Abstract:
The invention relates to a device for supporting electrodes in an electrolysis installation, said support comprising a busbar having electrodes fastened thereto, said electrodes being disposed on either side of the busbar and extending vertically below said busbar, the busbar and said electrodes being designed to be immersed at least in part in an electrolyte that gives off one or more gaseous species of a corrosive nature. The device further comprises a protective element of carbon/carbon material placed under the busbar, the protective element being of length and width that are not less than the length and the width of the busbar.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to the field of electrolysis cells or installations.  FIG. 1  is a diagram of an electrolysis installation  100  used for producing fluorine. The installation  100  comprises a tank  101  containing an electrolyte  102 , e.g. a solution of hydrofluoric acid (HF), and having two series of electrodes immersed therein, namely a first series of cathodes  103  and a second series of anodes  104 . The anodes  104  are fastened and electrically connected to each side of a busbar  105 . The busbar  105  serves both as a support and as a distributor of electrolysis current for the electrodes  104 . In well-known manner, the busbar  105  is connected to the positive terminal of a direct current (DC) generator (not shown in the figures) by conductors  106  placed in threaded rods  107 , while the cathodes  103  are connected to the negative terminal of the generator. The anodes  104  are distributed longitudinally on each side of the busbar  105  and they project beyond the bottom face  105 a of the busbar. 
         [0002]      FIG. 2  shows the electrolysis installation  100  while it is in operation, i.e. when the electrodes  103 ,  104  are immersed in the electrolyte and are powered by the DC generator. When the electrolyte is made up of hydrofluoric acid, for example, electrolysis leads to bubbles of gaseous fluorine  108  being given off at the anodes  104  and bubbles of hydrogen  109  being given off at the cathodes  103 . The bubbles of these two gaseous species rise to the surface of the electrolyte and they are collected by independent ducts (not shown in the figure) in the top portion of the electrolysis installation  100 . 
         [0003]    The bubbles of gaseous fluorine  108  give rise to corrosion and erosion of the elements of the installation with which they come into contact during electrolysis. Given their chemical nature, the bubbles  108  are very corrosive, and as they rise towards the surface of the electrolytes they give rise to an erosion phenomenon on the anodes  104  and more particularly on the busbar  105  whose bottom face  105   a  receives practically all of the fluorine bubbles given off by the inside walls of the anodes  104 , these bubbles then flowing along the bottom face  105   a  until they find a path to the surface of the electrolyte  102 . 
         [0004]    Consequently, in any electrolysis installation that produces one or more corrosive gaseous species, the corrosion and the erosion resulting from the gases being given off make it necessary to replace the busbar and the anodes frequently. 
         [0005]    To mitigate this problem, one solution consists in making the busbar and possibly also the anodes out of graphite, which is a material that is known to present good resistance to corrosion. Nevertheless, even though graphite does present improved resistance to the combined corrosion and erosion phenomenon compared with the metal materials commonly used, that is not sufficient to prevent the anodes and above all the busbar deteriorating during electrolysis. Thus, even when made of graphite, busbars need to be replaced frequently. On each replacement, the electrolysis installation, and consequently the production of the gaseous species, must be stopped. Busbar wear by the corrosion-erosion phenomenon thus leads to periods in which the electrolysis installation is not in operation and it is desirable for these periods to be shortened in order to improve the efficiency of the installation. 
       OBJECT AND SUMMARY OF THE INVENTION 
       [0006]    An object of the present invention is to propose a design solution that enables a busbar of an electrolysis installation to be protected against the corrosion-erosion phenomenon caused by gaseous species being given off during electrolysis, thereby increasing its lifetime. 
         [0007]    To this end, the present invention provides a device for supporting electrodes in an electrolysis installation, said support comprising a busbar having electrodes fastened thereto, said electrodes being disposed on either side of the busbar and extending vertically below said busbar, the busbar and said electrodes being designed to be immersed at least in part in an electrolyte that gives off one or more gaseous species of a corrosive nature, 
         [0008]    wherein the device further comprises a protective element placed under the busbar, and having a length and a width that are not less than the length and the width of the busbar, and wherein said protective element is made of carbon/carbon material. 
         [0009]    Thus, by placing a carbon/carbon element under the busbar, the busbar is protected against the bubbles of corrosive species given off by the electrodes during electrolysis. Since the protective element covers at least the bottom face of the busbar, it prevents the bubbles of corrosive species that are rising to the surface of the electrolyte from encountering the busbar, thereby protecting it from wear due to the above-described corrosion-erosion phenomenon. The lifetime of the busbar is thus considerably lengthened. 
         [0010]    Furthermore, the protective element is made of carbon/carbon which is a material that is particularly good at withstanding the corrosion-erosion phenomenon. Thus, in the presence of the corrosive gas that has been given off, the assembly formed by the busbar and the protective element withstands the corrosion-erosion phenomenon much longer than is possible with a busbar on its own, even if the busbar is made of graphite. Consequently, with the electrode support device of the invention, the frequency with which electrolysis installations are shut down for replacing worn busbars is significantly reduced compared with the usual frequency. 
