US2861938A - Electrolytic cells - Google Patents

Description

Nov. 25, 1958 R. D. s. GLAscoDlNE 2,861,938

ELECTROLYTIC CELLS Filed April 1, 1955 ATTORNEYS P Patented Nov. 25, 195d assigner to Imperial Chemical Industries Limited, Lon don,`England, acorporation of Great Britain Application April 1, 195s, serai No. 498,79 Claims priority, application Great Britain April 7, 1954 4 Claims. (Cl. 20ct- 243) This invention relates to an improvement in or relating to electrolytic cells and processes for the manufacture of sodium by the electrolysis of fused salts comprising sodium chloride and calcium chloride. More particularly it relates to a device for the removal of undesired metal deposits in conduits for the take-off of liquid alkali metals in electrolytic cells for the manufacture of sodium by the electrolysis of fused salts comprising sodium chloride and calcium chloride, to electrolytic cells in which the device is incorporated and to processes of electrolysis wherein su'chcells are employed. t t

in the production of an alkalimetal by electrolysis of fused salts comprisingl an` alkali metal halide, the electrolyte used is commonly a mixture of two or more salts, so chosen that the mixture has a relatively low melting point. For example in the production of sodium, the electrolyte commonly used consists of a fused mixture of sodium chloride and calcium chloride, which mixture has a melting point considerably lower than that of sodium chloride;`

A suitablecell for carrying' out the electrolysis of such a mixture comprises a graphiteanode projecting upwardly through the bottom of the cell, a steelcathode surrounding the anodea'nd a metal diaphragm disposed between anode and cathode. Situated abovethe anode is a dome for collection of evolved chlorine and abo-ve the cathode and conveniently made integral with the dome is an inverted annular collecting chamber provided with a vertical conduit communicating withy the exterior ofthe cell. The liquidmetallic products evolved at the cathode rise through the electrolyte, collect in theannular chamber and thence pass through the said` conduit to the exterior ofthe cell; The diaphragm,vconveniently made of steel wire gauze, is suitably supportedat the junction of dome and conduit. Such` a cell is well known and is described in UnitedStates patent specification No. 1,502,756.

However, when a lmixture of sodium and calcium chlorides is electrolysed in such acell considerable quantities of calcium are liberated'at theicathode together with the sodium, which'calciumat the temperatures existing in the electrolytic cell is ordinarily completely dissolved in the sodium. However, the solubility of calcium in molten sodium diminishes with decrease of temperature and it is customary to take advantage of this fact'tobring about the'separationof the calciumfrom the sodium before the latter leaves the cell; To this end it is arranged that a temperature gradient is set up along the vertical conduit leading `the sodium from thecell (commonly termed the sodium riser pipe) so that the molten metal is cooled to such an extent that most of the calcium is precipitated before the sodium passesV over a weir at the top of the riser and into a receiver. .Unfortunately however, this procedure in turn creates a further problem.. The cooling of the molten metal proceeds by heat transfer from the outer layers ofthe rising stream to the walls of the riser pipe and therefore the calcium is precipitated mainly `on the walls of the pipe. .in consequence a solid deposit of calcium is gradually builtup which, if not removed, tends seriously to obstruct the flow of sodium from the cell. f

`In United States patent `specilication No. 2,068,681 there is described a cell in which the vertical conduit leading the sodium from thecell is cooled by means of fins attached to the outside of the conduit and there is also described and illustrated a device to be installed in the sodium riser pipe whichis adapted to scrape the adherent calcium olfV the walls of the pipe and allow it to fall back into the cell where it reacts with sodium chloride to form sodium metal and regenerate calcium chloride.

