Silicon and aluminum removal from ilmenite concentrates by alkaline leaching

In this paper, a hydrometallurgical process for the treatment of ilmenite concentrates is developed, in order to reduce the silicon content to the maximum allowed value for the production of “grade B” ferrotitanium. The process was based on pressure leaching with sodium hydroxide and the effect of the main leaching parameters, namely the temperature, sodium hydroxide concentration and retention time, on the efficiency of silicon removal was studied. Caustic leaching of the ilmenite concentrates resulted in the removal of silicon, as well as of aluminum, without affecting the contents of titanium and iron. Leaching temperature and the retention time were proved as crucial parameters for the effective dissolution of silicon. Besides, these parameters affected directly the secondary precipitation of silicon and aluminum as a zeolitic phase, which polluted the treated ilmenite concentrate. Leaching kinetics determined the feasibility of the process, leading to the development of two optimized alternatives. The first alternative achieved the reduction of silicon and aluminum content to 0.32% and 0.61%, respectively, after 72 h leaching at 120 °C. This alternative necessitated a leached solids washing stage in order to separate the formed precipitate from the treated ilmenite concentrate. The second alternative achieved to reduce silicon and aluminum to 0.29% and 0.59%, respectively, after 1 h leaching at 200 °C, while avoided the formation of the secondary precipitate. Both alternatives achieved the selective leaching of silicon and aluminum without affecting titanium and iron content of the ilmenite concentrate. The applicability of each alternative is a matter of detailed technical and economic studies.