US3840453A

US3840453A - Rotary cathode cell

Info

Publication number: US3840453A
Application number: US00254866A
Authority: US
Inventors: R Cooper; C Putnam
Original assignee: Olin Corp
Current assignee: Olin Corp
Priority date: 1972-05-19
Filing date: 1972-05-19
Publication date: 1974-10-08
Anticipated expiration: 1991-10-08

Abstract

1. A ROTARY CATHODE CELL FOR THE ELECTROYLSIS OF AQUEOUS ALKALI METAL CHLORIDE BRINES, COMPRISING A HORIZONTALLY ARRANGED SEALED CONTAINER,; INLET AND OUTLET MEANS FOR BRINE AND LIQUID METAL CATHODE, OUTLET MEANS FOR CHLORINE GAS; SAID LIQUID METAL CATHODE BEING ARRANGED AT THE BOTTOM OF SAID CONTAINER; A ROTATABLE CYLINDER ARRANGED HORIZONTALLY WITHIN AND SPACED FROM SAID CONTAINER, SAID CYLINDER CONTACTING AND BEING WETTED BY SAID LIQUID METAL CATHODE; AT LEAST ONE ANODE SUPPORTED FROM THE UPPER PART OF SAID CONTAINER, SAID ANODE BEING SUBSTANTIALLY ABOVE AND HAVING ONE FACE CONCENTRIC WITH AND UNIFORMALY SPACED FROM A PORTION OF THE SURFACE OF SAID CYLINDER; A PARTITION SEPARATING SAID INLET AND OUTLET MEANS FOR SAID LIQUID METAL CATHODE; AND MEANS TO SUPPORT AND ROTATE SAID CYLINDER.

Description

Oct. 8, 1974 R. M. COOPER ETAL 3.840.453

ROTARY CATHODE CELL l y Filed May 19, 1972 Oct. 8, 1974 R. M. COOPER ETAL ROTARY CATHODE CELL Filed May 19 1972 Untecl States Patent Oltce 3,840,453 Patented Oct. 8, 1974 3,840,453 ROTARY CATHODE CELL Roy M. Cooper, Groton, and Clair H. Putnam, Madison, Conn., assignors to Olin Corporation Filed May 19, 1972, Ser. No. 254,866 Int. Cl. B01k 3/00;'C01b 7/06; C01c 1/08 U.S. Cl. 204-212 24 Claims ABSTRACT OF THE DISCLOSURE An electrolytic cell having a rotary cylinder contacting a liquid metal cathode is employed in the manufacture of halogen gas and caustic alkalies. The cell incorporates the advantages of high current density, high current efciency, low space requirement and low cost per ton of product.

This invention relates to novel structures suitable for use as electrolytic cells, particularly cells of the liquid metal cathode type and, more particularly, in liquid metal cathode electrolytic cells having a rotary cathode member. The use of the structure of this invention in other cells of similar construction is also contemplated.

Horizontal mercury cells usually consist of an enclosed, elongated trough sloping slightly towards one end. The cathode is a owing layer of mercury which is introduced at the higher end of the cell and ilows by gravity along the bottom of the cell toward the lower end. The anodes are suspended in such a manner that the bottom of the anode is spaced a short distance above the ilowing mercury cathode.

Cells of this design require an essentially tlat surface to be completely covered with mercury and necessitating the uniform ow of mercury in a layer having a thickness as small as possible. In fact, the volume of mercury in circulation represents a considerable capitalv outlay and must be controlled within the strictest possible limits.

It is known to use a rotating cylinder to transport sodium amalgam in the decomposer section of an electrolytic chlorine cell. See, for example, U.S. Pat. 1,538,390; 699,414 or 546,353.

U.S. Pats. 753,819 and 2,587,630 depict electrolytic cells having rotating cylinder cathodes where the anode is located below the cathode, the cathode current is conducted through the cylinder and the cell is open to the air.

U.S. Pat. 798,314 illustrates a rotating cylinder cathode cell where the anode substantially surrounds the cathode, the cathode current is conducted through the cylinder and the cell is likewise open to the air.

Electrolytically precipitating and amalgamating a metal from solution in a cell containing a rotating cylindrical cathode was disclosed in British Pat. 19,068, Sept. 24, 1901. The cathode surface is provided with radial projections or bands to enlarge the cathode surface and to advance the particles of ore precipitated on to it during electrolysis. At the Ibottom of the cylinder is a pool of mercury with which the ore comes in contact and in which the metal is amalgamated. The cathode surface is not covered with mercury or amalgam and chlorine is not manufactured in the cell.

