Process for producing high density sintered alumina

Claim 1

A process for producing sintered alumina shapes or grains of low porosity and reduced Na content which comprises treating a calcined alumina, resulting from the calcination of a Bayer process alumina hydrate precursor, possessing a 2-6 micron average particle size and having an Na 2 O content in the range of about 0.2-1.0% by weight of Al 2 O 3 with a synergistically acting combination of a boron-free magnesium salt and a boron compound, wherein the magnesium salt is selected from the group consisting essentially of water-soluble and essentially water-insoluble magnesium salts which during sintering of the alumina shapes decompose without leaving anionic impurities in the sintered alumina shape and wherein the boron compound is H 3 BO 3 or B 2 O 5 , wherein in the combination, the quantity of magnesium salt, calculated as Mg, is kept in the range of about 0.01-0.3% by weight of Al 2 O 3 and the amount of boron compound, calculated as B 2 O 3 , is maintained in the range of about 0.1-1.0% by weight of Al 2 O 3 , shaping and sintering the calcined alumina for a time period of 1 to 25 hours within the temperature range of about 1600° and 1900° C, and recovering a sintered alumina having a porosity of less than about 3% by volume and an Na content of less than about 0.03% by weight of Al 2 O 3 .