Effect Of Mechanochemical Treatment Of Lignite On Its Preparation Of Humic Acid And Thermal Reaction Properties

Inner Mongolia is rich in lignite resources,lignite is generally mined in open pit,and the near coal stripping layer close to the main coal seam has huge production,but it has not been effectively utilized due to its low carbon content（20-40%organic matter content in empty dry base）,low direct utilization efficiency and serious environmental pollution.The ash content of near-coal stripped layers of lignite coal mines is generally more than 50%or even 70%,and obtaining chemicals through depolymerization is an important way to improve the application value and non-energy utilization of poor quality lignite.Traditional depolymerization mainly adopts oxidation and hydrothermal reaction,which requires pressurized heating,resulting in high energy consumption and a large amount of wastewater and other pollutants,which is extremely unfavorable to the production of humic acid from poor quality lignite.In this paper,we propose to use mechanochemical method to treat poor quality lignite to extract humic acid,and study the effect of mechanochemical treatment on its structure and thermal reaction.The main research contents and results of this paper are as follows:（1）The optimum process conditions for the preparation of humic acid by mechanochemical treatment of lignite were explored by varying the KOH addition and grinding time as follows:the KOH addition to lignite was 10%（of the coal sample mass）and the grinding time was 60 min.Compared with the raw coal,the total humic acid and free humic acid yields of the mechanochemically treated lignite increased by 18.12%and 14.58%,respectively,to 45.80%and 39.93%,respectively.（2）The structure,molecular weight distribution and product types of the prepared samples were analyzed by elemental analysis,FTIR analysis,UV-Vis spectroscopy,gel permeation chromatography and gas chromatography-mass spectrometer characterization techniques.Mechanochemical treatment of lignite could change the molecular weight distribution and size of the prepared humic acids,with a substantial increase in the low-molecular-weight fraction and a decrease in the high-molecular-weight fraction;at the same time,the low-ring aromatic structure of humic acids increased,and compounds with oxygen-containing functional groups such as esters,carboxylic acids and phenols increased..（3）Structural characterization and thermal reaction tests were performed on raw coal and mechanochemically treated lignite.The mechanochemically treated lignite had lower volatile and fixed carbon content,higher oxygen element content,and lower carbon and hydrogen element content.The mechanochemical treatment broke the alkyl side chains and bridge bonds around the thick ring aromatic system in lignite,formed more oxygen-containing functional groups,and KOH and carboxylic acid functional groups formed potassium carboxylate structure,which promoted the pyrolysis,water vapor and CO₂gasification of coal,but inhibited its combustion reactivity.The results of kinetic fitting of thermal reactions of coal samples with selected homogeneous and nucleation reaction models showed that mechanochemical treatment of lignite reduced the activation energy of pyrolysis,water vaporization and CO₂ gasification reactions,but caused an increase in the activation energy of combustion reactions.