The novel cell of the present invention has a horizontally arranged axially rotating cylinder immersed in alkali metal chloride brine and contacting at its lower surface a body of conductive liquid metal cathode such that during rotation a continuous layer of the liquid metal is deposited on the rotating cylinder. At least one anode, concentric with a portion of the cylinder, is suspended above or to the side of the rotating cylinder and spaced therefrom by a uniform and optimum distance for electrolyzing brines to produce chlorine gas. 'Ihe anode is placed above and to the side of the rotating cylinder to prevent contact between the conductive liquid metal cathode and the anode and thus avoid short circuiting the cell. Current to the liquid metal cathode enters the cell thru the bottom of the container or thru the partition separating the inlet and outlet of the liquid metal cathode. The cell housing is a sealed container.

The electrolytic cell of the present invention operates advantageously at high current densities and current efiiciencies while greatly reducing the liquid metal cathode inventory required, the amount of tloor space required and the cost per ton of product.

This invention also provides means for the electrolysis of alkali metal chloride brines in a liquid metal cathode cell operated at elevated temperatures and pressures.

In addition, this invention, when using mercury as a cathode, produces an alkali metal amalgam of high purity by elimination of mercury butter which is formed in horizontal mercury cells.

More specifically the cell of the present invention for the electrolysis of aqueous alkali metal chloride brines comprises a horizontally arranged sealed container with inlet and outlet means for brine and liquid metal cathode and outlet means for chlorine gas. The liquid metal cathode is arranged at the bottom of the container. A rotatable cylinder, arranged horizontally within and spaced from the container, contacts and is wetted by the liquid metal cathode. At least one anode is supported from the upper part of the container and is substantially above and uniformly spaced from a portion of the surface of the rotatable cylinder. A partition separates the inlet means from the outlet means for the liquid metal cathode. Suitable means are provided for supporting and rotating the cylinder. Anode and cathode current-carrying means are attached to the anode and cathode respectively.

In one embodiment of the cell of the present invention, the rotating cylinder comprises an outer shell of an electrically conductive material which can be wetted with the liquid metal cathode. Suitable conductive materials include nickel, iron and alloys thereof. Metal composites, for example, nickel clad steel, can also be used.

The outer conductive shell is attached to an inner cylinder which can be either non-conductive or conductive. Suitable non-conductive materials which can be used for the cylinder include polymeric resins such as polyacrylates, polymethacrylates, polycarbonates, polyuoroolens, polyvinyl chloride and vulcanized rubber.

Where a conductive cylinder is used, a layer of nonconductive material is placed between the conductive shell and the conductive cylinder. Typical non-conductive materials include polylluoroolens, rubber or polyvinyl chloride.

Suitable means of attachment of the conductive shell to the cylinder include adhesives, e.g., epoxy resins, welding or fastening devices such as screws, pins or keys.

Likewise, the rotating cylinder can be of uniform composition where the conductive material has a coating or layer of a non-conductive substance, for example, a cement or adhesive, on the surface of the cylinder in contact with the c'ylinder support means.

The cylinder support means is rotatable and is axially arranged with respect to the cylinder. The cylinder support means is suitably a shaft which may be tubular or solid. Advantageously, it is fabricated of titanium or an alloy thereof which has superior resistance t-o the wet halogen gas and halogenated brine -to which it is exposed. Suitable means of rotation for the cylinder support are, for example, hydraulic or electric motors or other conventional means of rotation.

While mercury is the preferred conductive liquid metal cathode in contact with the rotating cylinder, other conductive metals which are liquids at ambient temperatures, e.g., gallium or liquid alloys can be used. It is particularly important that the amount of conductive liquid metal be sucient to provide a continuous layer on the rotating cylinder and yet not be excessive. A body of conductive liquid metal of from 1,500 to 2,000 lbs. is suitable. This represents in a cell using mercury a great reduction in the mercury inventory required, for example, in a cornmercial horizontal mercury cell of comparable capacity about 4,500 lbs. of mercury is required.

The conductive metal requirements can be minimized by the use of spacing elements as shown in FIG. 3, which also reduce the area of brine-conductive liquid metal interface. The spacing elements are made of any material which is chemically resistant to chlorine and `alkali metal chorides and not wettable -by mercury. Suitable materials include polymethacrylates, polyfluoroolens and rubber. They are secured by attachment means, for example, to fthe container wall or the anode support and are located in the lower half of the cell container, adjacent t but separate from the rotating cylinder.

