US2996446A - Apparatus for the electrolytic production of fluorine - Google Patents

Description

Aug. 15, 1961 A. DAVn-:S ET AL APPARATUS FOR THE ELECTROLYTIC PRODUCTION OF' FLUORINE Filed Deo.

United States This invention relates to improvements in or relating to a process for the electrolytic production of fluorine `and apparatus therefor, and particularly to its production by electrolysis of a fused substantially dry mixture of potassium fluoride and hydrogen uoride having a composition approximating substantially to KF.l.8HF KF.2.2HF.

It is well-known to manufacture fluorine by electrolysis of substantially dry fused mixtures of uorides, in particular mixtures of an alkali metal fluoride and hydrogen uoride (The Present Status of Fluorine Production, Chemistry and Industry, 1956, pp. S04-511). Further in such processes it is well known to use cells having anodes of carbon or graphite, the cathode being of mild steel or other metal resistant to the action of the electrolyte. Hydrogen is evolved at the cathode-and liuorine, with perhaps varying amounts of oxygen and other impurities, at the anode. Also `as mixtures of hydrogen and uorine give rise to violent explosions such fluorine cells customarily have a diaphragm or partition designed to prevent mixing of the gases evolved at the two electrodes. In some cells this diaphragm or partition extends downwards in the interelectrode space for a distance equal to or even greater than that of the downward extension of the electrodes. In other uorine cells, for example, in that described and claimed in United Kingdom specification No. 668,465, -a barrier, impervious to gases, extends downwards for -a short distance only into the interelectrode space. However most of such cells of the types described above have one feature in common, namely, an undesirably large distance between the `anode and cathode.

It is, of course, well-known that the greater the spacing between the electrodes, the greater must be the potential applied and the energy consumed to electrolyze a given amount of material and therefore it is desirable to diminish the interelectrode space as `far `as is commensurate with safety. Nevertheless it has hitherto been the consistent teaching in this `art that (except in certain cells with unconventionally shaped electrodes, which are more fully described hereinafter) it is not possible safely to diminish the distance between anode and cathode (hereinafter termed the electrode separation) or the distance between anode and gas barrier (hereinafter termed the anode gap) below certain limiting values. United Kingdom specication No. 668,465, for instance, which represents the most developed form of this teaching, propounds a rule that as the electrodes extend further downwards into the electrolyte below the bottom of the gas barrier, the interelectrode spacing must be increased. It prescribes as absolute minima for safe working that when the electrodes extend to 8 inches below the gas barrier the electrode separation should not be less than 2% inches nor the `anode gap less than 1 inch. The corresponding values when the electrodes are extended to 36 inches below the barrier are 4% inches and 11%,; inches. However, if a special louvred cathode is used, the figures for ice the electrode separations appropriate to these depths of 8 inches and 36 inches may be diminished to 2%. inches and 315/16 inches respectively. However, as said, these are prescribed as limiting minimum values and the spacings which it teaches should preferably be used for reliably safe working vare appreciably greater than those prescribed minima.

In Uni-ted Kingdom specication No. 675,209 there is described and claimed la uorine cell with an unusually small electrode separation, but in this cell an anode of unconventional design is employed. 'I'he upper portion of the anode, which lies wholly or partly below the electrolyte surface, is `of smaller cross section than the main portion and extends above the level of the upper extremity of the cathode. A gas-impermeable bell or hood, conveniently of diameter equal to that of the main portion of the anode, dips into the electrolyte in the space between the cathode and this upper portion of the anode, entirely surrounding but not making contact with -the latter. The fabrication of such special anodes is however diicult or inconvenient and in practice these anodes are found to be embarrassingly fragile.

