Aluminum Production -- the Hall - Héroult Process -- Electrolysis of Aluminum Fluoride

Manufacture of Aluminum

see also: The Bauxite Story

Note in a dental journal of 1872 on how to solder aluminum using zinc amalgam

(Source: Correspondenzblatt fuer Zahnaerzte, Vol. I, Jan. 1872, p. 32)

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The Patents

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1883

"On Feb. 23, 1883, exactly three years before Hall made his first aluminum, Charles S. Bradley filed a patent application which claimed broadly the idea of fusing ´ores´ by the electric arc and then passing a current through the ore to decompose it and maintain it in the fused condition. The decomposition of cryolite as an ore of aluminum was the specific example given by Bradley. The Patent Office promptly rejected his application on the basis of the prior art. After the 2 years´ period then allowed by the Patent Office, Bradley replied with an amendment. The case dragged on this way for six years with one rejection after another. The principal art relied on by the Patent Office was Davy´s preparation of metallic potassium by the electrolysis of potash. Davy had very definitely stated: ´I only obtained my object by employing electricity as the common agent for fusion and decomposition.´ The Patent Office did not then consider it constituted invention to apply Davy´s idea to the electrolysis of cryolite" [Edwards J. D., Frary F.C., Jeffries Z. (all of ALCOA): "The Aluminum Industry. I. Aluminum and its Production", McGraw-Hill Co., New York 1930, p. 27]:

Charles S. BRADLEY, of Yonkers, New York: "Process of obtaining metals from their ores or compounds by electrolysis", US Patent 464,933; filed Feb. 23, 1883; divided and again filed Sept. 14, 1889; renewed March 16,1891, patented Dec. 8, 1891:

"My invention relates to the process of effecting the reduction of minerals or other compound chemical substances while in a state of fusion by the electrolytic action of an electric current; and it is especially designed for the extraction of metals from their ores or compounds and their reduction to the metallic state - for example the extraction of aluminium from one of its ores, say cryolite. ... I employ an electric current of greater strength or intensity than would be required to produce the electrolytic decomposition alone, and I maintain the ore or other substance in a state of fusion by the heat developed by the passage of the current through the melted mass ... I have spoken several times of fluorine being set free, although I am aware that it is considered to be almost impossible to isolate that element. I use the term ´fluorine´ merely for convenience, meaning thereby whatever is set free at the anode, which may be fluorine or some compound of fluorine with the substance composing the anode, moisture of the air &c."

William FRISHMUTH, of Philadelphia, Pa., assignor to himself and George R. Blanchard and Emerson Foote, of New York, N. Y.: "Process of extracting aluminium from corundum, &c."; US Patent 282,621; filed March 9, 1883; pat. Aug. 7, 1883

"Heretofore the practical method of extracting aluminium from its ores or from substances containing it has been to first to change such substances into a chloride of aluminium, and then to separate the metal therefrom by the use of metallic sodium acting as a reagent. This method of process has been found to be impracticable in the treatment of large quantities of material on account of the great cost of metallic sodium. My method of process differs materially from those now in use; and it consists in first converting the material into fluoride of aluminium and then producing the metal directly therefrom by the action of sodium vapor or gas generated by heating a mixture of carbonate of soda or an equivalent thereof with charcoal and chalk or lime, said gas or vapor being brought into immediate contact with the fluoride."

A process of producing metallic aluminium is claimed which consists in "first calcining and roasting the compound containing aluminium, together with fluor-spar, fluoride of soda, or other material containing fluorine, then converting the fluoride of aluminium thus produced into chloride of aluminium, then mixing this chloride with carbonaceous material and then subjecting the mixture to a high temperature, and simultaneously to the action of sodium vapor produced from a mixture of a sodium compound and carbon ..."

William FRISHMUTH, of Philadelphia, Pa., assignor to himself and George R. Blanchard and Emerson Foote, of New York, N. Y.: "Process of extracting the metal aluminium"; US Patent 308,152; filed Dec. 5, 1883; pat. Nov. 18, 1884

"... I am aware that one Knowles, in English Patent No. 1,742 of 1857, describes a process by which metallic aluminium can be produced by subjecting chloride of aluminium to the action of cyanide of sodium as a reducing agent."

