Abstract:
An electrolysis cell for recovery of metals that are lighter than the electrolyte used in the cell. The cell makes use of multiple electrode assemblies, and each assembly is provided with an individual hood at the top forming a gas collection chamber. The hood of each assembly collects gas generated by the assembly and isolates the gas thus generated from gas generated by other assemblies and from metal collecting in the cell outside the hoods. The invention also relates to an integrated unit made up of an electrode assembly and an associated hood for use in a cell of the above kind, and a method of recovering metal by operating a cell of the above kind.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to electrolysis cells used for the production of light metals, such as magnesium, lithium, sodium and aluminum, by electrolysis of a molten electrolyte having a greater density than the metal produced. More particularly, the invention relates to electrolysis cells of this kind that have means for separating newly formed molten metal from reactive gases produced during the electrolysis process. 
     2. Description of the Related Art 
     The production of magnesium metal is typical of the procedures to which the present invention relates. Magnesium metal is generally produced from magnesium chloride by electrolysis in a suitable electrolytic cell held at a temperature high enough to keep both the electrolyte and the metal product molten during the process. The electrolysis creates droplets of magnesium metal and chlorine gas. Since magnesium is a very light metal, it floats to the surface of the electrolyte, as do the bubbles of the chlorine gas. It is therefore necessary to keep the floating pool of metal separate from the chlorine gas collecting in the atmosphere above the electrolyte, or these elements will merely recombine, thereby reducing the current efficiency of the cell. 
     A typical modern cell design is described in U.S. Pat. No. 5,935,394 to Sivilotti et al. which issued on Aug. 10, 1999 and was assigned to the same assignee as the present application. This cell utilizes a number of multi-polar cell assemblies per cell that are arranged in a line along one long side wall of the cell. As best shown in FIG. 2 of the patent, a longitudinal refractory curtain wall is provided adjacent to the cell assemblies to separate a compartment within the cell for the electrode assemblies from a metal collection compartment. Electrolyte containing metal droplets overflows the top end of the multi-polar electrodes of each assembly (driven by the buoyancy of entrained gas) and flows through an upper aperture in the refractory wall positioned below the surface of the electrolyte. Gas entrained in the electrolyte escapes into the atmosphere above the electrode assemblies before the metal-containing overflow progresses through the aperture in the curtain wall. After passing through that aperture, the electrolyte encounters a quiescent zone where the metal droplets can rise to the surface, coalesce and collect as a pool. A further aperture at the bottom of the curtain wall allows metal-depleted electrolyte to recirculate to the electrode compartment. In this way, the chlorine gas is kept separate from the floating pool of molten metal. 
     There are, however, a number of disadvantages with this type of cell. Firstly, the electrolyte overflows the interpolar electrodes of each electrode assembly at all points around the upper end of the assembly. The overflowing electrolyte is collected in a trough surrounding the assembly and brought around to a position adjacent to the aperture in the curtain wall so that it can proceed through the aperture into the metal collection chamber. However, electrolyte that overflows the electrodes at points remote from the aperture spend a considerable amount of time in the electrode compartment where the metal droplets may contact the chlorine gas and may react, thus reducing the current efficiency of the cell. 
     Secondly, the cell has to be shut down if one of the electrode assemblies has to be removed for maintenance or repair because the headspace within the electrode compartment is common to all electrode assemblies, and chlorine gas produced by functioning assemblies would escape from an aperture in the cell opened for the removal of another cell assembly. 
     Thirdly, the use of an elongated curtain wall extending the full length of the long dimension of the cell reduces the operational life of the cell and limits the maximum cell dimensions. Such walls are exposed to corrosive chemicals on both sides, so that failure is fairly frequent. When such an integral component fails, it is necessary to shut down the cell completely and to remove the cell contents so that the wall can be rebuilt. This is obviously time consuming, difficult and expensive. Moreover, an unduly long wall would be structurally weak and prone to mechanical failure. 
