US3491002A - Adjusting anode blocks in an electrolytic cell - Google Patents

Description

Jan. 20; 1970 L, 5 w 3,491,002

: ADJUSTING ANODE BLOCKS IN AN ELECTROLYTIC CELL Filed Sept. 21, 1964 F F r F 2 Sheets-Sheet 1 POLE OF BATTERY POLE 0F BATTERY IN VENTOR.

FIGMZ. JOHN L. DEWEY United States Patent 3,491,002 ADJUSTING ANODE BLOCKS IN AN ELECTROLYTIC CELL John L. Dewey, Florence, Ala., assignor to Reynolds Metals Company, Richmond, Va., a corporation of Delaware Filed Sept. 21, 1964, Ser. No. 397,755 Int. Cl. B01k 3/00; C22d 3/12 U.S. Cl. 20467 5 Claims ABSTRACT OF THE DISCLOSURE A method of adjusting individual anodes in a multiple anode reduction cell by first determining for each anode the size of a pseudo-area corresponding to the effective conduction area of that particular anode. Each pseudo-area may be determined by means of two-dimensional analogue models of the cell representing orthog-- onal vertical cross-sections thereof. A determination is made of the resistance value between each anode and the cathode of each model and these resistance values are related to a linear dimension corresponding to a side of each anode blocks pseudo-area. The vertical position of each anode is adjusted so that the current therethrough bears substantially the same relation to the total current through all of the anode blocks as that particular blocks pseudo-area bears to the sum of all of the pseudo-areas.

This invention relates to a method of and apparatus for adjusting the anode blocks in a reduction cell. More specifically, although not limited thereto, this invention relates to a method for adjusting the individual carbon anode blocks of a multiple anode alumina reduction cell to the natural current-carrying capacity of each block, whereby full advantage may be taken of the natural tendency of the anodes to self-adjustment and whereby variations in anode-to-cathode spacings among the blocks may be eliminated.

As far as known there is no similar method in the art. General reference is made to U.S. Patent 2,930,746 to Cooper and to U.S. Patent 2,958,641 to Reynolds which show alumina reduction cells.

The principal object of this invention is to provide a method of and apparatus for adjusting carbon anodes in a reduction cell or cells.

Another object is to provide a method of adjusting carbon anode blocks in an alumina reduction cell.

Yet another object is to provide a method of adjusting carbon anode blocks in a multiple anode alumina reduction cell to the natural current-carrying capacity of each block whereby variations in anode-to-cathode spacings among the blocks may be eliminated.

Still another object is to provide a method of preparing an electrical analog model of a reduction cell.

Other objects and advantages will clearly appear from a description of a preferred embodiment as shown in the accompanying drawings, in which:

FIG. 1 shows schematically an alumina reduction cell having multiple anodes;

FIG. 2 depicts an electrical analog of an alumina reduction cell; and

FIG. 3 represents pseudo-areas of anodes.

Similar reference characters are applied to similar elements throughout the drawings.

As is well known in the industry, the carbon from the anode of a reduction cell is converted into carbon dioxide gas during the electro-chernical reduction process so that the carbon blocks or anodes must be replaced periodically. Generally it is presently the practice to selectively replace one or more of the carbon blocks in each plural 3,491,002 Patented Jan. 20, 1970 anode cell during each work shift. A carbon block replacement schedule is followed so that the average age of the carbon blocks in each section of the cell is maintained substantially constant. Where a pot line includes a large number of multiple anod cells, for instance cells each having twenty-four carbons, anode positioning and replacement is a constant and costly process that permits much error and inefficient cell operation.

Those skilled in the art recognize that at perfect adjustment substantial differences in current carrying capacity exist among the several blocks that compris the multiple-block anode, but in conventional practice the operator still adjusts each block to about the same current loading, primarily because he has no method of accurately predicting what the current loading should be for each block, and because he has relied upon the natural tendency of the system to adjust itself during operation. I have found, however, that self-adjustment proceeds so slowly and is so frequently upset by attempts of the operator to adjust the individual blocks of a multiple block anode that a high degree of adjustment cannot be maintained and full benefit cannot be taken of the natural tendency of the anode to self-adjust. Even after the operator has attained an apparently satisfactory adjustment of the blocks he often finds, after he has had occasion to reduce the anode-to-cathode spacing, that the anode blocks are no longer in adjustment.

