Alternative alkalis for peroxide bleaching of chemithermomechanical pulp

Abstract A total substitution of sodium hydroxide by weak alkali sources, namely Mg(OH)₂, MgCO₃ and Ca(OH)₂, was studied in a two-stage peroxide bleaching of a maple chemithermomechanical pulp. For the same brightness, the optimised bleaching conditions for Mg(OH)₂, MgCO₃- and Ca(OH)₂- based P1-P2 processes enabled to decrease the COD to more than 25%. Moreover, the biodegradability of bleaching effluents was enhanced, as reflected by the reduction of 1/3 of the recalcitrant COD remaining after the secondary wastewater treatment with activated sludge. Additionally, alternative alkali-based bleached pulp presented highest bulk and opacity than the conventional two-stage peroxide bleached pulp. Nevertheless, the use of weak alkali affected the development of strength properties, the tensile and tear indexes decreased by 25% and 16%, respectively, compared to NaOH-based bleached pulp. Eventually, this study demonstrated that the use of alternative alkalis instead of sodium hydroxide had not detrimental impact on the residual peroxide recycling.