Novel reductive alkali roasting for Cr6+-free extraction of Cr2O3 from chromite ores

Chromite ore is the raw material used in the manufacturing of ferrochromium, which is an important metallurgical product employed in the stainless-steel industry. Chromium-based chemicals have also a wide range of applications, including pigments industry, leather tanning processes and surface treatments. The production of chromium chemicals is based on the alkali roasting of chromite ore in oxidising atmosphere in order to extract chromium as water soluble alkali chromate (Na2CrO4, K2CrO4). The main drawback of this process is the inevitable generation of a hazardous processing waste containing highly toxic hexavalent chromium. This residue needs to be landfilled and presents a serious environmental concern due to the lethal effects of Cr6+ in flora and fauna. With the purpose of avoiding the formation of highly toxic Cr6+, a novel process for extraction of Cr2O3 based on the carbothermal reduction of chromite ore in the presence of alkali, is proposed. During the reductive alkali roasting (or alkali reduction) of chromite, iron is reduced to metallic form and chromium reacts with Na2O to form sodium chromite (NaCrO2). Reduced samples are subjected to wet magnetic separation obtaining a magnetic fraction rich in metallic iron and a non-magnetic fraction which contains the NaCrO2 formed. The non-magnetic fraction is further treated to extract a Cr2O3-rich product, which may be used as raw material for manufacturing of high grade Cr2O3. 85.6% Cr2O3 recovery was achieved after chromite reduction with Na2CO3 and activated charcoal (1050oC, 2.5 hours), followed by magnetic separation and leaching of the non-magnetic fraction with water and 0.5 M sulfuric acid. A final 83.6% Cr2O3-rich product was obtained. This process does not generate toxic waste in gas, liquid or solid form, as the formation of Cr6+ is completely avoided by maintaining chromium in (3+)-state at all stages of the process. The fundamentals of the different stages of the process were investigated, including the thermodynamics, kinetics and physico-chemistry of the reduction reaction as a function of the process parameters. Mass balance and energy balance, and the environmental aspects of the process were also studied based on the proposed flowsheet of the reductive alkali roasting process.