Removal of Trace Aluminum Impurity for High-Purity GdCl3 Preparation using an Amine-Group-Functionalized Ionic Liquid
Aluminum, as an associated metal with rare-earth resources, is a common impurity in rare-earth separation products. In the traditional P507-kerosene-HCl system, tremendous aluminum impurity (8875 mg/L herein) is often enriched in GdCl3 solutions because of the similar extraction sequence during rare-earth separation. In this work, a new amine-group-functionalized ionic liquid (IL) was developed to remove trace aluminum impurity from high-purity GdCl3 solutions, overcoming the drawbacks of strong hydrophilicity and complicated compositions of the organic phase, as well as other impurities introduced and tremendous saline wastewater produced inevitably. Methyltrioctylammonium hydroxide (N1888OH), instead of the water soluble ammonium hydroxide (NH4OH), was used to dissociate the dimer of sec-octylphenoxy acetic acid (CA12) and synthesize cation- and anion-bifunctionalized IL [N1888][CA12]. The separation factor of Al/Gd was as high as 36. The influences of pH, salting-out agent, extraction time, temperature, and stripping efficiency on the extraction efficiency were evaluated. The result showed that high-purity GdCl3 solution was obtained with low aluminum impurity of 10 ppm after five-stage countercurrent extraction. Due to the weak interaction between the metal and IL, very dilute HCl solution (0.07 mol/L) and NaOH solution (0.07 mol/L) were in favor of metal stripping and IL regeneration.
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