US3037927A - Device for purifying alkali metal - Google Patents

Description

United States atent Q Filed July 25, 1958, Ser. No. 751,020 6 Claims. (Cl. 204-245) This invention relates to an apparatus for the recovery of sodium from a fused salt electrolysis cell. More particularly it relates to a device through which sodium metal rises from within and out of the cell after it has been generated from salt and collected within the cell.

The cell described by Downs in US. Patent 1,501,756 is basic with reference to the production of sodium by the electrolysis of salt. In order to permit operation at reasonable temperatures, the cell bath usually contains a large proportion of calcium chloride to reduce the melting point of sodium chloride. At the operating temperature of the cell usually around 600 C. an undesirable amount of calcium metal is generated along with the sodium. The sodium rising from the cathode and gathered in the collector usually contains several percent by weight of calcium and the amount is generally in proportion to the solubility of calcium in sodium. This solubility is a function of temperature and decreases to about 0.04% calcium in sodium at about 110 C.

During the time the liquid sodium passes from the collector within the cell bath to the outside of the cell the liquid is cooled and this results in the separation of solid calcium from the sodium. Since the specific gravity of the cell bath is greater than that of sodium, the metal rises to the top of the bath and on out of the cell through a tube or pipe known as the riser pipe. Solid calcium in contact with molten cell bath tends to react with the sodium chloride to form calcium chloride and sodium and promoting this reaction is highly desirable. The solid calcium has a higher specific gravity than the liquid sodium and tends to sink to the metal-molten cell bath interface where it can react with the salt. Unfortunately, as the calcium is precipitated it tends to solidify in small particles which settle only slowly. Further, these particles tend to adhere to the inner surfaces of the riser pipe and if pressure is applied, the deposited calcium agglomerates and adheres more and more firmly. In efforts to solve this problem, a number of devices have been proposed and used, such as larger diameter riser pipes, increased cooling or heating of the part of the riser pipe projecting outside the cell bath, various ticklers or agitators operated continuously or intermittently. But none of these has been as effective as is requisite for continuous operation and either repeated laborious and dangerous reaming of the inside of the riser pipe walls is required by means of rods or bars to prevent plugging or else hot sodium is withdrawn from the cell which then necessitates expensive separation procedures outside the cell, which in turn results in more rapid depletion of the calcium chloride flux in the cell bath and this interferes with maintenance of proper electrolyte composition.

According to the present invention, it is possible to overcome or greatly ameliorate these difiiculties by providing an improvement in the riser pipe whereby it is possible to prevent the adhesion of calcium to the riser pipe wall and to expedite the downward passage of the precipitated particles of calcium so that they may readily contact the cell bath for conversion to sodium.

My invention comprises a vertical straight-walled tubular riser pipe through which the sodium rises above the bath with minimum vertical displacement and an axially centered independently movable scraping knife edge fitting closely to the inner surface of the pipe over the length ice through which there is a significant temperature change in the rising sodium. Said knife edge is adapted to cut or scrape the inner riser pipe clean of adhering calcium deposits and is rotatable in only the cutting direction of the knife edge.

The drawing will further clarify my invention by showing in suflicient detail a specific embodiment thereof.

FIGURE 1 is a vertical cross-section of a cell for the electrolysis of a fused salt mixture of the type referred to as the Downs cell. FIGURE 1 is a plane view of a scraper for use in the riser pipe of the cell. FIGURE 3 is a horizontal cross-section of the riser pipe and scraper at AA of :FIGURE 1. Like parts are designated by the same numerals in all the figures.

The cell of FIGURE 1 consists of a cylindrical shell 1, lined with refractory brick 2. Entering from the bottom is cylindrical graphite anode 3 surrounded by cylindrical annular steel cathode 4 having connections 5 to the outside of the cell. The anode and cathode are separated by a porous (usually wire screen) metal cylindrical diaphragm 6 suspended from annular structure 7 termed a collector ring which is an annular inverted trough for the collection of liquid sodium generated at the cathode. Chlorine gas rising from the anode 3 is gathered in chlorine dome 10 and is Withdrawn from the cell by pipe 11. The collector ring 7 and dome 10 assembly is supported in the cell by means not shown. At a suitable point a vertical conduit 8, 9 rises from the collector ring 7 to conduct liquid sodium vertically out of the cell by virtue of the differential in hydrostatic pressure between the molten cell electrolyte and the sodium. Sodium flows out of the conduit 8, 9 over a weir 12 into receiver 13.

