Control of by-pass current in multi-polar light metal reduction cells

Abstract

The invention relates to a multi-polar reduction cell for production of a light metal by electrolysis. The cell has an anode, a cathode, and at least one current-conducting multi-polar electrode interposed between the anode and the cathode. The cell has a molten electrolyte containing a metal salt to be electrolyzed held within the cell and preferably has means to maintain an upper surface of the electrolyte at a predetermined level within the cell. The level is preferably maintained above the upper end of the multi-polar electrode(s), at least in use of the cell. The multi-polar electrode has an electrically insulating surface at its upper end that minimizes or eliminates by-pass current between the anode and cathode when the cell is operated. The invention also relates to the method of minimizing or eliminating the by-pass current.

1 . A multi-polar electrolytic cell for producing a light metal by electrolysis of a corresponding metal salt, the cell comprising:
 a molten electrolyte containing a metal salt that produces a light metal and a gas when electrolyzed; and   an arrangement of generally vertical electrodes surrounded by said molten electrolyte, including an anode, a cathode and at least one current-conducting multi-polar electrode interposed between the anode and the cathode, said at least one multi-polar electrode having an upper end;   wherein said at least one multi-polar electrode has an electrical insulator positioned to extend at least partially over said upper end, and wherein, in use of said cell, said insulator is immersed beneath said electrolyte.   
   
   
       2 . The cell of  claim 1 , wherein said insulator is attached to said multi-polar electrode. 
   
   
       3 . The cell of  claim 2 , wherein said insulator is attached to said multi-polar electrode by a fastening means selected from the group consisting of pins, dovetails, interposed members, and adhesives. 
   
   
       4 . The cell of  claim 1 , wherein said insulator has a width between 0.1 and 1.5 times the width of the multi-polar electrode, a length substantially equal to the length of the multi-polar electrode and a height of 1 to 20 times an electrode gap between adjacent electrodes. 
   
   
       5 . The cell of  claim 1 , wherein said insulator has a width between 0.5 and 1.0 times the width of the multi-polar electrode, a length substantially equal to the length of the multi-polar electrode and a height of 5 to 10 times an electrode gap between adjacent electrodes. 
   
   
       6 . The cell of  claim 1 , wherein an insulating refractory shield is provided on the anode at a position confronting said upper end of an adjacent multi-polar electrode. 
   
   
       7 . The cell of  claim 1 , wherein the anode, cathode and said at least one multi-polar electrode are planar and are arranged parallel to each other. 
   
   
       8 . The cell of  claim 1 , wherein the cathode and said at least one multi-polar electrode each form a continuous body surrounding the anode. 
   
   
       9 . The cell of  claim 1 , wherein the cathode and said at least one multi-polar electrode are in the form of hollow cylinders surrounding the anode. 
   
   
       10 . The cell of  claim 1 , wherein said insulator is made of a material selected from the group consisting of alumina, magnesia, Mg-aluminate spinel, aluminum nitride, silicon nitride and SIALON. 
   
   
       11 . A method of minimizing or eliminating by-pass current between an anode and a cathode in a multi-polar electrolysis cell suitable for production of a light metal, said method comprising:
 electrically insulating an upper end of at least one multi-polar electrode of said cell, and   conducting electrolysis with said insulated upper end maintained below an upper surface of molten electrolyte containing a metal salt to be electrolyzed held within said cell.   
   
   
       12 . A multi-polar electrolytic cell for producing a light metal by electrolysis of a corresponding metal salt, the cell comprising:
 a molten electrolyte containing a metal salt that produces a light metal and a gas when electrolyzed; and   an arrangement of generally vertical electrodes surrounded by said molten electrolyte, including an anode, a cathode and a current-conducting multi-polar electrode interposed between the anode and the cathode, said multi-polar electrode having an upper end and an electrical insulator positioned to extend over said upper end;   wherein, in use of said cell, said insulator is immersed beneath said electrolyte.   
   
   
       13 . The cell of  claim 12 , having at least one other multi-polar electrode interposed between said anode and said cathode. 
   
   
       14 . The cell of  claim 13 , wherein said at least one other multi-polar electrode is provided with an electrical insulator positioned over an upper end thereof. 
   
   
       15 . The cell of  claim 14 , wherein said insulators positioned above said multi-polar electrode and said at least one other multi-polar electrode are interconnected by a spacer made of refractory material. 
   
   
       16 . The cell of  claim 12 , wherein said insulator is attached to said multi-polar electrode. 
   
   
       17 . The cell of  claim 16 , wherein said insulator is attached to said multi-polar electrode by a fastening means selected from the group consisting of pins, dovetails, interposed members, and adhesive. 
   
   
       18 . The cell of  claim 12 , wherein said insulator has a width between 0.1 and 1.5 times the width of the multi-polar electrode, a length substantially equal to the length of the multi-polar electrode and a height of 5.0 to 10.0 times said width of an electrode gap between adjacent electrodes. 
   
   
       19 . The cell of  claim 12 , wherein said insulator has a width between 0.5 and 1.0 times the width of the multi-polar electrode, a length substantially equal to the length of the multi-polar electrode and a height of 1.0 to 10 times an electrode gap between adjacent electrodes. 
   
   
       20 . The cell of  claim 12 , wherein an insulating refractory shield is provided on the anode at a position confronting said upper end of the multi-polar electrode.