US3159561A - Method and means for reducing or eliminating level variations of molten metal in high amperage electrolysis cells - Google Patents

Description

F illedi Jimmy: 5, 15961 2&4

INVENTORS JACQUES BOC QUE N T/N JEAN c/war B ATTORNEY Deck 3 J1 Emanuel-WIN ETAL 95 METHOD: AND; MEANS FQRi REDUCING 0R5 ELIMINASTING LEVEL VAR A'EIONS an" MOETENZ Mama IN: HIGH AMPERAGE' smc raomsrsr mamas Eileidi JhIy" 5 I961 Sheets-Sheet 5* INVENTORS JACQUES BOCQUENT/N JEAN G/VRV ATTORNEY United States. Patent METHGD MEANS FQR. REDUCING OR ELHWI- VARIATIONS 0F MGL'I'EN METAL, IN HIGH AMPERAGE ELECTROLYSE NATING LEVEL The present application is a continuation-in-part of our application Serial No. 641,530, filed February 21, 1957,

' now abandoned.

In the manufacture of aluminum, there are used more and more frequently electrolysis cells operating with 100,- 000 amperes and higher, in which the elfect of the magnetic fields produce level variations of the upper surface of the liquid metal which lies on the bottom of the crucibles of these cells.

These level variations may be static, the upper surface of the metal remaining motionless. They mayalso result from the more or less rapid movements of the metal, which lowers appreciably the efliciency of the electrolysis.

Accordingly, it is an important object of the present invention to provide a method and means for reducingas far as possible these variations in the level oftheliquid metal, or even to suppress them altogether.

High amperage electrolysis cells generally have a rectangular elongated shape, one of the sides of the rectangle being appreciably longer than the other. In the following description, the magnetic field and the current density at any point of the liquid metal will be defined by their projections on three coordinate axes starting from the same origin 0, the central point of the bottom of the crucible of the electrolysis cell. The Ox axis is parallel to the longer side of the electrolysis cell; the 0y. axis lies in the same horizontal plane as the Ox axis and is perpendicular to the latter (hence, parallel to the smaller side of the rectangle). The Oz axis is vertical and, hence, perpendicular to the .xOy plane.

Let B denote the value of the magnetic field at a given point, and Bx, By, Hz the projections of B on the Ox, 0y and Oz axes. respestively; I

Let I denote the value of the current density, and Ix, 1y, 12, the projections of J on the Ox, Oy and vOz axes respectively.

The static level variations of the upper surface of the liquid metal are proportional to, the products of the vertical component of the field. by the horizontal components of the, current density, i.e.

It has been proposed to suppress the horizonal longitudinal currents I x by judiciously dimensioning (calculating) the conductors which connect one cell to the succeeding cell so as to distribute the current properly in the bottom of the crucible. This arrangement-which has been proposed by others and is not claimed hereis elfective.

It has likewise been suggested by others, and, hence, is not the subject of claims herein, to reduce. or suppress the transverse currents Jy by maintaining over the entireperiphery of the liquid metal sloping masses of solidified fluorides so as to obtain a cathodic surfacesubstantially equal to the anodic surface. Good electrolysis efliciencies are obtained in this manner, even though magnetic fields still exist. "However, the more the magnetic fields are reduced, the better and the more stableis the. operation of "ice the electrolysis cell, should the current happen to spread out accidentally, that is to say when the Jy component is not zero.

It is heretofore been proposed by others-and is not the subject of claims herein-to reduce the magnitude of the magnetic field B by dividing the conductors outside the cell into as large a number as possible of smaller conductors, arranged with their upper endssimulating sheets and spaced from the metal. A marked improvement is achieved in this manner, but these arrangements interfere sometimes with the Work of the operators.

Finally, it has been proposed to suppress the magnetic fields by placing the conductors far away from the liquid metal and by placing, side by side, conductors through which flow currents in opposite directions. Unfortunately, this suggestionmade by others and not claimed herein--requires a large investment in conductors and leads to excessive power losses.