The upper end of conduit 8, 9 which may be designated as the riser pipe, may be provided with a plurality of fins 14 to effect cooling. A device 15, called the scraper, is inserted into riser pipe 8, 9 from the top and is rotatable for scraping the internal wall of the riser pipe by means of handle 16. The scraper 15 in this instance comprises three curved blades welded together "at 18 to form a structurally sound unit which is rotatable in the direction of the knife edges 19 which are adapted to scrape any calcium deposits from the inner wall of riser pipe 8, 9. The scraper is centered at the top by bearings 24} and preferably requires no bearing at any other point because the three knife edges of the scraper blades spaced at angles of about and having a small clearance within the riser pipe provide for adequate centering. Excessive heat loss from surface 21 of the fused salt is prevented by cover 22. By handle 16 the scraper can be lifted several inches within limits set by bearings 20 and, if then the handle is released, the scraper will drop down with sufficient force to loosen any material that may have attached itself to the scraper blades.

Because of the tendency of precipitated calcium to adhere to solid surfaces under the application of even very light pressure, exerted either mechanically or by gravity, it is essential that there be no horizontal areas or ledges in the riser pipe on which calcium may settle or deposit.

During operation of the cell, sodium produced at the cathode surface opposed to the anode rises and collects in collector 7. From the collector the sodium, having a specific gravity of about .97 as against a specific gravity of about 2.25 of the molten salt mixture, is forced upward in the riser pipe 8, 9 to a height a considerable distance above the bath level at 21. As the sodium rises it is cooled and solid calcium metal is precipitated in rather finely dispersed form. Since cooling takes place through the wall of the riser pipe, the calcium tends to form on or near the wall and, if it is allowed to accumulate or is pressed against the wall, it will adhere and form resistant concretions which become more and more difiicult to exposing the riser pipe contents to air.

thereby causing the deposit to harden.

dislodge. However, by means of the rotation of the scraper with its sharp edges it is possible to scrape away the accumulated calcium from the riser pipe wall and these accumulations, being heavier than the sodium, gradually pass downward through the sodium eventually to make contact with the fused salt. The solid calcium in contact with the fused salt reacts to form sodium and calcium chloride.

To scrape calcium from the riser pipe wall it is necessary that the scraper blades be sharp and form as reasonably acute an angle with respect to thesurface to be scraped as possible. It is necessary :that the scraper be rotated only in the direction of the leading sharp scraper edge and that all the edges face in the same direction. If the scraper is rotated in the direction opposite to the leading scraper edge, calcium will be smeared or packed hard against the riser pipe Wall and then the calcium will rather quickly become so hard that scraping it oif becomes virtually impossible and the riser pipe must then be reamed clean by means of a sharp chisel pointed rod introduced with force from the top of the riser pipe, thus All this is costly, hazardous and time consuming but can'be largely avoided by proper operation of the scraper of this invention.

If the scraper is provided with fewer than three scraper blades, it will be necessary to provide a suitable bearing somewhere down in the riser pipe to keep the edge in proper position to scrape the inner riser pipe surface. If three or more scraper edges are provided, no other bearing than the one on top of the riser pipe will be necessary. In general, more than three scraper blades may be used but there is no advantage in this and three is the preferred number. It is preferable that the scraper blades be curved as shown in FIGURE 3, but they may be straight in which case greater care in construction will be necessary and, in any case, the sharp scraped edge. must be directed at an angle in the direction of rotation of the scraper so that it will cut or scrape away any deposit formed on the riser pipe wall without at the same time tending to smear the deposit against the wall and Because of temperature effects it will in general be necessary to provide a slight clearance between the sharp scraper edge and the inner wall of the riser pipe but, this should not be greater than about A of an inch and may be as small as about inch.

The riser pipe must approach as far as possible a perfeet cylindrical shape. The pipe may have a lower segment of a smaller diameter than the upper segment. Thus, if the upper portion of the riser pipe is 8 inches in diameter, the lower portion may be 6 inches in diameter a and the scraper will be shaped accordingly since all inwhich will effect quick vertical movement. The extent of the vertical movement willin general not be greater than the diameter of the riser pipe; it may be much shorter, the requirement being that the jolt be sharp enough to dislodge any calcium that may attach itself to the scraper blade surfaces. The rate or speed of rotation of the scraper is not critical and it may be at a rate of one revolution or less per second. It is, of course, obvious that the knife edges must be quite sharp to cut easily through any adhesions on the inner riser pipe surface. It is also preferable that the blades be curved as shown in FIGURE 3 for achieving maximum strength with minimum weight and greater ease of construction.

The scraper may be actuated manually or it may be operated by automatic means'set to operate at predetermined intervals both to rotate the scraper and to jolt it by a drop of several inches.

1 claim:

1. In a cell for the electrolytic production of sodium from fused salts, comprising sodium chloride and calcium chloride, the combination comprising a vertical, tubular, riser pipe extending above the cell for dischar ing liquid sodium, while cooling and precipitating calcium from the liquid sodium, and in said riser pipe an axially centered, vertically disposed, independently movable scraper adapted for rotation to scrape away calcium deposited on the inner'wall of said riser pipe, said scraper having a central axis and a plurality of blades each having a sharp edge leading in the direction of rotation of said scraper and fitting closely to the inner wall of said riser pipe over the length through which there is significant temperature change in the rising sodium at least one or said blades being imperforate.

