| In China,with the decrease of high quality coal resources,high sulfur coal began to show great economic value.Sulfur is an important factor affecting the clean utilization of coal.The research on crystal characteristics of inorganic sulfur and chemical speciation of organic sulfur can provide a theoretical basis for the “clean,efficient,and high value-added” utilization of high sulfur coal.Firstly,characteristics of high sulfur coal in Eastern Sichuan mining area were studied.The coal is coking coal with medium ash and high sulfur content.Pyrite and organic sulfur account for 66.5% and 23.2% of total sulfur,respectively.A large number of fine particles with high sulfur make desulfurization difficult.Conventional flotation can not reduce the sulfur content of clean coal on the basis of high yield.The two main reasons(entrainment and floatability of pyrite itself)for flotation of dissociated pyrite were explored respectively.The simplified calculation of flotation process shows that part of liberated pyrite was carried into clean coal by entrainment.Coal-derived pyrite has higher carbon content.Quantum chemical calculation shows that carbon atom can occupy the interstitial lattice site in pyrite cell,and the defect formation energy of 1at.%carbon-doped pyrite is-217.08 KJ/mol,which indicates that carbon-doped pyrite is thermodynamically stable.The calculation of mechanical properties shows that with the increase of carbon concentration,the hardness of pyrite decreases and it becomes easy to break,which leads to the increase of fine pyrite particles in flotation slurry,the increase of entrainment probability,and the increase of sulfur content in clean coal.The wettability differences of coal and pyrite crystal planes were investigated using coal powder,pyrite powder,coal flake and pyrite {100},{210},{111} single crystals as research objects.The hydrophilicity difference between coal and pyrite is greater than that of oil affinity.Hydrophobicity of pyrite is greatly affected by p H and its hydrophobicity/lipophilicity increase after acid treatment.Acid-treated {111} plane is the most hydrophobic plane.In aqueous environment,the difference of gas affinity between four surfaces is less than that of oil affinity.The spreading of oily collector droplet on coal surface is less affected by p H.The difference of oil affinity between pyrite crystal faces and coal is small in acidic solution,especially the {100} plane.Surface oxidation degree of coal after acid/alkali treatment is smaller than that of pyrite.Polishing process and alkali treatment lead to oxidation of pyrite,which increases the hydrophilicity.After alkali treatment,oxidation degree of sulfur is the lowest on {210}crystal plane,while that of iron is the lowest on {111} crystal plane.The oxidation degree of {100} crystal plane is higher than the other two crystal planes.Pytite(100),(210)and(111)crystal planes and Wiser bituminous coal molecular model were built.The difference of hydrophobicity between pyrite crystal planes and coal was explored by molecular dynamics simulation.Water droplets spread fast on pyrite crystal surfaces,but not spread on coal surface.The(111)plane has strong hydrophobicity due to the two layers of sulfur atoms on surface.The(210)plane has strong hydrophilicity because of the four coordinated iron on surface.In aqueous environment,the adsorption of collector in coal pores shows that water does not affect the interaction between collector and coal.Collector can also be adsorbed and spread on the(111)plane,and the adsorption energy is larger than that of coal.(111)plane has strong lipophilicity and floatability,which is one of the reasons that pyrite was collected into clean coal in coal flotation.The organic matter was depolymerized by “sequential extraction-column chromatography separation-continuous thermal dissolution” technology to study the chemical speciation and preliminary removal of organic sulfur.Raw and flotation clean coal were sequential extracted by petroleum ether,dichloromethane,acetone,methanol,acetone/dichloromethane mixed solvent(V:V=1:1).The extracion residues were thermal dissolved by cyclohexane,ethanol and isopropanol step by step.The properties of extracts and thermal soluble substances were analyzed with gas chromatography/mass spectrometer.The properties of extraction residues and thermal insoluble portions were analyzed by X-ray photoelectron spectroscopy,Fourier transform infrared spectroscopy and element analyzer.The main components in extracts and thermal soluble substances are polycyclic aromatic hydrocarbons,and the content of long-chain alkanes is low,indicating that coal sample has a high degree of metamorphism and aromatization.The sulfur in raw and clean coal extracts is mainly octathiocane and thiophene,which indicates that sulfur was collected into clean coal during floatation due to its hydrophobicity,resulting in the increase of sulfur content in clean coal.Dibenzothiophene,benzonaphthothiophene and their monomethyl/dimethyl substitutes are the main organic sulfur compounds in extracts.The sulfur-containing organic compound with the largest number of rings detected is dinaphothiophene.No benzothiophene with low aromatization was found.Column chromatography realized the enrichment and separation of sulfur-containing substances and the detection of low concentration sulfur-containing organic compounds in the extract.No sulfur was found in thermal soluble portions of extraction residue.Thiophene is still the main organic sulfur.Compared with extracts,benzonaphthothiophene and its substituents increased,because high temperature and pressure are favorable for the dissolution of polycyclic sulfur-containing organic compounds.There are also small molecular aliphatic sulfur compounds in thermal soluble substances of raw coal extraction residue,such as: dicyclohexyl disulfide,diethylsulfone,3,5-dimethyl-1,2,4-trisulfide,diethyltri sulfide,1,1-bis(ethylthio)-ethane.The content of pyrite in coal decreased after extraction and thermal dissolution,and thiophene was still the main form of organic sulfur in coal.The sulfur content of raw coal decreased by 0.71%,while that of clean coal only decreased by 0.22% after extraction and thermal dissolution.It is because that the aromatization degree of coal is high,and the content of small molecule sulfur-containing organic compounds is low.Sulfur closely combines with the aromatic core of macromolecules,and it is difficult to break the C-S bond by extraction at room temperature and continuous thermal dissolution at high temperature.This thesis contains 100 figures,48 tables,and 207 references. |
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