Alkali pretreatment effects on acid leaching recovery of rare earth elements from coal waste of the Western Kentucky No. 13 and Fire Clay seams

Abstract

The recovery of rare earth elements (REEs) from the Western Kentucky No. 13 and Fire Clay coal wastes was enhanced by alkali pretreatment with concentrated NaOH solutions. The enhancements in the recovery of light REEs (LREEs) are more significant than those of heavy REEs (HREEs). For example, after treating with 5 M NaOH at 90 °C, the recovery of LREEs from the Western Kentucky No. 13 coal waste increased from 26% to 71%, while the recovery of HREEs only increased from 29% to 41%. Based on mineralogical studies through scanning electron microscopy-energy dispersive X-ray spectroscopy and X-ray diffraction analyses, two mechanisms were proposed to explain the positive effect of alkali pretreatment: (1) decomposition of rare earth minerals (primarily crandallite-group minerals) during the alkali pretreatment, and (2) liberation of encapsulated REE-bearing particles due to the enhanced dissolution of clay minerals. The more significant enhancements in the recovery of LREEs were explained by the fact that the REEs comprised in the crandallite-group minerals were mainly LREEs. Compared with zircon, monazite, and xenotime, alkali pretreatment with 5 M NaOH led to a more significant decomposition of crandallite-group minerals. In order to further increase the recovery of REEs, particularly HREEs, harsher alkali treatment conditions are required.

Enhanced Coal Fly Ash Desilication Using Atmospheric NaOH Leaching with Simultaneous Magnetic Separation

“…The preliminary treatment of CFA, such as desilication and magnetic fraction extraction, is suitable, enriching the valuable components in the CFA and reducing the reagent consumption and temperatures of the leaching [22,23]. Using the desilication method, silicon (Si) from an amorphous glassy mass that accounts for almost half of the mass of the CFA is extracted into an alkaline solution, whereas Al, Fe, and REEs are enriched in the solid residue [24][25][26]. The primary drawback of this method is the formation of the sodalite desilication product (DSP) (Na 6 [Al 6 Si 6 O 24 ] NaX, where X represents various inorganic anions, which are primarily sulfate, carbonate, chloride, and aluminate) [13].…”

Investigation on the Value-Added Production of Silicon Dioxide and Synthesizing Zeolites as well as Extraction of Rare Earth Elements from Fly Ash

“…This method requires highly corrosive solutions and is hazardous, energy intensive, and multistage. Owing to the presence of Al-Si-based glasses, the REEs extraction rate of this method is not sufficiently effective [27][28][29]. Meanwhile, this leaching method is indiscriminate, resulting in an impure mixture of REEs and bulk elements that require further separation processes [20].…”