We have now found, most surprisingly, that it is possible safely, reliably and economically to work a uorine cell with a substantially vertical solid carbon anode of substantially uniform cross-section and barrier of simple, robust, conventional design and employing current densities up to for example 1.1 amp/sq. in., using an anode gap much less than those which have hitherto been thought indispensible to safely if, inter alia, said anode has a permeability of, for instance, between 1.0 and 30, permeability being here dened in terms of cubic feet of air per square foot of surface capable of passing through one inch thickness of the anode material per minute under an imposed pressure equivalent to two inches of water. The determination of the permeability is carried out on cylinders one inch diameter and one inch long. These cylinders are mounted tightly in a rubber holder and the mean of the measurements of the quantity of air passing through two cylinders, which are cut at right angles to one another `from rthe same block, under an imposed pressure equivalent to 2 inches of water, is used to calculate the permeability. yOrdinary electrode carbon measured in this way has la permeability of 0.05 and is unsuitable for the carrying out of the process of the present invention. Electrode carbon of such low permeability is considered to be impermeable for the purposes of the present invention.

According to the present invention the process for the production of fluorine by electrolysis at a temperature of -1l0 C., preferably 80-85" C., from a fused substantially dry mixture of potassium iluoride and hydrogen uoride having a composition approximating substantially to KF.1.8HF-KF.2.2HF under non-polarization conditions and so that the composition of the mixture is maintained substantially `at KF.1.8HFKF.2.2HF, for example at a cathodic current efficiency of greater than and preferably greater than with no evolution of iluorine as free bubbles at a substantially vertical solid gas permeable carbon anode of substantially-uniform cross section in conjunction with a gas impermeable barrier which entirely surrounds but is not in contact with the anode by employing current densities of e.g. 0.1 to 1.1 amperes per square inch and for instance effectivelengths of anode and cathode up to eg. 18 inches and in a cell in which the horizontal distance of the cathode and the anode from the Ybarrier is varied in accordance with the effective lengths of the anode andthe cathode below the level of th'e' barrier and the upper portion of the anode is above the level of the top portion of the cathode and is partially or wholly below the surface of the electrolyte is characterized in that the horizontal distance between the anode and the cathode is from 3A; inch to 1.5 inches and in that lat least the lower extremity of the barrier is distanced, measured horizontally, by not more than inch from the anode and preferably in' that at least the lower extremity -of-the barrier approaches to Within a distance measured horizontallyV ofJAG. inch to 3/4; inch from the anode vand no portion approaches more nearly than @A6 inch.A

Said barrier must not be in electrical contact with the anode.4 To make sure of this it is sometimes desirable to provide insulating material in the space within the electrolyte between `at least the lower extremity of the barrier and the anode. Y

The gas permeable carbon anode has to have at least a gaspermeability such that no free fluorine bubbles are liberated inter alia at the operating current density.

Preferably the permeability is approximately 30.

The term eftectiveranode length refers to that portionof the anode length which is below the gas barrier and is opposite to the eective length of the cathode.

Current density is determined with reference to that portionl of the anode surface which is directly opposite the'Y cathode.

'Ihe term polarization is used herein to denote the condition under which Vat a iixed voltage a sudden or gradual decrease occurs in the current ilowing through the cell to a value which is a small fraction of that-passing when the cell is operating normally. The effect may be temporary or permanent and is essentially an anodic phenomenon.

Oneform of an electrolytic cell adapted for the repro'- duction of fluorine according to the process of the inventionl by electrolysis at a temperature of 80-l05 C., and preferably V80".-85" C., of a fused substantially dry mixtureD of potassium fluoride and hydrogen fluoride having a composition approximating substantially to KFLSHF to KF.2.2HF comprises a container for the electrolyte, preferably provided with means serving for heating or cooling its contents, electrolyte of the nature indicated and a Vsubstantially vertical solid gas permeable carbon anode of substantially uniform cross-section and for instancek of a gas permeability between 1.0 and 30 surrounded at its upper portion by a gas impermeable barrier dipping below the surface of the electrolyte and at its lower portion by a wholly submerged cathode which is below Vthe said barrier and is characterized in that the horizontal distance between the anode and the cathode (that is, the electrode-separation, as hereinbefore defined) is in the range 1/2 inch to 1.5 inches when the cathode is a plain sheet cathode, and in the range -'/s inch to 1.5 inches when the cathode is louvred or is made of punched sheet or gauze and in that at leastrthe lower extremity of the barrier approaches to Within a distance (measured horizontally). from the anode which is in the range 1/16 inch to 1./8 inch and no portion approaches more nearly than ide IlCh- Y Y The dimension of the anode gap is related tothe electrode separation which it is proposed to use.V Thus a small electrode separation requires a small anode gap though any electrode separation above the minimum referred to herein may be used with the minimum anode gap quoted. f

Preferably the barrier in the aforesaid form of cell has a downwardly and inwardly projecting slope or curve so shaped Vas to direct any hydrogen that rises from the cathode andv comes into contact with the barrier upwardly and outwardly away from the anode.

