Research On The Oxy-cracking Of Lignite In Aqueous For Producing Humic Acid

At present,it is of great significance to attach importance to and develop non-fuel utilization ways of lignite to produce high-value-added chemicals for effective utilization of fossil resources and environmental protection.In order to obtain humic acid(HA),an novel oxy-cracking technique,which is a combination of cracking and oxidation reactions,was conducted to convert Xilingele lignite(XL)and Shengli lignite(SL)into HA,and the effects of oxy-cracking conditions such as reaction temperature,reaction time,KOH/coal and the initial pressure of O2 were investigated in this paper.Meanwhile,the HA and oxy-cracked residual coal(OR)were separated by the stepwise acidification and the alkaline extraction,respectively.Further,the correlation between lignite structure and its oxy-cracking reactivity,based on the analysis and characterization of the product structure,were studied.Results indicate that the active cross-linking bridge bonds in the macromolecular structure of lignite,such as methylene and methylene ether bonds(Ar(CH2)nAr’,n>2 or Ar(CH2)nO(CH2)mAr’,n≥1,m≥0),were the key structural factors affecting the oxidative depolymerization of lignite to obtain HA.Due to low metamorphism degree,XL consists of more active bridge bonds,and is easy to be oxy-cracked at a mild temperature,with a high HA yield.SL has a low oxy-cracking reactivity because its macromolecular structure is difficult to depolymerize.The conversion of SL and yield of HA are low.Reaction temperature is the main factor affecting lignite oxidation-cracking,Increasing reaction temperature can significantly promote the oxidation-cracking of lignite and its intermediates,and significantly affect the yield of target product HA.KOH has a good catalytic effect on lignite oxidation and cracking,and can also promote the dissolution of depolymerization products,which is conducive to the increase of HA yield.Reaction time is a kinetic factor affecting lignite oxy-cracking,Prolonging reaction time was beneficial to the oxidation and cracking of HA resulting to the decrease of HA yield.The effect of initial pressure of O2 on XL oxidation cracking is small,but it can promote the oxidation of SL and its HA and produce a large amount of CO2.Under optimized conditions,the HA yield of XL(120℃、KOH/XL=1:3、O2 5 MPa、2 h)could reach 81%without CO2 detected,and the HA yield of SL(160℃、KOH/SL=3:5、2 h、O2 4 MPa)was 48%with 5.1%of CO2 yield.The HA from the oxy-cracking of lignite is composed of aromatic compounds containing more oxygen-containing functional groups such as hydroxyl and carboxyl groups.However,it is impossible to effectively separate HAs by adjusting the pH of acidification.It is speculated that the representative structures of XL-HA are mononuclear and binuclear naphthalene polycarboxylic acids and their salts,while the representative structures of SL-HA are mononuclear tricyclic and binaphthyl polycarboxylic acids and their salts.There is a certain amount of alkali soluble matter in the OR,which can be further separated and recovered by dilute alkali solution.The carbon balance of XL and SL were 96.47%and 97.71%,respectively.Compared with conventional alkali-oxygen oxidation of lignite to produce aromatic carboxylic acid,the oxy-cracking of lignite for producing HA can carry out at lower reaction temperature and alkali/coal ratio,with a lower CO2 yield and higher carbon utilization efficiency.Meanwhile,it can also avoid complicated separation and purification.Therefore,the oxy-cracking of lignite to obtain HA is a high value utilization approach with good application prospects.