Chemical pulping

Abstract

Removing lignin from black liquor is one way of recovering valuable organic substances for alternative use as well as reducing the energy surplus in a modern, energy-optimised kraft pulp mill. By using the recently developed "LignoBoost" process it is possible to upgrade black liquor streams of different origin to form valuable chemicals. In this investigation a combination of membrane filtration and the LignoBoost concept was used. The primary objective of the work was to investigate whether or not the filtration properties of the black liquor were affected by its hemicellulose content. The hemicellulose content in the black liquors was lowered prior to precipitation using three different pre-treatment techniques, and the filtration properties were compared to the reference black liquors of evaporated softwood and hardwood black liquor. The pre-treatment methods used were heat-treatment, ultrafiltration and a combination of ultrafiltration and nanofiltration. It was shown that the filtration resistance was lowered considerably when the hemicellulose content in the black liquor was reduced prior to precipitation. The experiments also showed that it was possible to produce a hardwood lignin product of high purity i.e. low sodium (0.2 w-% of TIDS) and hemicellulose content (0.7 w-% of TDS) (Less)