US3196091A - Process for producing fluorine and sodium-lead alloy - Google Patents

Description

July 20, 1965 s. v. R. MAsTRANGELo 3,196,091

PROCESS FOR PRODUCING FLUORINE AND SODIUM-LEAD ALLOY Original Filed March 12, 1962 Flc.:

2 Sheets-Sheet 1 ATTORNEY July 20, 1965 s'. v. R. MAsTRANGELo 3,196,091

PROCESS FOR PRODUCING FLUORINE AND SODIUM-LEAD ALLOY Original Filed March 12, 1962 2 Sheets-Sheet 2 FIG. 2

Flc.

INVNTOR SEBASTIAN IV. R. MASTRANGELO ATTORNEY s claims. (ci. 20e-s0) This application is a division of my copending application Serial No. 178,938, led March 12, 1962.

This invention relates to a process for the production of elemental lluorine and sodium-lead alloy by the electrolysis of molten sodium lluoroborate in the presence of a molten lead cathode.

Fluorine is ya Valuable commodity which has a Wide variety of important uses, such as the preparation of inorganic fluorine compounds and organic fluorine-containing compounds. At the present time, elemental iluorine is prepared commercially by the electrolysis of hydrogen fluoride or of sodium biiluoride. These processes produce hydrogen as the cathode product which, while useful, is more readily and more economically produced from petroleum reiining and the electrolysis of .acidiied water or brine. It would be desirable to obtain a commercially more valuable product las the cathode product, such as sodium-lead alloy which is useful for a wide variety of purposes and particularly for reaction with alkyl chlorides to form tetraalkyllead compounds that are widely used as antiknock agents in hydrocarbon -uels for linternal combustion engines. Sodium biiluoride cannot be used for the electrolytic production of fluorine and sodium-lead -alloy because it produces hydrogen at the cathode and furthermore it decomposes appreciebly at the melting point of lead so that it cannot be used with a molten lead cathode.

It has been proposed to electrolyze various metal fluorides to produce fluorine and other products, sometimes at tempera-tures below 300 C. but usually at temperatures of 600 C. and above. However, at 600 C. and above, elemental tiuorine is highly corrosive Iand rapidly reacts with carbon 4to produce uorinated organic compounds. Accordingly, it is impractical to use carbon anodes lat such high temperatures for ythe production of elemental fluorine because of the production of fluorinated organic products with corresponding large decreases in the yield of elemental iluorine, and rapid consumption ofthe carbon anode.

It is an object of this invention to provide a new and improved electrolytic process for preparing elemental lluorine. Another object is to provide a process for preparing elemental tluorine which simultaneously produces sodium-lead alloy as the cathode product. A further object is to provide a process of the above character which produces an anode product which can be readily purified to produce substantially pure elemental l'luorine and to reform the electrolyte. Still another object is fto advance the art.

The above and other objects may be accomplished in accordance with this invention wherein elemental iluorine and sodium-lead alloy are produced simultaneously by the process which comprises electrolyzing an electrolyte consisting essentially of molten anhydrous sodium fluoroborate in lan inert atmosphere at a temperature of from 385 C. to 500 C. in an electrolytic cell having an anode composed of a member of the group consisting of graphite, microcrystalline carbon, explosively compacted graphite, silicon carbide, and gold, and a cathode consisting essentially of molten lead at substantially the same temperature as the electrolyte, employing -an electric cur- States Patent O rent of from 2 to about 25 volts to provide a current density of from 2 to 50 amperes per square decimeter (amps/dm?) of anode surface area, recovering a s0- dium-lead alloy as the cathode product and a gaseous mixture `of elemental fluorine :and boron tritluoride as the anode product, and separating the elemental iluorine from the boron trilluoride.

A further impor-tant feature of this invention comprises t-he steps of separating the elemental iluorine from the boron trifluoride by passing the gaseous mixture through sodium lluoride at a temperature of from about 200 4C. to about 500 C. whereby the boron trifluoride react-s with the sodium uoride to reform sodium tluoroborate and recycling Ithe reformed sodium tluoroborate to the electrolytic cell.

