Abstract:
The present invention relates to a metal cathode sheet as component of cathode equipment for an electrolysis tank for the electrolytic recovery of pure metals, especially copper. The cathode sheet is provided, at its side edges which come into contact with the electrolyte and are vertically aligned in the electrolysis tank, with an edge protector, made of a ceramic material. The edge protector is electrically insulating, of dense porosity and resistant to electrolyte. Sharp-edged breakthroughs and the outer cathode cutting edges are completely coated by edge protector. In this manner flux line concentrations at these locations are forestalled.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a metal cathode sheet, more particularly a metal cathode sheet having an edge protector.  
         BACKGROUND OF THE INVENTION  
         [0002]    In the refinement of crude metals with the aid of electrolysis for recovering pure metal, the metal is dissolved in an electrolysis tank from the impure anode and deposited in pure form on the cathode. The impurities remain dissolved in the electrolyte or form anode slime.  
           [0003]    Various constructions of electrolysis cathodes are counted among the related art. They differ mainly in the choice of material or material combination of bearing rail and metal cathode sheet with a view to relatively good corrosion resistance, mechanical stability, and electrical conductivity for minimizing energy losses.  
           [0004]    Typically, the side edges of the cathodes, which are vertically aligned in the electrolysis tank, are provided with an electrically insulating screen as edge protection. This helps to prevent the growing together of the metal layers deposited on both sides of the metal cathode sheet from reaching over the side edges.  
           [0005]    In this connection, it is known to coat the side edges with wax. The disadvantage of doing this is, first of all, that a large quantity of wax is required. Furthermore, if the wax is interspersed with contaminating particles, bridge formations with the electrolyte can take place anyway, and this may lead to uncontrolled growth of metal buds whereby the depositing performance drops and the course of the operation is disturbed. Metal cathode sheets are therefore maintained in rotation, and the metal buds are removed. This requires an operating interruption each time.  
           [0006]    It is also known to provide the side edges of the metal cathode sheets with edge protection made of plastic.  
           [0007]    In the metal cathode sheet known from U.S. Pat. No. 5,314,600, the edge protectors are plastic rails that surround the vertical side edges of the cathode sheet in clamping fashion. On the side edges of the cathode sheet bore holes are provided into which holding pins are fitted, thereby fixing the plastic rails.  
           [0008]    The edge protector has a very slack, loose connection to the cathode sheet. This has the disadvantage that electrolyte can penetrate into the edge protector. Then, at the boring edges in the cathode sheet and at the inside sheet cutting edges, high local field densities can appear with the result that, particularly at these locations, uncontrolled metal growth takes place. After longer-term application of the cathode sheet in the electrolyte, the plastic protector can be pried apart and damaged. This causes expensive repair work or possibly a complete renewal of the edge protector.  
           [0009]    U.S. Pat. No. 5,919,343 also describes a plastic edge strip for use as an edge protector. The strip is connected to the cathode sheet using plastic pins and fusion welding technique. Nevertheless, non-fused, faulty connection regions can result due to non-observance of construction prerequisites with respect to the parts to be connected, by non-observance of certain welding parameters, as well as by errors in preassembly. These oversites allow the passage of electrolyte and lead to uncontrolled formation of buds at the outer edge. The problem of local flux line concentration at sharp-edged borings in the cathode sheet, with its negative effects, has also not been solved.  
           [0010]    U.S. Pat. No. 6,017,429 describes a metal cathode sheet having an electrically insulating edge protector made of plastic resistant to electrolyte. The edge profile is chemically connected to the metal cathode sheet, preferably using an adhesive or a vulcanizing technique.  
           [0011]    Also in this particular embodiment, an intimate combination of cathode sheet and edge profile is not absolutely ensured. Thus, penetration by electrolyte under the wall of the edge profile can take place.  
         SUMMARY OF THE INVENTION  
         [0012]    The object of the present invention is to create an improved metal cathode sheet for use under operational conditions. Such a cathode sheet is designed to prevent uncontrolled metal growth at the side edges. Thus, on the one hand, operational interruptions and maintenance work can be reduced and, on the other hand, deposit performance can be increased.  
