US3463721A - Purification device for alkali metal electrolysis cells - Google Patents

Description

Aug. 26, 1969 L. l... HARRIS 3,463,721

PURIFICATION DEVICE FOR ALKALI METAL ELECTROLYSIS CELLS Filed March 3, 1967 2 Sheets-Sheet 1 FIG. 2.

6,1969 L. L. mms 3,463,721

PURIFICATION DEVICE FOR ALKALI METAL E LECTROLYSIS CELLS Filed March a, 1967 2 Sheets-Sheet 2 United States Patent 3,463,721 PURIFICATION DEVICE FOR ALKALI METAL ELECTROLYSIS CELLS Luther L. Harris, La Porte, Tex., assignor to Ethyl Corporation, New York, N.Y., a corporation of Virginia Filed Mar. 3, 1967, Ser. No. 620,482 Int. Cl. C22d 3/02 US. Cl. 204-245 7 Claims ABSTRACT OF THE DISCLOSURE Apparatus for removing undesired metal deposits from product and product conduit walls of fused melt electrolysis cells. A rotatable central shaft supports vertically spaced horizontal arms having radial extensions. Vertical scraper bars are movably mounted on the ends of said radial extensions.

Background of the invention Generally, cells for the production of alkali metals are of a type basically similar in design to the Downs cell, US. Patent 1,501,756. This cell is characterized by a bottom-mounted carbon anode surrounded by a cylindrical steel cathode. Within the annular portion formed between the anode and cathode a foraminous metal diaphragm is inserted usually. During electrolysis, chlorine is evolved from the anode and is removed from the upper portion of the cell through a gas dome situated above the anode, and alkali metal, or sodium, which is separated from the salt solution electrolyte, is displaced to the upper portion of the cell, manifolded within an annular collecting chamber and removed through a vertical take-off line or riser pipe communicating with the cell exterior.

Because of the high temperature required to melt alkali metals salts, particularly sodium chloride, it is customary to add a second salt thereto to lower the melting point of the electrolyte mixture. Generally, a fused mixture of calcium chloride and sodium chloride is employed for the manufacture of sodium. This necessitates a separation of calcium metal from the product sodium metal, and a major portion of the calcium metal is removed by passing the sodium metal upward through a cool riser pipe prior to discharge. The sodium metal solution cools gradually during its upward ascent causing precipitation of calcium metal from the molten sodium. Most of the calcium metal precipitated falls or is returned to the bottom of the electrolytic cell where it dissolves; however, some calcium metal is deposited on the inner wall of the riser pipe. Such deposits, if not removed, will eventually cause complete plugging of the riser pipe, and it is, therefore, essential that these deposits be removed. customarily, tickler devices are used to dislodge calcium deposits in riser pipes of sodium cells and automatically operated tickler devices are described, for example, in U.S. Patents 2,770,364, 2,770,592, 2,861,938 and 2,944,955. These devices usually comprise a cleaner assembly operatively connected to a drive mechanism. The cleaner assembly usually consists of a plurality of cleaner blades or scrapers mounted so as to act upon the inner wall of the riser pipe, generally over its entire length, and to dislodge forming, or already formed, calcium deposits. Generally, the blades or scrapers are allixed to a shaft whose axis of rotation corresponds to the center of the riser pipe and extends therethrough. The shaft and affixed scraper bar generally are capable of rotation around the axis of rotation and of limited vertical movement along the axis.

Various drive mechanisms have been proposed for these devices; however, there is a common tendency for "ice all devices to function for a limited period and then to fail due to their inability to withstand the operating condition or to remove successfully all deposits. After a tickler fails, or ceases to function, a new cleaner assembly must be inserted into an operating cell. The duration of time within which a tickler will continue to function properly before it must be removed from the cell is referred to as tickler life, and for present commercial ticklers this service period is of extremely short duration. When an automatic tickler fails to function, it is essential that the tickler be placed back in operation promptly to prevent a complete plugging of the riser pipe. Should this occur the blockage must be chipped away by hand and a new cleaner assembly placed in the operating cell. A commercial installation for producing sodium contains a large number of individual cells and the cleaning of riser pipes and replacement of ticklers is a costly operation. Therefore, extension of tickler life means large savings in labor and material costs, and obviously, the extension of tickler life is a very desirable goal.

