US3776823A

US3776823A - Process for starting operation of a fused salt electrolytic cell

Info

Publication number: US3776823A
Application number: US00191493A
Authority: US
Inventors: S Crowther
Original assignee: Individual
Current assignee: Individual
Priority date: 1971-10-21
Filing date: 1971-10-21
Publication date: 1973-12-04
Anticipated expiration: 1990-12-04

Abstract

PROCESS FOR STARTING OPERATION OF A FUSED SALT ELECTROLYTIC CELL COMPRISING FILLING A PREHEATED CELL WITH MOLTEN FUSED SALT ELECTROLYTE AND THEN APPLYING OPERATING CURRENT LOAD TO THE CELL TO BEGIN PRODUCT RECOVERY. THERE IS NO PERIOD OF ELECTROLYSIS PRIOR TO INSTALLATION OF PRODUCT COLLECTION APPARATUS OR MEANS.

Description

Dec. 4, 1973 a CROW-[HER 3375323 PROCESS FOR STARTING OPERATION OF A FUSED sALT ELECTROLYTIC CELL Filed Oct. 21, 1971 United States Patent O US. Cl. 204-60 Claims ABSTRACT OF THE DISCLOSURE Process for starting operation of a fused salt electrolytic cell comprising filling a preheated cell with molten fused salt electrolyte and then applying operating current load to the cell to begin product recovery. There is no period of electrolysis prior to installation of product collection apparatus or means.

CROSS-REFERENCE TO RELATED APPLICATION This application is a continuation-in-part of US. application Ser. No. 658,131, filed Aug. 3, 1967, now abandoned.

BACKGROUND OF THE INVENTION In the production of a metal by electrolysis of a molten salt of the metal, electrolytic cells having electrodes separated by relatively short distances generally are used to avoid undue power consumption. Also, in such an electrolysis, it is often necessary or desirable to use a mixture of a salt of the metal desired with one or more salts of another metal or metals to provide an electrolyte having a lowered melting point so that operating cells can be operated at lower temperatures to give a longer service life. For example, in the production of sodium by electrolysis of sodium chlorde, calcium chloride, either alone or in combination with other substances may be used to lower the melting point of the electrolyte. The amount of calcium compound employed generally is controlled to provide a mixture of lowered melting point and one in which the concentration of calcium is such that the formation of an undesired separate phase of calcium is avoided or minimized during operation of the cell. In a typical operation of an electrolytic cell for the production of sodium, a suitable fused salt mixture for electrolysis may consist of about 42 Weight percent sodium chloride and about 58 weight percent calcium chloride and such a mixture will have a melting point of from about 525 C. to about 575 C.

In starting operation of an electrolytic cell for electrolysis of a fused salt or a fused salt mixture, and specifically such cells constructed and arranged for the production of sodium from sodium chloride, a method has been employed which involves the use of starter blocks prepared from graphite in the form of elongated wedges. In this method, a number of starter blocks are wedged between an anode and its respective cathode in the cell, and the cell is packed to the top of the electrodes with a suitable mixture of solid granular sodium chloride and calcium chloride or a granular mixture of calcium chloride and crushed solidified fused salt electrolyte from another cell. Then, while the starter blocks are held in place, usually manually by use of tongs, current, at a low load, is applied to the cell by means of the anode and cathode and all but one starter block removed as quickly as possible with the remaining starter block being left in position to carry current through the cell until the cell has heated sufficiently and melted enough of the solid electrolyte mixture to carry the current. After the cell bath has melted sufliciently and is carrying the low current load, the remaining starter block is lifted out of the cell; the current load raised to the operating level as the electrolyte melts; product collection apparatus is installed, and product recovery is begun.