The cathode should, preferably, as indicated in the aforesaid form of electrolytc cell, be wholly below the gasfbarrier.'` The verticalY distance separating; the upper extremity ofthe cathode from the lower extremity of the barrier is not critical but it must be adequate to allow of ready escape of hydrogen liberated in the space between anode and cathode. Clearly, if this vertical separation is inadequate, then when high current density is employed there is a possibility of a particularly brisk evolution of hydrogen leading to crowding of hydrogen bubbles within this space, so increasing the danger of hydrogen iinding its way into the anode compartment. The requisite vertical separation of cathode and barrier is thus iniluenced not only by the spatial relationship of anode, cathode and barrier but also by the material of the anode and the current density at which the cell is to be operated. This vertical separation is not, however, a factor of major importance and a convenient practice is to make it of comparable magnitude with the electrode separation.

The anode, cathode and barrier may be cylindrical in form although other shapes, for instance of rectangular or square section, or evenA of hexagonal section, may be used if desired.

When working withV such small clearances between Vthe electrodes and the barrier as are speciiied above, robust and accurate construction, particularly of the gas barrier, is of considerable importance. One suitable type of barrier that may be employed when the anode is of cylindrical cross-section is a hollow cylinder made of a suitable metal with an inturned ange which is neither horizontal nor` sloping upwards towards they anode, at the lower extremity inwards for such distance as to leave a clearance from the anode of the desired size. The anode barrier gap is in this case Vmeasured from the inner circumference of the flange to the face of the anode. Obviously the barrier may, if desired, be a simple hollow cylinder; the flanged structure has, however, the advantage of greater rigidity which is obviously important when such small clearances are used. A flanged construction or one with a tapered or inturned lower extremity can be more reliably and precisely positioned with respect to the anode than a barrier which is only, say 1A@ inch, from the anode for its full length. Also not only is such an arrangement more resistant to deformation that would Y cause short-circuiting, but it virtually limits to very small dimensions the possible areas of contact that could be involved in short-circuiting between anode and barrier.

The body of the cell (i.e. the container), the gas barrier and the cathode may all conveniently be made from mild steel, although other materials resistant to the electrolyte and the products of electrolysis may be used if desired. For instance, the barrier may be made of Monel or nickel.

The anode may be a simple block of carbon, whose minimum transverse dimension is at least 11/2 inches, preferably 2 inches. For, for instance, 60-amp. cells it may be convenient to use cylindrical anodes of diameter up to 3 inches; for 10-amp. cells, 2 inches is a convenient value. The length of the anode block is not of major importance. The point to be borne in mind in this connection is simply that if this total length be unduly great there will be an appreciable voltage drop as one proceeds down the anode from the lead-in conductor at the top towards the lower extremity, so that the effective potential dierence between anode and cathode will not be as great in the lower portion of the cell as irr the Vupper portion. The barrier is conveniently made 4 inches to 8 inches deep but can be greater or smaller if desired. It must dip suiciently below the electrolyte surface to make an adequate liquid seal at the base of the anode (or iluorine) compartmenta depth of immersion of 2 inches is convenient. The length ofthe anode block that is opposite to, and so in operative relationship with, the cathode, i.e. the effective lengths of the -anode and cathode, is of more importance. We have used lengths ranging from 2 inches to 141/2 inches and find little difference in their eiect apart'from the volt age Vdrop due tothe ohrnic resistance. alluded to above.

5 However, an unduly long and narrow interelectrode space can more easily lead to crowding of hydrogen bubbles if a high current density is used and also an increased length of carbon anode requires greater consideration to be given to its fragility.

