Abstract:
A method of operating an aluminium electrowinning cell that has one or more metal-based anodes ( 5 ). The anodes ( 5 ) comprise metal-based foraminate anode bodies ( 10 ) which are suspended by metal-based anode stems ( 20 ) in a molten electrolyte ( 50 ) and which are spaced above a cathode ( 30 ). The method comprises electrolysing alumina dissolved in the molten electrolyte ( 50 ) by passing current via the anode stems ( 20 ) and the anode bodies ( 10 ) through the electrolyte ( 50 ) to the facing cathode ( 30 ) whereby aluminium ( 60 ) is cathodically produced and gas is anodically evolved. The gas promotes an electrolyte circulation ( 51 ) through the foraminate anode bodies ( 10 ) which facilitates dissolution of alumina. Each anode ( 5 ) has a foraminate anode body ( 10 ) suspended by least three anode stems ( 20 ) that are spaced apart from one another and distributed around a foraminate stemless central part of the anode body ( 10 ). These stems extend from the anode body ( 10 ) to above the molten electrolyte ( 50 ), the electrolyte ( 50 ) flowing up through and above said foraminate central part of the anode body ( 10 ) to enhance dissolution of alumina fed thereabove.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates to electrowinning aluminium in a cell having foraminate metal-based anodes which permit an improved electrolyte circulation. The invention also relates to such a cell and to the anode.  
       BACKGROUND ART  
       [0002]     The technology for the production of aluminium by the electrolysis of alumina, dissolved in molten cryolite, at temperatures around 950° C. is more than one hundred years old and still uses carbon anodes and cathodes.  
         [0003]     Using metal anodes in aluminium electrowinning cells would drastically improve the aluminium process by reducing pollution and the cost of aluminium production.  
         [0004]     Several attempts have been made in order to develop non-carbon anodes for aluminium electrowinning cells, resistant to chemical attacks of the bath and by the cell environment, and with an electrochemical active surface for the oxidation of oxygen ions to atomic and molecular gaseous oxygen and having a low dissolution rate. However, all attempts have failed mainly due to the anode materials which had a low electrical conductivity and caused unacceptable contamination of the aluminium produced. Many patents have been filed on non-carbon anodes but none has found commercial acceptance, also because of economical reasons.  
         [0005]     U.S. Pat. No. 4,614,569 (Duruz/Derivaz/Debely/Adorian) describes metal anodes for aluminium electrowinning coated with a protective coating of cerium oxyfluoride, formed in-situ in the cell or pre-applied, this coating being maintained during electrolysis by the addition of small amounts of a cerium compound to the molten cryolite electrolyte so as to protect the surface of the anode from the electrolyte attack.  
         [0006]     Several designs for oxygen-evolving anodes for aluminium electrowinning cells were proposed in the following patents all assigned to MOLTECH Invent S.A. U.S. Pat. No. 4,681,671 discloses vertical anode plates or blades operated in low temperature aluminium electrowinning cells. U.S. Pat. No. 5,310,476 discloses oxygen-evolving anodes consisting of roof-like assembled pairs of anode plates. U.S. Pat. No. 5,362,366 describes non-consumable anode shapes including roof-like assembled pairs of anode plates. U.S. Pat. No. 5,368,702 discloses vertical tubular or frustoconical oxygen-evolving anodes for multimonopolar aluminium cells. U.S. Pat. No. 5,683,559 describes an aluminium electrowinning cell with oxygen-evolving bent anode plates which are aligned in a roof-like configuration facing correspondingly shaped cathodes. U.S. Pat. No. 5,725,744 discloses vertical oxygen-evolving anode plates, preferably porous or reticulated, in a multimonopolar cell arrangement for aluminium electrowinning cells operating at reduced temperature.  
         [0007]     WO00/40781 and WO00/40782 (both de Nora) both disclose aluminium production anodes with a series of parallel spaced-apart elongated anode members which are electrochemically active for the oxidation of oxygen. Various anodes are disclosed, in particular WO00/40782 discloses an anode with a rectangular anode-member arrangement which is held by two spaced-apart large feet, each foot being integral with a large anode stem and located towards one end of the anode-member arrangement.  
       SUMMARY OF THE INVENTION  
       [0008]     A main object of the present invention is to enhance the circulation of electrolyte in an aluminium electrowinning cell having foraminate metal-based anodes and the dissolution of alumina by using a new cell configuration.  
