Preliminary Assessment of the Economic Potential of Rare Earth Elements Associated with Coal in Wahau Formation

Abstract

Coal and coal-associated materials are increasingly considered unconventional sources of rare earth elements and yttrium (REY), but concentration alone is insufficient to establish economic potential. This study assesses REY enrichment, composition, oxide-equivalent grade, and processing complexity in three Wahau low-rank coals and three laboratory-produced ash samples through preliminary comparative screening. Fifteen REY (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and Y) were quantified by ICP-MS. The assessment combines: (i) enrichment relative to world coal and coal-ash averages, (ii) the Seredin-Dai outlook coefficient and critical REY percentage, (iii) calculated rare-earth-oxide (REO) grade for ash, and (iv) literature-based technical processing complexity. Wahau coal contains 26.54-60.22 mg/kg REY, below the world-coal average of 68.47 mg/kg. Laboratory ash contains 109.44-767.93 mg/kg; Wahau Ash 2 is 1.90 times the world coal-ash average of 403.5 mg/kg and corresponds to approximately 913 mg/kg REO. However, critical REY percentages are only 11.83-18.09%, and none of the samples meet the 30-51% critical-REE composition window used for promising materials in the Seredin-Dai framework. Comparison with Indonesian coal, Indonesian CFA, and coal-gangue studies shows that coal-derived residues can attain substantially higher REY grades, although refractory aluminosilicate hosts commonly require activation and hydrometallurgical treatment. The results identify Wahau Ash 2 as the strongest concentration target, while confirming that grade, REY composition, and extractability must be evaluated together before full techno-economic analysis.