GRAIN SCALE DISTRIBUTION OF RARE EARTH ELEMENTS IN COAL ASH BY SHRIMP-RG ION MICROPROBE

During coal combustion, rare earth elements (REE) are strongly retained in the ash fraction and this behavior has drawn attention to coal ash as a possible source of REE. Knowing how REE are distributed in coal ash is essential to devising extraction strategies and for developing possible approaches to pre-concentrate REE. REE-bearing trace phases are much less commonly observed in coal ash than in the corresponding coal, leading to the suggestion that REE may be partitioned into the glass phase during coal combustion at boiler temperatures [1, 2].

To better understand the distribution of REE in coal ash, in the present study we used the Stanford/USGS SHRIMP-RG ion microprobe to determine grain-scale REE partitioning in coal ash derived from various U.S. and international coals. Prior to SHRIMP-RG analysis, samples were characterized by backscattered electron imaging and wavelength-dispersive elemental mapping, showing constituents of the ash and their major element distributions. SHRIMP-RG microanalysis used a nominal spot size of 15 micrometers and an oxygen negative-ion primary beam. NIST multi-element glass standards SRM 611 and SRM 613 were used for REE calibration.

Results confirm the occurrence of REE in aluminosilicate glasses at concentrations that span those of the respective bulk coal ash samples. Co-occurring quartz and/or high-silica glasses generally have much lower REE contents than aluminosilicate glasses, consistent with results for bulk coal ash samples showing a strong positive correlation between Al and REE content [3]. Fe-oxide spheres were found to have variable REE contents. Results suggest that total digestion of the aluminosilicate glass fraction in coal ash will yield a significant portion of the REE present and that limited pre-concentration may be accomplished by removing the least REE-enriched fractions.

 

[1] Hower, J.C. et al., 2013, Coal Comb. & Gas. Prod. 5, 39-47, DOI:10.4177/CCGP-D13-00001.1

[2] A. Kolker et al., 2015, Geol. Soc. of Amer. Abs. with Programs, Ann. Mtg., Baltimore, MD, https://gsa.confex.com/gsa/2015AM/webprogram/Paper264228.html.

[3] Taggart, R.K., et al., 2016, Environ. Sci. Technol., 50 (11), pp 5919–5926, DOI: 10.1021/acs.est.6b00085

Deonarine present address: Niesenweg 2, Bern, Switzerland 3012