The use of calcium hydroxide as alkali source in peroxide bleaching of kraft pulp

The feasibility of replacing sodium hydroxide with calcium hydroxide in peroxide bleaching of kraft pulp was assessed. The bleaching experiments were carried out in small scale on two different starting pulps, with a preliminary study and a more narrow follow-up study. The preliminary study showed that the required amount of Ca(OH)(2) was quite low compared to NaOH. The follow-up study showed that the same brightness gain and decrease in yellowness as 1% NaOH were achieved with 0.25% Ca(OH)(2), which could result in savings of 80-85% on the cost of bleaching alkali. Replacing NaOH with Ca(OH)(2) resulted in similar optical and mechanical properties as the reference pulp. The concentration of total organic carbon in the bleaching waters was in the same size range with NaOH and Ca(OH)(2). This study indicates that alternative alkali sources can be used in the peroxide bleaching stage of kraft pulp.