     As well as exhibiting the problems referred to above, the cell of the Sivilotti et al. patent also has the problem that the electrical bus bar for the cathode connection exits the cell directly through a side wall at a position below the surface of the electrolyte. While this protects the metal bus from attack by chlorine gas, it makes any removal of electrode assemblies more difficult since the cathode connections are not easily accessible. 
     Accordingly, there is a need for an improved design of electrolysis cells of this kind to alleviate some or all of the problems of this kind. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to make electrolysis cells used for the production of light metals, such as magnesium, easier to construct, maintain and/or to repair. 
     Another object of the present invention is to increase the operational life of electrolysis cells used for the production of light metals, such as magnesium. 
     A further object of the present invention, at least in preferred forms, is to reduce the contact time between metal and chlorine in electrolysis cells of the kind discussed. 
     A further object of the present invention, at least in preferred forms, is to make individual electrode assemblies operationally independent of each other within an electrolysis cell. 
     A still further object of the invention, at least in preferred forms, is to make repair or maintenance of electrolysis cells possible without a complete cell shut-down. 
     According to one aspect of the invention, there is provided an electrolysis cell for recovery of a metal by electrolysis from a molten electrolyte containing a metal compound, wherein the molten metal has a density lower than the molten electrolyte and the compound produces a gas during electrolysis that reacts on contact with the molten metal, the cell having a housing containing a plurality of electrode assemblies each including an anode, a cathode and at least one interpolar electrode disposed between the anode and the cathode so as to form interpolar spaces in which electrolysis occurs, and connections for conveying electrical current to and from the electrode assemblies; wherein each electrode assembly is provided with a hood enclosing an upper portion of the electrode assembly including the cathode of the assembly, such that the hood in operation provides a gas collection chamber such that the gas generated by each electrode assembly is isolated from other electrode assemblies and from metal collecting in the housing outside each hood. 
     According to another aspect of the invention, there is provided a method of recovering a metal by electrolysis from a molten electrolyte containing a metal compound, the molten metal having a density lower than the molten electrolyte and the compound producing a gas during electrolysis that reacts on contact with the molten metal, in which electrolysis is conducted in a cell having a housing containing a plurality of electrode assemblies each including an anode, a cathode and at least one interpolar electrode disposed between the anode and the cathode so as to form interpolar spaces in which electrolysis occurs, and connections for conveying electrical current to and from the electrode assemblies; wherein the gas from each electrode assembly is collected in a hood enclosing an upper portion of the electrode assembly including the cathode of the assembly and providing a gas collection chamber, such that the gas generated by each electrode assembly is isolated from other electrode assemblies and from metal collecting in the housing outside each hood. 
     According to another aspect of the invention, there is provided an integral electrolysis unit comprising an electrode assembly having an anode, a cathode and at least one interpolar electrode, and a hood encircling an upper end of the to electrode assembly, the hood including a lower end sealed in a gas-fight manner against a periphery of the cathode, except at at least one open aperture at a point on the periphery of the cathode. 
     According to yet another aspect of the invention, there is provided an electrolysis cell for recovery of a metal by electrolysis from a molten electrolyte containing a metal compound, wherein the molten metal has a density lower than the molten electrolyte and the compound produces a gas during electrolysis that reacts on contact with the molten metal, the cell having a housing containing a plurality of electrode assemblies each including an anode, a cathode and at least one bipolar electrode disposed between the anode and the cathode so as to form interpolar spaces in which electrolysis occurs and the cathode forms an electrically and mechanically continuous surface surrounding the outermost at least one bipolar electrode, and connections for conveying electrical current to and from the electrode assemblies; wherein each electrode assembly is provided with a hood enclosing an upper portion of the assembly such that (a) the hood in operation provides a gas collection chamber such that the gas generated by the electrode assembly is isolated from the remaining electrode assemblies and (b) the hood and outer surface of the cathode are in a spaced relationship so that in operation, electrolyte flow containing the metal formed on the electrodes can flow over the top of the cathode and under the edge of the hood substantially adjacent the cathode over which it flowed. 
