Coal deposits as potential alternative sources for lanthanides and yttrium

This paper presents data on widespread abnormal accumulations of lanthanides and yttrium (REY) in many coal deposits worldwide. High REY contents (>0.1%) have been found in coal seams and coal ashes, as well as in the host and basement rocks of some coal basins. For a preliminary evaluation of coal ashes as an REY raw material, not only the abundance but also the individual REY compositions were taken into account in this paper. Three REY distribution patterns for high-REY coal ashes are fixed, with LREY- (La-N/Lu-N>1), MREY- (La-N/Sm-N<1, Gd-N/Lu-N>1), and HREY- (La-N/Lu-N<1) enrichment. Four genetic types of REY enrichment in coal basin can be identified: 1) terrigenous type, with REY input by surface waters; 2) tuffaceous type, connected with falling and leaching of acid and alkaline volcanic ash; 3) infiltrational or meteoric ground water driven type, and 4) hydrothermal type, connected with ascending flows of thermal mineral water and deep fluids. It is shown that the main modes of REY occurrence in high-REY coals are in fine-grained authigenic minerals (REY-bearing aluminum phosphates and sulfates of the alunite supergroup, water-bearing phosphates and carbonates) and organic compounds. Stratabound and cross-cutting REY mineralization may occur in the host and basement rocks of some coal basins. There are tuffaceous and hydrothermal types of REY mineralization outside coal seams that are significantly different in geological settings, ore body shapes, and ore compositions, as well as in REY contents and distribution patterns. The data presented indicate that coal deposits should be regarded as promising objects for recovery of REY as economic by-products of coal mining and combustion. As REY are crucial metals for alternative power and energy-efficient technologies, identification of these resources during coal exploitation and utilization may not only increase beneficiation of coal deposits themselves but also will promote humanity's further movement on the green road. (C) 2011 Elsevier B.V. All rights reserved.