1885

Eugene H. COWLES and Alfred H. COWLES, of Cleveland, Ohio: "Electric process of smelting ore for the production of alloys, bronzes, and metallic compounds"; US Patent 324,658; filed April 7, 1885, patented Aug. 18, 1885

A process of producing alloys is claimed "which consists in mixing together ore of one of the metals of the alloy, broken or pulverized carbon, inserting wires or rods of the other metal of the alloy into the said mixture and then passing an electric current through the mixture in a transverse direction to the wires or rods, so that the said mixture is rendered incandescent and an alloy formed..."

Eugene H. COWLES and Alfred H. COWLES, of Cleveland, Ohio: "Process of electric smelting for obtaining aluminium"; US Patent 324,659; filed April 23, 1885, patented Aug. 18, 1885

Aluminium of higher purity is obtained by mixing aluminum ore with broken carbon and with a metal, and further "reducing the said ore by means of electricity, so that the aluminium forms an alloy with the metal, and finally separating the aluminium from the alloy by amalgamating the said metal ..."

1886

"Paul T. Héroult, of Paris, on April 23, 1886, applied for and was granted a French patent covering the electrolysis of alumina in cryolite, his patent [FR 175,711; April 23, 1886] having much the same scope as the Hall process. Héroult also applied for a patent in the United States, but the Patent Office awarded priority of invention to Hall, who had also applied for patent protection" [Edwards J. D., Frary F.C., Jeffries Z. (all of ALCOA): "The Aluminum Industry. I. Aluminum and its Production", McGraw-Hill Co., New York 1930, p. 13]:

Charles M. HALL, of Oberlin, Ohio: "Process of reducing aluminium by electrolysis", US Patent 400,766; filed July 9, 1886; patented April 2, 1889

"The invention described herein relates to the reduction of aluminium from its oxide by dissolving such oxide in a bath containing a fused fluoride salt of aluminium and then reducing the aluminium by passing an electric current through the bath; and in general terms the invention consists in the electrolysis of a solution of alumina in a fused fluoride salt of aluminium, substantially as hereinafter more fully described and claimed ... In the practice of my invention I prepare a bath for the solution of the alumina by fusing together in a suitable crucible the fluoride of aluminium and the fluoride of a metal more electropositive than aluminium - as, for example, the fluoride of sodium, potassium, &c. - these salts being preferably mingled together in the proportions of eighty-four parts of sodium fluoride and one hundred and sixty-nine parts of aluminium fluoride, represented by the formula Na2Al2F8 ... In order to render the bath or solvent more fusible fluoride of lithium may be substituted for a portion of the fluoride of sodium..."

1887

Charles M. HALL, of Oberlin, Ohio: "Process of reducing aluminium from its fluoride salts by electrolysis", US Patent 400,664; filed July 9, 1886; divided and this application filed Feb. 2, 1887; patented April 2, 1889:

"In the practice of my invention I form a fused bath or solvent for the alumina by melting in a crucible a combination of the salts known as the ´fluoride of aluminium´ and the ´fluoride of potassium´. This combination, which may also be termed the ´double fluoride of aluminium and potassium´ is preferably formed by mixing together one hundred and sixty-nine parts of aluminium fluoride and one hundred and sixteen parts of potassium fluoride, such proportions of the ingredients corresponding to the formula K2Al2F8. A variation in these proprtions with certain limits produces only immaterial changes in operativeness of my process - as, for example, (as I now believe the fact to be,) a larger proportion of potassium fluoride increases the capacity of the bath for dissolving alumina at the same time lessens its fusibility, whereas a larger proportion of aluminium fluoride renders the bath more fusible, but decreases its capacity for dissolving alumina. The combination of the fluorides of aluminium and potassium may be rendered more fusible and its capacity for dissolving alumina may be increased by the addition of lithium fluoride ... These compounds or combinations of the fluorides of aluminium, potassium and lithium can be conveniently made by saturating and neutralizing with hydrofluoric acid suitable proportions of hydrate of alumina and carbonate of potassium or lithium."

