Speciation and Recovery of Rare Earth Elements (REEs) from Coal Fly Ash

Recovery of rare earth elements (REEs) from coal fly ash (CFA) is a promising resource recovery and waste recycling option that might bring about significant economic and environmental benefits. However, many challenges need to be addressed in order to develop cost effective and environmentally friendly techniques for REE recovery from CFA. The overall goal of this dissertation is to develop a synergistic approach to simultaneously achieve REE recovery from CFA and waste reduction. To achieve this goal, a methodology framework was first developed to characterize the speciation of REEs and other trace metals in CFA. By employing complementary techniques across molecular to bulk scales, a range of REE-bearing phases are identified, including REE oxides, REE phosphates, apatite, zircon, and REE-bearing glass phase. REEs can occur as discrete particles, as particles encapsulated in glass phase, or distribute throughout the glass phase. Based on the fundamental understandings of the distribution, speciation, extractability of REEs and other trace metals in CFA, an integrated system for concurrent REE recovery and waste reduction of CFA was successfully developed and evaluated. The system involves organic ligand induced REE leaching and precipitation, as well as an downstream upcycling step to utilize the solid and liquid residues for zeolite synthesis. This process successfully harvests REEs from CFA and eliminates the production of secondary solid and liquid wastes from the overall treatment system.Ph.D