US1594362A - Electrothermal reduction of alumina - Google Patents

Description

QUE GOPY F. C. FRARY ELECTROTHERMAL REDUCTION OF ALUMINA Aug. 3 1926.

Filed April 10 1924 awuentoz m M 1 PM: aw M a Patented Aug. 3, 1926.

UNITED STATES FRANCIS C. FRARY, OF OAKMONT, PENNSYLVANIA, ASSIGNOB T0 ALUMINUM PANY OF AMERICA, A CORPORATION OF PENNSYLVANIA.

1,594,362 PATENT OFFICE.

ELECTROTHERMAL REDUCTION OF ALUMINA.

Application filed April 10, 1924. Serial No. 705,458.

In processes for producing aluminum alloy by electrothermal reduction of aluminum oxid (alumina, Al,O )'with carbon in an electric furnace,-the obvious course is to make up the charges in about such proportions of alumina, carbon, and the desired alloying attempt is made to overcome this by using more carbon in the charge mixture a more serious trouble is apt toarise, and in fact usually does. In such case there is formed in the furnace a considerable amount of aluminum carbid, which accumulates in the alloy as a drossy emulsion and interferes bothwith the operation of the furnace and with the production of the metal. One effect is to raise the upper electrodes higher and higher in the furnace, thus tending to produce a cold bottom, which in turn increases the difiiculty of tapping. At the same time the production of alloy falls off. This seems to be due to the increased viscosity (of the alloy) occasioned by the presence ofthe carbid, the increased viscosity militat-ing against the thorough mixing and agitation of the alloy. This agitation is similar to boiling, when the furnace is in normal operation, and appears to be an important factor in the process, probably in facilitating the reduction of alumina and the absorption of aluminum by the copper or other alloying metal. When the furnace gets into this drossy carbid condition it is very difficult to remove the alloy by tapping, with the result that production is decreased, while the elevation of the reaction zone by the raising of the upper electrodes causes greatly increased losses of heat by radiation and of alumina by volatilization. The condition mentioned can be recognized from the presence of considerable amounts of aluminum carbid in the dross which separates out of the tapped alloy, the carbid appearing in the form of bright yellow crystals.

As intimated above, attempts to remedy the foregoing difiiculties by varying the comosition of the charge sometimes give unsatlsfactory results. In the first place, when the furnace has reached an objectionable condition the production of metal and the consumption of char e is by that time very markedly decrease so that a change in the proportions of carbon and alumina in the COM- next charge (it being understood that the raw materials are not supplied in a single charge but in a succession of charges, to per- .mit accumulation of a suflicient amount of alloy to justify the labor and time involved in t e tapping operation) acts very slowly, because of the fact that the fresh charge is relatively small in comparison with the amount of material already in the furnace. In the second place the objectionable condition is not apt to be discovered until a very considerable quantity of carbid or of alumina slag has accumulated, and hence the new charge of altered composition would have to make up not only for the erroneous composition of the previous charge or charges remaining in the furnace but also for the carbid or slag already accumulated. In the third place, such change of com osition, if persisted in, will eventually resu lt in reversing the phenomenon, so that a furnace which has been producing a large amount of carbid will become c ogged with alumina, or vice versa.

The facts recited in the foregoing have become known as a result of extended experience with processes of the kind referred to, and in the course of my experiments I have discovered that in most cases the operation of the furnace can be properly controlled by using a charge mixture which is approximately correct, (in this connection it will be readily understood that in practical operation on a commercial scale it would be a waste of time and effort to aim at such accuracy of quantitative composition as might be feasible in the laboratory), and adding, from time to time, directly to the molten alloy in the furnace, a considerable quantity of alumina or carbon (preferably in the form of coke), the former to reactwith the aluminum carbid, the other to react with the alumina slag, according to the conditions found by testing the furnace conan electric arc furnace generally suitable for the production of aluminum-copper alloys by processes of the kind indicated. The furnace shown, and the process which I prefer to employ, are described in the copending application of William Hoopes, Junius D. Edwards and myself, Serial No. 608,283, filed December 21, 1922, now Patent No. 1,534,316, issued April 21, 1925.

In the drawing, 10 designates an opentopped steel shell, containing a carbon bottom 11 and a refractory side lining 12 composed of tire brick, bauxite brick, or other suitable material. The carbon bottom slopes toward a tap hole 13 for withdrawing the alloy, and a tap hole 14 at a highfeif' level may be provided for withdrawal oflunreduced material independently of the under-. All these openings may closed in any convenient manner. ample, a plug of pine wood may be driven lying metal.

For exinto the hole, as indicated at 15. The heat encountered causes the plug to burst into flame immediately, and it is converted into charcoal in a few minutes, but it lasts long enough to stop the flow and permit solidification at the back of the plug. The spout 1.6, preferably rather steep so as to prevent clogging by freezing due to rapid cooling of the metal, may be lined with a mixture of magnesite and fire clay, moistened with a solution of water-glass.

