RARE EARTH BEARING TRACE PHASES IN COAL ASH: WHERE ARE THEY?

With critical shortages of rare earth elements (REE) anticipated in the U.S., there is much interest in coal fly ash as a possible source of REE. During coal combustion, REE are strongly retained in the residual ash fraction so that it is typical for REE in fly ash to be enriched by a factor of ten over those in the respective coal. Knowing how REE are distributed in coal ash is essential to devising effective approaches for REE extraction from this potential resource.

In bituminous coals, REE are largely controlled by trace phases including zircon (melting point 1690 °C), allanite (melting temperature uncertain), apatite (melts at ~1600 °C), and the REE-and Y-phosphates monazite and xenotime (melting points exceed 2000 °C). Flame temperatures in coal-fired boilers can range from ~1300 °C (fluidized bed boilers) to 1700 °C (cyclones and pulverized coal boilers). Therefore, important REE-bearing trace phases such as monazite and xenotime should be stable at all nominal boiler conditions although local variations in coal particle temperature (which can exceed gas temperatures) may influence trace phase stability.

Whereas one recent study documents zircon, monazite, and xenotime persisting in fly ash [1], more commonly, REE-bearing minerals are far more evident in power station feed coal than in ash from burning that coal. Alternatives to post-combustion retention of REE-bearing trace phases include reaction or dissolution of trace phases in the melt (now glass) and diffusion of REE from solid trace phase lattices. A reconnaissance study using wavelength dispersive elemental mapping [2] shows counts for REE in glassy fractions of the ash, but these are at or near the limits of detection by electron beam methods. To help resolve the question of how REE in coal ash occur, we are investigating a suite of well characterized ash samples using field emission SEM to determine if REE are present as nanometer-sized particles. This will be followed by in-situ quantification of REE concentrations in glassy portions of the ash using ion-beam microanalysis.

[1] Smolka-Danielowska (2010) J Environmental Radioactivity doi:10.1016/j.jenvrad.2010.07.001.

[2] Hower, J.C. et al. (2013) Coal Combustion and Gasification Products doi:10.4177/CCGP-D13-00007.1