Study On The Element Chlorine Free Bleaching Of Wheat Straw Pulp

With the intensified consciousness of envioronment protection and the require of the environment protection law, the paper industy moves on gradually, from the tradional CEH processes to the low or no pollution ways-ECF or TCF bleaching. There are more and more attention paied to ECF bleaching which based on chlorine dioxide bleaching, due to its good strength and low cost, compared to TCF bleaching. Therefore ECF bleaching has been becoming a way to reduce pollution and improve the pulp quality. In our country, we have a lot of wheat straw material, but little wood, so in a long run, wheat straw will still be main material in the field of paper industry. Many researchers have made an extensive and deep study on how to use the cellulose material properly to improve the technique of bleaching and reduce the pollution and they have acquired certain achievement. The pollution of waste water of CEH bleaching is serious and the quality of bleached pulp is not good, so the study on ECF bleaching of wheat straw pulp is feasible in our country.In this thesis, the technical processes and mechanism about ECF bleaching of NaOH-AQ wheat pulp were studied. The technical conditions and methods of improving efficiency of chlorine dioxide single bleaching were determined. The optimal treatment conditions with xylanase and acid pretreatment on ECF bleaching were investigated. The effect of HuA for ECF bleaching of wheat straw pulp was studied. And the mechanism kinds of bleaching conditions was studied too. Furthermore, the change of pulp morphological structure was analyzed by SEM (scanning electron microscopy). It also studied the degradation of lignin and the change function group of wheat straw pulp after D and ECF bleaching with UV, FTIR. And the main conclusions are as the follows.Adding V2O5 or HPA to the process of chlorine dioxide bleaching could improve the pulp brightness, and the bleaching efficiency was advanced. The optimum conditions were: dosage of V2O5 0.014%, temperature 70oC, time of reaction 120min, initial pH value 1.0 and the dosage of HPA is 0.5%-0.6%.Acid pretreatment could reduce the content of HuA of wheat straw pulp from 11.8mmol/g to 6.4mmol/g, increase brightness about 0.7%ISO, improve the efficiency of chlorine dioxide. Through different processes of ECF bleaching, the brightness of pulp could reach 80%ISO, and the pulp property was better than that by CEH, what's more, brightness of pulp had excessed 83%ISO by DXP, DQP, DAP bleaching processes, and the pulp property was also better than CEH pulp. Oxygen bleaching had a significant influence during ODQP bleaching of wheat straw pulp and the best conditions were: dosage of alkali 0.2%, pressure of oxygen 0.6MPa, temperature 110oC, time 60min, pulp consistency 10%, dosage of MgSO4 0.5%.The content of the Cu2+, Mg2+, Ca2+, and Mn2+ in wheat straw was high, and they were difficult to remove, so we had to input more EDTA during chelated process. When the dosage of EDTA was 0.1%~0.2% in Q step, the brigtness and conglutination of pulp was higher, and the content of Cu2+ and Mn2+ fell.The input of MgSO4 during P bleaching process could improve the conglutination of the pulp, and the best dosage was 0.04%(calculate by Mg2+).Inputing Na2SiO3 during P bleaching process could improve the conglutination, brightness, and the stabilization of brightness. When the dosage of Na2SiO3 was 3~4%, the brightness, conglutination and the degree of colour reversion almost got the best situation.Compared the brightness of bleaching pulp by ODQP with CEH bleaching, the former pulp had higher brightness, better stabilization of brightness and conglutination, the CODcr,BOD5,SS and AOX of the white waste water were much lower.The pretreatment with xylanase could improve the bleaching effect, and the best dosage was 6U·g-1, degree of pH was 6.0, time was 60min, and consistency of pulp was 8%.The content of HuA in different material pulp is different. It is about 10mmol/kg in wheat straw pulp, and it's about 15-20mmol/kg in soft-wood, and it's about 60-70mmo1/kg in hard-wood.We can use acid pretreament to reduce the content of HuA in pulp, and the best condition is: pH value 3~4, temperature 80oC,and time 60min.Removing the HuA could improve the brightness and brightness stability. With the increasing of remotion, the brightness also increased, and the degree of return yellow reduced. The degree brightness was increased about 2%ISO, and the degree of return yellow fell about 0.1 units.