Patent Publication Number: US-2004055878-A1

Title: Anode for the electrolysis of aluminium

Description:
[0001] This invention relates to an anode for the electrolysis of aluminium. In particular, the anode is formed of a power supply head and a carbon electrode, the shape of the head and the electrode being designed to improve the electrical contact between the head and the electrode, thereby improving efficiency.  
       [0002] DD 284,908 teaches forming an anode from a power rail and a carbon electrode. The top of the carbon electrode has a raised cavity into which a power rail is inserted. The size of the cavity is greater than that of the power rail. The gap between the wall of the cavity and the power rail is filled with molten metal which is allowed to solidify. As the molten metal solidifies it shrinks and an air gap is formed between the solidified metal and the wall of the carbon electrode. The mechanical connection between the carbon electrode and the solidified metal, and hence the power rail, is limited to a small surface area and is mechanically weak. When the carbon electrode is suspended by the power rail in the electrolyte, the contact area between the carbon electrode and the solidified metal tends to decrease further under the weight of the carbon electrode. The small contact area and the lack of reliable electrical contact results in increased electric consumption during electrolysis. In order to reuse the power rails they must be extracted from the spent carbon electrodes. This extraction requires cast iron melting furnaces, a press for the removal of the cast-iron smelt from the power rails, magnetic separation plants to extract used cast-iron fragments from the spent carbon electrodes and large equipment for dismantling the spent anodes and installing new anodes to the electrolytic cell. These all result in an increase in the cost of production.  
       [0003] The present invention aims to overcome the problems associated with known anodes and to provide additional advantages.  
       [0004] The present invention in its various aspects is defined in the independent claims below. Advantageous features are set forth in the dependent claims.  
       [0005] The anode is assembled by sliding the head into the slot of the electrode. The shape of the slot in the electrode and of the head increases the area of contact between the electrode and the head. It improves mechanical and electrical contact between the head and the electrode and therefore reduces losses and improves efficiency. The shape of the slot and of the head eliminates the need to fill the gap between the head and the electrode with cast metal. Reclaiming the head is therefore much simpler, quicker and less costly than for prior art anodes. The provision of projections on the sides of the head further improves contact between the head and the electrode. 
     
    
    
     [0006] A preferred embodiment of the invention will now be described in more detail, by way of example, with reference to the accompanying drawings in which:  
     [0007]FIG. 1 is a cut away side view of an anode embodying the present invention; and  
     [0008]FIG. 2 is a cut away front view of the anode of FIG. 1; and  
     [0009]FIG. 3 is an enlarged view of the head of the anode of FIGS. 1 and 2. 
    
