Recycling rare earth metals from luminescent fibers by solvent extraction

Abstract

Rare earth elements (REEs) are known for their specialized optical and electronic properties that have led to their use in a wide range of applications, such as luminescent fibers employed in security papers (e.g., bank notes). Most of the security documents are disposed at the end of their life, losing the raw materials. Considering REEs are listed as critical raw materials, adoption of sustainable recycling and circular economy practices are imperative for ensuring supply chains in the future and aligning with United Nations sustainable development goals. In this contribution, a recycling method consisting of leaching and solvent extraction-based separation for recovering REEs from luminescent fibers is presented. An optimization of the separation method for extracting the metals of interest (Er, Y, La, Gd, Yb, Ho, Tm, Tb, and Eu) was carried out by assessing selective extractants (di-(2-ethylhexyl) phosphoric acid (HDEHP) and N-methyl-N,N,N-trioctylammonium chloride (Al336)), the ligand concentration and the acid concentration. The former showed high selectivity and extractive performance for most of the heavy REEs (>95 %) by using the HDEHP ligand (1.0 mol/L) and adjusting the acid concentration of the aqueous solution at 0.01 mol/L with hydrochloric acid. The developed extractive metallurgy process was applied to doped REE cellulose fibers after performing a digestion, showing impressive extraction profiles in most cases (>80 %). The solvent separation method presented in this work can be employed for other classes of doped fibers after a proper optimization process and can make security fiber industry circular with its resources use.