Fracture resistant electrodes for a carbothermic reduction furnace

Abstract

Graphite electrodes for the production of aluminum by carbothermic reduction of alumina are either submerged in the molten bath in the low temperature compartment or they are horizontally arranged in the side walls of the high temperature compartment. The electrodes are manufactured by using a mixture of coke particles covering the complete particle size range between 25 μm to 3 mm and by using an intensive mixer to effectively wet all coke particles with pitch. The electrodes have a flexural strength of at least 20 N/mm 2 . By using a complete range (continuum) of particle sizes in conjunction with an intensive mixer, the geometric packing of the particles is significantly improved, hence the material density is increased and thus a higher mechanical strength as well as improved electrical conductivity in comparison to conventional graphite electrodes is achieved.

1 . In a carbothermic reduction furnace, a graphite electrode, comprising: 
 a shaped graphite electrode body formed of coke particles having a substantially continuous particle size distribution of from 25 μm to 3 mm, in a matrix of carbonized coal-tar pitch binder, and graphitized to form a graphite electrode body.    
   
   
       2 . The graphite electrode according to  claim 1 , wherein said coke particles are anode grade particles with an iron content of less than 0.1% by weight, and the electrode body is graphitized at a final graphitization temperature of below 2700° C.  
   
   
       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 , 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.  
   
   
       5 . 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.  
   
   
       6 . In a reactor for direct carbothermic reduction of alumina, the carbon electrode according to  claim 1 .  
   
   
       7 . An intermediate product in the production of a graphite electrode, comprising: particles of coke having a particle size with a substantially Gaussian distribution in a range from 25 μm and 3 mm mixed with a pitch binder and formed into a green electrode to be baked and graphitized to form a graphite electrode.  
   
   
       8 . The intermediate product according to  claim 7 , wherein the pitch binder is approximately 15% by weight of the green electrode.  
   
   
       9 . A method of producing a graphite electrode, which comprises: 
 milling coke particles to a continuous distribution of particle sizes from substantially 25 μm to substantially 3 mm, and mixing the coke particles with a coal-tar pitch binder to form a mixture;    forming an electrode body from the mixture to form a green electrode;    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 for a time sufficient to cause carbon atoms in the carbonized electrode to organize into a crystalline structure of graphite; and    machining the graphitized electrode into a final electrode shape.    
   
   
       10 . The method according to  claim 9 , wherein the milling step comprises screening a batch of coke particles into a coarse-grain fraction and a fine-grain fraction, separately milling the coarse-grain fraction and the fine-grain fraction, and subsequently joining the milled fractions to a coke particle batch having a Gaussian distribution of particles sizes.  
   
   
       11 . The method according to  claim 10 , which comprises milling the coarse-grain fraction into particles having a particle distribution of from 200 μm to 3 mm and milling the fine-grain fraction into particles having a particle distribution of from 25 μm to 300 μm.  
   
   
       12 . The method according to  claim 9 , which comprises providing the coke in the form of anode grade coke, and graphitizing the electrode at a graphitizing temperature of up to 2700° C.  
   
   
       13 . The method according to  claim 12 , which comprises graphitizing at a temperature of between 2200° C. to 2500° C.  
   
   
       14 . The method according to  claim 9 , which comprises providing the coke in the form of needle coke, and graphitizing the electrode at a graphitizing temperature of between 2700° C. and 3200° C.  
   
   
       15 . The method according to  claim 9 , which comprises, after the baking step, 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.  
   
   
       16 . The method according to  claim 9 , which adding oils or other lubricants into the mixture and forming the green electrode by extrusion.  
   
   
       17 . The method according to  claim 9 , which comprises forming the green electrode by molding in a forming mold or by vibromolding in an agitated mold.  
   
   
       18 . The method according to  claim 9 , which comprises adding a relatively low proportion of carbon fibers or carbon nanofibers into the mixture for forming the green electrode.  
   
   
       19 . A method of producing a graphite electrode column, which comprises producing a plurality of graphitized electrodes with the method according to  claim 9 , producing a nipple configured to mesh with the graphitized electrodes, and connecting the electrodes and the nipple to form a graphite electrode column.