Oxidative and reductive bleaching of deinked pulp
Deinked pulps for hygienic paper production were bleached with hydrogen peroxide and sodium dithionite in single and two-stage bleaching. The brightness gain of high brightness deinked pulp in the peroxide stage was 5.7% ISO and of low brightness 4.4% ISO. In sodium dithionite stage, the brightness gain of high brightness deinked pulp was 4.0% ISO and of low brightness deinked pulp 3.8% ISO. The two-stage oxidative-reductive bleaching sequence resulted in an increase of brightness by 9.7% ISO for high brightness deinked pulp and by 9.1% ISO for low brightness deinked pulp. Brightness gain in the two-stage reductive-oxidative bleaching sequence was 6.7% ISO for high brightness deinked pulp and 5.6% ISO for low brightness deinked pulp. The coordinate b* value and yellowness of deinked pulps decreased in sodium dithionite bleaching stage more than in hydrogen peroxide stage. The two-stage oxidative-reductive bleaching sequence is more preferred than reductive-oxidative sequence regardless of the fibre composition. The tensile index and the tensile energy absorption index of deinked pulps slightly decreased after single-stage and two-stage bleaching sequences, their reduction in the peroxide stage was greater than in the dithionite stage. The fibre strength and fibre length characteristics of deinked pulps decreased after bleaching, while fibre bonding ability increased. The water absorption and bulk softness increased after bleaching, the increase was higher after peroxide stage than dithionite stage.