Conducting members are embedded in the carbon bottom or lower electrode 11 for connection with one terminal of a source of alternating current represented by the transformer 17 Preferably the conducting members are in the form of steel pipes, as indicated at 18, through which water may be passed for cooling purposes if necessary or desirable The upper electrode is a car'- bon cylinder 19, connected to the other terminal of the secondary of the transformer 17. Any suitablemeans, not shown, may he provided to raise and lower the electrode. The energy input can be regulated in any convenient way, for example by varying the number of ampere turns in the trans former, preferably in the primary thereof, as indicated by the adjustable primary terminal 2O The furnace may be cooled, if necessary or desirable, by water discharged upon the outer shell from an encircling pipe 21, arranged at the upper part of the shell and connected to a source of supply, not shown. The water running down the shell can be caught by a trough 22 and carried away by a drain ipe '23. To keep water out of the tap ho es the latter may be provided with suitable shields, as 24.

When in use the furnace builds u alining, as indicated at 25, for examp e, composed, especially in the lower part of the furnace, of charge and solidified alumina.

' eled in.

. produce the aluminum content of such an alloy from alumina about 88 per cent pure (the remainder belng made up of impurities, such as oxids of SIllCOIl, magnesium, iron,

titanium), the charge composition may ,beroughly, by weight, 100 parts alumina,

23 parts silica (which may be clean sand), 60 parts copper or 91 parts copper oxid, and 16 parts carbon in the form of coke. The ingredients are crushed to suitable size, say to about one inch, and thoroughly commingled.

The operation may be started by lowering the upper electrode or electrodes and striking arcs between the same and a layer of crushed coke on the carbon bottom. After the furnace is well heated the charge is shov- The reaction begins at about 1800 (1, and goes on rapidly at 1900 and higher. As the reaction proceeds, additional charge mixture is supplied to the furnace and from time to time more or less of the alloy, which accumulates on the bottom of the furnace (as indicated at 26 in the drawing), is tapped out. The dross recovered from the tapped alloy is tested; or better, the dross floating on the alloy in the furnace, is tested at appropriate intervals, as by thrusting an iron or steel sampling rod momentarily into the alloy in the furnace and examining the layer of dross adhering to it after the same has had time to cool. If the test shows an excessive accumulation of aluminum can bid I treat the furnace contents with alumina, preferably in rather finely crushed condition and shoveling it into such position that it will be promptly associated with the alloy so as to react immediately therewith. If the test shows an excessive amountof alumina in the dross, I add carbon (which may be in the form of coke) in the same manner. In either case the resulting reaction is usually violent. After two or three hours the dross is again tested and the treatment repeated if necessary. In this" way I'have been able to restore normal operating conditions in a furnace very badly clogged with dross and giving very poor production of alloy. On account of the high temperatures involved and the very considerable volatility uf alumina and aluminum at such temperatures, it is desirable, in practice, that the furnace operationbe kept gofrequently, in order to keep down to a commercial basis the power consumed per pound of aluminum. Hence my method of controlling the operation is of marked value in practical work, facilitating the smooth progress of the operation and the production of a uniform product.

The amount of alumina or carbon supplied per treatment and the frequency of the additions depend, of course, very largely upon the capacity of the furnace and the observed rapldity of development of the untoward conditions which my invention is designed to rectify or prevent, and hence no hard and fast rule can be given. Guided by experience the attendant will usually find that with a furnace producing an average of about 10,000 pounds of alloy per day (24 hours) the alumina or carbon needed per treatment may be from 100 to 300 pounds in amount. 4

It is to be understood that the invention is not limited to the specific procedure herein described but may be carriedout inother ways without departure from its spirit as defined by the following claims.

I claim:

1. The process of producing aluminum alloy, comprising heating in an electric furnace a mixture containing a1umina,a reducing agent, and 'an alloying metal, to reduce alumina and permit the resulting aluminum to alloy with the said metal; determining, in

the course of the operation, whether-aluminum carbid or oxid is'present 1n excess 1n the alloy; and adding directly to the alloy in the furnace a component of the said mixture to remove such excess by reaction therewith and conversion of the same into metallic aluminum.

2. The process of producing aluminum alloy, comprising heating to the reducing temperature of alumina in an electric furnace a mixture containing-alumina, a reducing agent, and copper in suitable form, determining from time to time whether an excess of aluminum carbid or oxid is present in the resulting alloy, and upon finding such an excess, adding to the alloy in the furnace one of the two first-named ingredients of said mixture to remove said excess by converting the same into metallic aluminum.

3. The process .of producing aluminum alloy, comprising heating to the reducing temperature'of alumina in an electric furnace a mixture containing alumina, carbon. and copper in suitable form, determining in the course of the operation whether aluminum carbid is present in excess in the furnace, and upon finding such excess adding alumina to the furnace contents to react with the aluminum carbid. I

4. In a' process of producing aluminum alloy by electrothermal reduction of alumina in the presence of copper, the method of controlling the operation of the furnace, comprising testing the furnace contents to determine the presence of aluminum carbid in excess, and upon finding such excess supplying alumina alone to the furnace for removal of the excess carbid by reaction therewith.

In testimony whereof I hereto aflix my signature.

FRANCIS (3.. FRARY.

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