Abstract:
An arrangement for installing and sealing an anode within a fluorine generating electrolytic cell is described, the arrangement comprising: an anode connection member, said anode connection member ( 32 ) passing through an aperture ( 70 ) in a skirt wall ( 20 ) and being in electrical connection with a skirt wall closure member ( 72 ) wherein the skirt wall closure member is sealingly engaged with said skirt wall and is electrically insulated therefrom.

Description:
FIELD OF INVENTION 
   The present invention relates to the construction of fluorine cells and particularly to the construction relating to the closing of the anode connection to the fluorine producing compartment in such a cell. 
   BACKGROUND OF THE INVENTION 
   Fluorine generating cells produce both gaseous fluorine and hydrogen by the electrolysis of hydrogen fluoride. Fluorine and hydrogen explosively recombine when they contact each other, therefore, it is necessary to keep them completely separate when generated during electrolysis. Fluorine cells are generally constructed such that the two gases are collected in two separate compartments above the surface level of the electrolyte. The compartments are often separated by means of a so-called skirt, the skirt often being part of and depending from an upper, generally horizontal wall of the cell, and extending into the electrolyte and surrounding the anode. However, the skirt must remain electrically neutral with respect to the anode which it surrounds and to the cathode which is often formed by the inner wall of the cell container vessel. It is, therefore, necessary that the skirt is electrically insulated from the anode (and cathode) and for the anode connection (often referred to as the “stud”) to pass through the skirt or be connected thereto whilst completely sealing the fluorine compartment against leakage of fluorine. 
   It has been the practice to have a stud for connection to the anode and which stud passes through an insulating member and seal, often made of plastics material such as a fluoroelastomer rubber, in the upper horizontal skirt wall portion. However, due to the high currents which are inherent in fluorine generation by electrolysis a considerable amount of heat is generated by resistance heating, this condition often being exacerbated by poor electrical connection between the stud and carbon anode which is generally used. The effect of this heating can be to cause a runaway chemical reaction between the plastics seal material and the fluorine with which it is in direct contact and which may result in a fluorine leak. In extreme cases even the stud metal may itself burn in the fluorine gas stream in the resulting leak causing a so-called “stud fire”. This has been somewhat alleviated by the construction shown in WO 96/08589 where, instead of the anode connecting stud passing through an insulating seal in the skirt wall, the stud is formed by welding stud members on either side of the skirt wall, so that there is no through hole, and closing the fluorine compartment by an insulating gasket remote from the anode stud. However, even in this construction, whilst an improvement on earlier constructions, the sealing gasket is still directly contacted by the fluorine gas and is still susceptible to attack especially when there is an unexpected temperature rise for any reason. 
   In the case of the semiconductor industry, for example, it is essential that any process plant utilising toxic or hazardous gases such as fluorine, for example, possesses the most stringent levels of leak tightness since the majority of people working in such plants generally wear only normal non-protective clothing. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide a fluorine cell construction where significant areas of polymeric sealing members exposed to direct contact with fluorine are eliminated or minimised. 
   It is a further object of the present invention to provide a fluorine cell construction having a very high degree of integrity against fluorine leaks. 
   According to a first aspect of the present invention, there is provided an arrangement for sealing an anode within a fluorine generating electrolytic cell, the arrangement comprising: an anode connection member, said anode connection member passing through an aperture in a skirt wall and being in electrical connection with a skirt wall closure member wherein the skirt wall closure member is sealingly engaged with said skirt wall to seal said aperture and is electrically insulated therefrom, the arrangement being characterised by a non-conductive spacer member being interposed between the closure member and the skirt wall. 
   In the present invention the skirt wall closure member is sealingly engaged with the skirt wall around the aperture and electrically insulated from the skirt wall by means of an electrically non-conductive spacer member such as, for example, a ceramic spacer member. The spacer member may surround the anode connection member. The spacer member is essentially non-porous in the sense that there is no significant interconnected porosity which allows the passage of unacceptable quantities of fluorine gas to diffuse therethrough. 
   Examples of ceramic materials which may be utilised include alumina, calcium fluoride and magnesium fluoride. The material of the spacer member must be resistant to the effects of fluorine gas containing hydrogen fluoride at concentrations of up to 10 volume %. 