Accordingly, it is an object of this invention to provide a new and improved tickler or cleaning assembly capable of operation for extended periods of time without failure. It is also an object of this invention to provide a new and improved tickler which will decrease operating cost greatly. A further object of this invention is to provide an apparatus more simple in construction and effective in cleaning riser pipe walls than has heretofore been possible.

Summary of invention These and other objects are accomplished in accordance with this invention, in a fused salt electrolysis cell having a riser pipe extending above the cell for concurrently discharging sodium metal and for cooling, precipitating and returning to the cell calcium impurity contained therein, said riser pipe having a tickler assembly in combination therewith which includes a cleaning device extending into said riser pipe and driving means operably connected therewith for at least rotating said cleaning device, by an improved cleaning device comprising, in combination,

(a) A rotatable elongated member extending centrally and substantially vertically from substantially the bottom through the top of said riser pipe for operable connection with said driving means,

(b) At least two arms mounted in vertical spaced relationshi substantially horizontally on and extending radially from said elongated member, and

(c) At least one vertical scraper bar .movably mounted on the radial extension of a vertically adjacent pair of said at least two arms and extending therebetween, said at least one vertical scraper bar being constructed and arranged to have one vertical side move toward said riser pipe when said elongated member is rotated in one direction and said one vertical side move away from said riser pipe when said elongated member is rotated in the op posite direction.

A preferred embodiment of the invention is the apparatus described above further characterized by said one vertical side being provided with a substantially knife-like edge.

Another preferred embodiment is the apparatus described above further characterized by having a plurality of vertical scraper bars attached to a plurality of arms 7 and spaced alternately on opposite sides of said rotatable said at least two arms extending in the same radial direction from said rotatable elongated member.

And another preferred embodiment is the apparatus described above further characterized by said at least two arms being slotted horizontally for mounting said at least one scraper bar.

A further preferred embodiment of the invention is the apparatus described above further characterized by the vertical scraper bar closest to the bottom of said riser pipe being rigidly affixed to said arms.

A still further preferred embodiment of the invention is the apparatus described above further characterized by said at least two arms having means for restricting the movement of said at least one vertical side away from said riser pipe.

Brief description of the drawing The details of the apparatus and of a preferred embodiment will be more readily understood from the following description and by reference to the accompanying drawing having FIGS. l-4 wherein:

FIG. 1 is a generalized elevation illustrating the relation of the apparatus of this invention to a fused salt electrolysis cell for which it is intended.

FIG. 2 is an elevation, in partial cross section and on a larger scale, of a cell riser pipe having the apparatus bf this invention incorporated therein, and

FIG. 3 is a sketch showing the assembly of at least one section of the apparatus of this invention.

FIG. 4 is a cross section 22 of FIG. 2 showing the movement of the scraper bars of the apparatus of this invention in greater detail.

Description of a preferred embodiment Referring first to FIG. 1, the principal parts of an electrolytic cell for the production of sodium and chlorine from a molten mixture of at least sodium chloride and calcium chloride include a cell body 11 comprising an external steel shell lined with refractory brick, a centrally mounted carbon anode 12 surrounded by a cylindrical cathode 13 having lugs 14 projecting through the cell walls to provide for connection to bus bars 15 for supply of electric current. Similarly, current is fed to anode 12 by an anode bus bar 16. Dome and collector assembly 17 is mounted centrally above the anode-cathode electrodes and includes a chlorine dome 18 in the form of a generally cylindrical member and a sodium collecting conduit 19. The chlorine dome terminates in a chlorine discharge line 20 to transfer gaseous chlorine to subsequent operations. Sodium collecting conduit 19 meets and is connected with riser pipe 21, shown in this figure in an external view, which is an elongated steel tube which may be provided with fins 22 or other means on at least a portion of the external surface. Surmounting riser pipe 21 is a guiding and actuating assembly 23 for supporting and moving the apparatus of this invention.

