Rare earth elements and yttrium in coal ash from the Luzhou power plant in Sichuan, Southwest China: Concentration, characterization and optimized extraction

Abstract

Coal ash is considered as a promising alternative resource for rare earth elements and yttrium (REE + Y or REY) recovery. Coal ash samples (fly ash and bottom ash), as well as corresponding feed coals, collected from the Luzhou coal-fired power plant in Sichuan, southwestern China were analyzed for their chemical and mineralogical characteristics. The fly ash was further tested for its feasibility of REE extraction. The feed coals are characterized as a low-volatile bituminous, medium‑sulfur, and high-ash coal, sourced from the Guxu coalfield (Late Permian Longtan Formation) in Sichuan, SW China. REE and Y, along with high-field-strength elements Zr(Hf) and Nb(Ta), and Li, F, Sc, V, Cr, Co, Cu, Zn, Ga, Ge, Se, Sr, Cd, In, Sn, Hg, Th, U, are all enriched in the feed coal. The mineralogical compositions of the feed coals are dominated by kaolinite and illite/smectite (I/S) mixed layers, followed by the carbonate minerals including calcite, siderite, ankerite, and minor amounts of anatase and jarosite. The combustion of the feed coals produces a Class F fly ash, characterized by an aluminosilicate composition with a low CaO content. The bottom ash is compositionally similar to the fly ash, except that it is more enriched in high-density elements (Fe and Mn), and more depleted in volatile elements. REY are enriched in both the fly ash and bottom ash, although heavy REY (HREY) are notably more enriched in the fly ash. Mineralogical analysis shows that the fly ash consists of >70% amorphous glass and <30% mineral phases such as mullite, quartz and iron oxides. The bottom ash is composed of ~60% amorphous glass but with a more complex mineralogical composition than the fly ash. Dissolution of the fly ash and bottom ash with 4% hydrofluoric acid (HF) showed that ~90% of all the REY are associated with the amorphous glass in the fly ash, whereas <50% of REY in the bottom ash is contained in the glassy component. An alkaline-acid-combined (NaOH-HCl) sequential leaching process was employed to test the extractability of REY in the Luzhou fly ash. The experimental variables, including extractant concentration, liquid-to-solid ratio, leaching temperature and leaching time, were optimized by an orthogonal array design. The optimal NaOH leaching conditions results in 41.10% of active silica removal from the fly ash and 39.43% of REY enrichment. The HCl leaching of desilicated fly ash achieves 88.15% of REY extraction efficiency under the optimal conditions, which is a dramatic increase as compared to the same leaching of raw fly ash. Even though the extraction efficiency may vary between different coal ashes from various sources, a complete REY extraction procedure from fly ash is suggested for future use.