Aluminas are used in the surface polishing field of application either as trimming agents (measured by the “Cut” effect) or as polishing agents (measured by the “Polish” effect). Unground calcined aluminas that have a hardness index of 9 on the Mohs scale deburr the parts that are to be polished.
Alternatively, low calcined aluminas that are finely ground have an excellent polishing power but little deburring action.
Also fused aluminas can be used for high stock removal or lapping on materials like glass.
Aluminas with intermediate to relatively high specific surface areas (BET 3- 75m²/g) with alpha alumina contents varying between 20% and 95%.These are produced by controlled rotary calcination of aluminium trihydroxide.
Ground calcined alumina
These aluminas are ground in continuous ball-mills or air-jet mills to median particle sizes (d50) 3, 4, 5 or 6 µm.
Hard calcined alumina
These aluminas have low specific surface area (BET < 1m²/g) and are almost entirely alpha alumina. Standard soda content is 3000-4000 ppm and intermediate soda content is 2000-3000 ppm. Hard calcined aluminas are produced in rotary kilns.