     In a preferred form of the cell, a current bus for the cathode of each assembly attaches to the cathode below a lower end of the associated hood and extends within the cell outside the hood to the cell roof, where it exits the cell through an aperture in the roof. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a vertical cross-section of a part of an electrolysis cell according to one preferred embodiment of the present invention showing an electrode assembly provided with a hood; 
     FIG. 2 is a partial cross-sectional view similar to that of FIG. 1 illustrating an alternative preferred embodiment of the invention; 
     FIG. 3 is a partial cross-sectional view similar to FIG. 1 illustrating a further alternative preferred embodiment of the invention; 
     FIG. 4 is a plan view from above of an electrolysis cell showing one example of the distribution of electrode assemblies and associated hoods therefor; 
     FIG. 5 is a perspective view of a cell partly in section with parts removed for clarity, showing two side-by-side electrode assemblies of the kind shown in FIG. 3; and 
     FIG. 6 is a partial cross-sectional view similar to FIG. 1 illustrating a further alternative preferred embodiment of the invention, using a unitary hood and electrode assembly. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a partial view of a magnesium electrolysis cell C of the type disclosed in U.S. Pat. No. 5,935,394 (the disclosure of which is incorporated herein by reference) including one embodiment of an improvement according to the present invention. The drawing shows the region of the cell housing H surrounding just one electrode assembly  10 , but it will be understood that the cell contains a plurality of such assemblies, usually four or more. The cell design may in many other respects be conventional, e.g. it is provided with a refractory layer  65  lining the interior of the sidewalls  60  and the cell floor  26 . 
     The electrode assembly  10  consists of a central anode  11  (usually made of graphite), a cathode  12  completely encircling the anode across an annular gap  13 , and a plurality of interpolar electrodes  14  positioned in the annular gap between the anode and the cathode in spaced relationship to each other to form annular channels  15 . The cathode consists of a cylindrical wall  16  and a flat bottom wall  17  having a central hole  18 . The interpolar electrodes are of similar shape to the cathode, but are progressively smaller in size. Each consists of an annular wall  19  and a flat bottom wall  20  provided with a central hole  21 . The bottom wall  17  of the cathode and the bottom wall  20  of the interpolar electrodes may also be sloped upwardly from the centre. The hole  18  of the cathode and the holes  21  of the interpolar electrodes are concentric and provide a route by which molten electrolyte  22 , may enter the annular channels  15 . 
     The central anode  11  is supported from above and is electrically connected to a current bus bar  24 . The cathode  12  is supported from a current bus bar  27  which, for this purpose, is provided with an end plate  66  at right angles to the remainder of the bus bar  27 . The end plate  66  engages the cylindrical wall of the cathode and a hook  67  on circumferential plate  30  holds the cathode assembly firmly in place. If desired, the cathode  12  may also be supported from below by spaced supports (not shown) extending upwardly from the cell floor  26 . The interpolar electrodes are supported by electrically-insulating spacers (not shown) positioned between the cathode and the outermost interpolar electrode, and then between adjacent interpolar electrodes. The cathode  12  is electrically connected to and supported from current bus bar  27  that supplies the electrolyzing current for cell operation. The upper surface  23  of the molten electrolyte corresponds in height to the upper end  28  of the cathode  12  so that electrolyte substantially fills all of the annular channels  15  where electrolysis takes place. Metal droplets and gas bubbles form within the electrolyte as electrolysis proceeds and the buoyancy created by the gas lifts the electrolyte to the top of the electrode assembly, where it overflows. Fresh electrolyte is drawn in through the holes  18  and  21  to replace the overflowing electrolyte so that there is a continual flow of electrolyte through the electrode assembly. 
     The electrode assembly includes circumferential plate  30  that is integral with and surrounds the cathode  12  and has an outer edge  31 . The plate may be horizontal, but preferably slopes slightly upwardly from the side having the hook  67  as shown or may slope slightly upward in all directions concentrically around the electrode assembly. Together with corresponding plates of other electrode assemblies, the plate forms a roughly horizontal partition in the cell and serves to prevent electrolyte containing magnesium droplets overflowing the top of the assembly from returning directly to the bottom of the assembly. 
