WHERE DOES THE U.S. STAND ON THE PROSPECT OF RARE EARTH ELEMENT EXTRACTION FROM COAL AND COAL ASH SOURCES COMPARED TO INDIA

The continuous rise in demand for rare earth elements and yttrium (REYs) permits exploration into potential alternative sources. Coal combustion residuals (CCR) are a potential alternative source due to the enrichment of REYs during the coal combustion process.

Compared to average world coal, coal from the United States is not enriched in REYs; but studies suggest specific regions on the Appalachian basin can contain enriched coal with an REY concentration up to 1461 mg/kg.¹ Coal ash derived from Appalachian coal used during the coal combustion process have an average REY concentration of 602 mg/kg (322-1222 mg/kg).² Here we report the REY concentrations in coal and coal ash samples from various coal fields and coal power plants in central and northeastern India. Most coals from central India (Gondwana, 250 Ma) contain total REY concentrations ranging from 107-245 mg/kg and coals from northeastern India (Tertiary 15-60 Ma) contain less <0-90 mg/kg of total REYs. Coal ash from central and northeastern India contain total REY concentrations in the range of 304-591 mg/kg and 24-227 mg/kg, respectively. The outlook coefficient (C_outl) of these coals, a ratio of critical REYs (Nd, Eu, Tb, Dy, Y, Er) to excessive REYs (Ce, Ho, Yb, Lu) in whole coal, can be used as an indicator for the prospect of these coal resources for REY extraction.³ The C_outl values for central and northeast Indian coals are within the range of 0.7-1.1 (promising) and 1.2-2.7 (promising to highly promising), respectively. Coals from the United States, in comparison, have an average C_outl of 1.12 (0.3-3.18), with a C_outl > 2 indicating highly promising for REY extraction.⁴ Data from this study do not indicate regions of high REY enrichment in coal from India. The REY concentrations of Indian coal from this study do not meet recommended thresholds for economically feasible recovery of REY as suggested by the U.S. Department of Energy and Seredin and Dai 2012 (300 mg/kg and 1000 mg/kg in whole coal and coal ash, respectively) unlike coals from specific regions on the United States.

[1] Progr. Energ. Combust. Sci., 88 (2022), 100954. [2] Int. J. Coal Geol., 94 (2012), 67-93. [3] Environ. Sci. Technol. 50 (2016), 5919-5926. [4] Int. J. Coal Geol., 160 (2016), 73-81.