When the individual blocks of the anode have been adjusted to their natural current-carrying capacity in accordance with this invention the anode maintains its adjustment over long periods of time. The need for frequently adjusting the level of individual blocks is substantially reduced, and the anode-to-cathode distance may be changed as desired Without the appearance of sodium flames or other manifestations of maladjusted anodes.

An important type of alumina reduction cell, indicated generally by the numeral 11, is shown schematically in FIG. 1. This type of cell, known in the industry by various names such as prebaked, Niagara, etc., is particularly distinguished from other types of reduction cells by virtue of its anode. The anode in this type of cell comprises a plurality of carbon blocks 12, each of which are individually connected to the positive side of a source of direct current electricity represented in FIG. 1 as an anode bus 13. Each carbon block 12 is individually adjustable with respect to a carbon cathode 15, and the set of carbon blocks that comprise the anode is vertically adjustable with respect to the cathode 15 in such a manner that the position of each block relative to the other blocks of the set is unchanged.

=Each carbon block 12 is connected through an iron stub 16 cast in the block to a copper anode rod 22 which in turn is clamped to anode bus 13 by a hand operated clamp 17. Anode bus 13 is supported at each end by a bridge jack 18 attached to cell frame 19 whereby the anode bus 13 is raised or lowered with respect to the carbon cathode 15 and a layer of molten aluminum 20 which overlies the cathode 15, effecting a uniform increase or decrease respectively in the thickness of a layer of molten cryolite 21 disposed between the neighboring surfaces of the carbon blocks 22 and the molten aluminum 20. Individual carbon blocks 12 may be raised or lowered by an operator with respect to the anode bus 13 and cathode 15, after loosening the appropriate clamp 17, with a conventional hand jack (generally used by the industry) that operates between clamp 17 and rod 22.

The electrical circuit through the cell consists (in the order named) of the anode 13, the anode rod 22, the stub 16, carbon blocks 12, the molten cryolite layer 21,

molten aluminum layer 20, carbon cathode 15, current :ollecting members 23, and the cathode bus 14. The anode bus 13 and the cathode bus 14 are connected to a. suitable source of DC power (not shown). The major resistance to electrical current flow resides in the molten cryolite layer 21, and the total cell current distributes itself among the plurality of parallel-connected carbon blocks 12 in inverse relation to the thickness of the molten cryolite layer 21 between each of the carbon blocks and the molten aluminum layer 20.

The current flowing in each of the anode rods 22 may be measured with a suitable current-measuring means which in this embodiment comprises a suitable millivoltmeter 24 connected by suitable circuit elements 25 to opposite ends of precalibrated lengths 26 of the anode rod 22. The relationship between the reading of the millivoltmeter 24 and the current flowing through section 26 of rod 22 is readily determined before installation in the cell by passing a known current through rod 22 while noting the reading on millivoltmeter 24. Various other means known to the industry may be used to measure the current in rod 22.

FIG. 3 shows the respective pseudo-areas and current loads of a representative portion of the blocks of a 24 carbon anode as determined at a time when the blocks had attained the listed ages expressed in units of 8-hour shifts. The cell was operating at 65,000 amperes of an 18-shift carbon changing schedule. The dashed box 27 represents the size of a pseudo-area having a short side 38 and a long side 39. Each anode has a corresponding pseudo-area.

The method of adjusting each of the individual blocks to its natural current-carrying capacity comprises the steps of:

(a) Determining the size of a pseudo-area for each of the individual blocks of the anode;

(b) Adjusting the vertical position of each block so that the current through each block bears the same relationship to the total current as the pseudo-area of each block bears to the sum of all the pseudo-areas.

The current may be measured for each individual block by means of the meter 24, and the adjustment of the block may be made by means of a hand jack after loosening the hand clamps 17.

