Browsing by Author "Yang, Xinbo"

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    A Comprehensive Review of Rare Earth Elements Recovery from Coal-Related Materials
    Zhang, Wencai; Noble, Christopher Aaron; Yang, Xinbo; Honaker, Rick (MDPI, 2020-05-17)
    Many studies have been published in recent years focusing on the recovery of rare earth elements (REEs) from coal-related materials, including coal, coal refuse, coal mine drainage, and coal combustion byproducts particularly fly ash. The scientific basis and technology development have been supported by coal geologists and extractive metallurgists, and through these efforts, the concept has progressed from feasibility assessment to pilot-scale production over the last five years. Physical beneficiation, acid leaching, ion-exchange leaching, bio-leaching, thermal treatment, alkali treatment, solvent extraction, and other recovery technologies have been evaluated with varying degrees of success depending on the feedstock properties. In general, physical beneficiation can be a suitable low-cost option for preliminary upgrading; however, most studies showed exceedingly low recovery values unless ultrafine grinding was first performed. This finding is largely attributed to the combination of small RE-bearing mineral particle size and complex REE mineralogy in coal-based resources. Alternatively, direct chemical extraction by acid was able to produce moderate recovery values, and the inclusion of leaching additives, alkaline pretreatment, and/or thermal pretreatment considerably improved the process performance. The studies reviewed in this article revealed two major pilot plants where these processes have been successfully deployed along with suitable solution purification technologies to continuously produce high-grade mixed rare earth products (as high as +95%) from coal-based resources. This article presents a systematic review of the recovery methods, testing outcomes, and separation mechanisms that are involved in REE extraction from coal-related materials. The most recent findings regarding the modes of occurrence of REEs in coal-related materials are also included.
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    Correlations between the Mineralogy and Recovery Behavior of Rare Earth Elements (REEs) in Coal Waste
    Ji, Bin (Virginia Tech, 2023-01-12)
    Many literatures have been published recently regarding the recovery of REEs from coal-related materials, such as coal waste, acid mine drainage, and coal combustion ash. The recovery of REEs from coal waste has been investigated by the author in recent years, and it was found that after calcination at 600 ℃ for 2 h, a significant improvement in REE recovery can be achieved. In order to reveal the mechanisms of the enhanced REE recovery after calcination, coal waste samples from two different seams, i.e., Western Kentucky No. 13 and Fire Clay, were selected to investigate the modes of occurrence of REEs. Scanning electron microscopy- and transmission electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS and TEM-EDS) analyses were conducted to investigate the mineralogy of REEs in two coal waste samples. Totally, 49 and 50 REE-bearing particles were found from the SEM specimens of Western Kentucky No. 13 and Fire Clay coal waste samples, respectively. Based on the elemental composition analyses and TEM-EDS characterization, it was found that apatite, monazite, and crandallite-group minerals were the major light REE (LREE) carriers, while the heavy REEs (HREE) primarily occurred in zircon and xenotime in these two coal waste samples. Further analyzing the REE content and number of REE-bearing particles, it was confirmed that monazite, xenotime, and crandallite-group minerals were the dominant contributors to the total REE (TREE) contents in both materials. In addition to the mineralogy of REEs, the morphology of REE-bearing particles was also investigated. The SEM images suggested that the particle size of most REE-bearing particles was less than 5 μm. Moreover, not only completely liberated particles, but particles encapsulated by the host minerals present in the two coal waste samples. To identify the changes of mineralogy of REEs after recovery, the leaching solid residues of the raw and calcined coal waste samples were also characterized by SEM-EDS analysis. After REE recovery, the same REE mineralogical results were observed from the leaching residues of the raw coal waste samples. However, as for the calcined samples, the crandallite-group minerals disappeared. These results suggested that the crandallite-group minerals were decomposed into easy-to-leach forms after calcination at 600 ℃, thus leading to the improved REE recovery. Moreover, the number of REE-bearing particles (N) found from per area of the calcined leaching residue was confirmed to be larger compared to that of the raw ones. A combination analysis of these results indicated that two mechanisms of the enhanced REE recovery after calcination can be confirmed: (1) decomposing the crandallite-group minerals into more soluble species; and (2) promoting the liberation of the REE-bearing particles encapsulated in the host minerals. The thermal decomposition of crandallite-group minerals was mainly responsible for the enhanced REE recovery from coal waste. However, as a result of the complex isomorphic substitutions and association characteristics, it is difficult to collect a pure endmember of crandallite-group mineral for characterization. Therefore, florencite-(Ce) was synthesized in this study. X-ray diffraction (XRD), SEM-EDS, TEM, thermogravimetric and differential thermal analyses (TGA-DTA), and acid leaching tests were conducted on the synthesized product. The results showed that the variation in Ce leaching recovery corresponded to the phase transformation of florencite. The gradual transformation of florencite from a crystalline mineral into an amorphous phase resulted in the increases in the solubility of Ce. In addition, the thermal transformation of florencite was an independent reaction, which was not interfered by the host materials, such as kaolinite and coal waste.