US3492208A - Electrolytic cells and methods of operating same - Google Patents

Description

G. C. SEAGER Jan. 27; 1 970 ELECTROLYTIC CELLS AND METHODS OF OPERATING SAME Filed Jan. 5, 1966 R mR A I E S C E G R O E G ATTORNEY United States Patent US. Cl. 20467 6 Claims ABSTRACT OF THE DISCLOSURE v A system for producing aluminum is provided and includes a cathodic structure in which molten aluminum is collected in a pool to a depth approaching two inches. The cathodic structure has a substantially horizontal molten aluminum contracting surface which has a cathodic expansion of less than about 3%, which is wettable by molten aluminum and which comprises a mixture of refractory hard substance with at least 5% calcined carbon, the surface having a tapping depression therein.

This invention relates to improvements in electrolytic cells and method of operating same and is concerned with electrolytic cells for the production of aluminum.

The aluminum reduction cell in common commercial use today is generally of the classic Hall/Heroult design with carbon anodes and a substantially fiat carbon lined bottom which functions as part of the cathodic system. An electrolyte is used in the production of aluminum by electrolytic reduction of alumina which consists primarily of molten cryolite with dissolved alumina and which may contain other materials such as fluorspar. Molten aluminum resulting from the reduction of alumina collects as a molten metal pool over the carbon lined bottom and acts as a liquid metal cathode. The carbon anodes extend into the cell from above and make contact with the electrolyte. Current collector bars usually of steel are embedded in the carbon lined bottom and complete the connection to the cathodic system.

Such commercial cells are generally operated by maintaining a minimum depth of liquid aluminum on the floor of the cell. This minimum depth is usually at least 2" and is required due to the fact that at lesser depths the aluminum would break up into globules, as it does not wet the carbon floor. If a continuous pool of molten aluminum is not obtained, it is not possible to establish a uniform interpolar distance, i.e., the distance between the anode and the surface of the molten aluminum. If the metal forms globules these move about, the inter-polar distance varies and eflicient operation at the optimum inter-polar distance cannot be established. Additionally, the surface areas presented by such globules to the electrolyte is considerably greater than the substantially continuous surface of the pool of molten metal and as the aluminum backreacts with the electrolyte and tends to go back into solution the result is that more metal goes back into solution than would otherwise be the case and the process becomes uneconomic. However, the depth of metal required to overcome these problems leads to another problem resulting from the electromagnetic effects which are usually associated with the reduction cell and which produce circulation of the liquid metal and cause the thickness or depth to vary and hence restrict the possible reduction of the inter-polar spacing. This also produces a loss in efliciency since power is lost to the electrolyte interposed between the anode and cathode. Any restriction on the reduction of the anode to cathode spacing also restricts the achieve- 3,492,208 Patented Jan. 27, 1970 ment of maximum power efliciency and limits the ability to improve electrolytic cell operation.

The use of refractory hard substances as materials for contacting the liquid metal pool and leading current from the cell has been previously proposed in US. 3,028,324. Such substances are broadly defined as being wettable by molten aluminum under electrolytic cell operating conditions, having a low solubility in molten aluminum and molten cryolite, having an electrical conductivity at least as good as carbon, having a resistance to attack by molten aluminum and other cell constituents at least as good as carbon and being substantially dimensionally stable in a cathodic structure in an electrolytic cell. Refractory hard substances include refractory hard metals and mixtures of refractory hard metals as well as mixtures containing refractory hard metals and aluminum compounds such as borides, nitrides and carbides, compounds of rare earth metals, chromium and combinations of the above. The expression refractory hard metals refers to the carbides, borides, silicides and nitrides of the transition metals in the fourth to sixth groups (Hubbards Periodic Chart of the atoms). The preferred refractory hard substances include the borides and carbides of titanium and zirconium and mixtures thereof.

