Pressure oxidative leaching of Indian chromite ore in concentrated NaOH solution

A novel process for leaching chromite ore has been proposed. Compared to the traditional alkaline roasting process, the new hydrometallurgical process lowers effectively the reaction temperature and enhances the leaching efficiency through pressure oxidative leaching with pure oxygen (O2) in concentrated sodium hydroxide (NaOH) aqueous solution. The chromium (Cr) leaching efficiency reaches 99% with ore particle size of 40 to 50 μm, NaOH-to-ore mass ratio of 4:1, NaOH concentration of 60 wt.%, O2 partial pressure of 3.2 MPa, and reaction temperature of 250 °C. By investigating the dependence of the Cr leaching efficiency on NaOH concentration, O2 partial pressure, and reaction temperature, the macrokinetics can be described by the shrinking core model under control diffusion through the solid product layer with apparent activation energy of 54.6 kJ/mol. It was also found that during the leaching process, the alkaline insoluble iron (Fe) and magnesium (Mg) in the chromite ore remain in the residue in the form of magnesium hydroxide (Mg(OH)2) and magnesium ferrite (MgFe2O4) while the divalent Fe is oxidized to trivalent Fe. Furthermore, in the initial stage of the leaching process, silicon (Si) is leached faster than aluminum (Al) and their concentrations in the aqueous solution increase rapidly with reaction time. However, as the leaching proceeds further, the Si concentration in the aqueous solution drops a little and then stays constant.