US2991235A - Method for supplying current to the anode of aluminum refining cells - Google Patents

Description

July 4, 1961 E. RAVIER 2,991,235

METHOD FOR SUPPLYING CURRENT TO THE ANODE 0F ALUMINUM REFINING CELLS Filed July 2, 1957 Q\ F IIIIIIIIIIIIIIIIIIIIIII'IIIIIIIIIIIIII"I'II'IIIIIIIIIllIIIllIIlIIIIIIIIIIIIII/II,

INVENTOR Emile Ravier BY 6(4/ m ATTORNEY United States PatentO METHOD FOR SUPPLYING CURRENT TO THE ANODE F ALUMINUM REFINING CELLS Emile Ravier, Mercus-Garrabet, France, assignor to Pechiney, Compagnie de Produits Chimiques et Electrometallurgiques, Paris, France, a corporation of France Filed July 2, 1957, Ser. No. 669,656 Claims priority, application France July 13, 1956 3 Claims. (Cl. 204-67) The present invention, which is based upon the results of applicants researches, relates to improvements in the method of and device for supplying current to the anode of aluminum refining cells using the three superposed layer process.

In this aluminum refining process by igneous electrolysis, there is used a soluble anode consisting of a molten alloy of aluminum with a heavy metal intended to impart to the alloy a suitable density so that it does not mix with the electrolyte layer superposed thereon, nor with the refined aluminum layer which is itself superposed on the electrolyte layer.

To supply direct current to this molten alloy, there are generally used electrolytic cells with conductive hearths. Such a conductive hearth is ordinarily made of a carbonaceous material, prebaked amorphous carbon blocks or rammed carbon paste, in which are embedded currentconducting metallic bars. The anodic alloy covers this hearth and the current is transmitted thereto through the carbonaceous material by means of metallic bars extending outside the furnace.

With flow of current, this hearth arrangement leads to a voltage drop which will be called anodic voltage drop in the following description.

In an 18,000 amperes electrolytic cell, there frequently occurs an anodic voltage drop of 0.5 volt corresponding to power loss of 9 kilowatts per second.

A portion of this loss assists in maintaining the temperature inside the electrolytic cells and thus constitutes useful heat, but the remainder constitutes a waste of power which it is desirable to avoid. However, in reducing this waste of power, it'is important not to alter the thermal balance of the electrolytic cell. 7

In order to restore the thermal balance of the electrolytic cell when the anodic voltage drop is reduced, the following means are available:

(1) Increase the heat insulation of the cell, thereby reducing heat losses;

(2) Increase the working rate (current), thereby increasing production while lowering power consumption;

(3) If it be impossible to increase either the heat insulation or the working rate, it is still possible, in any event, to increase the height of the electrolyte so as to generate more heat in the bath by the Joule efiect.

The production will then remain unchanged but the greater interpolar distance will enable refined aluminum to be obtained more easily. Therefore, a gain in the anodic voltage drop is an advantage in all cases.

Further, in the conventional construction of hearths in aluminum refining cells, it is almost impossible to obtain a tight joint between the refractory internal walls and the carbon bottom, because the thermal expansion coefiicient of carbon is appreciably difiterent from that of the refractory materials used for the walls. The result is that the anodic alloy infiltrates into the hearth and, as a consequence thereof, the mechanical strength and the heat insulation capacity of the bottom is lowered.

The present invention enables the above named difiiculties to be overcome and, in particular, makes it possible to supply current to the anodic alloy of a refining cell with a voltage drop which does not exceed 0.2 volt, without any resultant increase in the heat losses of the electrolytic cells as compared to those of cells provided with a carbonaceous anodic hearth.

According to the present invention, the current is brought directly to the anodic alloy by means of conducting metal bars which are arranged horizontally, vertically or obliquely, and without the interposition of carbon between the bars and the alloy. To this end, it is necessary to select a metallic material which does not dissolve at all, or dissolves only slightly, in the alloy at the normalworking temperature of electrolytic cells.

Iron-base alloys are suitable for this purpose. Indeed, it is known that the anodic alloy of modern electrolytic refining cells has an iron concentration which is always practically equal to the saturation concentration of that element. The iron is regularly eliminated in the form of iron-rich crystals in an anodic segregation pool. The result is that the capacity of the anodic alloy for dissolving iron is extremely reduced.

Moreover, there are grades of cast iron and stainless steel which are even more suitable than the ordinary ferrou s products.

Nevertheless, applicant has obtained satisfactory results by using common steel bars. After the ends of the bars have dissolved away to a certain extent, there is established an equilibrium zone between the liquid phase of the anodic alloy and the solid phase of the steel, which zone remains stationary without interposition of solidified alloy.

The accompanying figure shows, in schematic form, a vertical sectional view of the refining cell with an embodiment of the anodic device of the present invention. This embodiment is given by way of example, and not by way of limitation.

In this figure, 1 designates the metallic shell surrounding the refining cell; 2 the refractory heat insulating lining which constitutes the walls and bottom of the electrolytic cell; 3 is the metallic bar, for example, a steel bar, through which current enters the anodic alloy layer 4; 5 is the electrolyte layer disposed above the anodic alloy and on which lies the refined aluminum layer 6 which forms the cathode of the cell. 9, 9 designate the graphite electrodes by which the current leaves the cell; they are connected to the bar 10. 11 is the anodic segregation well, while 12 designates the iron-rich crystals which are deposited therein and which are removed through the upper end 13 of the well 11 The current density in the bars must be judiciously chosen in accordance with the desired result:

A low current density leads to a bar of large cross section and, as a consequence thereof, to high thermal losses across the connection;

A high current density leads to a higher voltage drop and toviolent agitation-caused by electromagnetic forces-of the molten alloy against the ends of the bars facilitating solution of the latter.

