Abstract
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.
Highlights
Red mud is an industrial byproduct produced by the Bayer process used to obtain alumina from bauxite
In the microwave-heated red mud slurry, the leached element concentrations were higher at a given heating condition in the following order: V > Cr > As (Figure 3). This is comparable to the total element concentrations of the as-received red mud sample (Table 1). These results suggest that the microwave heating on the red mud slurry is effective for leaching V, Cr, and As, but is not effective for leaching Co, Cu, Mn, Ni, Pb, and Zn
The leached V concentration from microwaveheated red mud slurry increased from 72,330 to 95,098 μg/kg when heating time increased from 5 to 25 min at an output power of 500 W, and from 95,098 to 158,256 μg/kg as the output power increased from 500 to 5000 W at the heating time of 25 min (Figure 3). These results suggest that the output power and heating time significantly affect Cr, V, and As leaching from the microwave-heated red mud slurry
Summary
Red mud is an industrial byproduct produced by the Bayer process used to obtain alumina from bauxite. Several studies on red mud as a secondary source for regaining important compounds via various extraction processes have been done [7,8,9,10,11,12,13,14,15]. Hydrometallurgical and bioleaching methods were utilized in these studies to extract elements from red mud [12,16,17,18,19,20,21,22,23,24,25,26]. Solvents were employed to recover important components from red mud, including sulfuric acid (H2 SO4 ), hydrochloric acid (HCl), nitric acid (HNO3 ), and other selective reagents such as cyanex and di [2-ehtylhexyl] phosphoric acid. Intense acid solutions are required for solvent extraction to maximize the element recovery, resulting in a high concentration of hazardous contaminants through the extracted solution, which must be treated as effluent [13]
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