         [0011]    The protective element may be held in grooves formed in the electrodes or it may be fastened to the busbar by fastener members. 
         [0012]    In an aspect of the invention, the face of the protective element opposite from its face facing the busbar presents a profile that is concave. This profile serves to channel the bubbles of corrosive gaseous species given off by the electrodes and to guide them towards the longitudinal ends of the protective element. The concave face may also have a slight slope inclined towards one of the longitudinal ends of the protective element in order to guide the bubbles to that end. 
         [0013]    In another aspect of the invention, the protective element includes fins on its two longitudinally-extending sides, the fins extending above the face of said element that faces the busbar. The fins present widths that correspond substantially to the gaps left between pairs of adjacent electrodes and they are spaced apart from one another by distances that correspond substantially to the widths of the electrodes. With such fins, the protective element also protects the flanks of the busbar where they are exposed between two electrodes. 
         [0014]    The protective element may be made as a single piece (one-piece structure) or as a plurality of adjacent sectors that are assembled together via overlapping portions. 
         [0015]    The present invention also provides an electrolysis installation including at least one electrode support device as described above. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    Other characteristics and advantages of the invention appear from the following description of particular embodiments of the invention given as non-limiting examples and with reference to the accompanying drawings, in which: 
           [0017]      FIG. 1  is an exploded diagrammatic view of an electrolysis installation; 
           [0018]      FIG. 2  is a section view of the  FIG. 1  electrolysis installation when assembled and in operation; 
           [0019]      FIG. 3  is a diagrammatic perspective view of an electrode support device in an embodiment of the invention; 
           [0020]      FIG. 4  is a section view of the  FIG. 3  electrode support device when bubbles of corrosive species are being given off by the electrodes; 
           [0021]      FIG. 5  is a fragmentary diagrammatic view in perspective of an electrode support device in another embodiment of the invention; 
           [0022]      FIG. 6  is a diagrammatic view in perspective of an electrode support device in another embodiment of the invention; 
           [0023]      FIG. 7  is a section view of the  FIG. 6  electrode support device when bubbles of corrosive species are being given off by the electrodes; 
           [0024]      FIGS. 8 and 9  are diagrammatic perspective views of an electrode support device in another embodiment of the invention; and 
           [0025]      FIG. 10  shows a variant embodiment of the  FIG. 3  electrode support device. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0026]    A particular but non-exclusive field of application of the invention is that of electrolysis installations for producing gaseous species of a corrosive nature such as fluorine or chlorine, for example. The present invention seeks to protect the busbars used as electrode carriers in such installations against the above-described corrosion-erosion phenomenon when a corrosive gaseous species is given off by the electrodes. For this purpose, the present invention proposes using a protective element made of carbon/carbon that serves to isolate the busbar from the corrosive gas given off during electrolysis. Embodiments of electrode support devices making use of such a protective element are described below. 
         [0027]    Each support element described below is made of carbon/carbon (C/C) composite material which, in known manner, is a material made up of carbon fiber reinforcement densified by a carbon matrix. C/C composite material presents very good resistance to corrosion and also to erosion. 
         [0028]    The manufacture of parts made of C/C composite material is well known. It generally comprises making a carbon fiber preform of shape close to that of the part that is to be fabricated, and then densifying the preform with the matrix. 
         [0029]    The fiber preform constitutes the reinforcement of the part and its essential function concerns mechanical properties. The preform is obtained from fiber textures: yarns, tows, braids, cloth, felts, . . . . Shaping is performed by winding, weaving, stacking, and possibly also needling two-dimensional plies of cloth or sheets of tow, . . . . 
         [0030]    The fiber reinforcement can be densified by a liquid technique (being impregnated with a resin that is a precursor of the carbon matrix and then transforming the resin by cross-linking and pyrolysis, which process can be repeated), or by a gaseous technique (chemical vapor infiltration of the carbon matrix). 
         [0031]      FIG. 3  shows a first embodiment of a support device  10  in accordance with the invention. The support device  10  comprises a busbar  11  in the form of a rectangular block presenting a top face  11   a , a bottom face  11   b , and two side faces  11   c  and  11   d . In the example described, the busbar  11  is made of copper. Nevertheless, the busbar could be made of some other conductive material, such as graphite. A first series of electrodes  12  and a second series of electrodes  13  are fastened respectively on the side faces  11   c  and  11   d  of the busbar  11 . The electrodes  12  and  13  are distributed uniformly along the busbar  11  with gaps between adjacent pairs of electrodes. The electrodes  12  and  13  are made of graphite. Each electrode is constituted by a rectangular plate that extends below the bottom face  11   d  of the busbar  11 . The electrodes  12  and  13  are electrically connected to the busbar  11  that serves to feed the electrodes with current for electrolysis. For this purpose, the electrodes may be fastened to the busbar by connection means that ensure electrical conduction. In particular, the electrodes may be fastened by the busbar by brazing or by bonding with a conductive adhesive. The busbar  11  consequently performs both the function of an electrode carrier and the function of delivering electrolysis current to the electrodes. 