         [0009]     In particular, the invention is concerned with the flow of the electrolyte resulting inter-alia from the shape and configuration of the anode. To achieve the present invention, instead of having foraminate anode bodies that are suspended usually from their centre by large prior art anode stems, foraminate anode bodies are suspended by a greater number of anode stems, preferably of smaller size, which are distributed around a central part of the anode and which do not significantly interfere with the electrolyte upflow. In comparison with the prior art anode configuration, the electrolyte can flow up through substantially the entire foraminate anode body and this upflow is not weakened by being diverted into a multitude of different directions from below a large main anode stem. Hence, during use, a strong electrolyte circulation is produced and electrolyte circulates from and to substantially the entire active anode surface which is substantially uniformly operative for the oxidation of oxygen.  
         [0010]     The present invention relates to a method of operating an aluminium electrowinning cell. The cell has one or more metal-based anodes that comprise metal-based foraminate anode bodies which are suspended by metal-based anode stems in a molten electrolyte and which are spaced above a cathode. The method comprises electrolysing alumina dissolved in the molten electrolyte by passing current via the anode stems and the or each anode body through the electrolyte to the facing cathode whereby aluminium is cathodically produced and gas is anodically evolved. The evolved gas promotes an electrolyte circulation through the or each foraminate anode body to facilitate dissolution of alumina fed thereabove.  
         [0011]     The or each anode has a foraminate anode body suspended by at least three anode stems that are spaced apart from one another and distributed around a foraminate stemless central part of the anode body. These stems extend from the anode body to above the molten electrolyte. Electrolyte flows up through and above this foraminate central part to enhance dissolution of alumina.  
         [0012]     As opposed to the abovementioned prior art anode disclosed in WO00/40782, the anode of the present invention has at least three anode stems that are located around a central part of the foraminate anode body and are so dimensioned and positioned as to minimise interferences with the electrolyte upflow through and above the central part and generally through and above substantially the entire anode body. Such a circulation also permits the supply of alumina-rich electrolyte to substantially the entire active anode surface so that electrolysis takes place over substantially the entire active anode surface, i.e. without having significant areas of the active anode surface that remain inoperative for lack of electrolyte circulation to and from these active areas.  
         [0013]     The alumina is preferably supplied to the electrolyte between the stems of at least one anode vertically above the central part of the anode body directly into the electrolyte main flow to maximise dissolution of the alumina due to the electrolyte&#39;s stirring effect.  
         [0014]     The cathode can be aluminium-wettable. The product molten aluminium can be drained on the cathode. Various aluminium-wettable cathodes and drained cathodes have been disclosed in the prior art which can be used for this invention. See for example U.S. Pat. Nos. 5,683,559, 5,888,360, 6,093,304, 6,258,246, 6,358,393 and 6,436,273, and in PCT publications WO99/02764, WO00/63463, WO01/31086, WO01/31088, WO01/42168, WO01/42531, WO02/070783, WO02/070785, WO02/096830, WO02/096831, WO02/097168, WO02/097168, WO03/023091 and WO03/023092 (all assigned to MOLTECH Invent S.A.).  
         [0015]     Another aspect of the invention relates to an aluminium electrowinning cell having one or more metal-based anodes that comprise metal-based foraminate anode bodies which are suspended by metal-based anode stems in an alumina-containing molten electrolyte and which are spaced above a cathode. This cell is arranged so as to permit an electrolyte circulation promoted by anodically evolved gas through the or each foraminate anode body to facilitate dissolution of alumina in the electrolyte fed thereabove.  
         [0016]     The or each anode has a foraminate anode body suspended by at least three anode stems that are spaced apart from one another and distributed around a foraminate stemless central part of the anode body. These stems extend from the anode body to above the molten electrolyte so as to permit an upflow of electrolyte through and above the foraminate central part to enhance dissolution of alumina.  
         [0017]     Usually, the anode has a metal-based active surface, in particular an oxide surface that can for example contain at least one iron, cobalt, nickel and copper. In one embodiment the active surface is made predominantly of cobalt oxide CoO which provides the advantages described below.  
         [0018]     The anode stems of one anode can be connected together by cross-members.  
         [0019]     Usually, the cell has a cover above the electrolyte. The cover can be an insulating cover, in particular for cell operation with a substantially crustless and/or ledgeless molten electrolyte. Covers and materials are disclosed in U.S. Pat. No. 6,402,928, WO02/070784 and WO03/102274 (all assigned to MOLTECH Invent S.A.).  