The size of the pseudo-area 27 for each block may be accurately determined from electrical analog models of the anode and cell, constructed in accordance with the principles used in the construction of such models for the determination of the electrical or thermal conductance of complex shapes. The pseudo-area 27 may be determined in accordance with the electrical analog model of FIG. 2 so that the pseudo-area is equivalent to the cross-sectional area of a hypothetical column of molten electrolyte, equal in height to the anode-to-cathode distance of the cell. Values of the pseudo-area 27 determined from such a model, and the desired current loadings of each block calculated therefrom, are shown for each of the blocks represented in FIG. 3. It is evident that the natural proportion of the total current that each block should carry properly adjusted is strongly dependent on the environment of the block as well as on its age, and that if the blocks are adjusted to carry substantially an average current load as in the conventional manner substantial variations of anode-to-cathode distance must exist under the set of blocks that comprise the anode.

To properly position the individual blocks 12 according to this invention the current in anode rod 22 is measured with a suitable current measuring means, for example the calibrated section 26 and millivoltmeter 24, and the block 12 raised or lowered as needed so that the ratio of the current in rod 22 of said block 12 to the total current of the cell is equal to the ratio of the pseudo-areas 27 of said block 12 to the sum of the pseudo-areas 2,7 for all of the blocks 12 in the cell.

When first applying this invention to a cell in which the blocks 12 have been adjusted in the conventional manner, it is preferred to make a first partial adjustment of all the blocks 12, raising or lowering each block about two-thirds of the amount needed, and then to make a second adjustment of each of the blocks 12, bringing each to its natural current loading. When a new block 12 is set in a cell which has been adjusted according to this invention it is adjusted to about percent of its natural current loading three hours after setting and to percent of its natural current loading four hours after setting. Usually the adjustment at three hours is all that is necessary.

When the blocks 12 of a cell have been adjusted according to this invention no further adjustment is required unless the position of one or more of the blocks is mechanically disturbed, for example, by slippage of clamp 17 or by bending of an anode rod 22. The anodeto-cathode distance of the set of blocks 12 can be adjusted substantially at will by operating bridge jack 18 without the appearance of sodium flames or other manifestations of carbon maladjustment.

The method of estimating an area YnZn which is proportional to the pseudo-area 27 of block it is illustrated in FIG. 2 in which is shown schematically a vertical plane passing lengthwise of the cell and near the centers of blocks 12 along one side of the cell. The elevation view is drawn to scale on electrically conductive paper outlined by the borders 28. The anode blocks 12-1, 12-2, 123, 12-n and the aluminum metal cathode 29 are painted on the paper with electrically conductive silver paint. Each block 121 12-n is connected in parallel with, say, the positive pole of a source of electrical potential (not shown) through a bus 30. In series with each block is a milliammeter 31. The aluminum metal cathode 29 is connected with the negative pole of said electrical source through a negative bus 37. A milliammeter 32 may be inserted in negative bus 37 if desired, for use in checking the readings and the computations. Voltmeter 33 is across the supply. The edge 34 of the paper is cut to represent, in accordance with measurements, the endwall ledge of the reduction cell. The anode-to-cathode distance is represented by the dimension 35 and the height of molten electrolyte on the vertical surface of the carbon blocks is represented by the distance 36. The electrical potential E between positive bus 30 and the negative bus 37 is measured by voltmeter 33. The blocks 12 are shown as rectangles 12-1, 12-2, 123, 12-n for ease of illustration. However, the precision of carbon adjustment may be improved as the shape of the blocks 12n more nearly conforms to the observed shape of the blocks 12 of the reduction cell.

When the busses 30, 37 are energized the resistances R1, R-2, R-3, Rn of the parallel circuits through blocks 121, 122, 12-3, 12-n are readily calculated as the ratio of the voltage read on voltmeter 33 to the individual circuit currents read on ammeters 31. From the resistivity R of the paper and the length (X) of the dimension 35, a length (Y) which is proportional to the length of the side 38 of pseudo-area 27 of FIG. 3 is obtained from the relation Y =R X/R Values of Y Y Y are similarly calculated for all of the blocks 12 represented in FIG. 2, which may comprise all of the blocks 12 along one side of the reduction cell.

Values of Y for the blocks along the other side of the reduction cell are determined in the same manner from a similar model wherein the blocks 12 are drawn according to the shape of the blocks on said other side.

