Roth, and 7 co-authors, reported in 2017 on their research efforts to investigate, characterize, and extract rare earth elements (REE) from coal and coal by-products, including coal ash. Characterization of geologic core samples, coal mine wastes, and coal ash has led to interesting findings on the mode of occurrence and fate of rare earth minerals, such as monazite, in coal-based materials. Mineral associations and elemental distributions have been investigated and results indicate that Th is associated with light rare earth elements (LREE), possibly as monazite, and Zr is associated with heavy rare earth elements (HREE), possibly as zircon. Preliminary extractions of REE from coal ash using 2M hydrochloric acid (HCl) and sodium hydroxide (NaOH) hydrothermal pretreatment have been investigated. Sodium hydroxide solution pretreatment followed by an acid digestion was shown to increase extraction of REE to nearly 90% for coal ash.
Despite the recent downturn in coal production in the U.S., coal continues to be a tremendous resource with approximately one billion tons of coal mined annually in the U.S. between 1990 and 2014, and ~740 million tons produced in 2016. Renewed interest in the extraction of valuable and critical materials and elements from coal and coal by-products has the potential to shift coal research from pollutant control to economic recovery of critical elements and metals. Recent studies on rare earth elements and yttrium (REY) in coal and coal ash have focused on REY enrichment and distribution in coals, REY size dependent variations in fly ash, anomalous REY behavior in coal, and extraction of REY from fly ash.
The National Energy Technology Laboratory (NETL) has created an open access website with REY concentrations in field samples from coal mining to coal combustion processes. This paper focuses on a pulverized coal fly ash sample from Ohio. Bulk REY concentrations in particle size-separated samples are measured using inductively coupled plasma mass spectrometry (ICP-MS) and discussed. Concentrations and distributions of the REY are also analyzed using laser ablation ICP-MS. Results from preliminary studies of REE extractions using both 2M hydrochloric acid (HCl) leaching and 5M sodium hydroxide (NaOH) hydrothermal pretreatment followed by 2M HCl leaching are presented. Hf, Yb, and La concentrations are plotted against Zr concentrations for each laser ablation spot, the results illustrate that Yb and Hf correlate with Zr. As indicated above, these data indicate that zircon is a source of HREY in fly ash. By comparing the La and Ce concentrations with Th, it appears that LREY are associated with minerals containing Th, such as monazite.
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