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Soda ash (Na2CO3) roasting of the chromite is commonly used worldwide for the production of chromium compounds. However, chromium conversion yield of the chromite only reached 70–80% in industry owing to slow chromium conversion rate in the late stages of Na2CO3 roasting process, resulting in serious waste of resources. Through XRD, SEM, EDS, TG-DTA and HRTEM analysis, phase transformation and structure evolution of chromite in the late stages of Na2CO3 roasting of chromite were investigated. The results revealed that the chromite transformed from MgCrFe2O4 phase to the MgCr0.4Fe1.6O4 main phase and Mg1.5Na9Si12Al12O48 and NaSiAlO4 minority phases first. The MgCr0.4Fe1.6O4 phase gradually transformed to MgFe1.8Cr0.2O4, MgFe1.9Cr0.1O4 and MgFe2O4 phase. Microstructure, especially of the vacancies, was proved to play the key role in chromium conversion. Vacancies generated by thermal vibration needed greater activated energy, resulting in the low chromium conversion rate of chromite in the late stages of Na2CO3 roasting process.

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