Bleaching of mechanical pulps with sodium bisulfite

The use of sodium bisulfite as a substitute for dithionite in mechanical pulp bleaching or as an adjunct to other bleaching methods has been investigated using radiata pine and eucalypt pulps. Bisulfite bleaching can achieve similar brightness levels to dithionite bleaching at considerably greater reagent application. Reduced levels of applied bisulfite for bleaching of radiata pine pulps could be achieved by recycling bisulfite. Although the application level could be reduced up to 50% it was still unacceptably high. Sequential bisulfite-peroxide bleaching and peroxide-bisulfite bleaching gave marginal levels of brightness improvement which were soon lost through light induced reversion. UV-Vis reflectance and absorbance spectroscopy showed that various bleaching treatments affect different pulp chomophores. Peroxide affects both conferaldehyde and quinoid type chromophores whereas bisulfite predominantly affects the conferaldehyde types. Dithionite largely affects the quinoid type chromophores but the coniferaldehyde types also undergo reaction, probably with bisulfite formed on dithionite oxidation.