    
     [0010]FIGS. 1 and 2 show an anode embodying the present invention. The anode is formed from two parts: an electrode  10  and a power connection unit  12 . The electrode  10  is formed from a rectangular block of burnt, or calcined, carbon. A slot  14  is formed in the top surface of the carbon electrode  10 . The slot  14  extends axially along the entire length of the carbon electrode  10  from one side to the opposite side. The slot  14 , therefore, has open ends at two sides of the electrode  10  and is open at its top. Each open end of the slot  14  provides means for inserting the head  22  into the slot  14 . The depth of the slot  14  is constant. The slot  14  has a trapezoidal cross-section with the sides  16  and  18  of the slot  14  diverging towards the base of the slot. The narrow portion of the slot at the top surface of the electrode  10  defines a neck. The top and sides of the electrode  10  may be chamfered as shown in FIGS. 1 and 2.  
     [0011] The power connection unit  12  for supplying electricity to the electrode  10  includes a rod  20  and a head  22 . The lower end of the rod  20  bears an external thread for attaching the rod  20  to the head  22 . The top of the head  22  is provided with a connector  24 . The base of the connector  24  is welded to the top of the head  22 . The connector  24  has a double wedge shape and has sloping front and back walls  28  and  30  respectively and vertical side walls  32 . The front wall  28  and back wall  30  of the connector  24  slope outwardly and downwardly from the top  26  of the connector  24  towards its base. A bore with an internal thread is provided in the centre of the top  26  of the connector  24 . The rod  20  is connected to the head  22  by screwing the lower threaded end of the rod  20  into the connector  24  thus forming a good mechanical and electrical connection between the rod  20  and head  22 .  
     [0012] The profile of the head  22  matches that of the slot  14  of electrode  10 , the head  22  having a trapezoidal cross-section with the sides  36  of the head  22  diverging from the top  34  to the bottom of the head  22 . The head  22  generally has a smaller cross-section than the slot  14 . A portion of the head  22 , in this case the bottom has a larger cross-section than the neck of the slot  14  in the electrode  10 . The head  22  therefore keys in the slot  14  and once inserted into the slot  14  cannot be removed from the top of the slot  14  but may be removed by sliding it out of the open end of the slot  14 . The head  22  may, therefore, be used to suspend the electrode  10  with the portion of the head  22  having larger cross-section than that of the slot  14  serving to retain the head  22  in the slot  14  when the head is suspended. The length of the head  22  is greater than the width of the head at its widest part. The head  22  is made of metal, preferably steel, and therefore has a greater thermal expansion than the carbon electrode  10 . In operation, the different thermal expansions of the carbon electrode  10  and the head  22  result in an improved electrical and mechanical connection between the head  22  and the electrode  10 . The shape of the head  22  in combination with the shape of the slot in the carbon electrode  10  provides an increased contact surface area thereby improving mechanical contact and reducing electrical resistance between the head  22  and the electrode  10 .  
     [0013] As shown in FIG. 3, the sides  36  of the head  22  have parallel longitudinal ribs  40  which run the length of the head  22  and which increase the surface area of the sides  36 . The ribs have sharp edges and act as barbs which cut into the carbon electrode  10 . The use of sharp ribs further improves the electrical and mechanical contact during operation of the anode. The ribs  40  are preferably also metal and the head  22  may be formed with the ribs  40  or the ribs  40  may be cut into or welded onto the head  22 .  
     [0014] The bottom  38  of the head  22  is flat. When the head  22  is disposed close to the bottom of the slot  14 , the head  22  may slide freely in the slot  14 .  
     [0015] To assemble the anode, the head  22  is aligned with the slot  14  of the electrode  10 . The trapezoidal cross-section of the slot  14  is the same shape as, but is slightly larger than, the trapezoidal cross-section of the head  22 . By aligning the bottom  38  of the head  22  with the bottom of the slot  14 , and the sides  36  of the head  22  with the sides of the slot  14 , the head  22  may be slid into the slot  14  of the electrode  10 . The head  22  is prevented from rotating within the slot  14  because the length of the head  22  is greater than the width of the slot  14 . The head  22  and slot  14  key together once assembled to retain the head  22  in the slot  14 .  
     [0016] Once the head  22  has been introduced into the slot  14 , fixing pins  42  are inserted underneath the bottom  38  of the head  22  to raise the head  22  within the slot  14 . The neck of the slot prevents the head  22  from being removed from the slot in the vertical direction. Raising the head  22  within the slot  14  forces the sides of the head  22  to come into contact with the sloping sides of the slot  14 . The contact area between the head  22  and the electrode  10  is many times larger than that of known industrial anodes and may be more than an order of magnitude larger. The electrical resistance of the head/electrode contact area is reduced which in turn reduces electric energy consumption. Contact between the head  22  and the electrode  10  is further improved by the ribs  40  biting into the sides of the slot  14 .  
     [0017] The fixing pins  42  are made from material which burns, for example wood, or from the metal which is being recovered by electrolysis, such as aluminium, which melts so that they do not contaminate the metal production. The fixing pins  42  are wedge shaped and are inserted into the slot  14  with the narrow end towards the head  22  so that pushing the fixing pins  42  further under the head  22  in the slot  14  causes the head  22  to be raised vertically. Preferably, four wedges are used, one in each corner at the front and back of the head  22 .  
     [0018] During the electrolysis operation, the anode or anodes are suspended by the rod  20  of the power connection unit. The rod  20  is connected to a power supply (not shown). The electrode  10  and head  22  are heated and the fixing pins  42  are expended by being burnt or melting.  
