US1535458A

US1535458A - Method of and apparatus for electrolytic refining

Info

Publication number: US1535458A
Application number: US672867A
Authority: US
Inventors: Francis C Frary
Original assignee: Aluminum Company of America
Current assignee: Howmet Aerospace Inc
Priority date: 1923-11-05
Filing date: 1923-11-05
Publication date: 1925-04-28
Anticipated expiration: 1942-04-28
Also published as: GB224488A

Description

April 2s, 1925. 1,535,458

F. C. FRARY METHOD OF AND 'APPARATUS FOR ELECTROLYTIC REFINING Filed Nov. 5, 1925 6mp/life INVENTOR Eme/'5 C Ffa/y ATTORNEY Patented Apr. 28, 1925.

yUNITED STATES 1,535,458 PATENT OFFICE.-

FRANCIS C. FRARY, F OAKMONT, PENNSYLVANIA, ASSIGNOR T0 ALUMINUM COMf PANY 0F AMERICA, OF PITTSBURGH, PENNSYLVANIA, A CORPORATION 0F PENN- SYLVANIA.

i METHOD 0F AND,APPARATUS FOR ELECTROLYTC REFINING.

Application filed November To ZZ tlf/mm, it may @mwa/"n:

Be it known that I, FRANCIS C. Farini', a citizen of the United States of America, residing at Oakmont, in the county of Allegheny and State of Pennsylvania, have invented certain new and useful Improvements in Methods of and Apparatus for Electrolytic Refining, of which the following is a full, clear, and exact description.

In the electrolytic refining of aluminum by methods such as described in the copending applications (all filed December 2l. 1922) of William Hoopes, Serial No. 608,287; lVilliam Hoopes, Junius D. Edwards and myself, Serial Nos. 608,284 and 608,285; William Hoopes and myself, Serial No. 608,286, and in similar methods in which a layer of molten metal as cathode floats on a bath of molten electrolyte containing a salt of an alkali metal, such as sodium ilnorid, great difficulty has been experienced in making satisfactory electrical connection between the cathode and the external circuit. So far as I am aware, none of the methods 25 heretofore proposed is wholly successful in practice. For example, metal electrodes are attacked and dissolved by the molten metal, especially in the Case of aluminum. Tater-cooled metallic electrodes (preferably iron) covered with a thin layer or coating of frozen electrolyte, such as described in United States patent of von Kugelen and Seward, No. 858,400, can be used, but such devices possess certain disadvantages because of the high contact resistance between the metal of the electrode and the metal of the cathode, causing greater power consumption. This is further increased by the loss of the heat carried away by the cool ing water. Such factors as these seriously affect the efficiency of the process. Moreover, there is always danger of the protective coating being penetrated by the molten metal, which may dissolve and alloy with the iron and thus produce a leak, to say nothing of contaminating the cathode. This is especially liable to occur if anything happens to produce an are at any point between the electrode and the molten metal. Electrodes of amorphous carbon, such, for example, as are employed as anodes in the electrolytic reduction of alumina and for leading current into'ordinary electric furnaces, have been found wholly unsuitable 5, 1923. serial No. 672,867.

for my purpose, for th\e reason that when used as cathode connections in a process of the kind referred to they undergo a peculiar and rapid destruction, due apparently to the action of alkali metal or vapor upon the carbon connector or upon the binding material therein. Under some circumstances such carbons, if in contact with the molten bath', will flake ofi' in thin flakes, say about an eighth of an inch in thickness, so rapidly that the carbon will be rendered completely useless in a few minutes. Under other circumstances the carbon connector, particularly where it is in contact with a relatively small amount of the bath such as may work up' through the layer of moltenmetal, is subject to a rapid disintegration which causes it to swell up and become. simply a mass of loosely adhering particles. y

In the course of extended experience with the refining processes described in the above mentioned applications, in which the b-ath or electrolyte contains `fluorids of aluminum, sodium, and an alkaline earth metal such as barium, I have found that connectors made of graphite serve admirably, and indeed are the only ones suitable for the purpose., Such connectors may be made by graphitizing amorphous carbon rods at such a high temperature that all or substantially all of the ash is volatilized and the carbon converted into graphite. Such electrodes or connectors are also attacked in some degree by sodium liberated under some conditions in the electrolysis, but they are so resistant that the attack isv very slow, and unde-r most circumstances they will last several days, while if care is taken to maintain favoi-able bath conditions, for example by the use of excess aluminum fluorid, as described` in the above mentioned application of William Hoopes, Junius D. Edwards and myself, Serial No. 608,285, they often show but slight deterioration after a week or more of continuous use.