Commercially available refractory hard metals frequently contain up to /z% carbon as impurities and this is generally considered to be pure refractory hard metal. The amount of refractory hard metal in a cathodic structure may, however, be reduced virtually without sacrifice of the function thereof. Thus, in co-pending application Ser. No. 325,228, filed Nov. 21, 1963, now US. 3,400,061, in the names of Robert A. Lewis and Richard D. Hildebrandt, there has been disclosed a cathodic structure having a surface intended to contact molten cell constituents, which surface is formed from a mixture of refractory hard substance and at least 5% carbon and is wettable by molten aluminum and has a cathodic expansion of less than about 3% It is preferred, however, that the mixture should contain at least 20% by weight of the refractory hard substance. The carbon component of the mixture preferably comprises a carbonaceous material which has been heat treated at a temperature about 1500 C. to improve its stability under cathodic conditions in the cell. In the prior proposal the refractory hard substance is disposed as a molten constituent contacting layer on the floor of the cell to provide a cathodic structure which is drained, i.e., the molten metal is prevented from forming a pool on the cell floor and is drained into a collecting well. Specifically, the cathodic structure is provided with an inclination to the horizontal so that the metal produced by electrolysis at the face of the cathodic structure is drained off into a substantially centrally disposed well having an appreciable capacity so that the cell need only be tapped at periodic intervals of time.

An electrolytic cell using a drained and wetted cathode according to the previous proposal represents a considerable improvement over conventional cells. By draining the I cathodic surface so that only a thin film of molten aluminum remains in contact with the cathode and functions as part of the electrical circuit, it is possible to employ very short anode to cathode spacing and at the same time maintain high current efficiency. Power losses can be reduced by decreasing the electrical resistance in the cell. Electrical resistance can be decreased without sacrificing current efiiciency by decreasing the interpolar distance and thereby decreasing voltage losses due to electrolytic resistance, or enabling increased current densities to be employed to increase the output from a given cell by increasing the current through it. Aluminum which is produced in the drained and wetted cathode cell by the electrolysis of alumina is drained off the cathode surface so that only a thin substantially uniform film of molten metal remains thereon, since the surface is wettable by molten aluminum, i.e., molten aluminum adheres as a liquid to the surface. Molten aluminum drained from the cathode surface collects in a pool in the well which is located so that the molten metal pool is not an essential part of the electrical system, i e., the molten metal pool is not essential for conducting cathodic current from the cell and the well may be periodically tapped dry of aluminum.

One of the disadvantages of the prior proposal referred to above is that the well must be capable of holding a substantial volume of molten metal so that it need only be tapped at acceptably long intervals of time. Also, the level of molten aluminum in the well must be necessarily lower than that of the sloping cathode surface. The concentration of a considerable volume of aluminum in the well results in an imbalance of the temperature conditions existing Within the cell and can act as a heat sink. If the collecting well is located outside the active area of the anodes, i.e., remote from the shadow of the anodes, it will be at a temperature lower than the temperature prevailing in the active area and this can lead to precipitation and collection of solidified bath constituents in the well in addition to unbalancing the cell temperature gradients. If the well is located in proximity to the active area of the anodes it will have an adverse effect causing imbalance in the anode consumption and would prevent the use of Soderberg anodes. Another disadvantage of such a cell is that the size of well required to accommodate the molten metal requires a larger cell to be constructed with consequential increase in the capital cost and increase in the heat losses from the larger area of the cell during operation.

It is an object of the present invention to provide an improved electrolytic cell for the production of aluminum and method of operating such cell which will substantially reduce the disadvantages discussed above in relation to the generally used commercial cell and the improved cell design proposed in the co-pending application Ser. No. 325,228.

According to one aspect of the present invention there is provided an electrolytic reduction cell for the production of aluminum having a substantially level floor surface composed of refractory hard substance.

Preferably, a small depression is provided to facilitate tapping of the molten metal but having a volume which is insignificant in relation to the volume of molten metal which accumulates on the floor of the cell during cell operation.

According to another aspect of the present invention there is provided a method of operating an electrolytic reduction cell for the production of aluminum which comprises the steps of leading current from the cell through a substantially level floor surface exposed to the interior of the cell and composed of refractory hard substance and maintaining the depth of molten aluminum which accumulates on the floor surface at a value of not greater than 2 inches.

With the method according to the invention, the fact that the floor of the cell is composed of a refractory hard substance which is wettable by molten aluminum prevents the formation of globules of aluminum at the small depths envisaged and the step of maintaining this depth at a value of below 2 inches avoids the disadvantages associated with the electromagnetic elfects and enables small inter-polar distances of less than 1.75 inches and preferably from A to 1 /2 inches to be maintained with consequent increase in the power efiiciency of the cell without decreasing the current efficiency appreciably.

