PULP BLEACHING

Abstract: Bleaching of chemical pulp in several steps comprising final bleaching with chlorine dioxide and, respectively, hydrogenperoxide. The pulp is treated directly in connection to the chlorine dioxide step (3) without intermediate washing with chelating agent (2) for the elimination of metal ions, preferably manganese. The pulp is thereafter washed (4) and bleached with hydrogenperoxide (5).

Description: Pulp bleaching This invention relates to the bleaching of lignocellulosic material in the form of chemical pulp. The invention, more precisely, refers to the final bleaching of the pulp by using chlorine dioxide and hydrogenperoxide as bleaching chemicals. For being able to carry out peroxide bleaching effectively, the pulp must be substantially free of metal ions, especially manganese ions. The metal ions normally are solved out in the chlorine dioxide step as a consequence of the conditions in this step. The metal ions then are removed in subsequent wash¬ ing steps, whereafter the pulp can be peroxide bleached. This is in most cases sufficient for bringing about a satisfactory ISO- brightness at reasonable peroxide consumption. It was now found possible to additionally lower the metal ion content in the pulp and thereby to reduce the peroxide consumpt¬ ion and/or increase the ISO-brightness. According to the invent¬ ion, this is achieved in that the pulp is treated with chelat¬ ing agent in direct connection to the chlorine dioxide step. The characterizing features of the invention are apparent from the attached claims. The invention is described in greater detail in the following, with reference to the Figures showing two embodiments of the invention. Figs. 1 and 2 show two examples of a flow sheet for the process according to the invention. The bleaching process implies bleaching in several steps where the final bleaching is carried out with chlorine dioxide and peroxide in separate steps. At the embodiment according to Fig. 1, the pulp after a preced¬ ing washing step 1 is first treated with chelating agent (Q-step) 2 and thereafter with chlorine dioxide (D-step) 3, at which steps these chemicals are charged sequentially. After the D-step 3 the pulp is washed in a washing step 4 and bleached with hydro- peroxide (P-step) 5 and again washed in a washing step 6. Accord¬ ing to the alternative embodiment in Fig. 2, the chlorine dioxide is charged in the D-step sequentially prior to the chelating agent in Q-step 2. In other respects, this embodiment corresponds to that in Fig. 1. It is, thus, essential that the charging of chlorine dioxiαe and chelating agent takes place sequentially, i.e. not together, but in direct connection without any washing therebetween. Tne charging of the chelating agent together with the chlorine dioxide was found to yield a deteriorated result, while se¬ quential charging has produced the above-mentioned effect. The invention, thus, implies an improved bleaching process in that the bleaching chemicals, especially the hydrogenperoxide, can be utilized more effectively. The conditions in the Q-step as well as D-step can be normal. This implies for the Q-step a pulp concentration of 1-20%, preferably 8-15%, a temperature of 45-100°C, preferably 70-90°C, pH 1,5-8, preferably 2-4, and a time of 10-120 iπ, preferably 10-60 min. In the D-step the conditions can be a pulp concen¬ tration of 1-20%, a temperature of 45-70°C, pH 1,5-4, and a time of 15-60 min. The subsequent P-step can also be carried out in a conventional manner with or without addition of oxygen gas. The peroxide charge, however, as stated above, can be reduced, alternatively the ISO-brightness be increased as a result of the process according to the invention. The fact, thus, is a more effective utilization of the peroxide. The invention, of course, is not restricted to the embodiments described, but can be varied within the scope of the invention idea.

Claims: Claims 1. A method of bleaching chemical pulp in several steps com¬ prising final bleaching with chlorine dioxide and, respectively, hydrogenperoxide, c h a r a c t e r i z e d in that the pulp in direct connection to the chlorine dioxide step (3) without intermediate washing is treated with chelating agent (2) for the elimination of metal ions, preferablx manganese, and that the pulp thereafter is washed (4) and bleached with hydrogenperoxide (5. 2. A method as defined in claim 1, c h a r a c t e r i z e d i n that the pulp is treated with chelating agent (2) dir¬ ectly before the chlorine dioxide step (3) . 3. A method as defined in claim 1, c h a r a c t e r i z e d i n that the pulp is treated with chelating agent (2) dir¬ ectly after the chlorine dioxide step (3) . AMENDED CLAIMS [received by the International Bureau on 17 January 1996 (17.01.96); original claim 1 amended; remaining claims unchanged (1 page)] 1. A method of bleaching chemical pulp in several steps comprising final bleaching with chlorine dioxide and, respectively, hydrogen peroxide, c h a r a c t e r i z e d in that the pulp in direct connection to the chlorine dioxide step (3) without intermediate washing is treated with chelating agent (2) during 10-120 min for the elimination of metal ions, preferably manganese, and that the pulp thereafter is washed (4) and bleached with hydrogen peroxide (5) . 2. A method as defined in claim 1, c h a r a c t e r i z e d in that the pulp is treated with chelating agent (2) directly before the chlorine dioxide step (3) . 3. A method as defined in claim 1, c h a r a c t e r i z e d in that the pulp is treated with chelating agent (2) directly after the chlorine dioxide step (3) .

Inventors: DAHL MAARTEN, DAHL, MAARTEN
Applicants: DAHL MAARTEN, SUNDS DEFIBRATOR IND AB, DAHL, MAARTEN, SUNDS DEFIBRATOR INDUSTRIES AB
Application year: 1995
Main classification: D21C9
IPC codes: D21C9