US1464625A - Electrolyte for aluminum production and method of preparing same - Google Patents

Description

Patented Aug. 14, 1923.

UNITED STATES PATENT OFFICE.

ALADAR PACZ, OF CLEVELAND HEIGHTS, OHIO ELECTROLYTE FOB ALUMINUM PRODUCTION AND METHOD OF PREPARING SAME.

No Drawing.

To all whom. it may concern.-

Be it known that I, ALADAR Pacz, a citizen of the United States, residing at Cleveland Heights, in the county of (uyahoga and State of Ohio, have invented a certain new and useful lmprovement in Electrolytes for Aluminum Production and Methods of Preparing Same, of which the following is a full, clear, and exact description.

This invention relates to the production of metallic aluminum and especially to the composition, source and nature of the electrolyte employed in such production. According to all commercial practices, aluminum is obtained by the electrolysis of a so lution of alumina in fused cryolite. Cryolite has been employed heretofore because of the fact that it is anhydrous, is lighter than aluminum when molten and thus shields the same from the atmosphere, fuses at a fairly low temperature, and dissolves alumina with much ease. The melting point of aluminum is 657 (1, and that of cryolite is about 1000 C. but the latter is reduced to 915 by the presence of 5% of alumina, while the addition of certain other substances such as calcium fluoride and aluminum fluoride will reduce the melting point to about 800 C. It is desirable to perform the electrolysis at the lowest possible temperature in order to minimize heat loss and current loss. Theoretically, 42 grams of aluminum per kilowatt hour should be reduced by the electrolysis of alumina but practically the best figures obtainable for aluminum production show about 30 grams per kilowatt hour .which corresponds to an efficiency of 71%.

These is a difference of opinion as to what occurs in the bath during electrolysis, some asserting that the alumina is directly decomposed into its elements and others holding that the aluminum fluoride component of the cryolite is decomposed by the current and continuously regenerated by a secondary reaction of the dissociated fluorine upon the added alumina. The composition of the bath employed is kept Secret by most producers but always heretofore has comprised cryolitc with other materials to reduce the melting point to which pure aluinina is continuously added. heorctically, the best. melting: point reducer is aluminum fluoride as set forth by the most authoritative patentecs and writers, this being in Application filed March 18. i920. Serial No. 366,867.

conformity with the hypothesis that the aluminum fluoride is the primary active material; but this substance has never come int wide use owing to its high cost, as it is not a natural product nor even a by-product of any operation hitherto conducted. The melting point reducer ordinarily used is calcium fluoride, which however, is of no value in the reaction but is to some extent ficial cryolite could he produced at a cost to compete with the natural product.

The objects of my invention are the provision of a new and improved electrolyte for'the production of metallic aluminum, independently of natural cryolite; the provision of an electrolyte for this purpose which shall have a much lower melting point and a much higher current eliiciency; the provision of a new and improved electrolyte for this purpose which shall be made from substances at present a waste by-prodact of large industries, and which supply is capable of unlimited expansion; the provision of a new and improved method for the production of silicon especial] in company with other metals such as a uminum; the provision of a new and improved method for reducing and obtaining other metals in combination with aluminum such as boron, titanium, zirconium, thorium, beryllium, and rare earth metals, vanadium, columbium, tantalum, iron, manganese, nickel. chromium, cobalt, copper, uranium; the provision of a process for the produc tion of artificial cryolite at a cost to compete with the natural product; while further objects and advantages of my invention will become apparent as the description proreeds.