           [0013]    According to the present invention, the object is attained by providing a metal cathode sheet as a component of cathode equipment for an electrolysis tank for the electrolytic recovery of pure metals, especially copper, which is provided, at least at its side edges which come into contact with the electrolyte and are vertically aligned in the electrolysis tank, with an edge protector, wherein the edge protector is made of a ceramic material.  
           [0014]    Preferably the edge protector is made of an oxide-ceramic material such as aluminum, zirconium or magnesium oxide. The material has great hardness, strength and insulating capacity. In addition, especially aluminum oxide has very good chemical stability.  
           [0015]    The present invention is described in detail below, using an exemplary embodiment represented in the drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    [0016]FIG. 1 illustrates a horizontal cross-section through the side edge of a metal cathode sheet according to the present invention;  
         [0017]    [0017]FIG. 2 illustrates a second specific embodiment of a metal cathode sheet in horizontal section through the side edge;  
         [0018]    [0018]FIG. 3 illustrates the representation of a horizontal section through the side edge of a third specific embodiment; and  
         [0019]    [0019]FIG. 4 a fourth specific embodiment of a metal cathode sheet in horizontal section through the inside edge. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0020]    In FIGS. 1 through 3, three cathode sheets are marked  1   a ,  1   b  and  1   c , and shown in section respectively through breakthroughs  5 , 11 , and  18 . Cathode sheets  1   a ,  1   b ,  1   c  normally have a rectangular cross-sectional configuration. They are advantageously formed from corrosion-resistant stainless steel. The cathode sheets  1   a ,  1   b ,  1   c  are hung, for example, on bearing rails (not shown) made of copper, in an electrolysis tank (not shown) for refining crude copper. In one configuration, the ends of the bearing rails reach current rails running parallel to the electrolysis tank while making electrical contact with the installation.  
         [0021]    Cathode sheet la has on its inside edge  2   a  formed by breakthrough  5 , an edge protector  3  made of a ceramic material. Edge protector  3  is applied so that it adheres firmly to side edge  2   a  as a monolayer. Edge protector  3  preferably has a relatively dense porosity. It overlaps the outer cathode edge of cut  4  all-over and completely lines the wall of a breakthrough  5  in cathode sheet  1  a. It can be seen that the transitions  6 ,  7  on the cathode edge  4  and at breakthrough  5  are rounded. Thereby local flux li rie concentrations in these regions can be avoided or strongly suppressed. As such, uncontrolled outgrowth of metal is forestalled.  
         [0022]    The specific embodiment seen in FIG. 2 has a ceramic edge protector  8  constructed in two layers. It includes a half layer  9  and a cover layer  10 . Half layer  9  functions as adhesive agent and expansion adjuster for compensating changes in length arising from temperature fluctuations. A cover layer  10  is applied on half layer  9 . All sharp-edged breakthroughs  11  and the outer cathode edges  12  are completely coated with the electrically insulating ceramic edge protector  8  having rounded transition areas  13 .  
         [0023]    Preferably, edge protector  8  is developed in several layers—at least two-layered. Further advantages of properties can be achieved by multiple layers, especially an increase in imperviousness to diffusion and an increase in adhesive strength of the edge protector to the cathode sheet.  
         [0024]    In one embodiment, edge protector  8  is made of adhesive layer  9  and a covering layer  10 . In this connection, a first layer is applied as adhesion promoter and expansion adjustment layer in the form of a single or multiple layer, over which a cover layer is provided. Cover  10  layer can be developed as a single or multiple layer. It is essential for all sharp-edged breakthroughs  11  and the outer cathode edges to be coated completely with electrically insulating ceramic edge protection. Flux line concentrations at these locations are prevented.  
         [0025]    It is not essential but expedient to provide support for the edge profile in the region of the cathode sheet&#39;s side edges. The supports are preferably executed in the form of borings or breakthroughs in the cathode sheet.  