The process of the Iinvention is more economical than those processes which produce hydrogen as the cathode product, particularly because of lthe greater value of the sodium-lead alloy produced. Under the conditions ernployed, the sodium lluoroborate and the anode products, elemental lluorine and boron trifluoride, do not react with carbon to a signicant extent, whereby a carbon anode can be employed without consumption of the anode and the production of signican-t amounts ot ilumine-containing organic compounds. The gaseous mixture of elemental iluorine and boron tritluoride can be readily `separated by reaction of the boron trifluoride with sodium iluoride to give essentially pure elemental uorine and to reform sodium lluoroborate from the boron tritluoride, which contributes materially to the economics of the process and is particularly advantageous for continuous operation of the process.

The electrolyte should consist essentially of molten anhydrous sodium flu-oroborate. Small amounts of sodium fluoride up to Iabout 2% by weight can be tolerated, but larger amounts have the tendency t-o raise lthe melting point of the electrolyte to above the temperatures required for the presen-t process. Also, the sodium fluoroborate should be free of chlorine-containing materials so las to avoid contamination of -the anode product with chlorine. Water must be completely absent because it tends to hydrolyze the sodium iluoroborate to boric oxide and because water reacts with tluorine to form oxygen diiluoride which -is highly explosive.

Sodium lluoroborate has a melting point of about 383 C. to 385 C. The electrolysis should be carried out at a tempera-ture of from 385 C. to about 500 C. and preferably from about 440 C. to about 480 C. At temperatures above 500 C., sodium iluoroborate tends to decompose to sodium luoride and boron tritluoride. Also, at temperatures materially above 500 C., sodium iluorob ora-te and the anode products become highly corrosive and materially more reactive and tend to react with carbon and, Where a carbon anode is employed, produces ilumine-,containing carbon compounds resulting in loss in the production of elemental tluorine and consumption of the carbon anode.

In the process of this invention, the cathode consists essentially of a pool of molten lead. The molten lead should be substantially pure, as significant amounts of other metals cannot be permitted in the sodium-lead alloy, particularly when such alloy is to be used for making tetraalkyllead compounds.

The anode should consist of a conductor composed of graphite, microcrystalline carbon, explosively compacted graphite, silicon carbide or gold. in most cases, graphite, particularly explosively comp-acted graphite, is preferred. Explosively compacted graphite and its preparation are disclosed in Belgian Patent No. 598,362, granted January 13, 1961, to E. I. du Pont de Nemours and Company as assignee of Chrisp, Dieter, Jr., and

Jenkins. In some cases, such as in the use of a skull furnace, it will generally be preferred to employ a gold anode. In the gold anode, the gold may be plated as an external coating on another material such as graphite, copper or the like.

The reactions involved in the process are represented by the following equations wherein e represents an electron, (g) means gas, (s) means solid, and (liq.) means liquid:

Cathode reaction:

Anode reaction, carbon anode:

Unlike the other anodes which merely act as conductors of electricity, part of the gold electrode takes part in the reaction, forming auric fluoride which decomposes to gold and fluorine at the reaction temperature.v More specifically, BF( reacts atthe gold anode to form EP3 and F2. The luorine immediately reacts with the gold to form AuF3 (auric fluoride). rlhe auric fluoride apparently does not adhere to the gold electrode but floats away on top of the molten electrolyte. At temperatures greater than 385 C. auric fluoride is not stable and decomposes to gold and lluorine. The resulting gold is in a very finely divided colloidal state which lloats on the molten electrolyte, possibly due to surface tension effects of the melt. After a short time, the gold film on top of the molten electrolyte is continuous and forms part of the anode. The lilm then is continuously being converted to auric fluoride and back to gold. Colloidal gold, in spite of the high density of gold, is known to remain suspended in water indenitely, so it is not surprising that it floats on the molten electrolyte.

In the operation of the process, there should be employed a directV electric current of from 2 to Vabout 25 volts to provide a current density of from 2 to 50 arnperes per square decimeter (amps/dm?) of anode surface area, usually 2 to about 25 amps/dm?, preferably from 2 to about 7 volts and a current density of from 2 to about 20 amps/dm.2 and most preferably between 2 and 3 volts and current densities of from 2 to about 4.7 amps/dm?. The current densities and voltages are interrelated. The voltage required to obtain any currentA density is primarily a function ofthe anode material used. Current densities lower than about 2 amps/dm.2 cause a very slow production rate, although they are operable. At current densities above 25 amps/dm?, polarization of the electrodes tends to produce undesirable effects and, at current densities materially above 50 amps/dm?, erosion of the anode and severe polarization of the electrodes occur which require higher voltages to obtain the same current densities or render the system inoperative.

