In-digester reduction of organic sulfur compounds in kraft pulping

This experimental research examines the relationship between the formation of organic sulfur compounds and delignification during kraft wood pulping. A phase transition point (PTP) was discovered below and above which the formation of total organic sulfur compounds, with respect to delignification, changes significantly. The PTP occurs at approximately pulp kappa number 35 for softwoods and 20 for hardwoods. The PTP is directly related to the phase transition in kraft delignification from bulk to residual delignification. Significant in-digester odor reduction can be achieved using phase transition cooking (PTC), i.e. limiting delignification to the bulk delignification phase or the PTP. It was also discovered that anthraquinone (AQ) as a catalytic additive can reduce organic sulfur compound formation during kraft pulping. The reduction of sulfur compound formation is linearly proportional to the square root of the level of AQ addition (in percent). A 40% in-digester reduction in total reduced sulfur (TRS) formation can be achieved by using PTC and AQ in kraft pulping.