The disadvantages of such a startup operation are obvious in the necessary manual operations required and the attending safety hazards. Further, the use of starter blocks often results in overheating of the cathode which may cause cell distortion and cathode blistering which damages usually must be corrected before product recovery can be begun. Also, such a starting procedure subjects the anode to chemical, thermal and electrical shock as a result of localized overheating and may cause extended periods of low electrical load on the cell resulting in attending operating difiiculties and low produc tion rates. The principal damage is done by the oxidation of the anode during the melting process in which the heat for melting is supplied by the electrolytic operation of the cell.

One attempt to overcome this generally unsatisfactory and unsafe manual startup procedure is that disclosed in US. Pat. 2,913,381. In accordance with this procedure, an electrolytic cell is packed as before with the desired solid electrolyte in particulate form except that starter blocks are not used and the cell is not filled completely. A small portion of the anode and cathode at the top of the cell are left uncovered and the uncovered portion of the cathode is heated to a temperature greatly in excess of the melting point of the fused salt electrolyte. After the cathode reaches the excessively high temperature, a puddle of molten fused salt electrolyte is introduced into the cell at the heated portion and an electrical load substantially less than the normal operating load is applied to the anode and cathode. In this method, the small puddle of molten fused salt electrolyte is used as a first electrical load carrying material for the cell, and after operation is continued in this manner for a period sufficient to heat up the rest of the cell and melt the major portion of the fused salt electrolyte contained therein then the electrical load on the cell is increased to the usual operating level.

Although this method of startup for fused salt electrolytic cells offers a measure of improvement over the prior art manual method there are still many disadvantages to this method. Inparticular, oxidation of the anode occurs during the melting process because there is no collection means in the cell. Molten salt electrolytes must be prepared in solid form and then chipped or ground into particulate form for packing into a cell. This is especially laborious and time consuming when the electrolyte is obtained from an operating cell. To do so, the operating cell must be cooled down, the solidified electrolyte chipped out, then the chipped material ground and bagged for transport to the cell to be started.

Another disadvantage is that the cell cannot be operated at full operating load current until the gap area between the anode and cathode contains adequate molten electrolyte. In typical commercial operations, all cells in a production cell room are operated in series, and therefore during the startup of one cell when the current load on the starting cell is reduced, the current load on all other diaphragm are not installed on the cell being started up and there is product burning at the surface of the cell 3 Y causing undesirable and unsafe working conditions as well as attending product losses, and acting as a's'ource of oxygen supply to sustain the oxidation of the anode during periods of electrolysis without collecting means.

It can be seen that a process for starting operation of a fused salt electrolytic cell that substantially eliminates the above stated disadvantages is a worthwhile contribution to the art.

SUMMARY OF THE INVENTION In accordance with this invention, there is taught a process for starting operation of an electrolytic cell having at least an anode, a cathode and removable product collection apparatus and being constructed and arranged for the electrolysis of fused salt electrolytes which process comprises, in combination, the sequential steps of, (a) preheating an electrolytic cell having at least an anode, a cathode and removable product collection apparatus to a temperature sufficient to prevent the solidification of molten fused salt electrolyte, such preheating or drying being accomplished without applying any electrical current across the cell; (b) adding an operating quantity of molten fused salt electrolyte to the preheated electrolytic cell; and (c) applying operating current load across the molten fused salt filled electrolytic cell to begin recovery of the products of electrolysis. In an electrolytic cell in which the product collection apparatus is removed prior to preheating or addition of the molten fused salt bath, such product collection apparatus must be installed prior to the cell being connected to the electricity or any electrolysis being commenced.

The present invention provides complete avoidance of any electrolysis or cell electrical operation when no collection apparatus or means is in the cell.

BRIEF DESCRIPTION OF THE DRAWING In order to facilitate description of the invention, reference is made to the accompanying drawing in which FIG. 1 illustrates a typical electrolytic cell suitable for electrolysis of fused salts including electrolysis of sodium chloride for the production of sodium and chlorine. In the use of this drawing to aid in the description of this invention, it is not intended that the process of the invention be limited in its application to such a specific cell in that the invention may be practiced with electrolytic cells other than shown in the drawing which may be used or adapted for the electrolysis of fused salts.

