US3589989A - Electrolytic cell start-up procedure - Google Patents

Abstract

A START-UP PROCEDURE FOR AN ELECTROLYTIC CELL USING A MOLTEN METAL CATHODE IS PROVIDED WHEREIN A SALT OF THE CATHODE METAL IS MIXED WITH THE NORMAL ELECTROLYTE TO YIELD A MIXTURE WITH A MELTING POINT LOWER THAN THAT OF THE PURE NORMAL ELECTROLYTE, THE CELL OPERATION COMMENCING BY HEATING TO A TEMPERATURE OF THE MELTING POINT OF THE PURE NORMAL ELECTROYLTE USING ALTERNATING CURRENT WITH SUBSEQUENT UTILIZATION OF DIRECT CURRENT AND DECOMPOSITION OF THE CATHODE METAL SALT TO CATHODE METAL AND THE CORRESPONDING ANION LEAVING THE PURE NORMAL ELECTROLYTE FOR NORMAL OPERATION OF THE CELL.

Description

Patented June 29, 1971 3,589,989 ELECTROLYTIC CELL START-UP PROCEDURE George Gerald Day, New York, N .Y., assignor to F. Barry Haskett, New York, N.Y., and John Hlxon Martin, Washington, D.C., fractional part interest to each No Drawing. Filed Oct. 23, 1968, Ser. No. 770,095

Int. Cl. C22d 3/00, 3/16 US. Cl. 204--64 6 Claims ABSTRACT OF THE DISCLOSURE A start-up procedure for an electrolytic cell using a molten metal cathode is provided wherein a salt of the cathode metal is mixed with the normal electrolyte to yield a mixture with a melting point lower than that of the pure normal electrolyte, the cell operation commencing by heating to a temperature of the melting point of the pure normal electrolyte using alternating current with subsequent utilization of direct current and decomposition of the cathode metal salt to cathode metal and the corresponding anion leaving the pure normal electrolyte for normal operation of the cell.

The present invention relates to a procedure or method for starting-up electrolytic cells useful in the electrolysis of molten salts; more particularly, the present invention is directed to such a start-up procedure for an, electrolytic cell employing a molten metal cathode, in particular, an electrolytic cell employing a molten lead cathode in the electrolysis of molten sodium chloride.

In the electrolysis of fused salts, particularly sodium chloride and related materials, a serious problem has evolved with relation to the start-up of the electrolytic cell. Thus for example, because the melting point of such salt electrolytes, e.g. sodium chloride is extremely high, somewhere in the order of 800 C., it is sometimes quite diflicult for the electrolytic cell to reach satisfactory operating temperatures. In this respect, previous processes utilizing such molten or fused electrolytic materials, e.g. sodium chloride have involved the use of expensive equipment to reach the necessary temperature so as not to freeze the sodium chloride or similar electrolytic cell. This, of course, is disadvantageous from an economic standpoint. Accordingly, a great desire still remains to provide an efficient way of starting up an electrolytic cell using a molten electrolyte material which will eliminate the inherent deficiencies and disadvantages of previously employed methods. The process of the present invention involving such an improved start-up procedure is applicable to those electrolytic cells in which, in addition to the employment of the molten electrolyte, e.g. sodium chloride or similar material the electrolytic cell utilizes a flowing molten cathode. Such a molten cathode is generally one selected from lead, tin, and mixtures of these with other alloy materials.

Thus, in accordance with the present invention it has been discovered that the disadvantages and deficiencies of prior art methods, particularly associated with the start-up procedure of electrolytic cells utilizing a molten metal cathode and a molten salt electrolyte can be eliminated by employing as the electrolyte during the start-up of the electrolytic process a molten mixture, e.g. a molten eutectic mixture comprising the normal electrolyte and a salt of the cathode metal.

Accordingly, it is a principal object of the process of the present invention to provide an electrolytic cell process, particularly a process for the start-up of an electrolytic cell employing a molten metal cathode and molten salt electrolyte, such start-up procedure eliminating the inherent deficiencies and disadvantages of previously employed procedures.

It is a further object of the present invention to provide a start-up procedure for electrolytic cells wherein the electrolyte employed in such start-up procedure comprises a low melting mixture of the normal electrolyte and a salt of the cathode metal,

Yet, a further object of the present invention comprises an improved start-up procedure for electrolytic cells employing a molten salt electrolyte and a flowing metal cathode wherein the electrolyte employed in the start-up procedure comprises a eutectic mixture of the normal salt electrolyte and, a salt of the cathode metal, such electrolyte entering the electrolytic cell in a molten state.

