THE DEVELOPMENT OF DELIGNIFICATION AND LIGNIN-CELLULOSE SELECTIVITY DURING OZONE BLEACHING

A quantitative analysis of the problem of diffusion and reaction of oz one inside the fibre wall is presented. The progress of delignificatio n in the fibre wall is described by the shrinking-core model, whereby the rate of ozone consumption at a sharp lignin reaction front is limi ted by diffusion of ozone towards the front. A formula is given to cal culate the time for the lignin reaction front to traverse the fibre wa ll from the external fibre surface to the lumen for high-consistency o zone bleaching. Based on the shrinking-core model, and the understandi ng that hydroxyl radicals generated by the lignin-ozone reactions are mainly responsible for the degradation of cellulose, the delignificati on and cellulose degradation obtained during high-consistency hemlock kraft pulp ozone bleaching are interpreted. The development of the cel lulose molecular weight distribution of the ozonated kraft pulp is in agreement with the theoretical model.