     As already noted, in conventional cells of this kind, a refractory curtain wall is provided in the cell to separate an electrolysis chamber containing the electrode assemblies  10  from a metal collection chamber where droplets of molten metal coalesce and rise to the surface to form a metal pool that is tapped from the cell continuously or intermittently. The curtain wall contains strategically placed ports for transfer of molten electrolyte between the two chambers. Gas bubbles emerge from the molten electrolyte far more quickly than the metal droplets coalesce and rise to the surface, so the gas collects in the electrolysis chamber while the metal is carried through with the electrolyte to the metal collection chamber. Gas and metal are thus kept separate so that back-reaction is avoided. 
     In the illustrated embodiment of the present invention, there is no conventional curtain wall. Instead, each electrode assembly is provided with a hood  35  that forms a gas collection chamber  41  that encircles and encloses the upper end of the cathode  12 , the interpolar electrodes  14  and the anode  11 , and extends downwardly to a level below the upper surface  23  of the electrolyte  22  and the upper surface  29  of the metal pool  40  within the cell, but not completely to the bottom of the cell. As the electrolyte overflows the channels  15 , gas escapes and is collected within the hood  35 , and the metal-containing electrolyte then flows over the upper end  28  of the cathode  12  and down through an annular channel  36  formed between the cathode  12  and the lower end  37  of the hood  35 . From there, the metal-containing electrolyte flows out of confinement by the hood  35  into a common area  38  of the housing H of the cell C where metal coalescence and collection takes place to form the metal pool  40 . The electrolysis gases collected within the hood  35  are vented through suitable piping (not shown in FIG. 1, but see FIG. 5) for disposal as in conventional cells. 
     Consequently, instead of having a common electrolysis chamber containing all of the electrode assemblies provided with a common headspace for gas collection, the illustrated embodiment has an individual electrolysis and gas-collection chamber for each electrode assembly provided by the hoods  35 , and a common area  38  outside the hoods  35  forms a metal collection chamber that forms a common metal pool  40  collecting metal from all of the electrode assemblies  10 . 
     The lower end of the hood  35  is preferably the same shape as the cathode top to provide a substantially uniform gap around the periphery. It may be cylindrical, but could be of any shape (e.g. square, rectangular or elliptical) provided the necessary gas confinement is achieved. 
     One advantage of this arrangement is that metal-containing electrolyte overflowing the upper end  28  of the cathode  12  at any point around the periphery of the cathode may flow directly out of the electrolysis chamber through the annular channel  36  and under the lower end  37  of the hood. In conventional cells, there is usually a single port in the curtain wall adjacent each electrode assembly and metal-containing electrolyte from the far side of each assembly has to be channeled through the electrolysis compartment around the cathode to a point adjacent to the port. This means that the metal has a much longer residence time in the electrolysis compartment where reaction with gas in the headspace of the compartment may take place. The residence time of the metal-containing electrolyte in the electrolysis compartment is minimized in this embodiment of the present invention, so that back-reaction is also minimized. 
     Another advantage of the illustrated embodiment is that the hood  35  can, if desired, be removed from the cell without undue difficulty, e.g. for replacement or repair. The hood consists of a vertical annular wall  42  and an integral flat horizontal wall  43  that forms a removable part of the cell roof  44 . The hood may also have a top surface in the form or a hemisphere or a frustro-conical section or any other convenient form. The flat horizontal wall  43  has a central hole  45  allowing the anode  11  to enter the housing H. The gap  46  between the edges of hole  45  and the anode  11  are made gas-tight by insertion of a flexible and removable packing material  47 . The packing material also preferably provides electrical insulation between the anode and the hood. This arrangement makes it possible to remove the anode  11  (after removing the packing material) without disturbing the hood  35  and the cathode assembly  48  (which is the cathode  12  and interpolar electrodes  14  minus the anode  11 ), or both the anode and the hood, or all of the anode, the hood and the cathode assembly, as desired. 
     The hood may be made of any suitable material. For example, it may be formed as a single piece of refractory or jointed refractory blocks. It may be made from graphite provided the graphite is suitably protected from oxidation (in the known manner). It may also be made from steel lined by a suitable refractory such as an alumina or aluminum silicate refractory. This lining for the steel may be applied as a castable or gunned mixture, or as refractory blocks using attachment fixtures or techniques well known in the art. 