A still further object of the start-up procedure of the present invention comprises a method by which the electrolysis of the normal electrolyte is conducted subsequent to the decomposition of the cathode metal salt to provide additional cathode metal and the corresponding anion of the electrolyte material.

Still further objects and advantages of the start-up procedure of the present invention will become more apparent from the following more detailed description thereof.

The above objects and advantages of the present invention are achieved by a start-up procedure for the electrolysis of molten salt electrolytes in the electrolytic cell employing a molten metal cathode wherein the initial electrolyte employed in the start-up of the electrolytic cell comprises a mixture, e.g. eutectic mixture, of the normal electrolyte and a salt of the cathode metal. Preferably, the salt of the cathode metal has the same anion as the pure electrolyte so that upon decomposition such salt of the cathode metal forms additional metal cathode, plus the same anion as formed from decomposition of the normal electrolyte material.

As indicated previously, the type of electrolytic cell for which the improved start-up procedure of the present invention is applicable is one which contains a flowing molten metal cathode. Preferably, such molten metal cathode comprises lead, tin, mixtures thereof, or mixtures of such metals With other alloying materials. Such an electrolytic cell is generally of the horizontal type, the bottom of the electrolytic cell being slightly inclined so as to allow the flow of the molten cathode material. Such cells are designed for the electrolytic decomposition at high temperatures of inorganic compounds, e.g. sodium chloride and comprise a bottom surface as indicated above, side walls, anodes in spaced relationship to and disposed above the bottom and above the flowing cathode, and means separately electrically connected to the anodes and to the bottom of the electrolytic cell and adapted to pass electrical current to and from them. Such horizontal electrolytic cells are adapted to receive a molten inorganic electrolyte which floats on the cathode and is in contact with the anodes and decomposed by electrolysis during the passage of electrical current and operation of the electrolytic cell. Such an electrolytic cell which can advantageously employ the improved start-up procedure of the present invention is the type illustrated for example in Szechtman Pat. 3,104,213 as well as applicants co-pending application Ser. No. 542,002, filed Apr. 12, 1966, now US. 3,468,786 and directed to an improved cell bottom construction in such an electrolytic cell of the horizontal type employing for example, a flowing molten lead cathode. Of course, such an electrolytic cell is subject to various modifications, it being important for purposes of the present invention that the electrolysis of the molten salt electrolyte be conducted while in contact with a flowing molten metal cathode so as to take advantage of the improvement associated with the start-up procedure of the present invention.

In carrying out the improved start-up procedure of the present invention, the mixture of normal electrolyte and salt of the cathode metal material is melted outside the electrolytic cell in any convenient manner and run into the cell in the molten state. A suitable composition, for example, comprises sodium chloride, the normal electrolyte material, and lead chloride, the salt of the cathode metal. Such a composition has a melting point on the order of 410 C., as compared to the melting point of sodium chloride alone of 800 C. Accordingly, by employing the mixture of sodium chloride and lead chloride, it is possible to considerably lower the melting point of the initial electrolyte material, the melting point of the mixture being approximately one-half that of the pure normal electrolyte. The melting point of 410 C., is based upon a eutetic mixture of approximately 28 mol present sodium chloride and 72 mol percent lead chloride. However, other mixtures of such material can be conveniently employed, the result obtained being a slightly elevated melting point still considerably less, however, than the melting point of the pure normal electrolyte. It is preferred, however, in accordance with the present invention to operate at or near the eutectic point of the mixture of normal electrolyte and salt of the cathode metal. Again, such eutectic point appears with a mixture of sodium chloride and lead chloride at approximately 28 mol percent sodium chloride and 72 mol percent lead chloride. Generally, the mixture employed as the initial electrolyte in the start-up procedure of the present invention will comprise from about 25% to 35% by weight of the normal electrolyte material and from 65% to 75% by weight of the salt of the cathode metal, such salt of the cathode metal preferably having an ion in common with the normal electrolyte.

After the molten mixture of normal electrolyte and salt of the cathode metal enters the electrolytic cell, a low voltage alternating current is applied to heat the cell to the required operating temperature. While capable of heating the cell to the operating temperature, the low Voltage alternating current does not effect the decomposition or electrolysis of either the molten electrolyte or salt of the cathode metal within the initial electrolyte mixture.

When the proper temperature level is reached for electrolysis of the mixture of normal electrolyte and the salt of the cathode metal, the current is changed over to direct current capable of providing the electrolysis or decomposition of the mixture.

