US2913381A

US2913381A - Start up method for fused salt electrolytic cells

Info

Publication number: US2913381A
Application number: US529198A
Authority: US
Inventors: William F Mcfadyen; Charles E Buterbaugh
Original assignee: National Destillers and Chemical Corp
Current assignee: Millennium Petrochemicals Inc
Priority date: 1955-08-18
Filing date: 1955-08-18
Publication date: 1959-11-17
Anticipated expiration: 1976-11-17

Description

Nov. 17, 1959 w. F. MOFADYEN EIAL 2,913,381

START UP METHOD FOR FUSED SALT ELECTROLYTIC CELLS 2 Sheets-Sheet 1 Filed Aug. 18, 1955 FIG.I

WlLLIAM F. Mc FADYEN CHARLES E. BUTERBAUGH Nov. 17, 1959, w. F. MOFADYEN EI'AL 2,913,381

START up mamon FOR FUSED SALT ELECTROLYTIC CELLS 2 Sheets-Sheet 2 Filed Aug. 18, 1955 WILLIAM F. Mc FADYEN CHARLES E.BUTERBAUGH INVEN TORS BYWjM ATTORNEY United States START UP METHOD FOR FUSED SALT ELECTROLYTIC CELLS Application August 18, 1955, Serial No. 529,198

11 Claims. (Cl. 20468) The present invention relates to a novel method for starting up electrolytic cells for electrolysis of fused salts and, more particularly, to an improved method for starting up electrolytic cells adapted for production of a metal by electrolysis of a fused salt; and, still more particularly, an electrolytic cell adapted for production of a light metal by electrolysis of a halide of the metal, such as for production of sodium from sodium chloride.

In the production of a metal by electrolysis of a molten salt of the metal, electrolytic cells are generally employed in which the electrodes are separated by a relatively short distance, as fused salts are relatively poor conductors particularly when compared with aqueous solutions of electrolytes, so as to avoid undue power consumption per pound of metal obtained. Moreover, in the production of a metal by electrolysis of a fused salt, it is generally necessary to use a mixture of a salt of the metal desired with one or more salts of another metal or metals in order to provide for the electrolysis an electrolyte having a suitable melting point. For example, in the production of sodium by electrolysis of sodium chloride, a suitable substance such as calcium chloride is used in mixture with sodium chloride, the amount of the calcium compound employed being controlled so as to provide a mixture of suitable melting point and in which mixture the concentration of calcium is such that formation of an undesired separate phase of calcium is avoided or minimixed during operation of the cell. For example, in a typical operation of a sodium cell a suitable mixture for the electrolysis operation can consist of about 42 weight percent sodium chloride and 58 weight percent calcium chloride.

In starting up of electrolytic cells for electrolysis of fused salts, and specifically such cells adapted for production of sodium from sodium chloride, a method has been employed which involves the use of starter blocks comprised of graphite in the form of elongated wedges. In such a method a plurality of starter blocks are wedged between an anode and its respective cathode in the electrolytic cell, following which the cell is packed completely with a suitable mixture of sodium chloride and calcium chloride or a mixture of calcium chloride and crushed cell bath, e.g., such as a mixture of sodium chloride and calcium chloride obtained by removal of a cell bath from another cell and which bath has been cooled to solidification and crushed. While the starters are held in place, usually by means of tongs, current is passed to the cell and all but one starter pulled out as quickly as possible, the starter left in position being allowed to remain until sumcient bath has melted to carry current. The starter is then lifted out of the cell and the cell is maintained under normal operating conditions. In such a method for starting up the electrolytic cells, considerable manpower is required to hold the starters in place, to pull the starters and to provide the necessary close-up manual attention when the power load is applied. Additionally, such a starting procedure possesses several disadvantages inherent in the method. For example, use of starter blocks atent O "ice often results in overheating of the cathode resulting in cell distortion and cathode blistering, especially when a starter block sticks in position and causes melting of the cathode thereby requiring considerable work and expense for removal of blistered metal before the collector and diaphragrns normally used with such cells can be installed. Moreover, such a starting procedure subjects the anode to thermal and electrical shock primarily as a result of localized overheating, requires an extended period of time before the cell is brought up to full voltage and load, and oftentimes loss of load results whereby the cell fails to start. Of considerable importance, the aforesaid method using starting blocks involves a safety hazard as the possibility is always present that an operator pulling the starter block may be hit with fragments of the block and/or cell bath and, especially so, in cases wherein the starter block or blocks blow out by themselves before the necessary pool of molten bath is formed so as to maintain the cell in operation. Accordingly, it is highly desirable to provide a method for starting up of electrolytic cells for electrolysis of fused salts with avoidance or substantial minimization of disadvantages of processes heretofore employed and inclusive of the method aforedescribed. Thus, a primary object of this invention is the provision of such an improved method.