2. In a cell for the electrolytic production of sodium from fused salts, comprising sodium chloride and calcium chloride, the combination comprising a vertical, tubular, riser pipe extending above the cell for discharging liquid sodium, while cooling and precipitating calcium from the liquid sodium and in said riser pipe an axially cen tered, independently movable, rotatable scraper comprising at least three vertically disposed blades dividing the said riser pipe in vertical compartments, each of said blades being provided with a sharp edge leading in the direction of rotation and fitting closely to the inner wall of said riser pipe over the length through which there is significant temperature change in the rising sodium at least one of said blades being imperforate.

3. The combination of claim 2 in which the scraper V is provided with vertically slidable means whereby it can terior surfaces of the riser pipe in contact with sodium must be scrapablc. The size of the diameter will be in part a function of the size of the cell. For a cell producing about 1000 lbs. Na/day, a diameter of 6 to 10 inches will be suitable. V V

The scraper ispreferably provided in one or more blades but not all blades, with one or more openings or holes as shown in *F-IGURE 2 at 17. These openings, which should be at least about one-half square inch in area, will permit a slight pumping action in the essentially three vertical compartments formed by the three blades and this assists in breaking up, for example, matrix or gel-like but ill defined masses formed by precipitated calcium in the sodium at certain temperature ranges through which the sodium passes on its way up the pipe.

I-tihas been found advantageous to actuate the scraper at periodic intervals such as every thirty minutes. This activation will generally comprise from one to ten revolutions of the scraper interspersed'by one or more drops or joltselfected by raising the scraper about 3 to 4 inches and allowing it to drop back under its own weight to loosen any solids that might have accumulated on the blades. Such i-olting may be accomplished by any means be lifted vertically and allowed to drop.

4. In a cell for the electrolytic production of sodium from fused salts, comprising sodium chloride and calcium chloride, the combination comprising a vertical, tubular, riser pipe extending above the cell for discharging liquid sodium, while cooling and precipitating calcium from the liquid sodium and in said riser pipe an axially centered, independently movable, rotatable scraper comprising three vertically disposed blades dividing the said riser pipe in vertical compartments, each of said blades being provided with a sharp cutting edge leading in the direction of rotation of said scraper and fitting closely to the inner wall of said riser pipe with a clearance between said edge and said inner wall of about to inch over the length through which there is significant temperature change in the rising sodium at least one of said blades being imperforate.

5. The combination of claim 4 in which the scraper is provided with vertical slidable means whereby it can be lifted vertically and allowed to drop.

6. In a cell for the electrolytic production of sodium from fused salts, comprising sodium chloride and calcium chloride, the combination comprising a vertical, tubular, riser pipe extending above the cell for discharging liquid sodium, while cooling and precipitating calcium from the liquid sodium, and in said riser pipe an axially centered,

5 6 independently movable, rotatable scraper comprising at References Cited in the tile of this patent least three vertically disposed blades dividing the said riser UNITED STATES PATENTS pipe in vertical compartments, each of said blades being provided with a sharp edge leading in the direction of 859,217 Hanson July 1907 rotation and fitting closely to the inner wall of said riser 5 2'770592 Fentress 1956 pipe over the length through which there is significant 2850442 Cathcart, et a1 Sept 1958 temperature change in the rising sodium, at least one but 2,861,938 Glascodme 1958 not all of said blades :being provided with at least one opening to permit a slight pumping action in the vertical compartments. 10

Claims (1)

1. IN A CELL FOR THE ELECTROLYTIC PRODICTION OF SODIUM FROM FUSED SALTS, COMPRISING SODIUM CHLORIDE AND CALCIUM CHLORIDE, THE COMBINATION COMPRISING A VERTICAL, TUBULAR, RISER PIPE EXTENDING ABOVE THE CELL FOR DISCHARGING LIQUID SODIUM, WHILE COOLING AND PRECIPITATING CALCIUM FROM THE LIQUID SODIUM, AND IN SAID RISER PIPE AN AXIALLY CENTERED, VERTICALLY DISPOSED, INDEPENDENTLY MOVABLE SCRAPER ADAPTED FOR ROTATION TO SCRAPE AWAY CALCIUM DEPOSITED ON THE INNER WALL OF SAID RISER PIPE, SAID SCRAPER HAVING A CENTRAL AXIS AND A PLURALITY OF BLADES EACH HAVING A SHARP EDGE LEADING IN THE DIRECTION OF ROTATION OF SAID SCRAPER AND FITTING CLOSELY TO THE INNER WALL OF SAID

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