US3071532A

US3071532A - Cells for the electrolysis of fused salts

Info

Publication number: US3071532A
Application number: US13586A
Authority: US
Inventors: Koehl Anton; Kaegi Ernst
Original assignee: Ciba AG
Current assignee: BASF Schweiz AG
Priority date: 1959-12-07
Filing date: 1960-03-08
Publication date: 1963-01-01
Anticipated expiration: 1980-01-01
Also published as: CH374834A; SE305545B; DE1128673B; FR1248757A; NL249893A; ES257223A1; NL127725C

Description

Jan. 1, 1963 A. KOEHL ETAL 3,071,532

CELLS FOR THE ELECTROLYSIS OF FUSED SALTS Filed March 8, 1960 a a E 2" N INVENTORS Goflfr/Zed HJeI/emann Ernst Kaegi by kw AT TORNFYS 3,dll,532

Patented Jan. 1, 1%63 3,ll71,532 CELM; FUR THE ELE CTRQLYSES til FUSED SALTS Anton Koehl, Gattfried Fuellemann, and Ernst Kaegi, Montltey, Switzerland, assignors to Ciba Limited, Basel, witzerland, a Swiss firm Filed Mar. 3, H60, Ser. No. 13,586 Claims priority, application Switzerland Dec. 7, 1959 3 Claims. (Cl. 204-443) This invention relates to cells for the electrolysis of fused electrolytes, and particularly to cells used in the manufacture of sodium by electrolysis of common salt.

In large scale manufacturing processes, electrolysis of molten metal salts, and particularly of alkali halides, has been carried out for many years. The manufacture of metallic sodium from common salt or mixtures thereof with other salts, such as calcium chloride, barium chloride and the like, has become of especial importance and has resulted in the development of many types of cell. The majority of these cells consist essentially of a cylindrical steel container, lined with a ceramic material, through the bottom of which enters a central graphite anode surrounded by an annular cathode. The space between the anode and the cathode is divided by an annular diaphragm which supports suitable apparatus for collection of the deposited materials, such as, for example, chlorine and sodium. A cell of this type is described in US. Patent No. 1,501,756 to James Cloyd Downs (patented luly 15, 1924).

Although such cells generally have been fairly successful, they still have certain disadvantages, since the operation of a fused salt electrolytic cell involves technical problems, due to high working temperatures, corrosiveness of salts employed in the processes and the nature of the final products, the solution of which presents difiiculties.

One of these problems arises in the ceramic lining of such electrolytic cells. It is difiicult indeed to find any ceramic material which, for prolonged periods, can resist attack due to the effects of molten salts used in the process and due also to the products of electrolysis. Apart from the fact that the ceramic materials used are expensive, the fabrication of the lining itself tends to be a complicated and costly operation. Further, during renewal of the lining, these cells cannot be operated making economical utilisation of the plant difiicult.

The use of a diaphragm also gives rise to difiiculties in that materials used in manufacturing these diaphragms do not stand up to the demands made of them during operation of cells. The metal screens used as diaphragms have to be replaced at relatively frequent intervals during which replacement operation of the cells must cease, the latter must be partly dismantled and may even have to be emptied, thereby further complicating the operation of the plant.

However, no cell is at present known which can be operated without a diaphragm and which makes as efficient use of supply energy and current as does a cell using a diaphragm.

Conventional cells for the electrolysis of fused salts thus tend to have constructional complications and to be relatively expensive to maintain and operate; it is an object of this invention to provide an improved cell.

According to the present invention, a cell for the electrolysis of fused salts includes a container within which a cathode structure surrounds an anode structure, the anode structure comprising a supporting column and a plurality of anode plates removably and electrically connected thereto, the plates being spaced apart from the column and each having a face directed toward the cathode structure.

The anode plates may be mutually spaced apart and may be arranged to form an annular or polygonal ring around the column. Conveniently, the column may have radially extending arms to which the plates are attached. Preferably, the anode structure is so dimensioned and constructed, and together with the cathode structure so arranged within the container, that the electrolyte can freely circulate within the container. Further it is advantageous that the effective opposing surfaces of the cathode and anode are not separated by any intermediate surface, such as, for example, a diaphragm.

By way of example, a cell embodying the invention, and suitable for the production of sodium by the electrolysis of common salt, will be described in greater detail and with reference to the accompanying drawings, in which:

FIGURE 1 is a sectional side view of a cell embodying the invention, and

FIGURE 2 is a section on the line A-A of FIG- URE 1.

FIGURE 1 shows a cell comprising a container in which is mounted an anode structure including a centrally disposed column 1 2, made of graphite, and a number of plates 12. forming the working anode and which are renewable and attached to the central column llll by separate support arms 14. The anode plates 12 and support arms 14 are also made of graphite. Opposite and facing anode plates 12 there is an annular cathode 16, made of iron. The column 16 is mounted in the bottom 18 of the container and is also used to supply current to the anode plates 12, the positive side of a power supply being connected to a terminal 20 located on column llll on the underside of the container.

