From Ashes to Riches: Microscale Phenomena Controlling Rare Earths Recovery from Coal Fly Ash

Abstract

Coal fly ash is an alternative source of rare earth elements (REEs), which are critical in modern energy and electronic technologies. Current hydrometallurgical processes, however, yield variable recovery rates because of the limited understanding of the microscale phenomena controlling the extraction of REEs from fly ash. This work investigates the microscale processes that dictate the recovery of REEs from ash particulates via a spatiochemical analysis. We find that REE-bearing minerals are hosted in three modes with distinct recovery mechanisms: (i) REEs encapsulated in dense particles are recovered via the cation exchange between matrix metals (Al, Ca, Mg, etc.) and solution cations, (ii) REEs within permeable particles are recovered via intraparticle pore-scale fluid flow, and (iii) discrete and surface-bound REE-bearing minerals are recovered via direct exposure to reagents. The role of metal content and the limiting transport mechanisms are further probed for dense particles, the predominant mode of occurrence. This study highlights, for the first time, how the morphology and the elemental makeup of the ash matrix play a critical role in the accessibility of REEs, furthering the knowledge base required for the design of cost-effective and environmentally benign REEs recovery techniques.

Role of Nanoscale Crystallinity on the Recovery of Rare Earth Elements (REEs) from Coal Fly Ash

“…No other treatment of the ash was performed. The bulk and microscale properties of this ash sample were reported previously . In summary, the bulk mineralogy of the fly ash consists of amorphous aluminosilicate, quartz, hematite, mullite, and anhydrite .…”

“…The crystalline nature of the porous particle matrices explains previous observations of their unchanged composition throughout the recovery of REEs via leaching treatments. Crystalline aluminosilicates possess fewer interstices than their amorphous counterparts such that reactions with leaching solutions are primarily restricted to solid–fluid interfaces (as opposed to solid-state transport in the case of amorphous matrices). , Consequently, REE-bearing minerals in porous matrices are recovered via fluid transport through intraparticle pores, while the polycrystalline aluminosilicate matrix remains unaltered (Figure B).…”

“…The bulk and microscale properties of this ash sample were reported previously. 22 In summary, the bulk mineralogy of the fly ash consists of amorphous aluminosilicate, quartz, hematite, mullite, and anhydrite. 22 The total content of REEs in this sample is 384 ± 32 ppm.…”

“…The total content of REEs in this sample is 384 ± 32 ppm . Microvisual characterization of milled ash particles shows that approximately 90% of REE-bearing minerals were encapsulated in the dense and porous matrix aluminosilicate particles; the remainder were either discrete or bound to the surface of ash particles and were directly accessible to leaching reagents . Importantly, although our previous microvisual characterization work was performed in a two-dimensional plane, the large number of REE minerals analyzed (97 in total) and symmetry of the ash particles suggest full encapsulation in the unadulterated three-dimensional material.…”

“…Distinct transport and reactive phenomena therefore take place at the subparticle level during the recovery of REEs. Acid leaching [e.g., hydrochloric acid (HCl) and nitric acid (HNO 3 )], in particular, recovers up to ∼60% of total REEs from the dense particles with noticeable metals leached (Al, Mg, Ca, Na, and K) from the solid matrix, while porous particles yielded high recoveries of REEs (>90%) without compositional or physical changes . Such stark differences in matrix reactivities between the dense and porous aluminosilicate particles reveal that elemental composition and intrinsic porosity alone do not fully elucidate the phenomena that take place and underscore the need to investigate additional characteristics of the particle matrix.…”