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 b leached in the laboratory by a totally chlorine-free (TCF) bleaching p rocess based on a novel ozone bleaching technology developed earlier. Neither oxygen nor extended delignifications were included. The delign ification stage was carried out with the ZE(O) sequence. It was found that the presence of 1,4-dioxane in the impregnation liquor during ozo ne bleaching significantly improves the lignin-to-carbohydrate selecti vity. 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 t hat a one-stage peracetic acid or dimethyl dioxirane (DMD) treatment i ncreased the brightness to a maximum of about 82% ISO. When hydrogen p eroxide was used for brightening at a temperature of less than 90 degr ees C, the brightness was also limited to the low 80s. However, full b rightness (i.e., > 88% ISO) can be achieved when the final peroxide bl eaching is carried out at higher temperature and pressurized condition s at a charge as low as 2.0%. A ZE(O)QPPa sequence resulted in the pro duction of fully bleached pulp with a brightness above 88% ISO and a v iscosity similar to that of conventional chlorine-bleached pulp, thus representing a promising TCF bleaching sequence. A comparison of its p hysical properties with those of a DEDED-bleached pulp is also given.