Temperature control for low temperature reduction cell

Abstract

An improved method of producing aluminum in an electrolytic cell containing alumina dissolved in an electrolyte, the method comprising the steps of providing a molten salt electrolyte at a temperature less than 900° C. having alumina dissolved therein in an electrolytic cell having a liner for containing the electrolyte, the liner having a bottom and walls extending upwardly from the bottom, the liner being substantially inert with respect to the molten electrolyte. A plurality of non-consumable anodes and cathodes are disposed in the electrolyte and an electric current is passed through the anodes and through the electrolyte to the cathodes depositing aluminum on the cathodes and generating oxygen bubbles at the anodes, the bubbles stirring the electrolyte. Periodically, the electric current flow to the cell is reduced for extended periods. The electrolyte and aluminum in the cell is maintained in a molten condition during the extended periods of reduced current flow by application of heat to the bottom for purposes of heating the cell.

What is claimed is:  
     
         1 . An improved method of producing aluminum in an electrolytic cell containing alumina dissolved in an electrolyte, the method comprising the steps of: 
 (a) providing a molten salt electrolyte at a temperature less than 900° C. having alumina dissolved therein in an electrolytic cell having a liner for containing said electrolyte, said liner having a bottom and walls extending upwardly from said bottom, said liner being substantially inert with respect to said molten electrolyte;    (b) providing a plurality of non-consumable anodes and cathodes disposed in said electrolyte;    (c) passing an electric current through said anodes and through said electrolyte to said cathodes, depositing aluminum on said cathodes, and generating oxygen bubbles at the anodes, said bubbles stirring said electrolyte;    (d) periodically reducing electric current flow to said cell for extended periods; and    (e) maintaining said electrolyte and aluminum in said cell in a molten condition during said extended periods of reduced current flow by application of heat to said bottom for purposes of heating said cell.    
     
     
         2 . The method in accordance with  claim 1  including operating said cell to maintain said electrolyte in a temperature range of about 660° to 800° C.  
     
     
         3 . The method in accordance with  claim 1  including using an electrolyte comprised of one or more alkali metal fluorides.  
     
     
         4 . The method in accordance with  claim 1  including maintaining 0.2 to 30 wt. % undissolved alumina particles in said electrolyte to provide a slurry therein.  
     
     
         5 . The method in accordance with  claim 2  wherein undissolved alumina has a particle size in the range of 1 to 100 μm.  
     
     
         6 . The method in accordance with  claim 1  wherein said anodes and anodic liner are comprised of an Ni—Cu—Fe alloy.  
     
     
         7 . The method in accordance with  claim 1  including passing an electric current through said cell at a current density in the range of 0.1 to 1.5 A/cm 2 .  
     
     
         8 . The method in accordance with  claim 1  including using cathodes selected from the group consisting of titanium diboride, zirconium boride, titanium carbide and zirconium carbide.  
     
     
         9 . The method in accordance with  claim 1  including providing said anodes and said cathodes substantially vertical in said electrolyte and arranging said anodes and said cathodes in alternating relationship.  
     
     
         10 . A method of efficiently operating a low temperature cell for the electrolytic production of aluminum from alumina dissolved in a molten salt electrolyte in a manner which is regulated to consume electrical power in a more cost-effective basis, the method comprising the steps of: 
 (a) providing a molten salt electrolyte at a temperature less than 900° C. having alumina dissolved therein in an electrolytic cell having a metallic liner for containing said electrolyte, said liner having a bottom and walls extending upwardly from said bottom, said liner being substantially inert with respect to said molten electrolyte;    (b) providing a plurality of non-consumable anodes disposed substantially vertically in said electrolyte and a plurality of cathodes disposed vertically in said electrolyte, said anodes and said cathodes arranged in alternating relationship;    (c) passing an electric current through said anodes and through said electrolyte to said cathodes, depositing aluminum on said cathodes, and generating oxygen bubbles at the anodes, said bubbles stirring said electrolyte;    (d) periodically reducing electric current flow to said cell for extended periods; and    (e) maintaining said electrolyte and aluminum in said cell in a molten condition during said extended periods of reduced current flow by application of heat to said bottom for purposes of heating said cell.    
     
