Delignification of softwood pulps to give high brightness - using peracid, complexant and peroxide, with retention of pulp viscosity and strength

摘要

Lignocellulose-contg. pulp is delignified and bleached using a peracid (or a salt of a peracid). The pulp is then treated with a complexant, followed by a Cl-free bleaching agent comprising one or more of a peroxide, ozone or sodium dithionite. The pulp to be treated with peracid is a chemically digested pulp. Peracetic acid is pref., when delignification is performed at pH 3-10. The peroxide is H2O2 or a mixt. of this and O2. The pulp is washed (after treatment with complexant) at a pH of at least 4. The complexant is a nitrogenous cpd., esp. DTPA or EDTA. Complexation is effected at pH 2.5-11. In addn., delignification with peracid may be preceded by oxygenation.