Functions, advantages and disadvantages of bleaching chemicals Chemica Function Pul Advantage disadvantag pS es Na2S20 Reduce and MEasy to use;Decomposes decolorize ow capital readily Limited lignin in high cost brightness gain yield pulps Xylans Catalyze xylan c Easy to use; Limited hydrolysis and ow capital effectiveness aid in lignin cost COS removal NaOH Hydrolyze C Effective Darkens pulp chlorolignin and solubilize Economic lignin al EDTA Removal Improves Cost peroxide DTPA meta selectivity and
Chemica l Function Pul p Advantage s disadvantag es Na2S2O 4 Reduce and decolorize lignin in highyield pulps M Easy to use; Low capital cost Decomposes readily; Limited brightness gain Xylanas e Catalyze xylan hydrolysis and aid in lignin removal C Easy to use; Low capital cost Limited effectiveness; cost NaOH Hydrolyze chlorolignin and solubilize lignin C Effective Economic al Darkens pulp EDTA DTPA Removal metal ions C Improves peroxide selectivity and efficiency Cost Functions, advantages and disadvantages of bleaching chemicals
Reactions of different oxidants used in bleaching Cl2+2e→2C Naocl+h o+2e-NaCl+20H- Clo,+2H.0+5e CI-+40H- O2+2H2O+4e-40H H2O2+2H+2e→2H2O 0,+3H++6e-30H- South China University of Technology
South China University of Technology Reactions of different oxidants used in bleaching Cl2+2e-→2ClNaOCl+H2O+2e-→NaCl+2OHClO2+2H2O+5e-→Cl-+4OHO2+2H2O+4e-→4OHH2O2+2H++2e-→2H2O O3+3H++6e-→3OH-
■B| eaching sequence TWo improtant aspects of establishing the proper sequence of bleaching:★cost★ Selectivity I Typical bleaching sequences ●CEH●(C+ D)EODED ODEODED● OZEOD ●oQP YP Coventional sequences for KP CEDED CEHDED Softwood CEDED CEHD Hardwood South China University of Technology
South China University of Technology Bleaching sequence Two improtant aspects of establishing the proper sequence of bleaching: ★Cost ★Selectivity Typical bleaching sequences ●CEH ●(C+D)EoDED ●DEoDED ●OZEoD ●OQP ●YP Coventional sequences for KP CEDED CEHDED Softwood CEDED CEHD Hardwood
■ New sequences OCEODED CDEODE D OCDEOD OZED XEODDED New development ECF--OPDED ODEDD TCF-OZEP OQPZP OQPP South China University of Technology
South China University of Technology New sequences OCEoDED CDEoDEpD OCDEoD OZED XEOPDED New development ECF—OPDED ODEDD TCF—OZEP OQPZP OQPP
Typical process conditions for bleaching softwood KP chemical Naoh CIO laOh CiO2 Water Water Steam Water Steam Steam Unbleached pulp D Eo D LBI leached pulp Effluent to Acidic Alkaline effluent recovery effluent South China University of Technology
South China University of Technology Unbleached pulp Effluent to recovery NaOH O2 Water & Steam Acidic effluent ClO2 Water NaOH O2 Water & Steam Alkaline effluent Water & Steam O2 ClO2 Bleached pulp O D Eo D Typical process conditions for bleaching softwood KP chemical