Attached to the upper part of the container is one or more adjustable anodes. At least one surface of the anode is concentric with and uniformly spaced from -a portion of the rotating cylinder.

Optimum uniform spacing between the anode and the outer surface -of the rotating cylinder may be maintained by known methods of anode adjustment as disclosed, for example, in U.S. 3,390,070 and 3,574,073. In one embodiment of the present invention, uniform spacing can be maintained b'y the use of anode space pads. The pads can be located, for example, at the ends and/or the center of the anode and are attached to the anode frame. The thickness of the pads are such that they maintain the desired uniform space gap, for example, 2 to 3 millimeters and would contact very slightly the rotating cylinder during operation of the cell. Suitable non-conductive materials of construction for the space pads include polyacrylates, polymethacrylates, polycarbonates, polyfluoroolefins, polyvinyl chloride and vulcanized rubber. A preferred pad material 'is polytetrauoroethylene.

Anodes suitable for use in this invention are composed of graphite or metal, for example, zirconium, niobium, tungsten, tantalum or ti-tanium having a thin coating over at least part of its surface of a platinum group metal or oxide thereof. The term platinum group metal as used in this specification means an element of the group consisting of ruthenium, rhodium, palladium, osmium, iridium and platinum and alloys thereof. The term titanium includes alloys consisting essentially of titanium. These coated electrodes have the advantage of substantially complete resistance to corrosion and permit optimum spacing for the anode-cathode gap.

Anodes can be made in various forms, for example, expanded mesh lscreen or perforated plate. See, for example, U.S. Pat. 3,297,561. h

-Various anode designs are known in the art to facilitate the removal of bubbles and collection of the chlorine gas, including drill holes, channels, and slots variously arranged. See, for example, U.S. Pats. 3,062,733; 3,174,923; 3,268,427 and 3,558,464.

The inlet and outlet for the liquid metal cathode are located in the bottom of the container and are separated b'y a partition. The partition is suitably composed of a nonconductive or conductive material and is attached at its base to or is integral with the cathode bus bar. Nonconductive materials which can be used to form the partition include polymeric resins such as polyacrylates, polymethacrylates, polycarbonates, polyfluoroolens, polyvinyl chloride or vulcanized rubber. Suitable conductive `materials include nickel, iron land alloys and composites thereof.

Removal of the liquid metal cathode from the rotating cathode is facilitated by a doctor blade or similar device which directs the liquid metal cathode into its outlet and avoids mixing the amalgam with fresh mercury.

Following removal, the liquid metal cathode is transferred to a decomposer for the removal of the alkali metal. Modern decomposers are usually compact chambers packed with graphite. U.S. Pats. 2,083,648; 2,335,045; 2,422,351 and 3,215,614 show examples of suitable decomposers.

Suitabel alkali metal chloride brines for use in this invention include those of sodium, potassium or lithium. Brine concentrations employed are aqueous solutions containing from about 25 percent to about 28 percent by weight of the alkali metal chloride. Alkali metal bromides can suitably be used in place of alkali metal chlorides.

lIn operating the cell of this invention, brine inlet and outlet ows, brine preparation -and purication, liquid metal cathode ow to decomposers and recycle ow, hydrogen and chlorine collection and treatment are conventional. Y

The electrolytic cell of this invention can be operated at cylinder rotation speeds of from about 5 to 150 revolutions per minute (r.p.m.) with a preferred range of 10 to 70 r.p.m.

The cell of the present invention can be operated at atmospheric pressure as well as at elevated pressures, for example, from about 15 to about 150 p.s.i. absolute. Where pressure is employed, a range from about 25 to about 75 p.s.i. absolute is preferred.

Pressure is suitably provided, for example, by the controlled removal of gaseous chlorine produced in the cell or by supplying an inert gas such as argon, helium or nitrogen under pressure. In a pressurized cell, vaporization loss of liquid metal conductor is prevented and less mechanical compression is required in the conversion of gaseous to liquid halogen.

Temperatures ranging from about 50 to 150 C. are suitably used, with a preferred range of about to 130 C. Elevated temperatures may be provided, for example, by preheating the brine before introduction into the cell or the use of other conventional means for increasing temperatures.

