US3496089A

US3496089A - Electrode construction

Info

Publication number: US3496089A
Application number: US573099A
Authority: US
Inventors: Fin Enok Folkestad
Original assignee: NORSK HYDRO ELEKTRISK KVAELSLO
Current assignee: NORSK HYDRO ELEKTRISK KVAELSLO; NORSK HYDRO ELEKTRISK KVAELSLOF AB
Priority date: 1965-09-01
Filing date: 1966-08-17
Publication date: 1970-02-17
Anticipated expiration: 1987-02-17
Also published as: SE313548B; FI44585B; GB1162551A; IL26377A; DE1671462B1; DE6607971U

Description

Feb. 17, 1970 F. E. FoLkl-is-rAD 3, 9

ELECTRODE CONSTRUCTION Filed Aug. 17, 1966 3 Sheets-Sheet 1 INVENTOR FINN ENOK FOLKESTAD ATTORNEYS 3 Sheets-Sheet 2 F. E. FQLKE-IAD ELECTRODE CONSTRUCTION Feb. 17, 1970 Filed Aug. 17, 1966 I'NVENTOR ATTORNEYS FINN ENOK FOLKESTAD Feb. 17; 1970 F. E. FOLKESTAD 3,496,039

. I -ELEM:'IRQDE CONSTRUCTION Filed Aug. 17, 1966 s Sheets-Sheet 5 INVENTOR FlNN ENOK FOLKESTAD United States Patent M US. Cl. 204-288 6 Claims ABSTRACT OF THE DISCLOSURE An electrode construction for the electrolysis of salt fusions in an electrolytic cell employing electrodes having substantially vertical working surfaces with a cathode on each side of an anode having openings in the lower part. Cathodic members extend from one cothode to the other and run through the openings in the intermediate anode. The cathodic members are vertically arranged plates and the openings are downwardly open, vertically elongated slits. The anode is composed of alternate longer anode rods and shorter anode rods located side by side and the lower part of the shorter anode rods are shaped to guide the flow of the melt outwardly from the mid-plane of the anode. The lower part of the shorter anode rods is cut symmetrically to form an edge.

This invention relates to an electrode construction for the electrolysis of fused salts in an electrolytic cell employing electrodes having substantially vertical working surfaces, and more particularly for the production of magnesium and chlorine from a MgCP-containing fusion.

The prior art comprises a number of types of electrolytic cells for the electrolysis of molten chlorides, in particular chlorides of metals of groups 1 and 2 of the periodic system of the elements. With respect to magnesium more particularly, the preponderant part of the world production is being produced electrolytically from molten salts. The more common cells therefor have been described in Chimie et Industrie, vol. 92, No. 2 (August 1964), p. 125134. The electrolyte used in such cells usually consists of alkali metal chlorides and alkali earth metal chlorides, and possibly minor amounts of fluorides. The cathodes are usually made of iron or steel and the anodes of graphite.

It is an object of the invention to provide an electrode construction which enables the achievement of a high yield of metal per unit of electrical energy consumed.

Another object of the invention is to provide an electrode construction which permits a high production capacity of a cell of a given size.

Still another object of the invention is to provide an electrode construction which permits simplification and rationalization in operating the electrolytic cell.

A further object of the invention is to provide an electrode construction resulting in a relatively low overall cell resistance and thus in a low cell voltage at a given amperage, and permitting a relatively high amperage to be used at a given voltage.

Other objects of the invention will be apparent from the following description.

The invention thus relates to an electrode construction for the electrolysis of fused salts in an electrolytic cell employing electrodes having substantially vertical working surfaces, more particularly for the production of magnesium and chlorine from a Mgcl -containing fusion, and the electrode construction according to the invention is characterized by transverse cathode members arranged in the electrolysis zone between the electrodes, which cathode members extend into the anode zone.

3,496,089 Patented Feb. 17, 1970 According to a preferred embodiment of the invention, the electrode construction consists of an anode, a cathode on each side of the anode, and transverse cathode members connecting the two cathodes and running through recesses in the intermediate anode. Each of the transverse cathode members is preferably welded to the cathodes.

The term transverse as used herein to describe the cathode members of the invention means transverse relative to the substantially vertical electrodes. It will be understood, however, that this term is not limited to mean perpendicularly or normally arranged relative to the substantially vertical electrodes, as such mode of arrangement is not a necessary requirement for the usefulness of the transverse cathode members. The essential requirement is that the transverse cathode members extend sufficiently far in horizontal direction to extend into the anode zone, i.e. so that part of the transverse cathode member will be within the general contour of the anode, and preferably so that the transverse cathode members extend all the way through the anode as described above.

