BLEACHING OF MECHANICAL PULPS WITH SODIUM BISULFITE

The use of sodium bisulfite as a substitute for dithionite in mechanic al pulp bleaching or as an adjunct to other bleaching methods has been investigated using radiata pine and eucalypt pulps. Bisulfite bleachi ng can achieve similar brightness levels to dithionite bleaching at co nsiderably greater reagent application. Reduced levels of applied bisu lfite for bleaching of radiata pine pulps could be achieved by recycli ng bisulfite. Although the application level could be reduced up to 50 % it was still unacceptably high. Sequential bisulfite-peroxide bleach ing and peroxide-bisulfite bleaching gave marginal levels of brightnes s improvement which were soon lost through light reduced reversion. UV -Vis reflectance and absorbance spectroscopy showed that various bleac hing treatments affect different pulp chromophores. Peroxide affects b oth conferaldehyde and quinoid type chromophores whereas bisulfite pre dominantly affects the conferal-dehyde types. Dithionite largely affec ts the quinoid type chromophores but the coniferaldehyde types also un dergo reaction, probably with bisulfite formed on dithionite oxidation .