In accordance with this invention, an electrolytic cell adapted for electrolysis of fused salts is started up by packingv the cell in particular manner with comminuted solids, which are to provide initially the fused salt bath, heating the cathode at a portion or portions thereof that are not in contact with the packing material, introducing into contact with the heated portion of the cathode, and in a manner so as to provide contact between the cathode and anode, a suitable molten bath having sufficient conductivity to allow current to pass from the anode to the cathode, and applying electrical current to the cell. In such a method, markedly improved results are obtained by heating the anode whereby, during the starting procedure, the possibility of thermal shock of the cell and particularly the anode is obviated or substantially reduced.

In order to facilitate description of the invention, reference is made to the accompanying drawings in which Figure I illustrates an electrolytic cell suitable for electrolysis of fused salts including electrolysis of sodium chloride for production of sodium and in which Figure II is a cell similar to that of Figure I showing the packing material disposed in a manner suitable for practice of the start up method of this invention. In the use of such a cell to facilitate description of the present invention, it is not intended that the invention be limited in its application to such a specific cell as, in broad aspect, the invention may be practiced with cells adaptable for electrolysis of fused salts other than a cell as shown in the drawings.

In the drawings showing a diagrammatic, sectional view of a fused salt electrolytic cell, there is shown a cylindrical brick-lined steel casing 1, having a cylindrical graphite anode 2 projecting upwardly from the bottom thereof. Cathode 3, a steel cylinder, positioned vertically coaxial with the anode and spaced apart therefrom has two diametrically opposed steel arms 4 which project outside the cell casing to serve as electrode terminals. In such a cell a perforated cylindrical steel diaphragm 6 is suspended in the annular space between anode 2 and cathode 3. The annular collector ring 7 serves to support diaphragm 6 and to collect molten metal which rises in the fused electrolyte from cathode 3. Outlet 8 serves to carry the metal collected in collector ring 7 to the outside of the cell and gas dome 9 serves to carry out anodic products formed by the electrolysis. For the primary purpose of cooling or heating, the anode cell embodiment shown contains a jacketed casing 10 for a bottom portion of the anode with inlet means, such as conduit 11, and,

outlet means, such as conduit 12, for passing, as desired, a cooling medium (e.g. Water) or heating medium (e.g., steam) into jacket 10. To provide adequate sealing of the cell, the bottom portion thereof may be packed with a suitable refractory material, such as Firecrete, as shown by Firecrete packing 13. Although the cell has been illustrated primarily for purposes of completeness with a showing of gas dome 9 for collecting gaseous anodic prodnets and diaphragm 6, such elements are generally placed in the cell to be started in accordance with this invention following the start up of the cell as embodied herein. While the electrolyte is melting, sodium and chlorine are not collected, but a hood under suction is provided to draw away the latter to a caustic scrubber together with the sodium and calcium oxides formed above the melting bath. When the electrolyte is fully melted, the collector 7 with attached diaphragm 6, riser pipe 8 and dome 9 are inserted into the cell by means of a suitable guiding device. Such a cell, prior to insertion of the collector, diaphragm, riser pipe and dome is shown in Figure II which, as per dotted line 14, shows the surface of a suit ably disposed packing for the start up operation. As shown, the cell is preferably filled with the solid packing material up to a height close to the top of the anode and slightly banked up against the sidewall of the cell to, in effect, form a dished receptacle for the molten mass to be poured onto the surface of the packing for the start up operation.

By use of a cell embodiment as illustrated in the drawings and using, for purposes of illustration, the start up of such a cell for production of sodium by electrolysis of sodium chloride, the cell is packed to a point short of the top of the cathode, as for example as shown by dotted line 14 in Figure II to within about 6" of the top of the cathode, with a mixture of solid particles of materials that will provide a suitable fused bath for operation of the cell. For production of sodium, the mixture of solids may comprise from about 56 to 67% calcium chloride, and preferably 58 to 62% calcium chloride, with the remainder being sodium chloride. With respect to the packing, suitable therefor is a mixture of solids of coarse gravel size produced by crushing a bath obtained from another electrolytic cell that has been utilized for production of sodium by electrolysis of a fused bath of sodium chloride and calcium chloride and which crushed mixture contains sodium chloride and calcium chloride. Following such packing of the cell, the cathode is heated, preferably at a plurality of points in the portion of the cathode that is above the packing material, to a temperature above the melting point of a molten mass utilized in the start up procedure and referred to hereinafter. The heating of the cathode may be carried out by any of several suitable means with an example thereof being to heat the cathode with acetylene torches, preferably until the heated portion or portions are brought to red heat and, by use of acetylene torches, such heating is usually accomplished in a matter of several minutes. In preferred practice of the invention, and from another cell under operation for production of sodium by electrolysis of a fused bath containing sodium chloride and which cell may be momentarily cut out of its electrical circuit, a portion of its molten bath is removed and poured onto the heated portion or portions of the cathode of the cell to be started. The molten bath is poured onto the heated portion of the cathode in a manner whereby the poured bath lays on the packing, such as in the form of a pool, to contact both the anode and the cathode. The cell is then cut into its circuit and an electrical load, normally less than that at which the cell is operated under normal production conditions is applied. When the pool or pools of molten material, which possess sufficient conductivity to allow current to pass from the anode to the cathode, have taken the applied load, the load is increased rapidly to the normal operating value.

