US2958640A - Arc-heated electrolytic cell - Google Patents

Description

Nov. 1, 1960 M. T. CICHELLI 2,958,640

ARC-HEATED ELECTROLYTIC CELL Filed May 8, 1959 Ei g. 1

44 eoc-Ac.-22ov. socAc.-22ov. o I l I l 5| E E as INVENTOR MARIO T. CICHELLI BY y/ w w ATTORNEY United States Patent ARC-HEATED ELECTROLYTIC CELL Mario T. Cichelli, Wilmington, Del., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware Filed May 8, 1959, Ser. No. 811,886

"6 Claims. (Cl. 204-246) This invention relates to an are heated electrolytic cell, and particularly to such a cell wherein relatively high temperature electrolyses are to be conducted, such as those requiring fusion of the electrolyte, or the like. *One high temperature electrolysis to which the are heated cell of this invention is particularly suited is that of CaF which produces elemental calcium and fluorocarbons concurrently when a graphite electrolysis anode is employed, all as taught in application Ser. No. 811,- 887, filed of even date herewith. This electrolysis is preferably conducted within the relatively narrow temperature range of 14001500 C. and, of course, involves extremely reactive materials, so that it is desirable to employ a cell which affords its own corrosion-erosion shielding. This is achieved by the utilization of the skull furnace principle translated to the electrolytic cell art, all as hereinafter described in detail.

A primary object of this invention is to provide an electrolytic cell which incorporates integral electric arc heating facilities, thereby permitting quite precise temperature maintenance within the cell over a relatively wide range of operating temperatures. Another object of this invention is to provide an electrolytic cell wherein the symmetrical introduction of heat and electrolysis current are simultaneously achieved. Another object of this invention is the provision of a heated electrolytic cell which is particularly suited to the electrolysis manufacture of extremely reactive substances due to the corrosion-erosion shielding obtainable by the maintenance of a protective skull around the inside cell surfaces. The manner in which these and other objects of this invention are'attained will become apparent from the following detailed description and the drawings, in which:

i Fig. 1 is a partially schematic representation in vertical cross section of a preferred embodiment of electrolytic cell according to this invention wherein the lowermost electrolyzing electrode is a liquid phase material,

L Fig. 2 is a partially schematic representation in vertical cross section of a second embodiment of electrolytic cell utilizing an annular form of solid electrode as one of the electrolyzing pair, the cell superstructure being omitted from the showing, and

Fig. 3 is a schematic representation in plan cross section through the cell body of yet another embodiment of electrolytic cell utilizing a plurality of horizontal, symmetrically disposed heating electrode pairs and a solid, centrally disposed electrode as the common member of the electrolyzing pairs.

Generally, the objects of this invention are obtained by utilizing one or more shielded heating electrode pairs comprising a first electrode coaxially disposed within a second electrode having the shape of a closed end tube, thereby providing an electric arcing region adjacent the ends of these two electrodes, the tubular electrode being in contact externally with the electrolyte retained in the cell body, a third electrode immersed in the electrolyte and constituting, with the tubular electrode, an electrolyzing pair, and at least one direct-current power 2,958,640 Patented Nov. 1, 1960 source in electrical connection with the electrodes to provide the neceessary heating and electrolyzing energy.

Referring to Fig. 1, a preferred embodiment of my invention consists of an electrolytic cell indicated generally at 10, which incorporates a metal external shell 11, such as stainless steel provided with a refractory inside lining 12, which here constitutes a unitary graphite crucible. The electrolyte in process is retained within" 12 and consists of an enclosing solid shell or skull of material 13 containing a molten pool 14 of the electrolyte within which the electrolysis is conducted. As electrolytic dissociation proceeds, it will be apparent that the level of pool 14 drops, the level shown in Fig. 1 being that taken some time after electrolysis was initiated. It is preferred to introduce replenishing electrolyte to the cell at frequent intervals, and this can readily be accomplished by feeding powdered (2 mm. particle size or less) or premelted material into the cell through an inclined tube or the like provided with a conventional gas seal, not shown in the drawings in the interest of simplicity.

The cell shown in Fig. 1 is specifically intended for the electrolysis of CaF, at temperatures of about 1400-1600 C. to manufacture concurrently elemental calcium together with mixed fluorocarbons and incorporates a molten tin cathode 18 which is retained as a well within a ring 19 of highly refractory material, such as boron nitride, for example. The base of crucible 12 is recessed on the underside at 20 to provide space for thermal insulation 21, such as fibrous alumina or silica, which is packed therein as a protection for shell 11, the latter being cooled over its entire outside expanse by cooling coils 22 of conventional design.

