Pulping and Delignification of Sapwood and Heartwood from Eucalyptus Globulus

Eucalyptus globulus heartwood and sapwood were chemically characterized and their behaviour was investigated during isothermal kraft pulping at 130, 150 and 170 degrees C regarding mass loss and lignin removal. Heartwood contained more extractives than sapwood (9.8 vs 3.9%) but similar lignin content (23.5 vs 24.3%). Pulping yields of heartwood were lower than sapwood's by 8% at 130 degrees C, 4% at 150 and 170 degrees C after 180 min pulping time. Lignin removal was only slightly lower in heartwood in relation to sapwood (residual lignin of 2.0 and 1.1% after 180 mm at 170 degrees C, respectively). The delignification rate at 170 degrees C increased rapidly during the first 25 min until a constant residual lignin after 35 min. A similar behaviour occurred at 150 degrees C, but with a lower initial decrease rate in lignin content and stabilizing only after 95 min, while at 130 degrees C lignin removal was approximately linear with time without reaching a constant value during the tested period. E. globulus heartwood compared unfavourably to sapwood in kraft pulping owing to lower yields, mainly due to its higher content of extractives.