Removal of aluminum to obtain high purity gadolinium with pyridinium-based ionic liquids

As an associated element, aluminum (Al(III)) has a negative effect on the purity of rare earth gadolinium (Gd(III)). Three pyridinium-based ionic liquids [(CH2)(n)COOHpyr][NTf2], n = 3, 5, 7) were proposed as the solvent in liquid-liquid extraction for separation of the Al(III) impurity from Gd(III) in this study. The effect of pH value of the aqueous phase, diluent type, extraction time and temperature on the extraction separation of Al3+ from GdCl3 solution were investigated systemically. The results show that the separation factor beta of Al(III)/Gd(III) is up to 340 by adjusting the pH value of the aqueous phase which is higher than the values of other reported systems, and the removal efficiency of Al(III) could reach 97.1%, with the recovery efficiency of Gd(III) of 85.6% through one-time extraction using a mixture of ionic liquids [(CH2)(7)COOHpyr][NTf2]-[C(4)min][NTf2] as the extractant. The slope analysis and infrared spectroscopic characterization indicated that the extraction mechanism of Al(III) by the [(CH2)(7)COOHpyr][NTf2]-[C(4)mim][NTf2] system follows the cation exchange mechanism. After the extraction, 0.08 mol/L hydrochloric acid aqueous solution was used to strip Al(III) from the ionic liquid phase, and complete recovery of Al(III) could be realized. The extraction efficiency of the [(CH2)(7)COOHpyr][NTf2]-[C(4)mim][NTf2] system for separation of Al(III) is still more than 97.8% after recycling for 8 cycles.