Investigation on TCF Bleaching of Wheat Straw Soda Pulp

Pulp and paper production from non-wood fiber especially from agricultural residues, is accompanied by serious global challenges. Utilization of CEH bleaching sequence on non-wood chemical pulps not only causes serious environmental concerns, but also reduces the possibility of closed mill water circulation. Therefore the application of TCF bleaching (O/Q/OP sequence) on wheat straw soda pulp was investigated. Two application periods (90 and 150 minutes), sodium hydroxide variation of between 1-4.5% and consistency of 8, 10, 12, 14 and 16% were investigated in oxygen delignification stage. Bleaching with H2O2/O2 was studied in two stages. In stage 1, the influence of H2O2 variation (1-4%) and in stage 2, the application of DTPMPA, MgSO4, Na2Si2O3 along with two time periods of 90 and 120 minutes were studied at a constant charge of 4% H2O2¬. Statistical analysis revealed that the effect of NaOH, temperature and consistency in oxygen delignification on kappa number, viscosity and brightness was significant at 1% level, and an application of one percent NaOH for 90 minutes caused 43% reduction in lignin content. The effect of H2O2 charge, temperature and various additives in OP stage is statistically significant. The results of this stage indicate that if sodium silicate is applied, brightness can reach values higher than 80 %ISO while the use up of H2O2 being significantly reduced. The application of this compound must be at a minimum level of 2%. Strength properties of unbleached and TCF bleached pulps indicate that TCF bleaching was conducted selectively and the strength of TCF bleached pulp is superior to CEH bleached one. Finally the results indicate that the brightness and strength properties of O/Q/OP bleached wheat straw soda pulp are superior to CEH while the AOX of bleaching effluent is also kept at zero level.