Curt NETTO, Dresden, Germany: "Verfahren zur Darstellung von Aluminium", German Patent DE 45,198; filed March 26, 1887; patented Nov 15, 1888; "Process of making aluminium", US Patent 431,912; filed July 21, 1887; patented July 8, 1890:

"As early as 1855 Percy, Rose, Woehler and Deville proposed methods of manufacturing aluminium from cryolite by means of sodium. Though this mineral is found in nature in great quantities, it has not been sufficiently employed for the manufacture on a commercial scale of aluminium, for the reason that the chloride of aluminium is so difficult to produce that its double combination with chloride of sodium has generally been used. The reason for this is that the reduction of cryolite by means of alkaline metals is subject to the objection that its melting-point, being even in the presence of a suuitable flux between 750° and 800° centigrade, surpasses the evaporating - point of sodium and also of potassium. If, therefore, as in the Deville porocess, a charge of cryolite flux and alkaline metals is heated, the better part of the latter escapes in vapor before the temperature is reached necessary for the melting or the reduction of the cryolite..."

"It would appear as if Héroult failed to grasp the possibilities inherent in the alumina-cryolite process, for a year later, April 15, 1887, he took another French patent, as well as a series of foreign patents, on a totally different process which he proceeded to commercialize. His second process only produced aluminum alloys" [Edwards J.D., Frary F.C., Jeffries Z. (all of ALCOA): "The Aluminum Industry. I. Aluminum and its Production", McGraw-Hill Co., New York 1930, p.13]. Pure alumina was fused electrothermally by heat from a powerful electric current::

Paul HÉROULT, of Lauffen-Uhwiesen, Switzerland: "Process of preparing aluminium-bronze and other alloys", US Patent 387,876; filed Dec. 27, 1887; patented Aug. 14, 1888; priority France April 15, 1887, Patent No. FR 170,003:

"Alumina is a refractory oxide and cannot be conveniently melted in a vessel by external application of heat unless a flux be used, and to such use there are serious objections, which it is one object of my invention to obviate. Thus the flux may contain elements which will undergo electrolysis before the oxide, in which case not the product desired - namely aluminium - but some other substance (depending upon the character of the flux) will be produced; or the flux may contain substances such as phosphorus, iron, silicon, &c., which will enter into the product and deteriorate it; or the flux may increase the resistance of the bath, thus increasing the demands for power. I have tried many fluxes and found this to be the case with all of them. Again, the flux may evolve gases, which will settle on the carbon anode and tend to polarize it and by preventing free access of the oxygen to the carbon increase the resistance and the cost of production. For instance, I have used cryolite (which can be fused by external heat without a flux) as an electrolyte, and also as a flux in connection with alumina, but have found it objectionable because it evolves fluorine, which does not, like the oxygen evolved from alumina, combine with the carbon anode and pass off, but settles upon it with the effect above mentioned. Moreover, the use of a flux must always leave the quantity and quality of the product to be obtained uncertain, unless the character and composition of the flux used in each operation are positively known."

Ludwig GRABAU, Hanover, Germany: "Process of manufacturing aluminium", US Patent 400,449; filed Nov. 19, 1887; patented April 2, 1889:

"... it has heretofore been the custom to pour molten aluminium combinations upon a metal of the alkalies. The lighter alkali metal invariably rises to the surface and deflagration ensues. On the other hand, all refractory materials known and heretofore used in the construction of the reducing apparatus (converter, crucible, or furnace) are readily attacked and speedily destroyed by the aluminium alkali fluorides formed in the process of reduction or during the reaction, thus not only entailing expenses in repairs, but also resulting in an inferior and impure product."