` it is said that the device may be given a vertical reciprocating motion or a rotational `motion or both and that `it can be operated `continuously or, preferably, intermittently,`and that in the latter case a convenient time cycle is to operate the device every half-hour.

in practice it i'sl found necessary to operate this device every half-hour and to give it irst avertical reciprocating motion and subsequently a rotational motion. Further, numerous attempts have been made to. devise `means for operating the device automatically but s o far as we are aware, none has beensuccessful, and in practice it has to be operated manually. In View of the high. temperature at which the cells are run and the physical effort `involved in operating the device the provision of alternative meansfor its operation is a verydesirable objective but so far as we are aware there has not yet'been devised any s`atisy factorymeans for the automaticoperation of this or any other device for removing the calcium deposits from the sodium riser pipes. The problem is delicate, because inadequate scraping of the pipe leads to serious obstruction or blockage,` whereas unduly vigorous operation of a scraping device interferes with the requisite gravitational settling of the precipitated calcium and leads to excessive quantitiesof calcium beingcarried with the sodium overV the Weir and into the receiver,.thus increasing the calcium content of the sodium produced beyond the tolerable limit.

According to the present invention an improved scrap` ing device for the removal of undesiredmetal deposits in a verical conduit forthe take-off of liquid metal in electrolytic cells for the manufacture of sodium by the electrolysis of fused salts comprising sodium chloride and calcium chloride, which device is adapted to be inserted in said conduit, comprises one or more longitudinal, preferably substantially vertical, members bearing rigidly attached thereto and held in vertically spaced relationshipl to each other, preferably at approximately equal intervals, a number of hoops preferably each provided with a cutting edge on both its upper and lower rims, the said device being provided with means to eifect longitudinal reciproeating movement within said'conduitsuch that: the amplitude of the reciprocating motionis at least equal. to the" greatest distancebetween consecutive hoops, the diameter of each said hoop being slightly less than the internal diameter of that portion of the conduitwhich it is capable of traversing, the device being robustly constructedof a material capable of withstanding attack by the electrolyte and by the molten metal products.

Advantageously there is provided an upper and a lower 'stop adapted to check abruptly the reciprocating motion and so impart to the scraper a shock or jar calculated to dislodge therefrom any metal that is precipitated thereon or that having been dislodged from the walls of the riser pipe subsequently tends to adhere to the scraper.

A convenient form of such a device consists of some 6-8 hoops, like barrel hoops, of a diameter slightly less 'than that of the riser pipe and a height approximately one-third to one-fourth of the diameter, each having both the upper and lower edges internally charnfered to make a cutting edge, these hoops each being attached, suitably by welding at opposite ends of a diameter to a pair of parallel rods that lie within the hoops and are joined at the top by aat vertical plate or saddle which is of stepped form so that when the device is at the lowest point of its travel the upper portion of the plate seats on an internal ange or lip in the riser pipe which flange thereby provides the lower of the two stops referred to above. Rising from the centre of the saddle plate is an actuating rod which passes through a guide plate supported on the upper rim of the riser pipe and is attached to the piston of a double-acting pneumatic cylinder which also is conveniently supported on brackets or rods that are attached to the upper rim of the riser pipe. The internal flange in the riser pipe and the guide plate attached to the upper rim of the latter serve as the two stops previously mentioned that define the limits of travel of the scraper and therefore the distance between the ange and the guide plate determines the maximum distance between consecutive hoops. For eicient scrap'- ing of the whole surface of the riser pipe the distance between guide plate and flange diminished by an amount equal to the height of the upper portion of the saddle plate should be slightly greater than the distance between consecutive hoops; this relationship will be more readily visualized if reference is made to the drawing which illustrates a form of the invention described in more detail hereunder'.

Alternatively, the lower stop may be provided by means of a collar such as a nut securely attached to the actuating rod in such a manner that when the device is inserted in the riser pipe the nut rests on the upper surface of the guide plate and the saddle plate is just removed by a short vertical distance from the inturned flange of the riser pipe. In this case the distance between the top surface of the saddle plate when at its lowest point of travel and the lower surface of the guide plate determines the maximum vertical travel of the device; the distance between successive hoops is made slightly less than this figure. In its downward journey the device is brought to a jarring halt by contact of the nut with the guide plate.

