Influence of pulping conditions on eucalyptus kraft pulp yield, quality, and bleachability

This study evaluated the effect of cooking temperature and residual alkali on kraft pulp yield, bleachability, and quality. We cooked Eucalyptus grandis wood chips to kappa number 16-17 using high and low residual alkali (14-18 g/L and 3-4 g/L) and high and low temperatures (170degreesC and 160degreesC). We then quantified process yield and evaluated pulp quality through measurement of strength properties, carbohydrate composition, hexenuronic acid content,. alkali-leachable lignin content, and residual lignin nature. After oxygen delignifying the pulps and bleaching them to full brightness with the sequences D(EOP)DD and D(EOP)(ZE)D, we determined their bleachability and brightness stability. Low temperature and low residual alkali during pulping resulted in the highest screened yield (54.2%) and viscosity (64.6 mPa.s). Pulps cooked at low residual alkali showed about 1.7%-2.1% higher yield, 20%-30% higher Viscosity, and higher strength properties (except for tear index) than those cooked at high residual alkali both at 160degreesC and 170degreesC. Lower temperature and/or residual alkali during pulping favored pulp refining. Pulps produced at high residual alkali showed slightly lower oxygen delignification efficiency (2%-3%), but significantly higher (10%-15%) bleachability, as measured by active chlorine consumption, regardless of cooking temperature. Pulps cooked at high residual alkali showed higher initial brightness (6-8% ISO) for a fixed kappa, which carried on across oxygen delignification and ECF bleaching processes. Oxygen delignification filtrates of pulps cooked at low residual alkali contain. substantially more COD and color than those cooked at high residual alkali because the former pulps are richer in alkali-leachable lignin. Pulping conditions did not affect pulp brightness stability. Pulps produced at high residual alkali showed lower contents of xylans and of hexenuronic acids and higher quantities of lignin containing carboxyl and aliphatic hydroxyl groups. Pulps cooked at 160degreesC, presented higher xylan and,hexenuronic acid content in relation to those cooked at 170degreesC.