split into 20 kg subsamples which were kept in nitrogen to avoid superficial oxidation (Gonçalves, 2002). the primary grinding was done in a cylindrical rod mill (35.5 × 76.2 cm). the samples were ground at 60 per cent solids. table 2 presents the rod size distribution. the pH of the slurry was adjusted to pH 10.5 - 11.0 with lime as required.