Rare earth elements and yttrium (REY) in coal mine drainage from Southwest China: Geochemical distribution and resource evaluation

Abstract

The ever-intensifying global demand and supply shortage of rare earth elements (lanthanides and yttrium, REY) have led to investigations into alternative sources of REY. Coal mine drainage (CMD) has been suggested as an attractive supplementary source of REY primarily due to its high content of dissolved REY, and some CMD is considered critical-REY resource. Guizhou has the fifth largest coal reserve in China and a long history of coal mining activities with the continuous discharge of CMD, which is a long-overlooked likely source of REY. In the present study, 120 CMD samples were collected from the Guizhou coal mine areas of Southwest China to investigate REY concentrations, enrichment trends and potential resources for REY reclamation. The obtained concentrations of REY across all the CMDs vary from less than 0.02 μg/L to more than 8765.66 μg/L, with an average of 684.12 μg/L. The average ΣREY concentrations of CMD from different coal-bearing strata decrease in the following order: Liangshan Formation (LS Fm.; 2337.14 μg/L) > Xiangbai Formation (XB Fm.; 1591.82 μg/L) > Heshan Formation (HS Fm.; 1249.22 μg/L) > Longtan Formation (LT Fm.; 1243.48 μg/L) > Banan Formation (BN Fm.; 534.66 μg/L). When normalized to the North American Shale composite (NASC), most CMD samples generally display middle rare earth element (MREE) enrichment, but some are also enriched in light rare earth element (LREE) and heavy rare earth element (HREE). The REY distribution patterns, Mantel test and geochemical modelling results demonstrate that the geochemical compositions of CMD and the geological conditions are significantly correlated with the behaviours of REY in CMD. CMD with pH ≤ 5.5 contains elevated concentrations of dissolved ΣREY and is also enriched in critical-REY (Nd, Eu, Tb, Dy, Er and Y). Critical-REY in CMD with pH ≤ 5.5 accounts for 19.07%–68.21% of the total REY, and the outlook coefficients (Coutl) of REY reclamation in acidic CMD are in the range of 0.48–4.14. The data of high proportions of critical-REY and the Coutl values presented in this study suggest that CMDs with pH ≤ 5.5 from coal-bearing strata of the LS Fm., LT Fm. and XB Fm. could be a promising alternative source of REY, especially critical-REY.