In the drawing there is shown a diagrammatic sectional view of a typical fused salt electrolytic cell having a cylindrical brick-lined steel casing 10, a cylindrical graphite anode 11 projecting upwardly from the bottom of the cell and a steel cylinder-like cathode 12 positioned vertically and coaxially with the anode and spaced substantially uniformly apart therefrom. Cathode 12 has two diametrically opposed steel arms 13 which project outside the cell casing at 24 to serve as electrode terminals. A preforated cylindrical steel diaphragm 14 is suspended in the annular space between anode 11 and cathode 12 and annular collector ring 15 serves to support diaphragm 14 and to collect molten metal which rises, during opration, in the fused salt electrolyte along cathode 12. (Line 16 indicates the approximate surface level of a molten fused salt electrolyte within the cell when the cell is operating.)

'Outlet 17 serves to carry molten metal collected in collector ring 15 to the outside of the cell to a product recovery system or container. Gas dome 18 serves to collect anodic products formed by the electrolysis and outlet 19 serves to lead the collected product to a suitable recovery system or container.

In the embodiment shown, jacket casing 20 surrounds the bottom portion of anode 11 which extends through cell casing to provide heating or cooling for the anode during operation, startup or shutdown. Suitable inlet 21 and outlet 22 may be provided for jacketed casing 20 for the inlet and outlet of heating and cooling means such "a 'steam or water or other as necessary.

The bottom portion of cell casing 10 may be provided with a suitable refractory material 23 such as Firecrete or others to provide adequate sealing of the bottom of the cell and the opening around anode 11 which extends through the bottom of the cell casing.

It should be understood that a typical electrolytic cell has been described having only one anode and one cathode; however, it is clear that other configurations of anode, cathode, cell casing, and product collection apparatus are applicable to the process of thisinvention. In many instances more than one anode and cathode pair are located within the cell casing of an electrolytic cell, and in many commercial installations, electrolytic cells for the electrolysis of molten fused salts containing a group of four or more anode-cathode pairs arranged in a geometric pattern. Electrolytic cell arrangements such as these and others are clearly within the scope of the process of this invention.

DESCRIPTION OF PREFERRED EMBODIMENTS For the start of operation of an electrolytic cell, such as one shown in the drawing, reference will be made to the production of sodium metal and chlorine from a fused salt electrolyte containing sodium chloride; however, it is clear that the process of this invention is applicable to other embodiments of electrolytic cells and other fused salt electrolytes.

During the course of routine commercial operation for the production of a metal or metals by the electrolysis of a fused salt electrolyte containing the desired metal, anodes and cathodes may deteriorate or become misaligned due to heat distortion causing increased or irregular gap space therebetween, the brick lining of the cell casing may become corroded or heat eroded or other difliculties may occur. To alleviate these and other problems, it is necessary to remove or shunt the operating current load across the cell, pump out or drain the molten fused salt electrolyte contained therein, and tear the cell down to replace defective or misaligned parts. After an electrolytic cell is constructed or reconstructed, it is necessary generally to subject the cell to drying to remove the moisture contained in the mortar and bricks, the cell lining, the sealing material around the anode and cathode electrode inlets and other areas and the refractory material placed in the bottom of the cell. In accordance with the process of this invention, new or reconstructed electrolytic cells may be subjected to a drying step prior to the preheating step of the invention, or the drying and preheating steps may be advantageously one and the same. In this embodiment, after construction of a cell is complete, suitable heating means is provided within and around the cell first to remove the moisture contained in the celland then to preheat the cell to a temperature sufficient to prevent the solidification of the desired molten fused salt electrolyte to be added later. As can be seen clearly, this simplified operation prevents the regain of moisture by the cell and its possible deleterious effects on the cell by not permitting the cell to return to ambient temperature between the drying of the cell and the startup by methods practiced heretofore. During the drying and preheating step, it is preferred that steam or other heating medium be provided in the jacketed casing surrounding the portion of the anode outside the electrolytic cell to aid in the uniform heating of the anode.

