NZ246713A

NZ246713A - Bleaching of sulphate pulp comprising: oxygen delignification, pretreatment with complexing agent, washing then dewatering, and alkaline peroxide bleaching in presence of silicate or an organic complexing agent

Info

Publication number: NZ246713A
Application number: NZ246713A
Authority: NZ
Inventors: Lars-Ake Lindstrom; Solveig Norden; Lars Sjodin; Marten Dahl
Original assignee: Sunds Defibrator Ind Ab
Priority date: 1992-01-21
Filing date: 1993-01-12
Publication date: 1995-04-27
Also published as: AU3412593A; ZA93317B; SE469842C; WO1993014262A1; DE69309893D1; NO942718D0; FI943431A; DE69309893T2; SE9200152D0; SE9200152L; CA2124088A1; BR9305761A; ATE151826T1; ES2102008T3; FI943431A0; EP0621915A1; EP0621915B1; NO942718L; AU653467B2; JPH07503042A

Abstract

A method of bleaching sulphate pulp without using chlorine-containing bleaching agents. The sulphate pulp is oxygen delignified and can possibly also be ozone delignified to a kappa number below 9.5. Thereafter a pretreatment with complexing agent is carried out for 1-60 minutes, at which the pulp consistency is 1-15 %, the temperature 70-110 DEG C and pH 5-7. The pulp thus pretreated is washed and dewatered to a consistency of 18-40 %. Thereafter peroxide bleaching in alkaline environment takes place in the presence of silicate or an organic complexing agent of the type phosphonic or carboxylic acid.

Description

New Zealand No. 2 4 6 713 International No. PCT/SE93/00012 TO BE ENTERED AFTER ACCEPTANCE AND PUBUCATION Priority dates: 2. * v international filing date: ,2.

Classification: Publication date: 2 7 APR 1995 Journal No.: NEW ZEALAND PATENTS ACT 1953 COMPLETE SPECIFICATION Title of invention: BLEACHING OF CHEMICAL PULP Name, address and nationality of applicants) as in international application form: SUNDS DEFIBRATOR INDUSTRIES AB, S-851 94 Sundsvall, Sweden. |_ (FOLLOWED BY PAGE 1 A) - 1A- 2t^i(3 Bleaching of chemical pulp This invention relates to the bleaching of sulphate pulp without using bleaching agents containing chlorine. The technique of bleaching chemical pulps is now developing rapidly, because the bleaching chemicals heretofore mainly used, Cl2» C102> anc* hypochlorite, are feared to give rise to products being a risk for the environment and health.

Until now, the discussions have been concentrated on the emissions of chlorinated organic substance to the recipient. The majority of the countries in the world have now established limit values for these emissions, which normally are measured as AOX (Adsorbable organic halogens). As a new trend, the customers require to an ever increasing degree paper with a low content of chlorinated organic substance.

Conventional bleaching with CI2 disappears now more and more, and Cl2 is replaced primarily by C1C>2* Bleaching with C102 causes substantially lower AOX-emissions to the recipient, but the amount of chlorinated substance in the pulp is affected only insignificantly.

In recent years, the use of hydroperoxide at the bleaching of chemical pulps has increased considerably. Hydroperoxide can replace part of the chlorine chemicals.

In patent application SE 89 02058-0, a method is described at which the pulp is pretreated with complexing agent under neutral conditions. By this process, coniferous sulphate pulp can be bleached with peroxide-containing bleaching agents to 70-75% ISO at about 10% pul^ consistency. In order to achieve fully bleached pulp qualities, with higher ISO-brightness (above 83% ISO), this process can be combined with final bleaching with C102. As a result, tiris final bleaching then yields emission of AOX from the bleach plant and chlorinated organic susbstance in the bleached pulp. 2 PCT/S E93/00012 lltbio Other known methods of reducing emissions of chlorinated organic substance to the recipient are to extend the delignification further in the closed part of the mill, i.e. in the digester house and/or oxygen step.