The electrode separation, the depth of the interelectrode space, the current density employed, the material of which the gas permeable carbon anode is constructed and the size of the anode gap are all interconnected, but once the relevant principles are appreciated the `appropriate adjustment of these various factors is a routine matter easily -within the competence of the operator skilled in this art.

It is to be noted that the maximum current density which can be used will be dictated by the current density at which polarization occurs and/ or that at which breakaway of fluorine bubbles takes place.

Commercially available electrode carbons having a gas permeability of 2530 and 10 respectively are eminently suitable.

High current density, a long narrow interelectrode space and an inadequate vertical clearance between the lower extremity of the barrier and the upper extremity of the cathode all tend to produce hydrogen crowding, but this can be avoided without increasing the electrode separation by increasing the vertical cathode-barrier clearance, shortening the electrodes and diminishing the anode gap. Thus, for instance, in operating one cell with a plain steel cathode and an anode of gas permeability 25-30, 141A. inches long, using a current density of 1.0 am./sq. inch and an electrode separation of 3A inch, reliable working was obtained with -an anode gap of ls inch. On diminishing the electrode separation to 1/2 inch, the other conditions being unchanged, it was found necessary to reduce the anode gap to 1/16 inch to achieve reliable working.

By adjusting these various factors appropriately as indicated, We have been able to construct cells having electrode separations and anode barrier gaps of the surprisingly small dimensions defined `above which have given continuous trouble-free operation for periods as long as 9 months, and this without any need for electrode replacement or removal of sludge from the cell. The current eiciency was 97-9'8% using current densities up to 1.1 amp/sq. in.

The saving achieved on a standard 60-amp. cell consequent on reducing the electrode separation from the 2 inches hitherto employed to 1/2 inch with an 11 inch anode (ie. below the bottom of the barrier) Was 1.2 volts when operating at a current density of 6- amp/ sq. inch and 1.9 volts when working at a current density of 0.9 amp./ sq. inch.

The saving for an approx. 1400 amp. cell consequent on reducing the electrode separation from 21/2 inches to 11A inches with an 8 inch anode (i.e. below the bottom of the barrier) was 1.4 volts when operating at a cur rent density of 0.5 amp/sq. inch and 2.5 volts when working at a current density of 0.9 amp./sq. inch.

The more detailed practice of the invention is illustrated by the following description.

One form of cell suitable for carrying out the invention is illustrated in the diagrammatic drawing accompanying the provisional speciiication which represents a Vertical section through the said cell. Referring to the drawing, 1 is a container of mild steel or other suitably resistant metal, surrounded by a heating jacket 2 adapted for water, steam of electrical heating (water shown), and preferably thermostatically controlled. The lid 3 is linsulated from the container 1 and from the carbon anode 4 by insulating material 5. The carbon anode 4 is partly submerged in the electrolyte 6. An electrically conducting rod 7 insulated from the cell lid 3 by insulating material is connected to the anode 4. In close proximity to the anode 4 -is a cathode 8 which may be of mild steel, copper or other material substantially resistant to the electrolyte 6 and products of electrolysis. The cathode 8 may be a plain sheet, a louvred sheet, a punched sheet or gauze. When the cathode 8 is of punched sheet, it may be a sheet with V; inch diameter holes at 3A@ inch centres. The cathode 8 is supported by an electrically conducting rod 9 passing through the top of the container 1. Attached to the lid 3 is a gas impermeable barrier 10 |which surrounds that part of the anode 4 above the level of the top of the cathode 8. The pipe 11 for iiuorine take-olf is connected through the cell lid 3 to the space between the anode 4 and the gas barrier 10. The pipe 12 passing through the top of the container 1 is for take-oir of hydrogen. The pipe 13 passing through the top of the container 1 into the elec, trolyte 6 is for addition of hydrouoric acid.

The following examples of the working of such cells illustrate but do not limit the invention.

