Influence of lanthanide contraction effect on the interaction of REE and coal and the occurrence forms of organic rare earth element in coals

ICP-MS was used to examine the rare earth element (REE) in the raw coals from Yimin coalfield, East Inner Mongolia, and the samples were treated by acid washing and humic acid extraction. Based on the correlation between the REE concentration and the ash and organic elements(C, H, O) for raw coal and demineralized coal as well as the occurrence of REE in humic acids, it can be concluded that the interaction between REE and the organic matter should be controlled by deoxygenation, dehydrogenation and carburetion during the coalification. Four kinds of REE-organic compounds in these coals might be formed: (1) the REE-organic complexes with aromatic structures highly substituted by aliphatic side chains, (2) ones with hydroaromatic structures, (3) ones with lower substituted aromatic structures, (4) ones with oxygen-containing functional groups. The REE-organic compounds with lower substituted aromatic structures and oxygen-containing functional groups are less stable. Meanwhile, not only the lanthanide contraction effect but also the valences of REE can influence the stability of REE-organic compounds. Compared to the LREE(light rare-earth element), the HREE(Heavy rare-earth element)-organic complexes are more stable. The combination of LREE with humic acid is unstable contrary to the lanthanide contraction in raw coal. Like the combination of MREE(middle rare-earth element) and HREE with humic acids in raw and demineralized coals, however, the combination of LREE with humic acids in demineralized coal shows a good lanthanide contraction effect.