Extended digestion with maintained pulp quality can be achieved by impregnating the chips with black liquor in the initial phase of the cook, as described in Finnish patent application 90 0663. This method renders it possible to digest pulp down to a kappa number of about 10 and obtain strength properties equal to or better than with pulp digested to conventional level, kappa number 25-30. Combined with oxygen delignification, kappa numbers as low as 5-6 can be obtained for.further bleaching.

By combining conventional digestion, oxygen delignificat- 9 ion and ozone bleaching in acid environment, kappa numbers lower than 10 can be obtained.

It has, however, not been possible to produce fully bleached pulp qualities without final bleaching with C102» The present invention implies, that fully bleached pulp with a brightness above 83% ISO can be made without the use of chlorine-containing bleaching agents.

At the s.tart of the bleaching, the pulp shall have a kappa number below 9.5. This can be achieved by conventional or extended digestion followed by 02~delignification, alternatively by conventional or extended digestion followed by O2- and 0^-delignification. The bleaching is initiated by a pretreatment step with jiomplexing agent followed by treatment with hydroperoxide • in alkaline environment in the presence of silicate or an organic complexing agent of the type phosphonic or carboxylTc acid. 3 The characterizing features of the invention are apparent from the attached claims.

The pulp digested and delignified with oxygen and possibly ozone according to above shall have a kappa number below 9 . This can be achieved by known methods o£ digestion and delignification. The pulp shall thereafter be pretreated with complexing agent, for example EDTA or DTPA, at a pulp consistency of 1-15% for 1-60 minutes, preferably 5-30 minutes. The temperature shall be 70- 110°C, preferably 80-100°C, and the pH-value 5-7.

The pulp thus pretreated is washed and dewatered to a consistency of 18-4-0%, preferably 20-30%. The pH-value is' increased to alkaline level, and hydroperoxide is added together with silicate, for example sodium silicate, alternatively together with an organic complexing agent of the type phosphonic or carboxylic acid. The dwell time of the pulp in this hydroperoxide step shall be 1-/2 - 5 hours, preferably 1-4- hours, and the temperature 70-110°C, preferably 80-100°C. The silicate charge shall be 2-30 kg/ton pulp, preferably 5-15 kg/ton pulp, expressed as Si02 • Alternatively, the charge of the organic complexing agent shall be 0.5-5 kg/ton pulp.

Fig. 1 is a diagram, in which the brightness of the pulp is plotted as a function of the pulp consistency at the peroxide'bleaching,partly at conventional peroxide bleaching and partly at bleaching according to the invention. The starting pulp was oxygen-delignified coniferous wood sulphate pulp with kappa number 6.5 (10.5 after digestion). Curve 1 showg.. conventional peroxide bleaching, with a consumption of H2O2 of 2t) kg/ton pulp. Curve 2 and, respect- WO 93/14262 PCT/SE93/00012 4 l^nS It appears from the diagram how the brightness, after bleaching according to the invention and, respectively, after conventional bleaching, depends on the pulp consistency. At about 12% pulp consistency, the brightness obtained is equal in both cases, but at increased pulp consistency in the peroxide step an increased brightness is obtained according to the invention, while conventional peroxide bleaching results in a deteriorated bright-, ness.

The following examples have the object to additionally elucidate the invention and its advantages.

Example 1 An oxygen-delignified coniferous wood sulphate pulp (Scandinavian fir/pine) with kappa number 6.4- (10.5 after digestion according to above Finnish patent application 90 0663), brightness 4-3% ISO, intrinsic viscosity 605 dm^/kg, was pretreated with complexing agent and peroxide bleached according to the invention.