The present invention, which is. based upon applicants researches, makes it possible to avoid the above disadvantages while attaining a satisfactory stable operation of high amperage electrolysis cells even when they are arranged lengthwise, that is. to say, when the long sides of the rectangle of one cellare placed in the prolongation of the long sides of the rectangle of the preceding and of the succeeding cells in the direction of the current flow.

The invention, its advantages and the manner of its ap' plication, will be described with reference to the annexed schematic drawings whcih relate to an improved system for the igneous electrolysis of a molten bath of metallic compounds to produce molten metal in a cell comprising a crucible for containing the molten metal, an anode vertically supported above said crucible, a cathode, a feeder system for supplying current to the anode, current outlet means associated with said cathode, and conductor means connecting the current outlet means of the cell to the feeder system of asucceeding adjacent cell.

In the drawings:'

FIGURE 1 is a perspective diagrammatic view showing' the volume of liquid metal lying in the bottom of the crucible of the electrolysis cell;

FIGURE 2 shows the lengthwise arrangement of a line of' cells successively traversed by a current of the same intensity, the direction. of the current being indicated by an arrow;

FIGURE 3 is a view in vertical section and perpendicular to its greatest length of the cell crucible;

FIGURE 4 is a plan view of the cell of FIGURE 3;

FIGURE 5 is a view in elevation of two successive cells, in the direction of their greatest length, the current being supplied to the cells at one end only;

FIGURE 6 is a view similar to that of FIGURE 5, but with a modified current supply, and

FIGURE 7 shows a sectional view of the electrolysis cell as in FIGURE 3, together with curves, the intersection of which enables determination of the optimum position of. the conductors according to the present invention, as will be described below.

FIGURE 8 isalso a view in vertical section and perpendicular to its-greatest length of a cell in a lengthwise arrangement of a line of cells (as illustrated in FIGURE 2) wherein the conductors are shown disposed as they were according to an actualexample of the prior art practice, while FIGURE 9 is a similarview of the same cell, but with the conductors disposed in accordance with the teachings ofthe present invention. a

In the various figures, the same or equivalent parts are ceding-cell and distributes. it to. the anode system of the cell. 3, 3 represent the conductors in their means position disposed parallel to the greatest length of the cell. 3, 3' (FIG. 4) represent the ends of the conductor which collect the current leaving the preceding cell; they are electrically connected to the feeder system 2, 2 which, in the case of FIG. 4, is supplied with current at one end only. This is also true in the case of the two successive cells shown in FIGURE 5, where the feeder 2 is supplied at one end only.

- In contrast, FIGURE 6 shows an arrangement of two cells similar to those of FIGURE 5, but wherein the feeder system 2 is supplied with current at its both ends. The current leaving the preceding cell and brought by conductor 3 is divided in this case into two currents, and a conductor 4-parallel to 3-connects with the other end of feeder system 2. With such an arrangement, the conductors 3 in FIGURES 3 and 4 would represent the mean position of the system of conductors 3 and 4 of FIGURE 6. These conductors 3 and 4 can be arranged as a plurality of conductors of smaller size disposed to simulate sheets, as previously proposed.

The lengthwise arrangement of the electrolysis cells is adopted very often because it saves space and facilitates the operation; however, the magnetic eifects may be particularly large.

The method which is the object of the present invention consists in nullifying the magnetic effects at the center of the crucible of the electrolysis cell. While these effects will still exist on the rest of the cell surface, they will be very weak in the neighborhood of the center 0 and, in practice, their magnitude shows a certain symmetry with reference to that point, which insures sufiicient stability in the operation of the electrolysis cell.

The vertical component Bz of the field produced by the horizontal conductors situated near the cell is reduced to Zero by reason of symmetry.

The transverse component Jy of the current density is likewise reduced to zero at the center of the cell for reasons of symmetry, and so is Ix; hence, at that point, there are no magnetic effects of static origin.

The effects of dynamic origin depend on the value of a rotational vector R which can be calculated by means of the values of the components Bx, By, Hz and of the current densities Ix, Jy, Jz and their partial derivatives.