With reference to the molten mass employed for contacting the heated portion of the cathode, it should not only be sufliciently conductive to allow passage of current from the anode to the cathode, but should be of such composition that it is a suitable component of the packing which results in the fused bath for operation of the cell. Though use of molten bath from another cell is a particularly suitable material, other materials such as molten sodium chloride or molten calcium chloride may be employed in practice of the invention with cells for production of sodium by electrolysis of fused mixtures of sodiurn chloride and calcium chloride.

In starting up of a cell by use of the aforesaid method, an improvement has been found to result by heating the anode prior to starting the cell and, for such a purpose, using the embodiment in the drawings for illustration, the heating may be accomplished by passing steam into jacket 11 before the cell is started, and preferably, the heating of the cell is continued at least until the cell is in operation for production of sodium, following which the jacket may be utilized for cooling puruposes which may be accomplished by passing water into the jacket.

In starting up of electrolytic cells by the method of the present invention and which method may be used with cells consisting of a single anode and cathode or a plurality of anodes with their respective cathodes, it has been found that numerous advantages are obtained, including the elimination or substantial minimization of disadvantages of starting up procedures heretofore employed and inclusive of the method using starter blocks as described herein. For example, by practice of this invention, overheating of the cathode which results in cell distortion and cathode blistering has been eliminated, the time normally required to start up the cell and have it reach full voltage and load may be realized in about 5 minutes whereas methods such as those using starter blocks have normally required a period of 15 minutes or more; hazards to personnel inherent in heretofore employed methods, such as those using starter blocks, are eliminated; the tendency for the cell and anode to undergo electrical and thermal shock during the start up is eliminated or substantially minimized, and the possibility of loss of load, such as is often encountered in procedures using starter blocks with failure of the cell to start up, is also obviated or materially reduced.

In a specific embodiment and utilizing a cell which, under normal operations for production of sodium, opcrates at a voltage of about 6.5 to 7.5 and amperage of 30,000 to 40,000 or higher, the initial load placed on the cell during the start up procedure, following provision of the molten pool or pools contacting the anode and cathode, may be on the order of about 16,000 amperes but, in general, the initial load applied may be varied depending on the particular cell employed, power availability, etc. As to the improvements that result by having heated the anode for the start up of the cell, improved uniformity of the starting pool or pools at the top of the anode has been found to occur, the cell reaches full production capacity more quickly than cells started without heating of the anode, and cell distortions and thermal shock to the anode are substantially avoided. Although it is not intended that the invention be limited to the particular temperatures at which the anode should preferably be heated for the start up, particularly suitable results are obtained by heating the bottom portion of the anode to temperatures between and C.

While there are above disclosed but a limited number of embodiments of the invention herein presented, it is possible to produce still other embodiments without dcparting from the inventive concept herein disclosed, and it is desired therefore that only such limitations be imposed on the appended claims as are stated therein.

What is claimed is:

1. A method for starting up electrolytic cells adapted for electrolysis of a fused salt bath which comprises packing said cell with solid particles of a mixture of salts suitable for use as said fused bath, said packing being made in a manner whereby a portion of the anode and a portion of the cathode is not in contact with the packing, heating the cathode at a portion thereof that is not in contact with said packing to a temperature substantially above the melting point of a molten electrically conductive mass defined hereinafter, introducing into contact with said packing, said heated portion of said cathode and the anode a molten, electrically conductive mass comprising at least one salt that is a suitable component for said fused bath, and applying electrical current to the cell.

2. A method, as defined in claim 1, wherein the anode is heated prior to starting up of the cell.

3. A method for starting up an electrolytic cell adapted for production of sodium by electrolysis of a fused bath containing sodium chloride which comprises packing said cell with solid particles of a mixture of calcium chloride and sodium chloride, said packing being made in a manner whereby a portion of the anode and a portion of the cathode is not in contact with the packing, heating the cathode at a portion thereof that is not in contact with said packing to a temperature above he melting point of a molten mass defined hereinafter, introducing into contact with said packing, said heated portion of the cathode and the anode a molten, electrically conductive mass comprising sodium and calcium chloride, and applying electrical current to the cell.