The anode products, elemental fluorine and boron trifluoride, have very low boiling points whereby it is diflicult to separate them by fractional distillation or fractional condensation. However, in accordance with the present invention, they are readily separated by passing the gaseous mixture over anhydrous sodium fluoride, usually preheated to about 200 C.l to about 500 C. Under such circumstances, the boron trifluoride reacts to reform sodium fluoroborate, the elemental fluorine does not react with the sodium fluoride and hence passes on to be collected and stored for use. Usually, an excess of sodium fluoride is employed to insure the complete removal of the boron trifluoride from the gaseous mixture. The resulting sodium uoroborate therefore usually contains some sodium uoride and it is then treated with further amounts of boron trifluoride at about 400 C., employing suflicient boron trilluoride to substantially completely convert the sodium fluoride to sodium fluoroborate which can then be recycled to the electrolytic cell.

Suitable forms of electrolytic cells for practicing the process of this invention are illustrated in the accompanying diagrammatic drawings in which FlG. l is a view in vertical cross section of one form of cell; FIG. 2 is a horizontal cross section taken on line 2 2 of FIG. 1; and FIG. 3 is a view in vertical cross section of second form of cell.

Referring particularly to FIGS. 1 and 2, the cell is of a general H-form having two vertical tubular legs 10 and l2 connected by a horizontal tubular cross member 14 at approximately 1/3 the height of the legs, to provide a passage for the flow of liquid between said legs 10 and 12. Said members 10, 12 and 14 are made of a heat resistant metal such as nickel, Inconel or the like, and are provided with inner linings 15 of a refractory material such as solid sodium fluoride, solid calcium fluoride, or a solid fused mixture of NaF and CaF2. The lower '3A (approximate) of the cell is provided with a heating jacket 18 which as shown is in the form of insulated electric heating elements. Cooling coils 20' of tubing for the circulation of a cooling fluid, eig., water, surround the upper portions of the legs 10 and l2 to maintain the upper portions of the cell at a temperature such that the metal will not be seriously attacked by the gaseous products.

The leg l0 has a cap 22 having a charging port 24, a port cover 26, and gaskets 23 and 25. A discharge port 28 for discharging sodium-lead alloy is provided in the bottom of leg l0 and is closed by a suitable valve (not shown). An electrical connector 30 projects into the leg 10 near the bottom thereof and into the molten lead or molten sodium-lead alloy 32 which is maintained in leg 10 below the level of the passage formed by the cross member 14 and forms the cathode of the cell.

The leg 12 is closed at the bottom and may terminate at the level of the bottom of the cross member 14, if desired. The leg l2 is provided with a cover 34 which has a gas discharge port 36. The cover 34 also supports a carbon anode 38 fitted through a compression seal 40 which preferably is made of polytetrafluoroethylene or like inert material and which is held in place by a cap 42. Gaskets are also provided at 35 and 41. The lower end of the anode 38 dips into the electrolyte 44 above the level of the passage provided by the cross member 14. Means for introducing an inert gas, such as nitrogen, may be provided in cover 34, if desired. Y

In operation, sodium uoroborate is added via charg- `v ing port 24 and melted by means of heaters 18, the

amount being suicient to lill the cell for approximately 273 its height, i.e., at about the level indicated at 44, and so as to extend above the lower end of the anode. Lead is then added, via port 24, to the desired level indicated at 32, i.e., below the level of cross member le. Direct current is applied across the anode and the cathode. Sodium forms at the lead cathode 32 and dissolves therein to form sodium-lead alloy. Fluorine and boron trifluoride form at the anode 38 and pass out of the reactor via port 36. The fluorine and boron triiluoride are separated bypassing the mixture over anhydrous sodium fluoride. The boron trifluoride reacts with the sodium fluoride to reform sodium luoroborate and the nonreacting elemental fluorine passes on to be collected and stored for use. The sodium lluoroborate recovered is returned periodically to the reactorrvia port 24. Periodically, part of the liquid cathode 32 is Withdrawn as sodium-lead alloy, and fresh lead is added via port 24. It is best not to allow the concentration of sodium in the cathode 32 to exceed about 5% to avoid reaction of sodium with sodium uoroborate to form elemental boron which is undesired.

