Process for producing high density sintered alumina

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.sub.2 O content in the range of about 0.2-1.0% by weight of Al.sub.2 O.sub.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.sub.3 BO.sub.3 or B.sub.2 O.sub.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.sub.2 O.sub.3 and the amount of boron compound, calculated as B.sub.2 O.sub.3, is maintained in the range of about 0.1-1.0% by weight of Al.sub.2 O.sub.3, shaping and sintering the calcined alumina for a time period of 1 to 25 hours within the temperature range of about 1600.degree. and 1900.degree. 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.sub.2 O.sub.3.