Influence mechanism of leaching agent anions on the leaching of aluminium impurities in ionic-type rare earth ores: A DFT simulation combined with experimental verification

Abstract

The aluminium impurity present during the leaching process of ionic-type rare earth ores increases the complexity and cost of the subsequent dealumination procedure, causing the loss of rare earth elements. Understanding the interactions between minerals and impurities is crucial for grasping interfacial reactions. However, the mechanism by which anions influence the adsorption behaviour of Al3+ remains unclear. This study examined the impact mechanisms of four typical leaching agent anions (SO42-, Cl-, NO3–, and CH3COO–) on the interaction between Al3+ and the kaolinite (001) surface using density functional theory (DFT). According to DFT calculations, the order of adsorption energy of Al3+ on the kaolinite (001) surface in the presence of anions is as follows: Al3+ < Al3+ in aluminium sulfate < Al3+ in aluminium chloride < Al3+ in aluminium nitrate < Al3+ in aluminium acetate. The adsorption strength of Al3+ on the kaolinite (001) surface is weakened to varying degrees by the presence of anions. Specifically, under the influence of acetate anions, it is difficult for Al3+ to adsorb and form bonds on the kaolinite (001) surface. In contrast, Al3+ can still adsorb mainly through Al-O ionic bonds under the influence of the other three anions. The density of states analysis showed that the 3p orbitals of Al atoms and the 2p orbitals of O atoms primarily contribute to the Al-O bond near the Fermi energy level. Experimental results from adsorption capacity tests, microcalorimetry, and zeta potential measurements support the DFT simulations, indicating that Al2(SO4)3 is the most easily adsorbed on the kaolinite surface, followed by AlCl3 and Al(NO3)3, with (CH3COO)3Al being the most difficult to adsorb. Column leaching tests further verified that adding CH3COO– to the leaching agent solution at pH > 4.5 can inhibit the leaching of aluminium impurities, enabling the efficient separation of rare earth elements from aluminium impurities, and reducing the aluminium content in the leachate.