Acid solubility and affinities of trace elements in the high-Ge coals from Wulantuga (Inner Mongolia) and Lincang (Yunnan Province), China

Abstract

The Wulantuga and Lincang Ge ore deposits are both considered world-class, coal-hosted germanium deposits. Previous studies have suggested that the abnormally enriched trace elements (including Ge, W, As, Sb, Be, U, and Nb) and rare earth elements and Y (REY) in the Wulantuga and/or Lincang high-Ge coals are associated with organic matter to varying degrees.Unlike previous studies that have used untreated bulk coal for trace element analyses, element affinities in this study were evaluated based on a comparison of their concentrations before and after HCl-HF treatment. Thirteen bench samples from Wulantuga and Lincang were micronized to ~3μm using a fluid energy mill to ensure effective exposure of mineral matter to acids during subsequent HCl-HF demineralization. Trace element concentrations of both micronized coals and micronized-demineralized coals were determined using ICP-MS. Separately, a multi-stage acid leaching analysis was conducted to investigate the individual effects of HCl, HF, and HNO3 on trace element content.Most trace elements that are widely considered to have varying degrees of organic affinity in the Wulantuga and/or Lincang high-Ge coals, including Ge, W, As, Sb, Be, U, Nb, and REY, can be removed to a large extent by thorough HCl-HF leaching, which suggests that if these elements do have an organic association they are held only weakly by the organics. In addition, multi-stage acid leaching results show that HCl is highly effective in the removal of W, As, and REY in the Wulantuga high-Ge coal and As, Be, U, and REY in the Lincang high-Ge coal, whereas HF contributes most to the removal of Ge and Sb in the Wulantuga coal, and Ge, W, Sb, and Nb in the Lincang coal. The highest Ge contents are seen in those lignites that have the highest huminite contents.The most likely explanation for the observations made in this study is that the abnormally enriched trace elements (especially Ge) in the Wulantuga and Lincang high-Ge coals are weakly bonded to organic matter, possibly occurring in the form of chelates, thus allowing them to be removed by HCl-HF. Alternatively, these elements may be associated with fine-grained minerals (or poorly crystalline mineraloids or non-mineral inorganics) that are closely associated with organic matter but are difficult to detect.