Selective leaching trace elements from bauxite residue (Red mud) without and with adding solid NH₄Cl using microwave heating

Bauxite residue (red mud), which is an industrial byproduct, contains valuable trace elements. Solid NH₄Cl was used as a chlorinating agent during the microwave heating of red mud to convert trace elements into soluble metal chloride. Red mud was heated using microwave ovens under various conditions (i.e., with the addition of solid NH₄Cl and with a range of microwave output powers and microwave heating times). Leaching tests were then conducted using deionized (DI) water on the microwave-heated red mud to leach trace elements from red mud. V, Cr, and As were selectively leached from the microwave heated red mud slurry (30% water content), whereas Mn, Cu, Co, Ni, Zn, and Pb were selectively leached from the microwave-heated red mud with the addition of solid NH₄Cl. The oxides of V, Cr, and As in red mud could be transformed into metal chlorides by chlorination, which are insoluble in water, or could be easily volatilized when red mud was microwave-heated in the presence of solid NH₄Cl. On the other hand, the oxides of Mn, Cu, Co, Zn, Ni, and Pb in red mud could be heated rapidly by microwave irradiating, resulting in metal chlorides in the presence of solid NH₄Cl. Those metal chlorides are relatively soluble in water, leading to higher leaching efficiency for microwave-heated red mud with the addition of solid NH₄Cl. Experimental results suggest that trace elements from red mud can be selectively leached by microwave heating of red mud without or with the addition of solid NH₄Cl.