As shown in FIG. 2, molten sodium may be discharged from the cell at the top portion of riser pipe 21, over wier 25 into receiving box 26 mounted adjacent to riser pipe 21. Receiver box 26 provides a container for temporary recovery of produced sodium which may be collected subsequently in a transfer vessel or container not shown, from discharge spout 27.

In operation, bath 31 contains molten electrolyte chlorides to a level above the bottom of chlorine dome 18 and sodium conduit 19 and intermediate the full vertical elevation of chlorine dome 18. The molten electrolyte chlorides generally include sodium chloride and appreciable quantities of calcium chloride to reduce the melting point of the sodium chloride. A sodium-calcium mixture and chlorine are produced in the annular space between anode 12 and cathode 13 and rise on opposite sides of diaphragm 32. The sodium-calcium mixture collects in channel 19 and flows to the entrance of riser pipe 21, rises in the riser pipe and is discharged over wier 25 to receiver 26 for subsequent collection from discharge spout 27. Cooling of the sodium-calcium mixture in the riser pipe precipitates calcium from the mixture and a major portion thereof falls down through the rising mixture to be returned to the cell; however, a noticeable quantity tends to deposit on the interior walls of the riser pipe.

Details of the construction and the method of operation of the improved cleaning device of this invention will be more readily understood by reference to FIG. 2. Guiding and actuating assembly 23 located on top of riser pipe 21 carries and rotatably supports central shaft 41 and driving means 44 for the shaft. In the embodiment described, driving means 44 is manual, however, it is clear that such means may be pneumatic of electrically driven or a combination of such devices. A preferred pneumatic device driving means is described fully in US. Letters Patent 2,770,364.

Located in vertically spaced relationship on central shaft 41 are substantially horizontal arms 42 having radial extensions 45. In the preferred embodiment of FIG. 2, a plurality of arms are shown, however, it is clear that only at least two are necessary for the apparatus and method of the invention. Movably mounted on the radial extension of a vertically adjacent pair of arms 42 are vertical scraper bars 46. Scraper bar 46 may be provided in suflicient length so that the cleaning path of one bar overlaps the cleaning path of the bar immediately above and below, and it is clear also that in an embodiment having only two arms only one vertical scraper bar would be required. In an embodiment having two arms and one scraper bar or more than two arms and two or more scraper bars, it is clear that it is not necessary that the scraper bar be located vertically. It is within the scope of the invention that the arms and the radial extensions thereof may be located on shaft 41 at a position such that the radial extensions of the arms do not lie within the same vertical plane. In this embodiment, suitable connections may be provided to locate the movable scraper bar or bars between vertically adjacent arms not in the same vertical plane.

Referring now to FIG. 3, horizontal arm 42 consists of an upper section 47 and a lower section 48 for securing and movably mounting vertical scraper bar 46. For the assembly of the apparatus, vertical scraper bar 46, having knife-like edge 49, is joined securely by welding or other suitable means to support plates 50 with plates 50 being located on bar 49 with a proper distance between them to provide for sliding to the assembly freely into the horizontal slot between upper and lower sections 47 and 48 on a vertically adjacent pair of arms 42. Attached to each plate 50, is a stop bar 51 provided in a length sufiicient to overlap sections 47 and 48 when assembled. Fixed stop bar 52 is fixedly located by welding or other means on arm 42 across the slot between sections 47 and 48 to restrict the movement of plate 50 in a horizontal direction toward shaft 41 and to prevent the inadvertent disassembly of the apparatus. After locating the assembled vertical scraper bar, horizontal plates 50 and stop bar freely within the slotted area between sections 47 and 48 on a vertically adjacent pair of arms 42, end plate 53 supported by beam 54 is fixedly attached across the radial ends of sections 47 and 48; to prevent the disengagement of the vertical scraper ar assembly from the apparatus during the insertion and removal of the improved cleaning device from riser pipe 21 and during operation thereof. If necessary or desired, one or more scraper bars may be rigidly attached to a pair of arms 42 or to one or more permanent arms as indicated at 55.