Since the fluorocarbon products are gaseous, cell 10 must be enclosed and this is accomplished by providing a tight fitting cover 25 which is sealed against leakage past the top of the cell by O-ring 26, which may be fabricated from polytetrafluoroethylene. The electrolysis is advantageously conducted in an inert gas atmosphere, such as helium, and this is introduced through port 27 and exhausted, together wtih the gaseous products of electrolysis, through port 28. A sight glass 29 is provided in the cover to permit visual inspection of the cell interior.

The remaining electrode assembly is mounted vertically Within a central opening in cell cover 25 from which it is electrically insulated by bushing 30, which may also be fabricated from polytetrafluoroethylene or other temperature-resistant material. The bifunctional electrode 3-1, that is, the electrode cooperating as one electrode of both the heating and the electrolyzing electrode pairs, is a tube closed at the in-cell end and, in this instance, is fabricated from relatively non-porous carbon. The lower end of electrode 31 is intended to be immersed at all times within the molten electrolyte 14; however, the spacing of 31 with respect to liquid electrode 18 can be adjusted in part at least to maintain a satisfactory over-all thickness of skull 13 around the inside periphery of crucible 12.

The heating electrode 32 cooperating with electrode 31 is a solid graphite rod in the embodiment of Fig. l and is supported coaxially within 31 by annular metal support 33, which is provided with a flange 34 resting on the top of insulating bushing 30. To afford additional support to bifunctional electrode 31, the lower end of support 33 is preferably threaded thereto over the length 35. An electrical insulation bushing 38 is interposed between support 33 and the immediate holder 39 for electrode 32 wtih a sliding friction fit to the holder. Electrode holder 39 is represented schematically in Fig. l as a watercooled metal support which is provided on the exterior with a rack 40 engaging with pinion 41, thereby permitt ting progressive lowering of electrode 32 within the interior of electrode 31 as erosion of one or the other of these electrodes necessitates. Electrode '32 is attached at its upper end to holder 39 by thread engagement, clamps, or the like, not detailed in Fig. 1.

It is desirable to maintain cell cover 25 and the remaining superstructure of cell 10 cool and this is elfected by circulating a cooling liquid, such as water, through electrode holder 39. Accordingly, a water supply duct 42 is provided interiorly of holder 39 which discharges near the base, permitting reverse flow of the coolant, which thereafter flows upward and discharges through connection 43. Flexible coolant supply and discharge tubes 44 and 45, respectively, accommodate free vertical movement of holder 39 under operation of the rack and pinion set hereinbefore described.

The service life of the heating electrode pair 3132 is increased if the interspace therebetween is maintained in a non-oxidizing atmosphere, and this is effected by providing electrode support 33 with a gas-flooding port 46 through which a blanketing gas such as nitrogen, argon or helium may be introduced as desired.

From the foregoing, it will be understood that there are essentially three electrodes in my construction, 31 and 32 together constituting the arc heating pair while 18 and 31 together constitute the electrolyzing pair. Thus, electrode 31 is bifunctional, in that it is common to both the heating and the electrolyzing pairs. In the preferred embodiment where separate power supplies are utilized the power supply to electrodes 31-32 is provided from a heating current rectifier which receives A.-C. power from conventional mains and delivers direct current to electrode 32 via lead 51 and holder 39, and to electrode 31 via leads 52 and 52 and electrode support 33. The polarity of the connections is, in this instance, such as to make electrode 32 the anode and 31 the cathode, although it will be understood that the order can be reversed if the particular situation requirements dictate. Similarly, the electrolyzing electrode pair is supplied with power from a second rectifier 53 with connections to the bifunctional electrode 31 via lead 54 and lead 52', and to electrode 18 via lead 55, which is in tight threaded attachment at 56 with graphite crucible 12. The polarity in the specific installation described is chosen so that electrode 31 is the anode and electrode 18 the cathode of the electrolyzing pair, although it will be understood that this order too can be reversed, depending upon the particular electrolysis it is desired to efiect. Finally, a ground connection 57 is preferably provided at the point of joinder of leads 52 and 54 as a safety measure.