The transverse cathode members can have any suitable shape and cross section. Advantageously they are plates or rods, if desired with a somewhat streamlined cross section. I prefer, however, to use as transverse cathode members substantially vertically mounted plates.

In the case of an electrolytic cell employing anodes of rectangular cross section, such as a cell (developed by the LG. Farbenindustrie AG. and described in the abovementioned article in the Chimie et Industrie), the transverse cathode members are preferably arranged parallel with each other, evenly spaced from each other and at right angles with the general side plane of the anode. A more irregular mode of arrangement of course may be desirable when particular considerations so dictate, for instance to make room for other appurtenances of the cell, or to provide accessibility for means for removal of settlings from the cell, etc. For such and similar reasons it may also be desirable to give one or more individual transverse cathode members a particular shape.

Such and the like variations in the electrode construction of the invention may also have their motivation in the shape of the anode. In the case of cell constructions comprising cylindrical anodes and concentrically arranged cathodes, the transverse cathode members suitably can be star-shaped, the centre of the star being on the axis of the anode, so as to divide the lower part of the anode into a number of segments, preferably 3-6 segments However, also in the case of cylindrical anodes the transverse cathode members can be parallel plates. Such and the like variants and modifications will be understood to be embodiments of the invention all of which are intended to fall within the framework of the invention.

The distance between the transverse cathode members when the anodes are substantially plate-shaped, such as is the case in cells of the electrolytic type, can vary within a broad range, but more suitably is 10-40 cm., and preferably from 15 to 20 cm.

According to one embodiment of the invention, the abovementioned recesses in the anode for passing the transverse cathode members therethrough are substan tially vertical slits in the lower part of the anode. Especially when the cross section of the anode has the shape of an oblong rectangle the lower part of the anode will thus have a fork-like shape. The number of fork prongs i.a. will depend on the horizontal length of the anode. In the case of a cell of the electrolytic type, whereinthe anodes are placed across the cell, the inside width thereof being for instance about cm., the number of fork prongs is suitably from 3 to 15, preferably from 6 to 10. An anode of this shape is advantageously constructed by using alternately long and short graphite rods which are placed side by side and held together in conventional manner by suitable clamping means and bolts. According to one embodiment of the invention, the lower end of the short graphite rods are so shaped that the flow of the melt which near an anode is directed upward due to the electrolytic gas evolution on the anode surface, is deflected outward from the midplane of the anode, i.e. away from the anode, and so as to facilitate the upward flow of the melt and avoid turbulence. In order to obtain satisfactory efficiency when using the electrode construction of the invention, it is important that the upward flow of the metal-containing melt from the regions around the transverse cathode members is not excessively mixed with the gas in the anode zone, since recombination of the respective products of the electrolysis, such as magnesium and chlorine, can be tolerated only when occurring to a very small extent.

The electrode constructions will be further explained in the following description referring to the drawing, which illustrates an embodiment of the invention. (The cell itself is not shown in the drawing.)

FIG. 1 is a plan view of an electrode construction according to the invention, and FIG. 2 is a vertical section along the line A-A of FIG. 1, and FIG. 3 is a side view of the construction shown in FIG. 1 (although curtains 7 have been omitted in FIGS. 1 and 2 for the sake of clarity). Identical parts shown in the drawing have identical reference numerals in all the figures, which show cathode shaft 1, auxiliary or front cathodes 2, transverse cathode members 3, long anode rods 4, short anode rods 5 with the lower part 6 thereof, and in FIG. 3 also curtains or partitions 7 have been indicated be tween the anode and the cathodes.

The lower end 6 of the short anode rods 5 have been shown (see FIG. 3) cut symmetrically to form an edge. If this edge is made very sharp it will soon be consumed during electrolysis. I prefer, therefore, to make this edge less sharp, i.e. blunt or rounded. If desired, the entire lower end 6 of the short anode rods 5 can be rounded, for instance so as to give the end 6 a more or less streamline shape. The preferred shape will i.a. depend on the construction of cell and the size thereof.