         [0020]     The anode stems of one anode can be connected together by cross-members above or below the insulating cover.  
         [0021]     The cross-members can be joined to a main current conductor (or main stem) that is connected to an anode bus bar. The bus bar is usually part of the cell superstructure located above the cell cover. Such a main conductor should be located outside the electrolyte circulation to avoid interference therewith, for example this conductor can be located above the electrolyte.  
         [0022]     Usually, the alumina-containing electrolyte is a fluoride-based electrolyte, in particular an electrolyte containing predominantly aluminium fluoride and sodium fluoride.  
         [0023]     A further aspect of the invention relates to an aluminium electrowinning metal-based anode for use in a cell as described above. The anode comprises a metal-based foraminate anode body and metal-based anode stems which are connected to the anode body.  
         [0024]     The or each anode has a foraminate anode body connected by at least three of said anode stems that are spaced apart from one another and distributed around a foraminate stemless central part of the anode body. These stems extend during use from the anode body to above the molten electrolyte so as to permit an upflow of electrolyte through and above this foraminate central part to enhance dissolution of alumina.  
         [0025]     Four to eight anode stems can be connected to an anode body, in particular four to six stems.  
         [0026]     Usually, the anode body has a grid-like or plate-like foraminate structure that is parallel to the facing cathode. Examples of such anode bodies are disclosed in WO00/40781, WO00/40782 and WO03/006716 (all assigned to MOLTECH Invent S.A.).  
         [0027]     Typically, the anode body has an upper face to which the stems are connected around a central point of the upper face. Each anode stem can be located at a distance from the central point which is in the range of ¼ to ¾ of the length of a segment of a line extending from the central point to a side of the face and intercepting the anode stem, in particular ⅓ to ⅔ of said length.  
         [0028]     The anodes stems are preferably positioned on the anode bodies taking into account the current distribution in the anode bodies during use so as to optimise this distribution.  
         [0029]     In one embodiment, the anode body has polygonal, in particular a square or rectangular upper face. For example, the anode body is suspended by four anode stems.  
         [0030]     These stems can be generally located on crossing diagonals of the body&#39;s upper face, each stem being located generally on the upper face about half way between a corner of the body&#39;s face and the crossing point of the diagonals. In other words, the rectangular or square upper face can be notionally divided into four equal rectangular or square quadrants in the middle of which an anode stem is connected to the anode body.  
         [0031]     These four stems may be generally located substantially on crossing perpendicular median lines of the body&#39;s upper face, each stem being connected about half way between a side of the body&#39;s face and the crossing point of the median lines.  
         [0032]     In another embodiment an anode body has a generally circular upper face. Alternatively the upper face can be generally elliptic. The upper face may be connected to four stems evenly distributed around the body&#39;s central part. Each stem can be located substantially in the middle of a radius of the circular upper face.  
         [0033]     Usually, the anode stems have ends away from the anode body that are connected together by cross-members.  
         [0034]     In particular, the anode can have pairs of opposite stems that are connected by intercepting cross-members. For example, the anode has four stems that are connected by two cross-members that form an “X” or three cross-members that form an “H”.  
         [0035]     The cross-members can be joined to a main current conductor for connection to a busbar.  
         [0036]     Preferably, the anode stems connected to the anode body have a sufficient transverse cross-sectional area for passing a current that leads to a current density in the range of 0.5 to 1.5 A/cm 2  near the surface of the anode body with a voltage drop along the anode stems below 80 mV/cm, in particular in the range of 20 to 50 mV/cm. The total voltage drop along the stems can be of the order of 0.2 to 0.5 V. The anode body can have an active surface that has a total projected surface area AA and the anode stems connected to the anode body have a cumulated transverse cross-sectional area AS (equal to the sum of the transverse cross-sectional area of the individual anode stems), the area AS corresponding to a fraction of the area AA which is in the range of 0.1% to 2% of the area AA, in particular 1 to 1.5%.  
         [0037]     The diameter of each anode stem may be in the range of 2 to 8 cm, in particular 2.5 to 6 cm, such as 3 to 4 cm. This is significantly smaller than the usual diameter of anode stems, which is typically 10-15 cm.  