As shown in FIGURE 1, the bottom of the container consists of a shell 22 lined with a corrosion resistant material; this lining also forming the lower floor 24 of the cell. A drainpipe 25 is provided in the floor 24 through which the cell can be emptied.

A side wall 28, made of iron and forming the inner wall of the cell, is inserted into a groove 26 in the bottom of the container. To provide thermal insulation the side wall 28 is enclosed in a thermally insulating layer 30. The cathode 16 is supported by four circumferentially spaced bars 32 which pass, electrically insulated, through a collector channel 34. The bars 32 also act as current supply leads for the cathode 16.

The channel 3 3, which is of annular form and open on its underside, is located above the cathode l6 and is immersed in the electrolyte 46. in operation of the cell, channel 34 collects sodium deposited at the cathode 16. As shown on the right hand side of FlGURE 1, this channel supports at one point on its periphery a collector chamber 38 within which sodium collects above the electrolyte, the latter having a higher density, so that the so- (hum can be discharged through passage 40. At another point on the channel 34 there is an inlet opening through which the cell can be replenished with electrolyte. The collector channel 34- is made, as is the wall of the cell, of iron.

A collector dome for chlorine released at the anode surfaces rests upon the collector annulus 34. The lower edge 44 of the dome is immersed in the electrolyte 46 thereby sealing oil": the gas space from the exterior of the cell. At the highest point of the dome 42 there is an aperture 48 through which the collected chlorine can be discharged; if desired other apertures may be provided in the dome 42 through which electrolyte also may be admitted to the cell. The dome 42 preferably is made of a corrosion resistant ceramic material.

The anode plates are so attached to the column 10 that they may be renewed. For this purpose, as is shown in FIGURE 1, the support arms 14 have

pins

13 and 15, of which pins 13 fit into corresponding holes in the column 19 and pins 15 into the anode plates 12. It is to be understood that other methods of attachment may also be used, for example, the pins of arms 14 may be screw threaded and holes tapped in plates 12. The anode plates are arranged to form a circular or polygonal ring, located within the annular cathode, in general with the anode plates 12 being mutually spaced apart. The supporting arms 14 are of adequate length to ensure that an area of sufiicient section is available at the rear side of the anode plates in order to allow circulation of the electrolyte around the anode plates, as indicated by arrows 50, to take place. The cathode is likewise spaced from the walls of the container. Further, the anode plates the cathode are displaced from the bottom of the Such a design al ows a free flow of a melt within the cell, as indicated by arrows t) and 52, which is well defined and which particularly in the space between the anode plates and the cathode is free from turbulence. Chlorine forming at the anode plates and sodium deposited at the cathode can rise up without any turbulence causing mixing of these two substances. The thermal convection taking place between the anode and cathode aids a comparatively rapid flow of the electrolyte in an upwards direction which is of importance in so far as it prevents overheating of the electrolyte in the vicinity of the electrodes and thereby increases the eiiiicency of the cell. The rising melt thus circulates between the anode column and the anode plates and also between the cathode and the side walls of the cell Where it becomes cooled. The favorable flow pattern thus achieved also makes it possible to arrange the cathode and anode working surfaces directly opposite each other without separation by a diaphragm or other intervening surface. The efiiicency of the cell, as regards consumption of supply current and energy, however, is comparable with that obtained by use of a conventional diaphragm equipped cell.

Apart from the fact that the design or" anodes described above results in a favorable flow pattern, it also leads to a substantial saving in material and labor in the operation of the cells. In actual fact only the anode plates are subject to substantial wear during operation of the cell, the supporting arms and anode column being subject to much less wear. After a given period of operation it is suflicient, therefore, to renew only the worn anode plates, whereupon the cell can be refilled and operated again. This substantially simplifies maintenance operations and reduces the time during which the cell need be out of operation during such maintenance.

Elimination of the need for a diaphragm also results in a simplification of construction and reduction of cost in the operation of the cell.

Another advantage is that the side walls are not lined and that lining is restricted to the floor of the cell. This is important as the ceramic lining of a fused salt electrolytic cell is a difficult operation. No known material will resist for a prolonged period the chemical attack by the contents of the cell. The lining must therefore be renewed at comparatively short intervals of time, thereby increasing the cost of operation of the cell. In the present case, normal maintenance work is limited to the replacement ofthe top layer of the floor lining which may consist of ceramic slabs or a compressed material.

The elimination of the ceramic lining is also advantageous from the point of view of operation of the cell. In fact, the attack by the melt and the sodium on the ceramic material forms decomposition products in lined cells which collect on the floor of the cell forming there a viscous mass which in time reduces the circulation of the melt, become electrically conductive and hence causes short circuits which gradually reduce the efiiciency of the cell, necessitating its drainage and cleaning. In the case of a cell constructed in accordance with this invention, the ceramic lining is limited to the bottom of the cell, and thus the surface exposed to attack by the melt is greatly reduced. The formation of decomposition products is also reduced, resulting in a long operating life at a substantially constant rate of yield of the cell.

