Supercritical fluid extraction of rare earth elements from luminescent material in waste fluorescent lamps

Abstract

Rare earth elements were extracted from luminescent material in waste fluorescent lamps using supercritical carbon dioxide (SF-CO₂) containing tri-n-butyl phosphate (TBP) complexes with HNO₃ and H₂O. The determined Y, Eu, La, Ce and Tb in the employed luminescent material were 29.6, 2.3, 10.6, 5.0 and 2.6 wt.%, respectively. Aqueous droplets were generated in an extraction experiment from a reaction with metal oxides and a complex prepared from vigorous mixing of TBP with concentrated nitric acid (15.5 mol dm⁻³ (M)). These droplets were generated because the complex was saturated with water and the excess water separated, leading to low recovery yields. The molecular ratio of TBP:HNO₃:H₂O in the complex was 1.0:1.8:0.6. When the ratio was changed to 1.0:1.3:0.4 by dilution with TBP anhydrate, the separation of water was prevented. Using this complex, extraction efficiencies for Y and Eu increased to over 99% after the static extraction for 120 min at 15 MPa, 333 K.

Introduction

Rare earth elements have been widely used as functional materials such as luminescent materials and catalysts. Notably, these materials have been proven indispensable for use in fluorescent lamps that inside is coated with a luminescent material derived from rare earth elements. While mercury and glass cullets are recovered from the waste fluorescent lamps, luminescent materials containing rare earth elements are dumped into landfills without recovery. Recycling of rare earth elements is not considered practical due to the lack of an available method.

Supercritical fluid extraction (SFE) technology using carbon dioxide (CO₂) exhibits several advantages compared with traditional solvent extractions [1], [2], [3], [4], [5], [6], [7], [8]. The extraction efficiency is expected to be improved due to rapid mass transfer in supercritical fluid. Furthermore, the rapid and complete removal of the solute from the solvent is achieved by gasification of CO₂ at atmospheric pressure after extraction. Additionally, the CO₂-soluble TBP complex with HNO₃ and H₂O are effective for dissolution and extraction of metal oxides [8], [9], [10], [11], [12], [13], [14], [15].

The CO₂-soluble TBP complex, however, is problematic in that an excess amount of water is generated from the reaction of the complex with metal oxides. The complex is prepared by a vigorous mixing of TBP and concentrated nitric acid, which is a mixture of HNO₃ and H₂O, therefore, the complex gets saturated with water. The excess water separates, forming small droplets in the supercritical fluid. When the metal ions are distributed into the droplets, effective extraction is not achieved.

In the present investigation, a method of preventing the droplet formation is proposed, and SFE is applied to recovery of the rare earth elements in luminescent materials removed from the actual waste from fluorescent lamps. Droplets formation was prevented by controlling of the molecular ratio of TBP:HNO₃:H₂O in the complex.