My improved electrolyte is prepared by the action of aluminum upon the double fluoride of sodium and a metal whose fluoride is reduced by aluminum under the onditions which surround the reaction as actually performed. The substances named are mixed in finely divided condition, the amount of aluminum being such as to displace the other metal from the double fluoride. In order to hold the ingredients in their properly mixed condition. prevent segregation, and enable their simultaneous sulunergence in the molten bath preterably agglomeratc The. mlxture into lumps, which may take the form of bricks, briquettes, blocks chunks, or pellets and be accomplished either by the use of a suitable binder, such as sodium silicate or even by pressure alone in some instances. The reaction is effected by adding these lumps to a bath of molten metal whose temperature is at least as high as that of fused aluminum. I

The cheapest and most readily available double fluoride i's that. known as sodium silico-fiuoride (Na SiF which isproduced in great quantites in the commercial manufacture of certain fertilizers known as super phosphates and which is very largely wasted at the present time. its only commercial use which is at all extensive being in connection with the enameling of sheet metal which utilizes only a very small part of the output. I take a quantity of this substance in the powdered form (it is in this form that it is normally produced) and mix it Int lmately with powdered or blown metall c aluminum in the proportion of approximately five parts of the former to one of the latter. In order to hold the two ingredients in intimate relatiorl with each other and to prevent segregation which might easily occur as the result of their different specific gravities, I preferably briquette or agglomerate the mixture into lumps by means of a suitable binder such as a diluted solution of sodium silicate.

This material is then introduced into a bath of pure molten aluminum, maintained at any temperature above its fusion point, whereupon the following reaction o-curs sua sir cuzasi+eNar+4iuF The reaction is slightly but not strongly exothermic and occurs very quickly. the bath being preferably stirred continuously while it is taking place. The silicon released by the reaction becomes alloyed with the aluminum of the bath while the remaining ingredients rise to the top of the bath in the form of a clear molten slag. The alloy of aluminum and silicon thus produced when treated as described and claimed in my earlier application filed February 13. 1920, Serial No. 35 .555. exhibits features of the very highest commercial value. so that the silicon .illoy constitutes a very valuable hy-product. The slag which is pl fultltefl by this reaction is easily removed either by ladling or pouring and forms a most'perfeet electrolyte for the production of aluminum. consisting as itdoes essentially of artificial cryolite mixed with an excess of aluminum fluoride. the'latter constituting about i-ltl of the whole. This is not to assert that the ingredients of this slag or batlrarc combined together in the way that natural cryolite is combined. This is a theoretical point and its discussion is not material at the present time. \Vhether the bath be considered as a mixture of cryolite and aluminum fluoride or whether it be considered as consisting of some other combL nation of the various ingredients the fact. remains that its melting point is more than 200 less than that of any natural cryolite bath ever before produced and at least as low as that of pure aluminum; its electrical conductivity is at least as great, its content of active ingredients is greater than any commercially employed electrolyte. and the solubility of alumina therein is at least as great. As a result of these advantages I find it possible to produce aluminum at a considerably higher current efficiency than is now obtained in any practical establishment. and higher than any obtained under the most perfect laboratory conditions of which I am aware. In every respect this electrolyte is superior to the best natural cryolite.

As to the cost of this bath. the use of 564 pounds of sodium silico-fiuoride with 108 pounds of aluminum (these being the strictly theoretical proportions) will produce 84 pounds of silicon (alloyed with the bath) and 588 pounds of the improved electrolyte and nasmuch as the best available figures provide for a wastage of the crvolite bath at the rate of pound of the bath for each pound of aluminum produced. this amount of electrolyte will serve for the production of nearly 6000 pounds of aluminum. Consider ng the fact that the sodium silicofluoride is practically a Waste product and that the electrical efficiency is so much greater'and the heat loss so much less with my improved electrolyte. this more than of?- sets the cost of the aluminum originally em ployed; in addition to which the value of the alloy-silicon produced largely offsets the cost of the aluminum employed. It

In fact my observation is that the rate of waste of the bath is much less with my improved electrolyte than with the baths here tofore used and I believe that the absence of su h foreign substances as calcium is largely responsible for this longer life.