         [0038]     The anode body has an active face that usually has a total projected surface area in the range of 0.2 to 2 m 2 , in particular 0.5 to 1.5 m 2 .  
         [0039]     The total projected surface area mentioned above refers to the overall active surface without deduction of the holes present at the surface due to the fact that the anode body is foraminate.  
         [0040]     The active anode surface can be made of any prior art non-carbon materials, in particular metal-based materials such as materials containing at least one of iron, nickel and cobalt and oxides thereof. For example the anode&#39;s active surface can be made of the materials disclosed in any of the following publications: WO99/36591 and WO99/36592, WO99/36593 and WO99/36594, WO00/06800, WO00/06801, WO00/06802 and WO00/06803, WO00/06804, WO00/06805, WO00/40783 and WO01/42534, WO01/42536, and WO01/43208, WO02/070786, WO02/083990, WO02/083991, WO03/078695, WO03/087435, WO2004/018731, WO2004/024994 and WO2004/044268, WO2004/050956 (all in the name of MOLTECH Invent S.A.). 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0041]     The invention will now be described by way of example with reference to the drawings, wherein:  
         [0042]      FIG. 1  is a perspective view of an anode body of an anode according to the invention;  
         [0043]     FIGS.  2  to  5  show schematic plan views of anodes according to the invention; and  
         [0044]      FIGS. 6 and 7  are cross-sectional views of two aluminium electrowinning cells according to the invention.  
     
    
     DETAILED DESCRIPTION  
       [0045]      FIG. 1  shows an anode body  10  of an anode according to the invention. The anode body  10  has a generally rectangular upper face and comprises a series of parallel spaced apart anode members  11  that have a generally pentagonal cross-section with a tapered upper part for guiding an electrolyte circulation. Examples of suitable anode members  11  are disclosed in WO03/006717 (de Nora). Variations for the anode members are also disclosed in WO00/40781 and WO00/40782 (both de Nora). The anode members  11  are connected by two end-members  12  and two intermediate cross-members  13  as shown in  FIG. 1 .  
         [0046]     The anode body  10  has in the cross-members  13  four sockets  15  each with an opening  16  for receiving anode stems (not shown). An anode stem can be screwed, force-fitted or welded into opening  16 . In a variation, an anode stem can be made integral with the cross-member  13 .  
         [0047]     Each socket  15  is located in the middle of a notional quadrant delimited by dashed lines A and B which divide the upper face of the anode body into four equal quadrants. In other words, each socket  15  is located on a diagonal of the body&#39;s upper face, half way between a corner of the upper face and the intersection of the diagonals (which coincides with the intersection of dashed lines A and B). Sockets  15  are joined to cross-members  13  which permits a better current distribution from anode stems and sockets  15  to anode members  11 .  
         [0048]     For instance, the anode body  10  has a total length of 65 cm along the direction of the anode members  11  and a total width of 61 cm across the direction of the anode members. Each anode member  11  has a width of 20 mm and a height of 25 mm. The anode members are spaced apart by inter-member gaps of 20 mm. The total projected active bottom surface of the anode body (without deduction of the inter-member gaps) is about 65 cm×61 cm, i.e. approximately 0.4 m 2 . Each socket  15  has an outer diameter 60 mm and the opening  16  has a diameter of 40 mm. This opening can receive an anode stem whose lower end has a corresponding diameter, i.e. 40 mm. The remaining part of the anode stems can have the same diameter or a different diameter, in particular a larger diameter. However, the diameter of this remaining part should not be too large, in any case no larger than the outer diameter of socket  15 , in order to avoid noticeable interference of the anode stems with upflowing electrolyte during operation in a cell.  
         [0049]     In any case, these sockets  15  and the corresponding anode stems are significantly smaller than prior art central anode stems which have a typical diameter of the order of 120 mm. The cross members  13  have a width of 20 mm.  
         [0050]     Cell operation with an anode body  10  as shown in  FIG. 1  will be described in greater detail in connection with the cells shown in  FIGS. 6 and 7 .  
         [0051]     FIGS.  2  to  5  show schematically plan views of anodes  5  having variations of the anode body shown in  FIG. 1 .  
