Influence of raw-material and process variables in the kraft pulping of Cynara cardunculus L.

Whole stalks and manually depithed stalks of the thistle Cynara cardunculus L. were studied at 150, 160 and 170 °C regarding their kraft pulping kinetics. Three pulping phases were observed: initial, bulk and residual, corresponding to the removal of approximately 25, 15 and 5% of the material, respectively. The rate constants of the initial phase were approximately 10 times higher than that of the bulk phase (5.3 × 10−2 min−1 versus 5.9 × 10−3 min−1 for the whole stalks pulping at 160 °C). The activation energy was 74, 45 and 24 kJ mol−1, respectively, for the initial, bulk and final delignification phases. The pulp yields from depithed stalks were somewhat higher (57% versus 54% at 160 °C, 120 min). The presence of pith was related to lower yields and to higher diffusion resistance of parenchyma cells. Depithing of stalks should be included as a raw-material preparation before pulping