Another advantage having a similarly favorable effect is the fact that both the working anodes and the cathode are arranged at some distance above the bottom of the cell, since some amount of deposit can form on the floor without being in the actual neighbourhood of the electrodes. This features helps to diminish dangers of a short circuit.

The thermal insulation layer is provided on the outside of the cell thus not coming in contact with the electrolyte. Hence any desired type of insulating material can be used since it is not subject to any particular chemical requirement. It is possible also to vary the heat insulation of a cell during operation as required, thereby ensuring optimum, or near optimum temperature conditions for any particular cell.

Another advantage resulting from the elimination of the inside lining of the cell is the very simple and efficacious construction of the cell. The sidewall is not attached rigidly to the bottom but rests in a sealing groove so that it can be lifted up, together with the chlorine dome, cathode and sodium collector without difiiculty from the floor of the cell, without involving the dismantling of the thermal insulation. This provides ready access to the floor portion of the cell, so that both inspection and maintenance of the cell are greatly facilitated. Thus, for example, it is possible within a very short time to lift up the upper portion of the cell, remove any sediment collected on the bottom, replace the upper portion, refill the cell and put it into operation again.

What is claimed is:

1. A cell for the electrolysis of fused salts, comprising a container having a bottom to hold electrolyte, a central support column in said container, a plurality of anode plates in said container having their active faces disposed vertically and being spaced apartfrom each other and from said container bottom and forming together a ringshaped structure concentrically arranged with respect to said central support column, said anode plates being removably attached to said central column by means of radial arms extending from said column in spoke-like configuration, said radial arms located substantially equidistantly between the top and bottom of said container, a ring-shaped cathode inside said container concentrically surrounding said anode plates to provide a free space between said oathode and anode plates, said anode plates being spaced with respect to each other and said cathode plate whereby fluid flow mutually therebetween is possible, said central column piercing said container bottom and providing electrical connection means with said anode plates and a current supply.

2. A cell as described in claim 1, wherein said container comprises a base portion forming the bottom of the container, an upstanding cylindrical iron side wall, a thermal insulation provided only on the exterior surface of said side wall, and a covering dome for the gases liberated at said anode plates, each of the three parts representing a self supporting unit easily detachable from the other container parts; said cell further comprising an an- .nular collecting channel for gathering the material deposited on the cathode, said channel located above said cathode and extending downward below the electrolyte fluid level maintained during normal cell operation.

3. A cell for the electrolysis of fused salts comprising a container, a plurality of anode plates in said container having vertically-oriented active faces, a support column for said anode plates, said column piercing the bottom of said container and providing electrical connection between said anode plates and an electrical current source, saidanode plates being removably connected with said support column by means of radial arms extending from said column in spoke-like configuration, said radial arms being of substantially equal length and located substantially equidistantly between the top and bottom of said container, said anode plates forming a concentric ring with respect to said support column, and a ring-shaped cathode concentrically and exteriorly arranged with respect to said column and said anode plates to provide a free space between said anode plates and said cathode.

References Cited in the file of this patent UNITED STATES PATENTS Castner May 12, Blackrnan Sept. 22, Ward et al. Mar. 29, Hardy et a1. Mar. 19, McNitt Dec. 4,

Claims

1. A CELL FOR THE ELECTROLYSIS OF FUSED SALTS, COMPRISING A CONTAINER HAVING A BOTTOM TO HOLD ELECTROLYTE, A CENTRAL SUPPORT COLUMN IN SAID CONTAINER, A PLURALITY OF ANODE PLATES IN SAID CONTAINER HAVING THEIR ACTIVE FACES DISPOSED VERTICALLY AND BEING SPACED APART FROM EACH OTHER AND FROM SAID CONTAINER BOTTOM AND FORMING TOGETHER A RINGSHAPED STRUCTURE CONCENTRICALLY ARRANGED WITH RESPECT TO SAID CENTRAL SUPPORT COLUMN, SAID ANODE PLATES BEING REMOVABLY ATTACHED TO SAID CENTRAL COLUMN BY MEANS OF RADIAL ARMS EXTENDING FROM SAID COLUMN IN SPOKE-LIKE CONFIGURATION, SAID RADIAL ARMS LOCATED SUBSTANTIALLY EQUIDISTANTLY BETWEEN THE TOP AND BOTTOM OF SAID CONTAINER, A RING-SHAPED CATHODE INSIDE SAID CONTAINER CONCENTRICALLY SURROUNDING SAID ANODE PLATES TO PROVIDE A FREE SPACE BETWEEN SAID CATHODE AND ANODE PLATES, SAID ANODE PLATES BEING SPACED WITH RESPECT TO EACH OTHER AND SAID CATHODE PLATE WHEREBY FLUID FLOW MUTUALLY THEREBETWEEN IS POSSIBLE, SAID CENTRAL COLUMN PIERCING SAID CONTAINER BOTTOM AND PROVIDING ELECTRICAL CONNECTION MEANS WITH SAID ANODE PLATES AND A CURRENT SUPPLY.