     
         11 . The method in accordance with  claim 10  including operating said cell to maintain said electrolyte in a temperature range of about 660° to 800° C.  
     
     
         12 . The method in accordance with  claim 10  including using an electrolyte comprised of one or more alkali metal fluorides.  
     
     
         13 . The method in accordance with  claim 10  including maintaining 0.2 to 30 wt. % undissolved alumina particles in said electrolyte to provide a slurry therein.  
     
     
         14 . The method in accordance with  claim 11  wherein undissolved alumina has a particle size in the range of 1 to 100 μm.  
     
     
         15 . The method in accordance with  claim 10  wherein said anodes and anodic liner are comprised of an Ni—Cu—Fe alloy.  
     
     
         16 . The method in accordance with  claim 10  including passing an electric current through said cell at a current density in the range of 0.1 to 1.5 A/cm 2 .  
     
     
         17 . The method in accordance with  claim 10  including using cathodes selected from the group consisting of titanium diboride, zirconium boride, titanium carbide and zirconium carbide.  
     
     
         18 . An improved method of producing aluminum in an electrolytic cell containing alumina dissolved in an electrolyte, the method comprising the steps of: 
 (a) providing a molten salt electrolyte at a temperature less than 900° C. having alumina dissolved therein in an electrolytic cell having a metallic liner for containing said electrolyte, said liner having a bottom having an outside surface and having walls extending upwardly from said bottom, said liner being substantially inert with respect to said molten electrolyte;    (b) providing a plurality of non-consumable anodes and cathodes disposed in said electrolyte;    (c) passing an electric current through said anodes and through said electrolyte to said cathodes, depositing aluminum on said cathodes, and generating oxygen bubbles at the anodes, said bubbles stirring said electrolyte;    (d) removing heat from said cell through said bottom of said liner by passing an air sweep from outside said cell over said outside surface of said bottom to remove heat from said bottom to provide a heated air sweep; and    (e) discharging said heated air sweep to the atmosphere outside said cell thereby maintaining said cell at said temperature.    
     
     
         19 . The method in accordance with  claim 18  including operating said cell to maintain said electrolyte in a temperature range of about 660° to 800° C.  
     
     
         20 . The method in accordance with  claim 18  including using an electrolyte comprised of one or more alkali metal fluorides.  
     
     
         21 . The method in accordance with  claim 18  including maintaining 0.2 to 30 wt. % undissolved alumina particles in said electrolyte to provide a slurry therein.  
     
     
         22 . The method in accordance with  claim 19  wherein undissolved alumina has a particle size in the range of 1 to 100 μm.  
     
     
         23 . The method in accordance with  claim 18  wherein said anodes are comprised of an Ni—Cu—Fe alloy.  
     
     
         24 . The method in accordance with  claim 18  including passing an electric current through said cell at a current density in the range of 0.1 to 1.5 A/cm 2 .  
     
     
         25 . The method in accordance with  claim 18  including using cathodes selected from the group consisting of titanium diboride, zirconium boride, titanium carbide and zirconium carbide.  
     
     
         26 . The method in accordance with  claim 18  including providing said anodes and said cathodes substantially vertical in said electrolyte and arranging said anodes and said cathodes in alternating relationship.  
     