The temperature to which the electrolytic cell and, its components must be heated during a preheating step is dependent upon the composition of the fused salt electrolyte to be used within the cell during operation and the temperature and quantity of the molten fused salt electrolyte which is added. During the preheating step of the process of this invention, it is necessary only to supply sutficient heat to the electrolytic cell and its components so that when the molten fused salt electrolyte is added to .5 the cell in a subsequent step of the process, the molten fused salt electrolyte will not solidify within the cell. Preheating of the cell provides assurance that addition .of molten bath thereto will not cause thermal and mechanical expansion damage to the cell. In most cases, preheating of the electrolytic cell to a temperature of at least 200 C. will be adequate in that sensible heat of the molten fused salt electrolyte added will be suificient to raise the temperature of the electrolytic-cell to one above the solidification temperature of the fused salt electrolyte being added.

As is well known, graphite, either alone or in combination with other materials, is used extensively for construction of anodes in electrolytic cells. During the preheating step of the process of this invention, care should be exercised to prevent substantial oxidation of the anode and any other oxidazible components of the unfilled electrolytic cell, and accordingly, it is within the scope of this invention to provide a substantially inert or oxidation preventing atmosphere surrounding the anode and other oxidation susceptible components during the preheating step when necessary or desirable. In an apparatus embodying one or more graphite components, an inert gas blanketing atmoshpere such as nitrogen may be necessary or desirable when the temperature surrounding the graphite component exceeds 300 C. for extended periods of time.

The product collection apparatus, including any diaphragm located between the anode and cathode, may be assembled within the cell during the preheating step, and operation of the process of this invention in this manner is a preferred embodiment of this invention. Because of variations in design of electrolytic cells, and product collection apparatus therefor, it may be advantageous to have product collection apparatus removed from the electrolytic cell during atleast a part of the preheating step so that sufiicient space is available at the top or opening of the cell for the insertion of necessary heating means within the cell and the addition of the molten fused salt electrolyte. In accordance with this invention, when the product collection apparatus is removed during at least a part of the preheating step, the cellection apparatus may be installed in the cell at a time following a part of the preheating step and before or after the addition of molten fused saltelectrolyte and before the application of substantially operating current load across the electrolytic cell.

' In a preferred embodiment of this invention, following the preheating of the cell to a temperature sufficient to prevent the sodification of the molten fused salt electrolyte, an operating quantity of molten fused salt electrolyte is pumped or transferred, by apparatus well known in the art, into the cell and operating current load is applied across the molten fused salt filled cell to begin recovery of the products of electrolysis. In the practice of this invention for starting operation of an electrolytic cell for the production of sodium metal and chlorine the fused salt electrolyte may comprise from about 56 to about 67 percent calcium chloride with the remainder being sodium chloride. In this embodiment a cell containing the product collection apparatus may be heated to at least about 200 C. and then the molten fused salt electrolyte comprising sodium chloride and calcium chloride at a temperature of from about 540 C. to about 620 C. is transferred to the cell to the desired operating level from a suitable container or another operating cell. On completion of the transfer, the cell may be placed immediately on full operating load current to begin production.

The premolten bath is particularly advantageous in that such bath can be treated to remove oxides and other impurities prior to its addition to the cell. Purification of the bath can be accomplished by filtering, scrubbing with chlorine, electrolysis in special means or by a combination of these treatments.

The molten fused salt electrolyte added to the cell during the process of this invention may be obtained all or in part from another operating cell. When the molten fused salt electrolyte is obtained from another operating cell, the operating cell, prior to the removal of the electrolyte, is shunted to remove the operating current load from the cell and the molten cell bath may be pumped directly from the cell being shut down to the one being started up. When a two or more component system is used as the fused salt electrolyte, additional quantities of one or more of the components in solid particular form may be added as needed or desired to adjust the concentration of the components in the fused salt electrolyte as desired. The molten bath may also be treated at this point to remove oxides, particulates and other impurities as desired.