The electrode construction illustrated in FIGURES l-3 is suitable for use in an electrolytic cell. The anodes are placed transversally of the cell, each anode having, suitably spaced therefrom, a cathode at each side. Around the upper portion of each anode there is provided a gas-collectin cover or hood usually made of a ceramic material and forming said curtains or partitions 7, under which cover the gas evolving from the anode accumulates to be withdrawn through suitable means.

The conventional cell usually has 3-5 anodes and twice as many cathodes, a curtain or partition being provided everywhere between an anode and a cathode. Thus, the cell will be divided into a number of anode chambers and cathode chambers, the metal produced on the cathodes accumulating in the cathode chambers on top of the melt.

Dividing a cell into a relatively high number of such chambers results in a relatively large portion of the surface area of the bath being taken up by the many partitions between anode and cathode chambers. In the case of a conventional cell having 4 anodes, the partitions would normally take up more than 20 percent of the surface area of the bath. Such a cell has 5 cathode chambers from which produced metal is to be withdrawn at intervals, and the high number of metal withdrawal operations required per ton of metal produced is a drawback.

Another rather serious drawback ensuing from a relatively high number of partitions or curtains in a given cell is the fact that the frequency of curtain breakages resulting in interruption of the operation of the cell is likely to increase with the number of curtains per cell.

The electrode construction of the invention permits the useful surface area of the electrodes to be considerably increased over that of the conventional electrode construction. This is partly due to the fact that when using the electrode construction of the invention, one can advantageously increase the thickness of each anode and reduce the number of anodes per cell of a given size, which means fewer curtains per cell than what has been found suitable using the conventional electrode construction. In the case of a cell in which one would conventionally use 3 to 5 anodes and 6 to 10 cathodes, one can advantageously use one, two or three of the electrode assemblies shown in the drawing. The latter arrangement permits a considerable increase in the amperage of the cell.

The electrode construction of the invention also provides other advantages. Thus, when the bath is less divided up, as explained above, the assembly and operation of the cell can be simplified. More particularly, the withdrawal of metal from the cell can be rationalized.

The application of the electrode construction of the invention is not limited to electrolytic cells of a special cell type. Further, it is not necessarily required that the partitions of the cell be situated between anode and cathode as shown in FIG. 3 (curtains 7). Such partitions primarily serve the purpose of isolating from the atmosphere the gas evolved on the anodes and the partitions can well be placed in the peripheral parts of the cell outside of the electrode construction of the invention.

As mentioned above, an anode is constructed, according to an embodiment of the invention, from alternate long short anode rods. Comparing the surface area of such an anode with that of a similar anode wholly constructed from long rods, it will be understood that the first-. entioned anode (having a fork-like shape as shown in FIG. 2) will have the larger surface area if the thickness of the anode is greater than the width of the rod. I prefer, therefore, when constructing the anode in this way, to make the anode thickness greater than the width of the rod. It will be understood, however, that this is not a necessary requirement to obtain an improvement with respect to the overall electrical resistance of the cell, and even less is it a necessary requirement for the usefulness of the electrode construction of the invention when all the advantage thereof are taken into consideration. It should be recalled, in this connection, that the cathode surface area is increased very considerably in any case due to the transverse cathode members. Therefore, the invention is not limited to electrode constructions in which the thickness of the anode is greater than the width of the rods from which the anode is made.

Generally speaking, recesses of the shape and size desired can of course be provided in any suitable way, for instance, by cutting out recesses from an integrally made anode, or from any conventionally made anode.

Even when the transverse cathode members, for inst. between two cathodes in an electrolytic cell, are arranged underneath the intermediate anode and not through recesses in the latter, the advantages of the electrode construction of the invention can be obtained in some measure, partly because the area of the lower part of the anode can then be utilized more efiiciently in the electrolytic process. However, as mentioned, I prefer to place the transverse cathode members through recesses in the anode. The recesses, as well as the transverse cathode members, can extend upward as far as is desired, although not above the bath level, and, if desired, the upper contour of the transverse members can be suitably adapted in shape to the lower end of the curtains 7 and to the lower end of the short rods 5, etc.

The following examples, in which the electrode construction of the invention was used for the production of magnesium and chlorine from magnesium chloride, will illustrate the utility of the invention, and are not intended for purposes of limitation.

EXAMPLE 1 In an electrolytic cell having four sets of electrodes, each set comprising one anode, two cathodes and 3 transverse cathode members, the arrangement being essentially as shown in FIG. 3, a fused salt mixture was electrolyzed consisting substantially of MgCl (about by weight), CaCl (about 35%), NaCl (about 30%) and KCl (about The transverse cathode members were vertically arranged steel plates, 30 cm. in height and 1 cm. thick, the ends of which were welded to the respective cathodes. The distance between the successive transverse members was 40 cm.

