| 摘要 |
A method of total chlorine-free bleaching to a brightness of ISO 82-90 uses an acid stage and a superatmospheric pressure peroxide stage. The acid stage (which may be the only acid stage) may comprise a Q stage or a Z (ozone) stage, in which metals in the pulp are freed from pulp fibers and become dissolved in the surrounding liquid. The pulp is then subjected to fractional washing to produce at least two wash filtrates, the first filtrate having over 50% of the dissolved metals from the pulp. The first filtrate is ultimately returned (e.g. through brown stock washing) to the black liquor recovery system of the pulp mill so that the metals precipitate on organics in the black liquor recovery system due to the alkaline pH of the black liquor. Other filtrates from the fractional washing are used to wash the pulp just before the acidic (e.g. ozone) stage. Acid and/or a chelating agent are added tothe wash liquid during fractional washing so that the pH of the first filtrate is six or less, and magnesium may be added to the pulp while it has an acidic pH just prior to the peroxide bleaching stage. From the fractional washing stage the filtrates may each have a flow volume of 0.5-7 cubic meters per ton of pulp. A further (ZP) stage may be added if it is desired to achieve a brightness of ISO 89 or above. |