Example 1 The cell in this example comprises a jacketed mild steel vessel which is heated by hot water. The anode is cut from a block of carbon Whose permeability is appriximately 30, permeability being defined in terms of cubic feet of air per square foot of surface capable of passing through one inch thickness ofthe carbon per minute under an imposed pressure equivalent to two inches of water. The anode is of circular cross-section and of the following dimensions: Diameter 3 inches, efective length /8 inches. An impermeable gas barrier of mild steel is situated only 1A@ inch from the anode, while the cathode of mild steel is situated at a distance of only 5A; inch from the anode. The electrolyte is a mixture of hydrogen fluoride and potassium iiuoride in the ratio HF:KF=1.S. The temperature of operation is C. Y

The cell operates very smoothly at a current efficiency on fluorine of 90-95% for a period of 24 hours at an anode current density of 0.6 amp./ sq. in. with an applied voltage of 7.3 volts. It may be mentioned by way of contrast that with an anode/cathode separation of three inches, as in a conventional cell, the applied voltage necessary to attain the same current density is 9.4 volts.

Example 2 The following table illustrates the voltage saving which is obtained with a 60-amp. cell having an `anode gap of 1A@ inch as a result of reducing the electrode separation. The anode is 141/2 inches long and the electrolyte contains 40% HF. The cell is operated at 90 C. and the current efliciencies range from 98 and 100%.

Cell Voltage at Separation of Anodic Current Density (Amp./sq. 1n.)

3l! 1l! ll %/l Example 3 The cell in this example comprises a rectangular mild steel vessel which is heated electrically by means of strip heaters on the outside of the vessel. The anode is cut from a block of carbon whose permeability is approximately 30. lt is of rectangular cross-section, 2% by 11, and length 131/2, the bottom 8" length extending below an impermeable gas-barrier of nickel. The lower portion of the barrier is situated only 1/16 from the anode, while the mild steel cathode is situated at a distance of only 1/2" from the anode. The electrolyte is a mixture of hydrogen iluoride and potassium floride in the ratio HF:KF=2. The temperature of operation is 85 C.

The cell can be operated at loads up to 200 A. (anodic current density of 0.9'5 A. per square inch) for a total 7 time of .l030zhours overa period of 6 months. rentiefciencies when determined during theperiod glve results of 994.00%. on lluorine. At 200 A. the-cell operates autothe1mally,"with.an applied voltage of 8.2 volts.

Afsecondl cell of identical size,V but with the barrier placed 11/1" from the anode, and, the cathode situated Zjfrom the anode, operates autothermally at 170 A.

lan applied voltage of 9.9 volts. Theoperatingvoltageof the' first; cell atg170 A. Vload is 8.0, showing a savingof 1.9 volts.,

Theuorine from the. celliwith 1/2" electrode separationv `also contains less hydrogen tluoride than that from the :cell with 2" electrode separation. At 180 A. load the resultsare 9.8 and 26.6% by volume, `respectively.

Example 4 The cell in this example comprises a mild steel vessel containing twelve anodes of the same type and sizeof carbon as that in Example 3. The anodes are in pairs. Six impermeable gas barriers of nickel surround the upper portions of each of these pairs of anodes. The barriers are situated 3/8" from the anodes. Each pair of anodes has a pair of mild steel cathodes and each cathode is situated at a distance of 11A" from a pair of anodes. Provision is made for heating and cooling of the cell by circulating water through coils in the cell. Thek electrolyte is a mixture of hydrogen tiuoride and potassium uoride in the ratio HF:KF=2. The temperature of operation is 85 C.

TheV cell can be operated continuously at a load of 2500 A. (anodic current density of 1.1 A. per square inch) for a period of at least 2 Weeks. Current eiciency determinations during the period give results of 97-98% on hydrogen.

Example 5 The following table is a record of the results which have been obtained with an electrolytic cell according to the invention having a carbon anode of 1 inch diameter and of permeability 1.0 with an anode/cathode separationof one 4inch and an anode/barrier gap Vof 1&6 inch.

Example 6 The .following table is a record of the results which have-been obtained with an electrolytic cell according to the invention having a carbon anode of l inch diameter, of 2 inches Veffective length and of permeability V2.0

7 with-an anode/cathode separation of one inch and an anode/ barrier gap ofgl inch.