FIG. 3 shows an electrolytic cell of the so-called skullfurnace type in which the molten electrolyte and the molten lead cathode are contained in a shell formed of solid electrolyte. This type of cell is particularly useful with a gold anode, but may also be used with a carbon anode. 'lfhis cell comprises a cylindrical vessel 46 and a cover 48, both made of a heat resistant metal such as nickel, Inconel, or the like. The cover is provided with a charging port 50 closed by a cover 52, and with a gas discharge port 54. Also, the cover 48 carries at its center an anode 56 fitted through a compression seal 5S which preferably is made of polytetrafluoroethylene or like inert material and which is held in place by a cap 60. Gaskets are also provided as in the structure of FIG. l. A vertical pipe 62, made of a metal which is a good conductor of electricity and is heat resistant, passes up through the bottom of the vessel 46, offset from but near the center of said bottom, and has its open end terminating approximately 1/3 the height of the vessel 46. Said pipe 62 is provided with a valve 64 and is separated from the bottom of the vessel by an electrical insulator 66. Also, said pipe 62 is connected to the source of electric current so as to form an electrical connector for the molten lead cathode 68. Means for introducing an inert gas, such as nitrogen, may be provided in the cover 48, if desired.

The cell of FlG. 3 is operated by iilling the vessel 46 to near the top with solid sodium iluoroborate and passing current between the anode 56 and the pipe 62, generating sulcient heat to melt part of the solid sodium fluoroborate to form a pool 70 of liquid sodium fluoroborate in the center of the upper portion of the solid sodium iiuoroborate 72, said pool extending from below the upper end of pipe 62 to considerably above said end. With thecurrent oi, lead is quickly added through port 50. The lead melts and sinks to the bottom of the pool YIlle cell shown in FIG. 3 has the advantage that it permits the use of a gold anode. In the cell of FIG. 3, the gold which forms a film on top of the molten electrolyte 70, cannot come into contact with the metal walls of the vessel 46. Otherwise, the whole vessel 46 would become part of the anode and the cell would cease to operate.

The hazards of handling elemental iiuorine are well known. In particular, materials which react with fluorine rapidly should be scrupulously excluded from the reaction system. These particularly include organic materials such as oils, greases, waxes, pipe clopes and the like, water, oxygen, reactive metals, and the like. Therefore, the process is carried out under an inert atmosphere in the cell. Any air, which may be present in the cell after charging the cell with the electrolyte and the lead cathode, will be small in amount and is quickly swept out with the iirst formed portions of the gaseous anode products, or can be largely eliminated by evacuation before the operation is started. Usually, it will be preferred to sweep the system with an inert gas such as nitrogen, helium, GF4, iluorine, and the like, before the operation of the cell is started, preferably before the electrolyte is charged to the cell. The health hazards of iluorine are well known and all appropriate precautions should he taken.

In order to more clearly illustrate this invention, preferred modes of practicing it, and the advantageous results to be obtained thereby, the following examples are given in tabular form in Table I. In these examples, the cell corresponded to that shown in FIGS. 1 and 2 of the drawing, and the various columns of Table I show the anode materials, the temperatures, the current densities, the volts, the times for which the cell was operated, and the results obtained as indicated by the current eliciencies. Boron tritiuoride was removed from the olf-gas by passing through solid sodium iuoride at 200 C. to 500 C. Fluorine production was determined by passing the offgas, freed of boron tritiuoride, through a sodium iodide solution and titrating the iodine formed with standardized sodium thiosulfate solution. Sodium, in the sodiumlead alloy, was detected by carefully treating the alloy with water.