The method of operation of the improved cleaning device of this invention may be understood more clearly by reference to FIG. 4 which is a cross section 2-2 of FIG. 2 enlarged. As can be seen on FIG. 4, when shaft 41 is rotated in a counterclockwise direction, scraper bar 46 contacts the inside wall of riser pipe 21 at its knifelike edge 49 sweeping away deposited material which may then fall freely to the bottom of the riser pipe for return to the cell. When shaft 41 is stopped or is rotated in the opposite direction, the scraper bar, and the parts fixedly attached, move toward the shaft in position Within riser pipe 21 to permit easy removal and insertion of the improved cleaning device. It is clear that a scraper bar assembly may be attached to the arms in a manner so that the cleaning device may be operable by rotation in either or both directions and vertical movement may be made of the entire cleaning device during operation also.

Any suitable material or materials which will Withstand the service conditions of temperature and contacting materials of the application for the cleaning device are suitable for the construction of the improved cleaning device of this invention. When the improved cleaning device is used in the riser pipe a fused salt electrolysis cell for the manufacture of sodium and chlorine, it is preferred that this apparatus be constructed from a material such as stainless steel.

The improved service life resulting from the use of the improved cleaning device and method of this invention is shown most clearly in Table I below wherein days of service of the improved cleaning device are shown for seven full size test electrolytic cells for the manufacture of sodium chloride by the electrolysis of a fused bath including sodium chloride and calcium chloride. The days of service are days that the improved cleaning device of this invention, as described in the drawings, remained in service on a 24-hour basis prior to removal for the reasons shown under the column. For those cells having a listing of in service, it is meant that the improved cleaning device was still in service and operating successfully at the time of the termination of the test to obtain the data for the table.

TABLE I Days Cleaning device Cell service Reason removed condition 61 Diaphragm change No. 5 blade broken. 77 ...do OK. 70 Operator decision OK. 17 Diaphragm change. N o. 5 blade broken. gig In service pipe having a tickler assembly in combination therewith which includes a cleaning device extending into said riser pipe and driving means operably connected therewith for at least rotating said cleaning device, an improved cleaning device comprising, in combination,

(a) a rotatable elongated member extending centrally and substantially vertically from substantially the bottom through the top of said riser pipe for operable connection with said driving means,

(b) at least two arms mounted in vertical spaced relationship substantially horizontally on and extending radially from said elongated member, and

(c) at least one vertical scraper bar movably mounted on the radial extension of a vertically adjacent pair of said at least two arms and extending therebetween, said at least one vertical scraper bar being constructed and arranged to have one: vertical side move toward said riser pipe when said elongated member is rotated in one direction and said one vertical side move away from said riser pipe when said elongated member is rotated in the opposite direction.

2. The apparatus of claim 1 further characterized by said one vertical side being provided with a substantially knife-like edge.

3. The apparatus of claim 1 further characterized by having a plurality of vertical scraper bars attached to a plurality of arms and spaced alternately on opposite sides of said rotatable elongated member, each of said plurality of vertical scraper bars being of suflicient length so that the vertical distance encompassed by one vertical scraper bar overlaps the vertical distance encompassed by the next alternate vertical scraper bar.

4. The apparatus of claim 1 further characterized by said at least two arms extending in the same radial direction from said rotatable elongated member.

5. The apparatus of claim 1 further characterized by said at least two arms being slotted horizontally for mounting said at least one scraper bar.

6. The apparatus of claim 3 further characterized by the vertical scraper bar closest to the bottom of said riser pipe being rigidly aflixed to said arms.

7. The apparatus of claim 5 further characterized by said at least two arms having means for restricting the movement of said at least one vertical side away from said riser pipe.

References Cited UNITED STATES PATENTS 3,037,927 6/1962 Gallinger 204245 JOHN H. MACK, Primary Examiner D. R. VALENTINE, Assistant Examiner U.S. Cl. X.R. 204--68

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