Abstract
It is reported that Douglas-fir is not suitable for mechanical pulping. Extractives such as dehydroquercetin and quercetin contained in the heartwood of Douglas-fir are known to be deleterious by consuming costly bleaching chemicals with the result that the end product may be a pulp with lower brightness. The objective of this study was to improve bleachability of thermo-mechanical pulp (TMP) of Douglas-fir. The TMP interstage bleaching was selected as a model process, where hydrogen peroxide bleaching took place between the primary and the secondary refining. Two sets of experiments were carried out to evaluate pulp washing and chip pretreatment in various conditions. The first experiments showed that pulp washing with water after the primary refining prior to the interstage bleaching was effective in improving bleachability. The second experiments found that chip pretreatment with diethylenetriaminepentaacetic acid (DTPA) or sodium hydroxide (NaOH) prior to the primary refining improved bleachability and saved 35-40% hydrogen peroxide in the interstage bleaching to achieve 55% ISO brightness. Precautions were taken to optimize the chip pretreatment conditions. The pretreatment efficiency depended on the initial pH of the chemical liquor and the optimal pH range was found to be around 11.5. DTPA or NaOH, which were added for the chip pretreatment, showed the same effect. The experimental results were successfully applied to the energy efficiency project in one of the largest TMP plants in North America.
