The Extraction And Purification Of Lignin From Alkaline Pulping Black Liquor

Alkaline pulping is a chemical pulping process for cooking plant fiber with alkaline solution.The black liquor produced in this process contains a large amount of organic compounds such as lignin and sugar.Among them,lignin in alkaline pulping black liquor is a kind of renewable natural macromolecule compound,which has great development and utilization value.However,because of the complexity of lignin’s structure and the instability of its chemical properties,the way to get the complete structure of the lignin hasn’t been found.From different sources,different separation methods and separation conditions of lignin,the structural units of the connection between the bond type and functional group composition are different,so there are still some difficulties encountered for the analysis of lignin’s structure and making good use of it.The components of black liquor are complex and crude lignin separated from pulping black liquor contains lots of impurities,such as ash and sugar,which are not conductive to the further reform and utilization of lignin.Therefore,how to maximize the purification of lignin is of great significance for the research and development of lignin.Improving the purity of lignin contributes to the further analysis of lignin’s structure and characteristics,which set the basis for the further development and utilization of alkaline lignin.The subject is divided into two parts.The first part is treating alkaline pulping black liquor with different kinds of acid under the application of acid precipitation method,replacing the sodium ion in the alkaline lignin by hydrogen ion in the acid.As a result,the alkaline lignin again becomes insoluble precipitate,so as to realize the separation of the crude lignin and black liquor.The second part is to use crude lignin extracted from black liquor as raw material and apply alcohol deposition method,cresol sulfuric acid method,liquid-liquid extraction and weak acid method to purifying it.Then compare the purification effect on crude lignin by these four methods in order to select the best method and reaction conditions.The advantages and disadvantages of different methods in separating and purifying lignin were compared mainly through the yield of crude lignin and purified lignin and the application of infrared spectroscopy,scanning electron microscopy,X ray fluorescence and DTA-TGA in this paper.The results show that when it comes to the separation of crude lignin from black liquor,the highest yield of crude lignin happens in the sulfuric acid precipitation method,which is 54.3%,while the yield of crude lignin is between 44.0%-54.3%in the use of inorganic acid precipitation method.In the use of organic acid precipitation method,the highest yield of crude lignin happens in the acetic acid precipitation method,which is 44.4%,while the yield of crude lignin is between 38.1%-44.4%.Aiming at the shortcomings of traditional acid precipitation method,the highest yield of crude lignin can reach at 62.6%in the optimized methods.In addition,among the four kinds of purification methods used in the second part,the highest recovery rate of purified lignin can be up to 79.2%and the inorganic salt content is only 1.8%in the weak acid method.Therefore,not only the highest recovery rate of purified lignin but also the highest removal rate of inorganic salts happens in the weak acid method.The thermal stability of purified lignin obtained by cresol sulfuric acid method is the best,whose highest degradation rate is at about 350 ℃.