The molten metal is collected as a pool over the floor of the cell so that there is no localized heat sink to disturb the temperature gradients. The build-up in depth of the molten metal on the floor of the cell during cell operation compensates for the consumption of the anode material during operation so that the inter-polar spacing is substantially maintained at a reasonably constant value. When the cell is tapped and the depth of molten metal reduced, the anodes are readjusted to the desired spacing between their active face and the surface of molten metal. Although a small depression is desirably provided to facilitate entry of a tapping nozzle, its capacity is insignificant in relation to the volume of metal which is allowed to collect in a pool on the floor of the cell before tapping. Additionally, during cell operation this small depression is full and the metal pool on the cell floor presents a continuous surface to the electrolyte. This depression may therefore be partly or wholly within the active area of the anodes, i.e., below the shadow of the anode or anodes, which may be either of the Soderberg or pre-baked type.

One embodiment of the invention will now be described by way of example, reference being made to the accompanying drawings in which:

FIG. 1 is a plan view of an electrolytic cell according to the invention,

FIG. 2 is a section taken on the line 11-11 of FIG. 1, and

FIG. 3 is a section taken on the line 111111 of FIG. 1.

The electrolytic cell of this example comprises a bottom 1 of refractory material such as thermal insulating brick over which is disposed a layer 2 of firebric-k. On top of the firebnck are supported a number of steel cathode bus-bars 3 over and between which is a layer 4 of rammed and fired carbon of a depth of about 12 inches. The side walls 5 of the cell are formed from alumina and lined with carbon bricks 6. The carbon layer 4 is identified as the bottom layer in Table 1 below and has the composition there described. It is formed with a central depression 7 of about 9 inches in diameter and from 1 to 2 inches deep. A graded layer 8 of refractory hard substance is disposed over the rammed carbon layer 4 and is effectively formed in three layers, the top layer of which is the richest in refractory hard metal. Typically, the lowest or third layer of the layer 8 may have 30 by weight of refractory hard metal and the balance carbon and binder, the intermediate or second layer 60% by weight of reefractory hard metal and the balance carbon and binder, and the top layer by weight of refractory hard metal and the balance carbon and binder. Typically, the refractory hard metal component may consist essentially of 70% by weight of titanium diboride and 30% by weight of titanium carbide although higher percentages of TiB may be preferred and commercially pure TiB can be used.

The carbon constituent of the layer 8 is desirably a carbonaceous materialsuch as anthracite or petroleum coke heat treated at a temperature above 1500" C. to improve its stability under cell operating conditions. The layer 8 has a cathodic expansion of less than about 3% as defined in the co-pending application Ser. No. 325,228.

The composition of the layers 8 and 4 is set out below in Table I.

TABLE I.WE'TTEZD CATHODE COMPOSITION (IN PERCENT) The depression in the layer 4 is reproduced at 9 in the layer 8 and is provided for the purpose of accommodating a tapping nozzle. Four carbon anodes 10 extend into the cell to afford an operative area of about 1000 square inches and have feet 11 embedded therein connected to hangers 12 supported on anode bus-bars indicated by the lines 13.

In operation of the cell described, the cell contains the usual molten flux constituents with dissolved alumina and an electric current passes from the anodes through the layers 8 and 4 to the bus-bars 3. Molten aluminum is deposited on the floor of the cell which is wetted thereby. The molten aluminum forms a pool the level of which is indicated at 14 and the depth of which is maintained at a depth of less than 2 inches. As the depth of metal approaches this value the cell is tapped by inserting a tapping spout into the depression and the metal is sucked out, care being taken that no molten flux constituents are removed. The anodes are lowered to the desired interelectrode spacing between their operative faces and either the wetted surface of the layer 8 or the surface of any remaining molten metal as the case may be, this spacing being less than 1.75 inches and desirably less than 1 /2 inches. As the molten metal pool builds up the operative faces of the anodes burn away so that the inter-electrode spacing is approximately maintained below the Value specified.

Table II below illustrates the comparable operations of a conventional cell, a wetted drained cathode cell as described int-he co-pending application Ser. No. 325,228 and a fiat wetted cathode cell in accordance with the present invention. The inter-polar distances quoted in the table represent an average value over the anode surface which becomes curved during use and the minimum distance may be A to /2 inch less than those given in the table.