In operation, it is necessary to maintain a heating are struck between electrodes 31 and 32 principally in the end-to-end region denoted A in Fig. 1, and this can be accomplished with a power delivery from rectifier 50 of the order of 100 amperes at 25-30 volts. The electrolysis of molten CaF to produce fluorocarbons and elemental calcium can be conducted at a voltage applied across electrodes 18 and 31 below 15 v. (preferably as low as 4.0-5.0 volts), and a current level which provides a current density of about 40-70 amps/sq. dm. at anode 31 and S02OO amps/sq. dm. at cathode 18, although these densities are not especially critical.

In the electrolysis of CaF a molten tin cathode 18 is preferably utilized, although tin alloys with aluminum, bismuth, lead, or silicon are also useful, provided that the ratio of tin to other alloying elements is not less than about 95:5. The calcium dissociated during the electrolysis is liberated at cathode 18 and alloys with the tin, from which it may be recovered later by conventional techniques known to the art. In the electrolysis of other substances mercury metal is, of course, an ideal liquid phase cathode material, as are also many of the commercially available low-melting alloys of bismuth and other elements.

At start-up it is often desirable, particularly for very high temperature elcctrolyses, to preheat the contents of electrolytic cell 10 prior to striking the heating are across the 3132 electrode pair. This can be conveniently done by providing a removable section of cooling coils 22 and thermal insulation 21 at the base of the cell, so that crucible 12 can be exposed to the direct heat of gas flames or even rested within a furnace until an initial temperature of 800-900 C. is reached. During this period the interior of cell 10 and the charge of solid electrolyte within crucible 12 are both evacuated to about 0.1 mm. Hg as a purging step preliminary to the electrolysis which is to follow. Then the cell is flooded with inert gas to near atmospheric pressure level and the heating arc struck between the electrode pair 3132. The temperature in the region immediately surrounding the end of electrode 31 and in the space between electrodes 31 and 18 rises rapidly. At above 1330 C. the CaF in this region fuses and the electrolyzing current is thereafter supplied from rectifier 53 as hereinbefore described.

The fluorocarbons, predominantly carbon tetrafluoride, generated at anode 31 by reaction of the fluorine electrolysis product with the carbon exterior of the electrode bubble off and are removed through exhaust port 28, while the calcium product alloys with the tin of cathode 18. The temperature of the molten electrolyte pool 14 is preferably maintained at between 1400 and 1500 C., which insures the preservation of a shielding skull 13 of solidified glazed electrolyte adjacent the inside walls of crucible 12, thereby protecting the crucible from the severe erosive and corrosive deterioration it would otherwise suffer. Preservation of skull 13 and protective cooling of metal cell shell 11 is further facilitated by the supply of cooling water through one or more sections of cooling coils 22.

Shielded arc electrodes per so are old in the art and several preferred designs are taught in US. Patent 1,552,143. However, insofar as I am aware, such enclosed arcs have not hitherto been incorporated into electrolysis cells with the realization of the very important advantages of simultaneous symmetrical heat and electrical current input thereby obtained. Moreover, it hereby becomes practicable to utilize the protective skull principle in electrolyses which, until now, has been limited in application solely to furnaces.

Referring to Fig. 2, another embodiment of this invention is shown wherein the arc heating electrode 61 internal of bifunctional electrode 31 is tubular in form and fabricated from tungsten metal, although it could as well be made from graphite or other material. With this construction it is possible to supply the inert gas fed to the arc heating region through the bore 62 of the electrode, all other details of the superstructure and power sup ply being identical with those shown in Fig. l and hereinbefore described, and therefore not further elaborated upon. In this embodiment a solid phase electrolyzing cathode 63 is utilized which is annular in form and immersed in the electrolyte through the top of cell 10'. For simplicity of representation cell 10 is shown open at the top and no details of support for electrode 63 are shown, it being understood that means resembling those of Fig. 1 are completely adequate for the purpose. With the construction of Fig. 2 an unbroken skull 13 is maintained over the entire inside, including the bottom, of crucible 12, which affords even greater protection than the skull of Fig. 1, while maintaining at all times a bath 14' of fused electrolyte in the electrolyzing electrode interspace.

Yet another embodiment of my invention is shown in Fig. 3 wherein the relative locations of the electrolyzing and heating electrode pairs are, in effect, reversed with respect to the crucible 12". Thus, there can be a plurality of bifunctional electrodes 64 which are disposed in a common horizontal plane radially of the center of the cell body and preferably completely immersed'in'theelectrolyte, while the remaining electrolyzing electrode 65 may befa solid rod disposed at the center of the cell perpendicular to electrodes 64. The in-cell ends of electrodes 64 are concavely formed on circular arcs of common radius from the center of electrode 65' in order to provide balanced electrolyzing current paths for each of the four symmetrically disposed bifuntional electrodes. The internal heating electrodes 66 are in all respects identical with electrodes 32 of Fig. l, and all other details of construction and electrical circuitry can be either identical, or modifications within ordinary skill, of those hereinbefore described with reference to Figs. 1 and 2.