The temperature of the bath during electrolysis was 760 C., and the current strength was 38,000 amperes. The results of the operations were (average) Energy consumption kwh./kg. mg 17.6

Loss of chlorine kg/kg. mg 0.04

Current efficiency percent 86 EXAMPLE 2 In an electrolytic cell of substantially the same size and construction as that of Example 1, but provided with three sets of electrodes, each set comprising one anode, two cathodes and seven transverse cathode members, a similar fusion as that of Example 1 was electrolyzed. The temperature of the bath was 770 C. and the current strength was about 61,000 amperes. The results of the operations were (average):

Energy consumption kwh./kg. mg 15.0

Loss of chlorine kg./kg. mg 0.06

Current efiiciency percent 80 EXAMPLE 3 In an electrolytic cell of substantially the same size and construction as that of Example 1, but provided with three sets of electrodes each set comprising one anode, two cathodes and 8 transverse cathode members, a similar fusion as that of Example 1 was electrolyzed. The temperature of the bath was 780 C. and the current strength was about 65,000 amperes. The results of the operations were (average):

Energy consumption kwh./kg. mg 14.2 Loss of chlorine kg/kg. mg 0.07 Current efiiciency percent 85 The examples given above refer to the application of the electrode construction of the invention in electrolytic cells wherein the cathodes, arranged parallel with the short side of the cell, are conventionally suspended so as to be immersed in the melt to a suitable level, the electrical connection being provided at the top of the cathode. However, the electrode construction of the invention can be mounted in alternative ways, for instance so as to supply the current to the transverse members through cathodic conductor bars or plates provided in the walls of the cell; or the walls of the cell can be made of steel, in which case the transverse members can be arranged to extend between opposite cell walls, either longitudinally or transversely of the cell. In these cases, the conductor bars, plates or walls would have cathodic function, to a smaller a larger extent; however, this cathodic function would be transferred to the transverse electrode members to an increasing degree as the number and size of such transverse members are increased, and the relative importance of the conventional cathodes qua cathodes is then correspondingly reduced. In the extreme case one can even imagine that the conventional cathodes are reduced to current supply means, which could be arranged passing through the wall or the bottom of the cell, or in any other suitable way obvious to those skilled in the art.

In a steel cell, wherein the steel walls and bottom are cathodic, it is possible for instance to use the bottom of the cell as support for the transverse electrode members. Conveniently, the latter can then be mounted on suitable footings welded to the bottom of the cell.

While the invention has been described with particular reference to a certain embodiment thereof directed to the production of magnesium, it is to be understood that it is not to be limited thereto.

I claim:

1. Electrode construction for the electrolysis of salt fusions in an electrolytic cell employing electrodes having substantially vertical working surfaces, comprising a cathode on each side of an anode having openings in the lower part thereof, and cathodic members extending from one cathode to the other and running through said openings in the intermediate anode.

2. Electrode construction as set forth in claim 1 wherein said cathodic members are vertically arranged plates.

3. Electrode construction as set forth in claim 2 wherein said openings in said anode are downwardly open, vertically elongated slits.

4. Electrode construction as set forth in claim 3 wherein said anode is composed of alternate longer anode rods and shorter anode rods located side by side.

5. Electrode construction as set forth in claim 4 wherein the lower part of said shorter anode rods are shaped to guide the flow of the melt outwardly from the mid-plane of said anode.

6. Electrode construction as set forth in claim 5 wherein said lower part of said shorter anode rods is cut symmetrically to form an edge.

References Cited UNITED STATES PATENTS 2,987,463 6/1961 Baker et a1. 204-286 1,296,046 3/ 1919 Charbonneaux 204-267 FOREIGN PATENTS 573,870 4/ 1959 Canada.

I OHN H. MACK, Primary Examiner D. R. JORDAN, Assistant Examiner US. Cl. X.R. 204245 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION P t nt N 3,496,089 Dated February 17, 1970 Inventor) FINN ENOK FOLKESTAD It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, change the first name of the inventor from "Fin" to -Finn- SIGNED AND S EALED JUN231970 i Anew MM. Fletcher Ir. Ed WILLIAM E. SW, at. Ami- 5 Officer Gomissionar of Pamll