' Anodic Current Time-ofrun (hour). current Efficiency density (percent) In one experiment an electrolytic cell according to the invention having two carbon yanodes each of permeability 2.0 and of rectangular cross-section 2%" X 1l and length 131/2, the bottom 8" extending below an impermeable gas-barrier of nickel has operated for 200 days at a current density of 0.7-1.1 amp/sq. in. without the carbon anodes showing any sign of polarization.

In another experiment, in which the same carbon was used as anode, an electrolytic cell according to the invention, having an electrode separation of l inch and an anode/barrier gap of V16 inch, had a current e'iciency of 98 to 100% when operated at an auodic current density of up to 2.0 amp/sq. in.

What we claim is:

l. An electrolytic cell for the production of iluorine by electrolysis of a fused, substantially dry mixture of potassium fluoride and hydrogen fluoride having a KF to HF ratio between about 121.8 and 1:2.2 comprising a container, a substantially uniform cross-sectioned, solid, gas-permeable carbon anode, means mounting said anode in a substantially vertical position in said container, a cathode, means mounting said cathode adjacent the lower portion of said anode, at a distance between three-eighths and 1.5 inches, a gas impermeable barrier, means mounting said barrier to surround the upper portion of said anode within said container and be immersed in electrolyte, the lower end of said barrier being a short distance above the cathode and `horizontally spaced Ifrom said anode by a distance of between one-sixteenth and three-eights inch and no portion of said barrier being closer than one-sixteenth inch, means operatively connected with the upper end of said barrierV forming an enclosed fluorine-receiving chamber and means for withdrawing fluorine from said chamber.

2. An electrolytic cell as set forth in claim l in which the carbon anode has a gas permeability of 1.0 to 30 cubic feet oi' air per square foot of surface capable of passing through a thickness of one inch of anode per minute under an imposed pressure equivalent to two inches of water.

3. An electrolytic cell as set forth in claim l in which the lower end of the barrier has a downwardly and inwardly projecting member so shaped as to direct any hydrogen rising from the cathode and contacting the barrier upwardly and outwardly from the anode.

4. An electrolytic cell as set forth in claim l in which the cathode is a plain sheet cathode mounted at a distance of 0.5 to 1.5 inches from the anode.

5. An electrolytic cell las set forth in claim 1 in which the cathode is apertured and mounted at a distance of three-eighths to 1.5 inches from the anode.

6.`An electrolytic cell as set forth in claim 5 including electric insulating means separating at least the lower end of said barrier 'from the anode.

References Cited in the file of this patent UNITED STATES PATENTS 2,534,638 Swinehart Dec. 19, 1950 2,684,940 Rudge et al. July 27, 1954 2,693,445 Howell'et al. Nov. 2, 1954

Claims (1)

1. AN ELECTROLYTIC CELL FOR THE PRODUCTION OF FLUORINE BY ELECTROLYSIS OF A FUSED, SUBSTANTIALLY DRY MIXTURE OF POTASSIUM FLUORIDE AND HYDROGEN FLUORIDE HAVING A KF TO HF RATIO BETWEEN ABOUT 1:1.8 AND 1:2.2 COMPRISING A CONTAINER, A SUBSTANTIALLY UNIFORM CROSS-SECTIONED, SOLID, GAS-PERMEABLE CARBON ANODE, MEANS MOUNTING SAID ANODE IN A SUBSTANTIALLY VERTICAL POSITION IN SAID CONTAINER, A CATHODE, MEANS MOUNTING SAID CATHODE ADJACENT THE LOWER PORTION OF SAID ANODE, AT A DISTANCE BETWEEN THREE-EIGHTS AND 1.5 INCHES, A GAS IMPERMEABLE BARRIER, MEANS MOUNTING SAID BARRIER TO SURROUND THE UPPER PORTION OF SAID ANODE WITHIN SAID CONTAINER AND BE IMMERSED IN ELECTROLYTE, THE LOWER END OF SAID BARRIER BEING A SHORT DISTANCE ABOVE THE CATHODE AND HORIZONTALLY SPACED FROM SAID ANODE BY A DISTANCE OF BETWEEN ONE-SIXTEENTH AND

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