Bleaching of Eucalyptus Kraft Pulp to Market Pulp Brightness in only 2 Stages

摘要

State-of-the-art in kraft hardwood pulp bleaching is a four stage sequence. The alternating application of chlorine dioxide and hydrogen peroxide, D_0-Eop-D_1-P, has the lowest demand for bleaching chemical, a low effluent load, a high yield, top brightness and good brightness stability as well as good strength. Compared with this reference shorter sequences require less investment and less electrical energy for less equipment. Their disadvantage is a higher chemical demand. An option for fewer stages is the combination of acid hydrolysis of hexenuronic acids with chlorine dioxide delignifi-cation. D_0A_(hot)-Eop-D_1 or DA_(hot)-P are these alternatives. With very high charges of chlorine dioxide it is possible to reach 90%ISO brightness; however, the resulting viscosity is low as result of the extended treatment at low pH and high temperature. A higher COD load indicates yield losses. A very attractive option is the combination of an ozone stage with a directly added chlorine dioxide treatment. Removal of double bonds, lignin and hexenuronic acids, is very good and a high brightness stability results. The total electricity demand of the Z/D-P treatment is lower as is investment. One high consistency ozone reactor is followed by two medium consistency stages for D and P and two washers complete the equipment. Less electrical energy is needed driving ma-chimes. This compensates a higher requirement for chemical generation.