Aluminium Electrowinning With Enhanced Electrolyte Circulation

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.

1 . A method of operating an aluminium electrowinning cell, the cell having 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, said method comprising 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 gas promoting an electrolyte circulation through the or each foraminate anode body to facilitate dissolution of alumina fed thereabove, 
 wherein 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, said stems extending from the anode body to above the molten electrolyte, electrolyte flowing up through and above said foraminate central part to enhance dissolution of alumina.    
   
   
       2 . The method of  claim 1 , wherein alumina is supplied to the electrolyte between the stems of at least one anode vertically above the central part of the anode body.  
   
   
       3 . The method of  claim 1  or  2 , comprising draining molten aluminium produced on the cathode.  
   
   
       4 . 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, said cell being 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, 
 wherein 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, said stems extending from the anode body to above the molten electrolyte so as to permit an upflow of electrolyte through and above said foraminate central part to enhance dissolution of alumina.    
   
   
       5 . The cell of  claim 4 , wherein the anode stems of one anode are connected together by cross-members.  
   
   
       6 . The cell of  claim 5 , wherein the anode stems of one anode are connected together by cross-members above the insulating cover.  
   
   
       7 . The cell of  claim 5 , wherein the anode stems of one anode are connected together by cross-members below the insulating cover.  
   
   
       8 . The cell of  claim 5 ,  6  or  7 , wherein the cross-members are joined to a main current conductor that is connected to an anode bus bar.  
   
   
       9 . The cell of  claim 5 , wherein the molten electrolyte is substantially free of any frozen crust.  
   
   
       10 . An aluminium electrowinning metal-based anode for use in a cell as defined in any one of  claims 4  to  9  that comprises a metal-based foraminate anode body and metal-based anode stems which are connected to the anode body, 
 wherein the or each anode has a foraminate anode body connected 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, said stems extending during use from the anode body to above the molten electrolyte so as to permit an upflow of electrolyte through and above said foraminate central part to enhance dissolution of alumina.    
   
   
       11 . The anode of  claim 10 , wherein the anode body has a grid-like or plate-like foraminate structure that is parallel to the facing cathode.  
   
   
       12 . The anode of  claim 10  or  11 , wherein the anode body has an upper face to which the stems are connected around a central point of the upper surface, each anode stem being 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.  
   
   
       13 . The anode of any one of  claims 10  to  12 , wherein the anode body has a square or rectangular upper face.  
   
   
       14 . The anode of  claim 13 , wherein the anode body is suspended by four anode stems.  
   
   
       15 . The anode of  claim 14 , wherein said stems are located substantially on crossing diagonals of the body's upper face, each stem being located about half way between a corner of the body's face and the crossing point of the diagonals.  
   
   
       16 . The method of  claim 14 , wherein said four stems are located substantially on two crossing perpendicular median lines of the body's upper face, each stem being connected about half way between a side of the body's face and the crossing point of the median lines.  
   
   
       17 . The anode of any one of  claims 10  to  12 , wherein the anode body has a circular upper face.  
   
   
       18 . The anode of  claim 17 , wherein each stem is located substantially in the middle of a radius of the circular upper face, the stems being evenly distributed on the circular upper face around the body's central part.  
   
   
       19 . The anode of  claim 17  or  18 , which comprises four anode stems.  
   
   
       20 . The anode of any one of  claims 10  to  19 , wherein the anode stems have ends away from the anode body that are connected together by cross-members.  
   
   
       21 . The anode of  claim 10 , wherein the anode has pairs of opposite stems that are connected by intercepting cross-members.  
   
   
       22 . The anode of  claim 20  or  21 , wherein the cross-members are joined to a main current conductor for connection to a busbar.  
   
   
       23 . The anode of any one of  claims 10  to  22 , wherein the anode stems have a transverse cross-sectional area that is sufficient for passing a current that leads to a current density in the range of 0.5 to 1.5 A/cm 2  at the surface of the anode with a voltage drop along the anode stems below 80 mV/cm, in particular in the range of 20 to 50 mV/cm.  
   
   
       24 . The anode of any one of  claims 10  to  23 , wherein the anode body has an active surface that has total projected surface area AA and wherein 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%.  
   
   
       25 . The anode of any one of  claims 10  to  24 , wherein each anode stem has a diameter in the range of 2 to 8 cm, in particular 2.5 to 6 cm, such as 3 to 4 cm.  
   
   
       26 . The anode of any one of  claims 10  to  25 , wherein the anode body has an active face that 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 .