Decomposition behavior and reaction mechanism of Ce0.67Tb0.33MgAl11O19 during Na2CO3 assisted roasting: Toward efficient recycling of Ce and Tb from waste phosphor

Abstract

Waste aluminate phosphor is a valuable secondary resource of rare earth elements (REEs). However, Ce and Tb in aluminate green phosphor can hardly be extracted by direct leaching in an inorganic acid. Therefore, Na2CO3 assisted roasting is adopted to decompose the stable spinel structure of Ce0.67Tb0.33MgAl11O19 in the present work and to achieve the transformation of REEs to simple oxides. Based on the thermodynamic calculations, systematic experiments of thermal decomposition have been conducted. The thermal decomposition behavior, phase evolution, valence state change, variations in micro and macro morphology of the green phosphor during Na2CO3 assisted roasting were examined by using TG-DSC/MS, XRD, XPS, SEM/EDS analyses. The results indicated that the green phosphor began to react with solid Na2CO3 at 800 °C, and the reaction was dramatically accelerated with temperature rising above 851 °C. At about 1000 °C, Ce0.67Tb0.33MgAl11O19 could completely decomposed into CeO2, Tb2O3 and MgO by roasting in an equivalent mass of Na2CO3 for 2 h, while α-Al2O3 was hardly attacked in roasting. The decomposition mechanism of Ce0.67Tb0.33MgAl11O19 in molten Na2CO3 could be depicted by the unreacted shrinking core model, and the reaction rate constant was estimated at approximately nanometers per second. The synergistic effect of cation-oxoanion ensures the successful extraction of CeO2 and Tb2O3 from the green phosphor via Na2CO3 assisted roasting method. The converted CeO2 and Tb2O3 can be extracted by using chlorination roasting and separated from non-REE residues. According to these investigations, a new efficient process technology is proposed for sustainable recycling of waste phosphor.