At elevated temperatures, the solubility of chlorine gas in brine is considerably reduced. Also, the volume of a halogen gas bubble becomes smaller as the pressure increases. This in turn makes the section of electrolyte available for the passage of current correspondingly greater and reduces the losses of energy due to the resistance of the electrolyte. These conditions, in combination, provide higher current efliciencies, reduced cell voltage and faster and more complete release of halogen gas.

In the operation of the cell of the present invention, very high current densities, e.g., from 8 to 16, and preferably from 12 to 14 kiloamperes per square meter are employed.

Accompanying FIGS. 1-3 illustrate the cell of the present invention. Corresponding parts have the same numbers in all figures.

FIG. 1 illustrates one embodiment of mercury cell of this invention where a cell generally U-shaped is viewed from the end with the end plate removed.

FIG. 2 shows a cross sectional View taken along the phase indicated by the line 2 2 in FIG. 1.

FIG. 3 depicts another embodiment of the cell of this invention where a generally cylindrically shaped cell having a plurality of anodes is viewed from the end with the end plate removed.

More particularly, FIG. 1 illustrates a U-shaped cell in which the liquid metal cathode enters thru inlet 2 and forms a reservoir below rotating cylinder 3. Alkali metal chloride brine enters the cell thru inlet 4 and leaves, along with chlorine gas via outlet 5. lImmersed in brine is anode 6 shaped concentrically and uniformly spaced apart from rotating cyclinder 3. Anode 6 is supported from the top of the cell by anode lead-in 7 attached to the upper part of the cell. Partition 8 separates liquid metal cathode outlet 9 from liquid metal cathode inlet 2 and is attached to the cathode bus bar (not shown). Doctor knife 10 deects the liquid metal cathode layer on rotating cylinder 3 thru outlet 9. Anode space pads 11 maintain uniform spacing between anode 6 and rotating cylinder 3.

FIG. 2 shows a side view of rotating cylinder 3 consisting of outer conductive layer 12 and inner non-conductive cylinder 13. Rotary cylinder support 14 is attached to non-conductive cylinder 13 by bolt 15 and nut 16. Rotary cylinder support 14 extends thru rear plate V17. Support 14 carries drive means, suitably a pulley 18 for rotating cylinder 3. Anode 6 is supported by anode lead-in 7 suspended from support 19 by

nuts

20 and 21. Current is supplied to the anode by bus bar connection 22.

FIG. 3 illustrates an embodiment where the center of the cylinder support shaft is below the center of the cylinder but parallel to it. Spacing elements 23 are depicted.

EXAMPLE I A mercury cathode cell substantially of the design shown in FIG. 3, using platinized titanium anodes, was operated at a current density of 13 kiloamperes per square meter, a temperature of 11S-120 C. and a pressure of 50-60 p.s.i. absolute in the electrolysis of a sodium chloride brine containing 25 percent NaCl by weight. A 12.5 inch diameter cathode cylinder consisting of an outer shell of nickel attached to a non-conductive cylinder of polymethylmethacrylate was rotated at a speed of 30 r.p.m. Upon rotation, the cathode cylinder contacted the mercury cathode so that a layer of mercury was deposited and retained on the nickel. A current efficiency of 96 percent was obtained at a cell voltage of 5.079 with the chlorine gas produced containing less than 1 percent hydrogen.

EXAMPLE II The cell of Example I employing graphite anodes was operated at a temperature of 95 C. and 105 p.s.i.a. pressure for electrolyzing 25 percent by weight NaCl brine. The 12.5 inch diameter cathode cylinder consisting of a steel shell attached to a non-conductive cylinder of polymethylmethacrylate was rotated at a speed of 16 r.p.m. Employing a current density of 13.0 kiloamperes per square meter, a current eiciency of 92 percent was obtained at a cell voltage of 4.6.

EXAMPLE III The cell of Example II was operated at a current density of 13 kiloamperes per square meter, a temperature of 60 C. and at atmospheric pressure for the electrolysis of 25 percent weight NaCl brine. A cathode cylinder rotation speed of 24 r.p.m. was employed. A current efficiency. of 92 percent was obtained with the chlorine gas contalmng 1.4 percent hydrogen.

6 EXAMPLE 1v The cell of Example I, having a steel shell cathode cylinder, electrolyzing 25 percent by weight sodium chloride brine, was operated at a pressure of 33 p.s.i.a., and a temperature of C. The voltage at an anode current density of 10.4 kiloamperes per square meter was 4.36 and a current eciency of percent was attained. Rotation speed for the steel shell cathode cylinder was 24 r.p.m.