1888

Charles M. HALL, of Oberlin, Ohio: "Manufacture of aluminium", US Patent 400,665; filed Aug. 17, 1888; patented April 2, 1889:

"The bath formed of the double fluoride of aluminium and an alkaline metal, as described in the above application [July 9, 1886, and Feb. 2, 1887], becomes less efficient after being subjected to electrolytic action for some time. This change does not result from any removal of aluminium fluoride or of fluorine from the bath, as no fluorine is separated; but a black or dark substance is formed in the bath, apparently from the alkaline constituent of the same, which interferes with a free electrolytic action and increases the electrical resistance; hence it has been found necessary when employing the salts named to change the bath after a continuous use of the same ... In carrying out my invention I prefer to employ a double fluoride of aluminium and calcium having a composition represented by the formula CaAl2F8.This composition is formed by one hundred aand sixty-nine parts of aluminium fluoride to seventy-eight parts of calcium fluoride or fluor-spar, the aluminium fluoride being artificially prepared by saturating hydrated alumina with hydrofluoric acid. ... It may be stated that means may be used to lower the specific gravity of the bath by the addition to it of other salts lighter than the double fluoride of calcium and aluminium, so that the pure aluminium will sink in the combination and thus be protected from oxidation. Thus by the addition to the bath above described of about two-thirds its weight of the double fluoride potassium and aluminium (K2Al2F8) which is comparatively a very light salt, is formed a composition lighter than aluminium, which will therefor sink when reduced and be thoroughly protected."

Charles M. HALL, of Oberlin, Ohio: "Process of electrolyzing crude salts of aluminium", US Patent 400,666; filed Aug. 17, 1888; patented April 2, 1889:

"In the practice of the present invention I form an electrolyte or bath of the fluorides of calcium, sodium and aluminium, the fluorides of calcium and sodium being obtained in the form of fluor-spar and cryolite, respectively, and the fluoride of aluminium being obtained by saturating hydrated alumina with hydrofluoric acid."

Charles M. HALL, of Oberlin, Ohio: "Process of electrolyzing fused salts of aluminium", US Patent 400,667; filed Sept. 21, 1888; patented April 2, 1889:

"..the baths described in the applications filed July 9, 1886, and Feb. 2, 1887, and consisting of the fluorides of aluminium and sodium and the fluorides of aluminium and potassium, become clogged when the negative electrode is formed of carbon, as is necessary in reducing pure aluminium on account of the affinity of aluminium for other metals; but when the baths composed of the fluorides of sodium and aluminium is employed in the manufacture of aluminium alloys - e.g. aluminium bronze, in which case the negative electrode is formed of the metal with which the aluminium is to be alloyed, e.g. copper, when aluminium bronze is desired - no such deterioration of the bath occurs."

1900

Considerable efforts had to be undertaken to purify the alumina (bauxite) used in the electrolysis, as it usually contains impurities such as compounds of silicon, iron and titanium.

Charles M. HALL, of Niagara Falls, New York: "Method of making alumina", US Patent 663,167; filed May 26, 1900; pat. Dec. 4, 1900

A method is calimed for converting bauxite or impure alumina into sodium aluminate by treating the material in a digester. "An essential part of my invention consists in the use of lime in the digester. Its function is to combine with the carbonic acid of the carbonate of soda and to convert the carbonate of soda into caustic soda and also to combine with the silica. Within my invention I may make use of lime not only where carbonate of soda is charged with the mixture into the digester, as above described, but also in processes in which bauxite is introduced with caustic soda or with the caustic aluminate liquor which results from the precipitation of alumina in the Bayer process. In such processes the mixture always contains a considerable proportion of silica and also some carbonate of soda, either introduced originally as an impurity or formed by exposure of the caustic-soda solution to the air. The silica and carbonate of soda in these processes are both harmful, the carbonate of soda because it decreases the efficiency of the mixture and the silica because it combines with and wastes the caustic soda ..."

