Effect Of Hot-Water Pre-Extraction On Alkaline Pulping Properties Of Wheat Straw

The forest resources of China are scarce. Non-wood fiber rawmaterial, especially wheat straw has been an important raw material ofpulping and papermaking. However, there are many problems inpapermaking with grass fiber raw materials, such as small scale, lowquality and serious pollution problems. With the increase of China’senvironmental protection awareness, grass fiber pulping enterprises,especially small and medium-sized enterprises are facing severechallenges.In recent years, with the global shortage of resources and energycrisis, biomass energy has been paid more and more attention becauseof its renewable and environmentally-friendly. Biorefinery technologyhas made great progress, which provides a new opportunity for thetransformation and upgrading of the papermaking enterprises.Pulping and papermaking industry is one of the main uses ofwoody biomass resources. Through the development of biomass refining,we can transformed the traditional chemical pulp mill into a factorywhich have forest industry, pulping and papermaking and biomassrefining (Integrated Forest Products Biorefinery, IFBR enterprise). As aresult, in addition to the production of pulp, the factory can also extracthemicellulose and lignin and other biomass components from the wasteor pre-extract. And these extracts can be further transformed intobiomass products with high value-added. It has a realistic and long-termsignificance for the efficient use of plant fiber resources, environmentalprotection and improving the economic efficiency of the pulp and paperindustry.In this study, the process of hot-water pre-extraction at differenttemperatures, reaction time and liquor ratios were investigated. The results of orthogonal analysis showed that the most important factor forstraw yield after hot-water pre-extraction is extraction temperature,then the reaction time, and finally the liquid ratio. The straw yieldsignificantly decreased with the increase of extraction temperature andthe reaction time. We believed that the hot-water pre-extractiontemperature should not exceed150℃, in order to avoid the seriousdecline of the straw yield. The most important factor for the solidscontent and acid soluble lignin content of pre-extract is extractiontemperature, then the liquid ratio, and finally the reaction time. Thesolids content and acid soluble lignin content are reduced with theliquor ratio increase. Liquid ratio is too large to make the pH value ofextraction decreased, which affects not only the dissolution ofhemicellulose, but also increases the cost of water.The NaOH-AQ pulping properties of wheat straw which had beenpre-extracted with hot-water were studied. The straws extracted or notwere made into pulp with NaOH-AQ at the same conditions. Then theeffects of extraction on properties of straw pulp were studied. Results ofpulping showed that pulp from pre-extracted wheat straw have a lowerKappa number compared with that without pre-extraction, while theyield and viscosity decreased to some extent. Through the orthogonalanalysis, we may reached the most important factor for the pulp yieldand Kappa number is the maximum temperature, followed by the alkalicharge, and finally the holding time at maximum temperature. In thisstudy, the optimized conditions of hot-water pre-extraction are:maximum temperature140℃, heat-up time30min, holding time atmaximum temperature60min and liquor ratio1:10. The optimal pulpingconditions are: alkali charge12%, maximum temperature155℃,heat-up time90min, and holding time at maximum temperature60minand liquor ratio1:5.The Chemi-mechanical pulping properties of wheat straw, whichhad been hemicelluloses pre-extracted with hot water, were studied. Inthis work, the process of hot-water extraction at various temperatureswas investigated. The straws pre-extracted or not were made into Chemi-Mechanical Pulp (CMP) at the same conditions. Then the effectsof pre-extraction on properties of straw pulp were studied. The resultsshowed that the Hot-water pre-extraction can improve tensile strength,tearing strength and bursting strength of the CMP, decrease sodaconsumption in chemical treatment stage, and reduce energyconsumption in the process of beating. However, the pulp yielddecreased in various degrees.The principle of the hot-water pre-extraction was analyzed. Thephysical and chemical properties of hot-water extract and black liquorwere characterized by the Fourier Transform Infrared Spectrometer(FT-IR) and Thermo Gravimetric Analysis (TGA). Hot-waterpre-extraction can destroy acetyl and furfural acid substitutions ofhemicellulose, generate acetic acid and other organic acid. Theformation of organic acids may contribute to damage the ether bonds offiber cell wall material, which is good for the formation and removal ofoligosaccharides. Under the acidic conditions, the dissolution ofhemicellulose destroyed the structure of lignin carbon hydrate complex(LCC), so it can increase the reactivity of the lignin and accelerate therate of delignification.