We have found that the exposed surface of the solid drained and wetted cathode in accordance with the copending application Ser. No. 325,228, and the discontinuity in the structure introduced by the depression makes the lining of the cell more vulnerable to penetration by bath constituents. Normally, in the early stages of the life of a cell a sodium compound has to be added to the bath to compensate for losses into the lining and to maintain the electrolyte at the desired composition while cryolite has to be added to maintain the volume of the bath. The extra losses of alkali and of cryolite into a cell having a drained and wetted cathode as compared with a cell having a flat wetted cathode according to the present invention are illustrated by the following figures in Tables III and IV.

Age, 16-30 days In conventional cells of normal current density, acceptable current efliciencies cannot be obtained at average inter-polar distances less than 1.75 inches. If attempts are made to improve the current efiiciency by increasing current density, the voltage becomes uneconomically high and the amount of the heat dissipated excessive. By employing cells having wetted cathodes, inter-polar distances of less than 1.75 inches can be achieved without difficulty and cells according to the present invention are desirably operated with an inter-polar distance in the range of A to 1 /2 inches.

It will be appreciated that in a cell according to the present invention, the pool of molten aluminum presents a continuous surface to the anodes so that there is no significant inequality in the erosion or burning away of the latter so that either pre-baked or Soderberg anodes may be used. By maintaining the depth of molten aluminum at a value less than 2 inches electromagnetic effects are avoided. The depression 9, which is merely provided to facilitate tapping, has a volume which is insignificant in comparison with that of the aluminum which is collected on the floor of the cell and has no effect on cell operation and does not interrupt the continuity of the surface of the molten aluminum pool. Current flows from the anodes 10 through the molten flux to the continuous surface of the molten metal and out through the layers 8 and 4 and the cathodic bus-bars 3. Even when the cell has been tapped, there will still be a continuous film of molten aluminum over the layer 8. A not insignificant advantage of the present invention is that existing conventional cells may be readily modified by incorporating a layer 8 over the existing carbon floor without any great capital cost and without any requirement for structural alterations.

It is apparent that various changes and modifications may be made without departing from the spirit and scope of the invention, the invention being limited only as defined in the following claims, wherein what is claimed is:

1. The method of operating an electrolytic cell for the production of aluminum which comprises a shell defining a receptacle, electrolyte containing dissolved aluminum compound within the receptacle, an anodic system including at least one anode, and a cathodic system comprising a cathodic structure disposed within the receptacle contacting a molten aluminum pool exposed to molten constituents during cell operation, the cathodic structure comprising a continuous horizontal molten aluminum pool contacting surface, having a shallow tapping depression therein, the molten aluminum pool contacting surface comprising composite cathode material said material being a mixture of refractory hard substance and at least about 5% calcined carbon, and having a cathodic expansion of less than about 3% and being wettable by molten aluminum comprising:

(a) connecting the anode and cathodic structure to a source of electric power such that current passes through the anode and electrolyte causing molten aluminum to collect in the molten aluminum pool and then passes through the molten aluminum pool into the cathodic structure;

(b) removing molten contents from said tapping depression in a manner such as to maintain the molten aluminum pool with a continuous surface at a substantial depth of less than 2 inches.

2. The method according to claim 1 wherein the electrolytic cell is operated with an inter-polar distance between the anode and molten aluminum pool of less than 1.75 inches.

3. The method according to claim 2 wherein the electrolytic cell is operated with an inter-polar distance between the anode and molten aluminum pool between 0.25 and 1.5 inches.

4. The method according to claim 1 wherein said composite cathode structure comprises a bottom layer of about 30% by weight refractory hard metal and the bal- 7 ance carbon and binder, an intermediate layer of about 60% by weight of refractory hard metal and the balance carbon and binder, and a top layer of about 90% by weight refractory hard metal and the balance carbon and binder.

5. The method according to claim 1 wherein the refractory hard substance component of the composite cathode material comprises 70% by weight of titanium diboride and 30% by weight of titanium carbide.

6. The method according to claim 1 wherein the composite cathode material comprises a mixture of at least 20% refractory hard substance and at least about 5% carbon.

References Cited JOHN H. MACK, Primary Examiner 10 D. R. VALENTINE, Assistant Examiner US. Cl. X.R.

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