In a form cell frequently employed for making sodium the riser pipe is made in two sections joined by a flange, this construction being adopted for ease of fabrication, adjustment and maintenance. The lower section of the pipe is made integral with the chlorine collecting dome and the annular sodium collecting channel and to this unit the diaphragm is attached; the upper section of the riser pipe is itself composite, the lower portion thereof being the same bore as the lower section of the pipe but the upper portion being of wider bore. The junction of these two portions provides' the internal flange referred. to above. The upper section of the pipe is provided with; external tins to increase the cooling surface and the wide bore portion of this upper section is coupled to or made integral with'the sodium receiver and includes the weir over which the rising stream of molten sodium ows into` the receiver. When such a cell is used it is difcult, by reason of the size and weight of the two units to be coupled together and the high temperatures at which the cell operates, to achieve and maintain exact alignment of the two sections of the riser pipe. Accordingly it is. preferable in such a case to make the hoops that traverse,

the lower section of the riser and the junction of the two sections slightly smaller in diameter than those in the upper section. The difference need not be great and 1n practice this change does not appear to impair the efficient functioning of the device.

Nevertheless, if desired the whole of the upper section of the riser pipe may be of the same cross-section, being provided of course with a suitably placed orifice through which the sodium passes to the receiver. With such a pipe the stops may adequately be provided for by the saddle plate coming in its upward journey into contact with the lower surface of the guide plate by a nut on the actuating rod coming in its downward journey into contact with the upper surface of the guide plate.

In the specic form of this invention described in more detail hereunder the activating mechanism that effects and controls the reciprocating movement of the scraper is a double-acting pneumatic cylinder. However, the invention is not limited to the use of this particular form of controland other means which may be, for

instance, mechanical, electrical or magnetic, may likewise be used. The operating schedule is, however, important. Insuiiicient scraping can obviously lead to obstruction that impedes the operation of the scraper and so culminates in a substantial blockage of the riser pipe, while on the other hand unduly frequent or unduly vigorous operation may result in a pumping action that brings excessive amounts of calcium4 over the ,Weir with Ithe sodium. Unduly vigorous action may likewise lead to subdivision of the precipitated calcium into particles so fine that they tend to be more readily carried up in the rising stream of molten metal. The appropriate operating schedule in any given case will clearly depend on the design of the riser pipe, which, coupled with the conditions under which the cell is operated, will determine the natural rate of upward flow of the molten metal from the cell, the precise construction of the scraper device (including, for instance, such factors as its clearance from the walls and the relationship of the volume swept out by the device with each stroke to the volume of molten metal contained in that portion of the riser pipe within which the device operates) and the speed of both the upward and downward movements that constitute each reciprocatory cycle. Bearing in mind the effects of these various factors one can arrive at an appropriate operating schedule in any given case by routine experimentation.

The accompanying drawing (not to scale) shows a vertical section through the centre of a riser pipe with the scraping device installed therein. Referring to the drawing, the upper and lower sections of the riser pipe, 1 and 2 respectively, are united by the ilanged joint 3. At its upper end, the upper section 1 opens out into a portion of wider bore 4, the two portions being joined by the internal llange 5. At the upper extremity of the wide-bore section 4 is a flange 6 which forms a seating and point of attachment for a cover 7 that also serves as the guide plate. Near the bottom of the wide-bore section 4 is the weir 8 over which the rising stream of molten sodium ows into the receiver 9 which is provided with the customary bottom run-off 10, cover plate 11 and paddle (not shown). To facilitate cooling, the upper section of the riser pipe is provided with integral external cooling ins, but for the sake of simplicity these are not shown in the drawing. The scraping device comprises two stainless steel bars represented by numeral 12, which bars are bent inwards as shown at the region near to the anged joint 3 between the upper and lower sections of the riser pipe, so that the distance between the lower portions of the bars is somewhat smaller than the distance between the upper portions. Stainless steel hoops represented by numerals 13 and 14, having cutting edges not shown in the drawing, all of which hoops are equidistant from one another, are welded at points on the inner periphery thereof to the bars. The hoops represented by numeral 14 are of smaller diameter than the upper hoopsyrepresented by numeral 13` so` that if the two sections of the riser pipe are not in perfect alignment there will still be a suflicient clearance between the lower hoops and the interior of the pipe when the hoops are moved upwards or downwards inside the pipe. The saddle plate 15 is welded to the upper extremity of the bars 12 and when at the lowest point of its travel rests on the internal flange 5. Four brackets, two` of which are shown and numbered 16, conveniently attached to the cover and guide plate 7, and an upper plate 17 form a framework within which is supported (by means not shown) the double-acting pneumatic cylinder 18. To the central portion of the saddle plate 15 isl welded a steel rod 19 which in turn is attached to the piston 20 of the cylinder. Inlet and vent pipes 21 and 22 communicating with the cylinder are provided as shown. The distance between the lower face of the guide plate 7 and the upper edge of the saddle plate 15 (when the latter is seated on the flange 5) is made slightly greater than the distance between consecutive hoops 13 or 14.

