The effect of carryover on medium-consistency oxygen delignification of hardwood kraft pulp

This study was conducted on hardwood brownstock pulp (predominantly eucalyptus wood) to determine the effect of carryover on medium consistency (MC) oxygen (O) delignification. Delignification experiments, performed in a laboratory MC mixer were designed To test the influence of mixing intensity and type of carryover on the reaction, and the influence of carryover from the pre-oxygen washing system on the kinetics of delignification. Even without fluidization of the fibre suspension, enough oxygen can be delivered to the fibre to effect most of the kappa number reduction possible. The degree of delignification can be increased to a small extent by increasing mixing time and mixing speed. Carryover of the filtrate from the cooking stage was more detrimental to delignification than carryover of the filtrate from the O stage. Delignification took place in two phases: a rapid initial phase and a slower secondary phase. In the case when carryover of the filtrate from the cooking stage was present, there could be competitive reactions between lignin in the pulp and the dissolved material in the entrained liquor. The kinetics of delignification was more dependent on pH than on the amount of residual alkali.