Aluminium collection in electrowinning cells

Abstract

A cell for the electrowinning of aluminium comprises an electrolysis chamber ( 20 ) in which alumina is electrolysed to produce aluminium ( 30 ) and a collection reservoir ( 40,40′ ) in which product aluminium is collected. The electrolysis chamber and the collection reservoir are in liquid communication so that aluminium produced in the electrolysis chamber can flow from the electrolysis chamber into the collection reservoir. The electrolysis chamber contains one or more metal-based anodes ( 15 ). Each anode has an active anodic surface ( 16 ) spaced above a facing cathodic surface ( 31 ) on which aluminium is produced. The cathodic surface is formed on a structural body ( 12 ) by a layer made of molten aluminium into which product aluminium is incorporated during operation. The anodic surface and the cathodic surface have a substantially constant operative position. The cell has means ( 60, 60′, 61, 61′, 62 ) for regulating the layer of molten aluminium so the layer forms a shallow or deep continuous cathodic pool ( 35 ) that extends continuously under the entire facing active anodic surface of at least one anode. The layer regulating means are arranged to maintain during operation the cathodic surface of the cathodic pool at a substantially constant position by periodic or continuous removal of molten aluminium from the aluminium pool to the collection reservoir at a rate corresponding substantially to the rate of production of the product aluminium that is incorporated into the aluminium pool.

1 . A cell for the electrowinning of aluminium from alumina dissolved in a fluoride-containing molten electrolyte, comprising an electrolysis chamber in which alumina is electrolysed to produce aluminium and a collection reservoir in which product aluminium is collected, the electrolysis chamber and the collection reservoir being in liquid communication so that aluminium produced in the electrolysis chamber can flow from the electrolysis chamber into the collection reservoir, the electrolysis chamber containing one or more metal-based anodes, each anode having an active anodic surface spaced by an anode-cathode gap from a facing cathodic surface on which aluminium is produced, said cathodic surface being formed on a structural body by a layer made of molten aluminium into which product aluminium is incorporated during operation, said anodic surface and said cathodic surface having a substantially constant operative position so that the anode-cathode gap is substantially constant, characterised in that the cell comprises means for regulating said layer of molten aluminium so the layer forms a shallow or deep continuous cathodic pool that extends continuously under the entire facing active anodic surface of at least one anode and that is contained within a cathodic cavity on the structural cathodic body, the layer regulating means being arranged to maintain during operation the cathodic surface of said cathodic pool at a substantially constant position by periodic or continuous removal of molten aluminium from the aluminium pool to the collection reservoir at a rate corresponding substantially to the rate of production of the product aluminium that is incorporated into the aluminium pool. 
     
     
         2 . The cell of  claim 1 , wherein the layer regulating means comprise a wall which delimits the cathodic cavity, the wall having an upper edge located at the level of said cathodic surface of the aluminium pool and arranged to allow the flow over the edge of product aluminium to the collection reservoir that extends below the cathodic surface. 
     
     
         3 . The cell of  claim 2 , wherein said collection reservoir has a generally U-shaped cross-section. 
     
     
         4 . The cell of  claim 2 , wherein said collection reservoir has a bottom part that extends underneath the cathodic surface. 
     
     
         5 . The cell of  claim 1 , comprising a reservoir chamber that contains said collection reservoir and that is adjacent to the electrolysis chamber and communicates therewith through an aluminium passage for the removal of molten aluminium from the aluminium pool to the collection reservoir, said layer regulating means comprising means for adjusting the amount of aluminium in the collection reservoir so that the position of the cathodic surface of the aluminium pool remains substantially constant during operation. 
     
     
         6 . The cell of  claim 5 , wherein the layer regulating means comprise means for measuring directly or indirectly the position of the cathodic surface. 
     
     
         7 . The cell of  claim 6 , wherein the measuring means comprise means for measuring the electrical characteristics of a current passing between the cathodic surface and at least one metal-based anode. 
     
     
         8 . The cell of  claim 6 , wherein the measuring means comprise one or more proximity sensors above the electrolyte for measuring the height of the electrolyte and/or aluminium pool. 
     
     
         9 . The cell of  claim 5 , wherein the layer regulating means comprise a means to adjust the pressure above aluminium in the collection reservoir. 
     
     
         10 . The cell of  claim 5 , wherein the molten aluminium in the collection reservoir is covered by a layer or molten electrolyte. 
     
     
         11 . The cell of  claim 1 , wherein the cathodic pool has a depth in the range of 1 to 15 cm. 
     
     
         12 . The cell of  claim 11 , wherein the cathodic pool has a depth in the range of 2 to 5 cm. 
     
     
         13 . A method producing aluminium in a cell that comprises:
 an electrolysis chamber containing one or more metal-based anodes in a molten electrolyte having alumina dissolved therein, each anode comprising an active anodic surface spaced by an anode-cathode gap from a facing cathodic surface that is formed on a structural body by a molten aluminium layer, said anodic surface and said cathodic surface having a substantially constant operative position so that the anode-cathode gap is substantially constant; and   a collection reservoir that is in liquid communication with the electrolysis chamber,   
       said method comprising electrolysing dissolved alumina in the anode-cathode gap to evolve gas anodically and produce aluminium cathodically, product aluminium being incorporated into said aluminium layer and flowing from the electrolysis chamber into the collection reservoir, 
       said method being characterised in that molten aluminium layer is regulated to form a shallow or deep continuous cathodic pool that extends continuously under the entire facing active anodic surface of at least one anode and that is contained within a cathodic cavity on the structural cathodic body, the cathodic surface of the cathodic pool being maintained at a substantially constant position by periodic or continuous removal of molten aluminium from the aluminium pool to the collection reservoir at a rate corresponding substantially to the rate of production of the product aluminium that is incorporated into the aluminium pool. 
     
     
         14 . The method of  claim 13 , comprising maintaining the electrolyte at a temperature in the range of 700° to 1000° C., 800 to 970° C., or 880 to 940° C. 
     
     
         15 . The method of  claim 13  or  14 , comprising maintaining the aluminium in the collection reservoir in a molten state in particular above 700° C.