Weber and k. Zeerleder, containing these same chlorides of aluminum, sodium and potassium,

with more than one molecule of aluminum chloride present for eachmolecule of the alkali chlorides.

The treatment of the electrodes with the described solvent consisting of or containing anhydrous sulfur dioxide may be carried out by dipping or immersion in any sort of extraction apparatus or vessel commonly usedv in the chemical industries, provided that they are adapted to use with the said solvent. I prefer the use of apparatuses which permit a continuous extraction of the salts as for instance known apparatuses constructed in accordance with the Soxhlet principle. However, the treatment may be otherwise. It is even feasible to bringthe electrodes into contact with gaseous sulfur'dioxide provided that the latter is caused to condense upon the dioxide S02,

, specla'li electrodes, since in the gaseous state it of course can not act as a solvent.

The present invention may be summed up specifically as relating to that process of producing refined metallic alum1num comprising removing aluminum electrolytlcally from a solid metallic anode containing 1mpure metallic aluminum on to a solid cathode through a molten electrolytic bath capable of acting selectively to dissolve aluminum from the anode, which bath contains aluminum halide, and the halides of one or more metals, such as the alkali metals or the alkali earth metals, the hydroxides of which have alkaline properties, and the improvement characterized in that the electrolyte material which is absorbed by or adheres to the'elecof solid electrodes in an electrolyte of molten salts, it is now with the present invention to perform the step of clearing or freeing one or both of the electrodes of salts adhering thereto upon their removal from the electrolyte by treatment with a suitable volatile solvent, thus not merely cleaning the electrodes but rendering the electrolyte material available for further use. While we have disclosed as a suitable volatile solvent anhydrous sulfur various erably anhydrous, solvents are available, anion which may be mentioned $001 and SO Ci, which however, although effective as solvents, are not so advantageous in a commercial sense on account of their higher cost and greater difiiculty in handling.

There has thus been described a process for producing pure metallic aluminum from the less pure metal, together with special treatments or procedure for the improvement of the process and the product thereof, and a agent or solvent for use therein, embodying the principles and attaining theobjects'of the present invention. Since various matters of process, ingredient and apparatus may be variously modified without departing om*the principles of the invention, it is not intended to limit the invention to such matters except so far as set forth in the appended claims.

at is claimed is:

1. The lic aluminum, comprising removing aluminum electrolytically from a solid metallic this solvent being afterwards process of producing refined metal= 103 other volatile, and prefanode containing impure'metallic aluminum, on to a solid cathode, through a molten electrolytic bath capable of acting selectively to dissolve aluminum from the anode, which bath contains aluminum halide, and halides of one or more metals the hydroxides of which have alkaline properties and characterized in that the electrolyte absorbed by or adher- I ing to the electrodes after their removal from lo the electrolytic'bath is separated from them and recovered by treating the electrodes with anhydrous sulfur dioxide, the anhydrous sulfur dioxide being afterwards distilled off from the resulting solution, leaving the salt mixture available for further use.

2. In the process of electrolytically refining aluminum comprising the employment of solid electrodes in a molten electrolyte comprising a mixture of halide salts, the step of clearing one or both electrodes of salts adhering thereto upon removal from the electrolyte by treatment with a volatile anhydrous solvent.

3. In the process of electrolytically refining aluminum comprising the employment of solid electrodes in a molten electrolyte comprising a mixture of aluminum halide and the halides of one or more metals the hydroxides of which have alkaline properties, the step of clearing one or both electrodes of salts adhering thereto upon removal from the electrolyte by treatment with a volatile solvent containing anhydrous sulfur dioxide.

4. In the process of electrolytically refiningaluminum comprising the employment of solid electrodes in an electrolyte of molten salts, the step of clearing one or both electrodes of salts adhering thereto upon removal from the electrolyte by treatment with 40 a solvent consisting of a volatile anhydrous agent. 5. In the process of electrolytically refining aluminum comprising the employment of solid electrodes in an electrolyte of molten salts, the step of clearing one or both electrodes of salts adhering thereto upon removal from the electrolyte by treatment with a solvent consisting of a volatile anhydrous sulfur compound. r In testimony whereof, this specification has i been duly signed by: l WILLIAM D.- TREADWELL.

cram-"learn or connection. Patent lio. 1,816,304; i Granted 1928, i931; to

VWILLIAM 'D. TREADWELL. it :is hereby certified that thename of the assignee, in the above numbered patent was'erroneously written and printed as "Aluminum Industrie Aktiengesell'schaft, of Neuhausen, Switzerland, a joint-stock company of Switzerland",

7 whereas said name should have been written and printed as Aluminium Industrie Aktiengesellschaft, of Neuhausen, Switzerland, a joint-stock company. of Switzerland, as shown by the records of assignments in this office; and that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 22nd day of September,.A. D. 1931.

\ M. J; Moore,

(Seal) 7 Acting Commissioner of Patents.