What is claimed is:

1. A rotary cathode cell for the electrolysis of aqueous alkali metal chloride brines, comprising a horizontally arranged sealed container; inlet and outlet means for brine and liquid metal cathode, outlet means for chlorine gas; said liquid metal cathode being arranged at the bottom of said container; a rotatable cylinder arranged horizontally within and spaced from said container, said cylinder contacting and being wetted by said liquid metal cathode; at least one anode supported from the upper part of said container, said anode being substantially above and having one face concentric with and uniformly spaced from a portion of the surface of said cylinder; a partition separating said inlet and outlet means for said liquid metal cathode; and means to support and rotate said cylinder.

2. The cell as claimed in Claim 1 which said liquid metal cathode is mercury.

3. The cell as claimed in Claim 1 in which said rotatable cylinder is a metal selected from the group consisting of nickel, iron and alloys and composites thereof.

4. The cell as claimed in Claim 3 in which said rotatable cylinder is a conductive shell attached to a nonconductive cylinder.

5. The cell as claimed in Claim 4 in which said conductive shell is a metal selected from the group consisting of nickel, iron and alloys and composites thereof.

6. The cell as claimed in Claim 5 in which said conductive shell is the metal nickel and alloys and composites thereof.

7. The cell as claimed in Claim 5 in which said conductive shell is steel.

8. The cell as claimed in Claim 5 in which said conductive shell is nickel clad steel.

9. The cell as claimed in Claim 4 in which said nonconductive cylinder consists of a polymeric resin.

10. The cell as claimed in Claim 9 in which said polyrneric resin is selected from the group consisting of polyacrylates, polymethacrylates, polycarbonates, polyuoroolens, polyvinyl chloride and vulcanized rubber.

11. The cell as claimed in Claim 1 in which said anode is graphite.

12. The cell as claimed in Claim 1 in Which said anode is titanium coated with a material selected from the group consisting of a platinum group metal or oxide thereof.

13. The cell as claimed in Claim 1 in which said partition is a polymeric resin selected from the group consisting of polyacrylates, polymethacrylates, polycarbonates, polyuoroolens, polyvinyl chloride and vulcanized rubber.

14. The cell as claimed in Claim 1 in which said partition is a metal selected from the group consisting of nickel, iron and alloys and composites thereof.

1S. The cell as claimed in Claim 14 in which said partition is nickel and alloys and composites thereof.

16. The cell as claimed in Claim 1 having means for maintaining a pressure therein of from 15 to 150 pounds per square inch absolute.

17. The cell as claimed in Claim 1 having means for maintaining a temperature therein of from 50 to 150 C.

18. The cell as claimed in Claim 1 having means for rotating said rotatable cylinder therein at from 5 to 150 revolutions per minute.

19. The cell as claimed in Claim 1 in which said brine is partially separated from said liquid metal cathode by one or more spacing elements.

20. The cell as claimed in Claim 19 in which said spacing elements consist of a polymeric resin selected from the group consisting of polyacrylates, polymethacrylates, polycarbonates, polyuorooleiins, polyvinyl chloride and vulcanized rubber.

21. The cell as claimed in Claim 20 in which said spacing element is polymethylmethacrylate.

22. The cell as claimed in Claim 1 in which said anode is uniformly spaced apart from a portion of the surface of said cylinder by means of one or more anode space pads.

23. The cell as claimed in Claim 22 in which said space pads are composed of a polymeric resin selected from the group consisting of polyacrylates, polymethacrylates, polycarbonate, polyfluoroolens, polyvinyl chloride and vulcanized rubber.

24. The cell as claimed in Claim 1 in which a doctor blade is attached to said partition.

References Cited UNITED STATES PATENTS 473,104 4/1892 Atkins 20'4-220 X 3,491,014 l/ 1970 Bianchi et al 204-242 2,836,400 5/1958 Jackson 204-213 X 1,536,569 5/1925 Cruse 204-212 1,538,390 5/1925 Ewan 204-59 2,234,967 3/1941 Gilbert 204-212 2,372,665 4/ 1945 Egli et al 204-25 3,096,272 7/1963 Beer 204-290 F 2,816,070 12/ 1957 Buchanan 204-279 FOREIGN PATENTS 204,995 1/1968 U.S.S.R. 204-219 U.S. Cl. X.R. 204-99, 219