         [0026]    The adhesive layer adheres by a direct interaction of the metal components in the adhesive layer, individual elements in the polymer chain and the stainless steel surface of the cathode sheet.  
         [0027]    In cathode sheet Ic, seen in FIG. 3, edge protector  14  is also constructed of two layers, a half layer  15  and a ceramic cover layer  16 . The edge protector  14  has a jacket  17  made of an electrolyte-resistant plastic. Jacket  17  completely embeds edge protector  14  and is connected to it with non-positive and positive locking. For this purpose, the breakthroughs  18  functioning as support in cathode sheet  1  c are completely filled with plastic. Outer cathode edge  19  is also embedded in the jacket.  
         [0028]    Jacket  17  guarantees impact protection for ceramic edge protector  14  besides guaranteeing an increase in the electrical insulating properties, pore density and resistance to electrolyte. Additionally, the plastic jacket guarantees impact protection which works out effectively during handling of the cathode sheets. This is advantageous especially during handling of cathode sheet lc outside the electrolysis tank.  
         [0029]    Plastic jacket  17  can be fixed to ceramic edge protector  14  by adhesion, vulcanization or fusion welding technique. Besides just adhesion, jacket  17  is expediently connected to edge protector  14  with positive locking. This is done preferably by having jacket  17  also engage with the supports at side edges  2   c  of the cathode sheet  16 .  
         [0030]    [0030]FIG. 4 shows a cathode sheet  20 , which corresponds to the basic construction of cathode sheets  1   a ,  1   b ,  1   c . It shows a inside edge  21  having an edge protector  22  made of a polymeric multi-layer system  23 . The polymeric multi-layer system  23  is made of a half layer  24  having embedded metal components  25  so as to increase adhesion by physical-chemical interaction, and a protective layer  26  made of a polymeric material. Adhesive layer  24  adheres by a direct physical-chemical interaction of metal components  25  and the surface of cathode sheet  25  made of stainless steel. As compared to known edge protection systems, the connection generates substantially higher and improved cohesive forces.  
         [0031]    A third attainment of the object is a cathode sheet having, an edge protector at the side edges which is constructed in multiple layers, at least one layer being made of a ceramic material, on which there is at least one further layer made of a polymeric material.  
         [0032]    In particular, the use of rubber-elastic polymer materials as cover layer avoids in an advantageous manner the mechanical influences, appearing during stripping off, on the basis layer made of a ceramic material.  
         [0033]    Edge protector  3 ,  8 ,  14 ,  22  of all the specific embodiments described above preferably has a medium thickness between 0.1 mm and 0.8 mm, particularly between 0.3 mm and 0.5 mm.  
         [0034]    In all instances, the edge protection can be applied to the side edges in fluid or powder form. After hardening, stable adhesion results. It is also possible to coat the side edges with a ceramic material in a gaseous or vapor condition. In practice, the application of the edge protection by using a sintering technique is available. Depending on the material, dip enameling, laser coating or powder coating can also be applied.  
         [0035]    The edge protection is electrically insulating, of dense porosity and resistant to electrolyte. Preferably, the ceramic material used is aluminum, zirconium or magnesium oxide. The edge protection has an absolutely fluid-tight intimate connection to the cathode sheet. Flux line concentrations at exposed metal edges can thus be reliably avoided. Metallic buds are thus forestalled. Waxing and dewaxing the edge regions is not required.  
         [0036]    The multiple layer system can further be constructed from at least two layers of various polymeric materials.  
         [0037]    For practical purposes, it is regarded as particularly advantageous to use rubber-elastic polymer materials. These are resistant to mechanical influences which appear during stripping off of the recovered pure copper.  
         [0038]    According to the features of claim  10 , the thickness of the edge protector is between 0.1 mm and 0.8 mm. A thickness of 0.3 mm to 0.5 mm is regarded as particularly advantageous. At this thickness the electrical insulation, pore density and resistance to electrolyte are reliably ensured. Beyond that, the edge protector is flexible enough to be able to stand deformations or mechanical shocks during stripping off of the recovered pure copper.