Spherical corundum particles, process for preparation thereof and rubber or plastic composition having high thermal conductivity and having spherical corundum paticles incorporated therein

A process for the preparation of corundum particles without cutting edges wherein the process comprises adding 0.1 to 4.0 parts by weight of one or more members selected from the group consisting of halogen compounds and boron compounds to 100 parts by weight of a pulverized product of at least one of electrofused alumina and sintered alumina, which has a maximum particle size smaller than 150 .mu.m and an average particle size of 5 to 35 .mu.m as single particles;

• heat treating the mixture at a temperature of at least 1000.degree. C.;
• and disintegrating the heat-treated product.

A process for the preparation of corundum particles without cutting edges, wherein the process comprises adding one or more members selected from the group consisting of halogen compounds and boron compounds to 100 parts by weight of a pulverized product of at least one of electrofused alumina and sintered alumina and 5 to 100 parts by weight of aluminum hydroxide, the amount of said one or more members of the halogen and boron compounds being 0.1 to 4.0 parts by weight based on 100 parts by weight of the total amount of the alumina and aluminum hydroxide, the alumina having a maximum particle size smaller than 150 .mu.m and an average particle size of 5 to 35 .mu.m as single particles;

• heat treating the mixture at a temperature of at least 1000.degree. C.;
• and disintegrating the heat-treated product.