Calcium and sodium recovery from microwave-pretreated red mud with added solid ammonium chloride

BACKGROUND Many studies have used an NH4Cl solution as a solvent during microwave-assisted metal leaching processes. However, in this study, solid NH4Cl instead of an NH4Cl solution was used as a chlorinating agent during the microwave pretreatment of red mud to induce a rapid temperature increase by minimizing heat loss by water. Red mud only and red mud-solid NH4Cl samples were pretreated by microwaves under various conditions. Leaching tests were then conducted on the microwave-pretreated red mud only and red mud-solid NH4Cl samples using deionized (DI) water as a leaching solution. RESULTS The leached Ca and Na concentrations from the microwave-pretreated red mud-solid NH4Cl samples were significantly higher than those from the microwave-pretreated red mud only samples at the given test conditions. In contrast to the red mud only samples, the leached Ca and Na concentrations from the microwave-pretreated red mud-solid NH4Cl samples increased with increasing microwave output power. The leached Ca and Na concentrations from the microwave-pretreated red mud-solid NH4Cl samples at 5000 W were significantly higher than those from the microwave-pretreated red mud-solid NH4Cl samples at 500 and 1100 W and from the non-microwave-pretreated red mud-solid NH4Cl samples. CONCLUSION Adding solid NH4Cl to the red mud during the microwave pretreatment process is effective to recover Ca and Na at a microwave output power of 5000 W. The leached solutions with elevated Ca and Na concentrations from the microwave-pretreated red mud-solid NH4Cl samples at 5000 W can be used for aqueous mineral carbonation to sequester CO2. (c) 2019 Society of Chemical Industry