     
         27 . A method of efficiently operating a low temperature cell for the electrolytic production of aluminum from alumina dissolved in a molten salt electrolyte in the method comprising the steps of: 
 (a) providing a molten salt electrolyte at a temperature less than 900° C. having alumina dissolved therein in an electrolytic cell having a liner for containing said electrolyte, said liner having a bottom having an outside surface and have walls extending upwardly from said bottom, said liner being substantially inert with respect to said molten electrolyte;    (b) providing a plurality of non-consumable anodes disposed substantially vertically in said electrolyte and a plurality of cathodes disposed vertically in said electrolyte, said anodes and said cathodes arranged in alternating relationship;    (c) passing an electric current through said anodes and through said electrolyte to said cathodes, depositing aluminum on said cathodes, and generating oxygen bubbles at the anodes, said bubbles stirring said electrolyte;    (d) removing heat from said cell through said bottom of said liner by passing an air sweep over said outside surface of said bottom to provide a heated air sweep;    (e) discharging said heated air sweep outside said cell;    (f) sensing the temperature of said electrolyte to provide a reading;    (g) relaying said reading to a controller;    (h) in said controller, comparing said reading to a set reading to provide a comparison; and    (i) in response to said comparison, increasing, decreasing or maintaining air flow rate in said air sweep to maintain said cell at temperature.    
     
     
         28 . The method in accordance with  claim 27  including operating said cell to maintain said electrolyte in a temperature range of about 660° to 800° C.  
     
     
         29 . The method in accordance with  claim 27  including using an electrolyte comprised of one or more alkali metal fluorides.  
     
     
         30 . The method in accordance with  claim 27  including maintaining 0.2 to 30 wt. % undissolved alumina particles in said electrolyte to provide a slurry therein.  
     
     
         31 . The method in accordance with  claim 28  wherein undissolved alumina has a particle size in the range of 1 to 100 μm.  
     
     
         32 . The method in accordance with  claim 27  wherein said anodes and anodic liner are comprised of an Ni—Cu—Fe alloy.  
     
     
         33 . The method in accordance with  claim 27  including passing an electric current through said cell at a current density in the range of 0.1 to 1.5 A/cm 2 .  
     
     
         34 . The method in accordance with  claim 27  including using cathodes selected from the group consisting of titanium diboride, zirconium boride, titanium carbide and zirconium carbide.  
     
     
         35 . An improved method for startup of a low temperature, electrolytic cell for producing aluminum from alumina dissolved in an electrolyte at less than 900° C., the method comprising the steps of: 
 (a) providing an electrolytic cell having a metal liner for containing electrolyte, said liner having a bottom having an outside surface and having walls extending upwardly from said bottom;  
 (b) providing a plurality of non-consumable anodes and cathodes disposed in said electrolyte;  
 (c) adding solid electrolyte and alumina to said cell;  
 (d) placing at least one heater adjacent said outside surface of said bottom;  
 (e) adding heat to said bottom until said solid electrolyte is melted; and  
 (f) when said electrolyte is in molten form, passing an electric current through said anodes and through said electrolyte to said cathodes, thereby depositing aluminum at said cathodes and generating oxygen bubbles at the anodes.  
 
     
     
         36 . The method in accordance with  claim 35  including adding heat until said electrolyte is in a temperature range of 660° to 800° C.  
     
     
         37 . The method in accordance with  claim 35  including using an electrolyte comprised of one or more alkali metal fluorides.  
     
     
         38 . The method in accordance with  claim 35  including maintaining 0.2 to 30 wt. % undissolved alumina particles in said electrolyte to provide a slurry therein.  
     
     
         39 . The method in accordance with  claim 2  wherein undissolved alumina has a particle size in the range of 1 to 100 μm.  
     
     
         40 . The method in accordance with  claim 35  wherein said anodes and metal liner are comprised of an Ni—Cu—Fe alloy.  
     
     
         41 . The method in accordance with  claim 35  including passing an electric current through said cell at a current density in the range of 0.1 to 1.5 A/cm 2 .  
     
     
         42 . The method in accordance with  claim 35  including using cathodes selected from the group consisting of titanium diboride, zirconium boride, titanium carbide and zirconium carbide.  
     
     
         43 . The method in accordance with  claim 35  including providing said anodes and said cathodes substantially vertical in said electrolyte and arranging said anodes and said cathodes in alternating relationship.