Table I Anode Current Current Example Anode Material Area Temp., Density, Volts Time, Etli- NO, (dm) C. amp/dm 2 Min. ciency, percent 1 Silicon carbide 0. 0l26 450 2. 2 2. 2 20 58 2 d 0. 0126 480 4. 0 2. 2 30 58 3 0. 02 440 5. 0 2. 2 120 25 4 0. O2 445 5. 0 18. 6 26 25 5 0. 0064 430 28. 3 2. 2 15 l0. 8 0. 0064 430 15-23. 3 2. 2 l5 14. 7 0. 0065 450 4. 5 2. 2 1l 72. 5 0. 0065 450 4. 2. 2 10 92. 0 0. 10 440 2. 4 2. 2 l5 1l. 3 0. l0 440 2. 6 2. 2 l5 10. 7 0. 10 440 2. 2 2. 2 15 32. 0 0. l0 440 2. 2 2. 2 l5 23. 5 Microcrystalline Carbon 0. 06 440 15-20 6. 75 49. 5 Explosively compacted graphite, 0. 20 440 10-15 2-2. 3 54-67 hr. 5-42 density 2.1 glee.

of molten electrolyte to form a pool 63 of molten lead which extends above the upper end of pipe 62 and forms the cathode for the further operation of the cell. The current is then resumed. Sodium forms at the molten lead cathode 68 and dissolves therein. Fluorine and boron triliuoride form at the anode 56 and pass from the reactor via port 54. The iluorine and boron triuoride are separated by passing them through sodium fluoride as previously described in connection with the operation of the cell of FIGS. l and 2. Liquid sodium-lead alloy is periodically withdrawn from the cathode pool 68 via pipe 62 and valve 64 and more lead is added via port 50. Sodium iiuoroborate is also added periodically via port 50.

The current e'iciency indicates the order of eiic-iency of utilization of electric current in forming iiuorine. Variations in observed current efficiencies, under substantially identical conditions, were due to variations. in naturally occurring impurities, e.g. moisture, in the starting materials which impurities are removed by the electrolysis, and to incomplete recovery of the fluorine due to mechanical diiliculties encountered in the recovery system. The current eliciencies were calculated in this manner; when a current of (c) amperes has passed through the cell for (x) seconds a total of (ax) coulombs of electricity has passed through the cell. According to Faradays law, the formation of one equivalent of suesser 96500(g) (100) (aw) Taking Example 1, where the current efliciency was 58%, the time was 20 min.:1200 sec.; the current was 2.2 amps./dm.2; the current ow was therefore 2.2 l200 ==2640 coulombs/dm-2. Hence equivalents of fluorine/drn.2 or 301.3 milligrams of flucrine/dm.2 or about 15.07 milligrams of fluorine per minute per dm?.

It will be understood that the foregoing examples have been given for illustrative purposes solely and that this invention is not limited to the specific embodiments described therein. Also, the types and structure of the cell may be Widely varied in the manner well known to those skilled in the art in the electrolysis of inorganic fluorine compounds. The materials employed, the temperatures, current densities, volts and the like may be varied within the limits set forth in the general description without departing from the spirit or scope of this invention.

From the foregoing description, it will be apparent that this invention provides a new and improved process for the electrolytic production of fluorine which also produces a valuable cathode product and boron triuoride as an anode product, which boron tritiuoride is readily separated from the elemental iuorine and readily reconverted to sodium tluoroborate which can be recycled to the process, whereby loss of materials as undesired by-products is avoided. The process is simple and readily carried out and results in material economic advantages. Therefore, it will be apparent that this invention constitutes a valuable advance in and contribution to the art.

The embodiments of the invention in which an exclusive property or privilege is claimed are dened as follows:

1. The process for preparing elemental fluorine and lyzing an electrolyte consisting essentially ofV molten anhydrous sodium fluoroborate in an inert atmosphere at a temperature of from 385 C. to 500 C. in an electrolytic cell having an anode composed of gold and a cathode consisting essentially of molten lead at substantially the same temperature as the electrolyte, employing an electric current of from 2 to about 7 volts to provide a current density of from 2 to about 20 amps/dm.2 of anode surface area, recovering a sodium-lead alloy as the cathode product and a gaseous mixture of elemental fluorine and boron triiluoride as the anode product, and separating the elemental tluorine from the boron trifluoride.