Pretreatment Pulp consistency % Time min • Temperature °C 90 EDTA kg/ton 2 Final pH 6.1 Peroxide step Pulp consistency % Time min 24.O Temperature °C 90 Si02 kg/ton H2O2 consumption kg/ton 13-5 32.0 .Final pH_ .2 .3 Brightness % ISO 79-6 84- • 2 Intrinsic viscosity dm-Vkg 520 U7 2 mis Example 2 A coniferous wood sulphate pulp with kappa number 13-3 (digested of Scandinavian fir/pine according to Finnish patent application 90 0663), oxygen delignified to kappa number 8.3"and ozone bleached in acid environment to kappa number 3.2, brightness 61% ISO, intrinsic viscosity 582 dm^/kg, was bleached according to the invention.

Pretreatment as in Example 1 Peroxide bleaching Pulp consistency % Time min 24.0 Temperature °C 90 Si02 kg/ton H202 consumption kg/ton 7.3 14- • 2 .1 Final pH • 8.9 9-5 .1 Brightness % ISO 85.3 88.4- 90.4 Intrinsic viscosity dm3/kg 536 498 420 Example 3 An oxygen delignified coniferous wood sulphate pulp (digested of Pinus taeda according to Finnish patent application 90 0663) with kappa number 9.9, ozone bleached to kappa number 4-. 8, brightness 54.-5% ISO, intrinsic viscosity 607 dm^/kg was pretreated and bleached according to the invention.

Pretreatment as in Example 1 Peroxide bleaching Pulp consistency % Time min 240 Temperature °C 90 Si02 kg/ton H202 consumption kg/ton 7.9 U.6 29.6 Final pH .2 .4- .6 WO 93/14262 PCT/S E93/00012 6 ■ Brightness % ISO 81.3 85-3 87.8 Intrinsic viscosity dm^/kg 570 534. 4-59 Example 4.

An oxygen delignified birch sulphate pulp with kappa number 9.0, brightness 59.4-% ISO and intrinsic viscosity 996 dm^/kg was bleached according to the invention.

Pretreatment as in Example 1 Peroxide bleaching Pulp consistency % Time min 240 Temperature °C 90 Si02 kg/ton H2O2 consumption kg/ton ,• 8.5 17.5 Final pH .2 .4 .5 Brightness % ISO 82.2 85.5 86.5 Intrinsic viscosity dm^/kg 903 975 822 The invention is not restricted to the embodiments set forth above, but can be varied within the scope of the invention idea. 7

Claims

Claims 1

1. A method of bleaching sulphate pulp without using chlorine-containing bleaching agents, comprising oxygen delignification and possibly ozone delignification of the pulp to a kappa number below 9-5 and subsequent peroxide bleaching, wherein directly after delignification the pulp is pretreated with a complexing agent for between 1 to 60 minutes, the pulp consistency being between 1 to 15%, the temperature being between 70 to 110°C and the pH being between 5 to 7, wherein the pulp thus pretreated is washed and dewatered to a consistency of between 18 to 40% and thereafter peroxide bleached in an alkaline environment in the presence of silicate or an organic complexing agent of the type phosphonic or carboxylic acid.

2. A method as defined in claim ,1, wherein the peroxide bleaching is carried out for between 1/2 to 5 hours at the temperature of between 70 to 110°C.

3. A method as defined in claim 1 or 2, wherein the complexing agent is EDTA or DTPA.

4. A method as defined in any one of the preceding claims, wherein the peroxide bleaching silicate is added in an amount of between 2 to 30 kg/ton of pulp, expressed as Si02.

5. A method as defined in any one of claims 1 to 3, wherein an organic complexing agent of the type phosphonic acid or carboxylic acid is added in an amount of 0.5 to 5 kg/ton of pulp during the peroxide bleaching.

6. A method as defined in any one of the precedJ claims, wherein the bleaching is carried out to brightness exceeding 83% ISO.

7. A method as claimed in claim 1 substantially as herein described.

8. A method according to any one of the examples.

9. Pulp produced according to the method of any one of the preceding claims. I SlUSttbS DEFIBRATOR INDUSTRIES AB By Tn BALDW ^r\^ EN. S ttorneys ON & CAREY V \