The mathematical expressions for the projections of the vector R on the Ox, Oy and Oz axes are as follows:

..as Jx=Jy=O. The derivative variation of the vertical component of the field along the 4 height of the liquid metal, is also zero.

dJz y variation of the vertical component of the current density along the transverse direction, is zero, Jz being practically constant over a very wide zone about the center 0.

Hence, at the center 0 of the cell, the components Rx and Rz of the rotational vector are zero and there re mains:

The derivative dJy y dBy Ry=By Therefore, rotational movement of the metal about an axis parallel to 0y may occur at the center of the cell. To prevent this, Ry must be reduced to zero. However, 12, the vertical component of the current density, cannot be reduced to zero, neither can which expresses the variation of this transverse component of the field along the vertical.

In order to compute the component By, in FIGURE 3 let a be the distance between the feeder 2 and the Oz axis;

b the vertical distance of the line passing through the middle (mean) points of conductors 2 to the center of the cell;

c the distance between conductor 3 and the Oz axis; and

d the vertical distance between 0 and a line passing through the middle (mean) points of conductors 3.

a1 will designate the intensity of the current supplied to feeder 2 at the side of the preceding cell (FIG. 4), and (1a)l theintensity of the current supplied to feeder 2 from the opposite side (FIG. 6). In the case of FIG- URE 5, 11:1.

The field produced at O by the current flowing through conductors 2 and 3 is calculated in the same way as if it were the case of conductors of unlimited (indefinite) length carrying the currents which actually flow through the conductors in the YOZ plane. Experience proves that there is obtained an approximation which is sufficient for practical purposes. I

The following results are obtained:

Conductors 2,2 Conductors 3,3

131! V 2I(a}) 2I(ad dBy (h -a 2+ 2 z 2+ 2 2 To reduce simultaneously to zero By and dBy it is necessary to have the following two conditions:

Equation 2 The term on can be eliminated from the two equations.

which can replace the second of the above equations.

Given a and b, i.e. the position of the feeders 2-2 which is generally determined by the requirements of construction and operation of the cell, it will be seen that the third equation represents a cubic curve independent of the current distribution a in the feeders.

The first equation represents a circle the radius of which varies with a. The distance a is generally small; if we assume that it is zero, then, we obtain conditions as illustrated in FIGURE 7. Here, 5 is the cubic curve on which the conductors 3, 3 must always be placed; 6 is the circle corresponding to oe=1, ie the situation where current is. supplied at one side only, with circle 6 having a radius of b/Z; 7 is the circle corresponding to oc= /3 and a radius of 512/2. The mean position of conductors 3 will then be at 8 in the case where the feeder system is supplied with current at one end only, and at 9 when the feeder system is supplied with current at both ends, with We of the current entering at the end nearest the preceding cell, /3 by the other end.

The position of the conductors is then determined by the following table:

and

all: (1/!) It has been assumed that the height b of the feeder 2 above the bottom of the crucible is known. This is the case most often; on the other hand, it is possible to assume that one of the dimensions 0 or d is known and the above equations can then be solved without departing from the scope of the invention.

The above defined position for the side conductors represents the mean point of the conductors which can be divided into as large a number as possible of parallel conductors arranged in a manner simulating sheets about this mean point.

The examples given below, which are not given by way of limitation, Will enable a better understanding of the advantages resulting from the application of the present invention.

Example 1 A 100,000 ampereelectrolysis cell, arranged lengthwise and the feeder 2 of which was supplied with current at both ends (FIG. 6), presented a bad distribution of the magnetic fields, namely, a weak field at the side of the preceding cell, a strong field at the center and very strong field at the side of the succeeding cell. The liquid metal on the bottom of the cells crucible was subjected to rota site side (the side of the next cell). By disposing the conductors 3, 3 according to the method of the present invention, the magnetic efiects 'beperiphery ofthe crucible.

much more stable andthere is obtained a reduction of the energy consumed amounting to 800 kwh. per ton come zero at the center 0 of the cell. They are substantially symmetrical with reference to the, point 0. The variations of level of the upper part of the liquid metal practically disappear and it is possible to maintain regular inclined masses of solidified fluorides over theentire The operation of the cell is Example 2 A $0,000 ampere electrolysis; cell, equipped with-prebaked anodes, arranged lengthwise andhaving feeders 2, 2 supplied with current on one side only (FIG. 5), and with the conductors 3, 3 placed on a. level with the bottom of the crucible (d=0r), was subjected to. magnetic fields having a very large horizontal'component- By at the side of the preceding cell, a component. half this value in the vicinity of point Owhich is reduced to zero onthe opposite side but having, in contrast, a large vertical component Bz.

