Isotopic Geochemical Constraint Of Environmental Coal-derived Sulfur And Experimental Study On Pyrite Pyrolysis

The consumption of coal resources in China is huge.With the growth of the national economy,the development and utilization of high-sulfur coal has become a major problem.Pyrite is the most common inorganic sulfide mineral in coal.If the pyrite cannot be removed from the coal,the amount of sulfur in the environment will increase greatly.Not only that,some harmful elements in coal are prone to occur with pyrite.The reaction caused great harm to the environment.Removal of pyrite by pyrolysis magnetic separation is an effective method for pre-combustion physical desulfurization.This method has the characteristics of low energy consumption,simple process,and economical and environmental protection.This thesis analyzes the sources of sulfur in coal in North China and Southwest China from the perspective of isotopes,and identifies the relationship between sulfur and coal in the environment of Xuzhou.In addition,this thesis used thermal gravimetric analysis（TGA）experiments and microwave pyrolysis experiments,magnetic susceptibility testing and X-ray diffraction（XRD）analysis,combined with crystal field theory,to explain the pyrolysis process from macro and micro aspects.The main understanding is as follows:（1）The sulfur isotope geochemistry of coal in North China and Southwest China is compared.Southwest coal has been affected by the eruption of the late Paleozoic Emeishan basalts.The sulfur isotope composition in coal is negative with respect to North China coal.（2）Sulfur isotopic compositions in the atmosphere,water and coal of Xuzhou is explored.The sulfur isotopes in the particles of Xuzhou City have the characteristics of coal combustion,and the sulfur content in the atmospheric precipitation is small.The sulfur in lake and groundwater comes from dissolved sulfate or water-rock reaction,and there is no obvious coal-combustion effect.（3）The pyrolysis process of pyrite in coal is described from a macroscopic and microscopic perspective.The pyrolysis of pyrite-containing coal generally goes through three major stages.The pyrolysis of pyrite occurs in the range of 400°C to640°C.The higher the pyrite content,the lower the decomposition temperature.（4）The magnetic generation mechanism of pyrite change in coal during pyrolysis was revealed.During the pyrite pyrolysis,the S/Fe ratio decreases with the increase of temperature,and the coal has a catalytic effect on pyrite decomposition.The change of S/Fe ratio is FeS₂→FeS_x→FeS_1.25（1.25<x<2）.After 1000°C,the S/Fe ratio may also decrease.The change in the magnetic susceptibility curve basically corresponds to the change of pyrite decomposition in DTG and DSC curves（400°C⁶40°C）.The XRD phase analysis also basically shows that there is no obvious change in the pyrite at 400°C.The crystal shape of pyrite begins to change at500°C,and pyrrhotite（Fe₇S₈）is formed at 600°C,and finally at 1000°C,pyrrhotite(Fe_0.95S_1.05)with a lower S/Fe ratio was formed.The essence of the pyrolysis of pyrite is the changing of Fe atoms from low spin to high spin.（5）The difference between ordinary pyrolysis and microwave pyrolysis is compared.In the microwave pyrolysis,the activation energy required for the pyrite decomposition is greatly reduced under the combined effect of the thermal effect and the microwave effect,so that the decomposition temperature of the pyrite is advanced.Microwave pyrolysis is an efficient desulfurization technology,but the cost is too high.