   In one embodiment of the arrangement of the present invention, the ceramic spacer is of annular form with flat, sealing faces and surrounds both the aperture through which the anode connection member extends and the anode connection member itself. 
   It is further preferred that the ceramic spacer member is sandwiched between two gaskets, one on each radial face thereof. In a preferred embodiment of the arrangement of the present invention the gaskets are so-called spiral wound gaskets which are supplied by many different manufacturers. Spiral wound gaskets comprise a spiral winding of a strip of at least one material which may be of “V” shaped cross section and known as the winding element. Frequently, there is a second element of similar cross sectional shape known as a sealing element, the two strips being nestled together and wound together so as to form alternate elements when viewed in cross section. The winding element is generally a metal which can be any metal of sufficient ductility and in any suitable metallurgical condition, e.g. annealed or cold worked, for example, and compatible with fluorine such as stainless steel or nickel, for example. The sealing element strip may be of a non-metallic material such as PTFE, expanded graphite or asbestos, for example, or may be of another, softer metal such as copper, for example. The spiral wound portion of the gasket may have inner and/or outer keeper rings to prevent the spiral wound portion from unwinding or deforming. 
   Where spiral wound gaskets having metallic elements in their construction are employed, a non-conductive spacer member such as the ceramic spacer member, for example, is necessary to provide electrical insulation of the anode from the skirt wall to render the latter electrically neutral. 
   In the present invention where, for example, the sealing element strip may be PTFE for example, because of the geometry of the gasket only a very small area of PTFE is exposed to the fluorine gas. However, spiral wound gaskets of fluorine-resistant all metal construction may be employed. 
   Such spiral wound gaskets as are contemplated in the present invention are by their nature generally of annular shape and construction and their size may be chosen to suit the radial face dimensions of the insulating spacer member. 
   The anode connection member may be welded to the inner surface of the skirt wall closure member. However, for reasons of dimensional accuracy, it is preferred that the anode connection member is a machined member which is attached to the skirt wall closure member by mechanical fasteners so that it and the anode may be easily removed for repair or maintenance. Such a construction generally requires that through holes be made in the skirt wall closure member and fasteners such as screws for example pass through holes to locate in suitable receiving holes in the anode connection member. It is possible to provide the fastening means with washers which seal against egress or leakage of fluorine through the fastener location holes via co-operating screw threads for example. However, in the interests of safety it is preferred that the anode fastening means may themselves be sealed from the ambient atmosphere with an auxiliary closure member which is sealed to the skirt wall closure member by means of a further gasket which surrounds the anode connection member fastening means. Such a further gasket may also advantageously comprise a spiral wound gasket. 
   Whilst spiral wound gaskets have proved to be exceptionally effective in the sealing arrangement of the present invention other types of gasket made from sheet metal such as copper, copper-nickel alloys or steel, for example, may be used in the form of embossed bead gaskets where elongate beads surrounding portions to be sealed are embossed into a metal sheet and which beads are then compressed during assembly to provide a seal. 
   The anode connection member may be for connecting to a separate anode such as a carbon anode by any known means or may itself be integrally formed with an anode portion which is adapted to be at least partially immersed in an electrolyte in the cell. 
   According to a second aspect of the present invention, there is provided a method of sealingly installing an anode in a fluorine generating electrolytic cell, the method comprising the steps of: providing a skirt member for a fluorine generating electrolytic cell, the skirt member being of open-ended construction which, when in use, a lower extremity of said open end is immersed in an electrolyte and forms a closed volume; forming an aperture in said skirt member to permit an anode connection member to pass through; suspending said anode connection member from a skirt wall closure member and sealing said aperture with said skirt wall closure member by providing at least one sealing, non-electrically conductive spacer member therebetween. 
   As described hereinabove, the at least one sealing, non-electrically conductive member may comprise a ceramic spacer member which is electrically non-conductive. In the interests of safety further sealing may be effected by suitable gaskets such as spiral wound gaskets, for example, preferably on each radial face of said spacer member. 
   The skirt wall, skirt wall closure member and an auxiliary closure member, where fitted, may be held together by conventional mechanical fastening means such as threaded studs, nuts and bolts and the like. The materials from which the constituent parts of the anode sealing arrangement may be made are those known and used in the fluorine generating art. 