Since the decomposition potential of the salt of the cathode metal, lead chloride, will be lower than that of the normal electrolyte, the cathode metal will be deposited first at a lower voltage than required for electrolysis of the salt generally employed in the normal operation. In this way, the lead deposited from the electrolysis of the lead chloride builds up the cathode metal while the chloride given oif from such electrolysis when employing, for example, lead chloride, merely adds to the quantity of this product of the electrolysis that is produced. Since the electrolysis of the salt of the cathode metal comprising a portion of the initial electrolyte mixture employed in the start-up procedure of the electrolytic cell adds to the metal cathode that is employed, it is not necessary to initially start with the same quantity of cathode metal as desired during the normal operation of the electrolytic cell. Accordingly, it can easily be calculated from the amount of salt of the cathode metal in the initial electrolyte mixture how much of such cathode metal will be deposited to add to the flowing metal cathode. After the electrolysis of the salt of the cathode metal during the start-up procedure of the electrolytic cell the pure electrolyte material, e.g. sodium chloride will be left. Accordingly, at this point the cell can continue normal operation evolving chlorine gas and sodium metal when employing sodium chloride as the electrolyte.

In addition to the use of slightly less cathode metal than required during the normal operation of the cell, the full cathode metal can be used, and some withdrawn during and after electrolysis of the initial electrolyte comprising the normal electrolyte salt and salt of the cathode metal. In addition, suitable level maintaining or adjusting devices can be included within the conventional horizontal cell apparatus so as to always maintain the level of the cathode metal with respect to the molten electrolyte and anodes at that level which most efiicaciously promotes the electrolysis of the electrolyte material.

After adjusting the direct current voltage to that which is normally employed in the electrolysis of the pure normal electrolyte after the electrolysis of the salt of the cathode metal in the mixture of the initial electrolyte, more pure electrolyte material can be added as required.

While the above description has been based primarily upon an electrolytic system wherein electrolysis of sodium chloride is carried out, it is, of course, quite obvious that the improved start-up procedure of the present invention is applicable to the use of other normal electrolyte materials. Thus, for example, similar electrolyte materials which can be employed in the startup procedure of the electrolysis process of the present invention include, for example, potassium chloride, calcium chloride, lithium chloride, or magnesium chloride, each of which can be mixed with a salt of a normal cathode metal so as to produce a mixture of materials having a melting point lower than the normal electrolyte itself. Again, as with the mixture of sodium chloride and the salt of the cathode metal it is preferred to employ a eutectic mixture of the electrolyte and cathode metal salt; however, mixtures not having the eutectic characteristic and containing varying amounts of normal electrolyte and salt of the cathode metal can be advantageously employed Where desired.

In addition, while the above description has been given primarily with respect to the use of lead as a flowing metal cathode in the horizontal electrolytic cell electrolysis of a molten salt electrolyte, the improved start-up procedure of the present invention is applicable to the use of other low melting metals as the cathode metal. Thus for example, tin and other low melting metals can be advantageously employed as the cathode metal in the electrolysis process. Thus, under certain circumstances it is most advantageous to employ tin as the cathode metal with a salt such as tin chloride being added to the normal electrolyte in the production of the low melting mixture initially employed in the start-up procedure of the electrolysis. In this respect, for example, in the electrolysis of lithium chloride the use of tin chloride is advantageous since the use of tin as the cathode metal facilitates the separation of lithium by distillation. Thus, it is a fairly simple matter when employing a cathode metal material other than lead and/or a normal electrolyte other than sodium chloride to develop a mixture of normal electrolyte material and salt of cathode metal which has the necessary low melting point. In addition, it is a fairly simple matter to provide such compositions which because of the proper selection of the amount of the cathode metal salt and normal electrolyte provide a eutectic mixture for use in the start-up procedure.

It is pointed out again that it is advantageous in accordance with the process of the present invention to employ a salt of the cathode metal having an ion in common with the normal electrolyte. Thus for example, when employing sodium chloride as the normal electrolyte material, it is advantageous to employ the chloride salt of the cathode metal. This of course, allows for an increase in the chlorine production while providing a chlorine product not contaminated by other products of the decomposition and electrolysis of the cathode metal salt. Of course, it is possible to employ a salt of the cathode metal in which the ion is not in common with the normal electrolyte. The use of such salt, however, provides a minor amount of impurity into the product of the electrolysis of the normal electrolyte, which when the salt of the cathode metal is employed in a minor amount in the low melting composition, does not appreciably effect the yield or impurity from the electrolysis.