The embodiment of Fig. 3 is particularly advantageous where either a very high heat input to the cell is desired or where it is desired to enlarge the effective heated zone 14". Again, with this design, there is obtained a continuous protective skull 13" resembling that shown in Fig. 2.

Usually it is preferred to constrain the electric arcing to a region central of the entire apparatus assembly, and such a limitation is averred in the claims, which prescribe that the end-to-end clearance between electordes 31 and 32 shall constitute the principal inter-electrode arcing region of the heating electrode pair. However, it will be particularly understood that the occasional transitory or relatively short term striking of the are between these electrodes in any other region can occur, especially just prior to corrective readjustment of intra-electrode position one with respect to the other during operation, and this is not objectionable, provided that it is not so sustained in duration that pronounced non-symmetrical heat introduction occurs with accompanying interference to the electrolysis in progress.

The electrode which constitutes the anode of the arc heating pair is the one which is subjected to the most severe service during operation, this electrode being 32, 62 and 66 in Figs. 1, 2 and 3, respectively. However, where polarity is reversed and the bifunctional electrodes 31, 31' and 64, respectively, are connected as anodes it is possible to protract their lives by introducing granular graphite or the like, such as chunks of broken electrode material, into the end-to-end electrode region, whereupon the arc strikes between the internal electrode and the granular graphite without deleterious effects to the bifunctional electrode. It will be understood that operation with this protective filling makes it possible to maintain a substantial axial separation between the electrodes of the heating pair, with the result that the heat introduction zone is not inherently limited to the ends of the bifunctional electrodes per se but may be preserved anywhere along the lengths thereof which may be convenient to the operations at hand. Nevertheless, the arc is preferably still struck in the end-to-end region between the internal electrode and the bifunctional electrode eflectively built up in end thickness by any added wear-receiving material, and such operation is intended to be comprehended within the term end-to-end clearance as employed in my claims.

As hereinbefore dmcribed for the embodiments of Figs. 1-3, inclusive, it is preferred to utilize separate power sources for the heating are and for the electrolysis, because it is thereby possible to maintain complete independence of adjustment as well as sequential operation in the conduct of the arc heating on the one hand and the electrolysis on the other hand. This may be necessary during start-up, as in the electrolysis of CaF described, because of the fact that the solid phase material is a poor electrical conductor and there must therefore be melting before there can be electrolysis. However, in some instances these limitations do not apply and, accordingly, it is then practicable to utilize only one power source for both the arc heating and the electrolysis, with resulting simplification of the circuitry. Thus, where the are heatand electrolysis are to be carried out concurrently, electrical connection can be made with shielded electrode 32 and external electrolyzing electrode 18, whereupon the same electrical current functions to both heat and dissociate. Also, where it is feasible to conduct the arc heating in sequence with the electrolysis a single power source can again be utilized, employing the electrode connections detailed in Fig. 1 and using only a single power source by connecting this source with the heating and electrolyzing electrode pairs in alternation for appropriate periods of time through a suitable switching arrangement. Normally, this latter arrangement is not as satisfactory as concurrent operation for the reasons that the temperature within the cell will vary over wider limits than would otherwise be the case and, also, the conduct of the electrolysis must be interrupted during the several periods of heating.

From the foregoing it will be understood that the electrolytic cell of this invention may be modified widely within the skill of the art, particularly as regards various arrangements of the electrodes with relationship to one another and in numerous other respects, and it is intended to be limited only by the scope of the following claims.

What is claimed is:

1. An electric arc heated electrolytic cell comprising in combination a cell body, at least one heating electrode pair consisting of a first electrode mounted coaxially within but out of contact with a second electrode of tubular configuration having a closed end adjacent the cell body end of said first electrode providing an end-to-end clearance between said first and said second electrodes such as to constitute the principal intra-electrode arcing region of said heating electrode pair, said second electrode being disposed so as to contact externally with the electrolyte processed in said electrolytic cell, a third electrode adapted to be at least partially immersed in said electrolyte constituting, with said second electrode, an electrolyzing electrode pair, and at least one direct-current power source in electrical connection with said electrodes so as to maintain an electric are between said first and second electrodes and a flow of electrolyzing current between said second and third electrodes.

