Highly efficient separation and recovery of heavy rare earths by((dioctylamino)methyl)phenylphosphinic acid

To date, no extractants have shown good heavy rare earth elements (HREEs) extraction and separation performance within a large acidity window. Herein, a strategy to introduce benzene and tertiary amine functional groups with both an electron donation effect and large steric hindrance effect into the structure of phosphonic acid was proposed, and a new extractant, ((dioctylamino)methyl)phenylphosphonic acid (DOAM-PPA), has been successfully synthesized by a simple one-step process. The pK a value of DOAM-PPA is higher than those of 2-ethylhexylphosphoric acid mono-2-ethylhexyl ester (P507), di-(2,4,4′-trimethylpentyl)phosphinic acid (Cyanex 272), and bis(2-ethylhexyl)phosphate (P204). DOAM-PPA exhibited good separation of HREEs. DOAM-PPA enabled efficient extraction and separation of HREEs at initial pH = 4.00–1.25 and 0.1–2 mol/L HCl. Its operating window in the acidic region was much larger than those of P507, Cyanex 272 and P204. Under 0.5 mol/L HCl and 0.1 mol/L NaCl, the separation factor (β Lu/Y ) of DOAM-PPA for Lu and Y reached 53.55, which was much greater than those of P507, Cyanex 272 and P204. The saturation capacity of DOAM-PPA for Lu is 2.45 times greater than that of P204 at 0.5 mol/L HCl. When DOAM-PPA was used to directly extract LYSO leachate, although the concentrations of Lu and Y reached 15.5 g/L and 0.75 g/L, respectively, the β Lu/Y still reached 20.22, the β Lu/Ce was 59.37–116.25, and the extraction percentages of Lu were all >86%. Considering the synthetic conditions, separation performance, saturation capacity, stripping performance, recycling performance and operating window in the acidic region, we believe that DOAM-PPA has great potential for HREEs separation and recovery applications.