MEDIUM CONSISTENCY OZONE BLEACHING - LABORATORY AND MILL EXPERIENCE

In Lenzing beech dissolving pulp has been bleached according to the EOP-H-P sequence for more than ten years. The replacement of hypochlorite by a non-chlorine bleaching sequence was recognized as the only means of fulfilling the Environmental Authority's demand for the total elimination of AOX emissions. Ozone bleaching proved to be the best nonchlorine alternative to the hypochlorite stage. The special conditions of this bleaching step - low kappa, low specific ozone requirement - encouraged us to examine ozone bleaching under MC conditions. A high-shear mixer developed for oxygen bleaching was used as the reactor. In order to achieve a gas to liquid phase-ratio lower than 0.6 (the upper limit for the MC mixer) the ozone-containing gas was compressed to 5-6 bar. The concept of MC ozone bleaching turned out successful. The influence of the process variables was examined extensively. On the basis of these results a pilot plant in a production scale (3-5 adt/h) was planned and completed with Kamyr. Since August 1990 the tests on this plant have been successfully.