Recovery of Eu from waste blue phosphors (BaMgAl10O17: Eu2+) by a sodium peroxide system: Kinetics and mechanism aspects

Abstract

An efficient process for recycling Eu from BaMgAl11O19: Eu3+ (BAM) waste phosphors has been developed. The treatment of BAM by the sodium peroxide (Na2O2) calcining process followed by acid leaching was investigated. The results demonstrated that the calcination temperature and time more significantly influence the recovery efficiencies of metals than does the mass ratio of Na2O2 to BAM. More than 99% of Eu was recovered under the optimal conditions of 425 °C, mass ratio 1:1 and 30 min duration. The decomposition kinetics of BAM were studied using a series of typical solid-solid reaction kinetic models. The apparent activation energies Ea for the decomposition reactions of Eu, Ba, Al and Mg were calculated as 52.3 kJ/mol, 64.6 kJ/mol, 87.1 kJ/mol and 88.9 kJ/mol, respectively. XRD analysis indicated that the decomposition of BAM involved a multistep reaction. The reaction mechanism was also analyzed at the crystal structure level, which indicated that Ba and Eu atoms in the mirror plane of the unit cell of BAM were replaced by Na from Na2O2 at the beginning, and that BaO2 and Na2MgAl10O17 were produced. Then, Na2MgAl10O17 was decomposed to NaAl7O11 and MgAl2O4, and finally, NaAlO2, MgO, BaCO3 and Eu2O3 were generated at a higher temperature (600 °C). SEM and EDS analysis also demonstrated the proposed substitution and decomposition reactions. This work is expected to open new avenues and provides a theoretical basis for the highly efficient recovery of Eu from waste rare earth phosphors.