High-temperature extraction in a bleaching sequence: Comparing single and double pressurized tower configurations

• Published in: Pulp & Paper Canada Vol. 109; no. 3; p. 28
• Main Authors: Lierop, B van, Faubert, M, Jiang, Z-H
• Format: Trade Publication Article
• Language: English
• Published: Westmount Annex Publishing & Printing, Inc 01.03.2008
• Subjects: Pressure; Pulp & paper mills; Studies; Temperature
• ISSN: 0316-4004; 1923-3515

The brightnesses after DEopDEpD, D(OE)DEpD, D(OOE)DEpD and OD(OE) (Dn)D are compared in Fig. 2. The same brightness was obtained after the sequences containing (OE) and (OOE), indicating that the small difference in kappa number obtained as a result of extra retention time at pressure in the (OOE) stage did not increase the final brightness at the ClO^sub 2^ charges applied in D^sub 1^ and D^sub 2^. The brightnesses of these pulps, however, were higher than that after the sequence containing DEop with an ACM of 0.13 in D^sub 0^. The D^sub 0^EopDEpD sequence with an ACM of 0.20 in D^sub 0^ and 0.44% H^sub 2^O^sub 2^ in the Eop stage performed as well as the sequences containing (OE) and (OOE), but with an 0.13 ACM in D^sub 0^ and no H^sub 2^O^sub 2^ in the extraction stage. The former sequence required 39.2 kg/t pulp of ClO^sub 2^ to achieve this brightness level, while the latter sequences with the enhanced extraction stage required only 32.1 kg ClO^sub 2^/t pulp, a saving of 7.1 ClO^sub 2^/t pulp. The brightnesses after D^sub 1^ and D^sub 2^, Fig. 5, indicate that the brightness limit is greater when the kappa number after the extraction stage is 4.5 than at 6.6-6.7. At less than maximum brightness, however, the chlorine dioxide savings can be substantial, Fig. 6. For example, a brightness target of 90% can be achieved with a total ClO^sub 2^ charge of 37 kg/tp using conventional DEoDED, 32 kg/tp ClO^sub 2^ using D(OE)DED, and 30 kg/tp ClO^sub 2^ using D(OOE)DED, Fig. 6. The net saving of chlorine dioxide compared to conventional DEoDED, therefore, is 5 kg/t pulp with an (OE) extraction, and 7 kg/t pulp with an (OOE) extraction. The temperature combination in (OE) was 95/90°C at 0.50/0 MPa in the respective two towers with a 15 minute retention time in the first tower. In (OOE), the temperature combination in the respective three towers was 90/90/90°C at 0.50/0.40/0 MPa, with a retention time combination of 15/30/50 minutes. The second 30minute pressurized tower contributed to a saving of 2 kg ClO^sub 2^/1 pulp. It is anticipated that raising the temperature in the (OE) stage can achieve the same savings of 7 kg ClO^sub 2^/t pulp as obtained by the (OOE) stage. Résumé: Une étape d'extraction oxydative conçue pour simuler deux tours sous pression avant une tour de rétention à pression atmosphérique n'offrait qu'un avantage mineur comparativement à la délignification obtenue dans une seule tour sous pression. La séquence D(OE)DED a permis d'économiser le ClO^sub 2^ d'environ 5 kgA de pâte comparativement à une séquence DEoDED classique, tandis que la séquence D(OOE)DED a permis d'économiser le ClO^sub 2^ d'environ 7 kg/t de pâte. Le profil de température indique toutefois qu'une seconde tour sous pression de 30 minutes n'est pas requise si l'on élève de 5 à 10°C la température de la première. La séquence OD(OE) était plus efficace que les séquences D(OE) ou D(OOE) pour délignifier la pâte. L'ajout de MgSO^sub 4^ a amélioré la sélectivité tant à l'étape (OE) qu'à l'étape (OOE), tandis que le fractionnement du NaOH entre les tours sous pression a l'étape (OOE) n'a offert aucun avantage supplémentaire. Les propriétés de résistance n'ont pas été modifiées par les étapes d'extraction améliorées.