     [0019] The electrode  10  and head  22  are made from different materials and therefore have different thermal expansion coefficients. During operation, differences between the thermal expansion coefficients of the electrode  10  and the head  22  are accommodated by the vertical displacement of electrode  10  relative to the head  22  and by the relative shapes of the head  22  and the slot  14  in the electrode  10  which excludes the possibility of the electrode  10  being destroyed as a result of the difference between the thermal expansion coefficients of the electrode  10  and the head  22 . Additionally, the ribs  40  penetrate into the electrode  10  due to the different thermal expansion coefficients of the head and electrode. These actions, separately and in combination, further improve the contact between the head  22  and the electrode  10 .  
     [0020] The mechanical and electrical connection between the head  22  and the electrode  10  is increased during operation because the head  22  expands more than the carbon block  10  at operating temperature. The sides of the head  22  expand sideways into the electrode  10 . The sharp edges of the ribs  40  also cut into the electrode  10 . Additionally, the weight of the electrode  10  further causes the sloping sides  36  of the head  22  to move into the contact with the sides of the slot  14  and for the ribs  40  to bite into the electrode  10 . The shape of the head  22  and the complementary shape of the slot in the electrode  10  ensure that the contact area of the head  22  and electrode  10  is maximised. Electrical resistance in the head/electrode interface is, therefore, reduced with a consequential reduction in energy consumed.  
     [0021] During electrolysis, the electrode  10  is oxidised. Before the bottom  38  of head  22  contacts the molten metal ore, the electrode  10  must be replaced. To replace the electrode  10 , the power supply is disconnected from the rod  20  of the power connection unit  12 . The anode assembly is lifted out of the electrolytic cell. The head  22  is made from metal to ensure good electrical conductivity and its rate of thermal shrinkage is more rapid than that of the carbon electrode  10 . The head  22 , therefore, shrinks away from the sides of the slot  14  in the electrode  10  allowing the head to be easily removed from the slot  14  of electrode  10 .  
     [0022] The large contact area of the head with the walls of the slot of the electrode ensures that a good electrical connection is made between the electrode  10  and the head  22  of the power connection unit  12  without the need to fill the gap between the head and electrode with cast metal. Recovery of the power connection unit  12  from the spent electrode  10  is therefore much simpler, quicker and therefore less expensive. The head  22  may be reused after the electrode  10  has been consumed.  
     [0023] With respect to the above description, it is to be realised that equivalent apparatus is deemed readily apparent to one skilled in the art, and all equivalent apparatus to those illustrated in the drawings and described in the specification are intended to be encompassed by the present invention. Therefore, the foregoing is considered as illustrative only of the principles of the invention. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation shown and described and accordingly, all suitable modifications and equivalents may be resorted to, falling within the scope of the invention.  
     [0024] For example, in alternative embodiments of the invention, the head  22  may be lifted by pulling on the rod. No fixing pins or any number of fixing pins may be used. Square or round cross-section fixing pins may be used instead of wedges. The head  22  may be formed in one piece with the rod  20  rather than in two connectable pieces. Alternative means for fixing the connector  24  to the head  22  may be provided as long as the fixing means ensures efficient flow of current from the rod  20  to the head  22 . The sides of the head  22  may be stepped or a thread may be formed on the head to provide partial spiral ribs. The head  22  may be frusto-conical with a thread formed thereon. The head  22  may then rotate in the slot  14 , the thread cutting into the sides of the slot  14  and providing good mechanical and electrical contact between the head  22  and the electrode  10 .  
     [0025] Although in the preferred embodiment of the invention the slot is provided in the top of the electrode  10  allowing the electrode to be suspended vertically above the electrolytic cell by the power support rod, the slot may be provided in one or more sides of the electrode and the power supply rod could extend outwardly from the sides of the electrode  10 . The slot  14  may extend from one side along only part of a surface of the electrode  14  so that the head can only be inserted into the slot from that one side. The electrode  10  may be of any shape.  
     [0026] Instead of the slot extending to at least one side of the electrode, a cavity, larger than the head, may be formed in the same face of the electrode as the slot, with the slot opening into the cavity. The head is inserted into the cavity, aligned with the slot opening, and slid into and along the slot. The head may then be fixed in position using fixing pins as described.  
     [0027] A number of heads  22  may be provided in a single electrode  10 . The flow of current in the head  22  may be more accurately controlled to improve the current flow pattern and reduce energy consumption further.  
     [0028] Rather than ribs, small sharp projections protruding from the sides of the head may be provided. Any number and arrangement of such projections may be used. If ribs are used, they need not extend along the entire length of the sides of the head but only along a part of its length. The ribs may be non-uniform in spacing. Ribs and projections may be omitted altogether and the sides of the head may be smooth.  
     [0029] It should be noted that the features described by reference to particular figures and at different points of the description may be used in combinations other than those particularly described or shown. All such modifications are encompassed within the scope of the invention as set forth in the following claims.  
     [0030] Throughout the specification and the claims which follow, unless the context requires otherwise, the word “comprise”, and variations such as “comprises” or “including”, will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.  
     [0031] The reference to prior art in this specification is not, and should not, be taken as an acknowledgement or any form of suggestion that the prior art forms part of common general knowledge.