Extended Oxygen Delignification with the Oxy TracTM Process

Many mills have adopted oxygen delignification systems in order to decrease chemical consumption in a bleaching plant since 1970's.Generally, the maximum delignification obtained by oxygen delignification system keeping pulp strength has been considered about 50% for softwood pulp because possible delignification by oxygen is limited to certain groups of residual lignin after cooking.Valmet has reviewed the reaction of oxygen delignification factor by factor to meet the current demand of reducing environmental impact with ECF/TCF bleaching sequence and has developed a new two-stage oxygen delignification process named “OxyTracTM” which improves selectivity and enables to obtain 70-75% delignification on softwood without quality degradation.The OxyTracTM process has already been adopted in several mills in the world showing better results than laboratory results especially on bleachability, pulp strength and kappa reduction.One of the new facts found in the mills is an effect of oxidized COD recirculation. In the OxyTracTM process, COD is acting as chelating agent to protect a rection of hydrogen per oxide which is creating in OxyTracTM process.This paper reports the process of OxyTracTM and its performance on pulp quality, selectivity, bleachability and shive reduction based on data from running installations.