Prebake anodes for electrolytic production of aluminum and coating therefor

Claim 1

IN THE ELECTROLYTIC PROCESS FOR THE MANUFACTURE OF ALUMINUM WHEREIN AN ANODE IS IMMERSED IN AN ELECTROLYTE BATH COMPRISING MOLTEN CRYOLITE (ALF3.3NAF) CONTAINING ALUMINA (AL2O3) DISSOLVED THEREIN AND WHEREIN CURRENT IS PASSED THROUGH SAID BATH BETWEEN AN ANODE AND A CATHODE IN CONTACT WITH SAID BATH WITH THE RESULTANT PRODUCTION OF MOLTEN, ELEMENTAL ALUMINUM AT THE CATHODE, THE IMPROVEMENT WHICH COMPRISES EMPLOYING AS THE ANODE IN SAID PROCESS A MASS OF COHERENT, SUBSTANTIALLY SOLID, NON-POROUS CARBON, A METALLIC ELECTRICAL CONDUCTOR IMBEDDED IN SAID MASS OF CARB ON AND A COATING PROVIDED ON THE OUTSIDE OF SAID MASS OF CARBON, SAID COATING CONSISTING ESSENTIALLY OF A MAJOR AMOUNT BY WEIGHT OF SYNTHETIC CRYOLITE (ALF3.3NAF) HAVING AN EXCESS OF ALUMINUM FLUORIDE (ALF3) THEREIN TO YIELD A WEIGHT RATIO OF SODIUM FLUORIDE (NAF) TO ALUMINUM FLUORIDE THEREIN IN THE RANGE FROM ABOUT 0.6 TO ABOUT 1.2, A MINOR AMOUNT BY WEIGHT OF ALUMINA (AL2O3) AND NOT MORE THAN ABOUT 1% WEIGHT CALCIUM FLUORIDE, SAID COATING BEING IMPERMEABLE AND HAVING A MELTING POINT IN THE RANGE FROM ABOUT 700*C. TO ABOUT 1000*C.