         [0052]     The anode bodies  10  of  FIGS. 2 &amp; 3  have a circular shape. These anode bodies  10  can be made up of concentric circular anode members connected by radial cross-members (not shown). See for example WO00/40781 and WO00/40782 (both de Nora). The anode body  11  shown in  FIG. 2  is suspended by four anode stems  20 , each stem  20  being usually joined to or integral with a radial cross-member. Anode stems  20  are located on a radial cross-member and spread around the centre of the anode body  11 . The anode body  11  shown in  FIG. 3  is suspended by three anode stems  20 , each stem  20  being located close to the middle of a radial cross-member.  FIG. 2  indicates in dotted lines intercepting members  23  for connecting anode stems  20  above the anode as explained in  FIGS. 6 and 7 .  
         [0053]     The anode bodies  11  of  FIGS. 4 and 5  have a square shape. Four anode stems  20  are located on median lines A and B to the anode body  11  of  FIG. 4 . Six anode stems  20  are located on median line A and diagonals C and D in  FIG. 5 .  
         [0054]      FIG. 6  shows in cross-sectional view a drained-cathode aluminium electrowinning cell with two different anodes  5 .  
         [0055]     The cell has a carbon cathode  30  with an inclined aluminium-wettable drained cathode surface  31  and an aluminium collection reservoir  35 . The surface of the carbon cathode can be made wettable by applying thereto a layer of aluminium-wettable material, in particular a titanium diboride coating as for example disclosed in WO02/096831 (in the name of MOLTECH Invent S.A.). Cathode  30  is located on a bed of insulating material  36  and electrically connected to a busbar (not shown) by collector bars  37  that are usually made of steel.  
         [0056]     The drained cathode surface  31  is covered with a thin layer of product aluminium  60 .  
         [0057]     The cell has sidewalls  40  which are made of or covered with a material resistant to molten electrolyte, such as silicon carbide or fused alumina or aluminium-wetted porous aluminium as disclosed in WO02/070783 (de Nora). Sidewall  40  is lined with an insulating material  41  in steel shell  42 . Sidewalls  40  are joined to cathode  30  by a body  43  of solidified ramming paste.  
         [0058]     The cell is covered with an insulating cover  45  which can be of the type disclosed in U.S. Pat. No. 6,402,928, WO02/070784 and WO03/102274 (all assigned to MOLTECH Invent S.A.).  
         [0059]     The cell contains an electrolyte  50  and has a sufficient insulation  36 ,  41 ,  45  to maintain electrolyte  50  in a molten state substantially without crust or ledge.  
         [0060]     A suitable molten electrolyte can be at a temperature below 950° C., in particular in the range from 910° to 940° C., and consist of: 
        6.5 to 11 weight % dissolved alumina, in particular 7 to 10 weight %;     35 to 44 weight % aluminium fluoride, in particular 36 to 42 weight % aluminium fluoride, such as 36 to 38 weight;     38 to 46 weight % sodium fluoride, in particular 39 to 43 weight %;     2 to 15 weight % potassium fluoride, in particular 3 to 10 weight % potassium fluoride, such as 5 to 7 weight %;     0 to 5 weight % calcium fluoride, in particular 2 to 4 weight % calcium fluoride; and     0 to 5 weight % in total of one or more further constituents, in particular up to 3 weight %.        
 
         [0067]     The presence in the electrolyte of potassium fluoride in the above amount has two effects. On the one hand, it leads to a reduction of the operating temperature by up to several tens of degrees without increase of the electrolyte&#39;s aluminium fluoride content or even a reduction thereof compared to standard electrolytes operating at about 950° C. with an aluminium fluoride content of about 45 weight %. On the other hand, it maintains a high solubility of alumina, i.e. up to above about 8 or 9 weight %, in the electrolyte even though the temperature of the electrolyte is reduced compared to conventional temperature.  
         [0068]     Hence, in the above electrolyte, in contrast to other low temperature electrolytes which carry large amounts of undissolved alumina in particulate form, a large amount of alumina is in a dissolved form.  
         [0069]     Without being bound to any theory, it is believed that combining a high concentration of dissolved alumina in the electrolyte and a limited concentration of aluminium fluoride leads predominantly to the formation of (basic) fluorine-poor aluminium oxyfluoride ions ([Al 2 O 2 F 4 ] 2− ) instead of (acid) fluorine-rich aluminium oxyfluoride ions ([Al 2 OF 6 ] 2− ) near the anode. As opposed to acid fluorine-rich aluminium oxyfluoride ions, basic fluorine-poor aluminium oxyfluoride ions do not significantly dissolve the anode&#39;s surface, in particular when made of predominantly of cobalt or nickel oxide, and do not noticeably passivate or corrode the anode&#39;s metals, in particular metallic cobalt or nickel. The weight ratio of dissolved alumina/aluminium fluoride in the electrolyte should be above 1/7, and often above 1/6 or even above 1/5, to obtain a favourable ratio of the fluorine-poor aluminium oxyfluoride ions and the fluorine-rich aluminium oxyfluoride ions.  
