| 摘要 |
A method of pulp pretreatment by acidification, giving improved bleachability, comprises: (a) adjusting the pH of the pulp to 2-6 (pref. 3-4) using aminic acid, sulphuric acid or hydrochloric acid; (b) feeding pulp to an acid tower; (c) treating the pulp in the acid tower (0-20 (pref. 1-10) bar/75-130 (pref. 80-110) deg C/20-240 minutes) to decrease the kappa number by 1-9 (usually 2-6) units; (d) transferring the pulp to a second treatment stage tower; (e) treating the pulp with a complexing agent at pH 4-9 (pref. 5-6) or with an oxidising chemical, e.g. chlorine dioxide, Caro's acid or peracids; and (f) washing and/or pressing the pulp. Pref. the pulp is treated with a complexing agent, either in step (e) with an acidifying cpd. or in a separate step between (e) and (f). The pH of the pulp is adjusted by adding acid pref. between steps (c) and (e) or in step (d). In step (f) the pulp is pref. washed in a fractionating washer so that a filtrate contg. heavy metals is removed and a cleaner filtrate is recycled for use elsewhere in the process. In a step (g) the pulp is bleached or treated in an alkaline step, e.g. with H2O2. Peroxide bleaching is pref. effected in two interconnected towers of different sizes and comprises: (i) mixing at least peroxide with the pulp; (ii) feeding the pulp into a pressurised pretreatment reactor (3-20 bar/10-60 minutes); (iii) allowing the pulp to react with the peroxide; (iv) separating gas from the pulp; (vi) blowing the pulp by the pressure of the pretreatment reactor to the lower section of a bleach tower in which it flows upwards; and (vii) removing the pulp from the bleach tower. |
