Effect of pH on oxygen delignification of hardwood kraft pulp

摘要

Oxygen delignification in practice is either a one-stage or a two-stage process and conventionally carried out at high alkaline pH. This work studied the effect of pH in the ' range of 1.6 to 13.5 on the second stage of a two-stage oxygen delignification process of a hardwood kraft pulp while the first stage was run at high alkaline pH. The residual lignins in the two-stage oxygen-del-ignified pulps were characterized as well. The degree of oxygen delignification is highest at pH 1.6 followed by pH 2.7, pH 13.5 and lowest at pH 7. Both the bleacha-bility of the D100ED1D2 sequence and the viscosity of the resulting pulps are improved as the pH of the oxygen delignification increases from 1.6 to 13.5. At a given tensile index, the apparent density and tear index decrease with increasing the pH of the oxygen delignification from 1.6 to 13.5. The ultraviolet and infrared spectra of the residual dioxane lignin preparations from oxy-gen-delignified pulps are similar to those of Eucalyptus globulus wood. The content of total phenolic hydroxyl groups in the residual lignins of oxygen-delignified pulps is lowest at pH 1.6 corresponding to the highest oxygen delignification degree at this pH.