https://pubs.rsc.org/en/Content/ArticlePDF/2015/GC/C4GC02360A

Reclamation of reactive metal oxides from complex minerals using alkali roasting and leaching – an improved approach to process engineering

Reduction of Cr(VI) by hydrazine in solution saturated with KHCO3.

Chromium is mainly used for the production of stainless steel, in electroplating, as pigments and in leather and refractory industries. The natural resource of chromium is the chromite ore, which is a spinel (Fe2+, Mg)[Cr,Al,Fe3+]2O4, and is often present with gangue minerals such silicates. Traditionally chromium chemical extraction has been carried out with alkali and CaO roasting, which led to major environmental issues due to CaCrO4/Na2CrO4 release.

Physical Chemistry of Roasting and Leaching Reactions for Chromium Chemical Manufacturing and Its Impact on the Environment — A Review

The phase relationships of the complex multi-component systems in the liquid-phase oxidization and separation process related with the chromate cleaner production are investigated theoretically and experimentally. The phase diagram of main system FeCr2O4-KOH-O-2 of oxidization process and the relationship of its standard Gibbs free energy change with temperature are calculated. A series of phase diagrams of the separation process at different temperatures, such as K(Na)OH-K(Na)(2)CrO4-H2O, KOH-KAlO2-H2O, K(Na)OH-K(Na)(2)CO3-H2O, K(Na)OH-K(Na)HCO3-H2O and K(Na)HCO3-K(Na)(2)CrO4-H2O have also been calculated. Some experiments are carried out to verify the calculated results. Our results showed that the sub-molten salt media are suitable for the oxidization and separation processes; FeCr2O4 can be oxidized across a wide range of temperature; K(Na)(2)CrO4 can be separated from K(Na)AlO2 system at appropriate temperature and proper concentration; the crystallization of carbonate is easier than that of K(Na)(2)CrO4.

Phase relationship of complex multi-component system in chromate cleaner production

Low temperature isothermal kinetic studies were performed on alkali roasting of Cr2O3 with progressive replacement of Na2CO3 in the reaction mixture by NaNO3 in the temperature range of 573 to 873 K. The influence of the nitrate on the course of the reactions was studied. Statistical design of experiments was employed to asses the relative influence of different process variables. Kinetic analysis of the experimental data shows that the reaction with NaNO3 follows a simple first-order model, and the activation energy of the reaction is 22.7 kJmol-1, as compared to 72 kJmol−1 for Na2CO3 flux.

A. K. Tripathy

Papers

Kinetics of roasting of chromium oxide with sodium nitrate flux