| 摘要 |
A method in bleaching of pulp is described, in which the pulp is washed in connection with the bleaching and the washing liquid from the washing is separated from the pulp in the form of a filtrate, which is treated with an aluminium compound for precipitation of interfering substances, whereupon the precipitated material is removed from the purified filtrate, which is recirculated to the bleaching process. The method is characterised in that the purified filtrate, before being recirculated to the bleaching process, is treated by adding a magnesium compound, such as magnesium oxide, and adjusting the pH of the purified filtrate to above 7, preferably to 8.5-11, for precipitation of an aluminium-containing precipitate, which is removed, whereupon the remaining filtrate is recirculated to the bleaching process, preferably to a peroxide bleaching step. By this treatment the aluminium content of the filtrate can be further reduced, resulting in aluminium content levels of less than 5ppm. When recirculating the filtrate to a peroxide bleaching step, an advantageous supply of magnesium in combination with aluminium, preferably in the form of hydrotalcite, to the peroxide bleaching step is achieved owing to the fact that the recirculated filtrate contains small amounts of magnesium and aluminium, and preferably also carbonate. |
