Spherical balls of alpha-alumina (corundum) is produced by sintering ball-formed calcined alumina at a temperature just under the 3704°F (2040°C) fusion point of aluminum oxide. These tabular alumina balls are then crushed, graded or screened, and ground to a wide range of granule. The powder size must be controlled in good condition, meanwhile adding special film formation additive . Otherwise, the corundum gravel can not absorb the particle from liquid metal。 There are four kinds of filtration media: . 1XXX，3XXX，8XXX；2XXX，7XXX；5XXX，6XXX；High purity aluminium. The Film formation additive is different.