When using the apparatus as described above compressed air is passed in through pipe 22 the effect of which is to push the piston 20 upwards in the cylinder and to cause the upward movement by the same distance, and at the same time, of the members appendant to the piston, including the saddle plate 15 and hoops attached to the steel rods 12. At the top of its stroke the saddle plate 15, bearing the bars and hoops is brought to a sudden jarring halt by Contact with the guide plate 7. Compressed air is then passed into the cylinder through the pipe 21 and the piston and appendant members including the hoops travel downwards and again are brought to a jarring halt by contact of the saddle plate 15 on the flange. The continual sliding movements upwards and downwards of the hoops inside the pipe (also helped by the jolting effect when the moving member is brought to a halt) effectively remove metallic deposits adhering to the inner surface of the riser pipe. This procedure may be repeated from time to time if and when necessary. By means of suitable timing gear the reversal of the air feed to the pneumatic cylinder is brought about automatically. The device is preferably made to operate intermittently, an appropriate operating schedule for any given construction and set of operating conditions being determined experimentially.

The invention includes within its scope not only the scraping device described but also electrolytic cells for the manufacture of sodium by the electrolysis of fused salts comprising sodium chloride and calcium chloride, which cells comprise a graphite anode, a metal cathode surrounding the anode, a metal diaphragm interposed between anode and cathode, an inverted chamber situated above the cathode adapted to collect the alkali metal products, the chamber being provided with a vertical conduit leading to the exterior of the cell for take-olf of liquid sodium in which conduit the above described scraping device is incorporated, that portion of the vertical conduit outside of the cell preferably being provided with external cooling fins. The invention further includes processes for production of sodium by the electrolysis of fused salts comprising sodium chloride and calcium chloride when employing cells having the hereinbefore described device incorporated therein.

The following example illustrates but does not limit the invention.

Example In an electrolytic cell employing a fused mixture of sodium chloride and calcium chloride as electrolyte an inverted annular chamber for collection of alkali metals, essentially sodium, was situated above the cathode, the collecting chamber being provided with a vertical conduit (riser pipe) leading to the exterior of the cell for takeolf of sodium, that portion of the pipe outside the cell of the riser pipe was 6 inches.

-6 being provided with external cooling tins. The general appearance of the riser pipe and the` device for scraping adherent calcium off the walls of the pipe was substantially as illustrated and described in the accompanying drawing and the descriptive matter in the specification and will be more particularly described with reference thereto. The internal diameter of the lower section (1) The steel hoops of the scraping device were seven in number and were welded at points on the inner periphery thereof to the steel bars (12) which were 5/6 inch in diameter. The hoops were equally spaced from one another, the upper and lower rimsvof the hoops being internally charnfered to make cutting edges. The outside diameter of the hoops 13 was 57/16 inches, the outside diameter of the hoops 14.

being about /16 inches; the height and thickness of all the hoops were 11/2 inches and approximately 3/16 inch, respectively. The distance between the bottom edge of one hoop to the bottom edge of the next hoop was 57/8 inches, the total distance between the bottom edge of the lowest hoop to the upper extremity of the steel bars (l2) was approximately 4l inches. The distance between the top surface of the saddle plate 15 when at its lowest point of travel and seated on internal flange 5 and the lower surface of the guide plate (7), which represents the distance through which the hoops move, was 41/2 inches. The internal llange 5 and guide plate 7 serve to bring the scraper which is to move in a. reciprocating motion to a jarring halt.