The following examples are offered only for purposes of explanation and illustration and no inference on the scope of the invention is intended or should be made therefrom.

Example I An electrolytic cell, construced and arranged for the production of sodium metal and chlorine from a fused salt electrolyte comprising sodium chloride and calcium chloride, substantially as shown in the drawing and thus including product collection equipment, but with multiple cathodes and anodes in a geometric arrangement, was dried by installing a cell heater within the cell and operating the heater at low heat for 6 hours and then at high heat for 40 hours. During the low heat operation, the bottom of the cell reached a temperature of approximately C. and during the high heat operation, a temperature of approximately 250 C. was obtained. Prior to and during the drying and preheating, a 30,000 ampere shunt was located across the anode and cathode assembly of the cell, and following the preheating of the cell, a 30,000 ampere shunt was placed across an operating cell having a molten fused salt electrolyte at a temperature of approximately 610 C. Sufficient molten fused salt electrolyte was pumped from the shunted operating cell to substantially fill the preheated cell during an approximate 45 minutes period and the temperature of the molten fused salt electrolyte exiting the pump discharge was approximately 590 C. during the pumping. After completion of the pumping, the shunt on the cell being started was removed and sufficient particulate calcium chloride was added to the molten fused salt electrolyte to adjust the electrolyte to the desired component concentration. Following the starting of operation in this manner, the electrolytic cell operated at a voltage of about 6.5 to about 7.5 volts and about 30,000 to 40,000 amps or higher with a cell bath temperature of 566 C., and an efficiency of approximately 92 percent.

Example II The starting operation of Example I was repeated substantially as described except that the product collection apparatus, including the diaphragms for location between' the anodes and respective cathodes, was removed from the cell during the preheating and the electrolyte addition and was installed prior to the application of operating current load. The cell was operated at substantially the same voltage and amperage at a cell bath temperature of 550 C. and an efliciency of 94 percent.

Starting operation of an electrolytic cell for electrolysis of a fused salt in accordance with this invention offers many advantages. When molten fused salt electrolyte is transferred from an operating cell to a cell being started in the operation, the necessity to chip out solidified electrolyte from an old cell and then dry, grind and package the electrolyte for reuse are eliminated. By starting operation of the electrolyte cell with molten fused salt electrolyte, operating current load on other production cells connected in series need not be reduced and production capacities can be maintained at high levels. By preheating the cell and then adding molten fused salt electrolyte without an intermittent cooling period to permit the addition of starter blocks or other items, much dryer cells are provided and cell life is increased. Further, starting opera- 7 tion in accordance with the process of this invention provides more uniform heating and much less thermal shock to anodes and cathodes than when using hot starter blocks or localized puddling of a molten fused salt electrolyte. By starting operation of the cell with'the product collection apparatus installed, oxides and other impuri ties in the cell bath are reduced thereby increasing the servic e life of anodes and like materials which are corroded and attacked by such fused salt oxides. Of considerable importance is that the overall safety of the startup operation is improved immeasurably by eliminating substantially all manual operations and removing the necessity for having operating personnel in close proximity to the electrolytic cells. Finally, the premolten electrolyte can be treated as needed to remove impurities prior to transfer of the electrolyte to the cell to be started up.

In carrying out the operation of the present invention, it should be noted that the preheating or drying step is carried out without applying any electrical current across the cell. No electrolysis of the cell is commenced until after the product collection apparatus has been installed. This is extremely important as such procedure substantially prevents the accelerated anode attack that is characteristic of prior art methods.

The foregoing disclosure and description of the invention is illustrative and explanatory thereof and various changes in the size, shape and materials, as well as in the details of the illustrated construction, may be made within the scope of the appended claims without departing from the spirit of the invention.

