Process for making sintered abrasive grains

Claim 1

A PROCESS FOR MAKING SINTERED ABRASIVE GRAINS OF THEORETICAL DENSITY, BY COMMINUTING A BASE REFRACTORY MATERIAL SELECTED FROM THE GROUP CONSISTING OF BAUXITE, SPINEL, ALPHAALUMINA, ZIRCONIA AND MIXTURES THEREOF TO A POWDER HAVING AN AVERAGE PARTICLE SIZE OF LESS THAN 1 MICRON, PREFORMING SAID FINE POWDER TO THE APPROXIMATE SIZE AND SHAPE OF THE ABRASIVE GRAINS DESIRED, AND SINTERING SAID FINE POWDER MAKING UP SAID PREFORMED ABRASIVE GRAINS, WHEREIN THE IMPROVEMENT COMPRISES, ACCOMPLISHING SAID SINTERING STEP BY: (A) MIXING SAID PREFORMED ABRASIVE GRAINS WITH A QUANTITY OF POWDERED GRAPHITE ISOSTATIC PRESSING MEDIUM SUFFICIENT TO TOTALLY SURROUND AND SEPARATE EACH OF SAID ABRASIVE GRAINS, SAID ISOSTATIC PRESSING MEDIUM BEING ESSENTIALLY NON-REACTIVE WITH SAID REFRACTORY MATERIAL, AND HAVING A SINTERING TEMPERATURE ABOVE THE SINTERING TEMPERATURE OF SAID REFRACTORY MATERIAL, (B) SUBJECTING THE MIXTURE OF SAID PREFORMED ABRASIVE GRAINS AND SAID ISOSTATIC PRESSING MEDIUM TO A PRESSING TEMPERATURE BETWEEN 1200* AND 1600*C IN A NON-OXIDIZING ATMOSPHERE, AT A PRESSURE OF NOT LESS TAN 2000 POUNDS PER SQUARE INCH SUFFICIET TO CAUSE ESSENTIALLY COMPLETE DENSIFICATION OF SAID PREFORMED ABRASIVE GRAINS, FOR A TIME PERIOD SUFFICIETLY SHORT TO PREVENT ANY