   According to a third aspect of the present invention there is provided a fluorine generating cell having the anode sealing arrangement of the first aspect. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In order that the present invention may be more fully understood, examples will now be described by way of illustration only with reference to the accompanying drawings, of which: 
       FIG. 1  shows an elevation in cross section of a schematic electrolytic fluorine generating cell having the arrangement of anode installation according to the present invention; 
       FIG. 2  shows the arrangement of anode sealing installation of  FIG. 1  at a larger scale; and 
       FIG. 3  which shows a plan view of the arrangement of  FIGS. 1 and 2  in the direction of arrow  3  of  FIG. 2   
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring now to the drawings and where the same features are denoted by common reference numerals. 
     FIG. 1  shows an elevation in part cross section of a schematic electrolytic fluorine generating cell  10  having an anode sealing arrangement  12  according to the present invention. Most of the cell is conventional and is only shown and described to place the anode sealing arrangement of the present invention in context. 
   The cell  10  includes an outer cell vessel  14  which also forms the cathode  16  and contains the electrolyte  18 ; a skirt member  20  which comprises a generally horizontal top plate  22  and a depending gas separating skirt member  24  which extends below the surface  26  of the electrolyte  18  and completely encircles an anode  30  and anode connection member  32 ; and, an anode sealing arrangement  12 , in this case according to the present invention. The construction of the cell forms two separate compartments  40 ,  42 , each closed at the lower end by the electrolyte surface  26 , and which compartments receive hydrogen and fluorine, respectively upon electrolysis of the electrolyte  18  on passing a current therethrough. Outlet conduits  44  and  46  having valve means  48 ,  50  to control the flow of gas are provided for the two compartments. The anode sealing installation  12  according to the present invention also provides the anode connection  56 . A heating jacket (not shown) is normally provided around the cell vessel  14  in order to melt the electrolyte which is normally solid at ambient temperature; the heating jacket may comprise a steam jacket or an electrically heated blanket for example. A plate  58 , usually of a non-electrically conductive plastics material is fixed to the cell bottom to prevent hydrogen from being formed on the cathodic area below the anode compartment  42  and consequently rising into the fluorine compartment  42  and explosively recombining with the fluorine. The skirt  20  is electrically neutral being insulated from the cathodic vessel  14  by an insulating gasket  60  and from the anode sealing installation  12  by means which will be described in greater detail below. Insulating gasket  60  is not in contact with fluorine and skirt  20  is held to the top of the cell wall by mechanical fastening means (not shown), for example. 
   Referring now to  FIGS. 2 and 3  where the anode sealing installation according to the present invention is shown in more detail. The sealing arrangement  12  of  FIG. 2  is a section along the line  2 - 2  of  FIG. 3 . The anode connection member is ultimately connected to the anode  30  which is at least partially immersed in the electrolyte  18  as shown in  FIG. 1 , however, neither the anode nor its construction per se form any part of the invention and may be of any suitable construction or material according to the type of fluorine cell in which it is to be used. The anode connection member  32  (shown truncated in  FIG. 2 ) passes through an aperture  70  in the horizontal top plate  22  of the skirt member  20 . The anode connection member  32  is fixed to a skirt wall closure member  72  by means of screws  74  passing through holes  76  into threaded holes  78  in a boss  80  of the connection member  32 , however, any means of connection of the member  32  to the closure member  72  may be employed. The arrangement described provides accuracy for the depending anode  30  in maintaining the anode  30  out of contact with the depending skirt wall member  24 , but providing that accuracy can be assured, the connection member  30  may be welded, for example, to the underside  82  of the closure member  72 . Alternatively, the end of the connection member  32  may be provided with a screw threaded male portion, for example, and screwed into a co-operating hole in the closure member  72 , for example. The closure member  72  is insulated from the skirt  20  by a non-conductive annular spacer ring  90  which is positioned intermediate two annular gaskets  92 . In this case the gaskets  92  are spiral wound gaskets having inner keeper rings  94  and outer keeper rings  96 . Whilst the gaskets  92  are electrically conductive they are separated by the non-conductive spacer ring  90  which, in this case, is made of non-porous alumina ceramic. The spacer ring  90  and gaskets  92  provide complete sealing against leakage of fluorine and the spacer ring  90  provides electrical insulation of the anode  30  from the skirt  20 . However, the existence of the holes  76  may provide a potential path for fluorine leakage and in the interests of complete safety an auxiliary closure member  100  is provided which is itself sealed to the closure member  72  by means of a further spiral wound gasket  102 . A recess  104  is provided in the auxiliary closure member  100  to accommodate the heads of the screws  74 . The auxiliary closure member  100 , the closure member  72 , the spacer ring  90 , gaskets  92  and  102  are all held together by means of threaded studs  108  and nuts  110 . The studs are insulated from the closure member  72  and auxiliary closure member  100  by insulating sleeves  112 , in this case of Mylar (trade name) plastics material, extending along the length thereof and the nuts  110  are insulated from the auxiliary closure member top face  114  by insulating washers  116 , in this case, of phenolic material such as Tufnol (trade name), for example. The spacer ring  90  and gaskets  92 ,  102  all lie within a pitch circle  120  defining the axes of the studs  108  and, of course, within a circle  122  defining the innermost extent of the stud  108  diameters thus, the insulating sleeves  112  and washers  116  are not subject to contact with fluorine. An electrical connector  56  is provided for a positive connection to the anode from a power source and controller (both not shown). 