The use of graphite electrodes' or connectors in a process of the kind referred to is illustrated in the accompanying drawing, in which the ligure is a diagrammatic cross section showing the anode, the bath or electrolyte, and the molten aluminum cathode. with graphite connectors in the form of short rods dipping into the latter.

CTI

In the drawing, 10 represents a layer-of molten aluminum alloy as anode, which may contain copper and silicon, as for example the alloy described in the above mentioned copending application of William I-Ioopes, Junius D. Edwards and myself, Serial No. ($08,284. This layer is shown resting on a conducting hed of carbon, 11, which may be the bottom lining of the cell in which the electrolytic. refining process is carried out. The carbon bed serves as an electrode by which the anode allov is connected to the positive terminal of the external circuit, represented by the conductor 12.

Floating on the molten anode alloy is a layer 13 of molten electrolyte, which may be of about the following composition, as described in the above mentioned copending application of William Hooes, Junius D.

Edwards and myself, Serial o. 687,285:

Barium uorid 30 to 38 per cent Sodium fiuorid -25 to 30 per cent Aluminum fluorid -30 to 38 per cent Alumina 0.5 to 5 per cent Calcium and magnesium fiuorids (present as unavoldable impur1t1es) about 2 per cent vFloating on the electrolyte is the layer 14 of molten aluminum constituting the cathode.

The graphite connectors 15, dipping into the molten cathode 14, are preferably in the lform of short, thick cylinders7 about six inches long and from four to six inches in diameter. Any suitable means may be employed to connect them to the negative terminal of the external circuit1 represented by the busbar 16. In the drawing copper rods 17, screwed into the tops of the connectors, are provided for the purpose. These rods are clamped or otherwise secured to the busbar in any convenient manner, and may serve to support or suspend the graphite connectors therefrom.

At 18 is shown a layer or crust composed largely of electrolyte working up through or around the molten cathode metal and solidifying above the same'.

As before stated, the connectors may be made by graphitizing amorphous carbon at high temperature, so as to volatilize the ash and convert the carbon into graphite. This treatment may be performed upon carbon rods of suitable length, from which the connectors may be out afterwards.

The present invention is disclosed inthe copending application of William Hoopes, Ser. No. 608,287, and is stated therein to be the subject 'of my present application.

It is to be understood that the invention is not limited to the details herein specifically set forth but can be carried ont in other ways without departure from its spirit as defined by the appended claims.

I claim:

1. In a process of electrolyzing a fused electrolyte containing a fiuorid of an alkali metal, in which a layer of molten metal floating on the electrolyte acts as cathode, conveying electrolyzing current between the cathode and the external circuit through a connector the lower end of which is composed essentially of graphite in contact with said layer of molten metal.

2. In a process of electrolytic refining of aluminum, in which a cathode layer of inolten aluminum floats on a bath or electrolyte containing cryolite, conveying the electrolyzing current between the cathode and the external circuit through a connector having its lower end composed essentially of graphite in contact with the layer of molten aluminum.

8. In a process ol' refining aluminum electrolytically, in which a cathode layer of molten aluminum floats on a bath or electrolyte containing sodium fiuorid and a fluorid of an alkaline earth metal, conveying electrolyzing current between the cathode and the external circuit through a connector having an end composed essentially of graphite.

4. In an apparatus for electrolytically refining aluminum with an electrolyte containing alkali metal fluorid, and a cathode of molten aluminum floating thereon, au electrical connector between the external circuit and the cathode, comprising a body of graphite adapted to 'dip into the molten cathode, and a conductor in electrical contact with the graphite body.

5. In an apparatus for electrolytically refining aluminum with an electrolyte containing all-:ali metal fluorid, and a cathode of molten aluminum floating thereon, an electrical connector between the external circuit and the cathode, comprising a body of graphite adapted to dip into the molten cathode, and a metallic conductor having one end embedded in the graphite body.

G. In an apparatus for electrolytic refining of aluminum, a molten electrolyte containing an alkali metal fluorid, a molten aluminum cathode floating on the molten electrolyte, an external circuit hav'ing its positive pole connected with the electrolyte, and a connector electrically connected with the negative pole oi' the external circuit and having ali-end composed of graphite in direct contact with the molten aluminum cathode.

In testimony whereof I hereto affix my signature.

FRANCIS C. FRARY.