4. A method, as defined in claim 3, wherein the anode is heated prior to packing of the cell.

5. A method, as defined in claim 3, wherein the molten, electrically conductive mass is obtained from the bath of a cell in operation for production of sodium by electrolysis of a fused bath comprising sodium chloride and calcium chloride.

6. A method, as defined in claim 3, wherein the anode is heated prior to packing of the cell, and the molten, electrically conductive mass is obtained from the bath of a cell in operation for production of sodium by electrolysis of a fused bath comprising sodium chloride and calcium chloride.

7. A method for starting up an electrolytic cell, adapted for production of sodium by electrolysis of a fused bath containing sodium chloride, having a vertically elongated anode and a cathode coaxial with and spaced apart from said anode, which comprises packing said cell to a point short of the top of said anode and cathode with a mixture comprising solid particles of calcium chloride and sodium chloride, heating the cathode at the top portion thereof not in contact with said packing to a temperature above the melting point of an electrically conductive mass as defined hereinafter, pouring onto the top surface of said packing a pool of a molten, electrically conductive mass to contact the heated portion of the cathode and the anode, said molten mass comprising sodium chloride and calcium chloride, and applying electrical current to said cell.

8. A method, as defined in claim 7, wherein the anode is heated prior to packing of the cell.

9. A method, as defined in claim 7, wherein the molten, electrically conductive mass is obtained from the bath of a cell in operation for production of sodium by electrolysis of a fused bath comprising sodium chloride and calcium chloride.

10. A method, as defined in claim 7, wherein the anode is heated prior to packing of the cell, and the molten, electrically conductive mass is obtained from the bath of a cell in operation for production of sodium by electrolysis of a fused bath comprising sodium chloride and calcium chloride.

11. A method for starting up an electrolytic cell for production of sodium by electrolysis of a fused bath containing sodium chloride, said cell having a vertically elongated anode and a metal cathode coaxial with and spaced apart from said anode, which comprises packing said cell to a point short of the top of said anode and said cathode with a mixture comprising solid particles of calcium chloride and sodium chloride, heating by external heating means and to substantially red heat a portion of the cathode Which is not in contact with said packing whereby said portion of the cathode is substantially above the melting point of an electrolytic conductive molten mass as defined hereinafter, pouring onto the top surface of said packing a molten electrolytic conductive mass to form on said packing a pool of said molten mass that contacts the heated portion of the cathode and the anode, said molten mass comprising sodium chloride, applying to said cell an electrical load less than the normal production load for said cell and, when said molten pool has taken the applied electrical load, rapidly increasing the electrical load on said cell to its normal operating electrical load.

References Cited in the file of this patent UNITED STATES PATENTS 473,866 Bradley Apr. 26, 1892. 596,458 Inglis Dec. 28, 1897 704,393 Simon July 8, 1902 1,080,113 Von Kugelgen Dec. 2, 1913 1,202,534 Keyes et a1. Oct. 24, 1916 1,545,582 Cobb July 14, 1925 1,905,882 Balke Apr. 25, 1933 2,390,548 McNitt Dec. 11, 1945 2,398,591 Mitchell Apr. 16, 1946

Claims

1. A METHOD FOR STARTING UP ELECTROLYTIC CELLS ADAPTED FOR ELECTRYSIS OF A FUSED SALT BATH WHICH COMPRISES PACKING SAID CELL WITH SOLID PARTICLES OF A MIXTURE OF SALTS SUITABLE FOR USE AS SAID FUSED BATH, SAID PACKING BEING MADE IN A MANNER WHEREBY A PORTION OF THE ANODE AND A PORTION OF THE CATHODE IS NOT IN CONTACT WITH THE PACKING, TION OF THE CATHODE IS NOT IN CONTACT WITH THE PACKING, HEATING THE CATHODE AT A PORTION THEREOF THAT IS NOT IN ABOVE THE MELTING POINT OF A MOLTEN ELECTRICALLY CONDUCTIVE MASS DEFINED HEREINAFTER, INTRODUCING INTO CONTACT WITH SAID PACKING, SAID HEATING PORTION OF SAID CATHODE AND THE ANODE A MOLTEN, ELECTRICALLY CONDUCTIVE MASS COMPRISING AT LEAST ONE SALT THAT IS A SUITABLE COMPONENT FOR SAID FUSED BATH, AND APPLYING ELECTRICAL CURRENT TO THE CELL.