Differences in bulk and microscale yttrium speciation in coal combustion fly ash

Abstract

Yttrium speciation in coal fly ash differs depending on the scale of the analysis.

A Comprehensive Review of Rare Earth Elements Recovery from Coal-Related Materials

“…This characteristic along with the low concentration and dispersed nature of REEs have caused difficulties in characterizing the modes of occurrence of REEs in fly ash using traditional approaches. However, many recent studies have addressed this challenge using advanced characterization tools, such as X-ray Absorption Near Edge Structure (XANES), micro-X-Ray Absorption Near Edge Structure (µ-XANES), laser ablation inductively coupled plasma mass spectroscopy (LA-ICP-MS), multimodal image analysis, and sensitive high resolution ion microprobe-reverse geometry SHRIMP-RG [65][66][67][68][69][70]. In addition, systematic SEM-EDX, TEM-EDX, and sequential chemical extraction (SCE) studies have been performed on coal combustion ashes, which also provided valuable information regarding the REE occurrence modes and potential processing routes [71][72][73][74][75][76][77][78].…”

Nano-Scale Rare Earth Distribution in Fly Ash Derived from the Combustion of the Fire Clay Coal, Kentucky

“…The TEM approach employed here improved the spatial resolution of other analyses, such as the bulk ICP-MS chemical analysis and the XAS approach applied by Taggart et al [36], and subsequently revealed substantial heterogeneity of REE forms within an individual fly ash grain. Unlike the indications of minerals seen in other studies [12,13], as in the work by Hood et al [18], the mineralogy of the particles is elusive.…”

“…Nevertheless, the EDS scan of the mineral showed the presence of Ce and Nd (Figure 7). This fly ash sample was also examined by Taggart et al [36] (denoted as "App-FA3" in that study) for Y modes of occurrence through microprobe X-ray Absorption Spectroscopy (XAS) techniques. That study also reported distinct Y species (e.g., Y distributed in glasses, YPO 4 particles), suggesting that REE occur in widely variable forms in this fly ash.…”

“…We note that this previous microprobe XAS evaluated the fly ash sample at a fairly coarse spatial resolution (2 µm resolution) and was limited to a whole grain or collection of grains, rather than being performed within a grain, as shown in this study. This fly ash sample was also examined by Taggart et al [36] (denoted as "App-FA3" in that study) for Y modes of occurrence through microprobe X-ray Absorption Spectroscopy (XAS) techniques. That study also reported distinct Y species (e.g., Y distributed in glasses, YPO4 particles), suggesting that REE occur in widely variable forms in this fly ash.…”

“…In this study, the fly ash from a southeastern US utility power plant burning Fire Clay coal was investigated. This sample was part of the large collection investigated for total REE contents by Taggart et al [17] and also part of the fly ash samples analyzed for yttrium speciation by X-ray absorption spectroscopy methods [36]. While these studies provided insights on total REE contents for a wide range of fly ashes and Y-specific local coordination states, additional insights related to morphology and composition of discrete minerals would enrich our understanding of REE modes of occurrence in fly ashes of the Fire Clay coal.…”