Critical Elements Enrichment Mechanism Of The Lopingian Coal-bearing Strata In The Eastern Sichuan Basin

Coal-hosted rare metal deposits are the hot topics in the field of coal geology,which have attracted much attention in recent years.The abundances,distribution,enrichment mechanism and extraction of coal-bearing strata in the eastern Sichuan Basin(the late Permian Longtan Formation of the Zhongliangshan Coalfield and the late Permian Wujiaping Formation of the Southeastern Chongqing Coalfield occurred as the typical cases)were studied using the theory of coal geology,mineralogy,geochemistry and metallurgy.Some methods were applied in this study,including the X-ray fluorescence spectrometer(XRF),inductively coupled plasma mass spectrometry(ICP-MS),low-temperature ashing X-ray diffraction plus Siroquant analysis(LTA-XRD-Siroquant),microscope,scanning electron microscopy in conjunction with energy-dispersive X-ray spectrum(SEM-EDX)and chemical extraction.1.The characteristics and mechanism of the critical elements enrichment in the late Permian Longtan Formation coals in the Zhongliangshan CoalfieldThe Zhongliangshan coals are medium volatile bituminous in rank(random vitrinite reflectance,average 1.38%),characterized by a medium-ash yield(26.84%)and high sulfur content(3.38%).Macerals are dominated by the vitrinite group,followed by the inertinite group.Minerals in the Zhongliangshan coals are mainly composed of clay assemblages(kaolinite,the illite/smectite mixed layer(I/S)and chamosite),pyrite,quartz,carbonate minerals(calcite,marcasite,ankerite,and dolomite),and anatase,followed by rutile,jarosite,natrojarosite,bassanite,gypsum and orthoclase,with traces of apatite,rhabdophane and barite.Compared with the average concentrations of the world hard coals,some trace elements including Li,V,Co,Cu,Se,Zr,Nb,rare earth element and yttrium(REY),Cd,Ta,Hf and Hg,are enriched in the Zhongliangshan coals.The modes of occurrence of chamosite,barite,rhabdophane,quartz and calcite in the Zhongliangshan coals indicate that the coals have probably been affected by the injection of multi-stage low-temperature hydrothermal fluids.Based on the concentrations of Sc,V,Cr,Co,Ni,Cu and Zn,the ratios of Al2O3/Ti O2 and the upper continental crust-normalized REY distribution patterns of the Zhongliangshan coals,the dominant sediment source regions are probably the Dabashan Uplift,Hannan Upland,and Leshan–Longnvsi Uplift rather than the Kangdian Upland.The K7 and the upper portion of K1 coals have the potential as raw materials for the recovery of REY.2.The characteristics and mechanism of the critical elements enrichment in the late Permian Longtan Formation tuff in the Zhongliangshan CoalfieldSome trace elements including Li,Be,Sc,V,Cr,Co,Ni,Cu,Zn,Ga,Zr,Nb,Cd,Sb,REY,Hf,Ta,Re,Th,and U are enriched in the late Permian Longtan Formation tuff from the Zhongliangshan Coalfield.The minerals in the tuff mainly include kaolinite,illite,pyrite,anatase,calcite,gypsum,quartz,and traces of minerals such as zircon,florencite,jarosite,and barite.The tuff is of mafic volcanic origin with features of alkali basalt.Some minerals including florencite,gypsum,barite and a portion of anatase and zircon have been derived from the hydrothermal solutions.It is suggested that the Zhongliangshan tuff is a potential polymetallic ore(including Nb,Ti,Ga and REY).3.The characteristics and mechanism of the critical elements enrichment in the late Permian Wujiaping Formation K1 coal seam in the Southeastern Chongqing CoalfieldThere is only one minerable coal seam(K1)in the late Permian Wujiaping Formation in the Southeastern Chongqing Coalfield.The K1 coal seam in the Donggou mine is low volatile bituminous coal,characterized by high sulfur and low and medim ash.The K1 coal seam in the Lutang mine is semi-anthracite and has high sulfur and low and medium ash.The K1 coal seam in the Daping mine belongs to medium volatile bituminous coal,which has a high sulfur and low ash.The dominant maceral group of the K1 coal seam in the Southeastern Chongqing Coalfield is the vitrinite,followed by the inertinite.However,the liptinite is unable to be recognized owing to a higher reflectance.The minerals of the K1 coal seam in the Dongzhou mine are mainly composed by clay minerals(including illite,I/S,kaolinite and trace amount of chamosite)and pyrite,to a lesser extent,jarosite,quartz and orthoclase.The minerals of the K1 coal seam in the Lutang mine are dominated by the clay minerals(including illite,I/S and kaolinite)and pyrite,while the orthoclase,calcite,ankerite,quartz and anhydrite were observed in some samples.The clay minerals(including illite,I/S,kaolinite,and trace amount of chlorite)and pyrite are the primary mineralogical compositions of the K1 coal seam in the Lutang mine,where the basanite(gypsum),anatase,and anhydrite are the accessory minerals.Some trace minerals(including rhabdophane,florencite,goyazite,xenotime,zircon,baddeleyite,and celestite),which are below the detection limit of XRD,are identified using the SEM-EDX.The K1 coal seam in the Southeastern Chongiqng Coalfield is enriched in Li,Nb,Zr,Ga,and REY.Based on the preliminary assessment,the K1 coal seam could be a potential source of Li,Nb,Zr,Ga,and REY recovery.The modes of occurrence and extraction methods need further study.The dominant sediment source region of the late Permian Wujiaping Formation K1 coal seam in the Southeastern Chongqing Coalfield is probably the Jiangnan Upland,rather than the Kangdian Upland.The K1 coal seam in the Southeastern Chongqing Coalfield is also influenced by the multi-stage hydrothermal fluids injection and sea water.4.The leaching process of REY,Ga and Nb of the tuff in the Zhongliangshan CoalfieldThe leaching behavior of the late Permian Longtan Formation tuff in the Zhongliangshan Coalfield was studied using the method of the alkaline sintering-water immersion-acid leaching(ASWIAL).The results show that the ASWIAL method can separate and extract REY,Ga and Nb effectively under the optimized conditions of calcining at 860 °C for 0.5 h with a sample to sintering agent ratio of 1:1.5,immersing at 90 °C for 2 h with 150 m L hot water dosage,and leaching using 4 mol/L HCl at 40 °C for 2 h with a liquid-solid ratio of 20:1(m L: g).The final leaching efficiencies of REY and Ga are up to 85.81% and 93.37%,respectively,whereas the leaching efficiency of Nb is less than 1%,suggesting the high concentration of Nb in the leaching residue,which needs further extraction.