The celloperated in a very unstable manner and all the phenomena describedin Example: I were observed. By changing theposition of the. conductors 3', 3 in accordance with this invention to point 8 on FIG. 7, an improvement similar to the one already described. was obtained, together with a more-stable operation. Moreover, the efiiciency of the electrolysis was increased by more than 3%, changing from 84 to 87% of the theoretical efiiciency.

It should be mentioned that in the practical design of a cell system, applicants. generally first select convenient values of a and b and that is, the position of the feeders or bus-bars (monture"), these values being governed by practical considerations, such as ready access, etc. The

; values of c and d are then calculated, i-.e. determined for tors in one cell of a line of Soderberg cells arranged lengthwise which were in actual operation before the present invention. .FIGURE. 9 represents. the same cell but with the conductors disposed in accordance with the teachings of the present invention. In both instances, the cell operated with a current of 90,000 amperes. The cell of FIGURE 8 had the following characteristics:

The feeder system (monture) 2, 2 was supplied at one end only from the preceding cell, hence a the mean distance between the feeder 2 and the Oz axis, was 0.50 m.;

b, the vertical distance of the line passing through the middle (mean) points of conductors 2, 2, to the center of the cell, was 2.60 m.; 1

c, the mean distance between the Oz axis and the conductor 3 collecting the current from the cathode was d, the vertical distance between the center of the cell and a line through the middle (mean) points of conductors 3, 3, was zero, that is to say, the conductor 3, v3 were positioned on the 0y axis.

The cells sufiered. from severe magnetic effects: There took place an unlevelling, or level deformation, of the separating surface between the bath and the metal of the order of 10 ems, this surface rising in the direction of 7 the current flow in the cells, i.e. rising in the direction from A to B (cf. FIG. 2). Further, there were observed systematic temperature differences of the bath between the two ends of the cell, the zone where the level of the metal was highestbeing the coldest.

Additionally, the shape of the solidified lateral inclined mass (talus) of fluorides of the bath was asymmetrical. It was quite pronounced in the zone (region) B (FIG. 2) where the height of the metal was a maximum and where, also, several centimeters of mud appeared on the bottom of the crucible. An inclined frozen mass (talus) was practically non-existent on the opposite side A where the bath and the metal appeared to be in continuous agitation. At the same time, the operation of these cells were unstable and inefiicient; the operating results were inferior to those of smaller cells; the average voltage was higher (by about 0.15 volt) and the yield of the current smaller (by about 5%). Actually, the yield of the current (Faraday yield) was 82% in lieu of 87%.

The cell of FIGURE 8 was subsequently modified in accordance with the teachings of the present invention in the manner illustrated in FIGURE 9. That is, the feeders 2, 2 were left in the same position but were supplied with current at both ends. FIGURE 9 shows the cubic curve 5 satisfying the equation given earlier in the specification, and further shows the various circles corresponding re spectively to the value of =1 100-0 11:0.9 90-10 =01; (80-20) =07 70-30 and (1:05 (60-40) As will be seen from FIGURE 9, the most advantageous position of the conductors 3, 3 is that corresponding to 06:0.8 (8020), because the conductors are not under the sub-structures of the cell and, yet, are not too far away therefrom. The conductors 3, 3 are those which collect the current leaving the cathode, and 4 is the conductor which supplies the 20% of the amperes to the end B of the feeder system 2, 2. H

As shown on FIGURE 9, the point of intersection of the circle 8020 wdith the cubic 5 is the mean point of the conductor assembly 3, 3 and 4. The coordinates of the conductors are =0.50 m. b=2.60 m. c=2.80 m. d=1.56 m.

