Microwave-Assisted Pretreatment of Coal Fly Ash for Enrichment and Enhanced Extraction of Rare-Earth Elements

A microwave-assisted pretreatment (beneficiation) technique was developed as an approach to improve the enrichment and extraction of rare-earth elements from coal fly ash solid waste. The mixtures of coal fly ash and carbon lampblack were pretreated under 2 kW microwave irradiation within argon for 2–10 min. Ultrahigh heating rates, high temperatures (820–1350 °C), and drastic temperature changes resulted in both compositional and microstructural changes. Silicon carbide, alumina, and iron silicides were formed due to carbothermal reduction of the mullite and quartz phases of the coal fly ash at high temperatures. Microstructure defects within the coal fly ash particles, which included large cracks and macropores, were produced during the short irradiation event. More drastic changes were observed with increasing irradiation time and carbon lampblack content, which later enhanced the penetration of acid leaching solution into the aluminosilicate fly ash particles. The leaching efficiency of the total rare-earth elements thereby increased from 21.7 to 54.9–83.4%. The leaching efficiency of the most critical and abundant rare-earth elements (Nd, Y, Dy, Eu, Tb, Ce, and La) reached nearly 93%. The developed technique is proven to be highly efficient and cost-effective for the recovery of critical rare-earth elements from such a difficult to process solid waste form.