Study On The Affinity Of Abnormally Enriched Trace Elements In The Coal-hosted Germanium Ore Deposits

The Wulantuga?Inner Mongolia?and Lincang?Yunnan Province?Ge ore deposits are both superlarge coal-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.However,the exact modes and mechanisms of occurrence of these trace elements in the organic matter of high-Ge coals are still unclear.In this dissertation,based on the theories of multiple disciplines involving coal petrology,mineralogy,geochemistry,structural chemistry,and statistics,the affinities of abnormally enriched trace elements in the Wulantuga and Lincang Ge ore deposits were deeply investigated using a variety of analytical methods including micronization,HCl-HF demineralization,multi-stage acid leaching,low temperature ashing?LTA?,coal petrology analysis,powder X-ray diffraction?XRD?,inductively coupled plasma mass spectrometry?ICP-MS?,density-gradient centrifuge?DGC?,electron probe microanalysis?EPMA?coupled with energy disperse spectroscopy?EDS?,nuclear magnetic resonance?NMR?and Fourier transform infrared spectroscopy?FTIR?.The main works of this dissertation contain four aspects:?1?Evaluation on the organic affinities of abnormally enriched trace elements in the high-Ge coalsUnlike previous studies,element affinities in this study were redefined based on a comparison of their concentrations before and after HCl-HF treatment.Thirteen bench samples from Wulantuga and Lincang were micronized to³?m using a fluid energy mill to ensure complete dissolution of micro-scale minerals?except pyrites?to HCl-HF in the demineralization process.Trace element concentrations of both micronized coals and micronized-demineralized coals were determined using ICP-MS.Seperately,a multi-stage acid leaching analysis was conducted to investigate the individual effects of HCl,HF,and HNO₃ on trace element contents.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.?2?The distribution of abnormally enriched trace elements in the macerals of high-Ge coalsTwo bench coals?WLTG C6-2 from Wulantuga and LC S3-6 from Lincang?were selected to characterize the associations between density fractions?maceral concentrates?and trace elements using micronization,density-gradient centrifugation?DGC?,and inductively coupled plasma mass spectrometry?ICP-MS?,especially for the abnormally enriched trace elements?including Ge,W,As,Sb,Be,U,and Nb?and rare earth elements and yttrium?REY?in the Wulantuga and/or Lincang high-Ge coals that are widely considered to have varying degrees of organic affinities.Huminite and inertinite contents of WLTG C6-2 account for 58.4%and 40.7%,respectively,whereas LC S3-6 is dominated by huminite?98.7%?.Lower peak densities for huminite/inertinite and Gaussian distributions of DGC profiles after HCl-HF demineralization suggest that some minerals still exist in the micronized fractions.In the Wulantuga high-Ge coal,Be and As are preferentially enriched in inertinite-rich density fractions,perhaps due to association with minerals in the heavier fractions.Higher Ge contents are seen in huminite fractions relative to inertinite,possibly due to differences in sites capable of holding Ge.Antimony is slightly more enriched in huminite-rich fractions.A positive Ge-W correlation and variable W concentrations across the density profile suggest a mixed organic-inorganic affinity for W,and Nb shows a similar distribution to W.Barium is predominantly associated with minerals.In the Lincang S3 seam,Ge concentration increases versus fraction density,probably due to the availability of more bonding sites in denser huminite macerals.The preferential enrichments of Be,U,and Sb in heavier fractions could be due to more bonding sites in denser huminites and/or their inorganic affinities.Arsenic content generally varies smoothly across DGC fractions,and most pyrite particles where at least part of the As occurs were removed in the DGC process.Tungsten,Nb,and possibly Th may occur in similar phases due to their comparable variation trends.In addition,REY anomalies in some fractions mainly result from elevated LREY,and REY?especially LREY?in WLTG C6-2 and LREY in LC S3-6 may occur in minerals with some trace elements,including Sc,Sr,and Ba.?3?Microanalysis on the distribution and occurrence of abnormally enriched trace elements in the high-Ge coalsIn this study,an in-situ electron probe micro-analysis?EPMA?technique was applied in order to map the distribution of both light elements?C,O,and S?and selected trace elements?Wulantuga:Ge,W,and As;Lincang:Ge,W,As,and U?in the high-Ge lignite samples and their low-temperature ashes?LTAs?.The EPMA detection limit was stable for these light and trace elements commonly associated with organic matter,so that reliable point analysis data were acquired for individual macerals.Additional mineralogical and geochemical data were obtained by petrographic microscopy,XRD,EDS,and ICP-MS analyses.WDS in-situ element mapping and point analysis demonstrate that the huminite in the Wulantuga coal has a lower C content and higher O and S contents relative to inertinite,and Ge,As,and W show a preferential enrichment in huminite.Among the different huminite macerals in both the Wulantuga and Lincang coals,Ge,As,W,and U tend to occur in more compact huminites?ulminite or gelinite?,and their abundances show mild to considerable variations across the compact huminites.The in-situ concentrations of Ge and W display a direct correlation,suggesting their co-affiliation with organic matter.Elements such as Ge,W,and U have a considerable organic affinity,with a minor fraction associated with inorganic S-rich phases,such as iron sulfide/sulfate?e.g.,pyrite and copiapite?.Arsenic has a mixed affinity to organic matter and iron sulfide/sulfate.No Ge minerals have been detected in the LTAs,and Ge in the LTAs is likely to exist in an inorganically amorphous state.?4?Structural causes of abnormally enriched trace elements in the high-Ge coalsAccording to the qualitative and semi-quantitative analyses results for ¹³C-NMR and FTIR spectra,the organic structure responsible for Ge enrichment in the Wulantuga and Lincang high-Ge coals was speculated by the structural differences of huminites from high-Ge coals before and after HCl-HF treatment.The qualitative analysis could not provide useful information.However,the semi-quantitative analysis results showed that no aromatic bridgehead carbons?129-137 ppm?exist in the huminites of WLTG C6-3 and LC S3-6.The common changes of NMR spectra after HCl-HF leaching are reductions of aromatic-CH₃?16-25 ppm?,-CH₂?25-37 ppm?,and oxygen-bonded aromatic carbons?149-164 ppm?.The three selected regions from FTIR spectra for semi-quantitative analysis were 700-900 cm^-1,1560-1800 cm^-1,and 2800-3000 cm^-1,respectively.The common changes of FTIR spectra after HCl-HF leaching are reductions of aromatic C=C and phenolic hydroxyls(¹610 cm^-1)and asymmetric-CH₃(²960 cm^-1).Considering the Ge occurrence in high-Ge coals?Ge-O coordination?and the results of NMR and FTIR analyses,the organic structure responsible for Ge enrichment in the high-Ge coals is probably phenolic hydroxyl.This study could provide reliable evidences and new perspectives on the modes of occurrence of abnormally enriched trace elements?especially Ge?in the coal-hosted germanium ore deposits in China,and it could also provide theoretical support for finding better recovery methods for the organically associated trace elements from coal in the future,including Ge.