When the cell was modified in the manner just described, the detrimental etfects of magnetic origin disappeared in the cell: the upper surface of the liquid metal became substantially horizontal, the solidified inclined masses (talus) of the bath were uniformly shaped and easy to maintain. Moreover, the energy consumption (kw./hour) per ton of aluminum was reduced from 16,750 kw./h. to 15,250 kw./h. per ton, that is to say, a gain of 1500 kw. per hour or 9% In the appended claims, the several symbols have the meaning given to them in the present specification.

We claim:

1. An improved system for the igneous electrolysis of a molten bath of metallic compounds to produce molten metal, and wherein detrimental deformation of the level of the molten metal is substantially inhibited, comprising in combination: a plurality of successively lengthwise disposed cells each including a crucible for containing the molten metal, an anode vertically supported above said crucible, a cathode; a system of feeders for supplying current to each said anode, said feeders being spaced laterally from the center of the crucible by a distance a and vertically above said crucible by a distance b, the distance a always being smaller than b, said distances a and b being determined arbitrarily; current outlet means associated with each said cathode; conductor means connecting the current outlet means of each cell to the feeders of the succeeding adjacent cell; said conductor means being spaced laterally from the center of the crucible by the distance c and vertically below and relative to the center of the crucible by the distance d, said conductor means being located at the point of intersection of a cubic curve passing through the center of the crucible and satisfying the equation and a circle, likewise passing through said center and in which a designates the proportion of the intensity of the current supplied to the feeders at the end nearest the preceding cell, on being 1 when all of the current is supplied to the end of the cell nearest the preceding cell.

2. A system according to claim 1, wherein the conductor means comprises a number of parallel conductors, and the lateral distance c equals the mean position of the parallel conductors from the center.

References Cited in the file of this patent UNITED STATES PATENTS 2,761,830 Kibby Sept. 4, 1956 FOREIGN PATENTS 740,025 Great Britain Nov. 9, 1955 740,063 Great Britain Nov. 9, 1955 167,946 Australia July 12, 1956

Claims (1)

1. AN IMPROVED SYSTEM FOR THE IGNEOUS ELECTROLYSIS OF A MOLTEN BATH OF METALLIC COMPOUNDS TO PRODUCE MOLTEN METAL, AND WHEREIN DETRIMENTAL DEFORMATION FO THE LEVEL OF THE MOLTEN METAL IS SUBSTANTIALLY INHIBITED, COMPRISING IN COMBINATION: A PLURALITY OF SUCCESSIVELY LENGTHWISE DISPOSED CELLS EACH INCLUDING A CRUCIBLE FOR CONTAINING THE MOLTEN METAL, AN ANODE VERTICALLY SUPPORTED ABOVE SAID CRUCIBLE, A CATHODE; A SYSTEM OF FEEDERS FOR SUPPLYING CURRENT TO EACH SAID ANODE, SAID FEEDERS BEING SPACED LATERALLY FROM THE CENTER OF THE CRUCIBLE BY A DISTANCE A AND VERTICALLY ABOVE SAID CRUCIBLE BY A DISTANCE B, THE DISTANCE A ALWAYS BEING SMALLER THAN B, SAID DISTANCES A AND B BEING DETERMINED ARBITRARILY; CURRENT OUTLET MEANS ASSOCIATED WITH EACH SAID CATHODE; CONDUCTOR MEANS CONNECTING THE CURRENT OUTLET MEANS OF EACH CELL TO THE FEEDERS OF THE SUCCEEDING ADJACENT CELL; SAID CONDUCTOR MEANS BEING SPACED LATERALLY FROM THE CENTER OF THE CRUCIBLE BY THE DISTANCE C AND VERTICALLY BELOW AND RELATIVE TO THE CENTER OF THE CRUCIBLE BY THE DISTANCE D, SAID CONDUCTOR MEANS BEING LOCATED AT THE POINT OF INTERSECTION OF A CUBIC CURVE PASSING THROUGH THE CENTER OF THE CRUCIBLE AND SATISFYING THE EQUATION

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