Laboratory study on bleaching softwood kraft pulp by a totally chlorinefree process including the novel ozone bleaching

Conventional softwood kraft pulp with a kappa number of about 30 was bleached in the laboratory by a totally chlorine-free (TCF) bleaching process based on a novel ozone bleaching technology developed earlier. Neither oxygen nor extended delignifications were included. The delignification stage was carried out with the ZE(O) sequence. It was found that the presence of 1,4-dioxane in the impregnation liquor during ozone bleaching significantly improves the lignin-to-carbohydrate selectivity. The ozone delignification was performed by charging 2% ozone to a high-consistency pulp impregnated with an aqueous and acidified (pH = 2) solution of 70% dioxane. The kappa number and viscosity of the ZE(O) pulp were about 4.0 and 26.5 mPa . s, respectively. It was shown that a one-stage peracetic acid or dimethyl dioxirane (DMD) treatment increased the brightness to a maximum of about 82% ISO. When hydrogen peroxide was used for brightening at a temperature of less than 90 degrees C, the brightness was also limited to the low 80s. However, full brightness (i.e., > 88% ISO) can be achieved when the final peroxide bleaching is carried out at higher temperature and pressurized conditions at a charge as low as 2.0%. A ZE(O)QPPa sequence resulted in the production of fully bleached pulp with a brightness above 88% ISO and a viscosity similar to that of conventional chlorine-bleached pulp, thus representing a promising TCF bleaching sequence. A comparison of its physical properties with those of a DEDED-bleached pulp is also given.