KRAFT COOKING WITH VARYING ALKALI CONCENTRATION - INFLUENCE ON TCF-BLEACHABILITY

The TCF-bleachability of softwood pulps produced by kraft pulping proc esses with varying alkali concentrations was investigated. The cooks b egan with a hydrogen sulfide ion pretreatment at low alkalinity, i.e. simulating a black liquor preimpregnation. The alkalinity in the cooks was varied in three different ways: already at the beginning of the c ook, both at 40 % sulfidity and at a HS- concentration of 0.3 mol/l (2 -stage cooks), and in the final part of a modified kraft cook (4-stage cooks) simulating an ITC (Isothermal cooking) cook. The pulps had the same kappa number after the cook, about 17, and were oxygen delignifi ed before bleaching. The influence of the alkali concentration profile in the cook on the bleachability was studied with a QPQP-sequence (Q =chelating agent stage, P=peroxide-stage with the addition of 0.05 % Mg ions). The bleachability (pulp brightness achieved for a given cons umption of bleaching agent per kappa number) was found to reach a maxi mum for pulps with a residual alkali concentration in the vicinity of 0.5 mol/l. The bleachability was however affected to only a minor exte nt by the alkali concentration during the cook. The hexeneuronic acid content was found to decrease with increasing alkali concentration and this was taken into consideration when calculating the chemical consu mption per amount of lignin. The process selectivity (pulp viscosity a t a given brightness) decreased with increasing residual alkali. The b leachability was not improved by conducting the cook with a levelled o ut active cooking chemical profile, when the comparison was made at th e same residual alkali concentration, but the selectivity was improved .