GB1091124A - Process of bleaching paper pulp - Google Patents

Abstract

1,091,124. Bleaching paper pulp. LA CELLULOSE DU PIN. Feb. 11, 1965 [Feb. 11, 1964], No. 6015/65. Headings D1L and D1P. Paper pulp is bleached by treating it with chlorine, and then with milk of lime, followed by chlorination in the presence of chlorine dioxide, a further treatment with milk of lime and treatment with chlorine dioxide. The process may be effected by introducing the crude pulp having a density of 3.5% by weight by pump 21 into mixer 22 in which chlorine is admitted through 22a, chlorination being carried out at 18‹-20‹C in tower 23 which the pulp takes about one hour to traverse, and the pulp being concentrated to a density of 15-20% by weight in screw press 24 prior to the addition of milk of lime introduced into mixer 25 through 25a which reduces the density to not less than 5% by weight and heating to 50‹C by means of steam through 25b or -means of the milk of lime or both. The pulp is then passed to filter 26 where it is washed with water, passed into mixer 27 where chlorine and 0.15-0.2% by weight of chlorine dioxide based on the weight of the dry pulp are added at 27a, chlorination being effected in tower 28 at ambient temperature prior to the addition of milk of lime through 28a changing the density from 5% to approximately 4% by weight and then passed by pulp 29 to liming tower 30 where it is heated by steam injected through 28b to about 35‹C for approximately 1 hour, its pH on leaving the tower 'being 9-10 and its density 4% by weight. It is then passed to filter 31 where it is washed and brought up to a density at 15% by weight, to mixer 33 by high density pump 32 where chlorine dioxide is added through 33a and steam introduced through 33b to raise the temperature of the pulp to 70‹C, and to chlorine dioxide tower 34 which it traverses in 4-5 hours, and is then treated with a solution of SO 2 to neutralise excess chlorine dioxide and washed over filter 35. The tower 28 in which chlorination in the presence of chlorine dioxide takes place may be dispensed with, contact for 2-3 minutes between the pulp and these reagents being sufficient.