         [0032]    In accordance with the present invention, the support device includes a protective element  14  constituted by a plate made of C/C composite material. The protective element  14  is placed under the busbar  11  in the vicinity of its bottom face  11   b . More precisely, the protective element  14  is put into place by being slid in grooves  12 A and  13 A formed respectively in the electrodes  12  and  13 . These grooves serve to hold the protective element in place at a determined distance under the busbar. A certain amount of clearance is preferably conserved between the protective element and the busbar in order to compensate for differential expansion between the material of the busbar (copper or other metal) and the material of the protective element (C/C composite material). 
         [0033]    The protective element  14  presents length and width that are slightly greater than those of the busbar  11 . Consequently, the protective element forms a screen facing the entire bottom face  11   b  of the busbar and protecting it against the corrosion-erosion phenomenon when a corrosive gaseous species is given off by the electrodes. As shown in  FIG. 4 , during electrolysis, i.e. while the electrodes  12  and  13  are immersed in an electrolyte  16  and are being fed with electrolysis current, bubbles  15  of the corrosive species are given off on the lower portions of the electrodes and they are stopped from rising by the protective element  14  that forms a screen in front of the bottom face  11   b  of the busbar. The bubbles  15  are then evacuated to the surface of the electrolyte by going past the edges of the protective element  14 . Thus, with the protective element  14 , the bubbles of corrosive species that have been given off no longer strike the bottom face of the busbar, thereby considerably reducing the influence of the corrosion-erosion phenomenon thereon. 
         [0034]      FIG. 5  shows a variant embodiment of a support device  20  of the invention that differs from that described above in that it includes a protective element  24  that is held in position under a busbar  21  and between electrodes  22  and  23  by bolts  25 . The protective element includes oblong holes  24 A for passing the bolts  25  and for adjusting the position of the element, the bolts being received in tapped holes  21 A formed in the busbar  21 . 
         [0035]    The face of the protective element that is to receive the bubbles of the corrosive gaseous species given off by the electrodes may present a plane surface as shown in  FIGS. 3 to 5 . Nevertheless, as shown in  FIG. 6 , the protective element may also have a bottom face that presents a concave surface. More precisely,  FIG. 6  shows a support device  30  including, like the device of  FIG. 3 , a protective element  34  that is held under the busbar  31  by grooves  32 A and  33 A formed respectively in electrodes  32  and  33 , but having a bottom face  34 A that presents a concave profile. As shown in  FIG. 7 , the concave shape of the bottom face  34 A of the protective element serves to channel the bubbles  35  of the corrosive gaseous species given off by the electrodes  32  and to guide them towards the longitudinal ends of the protective element  34 . This reduces the quantity of bubbles  35  that escape through the gaps left between the electrodes  32  or  33 , thereby better protecting the flanks of the busbar where they are exposed in the gaps. The concave surface of the bottom face of the protective element may also slope a little so as to guide the bubbles better towards one longitudinal end of the protective element. 
         [0036]      FIGS. 8 and 9  show another embodiment of a support device of the invention, respectively before and after assembly of the protective element. The support device  40  shown in these figures differs from those described above in that the protective element  44  is also provided with lateral protective fins  45 . The spacing and the width of the fins  45  are selected to fill in the empty gaps left between the electrodes  42  and between the electrodes  43 . The protective element  44  may be fastened to the busbar  41  by adhesive or by fastener members of the screw-fastener type. Once the support device  40  has been assembled under the busbar  41 , the fins  45  cover the flanks of the busbar where they are exposed between the electrodes, thereby protecting them against the bubbles of corrosive species escaping between two electrodes. The protective element  44  shown in  FIGS. 8 and 9  has a bottom face  44 A with a concave profile serving to channel the bubbles given off by the electrodes towards the longitudinal ends of the protective element. Nevertheless, the protective element  44  may also have a bottom face that is plane. 
         [0037]    The protective elements of the invention described above can be made out of a single piece of carbon/carbon composite material. Nevertheless, particularly when making a protective element of large size, the element may be built up as an assembly of a plurality of sectors, each made individually out of carbon/carbon composite material.  FIG. 10  shows an embodiment of a protective element  140  similar to the protective element  14  of  FIG. 3 , but differing therefrom in that it is made up as an assembly as a plurality of sectors  141 . The sectors are preferably made with one or two overlap portions  141   a ,  141   b  (one overlap portion for each end sector, two for each intermediate sector), enabling the sectors to be assembled together, e.g. by brazing.