Advanced characterization of rare earth element minerals in coal utilization byproducts using multimodal image analysis

Multimodal microanalytical characterization techniques are applied to identify and quantify rare earth element (REE) and REE + Y (REY) bearing mineral phases in coal utilization byproducts (CUB) from various coal-fired power plants. The characterization work provides quantitative assessments of REE in coal and CUB as obtained from 2- and 3D imaging, elemental mapping, volumetric estimates, and advanced high-resolution pixel classification. REY-bearing phosphate minerals rhabdophane (Ce,La)(PO4·H2O), monazite (Ce,La,Nd,Th)(PO4,SiO4), xenotime (YPO4,SiO4), and apatite (Ca5(PO4)3(F,Cl,OH), as well as REE-enriched calcium oxide were identified via electron microscopy and microprobe analysis. The minerals generally occurred as 1-20 μm-long crystals in the rock and ash samples. The most notable finding was that REEs are present as monomineralic grains dispersed within the ash, as well as fused to or encapsulated by amorphous aluminosilicate glass particles, also referred to as slag. It is indicative that conventional coal combustion processes sequester REE-bearing mineral phases such as rhabdophane, monazite, and zircon from the coal feed into aluminosilicate glass phases. The advanced microscopy and image analysis techniques applied in this study make it possible to deduce the average density of ash particles and quantify REE phases encapsulated in glass. Consequently, these REE phases may be targeted and recovered via density separation. These findings advance resource recovery techniques and commercial REE separation technologies for coal and combustion byproducts.