ABSTRACT
Metallurgical bauxites from Paragominas-PA region (northern Brazil) commonly exhibit high amounts of kaolinite. During caustic leaching of bauxite in the Bayer process conditions, this mineral reacts with NaOH and undesirable sodalite is formed, which is insoluble and therefore discarded with the solid residue, causing significant losses of the leaching agent. In the present work, an attention was given to two main types of sodalite formed as function of differences in caustic concentration and residence time. Thus, the hydrothermal process was carried out at temperature of 150 °C, caustic concentration ranging from 2.5 to 5.0 M and time varying from 60 to 420 min. Results showed that these variables affect both the speed of kaolinite to sodalite transformation as the crystal chemistry of the two main types of sodalite phases formed: 1) tetrahydrate basic sodalite, which is the more stable phase initially formed and that remains during all process conditions; and 2) hydrosodalite, which is the secondary phase, less stable and observed only in intermediate conditions. This last phase is more desirable in the Bayer context due the less amount of NaOH consumed. The favoring of this phase formation depends on the long time, caustic concentration and the amount of kaolinite.
Keywords:
basic sodalite; hydrosodalite; kaolin; bauxite