Applicant has obtained satisfactory results with a current density in the iron ranging between 30 and 40 amperes per square centimeter; however, these values are not given by way of limitation.

The position of the solidification front 7 which separates the molten alloy from the connnection may, eventually, be adjusted by disposing across the connection and the portion thereof which is outside of the electrolytic cell-at a place where no danger existsa cooling device having an adjustable rate of flow.

This arrangement is easily adapted for the temperature regulation of the electrolytic cells with rapid response, as it enables almost instantaneous action in lowering the temperature of the anodic alloy without interference by the generally large thermal inertia of the electrolytic cell, which slows down the desired temperature reductions. This arrangement can be of interest, particularly in the case where the anodic alloy contains a substantial percentage of zinc because, as is known, the latter presents the risk of contaminating the refined aluminum by reason of its vapor pressure, which increases rapidly when the temperature rises above 750 C.

In aluminum electrolytic refining cells provided with the above described anodic connection, it is possible, if desired, to keep a hearth of carbonaceous material of conventional design if it be deemed necessary to have such a hearth for starting. In this case, one can advantageously have the carbonaceous hearth operate in parallel with the special connection, and this improves further the anodic voltage drop.

But it is not absolutely necessary to retain a carbonaceous hearth because, if the anodic alloy be initially poured into the electrolytic cell during starting, the special anodic connection described in the foregoing can function immediately and ensures, from the very beginning, flow of the current. All carbonaceous parts can then be eliminated from the electrolytic cell and the internal lining of the crucible can be formed in a homogeneous manner of suitable refractory materials, such as magnesia bricks. The sturdiness of the whole structure is improved and there is less fear of lack of tightness of the crucible.

Example In an 18,000 ampere aluminum refining cell, there was placed an anodic current supply device consisting of steel bars in which the current density attained 30 amperes per square centimeter. The bars were cooled by dropping water slowly, drop by drop, upon their ends 8. There was obtained in this way a voltage drop of 0.19 volt between the anodic alloy and the ends of the steel bars, outside the refining cell, while the solidification front 7 was perfectly stabilized.

In the foregoing example, there was used an anodic alloy comprising 63% of aluminum, 28% of copper, 4% of zinc, 2% of iron, 2% of silicon, 1% of other minor impurities. As electrolyte, there was used a bath containing 60% BaCl and 40% chiolite (2AlF .3NaF), a small amount of sodium chloride being added to this mixture.

As previously indicated, it is possible to use alloys of aluminum and of other heavy metals, especially richer in zinc. Further, there can be used a sodium-free electrolyte bath which contains barium chloride, calcium (a) a molten layer of an alloy of the impure metal to be refined and a heavy metal, said layer resting on the bottom of the cell and serving as the anode,

(b) a layer of a molten electrolyte, and

(c) a layer of the refined metal serving as a cathode,

the improvement of advantageously reducing in the operation of said method the anodic voltage drop in the cell, which consists in: supplying the current to the molten anode in the cell by a metallic bar constituted of a ferrous material in direct contact with said molten anode and regulating the strength of the current in the bar to at cur rent density of 30 to 40 amperes per square centimeter.

2. A method according to claim 1, where the temperature of the metallic bar is regulated.

3. A method according to claim 1, wherein the metal refined is aluminum, the alloy comprises aluminum and iron, whereby the solution of the bar in the molten alloy is substantially inhibited.

References Cited in the file of this patent UNITED STATES PATENTS 510,276 Lyte Dec. 5, 1893 795,886 Betts Aug. 1, 1905 1,833,425 Jessup Nov. 24, 1931 2,213,073 McNitt Aug. 27, 1940 2,512,206 Holden et a1. June 20, 1950 2,685,566 Schmitt Aug. 3, 1954 2,866,743 Schrnitt Dec. 30, 1958 FOREIGN PATENTS 593,980 Germany Mar. 7, 1934 38,159 Norway Oct. 29, 1923 162,900 Australia July 3, 1952 OTHER REFERENCES (A.P.C.), Serial No. 369,610, published May 18, 1943.

Claims (1)

1. IN THE ELECTROLYTIC REFINING OF METALS BY THE THREELAYER METHOD IN A CELL CONTAINING IN SUPERPOSED RELATION AND IN THE ORDER NAMED, (A) A MOLTEN LAYER OF AN ALLOY OF THE IMPURE METAL TO BE REFINED AND A HEAVY METAL, SAID LAYER RESTING ON THE BOTTOM OF THE CELL AND SERVING AS THE ANODE, (B) A LAYER OF A MOLTEN ELECTROLYTE, AND (C) A LAYER OF THE REFINED METAL SERVING AS A CATHODE, THE IMPROVEMENT OF ADVANTAGEOUSLY REDUCING IN THE OPERATION OF SAID METHOD THE ANODIC VOLTAGE DROP IN THE CELL, WHICH CONSISTS IN: SUPPLYING THE CURRENT TO THE MOLTEN ANODE IN THE CELL BY A METALLIC BAR CONSTITUTED OF A FERROUS MATERIAL IN DIRECT CONTACT WITH SAID MOLTEN ANODE AND REGULATING THE STRENGTH OF THE CURRENT IN THE BAR TO A CURRENT DENSITY OF 30 TO 40 AMPERES PER SQUARE CENTIMETER.

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