     The removable design of the hood makes it easy to replace or repair the hood, unlike the normal curtain walls that are subject to the same environments. While it would be prudent for safety reasons to interrupt the flow of electricity to all of the electrode assemblies of the cell when working on one assembly, and to tap off as much of the metal pool  40  as possible, the fact that the hood can be removed from above (i.e. from the top of the cell C) means that the cell does not have to be cooled and the electrolyte removed. The repair or replacement of the hood can therefore be carried out quickly and economically, while the cell is kept hot, working on one cell assembly at a time, or on more than one assembly concurrently. Moreover, the headspace  39  of the cell does not have to be cleared of dangerous gases because these gases are confined within the hood  35  of each cell assembly, and only the hood of a cell assembly undergoing repair need be flushed of electrolysis gases before work commences. Again, this simplifies and accelerates cell repair and minimizes down-time. 
     Since the corrosive electrolysis gases are confined within the hood  35 , it is not essential to seal the roof of the cell above the common headspace  39  for environmental or safety reasons, but this is still desirable for several reasons (e.g. to minimize the entry of air which would oxidize the molten metal collected in the metal collection areas of the cell). As shown in FIG. 1, this can be done by providing a cover panel which outwards from the the periphery of the hood  35  and is used to seal the hood  35 . The cover panel has a projecting lip  51  that sits in a trough  52  formed on the upper surface  53  of the cell roof  44  around the aperture  54  provided for receiving the hood  35 . The trough may contain a flexible sealing material or powder (not shown). For convenience, the trough may be supported in part by the upper edge  63  of the sidewall as well as the cell roof. The vertical position of the hood  35  can be adjusted by means of an adjustment screw  55  or by means of blocks or wedges. This permits the degree of immersion of each hood in the electrolyte to be adjusted, and this adjustment can be made independently of the other hoods. The hood immersion is used to help control the rate of electrolyte flow through and over the top of the electrode assembly and the individual hood adjustment permits such control to be accomplished for each electrode assembly independently. The adjustment of electrolyte flow is useful to compensate for performance changes caused be electrode wear or other reasons and the independent adjustment permits individual compensation for electrode assemblies, a feature not heretofore possible. 
     FIG. 2, which is a view similar to FIG. 1, shows an alternative preferred embodiment of the present invention. The embodiment of FIG. 2 differs from that of FIG. 1 most notably in the design of hood  35 . In this case, the hood is formed in two separable parts, i.e. a vertical annular wall  42  and separable flat horizontal wall  43 . This allows the flat horizontal wall  43  to be removed while the vertical annular wall is kept in place within the cell. The anode  11  and, if desired, the cathode assembly  48 , may be removed without disturbing the vertical annular wall  42  of the hood  35  so that the metal pool  40  can be prevented from overflowing into the electrolysis compartment, and gases in the common area  38  of the cell can be kept confined to that area. To ensure that this is achieved, the vertical annular wall  42  of the hood should be long enough to extend deeply enough into the molten electrolyte  22  so that the lower end  37  of the hood always remains below the upper surface  23  of the molten electrolyte and the upper surface  29  of the metal pool despite variations in the height of that surface due, for example, to the removal of an electrode assembly (the cell may be provided with a level control device to minimize such variations in practice). Such a level control device is described for example in U.S. Pat. No. 5,935,394 (the disclosure of which is incorporated herein by reference). 
     The junction  68  between the two separable parts  42  and  43  of the hood should be gas tight. A sealing gasket (not shown) may be provided, if necessary, to achieve this. 
     Of course, while the vertical side wall  42  of the hood may be left in place when removing the anode  11  and/or the cathode assembly  48 , the vertical side wall may also be removed from the cell if desired, e.g. for maintenance or repair. Separate hoists (not shown) may therefore be needed for the two hood parts. 
