Energy efficient refining of black spruce TMP by using acid hydrogen peroxide: Part 2. Washing, chelating and bleaching studies

The acid hydrogen peroxide system has the potential to significantly reduce the specific energy consumption in the production of softwood thermomechanical pulps (TMPs). A drawback of the chemical system is discoloration of the pulp during refining. The work presented in this study evaluates the possibility to regain the lost brightness by washing, chelating and sodium dithionite or hydrogen peroxide bleaching of the treated pulps. A washing or chelating procedure can reduce the metal ion content of the chemically treated TMPs considerably, though brightness can be increased by a maximum of two ISO units. The amount of iron can be further reduced to a level similar to that of untreated pulps by performing a reducing agent-assisted chelating stage (Q(Y)) with dithionite. The discoloration cannot,, however, be completely eliminated. The brightness decrease-of the treated pulps is thus not only caused by higher iron content in the pulp, but is also dependent on the type of iron compound and/or other coloured compounds connected with the acid hydrogen peroxide treatment. Oxidative bleaching with hydrogen peroxide (P) is more effective than reductive bleaching with sodium dithionite in regaining the brightness lost during the energy reductive treatment. By using a Q(Y) P sequence, a hydrogen peroxide charge of 3.8% was needed to reach an ISO brightness of 75% for the chemically treated pulps. The corresponding hydrogen peroxide charge for the untreated TMP reference was 2.5%.