   In the embodiment shown the auxiliary closure member  100  is used due the fact that there are through holes  76  in the closure member  72  for the screws  74  securing the anode connection member  32  thereto and which could possibly be a source of fluorine leakage. However, if a construction is employed where no through holes are provided in the closure member  72  for securing the connection member  32  thereto, the auxiliary closure member  100  would not be required. One important advantage of the construction shown with reference to the Figures is that accurate angular location of the anode  30  within the skirt wall portion  24  is automatically achieved. 
   In the embodiment described spiral wound gaskets have been used to seal the faces of the spacer ring  90  and the corresponding faces of the skirt top plate, closure member and auxiliary closure member. The construction of such gaskets is well known in the prior art and they are particularly suitable when used in the present invention. Where spiral wound gaskets constructed with a dual winding of metal and polymer elements such as described hereinabove are used, only a very small end face area at the end of the wound gasket is potentially exposed to fluorine gas. Where keeper rings are employed even this area is substantially eliminated. However, other types of metal gaskets may be used such as embossed bead gaskets where beads are embossed into an essentially flat metal plate and sealing is achieved by compression of the beads by the tightening loads. 
   The present invention effectively seals the fluorine compartment of the fluorine cell without the need to place large areas of polymer gasket materials in contact with fluorine and thus the risk of degradation causing fluorine leaks and the possibility of stud fires is greatly reduced compared with prior art sealing methods. 
   In the embodiment shown and described, the anode connection member  32  is further provided with a hole  130  to permit insertion of a retaining bar (not shown) therethrough for use when inspecting, servicing, replacing seals and the like or repairing the fluorine cell. When the anode sealing installation  12  is disassembled for servicing etc. the anode assembly may be lifted before removal of the closure member  72 , the retaining bar inserted in the hole  130  and rested across the skirt top face  22  to prevent the anode  30  from dropping into the vessel bottom. 
   In another embodiment of the present invention, the anode connection member may be at least partially formed as an integral part of the closure member  72 , e.g. as a spigot depending from the inner central part thereof and the anode joined thereto, for example, by an internally threaded collar co-operating with threaded ends on the anode connection portion at the upper end and a threaded end on a rod connected to an anode hanger at the lower end. Thus, in this embodiment the auxiliary closure member would not be required. Furthermore, the closure member and the anode connection member are a unitary item. 
   Fluorine cells constructed according to the present invention have been found to have very high integrity against fluorine leaks and have leak tightness sufficient to pass a helium leak test at better than 10 −5  sccm (standard cubic centimetre min). 
   The types of seal members which may be employed in the present invention include but are not limited to: all metal seals such as ring joints made from steels, nickel, copper, copper-nickel alloys and aluminium; spiral wound joints using all metal windings; and, profile joints such as the bead type gaskets described hereinabove. Alternatively, part metal seals may be employed and may include but are not limited to: ring joints made from soft sealing materials where the sealing material is encapsulated in a groove or tongue and groove; spiral wound joints incorporating metal windings combined with soft sealing windings; and envelope joints comprising a metal envelope within which a filler material is encapsulated.