Rare earth elements in superhigh-organic-sulfur Rasa coal ash (Croatia)

The Rasa coal from Istria belongs to a special type of superhigh-organic-sulfur coals, with organic sulfur content of 11%. This study encompasses culm and coal ashes from old landfills left behind after many decades of the Rasa coal use with regards to their rare earth element geochemistry. Regarding the studied coal ashes, obtained results suggest a substantial variability in the content of rare earth elements and Y (REY) in coal combustion products generated from the Rasa coal. Although samples of coal ash and culm of the Rasa coal have REO (oxide of REY) content (up to 464 mg kg(-1)) below the level considered as the cut-off grade for beneficial recovery of these metals (1000 ppm, ash basis), they contain relatively high share of critical REY (> 30%). The C-oud values (outlook coefficient of REY ores; based on Seredin-Dai's criterion) range from 0.78 to 1.45 while Eu and Ce anomaly display slightly positive and negative values, respectively. With La-N/Sm-N ratios below unity, from 0.65 to 0.88, and La-N/Lu-N and Gd-N/Lu-N ratios ranging from 0.53 to 1.17 and from 0.80 to 1.41, respectively, they dominantly correspond to M-type of enrichment and only sporadically to H-type of enrichment. Previous studies attributed the formation of superhigh-organic-sulfur Rasa coal to the seawater percolation. However, the geochemical data presented in this study suggest that the Raga coal-bearing strata, representing the Upper Paleocene lacustrine and brackish facies, were influenced by both the seawater and the hydrothermal solutions under conditions which led not only to sulfur enrichment but also to REY accumulation. From the economic standpoint and currently available data, Rasa coal ash cannot be considered as the secondary raw material for rare earth elements, but its significantly variable composition and relatively high REO found in culm highlights the need for future studies of Raga coal and related combustion products.