Rare Earth elements (REE) are types of trace elements, generally comprising less than 0.1 percent of a coal or other rock type. Because interest in domestic supplies of REEs has increased, KGS, the Department of Mining Engineering, and the Center for Applied Energy Research (CAER) at the University of Kentucky (UK) have begun exploring the REE contents of coal seams, rocks associated with coal seams, power plant ash (material left from power plant combustion of coal), and other potential sources of REEs in Kentucky.

KGS uses an approved American Society for Testing and Materials (ASTM) standard for analyzing REEs from coals and associated materials (ASTM standard test method D6357-21b). Analyzing for REEs takes time and expertise. In general, a variety of common elements in a sample must be removed so that the tiny amounts of what remains can be more accurately measured.

Samples (coal, ash, shale, etc.) are catalogued, crushed to -60 mesh (maximum particle size <250 micrometers) and ashed. Ashing involves placing a small amount of powdered sample in an oven, at 500°C for 255 minutes to remove any organic carbon in the sample.

    

Samples in crucibles in ashing furnace, and view of ashed sample

Then samples are weighed and labeled. Each sample may be split into multiple samples for different types of analyses (for example, coal quality and REEs). Next, 0.1 gram of sample is put into a test tube with a strong acid (a solution of HF, HCl and HNO₃) under a flume hood. The acid solution dissolves silicate (e.g., quartz, SiO₂) carbonate (e.g., calcite, CaCO₃) and sulfide (e.g., pyrite, FeS₂) minerals.

    

Ethan Davis (wearing saftey equipment) at KGS mixes samples with acids under a flume hood.

Next, the remaining material is placed on a hot block at 135°C to evaporate the liquid.

   

30 samples on hot block, and one removed to show dried remaining sample.

The dried sample is treated a second time with acid (HNO₃) to ensure complete digestion of the unwanted minerals. Again, the dried material is placed on a hot block to evaporate any remaining acid. Then, a small amount of deionized water is added to the sample to place it back in solution for analysis.

Jason Backus, KGS lab manager, pipetting water into the samples before placing in the elemental spectrometer.

The solution is run through an elemental spectrometer, of which there are several varieties. KGS uses an inductively coupled plasma optical emission spectrometer (ICP-OES). The Department of Mining Engineering at UK uses an inductively coupled plasma atomic emission spectrometer (ICP-AES). UKCAER operates an inductively coupled plasma mass spectrometer (ICP-MS). The choice of instrument largely depends on element abundance. The ICP-OES at KGS can be used for samples in which very low amounts (typically <20 parts per million, ppm) of a particular element are likely or expected. All ICP instruments work by first ionizing the liquid sample into a gaseous plasma and then detecting individual elements, and their relative abundance, with a spectrometer. The instrument recognizes and measures the amount of each REE from their unique position in the generated spectrum and reports values as weight percent.

    

Elemental spectrometer (ICP-OES) at KGS and detail of probe analyzing samples.

 

Examining automated results

Reference

ASTM International, 2021. ASTM D6357-21b - Standard Test Methods for Determination of Trace Elements in Coal, Coke, and Combustion Residues from Coal Utilization Processes by Inductively Coupled Plasma Atomic Emission Spectrometry, Inductively Coupled Plasma Mass Spectrometry, and Graphite Furnace Atomic Absorption Spectrometry. ASTM International, West Coshocton, PA, www.astm.org.

 

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