3. The process for preparing elemental fluorine and a sodium-lead alloy simultaneously which comprises electrolyzing an electrolyte consisting essentially of molten anhydrous sodium fluonoborate in an inert atmosphere at a temperature of from about 440 C. to about 480 C. in an electrolytic cell having an anode composed of gold and a cathode consisting essentially of molten lead at substantially the same temperature as the electrolyte, employing an electric current of from 2 to about 7 volts t0 provide a current density of from 2 to20 amps/dm.2 of :anode surface area, recovering a sodium-lead `alloy as the cathode product and a gaseous mixture' of elemental iiuorine and boron trifluoride as the anode product, and separating the elemental luorine from the boron trifluoride.

4. The process for preparing elemental iluorine and a sodium-lead alloy simultaneously which comprises electrolyzing an electrolyte consisting essentially of molten anhydrous sodium fluoroborate in an inert atmosphere at a temperature of from about 440 C. to about 480 C. in an electrolytic cell having an anode composed of gold Iand a cathode consisting of molten lead at substantially the same .temperatur-e as the electrolyte, employing an electric current between 2 and 3 volts to provide a current density of from 2 to about 4.7 amps/dm.2 of anode surface area, recovering a sodium-lead alloy as the cathode product and a gaseous mixture of elemental iluorine andv boron triuoride as the anode product, and separating the elemental iiuorine from the boron trifluoride.

5. The process for preparing elemental iiuorinel and a sodium-lead alloy simultaneously which comprises electrolyzing an electrolyte consisting essentially of molten anhydrous sodium fluoroborate in an inert atmosphere at a temperature of from 385 C. to 500 C. in an electrolytic cell having an anode composed of gold and a cath- 10de consisting essentially of molten lead at substantially Yso sodium-lead alloy simultaneously which comprises electroi lyzing an electrolyte consisting essentially of molten Eanhydrous sodium lluoroborate in an inert atmosphere at a temperature of from 385 C. to 500 C. in :an electroconsisting essentially of molten lead at substantially the the same temperature as the electrolyte, employing an electric current of from 2 to about 25 volts to provide a cur- Yrent density of from 2 to 50 amps/dm.2 of anode surface area, recovering a sodium-lead alloy as the cathode product and a gaseous mixture of elemental uorine and boron triiluoride as the anode product, separating the elemental fluorine from the boron trifluoride by passing the gaseous mixture through sodium fluoride at a temperature of from about 200 C. to about 500 C. whereby the boron tri- Y-iiuoride reacts with the sodium uoride to reform sodium same temperature as the electrolyte, employing an electric v l current of from 2 to about 25 volts to provide a current density of from 2 to 50 amps/dm.2 of anode surface area, recovering a sodium lead alloy as the cathode product and a gaseous mixture of elemental uorine and boron trifluoride as the anode product, and separating the elemental lluorine from the boron trifluoride.

. 2. The process for preparing elemental fluorine and a sodium-lead alloy simultaneously which comprises electro- WINSTON A. DOUGLAS, Primary Examiner,

Claims (1)

1. THE PROCESS FOR PREPARING FLUORINE AND A SODIUM-LEAD ALLOY SIMULTANEOUSLY WHICH COMPRISES ELECTROLYZING AN ELECTROLYTE CONSISTING ESSENTIALLY OF MOLTEN ANHYDROUS SODIUM FLUOROBORATE IN AN INERT ATMOSPHERE AT A TEMPERATURE OF FROM 385*C. TO 500*C. IN AN ELECTROLYTIC CELL HAVING AN ANODE COMPOSED OF GOLD AND A CATHODE CONSISTING ESSENTIALLY OF MOLTEN LEAD AT SUBSTANTIALLY THE SAME TEMPERATURE AS THE ELECTROLYTE, EMPLOYING AN ELECTRIC CURRENT OF FROM 2 TO ABOUT 25 VOLTS TO PROVIDE A CURRENT DENSITY OF FROM 2 TO 50 AMPS./DM.2 OF ANODE SURFACE AREA, RECOVERING A SODIUM LEAD ALLOY AS THE CATHODE PRODUCT AND A GASEOUS MIXTURE OF ELEMENTAL FLUORINE AND BORON TRIFLUORIDE AS THE ANODE PRODUCT, AND SEPARATING THE ELEMENTAL FLUORINE FROM THE BORON TRIFLUORIDE.

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