Community Research and Development Information Service - CORDIS

Chlorine or chlorine compounds free bleaching of kraft chemical pulps

The following mains results were found. Bleaching sequences have been tested in order to develop environmentally-friendly processes for the pulp industry. Ozone does not depolymerize the residual lignin very efficiently. Consequently, it must be associated with other chemicals that have this ability (oxygen). This suggests that OZ (O : oxygen Z : ozone) is a good bleaching sequence. For the final stage, chlorine dioxide (D) and hydrogen peroxide (P) should be equally good candidates. Lignin can react directly with ozone to form hydroxyl radicals which in turn can participate in a radical chain involving ozone, oxygen, superoxide and organic substrates. Thus, ozone bleaching of lignin containing pulps should give rise to direct radical formation making it very difficult to avoid unselective carbohydrate oxidation reactions. Magnesium ions are important for the protection of carbohydrates during bleaching with oxidative agents. The selectivity of the delignification by ozone is independent of the reaction temperature and seems to be dependent of the amount of lignin removed from the pulp. The most promising TCF bleaching sequence seems to be OZP, but a viscosity drop takes place when an ozone stage is carried out. With Eucalyptus kraft pulp, QZD seems to be the best bleaching sequence (Q : chelating treatment). By using enzymatic treatment in a TCF bleaching sequence, the brightness level can be increased when the same charge of hydrogen peroxide is applied. For ECF and TCF bleaching sequences of Eucalyptus kraft pulp a reduction in colour and chemical oxygen demand (COD) values of the effluents are observed when oxygen stage is used. With Softwood kraft pulp the extended cooking pulps can be bleached by the TCF bleaching sequence OQZRP (R = sodium borohydride) giving high level of brightness taking account the environmental constraints. The TCF bleaching sequence OQZP and OQZRP with Spruce kraft pulp shows that the reducing sodium borohydride stage limits the pulp degradation. With an oxygenated Douglas fir kraft pulp, it is possible to obtain a TCF bleaching pulp with mechanical properties comparable to an ECF bleached pulp.

Reported by

Centre Technique du Papier
Domaine Universitaire de Saint Martin D'Hères
38044 Grenoble
France
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