         [0070]     It follows that the use of the above described electrolyte with metal-based anodes that contain cobalt oxide and/or nickel oxide inhibits its dissolution, passivation and corrosion. Moreover, a high concentration of alumina dissolved in the electrolyte further reduces dissolution of oxides of the anode, in particular cobalt oxide and nickel oxide.  
         [0071]     The electrolyte may for example consist of: 7 to 10 weight % dissolved alumina; 36 to 42 weight % aluminium fluoride, in particular 36 to 38 weight %; 39 to 43 weight % sodium fluoride; 3 to 10 weight % potassium fluoride, such as 5 to 7 weight %; 2 to 4 weight % calcium fluoride; and 0 to 3 weight % in total of one or more further constituents. This corresponds to a cryolite-based (Na 3 AlF 6 ) molten electrolyte containing an excess of aluminium fluoride (AlF 3 ) that is in the range of about 8 to 15 weight % of the electrolyte, in particular about 8 to 10 weight %, and additives that can include potassium fluoride and calcium fluoride in the abovementioned amounts.  
         [0072]     The electrolyte can contain as further constituent(s) at least one fluoride selected from magnesium fluoride, lithium fluoride, cesium fluoride, rubidium fluoride, strontium fluoride, barium fluoride and cerium fluoride.  
         [0073]     Advantageously, The electrolyte contains alumina at a concentration near saturation on the active anode surface.  
         [0074]     In order to maintain the alumina concentration above a given threshold in the abovementioned range during normal electrolysis, the cell is preferably fitted with means to monitor and adjust the electrolyte&#39;s alumina content.  
         [0075]     The drained-cathode cell trough  30 ,  36 ,  37 ,  40 ,  41 ,  42 ,  43  shown in  FIG. 6  and suitable variations are disclosed in greater detail in the prior art, in particular in U.S. Pat. Nos. 6,682,643, 6,692,620 and 6,783,656, and in WO02/070783, WO02/070785, WO02/097168 and WO02/097169 (all assigned to MOLTECH Invent S.A.).  
         [0076]     According to the invention, the cell has anodes  5  with a foraminate anode body  10  that is suspended by four anode stems  20  distributed around a foraminate stemless central part of the anode body and that is held above the cathode  30  parallel to the drained cathode surface  31 . The anode bodies  10  are in particular of the type shown in  FIG. 1 . Anode stems  20  of each anode  5  are connected by cross-members  23  to a main current conductor  25 .  
         [0077]     Advantageously, anodes  5  have an active surface with an enhanced stability against corrosion by the highly aggressive circulating electrolyte and/or against oxidation by anodically evolved oxygen, the enhanced stability being provided by a layer that contains predominantly cobalt oxide CoO. Such a composition is particularly suitable for anodes  5  of the invention which during use are exposed to a strong central electrolyte circulation.  
         [0078]     There are several forms of stoichiometric and non-stoichiometric cobalt oxides which are based on: 
        CoO that contains Co(II) and that is formed predominantly at a temperature above 920° C. in air;     Co 2 O 3  that contains Co(III) and that is formed at temperatures up to 895° C. and at higher temperatures begins to decompose into CoO;     Co 3 O 4  that contains Co(II) and Co(III) and that is formed at temperatures between 300 and 900° C.        
 
         [0082]     It has been observed that—unlike Co 2 O 3  that is unstable and Co 3 O 4  that does not significantly inhibit oxygen diffusion—CoO forms a well conductive electrochemically active material for the oxidation of oxygen ions and for inhibiting diffusion of oxygen. Thus this material forms a limited barrier against oxidation of the metallic cobalt body underneath.  
         [0083]     The anode&#39;s CoO-containing layer can be a layer made of sintered particles, especially sintered CoO particles. Alternatively, the CoO-containing layer may be an integral oxide layer on a Co-containing metallic layer or anode core. Tests have shown that integral oxide layers have a higher density than sintered layers and are thus preferred to inhibit oxygen diffusion.  