Values of Z Z Z Zn, Which are proportional to the length of side 39 of pseudo-area 27, are determined in the same manner from scale models of planes bisecting opposite carbons in the rows, for example, carbons 3 and 6, carbons 11 and 2, etc., of FIG. 3.

The pseudo-area 27 is representative of the effective conduction area of the corresponding carbon block (/1),

and, is proportional to the product of Z and Y and the proportion of the total cell current I to be carried by carbon block (n) is given by the ratio of Y Z to Z [('Y )(Z,)] wherein the summation i is taken over all the blocks in the cell.

The block (it) in the cell is set in a vertical position, in accordance with the reading of the rod meter 24, t0 the current given by the relation I :1 -Y Z EKY Z whereby the block (11) is set to its natural currentcarrying capacity.

Values of the ratios may be determined, as set forth above, for the set of blocks at various ages over the carbon-setting cycle. These values may then be used repetitively for setting the carbons during each successive cycle so long as the carbon-setting cycle is not changed.

This invention rests on the discovery that the natural current-carrying ability of an anode which in operation shows a substantial electrochemical overvoltage, about 30 percent of the cell voltage for alumina reduction cells, can be determined in the same manner as the natural current-carrying ability of an electrochemically inert system, thereby eliminatin the difiiculty which has heretofore forestalled advances in the methods of operating electrochemical cells.

Those skilled in the art, being apprised of this discovery by virtue of my detailed description of one embodiment of this invention, may readily devise other means of practicing this invention without departing from the concepts of discovery and teaching. Accordingly, it will be apparent that the embodiment shown is only exemplary and that various modifications can be made within the scope of my invention as defined in the appended claims.

What is claimed is:

1. A method of adjusting individual carbon blocks of an anode that is adustably spaced with respect to the cathode of a reduction cell comprising the steps of:

determining for each anode block the size of a pseudoarea corresponding to the cross sectional area of a column of molten electrolyte having a height equivalent to the distance between the anode and cathode of said cell and a resistance equivalent to the resistance between the particular anode block and the cathode, said determination of each such pseudoarea comprising the steps of:

(i) determining a first resistance value between each individual anode block and the cathode, said first resistance value being determined for a first vertical cross section of said cell;

(ii) relating each of said first resistance values to a first linear dimension for each of said anode blocks, each of said first linear dimensions representing a first side of the corresponding anode blocks pseudo-area;

(iii) determining a second resistance value between each individual anode block and the cathode, said second resistance value being determined for a second vertical cross section of said cell orthogonal to said first vertical cross section;

(iv) relating each of said second resistance values to a second linear dimension for each of said anode blocks, each of said second linear dimensions representing a second side of the corresponding anode blocks pseudo-area;

(v) multiplying each of said first linear dimensions by the second linear dimension corresponding to the same anode block so as to determine each anode blocks pseudo-area; and

adjusting the vertical position of each individual anode block so that the current therethrough bears substantially the same reltionship to the total current through all of said anode blocks as the respective pseudo-area of each anode block bears to the sum of all said pseudo areas.

2. The method of claim 1 wherein said pseudo-areas are determined by means of analogue models.

3. The method of claim 2 including the steps of:

constructing a firsttwo-dimensional analogue model of said cell representing said first vertical cross section thereof; and

constructing a second two-dimensional analogue model of said cell representing said second vertical cross section. 4. The method of claim 3 wherein the determination of the resistance value between each of said anodes and the cathode of said analogue models includes the steps of: connecting a source of electrical power across said analogue models;

measuring the current and voltage values between each anode and the corresponding cathode of the analogue models; and

dividing each voltage value by the corresponding current value.

5. The method of claim 3 wherein the relating of each said resistance value to each said corresponding linear dimension comprises the steps of:

determining the resistivity between the related anode and the cathode of the corresponding model; determining the anode-to-cathode distance of the corresponding model; and

dividing the product of said resistivity and said anodeto-cathode distance by said corresponding resistance value.

References Cited UNITED STATES PATENTS 3,128,371 4/1964 Spaulding et a1. 3,345,273 10/1967 Brown 20467 XR JOHN H. MACK, Primary Examiner D. R. VALENTINE, Assistant Examiner US. Cl. X.R.

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