Graphite electrode for electrothermic reduction furnaces, electrode column, and method of producing graphite electrodes

Abstract

A graphite electrode for an electrothermic reduction furnace is formed from anode grade coke and graphitized at a graphitization temperature below 2700° C. The resulting electrode is particularly suited for carbothermal reduction of alumina. It has an iron content of about 0.05% by weight, a specific electrical resistivity of above 5 μOhm·m, and a thermal conductivity of less than 150 W/m·K. The graphite electrode is manufactured by first mixing calcined anode coke with a coal-tar pitch binder, and a green electrode is formed from the mixture at a temperature close to the softening point of the pitch binder. The green electrode is then baked to carbonize the pitch binder to solid coke. The resultant carbonized electrode, after further optional processing is then graphitized at a temperature below 2700° C. for a time sufficient to cause the carbon atoms in the carbonized electrode to organize into the crystalline structure of graphite.

1 . In an electrothermic reduction furnace, a graphite electrode comprising a shaped graphite electrode body formed from anode grade coke, graphitized at a graphitization temperature below 2700° C., and having an iron content of less than 0.1% by weight.  
   
   
       2 . The graphite electrode according to  claim 1 , wherein said electrode body has a specific electrical resistivity of above 5 μOhm·m and a thermal conductivity of less than 150 W/m·K.  
   
   
       3 . The graphite electrode according to  claim 1 , wherein said electrode body has an iron content of approximately 0.05% by weight.  
   
   
       4 . The graphite electrode according to  claim 1  configured for an electrothermic reduction furnace for producing one of aluminum, titanium, silicon, ferroalloys, and phosphorous.  
   
   
       5 . The graphite electrode according to  claim 1 , which further comprises an amount of carbon nanofibers incorporated in said electrode body for increasing a mechanical strength and adjusting a coefficient of thermal expansion thereof.  
   
   
       6 . The graphite electrode according to  claim 1 , which further comprises an amount of carbon fibers incorporated in said electrode body for increasing a mechanical strength and adjusting a coefficient of thermal expansion thereof.  
   
   
       7 . The graphite electrode according to  claim 1 , wherein said anode grade coke has a mean particle size of approximately 5 to approximately 10 mm.  
   
   
       8 . The graphite electrode according to  claim 7 , wherein said mean particle size is between 5 and 7 mm.  
   
   
       9 . In a reactor for direct carbothermic reduction of alumina, the carbon electrode according to  claim 1 .  
   
   
       10 . An intermediate product in the production of a graphite electrode, comprising: particles of anode grade coke having a mean particle size of between 5 and 10 mm and an ash content of less than 0.5% mixed with a pitch binder and formed into a green electrode to be baked and graphitized to form a graphite electrode.  
   
   
       11 . In combination with the graphite electrode according to  claim 1 , a graphite pin formed of anode grade coke, graphitized at a graphitization temperature below 2700° C., having an iron content of less than 0.1% by weight, and being formed to mate with said graphite electrode body to form an electrode column.  
   
   
       12 . In a self-baking composite electrode for an electrothermic reduction furnace, the graphite electrode according to  claim 1  disposed to form a central column of the self-baking composite electrode.  
   
   
       13 . A method of producing a graphite electrode, which comprises: 
 providing calcined anode coke with an average particle size of 5 to 10 mm and mixing the anode coke with a coal-tar pitch binder to form a mixture;    forming an electrode body from the mixture to form a green electrode at a temperature in a vicinity of a softening point of the pitch binder;    baking the green electrode at a temperature of between approximately 700° C. and approximately 1100° C., to carbonize the pitch binder to solid coke, to form a carbonized electrode;    graphitizing the carbonized electrode with a heat treatment at a final temperature between 2100° C. to 2700° C. for a time sufficient to cause carbon atoms in the carbonized electrode to organize into a crystalline structure of graphite.    
   
   
       14 . The method according to  claim 13 , which comprises graphitizing at a temperature of between 2200° C. to 2500° C.  
   
   
       15 . The method according to  claim 13 , which comprises baking the green electrode at a temperature between 800° C. and 1000° C.  
   
   
       16 . The method according to  claim 13 , which comprises baking the green electrode in a relative absence of air at a heating rate of approximately 1 K to approximately 5 K per hour to the final temperature.  
   
   
       17 . The method according to  claim 13 , which comprises, after the baking, impregnating the electrode at least one time with coal tar or petroleum pitch for depositing additional pitch coke in open pores of the electrode, and following each impregnating step with an additional baking step.  
   
   
       18 . The method according to  claim 13 , which adding oils or other lubricants into the mixture and forming the green electrode by extrusion.  
   
   
       19 . The method according to  claim 13 , which comprises forming the green electrode by molding in a forming mold or by vibromolding in an agitated mold.  
   
   
       20 . The method according to  claim 13 , which comprises adding a relatively low proportion of carbon fibers or carbon nanofibers into the mixture for forming the green electrode.  
   
   
       21 . The method according to  claim 13 , which further comprises machining the graphitized electrode formed in the graphitizing step to provide a final form of the graphite electrode.  
   
   
       22 . The method according to  claim 13 , which comprises providing the calcined anode coke with an average particle size of 5 to 7 mm.  
   
   
       23 . A method of producing a graphite electrode column, which comprises producing a plurality of graphitized electrodes with the method according to  claim 13 , producing a nipple configured to mesh with the graphitized electrodes, and connecting the electrodes and the nipple to form a graphite electrode column.