The hard diasporic bauxites are difficult to grind and difficult to digest during alumina production. The economy of their processing in this way is predominantly effected by the grinding, which is in a strong relationship with the chemical digestion An ongoing systematical research programme at the University of Miskolc in cooperation with S&B aims at developing more precise design of ball mills and comminution process for grinding of bauxites, decreasing operation cots. The grindability tests were carried out in Bond and Hardgrove mills. In both tests. the grindability indices were determined in dry, aqueous. alkaline (NaOH) media, as well as after high pressure comminution. The effect of additives (CaO) was also investigated. (C) 2004 Elsevier B.V. All rights reserved.