What is claimed is:

1. A method of starting up a substantially empty electrolytic cell having at least an anode, a cathode, and removable product collection apparatus installed therein, for electrolysis of a fused salt bath, wherein no electrolysis is commenced prior to applying an operating current load across the cell consisting essentialy of in sequential order, the steps of:

(a) preheating the substantially empty electrolytic cell without applying any electrical current across the cell to a temperature sufiicient to prevent the solidification of a molten fused salt electrolyte to be introduced thereinto;

(b) adding to the preheated cell an operating quantity of a molten fused salt electrolyte; and,

(c) applying an operating current load acros the molten fused salt filled electrolytic cell to begin recovery of the products of electrolysis.

2. The method of claim 1, wherein an inert gas is purged into the cell during the preheating step to prevent substantial oxidation of the anode.

3. The process of claim 1, wherein the molten fused salt electrolyte is pre-treated to remove impurities.

4. A process for starting operation of a substantially empty electrolytic cell adapted for production of sodium by electrolysis of a fused bath containing sodium chloride, said cell having at least an anode, a cathode and removable product collection apparatus installed therein, and wherein no electrolysis is commenced prior to applying an operating currentload across the cell, comprising, in sequential order, the steps of: p

(a) preheating the substantially empty electrolytic cell without applying any electrical current across the cell to a temperature sufficient to prevent the solidification of a molten fused sodium chloride electrolyte to be introduced thereinto;

(b) adding an operating quantity of molten fused sodium chloride electrolyte to the preheatedelectrolytic cell; and,

(c) applying an operating current load across the molten fused sodium chloride filled electrolytic cell to begin recovery of sodium and chlorine.

5. The process of claim 4 further characterized by said temperature being at least 200 C.

6. The process of claim 4 further characterized by said operating quantity of molten sodium chloride fused salt electrolyte being obtained at least in part from the sodium chloride fused salt electrolyte of an operating electrolytic cell.

7. The process of claim 4 further characterized by said removable product collection apparatus being removed from said electrolytic cell during at least a part of the preheating step and being installed in said electrolytic cell before applying substantially operating current load across said molten sodium chloride fused salt filled electrolytic cell.

8. The process of claim 4 further characterized by beginning an inert gas purge into said electrolytic cell at least in the area surrounding said anode'during the preheating step to prevent substantial oxidation of said anode.

9. The process of claim 8 further characterized by said inert gas being nitrogen or argon.

10. A method of starting up a substantially empty electrolytic cell having at least an anode and a cathode, for electrolysis of a fused salt bath, wherein no electrolysis is commenced until after product collection means has been installed, consisting essentially of in sequently order, the steps of (a) preheating or drying the substantially empty electrolytic cell without applying any significant anode attacking amount of electrical current across the cell, to a temperature suflicient to prevent harmful solidification of a molten fused salt electrolyte to be introduced thereinto;

(b) installing product collection means in cell;,

(c) adding to the preheated cell an operating quantity of a molten fused salt electrolyte;'and, v

(d) applying an operating current load f across, the

molten fused salt filled electrolytic cell to begin recovery of the products of electrolysis.

References Cited UNITED STATES PATENTS 11/1959 McFadyen et a1. 20468 3/1968 Gomes et a1. 20464 R JOHN H. MACK, Primary Examiner D. R. VALENTINE, Assistant Examiner US. 01. X.R. 20464 R, 68 I 1 $32353? I I UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,77 ,823 m December 4, 1973 Inventor(s) S m CI'OWthe r It is certifiedthat error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, in the Heading, the Assignee ,Ethyl corporation" has been omitted. Column l,' line 58, reads "sodium chlorde'f and should read sodium chloride Column 3, line 6-1, reads "opration" and should read operation Column 4, line 16, reads "containing" and should read contain Column 5, line 17, reads "oxidazible" and should read oxidizible Column 7, line 46, reads "acros" andshould read across Column 8, line 36, reads "sequently" and should read sequential Signecl and sealed this 11th day of September 1974.

(SLAL) Attest:

McCOY M. GIBSON JR. C. MAnSHALL. DANN Astesting Ufficer Cormnissioner of Patents