Charles M. HALL, of Niagara Falls, New York: "Process of purifying bauxite", US Patent 677,207; filed Dec. 19, 1900; pat. June 25, 1901

"It is a matter of great importance in the manufacture of aluminium that the alumina to be electrolyzed should be free from impurities, for the character of the ore determines largely the quality of the aluminium which is produced from it." Therefore Hall patented a "method of purifying bauxite or other impure oxid of aluminium containing silica, which consists in fusing the same with an electric current, and while fused reducing the silica therein in the presence of a metal capable of alloying with the silicon, causing the silicon and said metal to unite in a fused alloy, and then removing the purified alumina from the furnace."

Charles M. HALL, of Niagara Falls, New York: "Process of purifying bauxite", US Patent 678,732; filed Dec. 21, 1900; pat. July 16, 1901

"The object of my invention is to provide ccheap and efficient means for eliminating silica from bauxite or other impure oxid of aluminium and thus fit it for use as an ore for the production of aluminium. I have discovered that this can be done by fusing the bauxite or impure oxid of aluminium together with a metallic fluorid or with two or more such fluorids in less proportion than the oxid to be treated. In this process the elimination of the silica is accomplished by combination of the silicon with the fluorin of the flused fluorid; forming silicon fluorid, which escapes into the atmosphere as a gas ... according to the following reaction, which takes place when fluorspar is used: SiO2 + 2 CaF2 = SiF4 + 2 CaO. A like reaction takes place, to some extent at least, with the titanic acid, and when fluorids other than fluorspar are employed the operation is similar."

1901

Charles M. HALL, of Niagara Falls, New York: "Process of purifying bauxite, &c"; US Patent 677,208; filed Feb. 2, 1901; pat. June 25, 1901

"I have discovered that bauxite or other impure oxid of aluminium can be freed from the silica, iron oxid, and titanic acid with which it is ordinarily contaminated by mixing with it aluminium either in its unalloyed state or preferably in the form of a powdered ferroaluminium alloy and subjecting it to heat sufficient to cuase the mass to fuse. The aluminium acts as a reducing agent upon the impurities above mentioned and reduces them to their elemental form, when they combine as a melted alloy, easily separable from the alumina of the bauxite."

Charles M. HALL, of Niagara Falls, New York: "Purified crystalline alumina", US Patent 677,209; filed Feb. 20, 1901; pat. June 25, 1901

"Having calcined the bauxite, I place it in a suitable electric furnace, preferably lined with carbon, and fuse it therein, preferably first mixing with it powdered carbon sufficient to amount to eight to ten per cent., more or less in order to assist in the reduction, althogh this is not always necessary, since the carbon of the electrodes and of the furnace lining will suffice. A more impure bauxite will require a greater percentage of carbon. It is desirable to avoid a large excess of carbon, which causes a waste of the alumina and has a prejudicial effect."

Charles M. HALL, of Niagara Falls, New York: "Process of manufacturing alumina"; US Patent 706,553; filed Aug. 10, 1901; pat. Aug. 12, 1902

"I mix the bauxite or other impure oxid of aluminium (in which term I include the silicates) with an amount of carbon proper to reduce the iron oxid, silica, and titanic acid, and, if the iron is deficient, with sufficient iron or other heavy metal to alloy with the silicon and titanium to be reduced, and I then heat the mixture in an electric furnace to a high temperature, but below the melting point of alumina. By this means the iron oxid contained in the bauxite is substantially reduced to metallic iron, and the silica, and generally the titanic acid, seems to be partially reduced. ... After allowing the mass to cool it is taken out, broken up if fritted together, and it is then fused by means of an electric current, and while fused the reduction of the impurities is completed, and the reduced iron, silicon, and titanium agglomerate into fused masses, which at the end of the process can readily be separated from the purified alumina."

1902

Charles M. HALL, of Niagara Falls, New York: "Paint", US Patent 701,718; filed Jan. 13, 1902; pat. June 3, 1902

"In making the alloy of iron, titanium, and silicon I preferably employ an electrical furnace in which I may charge bauxite or other material containing iron, titanium, and silicon. I mix it preferably with carbon and fuse it with the electric current. The iron reduced from this mixture is collected as an alloy with silicon, titanium and some aluminum. The alloy is first pulverized to a very fine state by the use of suitable pulverizing-mills and is then mixed or ground with oil to constitute a paint."

