The Migration Of Trace Elements During The Pyrolysis And Combustion Process Of Mongolia Oil Shale

In order to study the migration of trace elements in the pyrolysis combustion process of Mongolian oil shale,we use step-by-step extraction method and microwave digestion method using tube furnace,microwave digestion instrument,ICP-MS,specific surface area analyzer and other instruments.Experimental methods such as BET determination method explored the content of trace elements in the oil shale in Mongolia,the morphology of the trace elements and the migration of trace elements after pyrolysis and the change of pore structure of oil shale.The pore structure change and the occurrence morphology were combined to analyze the mechanism of migration of trace elements in the pyrolysis combustion process.The results show that the content of rare earth elements in Mongolian oil shale samples is generally lower than that in North American shale,but there is still a significant enrichment phenomenon compared with coal.In the oil shale samples,LREE is significantly enriched compared with HREE,and the fraction of light and heavy rare earth elements is obvious,which is moderately enriched by light rare earth.The change trend of heavy metal elements in Mongolian oil shale samples is basically consistent with the change trend of crustal abundance.The arithmetic mean of U content in Mongolian oil shale is 8.16×10^-6,and the arithmetic mean of the content is 22.53×10^-6.The content of these two elements is much higher than the abundance of the crust and sedimentary rocks.This indicates that U and Th are enriched in oil shale in Mongolia.When the final pyrolysis temperature is below 900°C,the mobility of 8 heavy metal elements in the four samples does not reach 100%.The trace elements start to release below500°C,and the migration rate decreases and then decreases with increasing temperature.Trend.The occurrence of the same elements in oil shale is similar.But the differences between the different elements are obvious.The phenomenon that Cu and Zn are rapidly precipitated under high temperature conditions during pyrolysis is because the carbonate content of the two elements is significantly higher than other elements.For U and Th,mainly in inorganic minerals,distributed in carbonates,sulfides and aluminosilicates,they are the most abundant in the sulfided state.As the final temperature of the pyrolysis increases,the elements on the exchangeable and organic interface precipitate first.As the final pyrolysis temperature increases,the mobility of U and Th elements gradually increases.However,when the final pyrolysis temperature is lower than 900°C,the mobility cannot reach 100%,generally between 40% and 80%.Both elements are more volatile,and the volatility of Th is significantly higher than the volatility of U.The internal structure of the oil shale changes during pyrolysis.When the final pyrolysis temperature reaches 400°C,the internal pores of the oil shale begin to develop,and the specific surface area and total pore volume begin to increase rapidly.Therefore,U and Th start to precipitate rapidly at this temperature.When the final combustion temperature was below 900°C,the mobility of 8 heavy metal elements in the four samples did not reach 100%.The mobility of the eight trace elements in the combustion process is similar to that of the final combustion temperature,indicating that the final temperature of combustion has a strong correlation with the migration of trace elements.The final combustion temperature has a significant effect on the adsorption of oil shale semi-coke,and the maximum adsorption amount first increases and then decreases,reaching a maximum at 700°C.The specific surface areas of the four groups of oil shale and semi-coke have a strong consistency with temperature.Unlike the pyrolysis process,trace element migration is positively correlated with pore development during combustion.