     FIG. 3, which is a view also similar to FIG. 1, shows a further alternate preferred embodiment of the present invention. In this embodiment, the cathode  12  is supported from below be spaced supports  25  extending upwards from the cell floor  26 , which avoids the need to provide a supporting structure based on the cathode bus bar  27 . The embodiment illustrated in FIG. 3 also has the advantage that the cathode bus bar  27  exits through the cover panel  61  rather than through a cell wall (as in conventional cells of this kind). Metal bus bars are rapidly corroded by electrolysis gases and therefore, in a conventional cell, they may not pass through the headspace of the cell where electrolysis gases collect. They are therefore routed directly through an adjacent cell wall at a position below the upper surface of the electrolyte, but this creates a possible point of cell wall failure. In the illustrated embodiment, corrosive electrolysis gases are confined within the hood  35  and there are no such gases within the general headspace  39  of the cell. Accordingly, a short horizontal section  49  of the bus bar  27  projects beyond the “shadow” of the hood  35  within the protection of the molten electrolyte  22  and then a vertical section  50  extends upwardly through the general headspace  39  and through a portion of the cover panel  61  where (like the anode bus bar  24 ) it can conveniently be routed above the cell to the electrical supply (not shown). 
     In the region where the cathode bus bar  27  extends through the cover panel  61 , the cathode busbar  27  has a panel  57  having a peripheral lip  58  which sits in a trough  59  provided around the opening in the cover panel  61  through which the cathode bus bar passes. Again, a seal is created by means of a flexible packing material or powder (not shown). In order that the illustrated design permit the hood  35  to be vertically adjustable, this packing material must permit vertical movement of the lip  58  within the trough  59 . The aperture  54  is large enough to allow the entire electrode assembly  10  and bus bar  27  to be removed from the cell when desired. 
     There is no need to arrange the electrode assemblies  10  in the conventional manner within the housing H of the cell C, i.e. there is no need to arrange the electrode assemblies along one longitudinal cell side wall, although this arrangement may be retained, if desired. As previously noted, the conventional cell requires an longitudinal electrolysis compartment along one long side of the cell and a metal collection compartment along the opposite long side wall of the cell with a refractory curtain or partition wall extending between and defining these compartments. The metal collection compartment has traditionally required a large surface area to reduce the downward electrolyte velocity to limit the amount of metal that is recirculated to the electrolysis compartment. In the present invention, the area can be increased, without changing the outside dimensions of the cell. This is because hoods  35  of the electrode assemblies  10  take up less surface area of the cell than a common electrolysis compartment of the conventional cell. Moreover, the assemblies of the present invention can be located in any arrangement within the cell because each assembly has its own electrolysis compartment formed by hood  35 , and the metal collection chamber is common to all electrode assemblies and is merely the area of the cell outside the hoods  35 . This freedom of positioning of the electrode assemblies means that it may be possible to introduce more electrode assemblies within a cell of a given size than is possible with conventional cells, and/or to optimize the routing of bus bars. 
     It should also be noted that the size of a conventional cell is limited because an increase in size requires an increase in length of the longitudinal curtain wall, which becomes relatively weaker and thus reduces the operational life of the cell. In the present invention, the cell size may be increased to accommodate more electrode assemblies without any penalty to the operational life of the cell. Cells according to the present invention may therefore be made larger than conventional cells, if desired. 
     A densely packed, uniformly oriented array of electrode assemblies within an electrolysis cell C is shown in FIG. 4, which is an overhead plan view of the roof  44  of the cell. The cell has an elongated rectangular upper surface that includes hoods  35  and cover panels  61  covering apertures  54 . Anode bus bars  24  and cathode bus bars  27  are visible and are arranged with good separation. In a cell that would conventionally hold four such assemblies, seven such electrode assemblies  10  are provided, four along one side wall and  3  along the opposite side wall of the cell. This means that the output of the cell may be almost double that of a conventional cell of the same size. Of course, other arrays of electrode assemblies may be preferred, as will be apparent to a person skilled in the art. In this embodiment, the cover panels are rectangular or square in shape, whereas the hoods are circular. The projecting lips and troughs are provided only on two, or at most three sides of the hood  35 , and the remaining edges  64  are sealed using metal plates, troughs, heat resistant fabric, pitch, packing material, powder or similar means. 