Charles M. HALL, of Niagara Falls, New York: "Manufacture of carbon electrodes"; US Patent 705,076; filed Feb. 19, 1902; pat. July 22, 1902

Hall claimed a method "of baking carbon articles, which consists in placing them in proximity to a conducting path or core, insulating the articles or groups of articles, and then passing a current through the core."

Charles M. HALL, of Niagara Falls, New York: "Stopper for electrolytic pots containing fused baths", US Patent 796,325; filed April 30, 1902; pat. Aug. 1, 1905

"It is necessary that the workmen should be able to open quickly and easily the tap-holes of such pots when it is desired to withdraw the metal. Any delay in inserting the stopper enatils waste of the molten bath which overlies the layer of metal to be withdrawn, and if the stopper is hard to dislodge it requires an expenditure of labor, and the effort required to dislodge it may occasion injury to the pot or to its lining. ... "

Charles M. HALL, of Niagara Falls, New York: "Process of removing silica from coke", US Patent 733,389; filed Sept. 24, 1902; pat. July 14, 1903

"In the manufacture of aluminium and other metals by electrolytic processes it is well known that the carbon used in the electrodes sshould be substantially free from silicon compounds and particularly silica. ... The process which forms the subject of this application is a novel method of removing all or the greater part of the silica contained in coke; and it consists, broadly stated, in mixing with the material to be treated a metallic fluorid which will react with the silica, giving a fluorid of silicon which goes off as a gas during the baking, hereinafter to be described. For this purpose I have used a number of different fluorids, good results being obtained with sodium fluorid, cryolite, and fluorspar."

1906

Charles M. HALL, of Niagara Falls, New York, assignor to Aluminum Company of America, of New Kensington, Pennsylvania, a corporation of Pennsylvania: "Process of making alumina"; US Patent 921,609; filed Jan. 11, 1906; pat. May 11, 1909

An improvement in the manufacture of alumina is claimed "which consists in heating a mixture of bauxite and chlorid of calcium in an atmosphere containing oxygen and digesting the resultant product with a solution of carbonate of soda to form aluminate of soda, and then recovering the alumina therefrom ... "

1914

Charles M. HALL, of Niagara Falls, New York; Arthur V. DAVIS, of Pittsburgh, Pennsylvania, and Homer H. JOHNSON, of Ckeveland, Ohio, executors of the will of said Charles M. Hall, deceased: "Method of treating aluminous materials of high silica content"; US Patent 1,282,222; filed Nov. 25, 1914; pat. Oct. 22, 1918

A continuous process is described "of treating aluminous materials of high silica content, comprising mixing the material with common salt in about the proportion of four molecules of salt to one of the alumina present; heating the mixture to a temperature of about 1600 to 1700° F, for about four hours, in the presence of steam, whereby the salt is decomposed with the formation of a compound of soda, alumina, and silica, and the evolution of hydrochloric acid; mixing the solid powder with lime and soda; and heating this mixture to a temperature of about 1300 to 1400° F, for about two to three hours, to produce soluble sodium aluminate and insoluble calcium silicate, and thereafter leaching the product."

Charles Martin Hall died December 27, 1914, at Daytona, Fla. [Edwards et al. 1930].

See also:

ALCOA chemist discovers fluoride as cause of mottled teeth

- http://www.fluoride-history.de/bauxite.htm

Bauxite Mill, American Bauxite Company, Bauxite, Arkansas, ca. 1925

- http://www.cals.lib.ar.us/arkansas/postcard/bauxite.htm

Short history of aluminium (key data)

- http://www.world-aluminium.org/history/index.html#

History of the Aluminum Cap of the George Washington Monument:

- http://www.tms.org/pubs/journals/JOM/9511/Binczewski-9511.html

The Brooks - Hall family history

- http://kinnexions.com/kinnexions/johnson/rr01/rr01_419.htm