An electrolytic cell equipped with such a riser pipe and scraper was operated under the following conditions: sodium was being delivered to the bottom of the riser pipe at the rate of 47 lb./hr. at a temperature of 576 C. (the solubility of calcium in sodium at 575 C. is 5.9% by weight), the average temperature at the top of the riser pipe from which the sodiumleft the cell was about 250 C. at which temperature the solubility of calcium in sodium is 0.42% by weight. Under such conditions in the riser pipe, air was fed to the double acting air cylinder at 60 lb. per square inch. The operating schedule of the tickler was five down and up strokes, taking a total time of 71/2 seconds, every four minutes. The sodium leaving the cell had an average calcium content of 0.88% by weight and was subsequently cooled and filtered.

What l claim is:

l. An improved scraping device for removing undesired metal deposits from the inner wall of a vertical riser pipe of a sodium producing electrolytic cell comprising, a substantially vertically disposed supporting bar extending substantially the length of the riser, a plurality of vertically spaced cutting hoops secured at substantially equal intervals to said supporting bar, means mounting said bar for reciprocating movement within said riser, upper and lower stop means for limiting the reciprocating movement of said bar to a predetermined amplitude, said amplitude being at least equal to the greatest distance between consecutive hoops, and driving means being operable to impel said bar forcefully against said upper and lower stop means to produce a jarring eifect at both ends of the reciprocatory movement of said bar.

2. An improved scraping device for removing metal deposits from the riser pipe of an electrolytic cell used in producing sodium metal, the riser pipe having an enlarged upper end of slightly larger diameter than the lower end to thereby form an internal flange, said scraping device comprising, a pair of parallel supporting bars, a plurality of cutting hoops having an outer diameter slightly less than the inner diameter of the riser and having a height approximately one-third to one-fourth of the diameter, said hoops being separate and independent and supported on said supporting bars at equal intervals throughout the length thereof, a saddle plate secured to and joining the upper ends of said supporting bars', said saddle plate making contact with the internal flange when in its lowermost position, a guide plate attached to the upper end of the riser, an actuating rod extending upwardly from said saddle plate through the guide plate and means attached to said actuating rod for effecting reciprocating movement thereof, said means being effective Ato forcefully impel said saddle plate against said flange to produce a jarring effect.

3. A scraping device as dened in claim 2 wherein the ris'er pipe is constructed of an upper and lower section and wherein the cutting hoops traversing the lower section of pipe are of smaller diameter than those hoops which traverse the upper section.

u8 4. A scrapping device as defined in claim 2 wherein the actuating rod is attached to the piston ofa double acting fluid cylinder.

References Cited in the file of this patent UNITED STATES PATENTS 813,739 Ross Feb. 27, 1906 2,068,681 Hulse Ian. 26, 1937 2,390,115 McNitt Dec. 4, 1945 2,542,989 Carter Feb. 27, 1951

Claims (1)

1. AN IMPROVED SCRAPING DEVICE FOR REMOVING UNDESIRED METAL DEPOSITES FROM THE INNER WALL OF A VERTICAL RISER PIPE OR A SODIUM PRODUCING ELECTROLYTIC CELL COMPRISING, A SUBSTANTIALLY THE LENGTH OF THE RISER, A PLURALITY EXTENDING SUBSTANTIALLY THE LENGTH OF THE RISER, A PLURALITY OF VERTICALLY SPACED CUTTING HOOPS SECURED AT SUBSTANTIALLY EQUAL INTERVALS TO SAID SUPPORTING BAR, MEANS MOUNTING SAID BAR FOR REDCIPORCATING MOVEMENT WITHIN SAID RISER, UPPER AND LOWER STOP MEANS FOR LIMITING THE RECIPROCATING MOVEMENT OF SAID BAR TO BE PREDETERMINED AMIPLITUDE, SAID AMPLITUDE BEING AT LEAST EQUAL TO THE GREATEST DISTANCE BETWEEN CONSECTIVE HOOPS, AND DRIVING MEANS BEING OPERABLE TO IMPEL SAID BAR FORCEFULLY AGAINST SAID UPPER

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