     FIG. 5 shows part of an electrolysis cell C having a housing H provided with several side-by-side electrode assemblies  10  and encircling hoods  35  (two are shown) constructed according to FIG.  3 . In addition to the features shown in these figures, the illustration also shows vents  76  for removal of electrolysis gases from the interior of the hoods  35 . Piping  77  connects with the vents  76  for conveying the electrolysis gases to treatment or storage facilities (not shown). Flanges are provided in the piping  77  to permit demounting and isolating of individual hoods should the need arise. It is normal to maintain a slight vacuum in the vents to facilitate chlorine removal and it is advantageous in the present invention to place manual or automatically controlled dampers  80  in each pipe  76  to permit the vacuum level to be independently adjusted for each electrode assembly. The vacuum level provides a fine control of the electrolyte level in each electrode assembly (the level control means described above providing an overall control) and this in turn provides control of the electrolyte flow through the assembly either in conjunction with the immersion of the hood already described or separately in order to compensate for differences in operating conditions between electrode assemblies. The vacuum control may be accomplished, for example, by applying the methods described in European Patent Application EP0915187 (the disdosure of which is incorporated herein by reference) where the methods are, however, in this case applied to each hood and electrode assembly within a cell rather than to separate cells. 
     While, as explained above, there is a significant advantage in providing a hood  35  having a lower end  37  that is spaced from the cathode around the entire cathode periphery, an alternative design according to the present invention has a hood contacting the periphery of the cathode  12  or the plate  30 , except at an exit port for the metal-containing electrolyte. An embodiment of this kind is shown in FIG. 6, which is a view similar to that of FIG.  1 . As shown, the lower end  37  of the vertical wall  42  of the hood  35  rests directly on and may be attached to the plate  30  extending from the cathode  12 , thus sealing off the interior of the hood forming the gas-collection chamber  41  from the remainder of the cell around the cathode periphery. At a position opposite the side wall  60  of the cell, the vertical wall  42  has a foreshortened portion  69  forming an aperture  70  through which metal-containing electrolyte overflowing the cathode may pass to the metal collection area of the cell outside the hood  35 . This aperture is positioned below the normal height of the surface  23  of the molten electrolyte  22  and the surface  29  of the metal pool  40  so that electrolyte gases collecting within the hood  35  cannot escape to the common area  38  of the cell outside the hood  35 . Naturally, electrolyte may still flow into the electrode assembly from below through aligned holes  18  and  21  to replace the electrolyte leaving the aperture  70 . 
     Projecting from the lower front edge of the aperture  70  into the interior of the cell is an upwardly angled metal plate  71 . The metal-containing electrolyte exiting aperture  70  is deflected upwardly by the angled plate  71  and comes into contact with the underside of the metal pool  40 . 
     While this embodiment has the disadvantage that metal-containing electrolyte overflowing the upper end  28  of the cathode  12  at points remote from the aperture  70  must flow around the cathode periphery in the annular channel  36  formed between the cathode  12  and the hood  35 , which means that there is greater residence time of the metal in the electrolysis chamber formed within the hood and hence more risk of reaction with the electrolysis gases than is the case for the earlier-described embodiments, this embodiment has the advantage that the electrode assembly may be produced as a unitary structure that can be inserted into and removed from the cell as a single unit. The embodiment still has the advantage that the corrosive and dangerous electrolysis gases are confined to the interior of the hood  35 , and the metal is collected in the remainder of the cell. 
     The disadvantage of having a single aperture can be overcome to some extent by providing a plurality of apertures similar to the aperture  70  around the periphery of the hood and at a same level. If uniformly spaced, such apertures permit electrolyte to exit from the space between the other surface of the cathode and the hood in a substantially uniform manner thus retaining some of the benefits of the previous embodiments while permitting a unitary structure to be used. 
     Having described preferred embodiments of the invention, other modifications, alterations and improvements will readily occur to persons skilled in the art. All such modifications, alterations and improvements within the spirit and scope of the present invention are to be regarded as forming part of the invention.