         [0084]     When CoO is to be formed by oxidising metallic cobalt, care should be taken to carry out a treatment that will indeed result in the formation of CoO. It was found that using Co 2 O 3  or Co 3 O 4  in a known aluminium electrowinning electrolyte does not lead to an appropriate conversion of these forms of cobalt oxide into CoO. Therefore, it is important to provide an anode with the CoO layer before the anode is used in an aluminium electrowinning electrolyte.  
         [0085]     The formation of CoO on the metallic cobalt is preferably controlled so as to produce a coherent and substantially crack-free oxide layer. However, not any treatment of metallic cobalt at a temperature above 895° C. or 900° C. in an oxygen-containing atmosphere will result in optimal coherent and substantially crack-free CoO layer that offers better electrochemical properties than a Co 2 O 3 /Co 3 O 4 .  
         [0086]     For instance, if the temperature for treating the metallic cobalt to form CoO by air oxidation of metallic cobalt is increased at an insufficient rate, e.g. less than 200° C./hour, a thick oxide layer rich in Co 3 O 4  and in glassy Co 2 O 3  is formed at the surface of the metallic cobalt. Such a layer does not permit optimal formation of the CoO layer by conversion at a temperature above 895° C. of Co 2 O 3  and Co 3 O 4  into CoO. In fact, a layer of CoO resulting from such conversion is not preferred but still useful despite an increased porosity and may be cracked. Therefore, the required temperature for air oxidation, i.e. above 900° C., usually at least 920° C. or preferably above 940° C. should be attained sufficiently quickly, e.g. at a rate of increase of the temperature of at least 300° C. or 600° C. per hour to obtain an optimal CoO layer. The metallic cobalt may also be placed into an oven that is pre-heated at the desired temperature above 900° C.  
         [0087]     Likewise, if the anode is not immediately used for the electrowinning of aluminium after formation of the CoO layer but allowed to cool down, the cooling down should be carried out sufficiently fast, for example by placing the anode in air at room temperature, to avoid significant formation of Co 3 O 4  that could occur during the cooling, for instance in an oven that is switched off.  
         [0088]     Further details regarding CoO-containing anodes and cell operation therewith are disclosed in MOL0679, MOL0680, 681 and 682.  
         [0089]     An anode with a CoO layer obtained by slow heating of the metallic cobalt in an oxidising environment will not have optimal properties but still provides better results during cell operation than an anode having a Co 2 O 3 —Co 3 O 4  layer and therefore also constitutes an improved aluminium electrowinning anode according to the invention.  
         [0090]     The section of main current conductor  25  shown in  FIG. 6  corresponds to the section of prior art stems whereas the section of anode stems  20  is a fraction of the section of conductor  25 , usually about the size of the section of the conductor  25  divided by the number of anode stems  20  connecting each anode body  10 .  
         [0091]     On the left-hand side of  FIG. 6 , anode stems  20  extend through cell cover  45  and are connected by cross-members  23  above cover  45 . On the right-hand side of  FIG. 6 , anode stems  20  are connected to cross-members  23  below cover  45 . The connection can be achieved by screwing, welding or force-fitting. In a variation the anode stems are integral with cross-members.  
         [0092]     During use, dissolved alumina is electrolysed between anode bodies  10  and cathode  30  to produce aluminium  60  cathodically and oxygen anodically. The oxygen released at the anode body  10  promotes an electrolyte upflow in the direction of arrow  51  through anode body  10 . This upflow is strongest through and above the central part of anode body  10 . However, electrolyte can circulate through substantially the entire anode body  10  and is electrolysed over the entire active surface of anode body  10 . The main current conductor  25  being located above electrolyte  50 , it does not interfere with this electrolyte circulation.  
         [0093]     Alumina is fed to the electrolyte  50  vertically above the central part of each anode body  10  between anode members  20  of each anode  5  where the stirring effect of the electrolyte is highest. The alumina dissolves as it enters the electrolyte and is circulated with the electrolyte  50  to the gap spacing the anode body  10  and cathode  30  mainly around anode body  10  and is electrolysed substantially uniformly under anode body  10 .  
         [0094]      FIG. 7 , in which the same references refer to the same elements, shows a variation of the cell shown in  FIG. 6 . In  FIG. 7 , the cell operates with a shallow aluminium pool  60  on a horizontal cathode  30 .