In this respect, it is pointed out that the cathode metal for purposes of illustration only and the present invention is in no way to be deemed as limited thereto.

EXAMPLE I Using a horizontal electrolytic cell employing a molten lead cathode, the cell being of the type described in U.S. patent application Ser. No. 542,002, filed Apr. 12, 1966, a molten mixture of lead chloride and sodium chloride Was employed as the initial electrolyte for the electrolysis process. Such a mixture of lead chloride and sodium chlo ride was prepared by melting a mixture of 28% sodium chloride and 72% lead chloride, the mixture having a melting point of approximately 410 C. The melted mixture is heated to a temperature of 425 C.,' and run into the horizontal electrolytic cell, preheat the cell to 425 C., until the normal salt level in the cell is reached. Alternating current was then applied to the anode and cathode terminals at low voltage, alternating current, of course, does not electrolyze but, the current is sutficient to heat the pure electrolyte to its melting point of approximately 80l C. At this point, the current is changed to direct current with electrolysis of the lead chloride portion of the eutectic mixture taking place first. Under such conditions, lead is deposited at the cathode of the electrolytic cell and chlorine released at the anode and withdrawn in conventional manner. The lead deposited at the cathode adds to the cathode metal employed in the horizontal electrolytic cell, the proper amount of cathode metal being maintained through the use of a conventional level adjusting device. After electrolysis of the lead chloride portion of the eutectic mixture direct current voltage is applied in that amount necessary to eifect the normal electrolysis of the normal electrolyte material. Additional pure electrolyte is added as required during hydrolysis as in conventional electrolytic processes.

EXAMPLE II A similar process as in Example I is utilized except that the initial mixture comprises a mixture of lithium chloride and lead tin chloride, molten lead tin being employed as the flowing metal cathode in the electrolysis process. The molten mixture of electrolyte material and cathode salt was prepared by melting a mixture of 42% mol percent lithium chloride and 58% mol percent lead in chloride to provide a low melting composition. When the electrolysis of the salt of the cathode metal increases the tin 6 content of the cathode after further electrolysis conducted in a conventional manner.

While the present invention has been described primarily to the foregoing specific examples, it is to be understood that the present invention is in no way to be deemed as limited thereto, but must be construed as broadly as all or any equivalents thereof.

What is claimed is:

1. A start-up procedure for electrolytic cells employing a molten electrolyte disposed between anodes and a molten metal cathode, said start-up procedure comprising:

(a) mixing an electrolyte salt of a normally high melting point with a salt of the cathode metal so as to produce a mixture having a melting point lower than that of the electrolyte salt;

(b) melting said mixture;

(c) running the molten mixture of (b) into the electrolytic cell;

(d) heating the electrolytic cell to a temperature of the melting point of the electrolytic salt using alternating current;

'(e) eifecting the decomposition of the salt of the cathode metal byemploying direct current sufficient to eifect the same; and

(t) after the decomposition of said salt of the cathode metal, efiecting normal electrolysis of said electrolyte salt.

2. The start-up procedure of claim 1 wherein said salt of the cathode metal has an ion in common with said electrolyte salt.

3. The start-up procedure of claim 2 wherein said salt of the cathode metal comprises lead chloride and said electrolyte salt comprises sodium chloride.

4. The start-up procedure of claim 1 wherein said mixture of said salt of the cathode metal and electrolyte salt comprises an eutectic mixture.

5. The start-up procedure of claim 4 wherein said salt of the cathode metal has an ion in common with said electrolye salt.

6. The start-up procedure of claim 5 wherein said salt of the cathode metal comprises lead chloride and said electrolyte salt comprises sodium chloride.

References Cited UNITED STATES PATENTS 2,913,381 11/1959 McFadyen et al. 20468 3,071,523 1/1963 Zadra et a1 204--64 3,271,277 9/ 1966 Yntema 20464 3,382,166 5 1968 DeVarda 204-246X 3,383,294 5/1968 Wood 20471X JOHN H. MACK, Primary Examiner D. R. VALENTINE, Assistant Examiner U.S. Cl. X.R. 20466, 68, 71

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION patent 3,589,989 Dated June 29, 1971 Inventor(s) George Gerald Day It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

In the Heading, second line after the title, please amend the name of the second assignee to read as follows:

JOAN HIXON MARTIN Signed and sealed this 22nd day of February 1972.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GO'ITSCHALK Attesting Officer Commissioner of Patents FORM PC4050 (10-69) USCOMM-DC 60376-P69 9 U 5 GOVERNMENT PIHNYING OFFICE I969 0-35 334