Abstract:The reaction between Na₂O·Al₂O₃ and chromium spinels or chromite in the roasting process was systemically investigated to reveal the effects of Na₂O·Al₂O₃ on chromium oxidation. The results of Na₂O·Al₂O₃ roasted with MgO·Cr₂O₃ illustrate that only about 50% of the chromium in MgO·Cr₂O₃ can convert into Na₂CrO₄, while the remaining chromium converts into an Al-bearing spinel Mg(CrAl)O₄. Mg(CrAl)O₄ is found to be difficult to further react with Na₂O·Al₂O₃. The oxidative roasting of Na₂O·Al₂O₃ with chromite further confirms that Na₂O·Al₂O₃ cannot fully react with some chromium spinels, which reveals the cause of the low chromium oxidation rate in the traditional chromite lime-free roasting process. Based on the experiment results, a strategy for eliminating the impacts of Na₂O·Al₂O₃ in the lime-free roasting process was proposed, in which adding more Na₂CO₃ for forming Na₂O·Al₂O₃ is considered. The chromium oxidation rate can rise close to 100% with the new strategy. This study contributes to better understanding the chromite lime-free roasting process and provides a new idea for developing the chromate salts manufacturing process more efficiently and cleanly.

Key words: chromate; sodium aluminate; chromite; chromium spinel; oxidative roasting