Sodium Silico-fiuoride is not the only source of my improved electrolyte and I have only instanced the same as being )6:- culiarly available. Nearly every don le fluoride of sodium and some other metal can be employed, for example boron, beryllium. scandium, ytterbium, lanthanum,

titanium, zirconium, cerium, thorium, vanadium, niobium, tantalum, chromium, manganese. iron, cobalt, nickel, copper. or uranium. shown above. the silicon aluminum replacing the metal in question which appears alloyed with the aluminum bath, and I apprehend that a considerable source of this electrolyte will be as a by-product in some of these reactions inasmuch as this constitutes an easier mode of reducing many of those metals than any other mode now known. The fact that the metals are obtained in admixture with another metal constitutes-no serious disadvantage since those alloys are frequently valuable for themselves and the metals can be separated therefrom by dissolving out one substance with some reagent that does not attack the other substance. For example aluminum will dissolve in both hydrochloric acid and sodium hydroxide and almost no other metal will succumb to both of them.

In fact I do not restrict myself to the use of an aluminum bath since the same reactlon occurs when the mixture of double fluoride and aluminum powder is introduced into any other molten metal and in this way numerous very valuable alloys are made but always with the production of this peculiar and valuable electrolyte as a slag.

In this specification and claims I have included silicon under the term metal inasmuch as its properties, both when al- 10 ed with aluminum and when combined with other metals in difl'erent chemical compounds are clearly those of a metal.

It will be understood that I do not restrict myself to an electrolyte of the exact composition described since the same can be either diluted or fortified by the addition of other substances, but I claim and assert as my invention all electrolytic baths wherein any part is produced in accordance with the process here described, limiting myself in no wise whatsoever except as specifically set forth in my claims.

Having thus described my invention. what I claim is:

1. Method for the production of electrolyte for the anhydrous reduction of metals which contains the step of reacting with aluminum upon a double fluoride of sodium and a second non-aluminous metal at a.

temperature above the melting point of In all cases the reaction is asaluminum upon sodium fluo-silicate at a temperature above the melting point of aluminum and in the presence of an excess of aluminum.

4. Method for the production of electrolyte for the anhydrous reduction of metals which contains the step of introducing into a bath of fused aluminum an intimate mixture of aluminum powder and sodium fluosilicate.

' 5. Method for the production of electrolyte for the anhydrous reduction of metals which contains the step of introducing into a bath of fused aluminum an intimate mixture of aluminum powder and a double fluoride of sodium together with yet another metal.

6. Method for the production of electrolyte for the anhydrous reduction of metals which contains the step of introducing into a bath of fused metal an intimate mixture of aluminum powder and a double fluoride of another metal wit sodium.

7. Method for the production of artificial cryolite which contains the steps of introducing the double sodium fluoride of an other metal intimately mixed with the amount of powdered aluminum necessary to replace such other metal, into a bath of molten metal whose temperature is at least as high as the melting point of aluminum, and subsequently separating the slag from the metallic constituents.

8. Composition of matter for the production of artificial cryolite comprising the double fluoride of sodium and a. metal intimately mixed with the amount of powdered aluminum required to displace such other metal.

9. A composition of matter for the purpose described comprising the double fluoride of sodium and some metal whose halogen compound is reduced by aluminum. mixed with the amount of powdered aluminum to displace such metal and agglomerated into lump form.

10. A composition of matter for the purpose described comprising sodium silicofluoride mixed with a suflicient amount of powdered aluminum to re lace the silicon content and agglomerated into lump form.

11. A-composition of matter for the purpose described comprising an intimate mixture of sodium fluoride, aluminum powder, and the fluoride of some metal which is replaoeable by aluminum, the aluminum bemg present in sufficient quantity to replace such other metal and the sodium fluoride being present in such quantity as to form with the resulting aluminum fluoride a composition which is substantially cryolite mixed with an excess of aluminum fluoride.

12. Method for the production of electrolyte for the anhydrous reduction of aluminum which contains the step of subjecting a solid mixture of sodium fluoride and the fluoride of some metal to the action of powdered aluminum at. a temperature above the melting point of aluminum, unde air ex- 5 eluding conditions, said metal being of a nature whose fluorine compound is reduced by aluminum under the conditions stated.

13. Process of making electrolyte for aluminum reduction which contains the step of decomposing sodium silica-fluoride bv 10 metallic aluminum in the presence of heat. In testimony whereof, I hereunto aflix my signature.

ALADAR PACZ.