2. An electric are heated electrolytic cell comprising in combination a cell body, at least one heating electrode pair consisting of a first electrode mounted coaxially within but out of contact with a second electrode of tubular configuration having a closed end adjacent the cell body end of said first electrode providing an end-to-end clearance between said first and said second electrodes such as to constitute the principal intra-electrode arcing region of said heating electrode pair, said second electrode being disposed so as to contact externally with the electrolyte processed in said electrolytic cell, a third electrode adapted to be at least partially immersed in said electrolyte constituting, with said second electrode, an electrolyzing electrode pair, and separate direct-current power sources in electrical connection with the two electrodes of said heating electrode pair and with the two electrodes of said electrolyzing electrode pair respectively.

3. An electric are heated electrolytic cell comprising in combination a cell body, a vertically disposed heating electrode pair consisting of a first electrode substantially centrally located with respect to said electrolytic cell and mounted coaxially within but out of contact with a second electrode of tubular configuration having a closed end adjacent the cell body end of said first electrode providing an end-to-end clearance between said first and said second electrodes such as to constitute the principal intra-electrode arcing region of said heating electrode pair, said second electrode being disposed so as to contact with the electrolyte processed in said electrolytic cell, a third electrode adapted to be at least partially immersed in said electrolyte constituting, with said second electrode, an electrolyzing electrode pair, and separate direct-current power sources in electrical connection with the two electrodes of said heating electrode 7 pair and with the two electrodes of said electrolyzing electrode pair respectively.

4. An electric are heated electrolytic cell according to claim 3 wherein said third electrode consists of a liquid phase electrically conductive material.

5. An electric arc heated electrolytic cell according to claim 3 wherein said third electrode comprises an annular member fabricated from an electrically conductive solid material and disposed substantially coaxially with respect to said first and second electrodes.

6. Anelectric arc heated electrolytic cell comprising in combination a cell body, a plurality of heating electrode pairs disposed substantially radially of said cell body, each consisting of a first elect-rode mounted coaxially within but out of contact with a second electrode of tubular configuration having a closed end adjacent the cell body end of said first electrode providing an end-toend clearance between said first and said second electrodes such as to constitute the principal intra-electrode arcing region of said heating electrode pair, said second electrode being disposed so as to contact externally with the electrolyte processed in said electrolytic cell, a electrode adapted to be at least partially immersed insaid electrolyte in a position substantially central of said heating electrode pairs constituting, with said second electrodes, a plurality of electrolyzing electrode pairs, and separate direct-current power sources in electrical connection with the two electrodes of said heating electrode pairs and with the two electrodes of said electrolyzing electrode pairs respectively.

References Cited in the file of this patent UNITED STATES PATENTS 1,080,113 Von Kugelgen Dec. 2, 1913 1,521,002 Brace Dec. 30, 1924 1,545,582 Cobb July 14, 1925

Claims (1)

1. AN ELECTRIC ARC HEATED ELECTROLYTIC CELL COMPRISING IN COMBINATION A CELL BODY, AT LEAST ONE HEATING ELECTRODE PAIR CONSISTING OF A FIRST ELECTRODE MOUNTED COAXIALLY WITHIN BUT OUT OF CONTACT WITH A SECOND ELECTRODE OF TUBULAR CONFIGURATION HAVING A CLOSED END ADJACENT THE CELL BODY END OF SAID FIRST ELECTRODE PROVIDING AN END-TO-END CLEARANCE BETWEEN SAID FIRST AND SAID SECOND ELECTRODES SUCH AS TO CONSTITUTE THE PRINCIPAL INTRA-ELECTRODE ARCING REGION OF SAID HEATING ELECTRODE PAIR, SAID SECOND ELECTRODE BEING DISPOSED SO AS TO CONTACT EXTERNALLY WITH THE ELECTROLYTE PROCESSED IN SAID ELECTROLYTIC CELL, A THIRD ELECTRODE ADAPTED TO BE AT LEAST PARTIALLY IMMERSED IN SAID ELECTROLYTE CONSTITUTING, WITH SAID SECOND ELECTRODE, AN ELECTROLYZING ELECRODE PAIR, AND AT LEAST ONE DIRECT-CURRENT POWER SOURCE IN ELECTRICAL CONNECTION WITH SAID ELECTRODES SO AS TO MAINTAIN AN ELECTRIC ARC BETWEEN SAID FIRST AND SECOND ELECTRODES AND A FLOW OF ELECTROLYZING CURRENT BETWEEN SAID SECOND AND THIRD ELECTRODES.

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