Decomposition behavior and reaction mechanism of CeTbMgAlO during NaCO assisted roasting: Toward efficient recycling of Ce and Tb from waste phosphor.

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, NaCO assisted roasting is adopted to decompose the stable spinel structure of CeTbMgAlO 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 NaCO 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 NaCO at 800 °C, and the reaction was dramatically accelerated with temperature rising above 851 °C. At about 1000 °C, CeTbMgAlO could completely decomposed into CeO, TbO and MgO by roasting in an equivalent mass of NaCO for 2 h, while α-AlO was hardly attacked in roasting. The decomposition mechanism of CeTbMgAlO in molten NaCO 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 CeO and TbO from the green phosphor via NaCO assisted roasting method. The converted CeO and TbO 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.