Highly-efficient and sequential recovery of rare earth elements, alumina and silica from coal fly ash via a novel recyclable ZnO sinter method

A novel sinter method using ZnO as the activator instead of the conventional Na 2 CO 3 /CaCO 3 , (NH 4 ) 2 SO 4 , and K 2 S 2 O 7 was developed to achieve efficient sequential extraction of rare earth elements (REEs), alumina (Al), and silica (Si) from coal fly ash (CFA). Up to 93.3% Si, 87.1% REEs (70.7% Ce, 82.5% La, 83.2% Gd, 87.1% Nd, 62.3% Dy, and 81.7% Y), and 92.9% Al were extracted from CFA, respectively. Moreover, 93.1% of the ZnO activator was efficiently recycled, and the yield of red mud was only 14.9%. X-ray diffraction (XRD) and X-ray absorption near edge structure (XANES) results showed that the speciation transformation of Al/Si during CFA/ZnO roasting was as follows: mullite , quartz, amorphous Al 2 O 3 , and SiO 2 → Zn 0.75 Al 1.5 Si 1.5 O 6 , kyanite and willemite → gahnite and quartz/cristobalite solid solutions . The change in the REEs occurrence mode hinted at the migration of most REEs in aluminosilicates forms with Si during roasting, and disassociation with Si into the acid-soluble form after alkali leaching. These results indicate that the coupling of Al-Si-REE in CF was broken by this ZnO sinter method, promoting the sequential and efficient extraction of REEs, Al, and Si from CFA. This study provides a green and efficient strategy for element recovery from CFA, substantially reducing residues and favoring REEs concentration.