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

Abstract

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. © 2019 Society of Chemical Industry

Selective Leaching Trace Elements from Bauxite Residue (Red Mud) without and with Adding Solid NH4Cl Using Microwave Heating

“…After cooling down the red mud sample, the solidified hard parts were collected for the leaching test. For the leaching tests, each of the red mud samples heated with microwaves was mixed with DI water (pH = 5.6) at a solid dosage of 100 g/L and stirred at 300 revolutions per minute (rpm) at room temperature (approximately 25 • C) for 24 h. The solid dosage of 100 g/L was used because no significant solid dosage effect on the leachability was observed in the previous study [38]. To evaluate the effects of microwave heating, the leaching tests were also conducted on the non-microwave-heated red mud with approximately 30% water content (i.e., red mud slurry) and a non-microwave-heated red mud with the addition of solid NH 4 Cl (i.e., red mud-solid NH 4 Cl).…”

“…In contrast to Cr, V, and As, little to no amounts of Mn, Cu, Co, Zn, Ni, and Pb were leached from the microwave-heated dried red mud without solid NH 4 Cl (Figure 3 and Table 2), indicating microwave heating had no effect on their leachability. At 5000 W, red mud can be heated to 321 • C after only 5 min of microwave treatment and heated to approximately 1600 • C after 25 min of microwave treatment [38]. The increase in temperature can be sufficient to cause changes in mineral phases of red mud.…”

“…Microwaves have been utilized as a heat source to recover precious metals from natural resources, including heavy metals including Al, Co, Cu, Pb, Ni, Mg, and Zn, and REEs such as Ce, La, Sc, Nd, and Sm. Microwave pretreatment increases metal recovery and reduces process time [24,[35][36][37][38]. Although most researchers who have undertaken microwave-assisted leaching have employed acid solutions as additives to leach metals from raw materials, a few have made use of solid salts.…”

“…This approach is analogous to the reductive roasting processes in which carbon and chloride salt are used to remove components from insoluble materials through roasting, reduction, and leaching. Kim et al [38] recently developed a microwave-assisted leaching method for extracting Ca and Na from red mud utilizing solid NH 4 Cl as an additive. They demonstrated that adding solid NH 4 Cl to the red mud throughout the microwave heating process effectively leached calcium and sodium at the high microwave output power of 5000 W.…”

Selective leaching of scandium and yttrium from red mud induced by hydrothermal treatment

“…By the application of pyrometallurgy, the iron was recovered under high temperatures in fluid–solid form, but in co‐existence with other minerals 2 . As for hydrometallurgical recovery, the separation of valuable elements from red mud was obtained using inorganic acidic solutions (HCl, HNO 3 , or H 2 SO 4 ) 8,9 or a combination of acids, ionic liquids, 1 and organic acids 10 . In some cases, the effect of heating was studied 11 .…”