Research On The Cleaner Bleaching Of Pulp And Preparation Of Nanofibril From Forestry Residues By Ozone Treatment And Depolymerization

Natural resources are the important basis for the operation of economic systems,and environmental pollution is the key issue limiting future economic development.The development of renewable and degradable biomass resources is the key to solve environmental problems and sustainable development.The low-cost,large and stable forestry residues,and clean,efficient and low-cost processing are the key criteria to promote high value utilization and cleaner production of forestry residue fiber.In this paper,good bleached eucalyptus slab pulp（BESP）and high performance thermomechanical pulp（TMP）were cleanly prepared from wood processing residues and forest pruning residues with treatment of green,strong oxidizing ozone.The cellulose nanofibril（CNF）and lignin-containing cellulose nanofibril（LCNF）and film materials were cleanly prepared from the MC-BESP and HC-TMP with further treated by ozone and mechanical treatment.Based on ozone treatment,the whole process and mechanism research of clean production from forestry residues to two kinds of pulp to two kinds of cellulose nanofibrils and films had been realized.It provided new ideas for solving environmental problems such as solid waste pollution and carbon emission from forestry residues,solving the shortage of pulp raw materials,providing theoretical support for enriching the clean production routes of pulp and cellulose nanofibril,and providing technical support for high value utilization of forestry residues.（1）The oxygen delignified MC-ESP as raw material,the self-designed and built MCP-Ozone bleaching system was used for bleaching.The effects of ozone concentration,dosage and mixing intensity factors were studied,and the mass transfer mechanism of ozone was analyzed.The results showed that the higher concentration and dosage of ozone had better bleaching effects.The mass transfer process of ozone bleaching MC-ESP was consistent with the double film theory model under static and the surface renewal theory model under turbulent mixing.The optimized process for ozone bleaching MC-ESP was 170 mg/L ozone concentration,1.0%ozone dosage,1200 rpm mixing speed,with the bleached pulp of65.4%ISO brightness,4.8 kapper number and 410.6 ml/g viscosity,and with the wastewater of 739 mg/L COD and 195 mg/L TOC.The designed TCF（O_PZE_P）bleaching process was less costly and the bleached pulp met the production requirements and national standards.（2）The study was conducted with TMP prepared from forest pruning residues,using the self-designed and built HCP ozone treatment system.The surface properties and strength properties of ozone-treated HC-TMP were studied,and the degradation products and wastewater characteristics were analyzed.The results showed that the surface lignin content（SLC）of fiber decreased with the increase of ozone dosage,and when the ozone dosage increased to 7%,the lignin content on the fiber surface decreased to 39.63%.Compared with the untreated ones,the tensile index,tear index and burst index of 7%dosage ozone-treated TMP increased by 40.4%,56.5%and 47.4%,respectively.Ozone treatment effectively destroyed the lignin barrier on the fiber surface and promoted inter-fiber bonding.The wastewater of 7%dosage ozone-treated TMP mainly contained lignin fragments and the BOD₅/COD ratio was 0.46,which meant the very good biochemical degradability.Compared with biological pretreatment technology,the TMP prepared by ozone treatment had the similar performance and better water filtration capacity.Ozone treatment was suitable for the TMP of agricultural residues.（3）With the ozone treatment of MC-BESP in the MCP ozone treatment system,cellulose degradation kinetics and the reaction mechanism of cellulose depolymerization were further investigated.The CNFs and films were prepared by combining high pressure homogenization.The results showed that at 1%ozone dosage,the degree of polymerization（DP）and fiber size decreased with increasing initial p H.At the initial p H of 10 and the reaction temperature of25℃,the fiber size and DP decreased rapidly and the carboxyl content（CC）increased rapidly with the increase of ozone dosage.The fitted equations between the DP and CC and ozone dosage were:DP=331.8052-86.0662×ln（D_Z-0.2115）and CC（mol/kg）=112.2359+7.2431×ln（D_Z-0.3166）.The reaction kinetics of the cellulose depolymerization by ozone treatment was modeled and analyzed with apparent activation energies of 14.8450k J/mol and 7.0120 k J/mol for the early（less than 4 min）and late（more than 4 min）phases of the reaction,respectively.The combined depolymerization factor（CDF）of cellulose was proposed as CDF=C_Z·t·exp（8.3925-28.5689/（RT））.The equation for predicted the degree of cellulose depolymerization was:DP/DP₀=（1-0.4745）·exp（-CDF）+0.4745·exp（-0.0068·CDF）.The initial p H was 10 and the CNF prepared from 3.8%dosage ozone-treated fibers was smaller in size and more uniform.The diameters of the four CNFs（BESP、BESP+O₃、BESP+O₃+Na OH-5 and BESP+O₃+Na OH-10）were 68 nm,49 nm,42 nm and 34 nm,respectively,and the heights were 11 nm,8 nm,7 nm and 5 nm,respectively.The CNF films had some transparency and haze,and good hydrophilicity.The mechanical properties of the films increased with the ozone treatment,with the maximum tensile strength of 117.43 MPa and the elastic modulus of 4992.79 MPa.（4）The four LCNFs and films were prepared by ozone treatment of raw TMP combined with the grinding and high pressure homogenization.The results showed that ozone treatment could effectively reduce the lignin content and size of TMP fibers.When the ozone dosage was 7%,the lignin content of the TMP fiber was 13.86%,the fiber length was 618μm and the fine content was 67.57%.At the-80μm grinding gap,the length of 7%Z-TMP fibers was 374μm and the fine content was 97.65%.As the dosage of ozone increases,the size of LCNF became smaller and more uniformly distributed.The diameters of the four LCNFs（Blank-TMP、3%Z-TMP、7%Z-TMP and 11%Z-TMP）were 73 nm,64 nm,52 nm and 41 nm,respectively,and the heights were 7.7 nm,6.2 nm,5.4 nm and 4.3 nm,respectively.The SLC of the LCNF decreased with the increase of ozone dosage,and the water contact angle of its film decreased with the better hydrophilicity.The thermal stability of LCNF prepared by different ozone dosage treatment was similar.LCNF film had excellent UVA blocking performance,with some light transmittance and haze.The mechanical properties of the films increased with the ozone dosage and then decreased slightly,with the maximum tensile strength of 132.56 MPa and the elastic modulus of 3930.54 MPa.