BLEACHING OF CHEMICAL PULP

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: 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, CI2, Clθ2 > a 110 - hypochl-orite, 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 establ¬ ished 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 Clp disappears now more and more, and CI2 is replaced primarily by C102** Bleaching with CIO2 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% pulp_ 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 CIO2. As a result, th-is final bleaching then yields emission of AOX from the bleach plant and chlorinated organic susbstance in the bleached pulp. 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 or.further bleaching. By combining conventional digestion, oxygen delignificat- ion and ozone bleaching in acid environment, kappa numbers lower than 10 can be obtained. It has, however, not been possible to produce fully bleach¬ ed 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~ elig ification, altern¬ atively by conventional or extended digestion followed by O2- and 0^-delignification. The bleaching is initiated by a pretreatment step with ^ complexing agent followed by treatment with hydroperoxide * in alB-aline environment in the presence of silicate or an organic complexing agent of the type phosphonic or carboxylTc acid. The characterizing features of the invention are appar¬ ent from the attached claims. The pulp digested and delignified with oxygen and possibly ozone according to above shall have a kappa number below 9. 1. 5. This can be achieved by known methods of digestion and delignification. The pulp shall thereafter be pretreated with complexing agent, for example EDTA or DTPA, at a pul£> consistency of 1-15% for 1-60 minutes, preferably 5-30 minutes. The temperature shall be 70-100°C, preferably 80---100°C, and the pH-value 5-7. The pulp thus pretreated is washed and dewatered to a con¬ sistency 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, alt¬ ernatively 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 70-100°C, pref¬ erably 80-100°C. The silicate charge shall be 2-30 kg/ton pulp, preferably 5-15 kg/ton pulp, expressed as Siθ « Alternatively, the charge of the organic complexing agent shall be 0.£-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 bleach¬ ing 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 con¬ sumption of H2θ 2 of 20 kg/ton pulp. Curve 2 and, respect¬ ively, 3 refe-3?- to bleaching according to the invention itith silicate addition and a consumption of H2O2 of 20 and, respectively, 30 kg/ton pulp. It appears from the diagram how the brightness, after bleaching according to the invention and, respectively, after conventional bleaching, depends on the pulp con¬ sistency. 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 convention¬ al 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 applic¬ ation 90 0663), brightness 4-3% ISO, intrinsic -viscosity 605 dur/kg, was pretreated with complexing agent and peroxide bleached according to the invention. Pretreatment 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 kap¬ pa number 3.2, brightness 61% ISO, intrinsic viscosity 582 dm-Vkg, was bleached according to the invention. 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 bleach¬ ed to kappa number 4*.8, brightness 54*.5% ISO, intrins¬ ic viscosity 607 dm-Vkg was pretreated and bleached according to the invention. Brightness % ISO 81.3 85-3 87.8 Intrinsic viscosity dm^/kg 570 534 459 Example 4 An oxygen delignified birch sulphate pulp with kappa number 9-0, brightness 59.4% ISO and intrinsic viscos¬ ity 996 dm^/kg was bleached according to the invention,, Pretreatment as in Exam le 1 The invention is not restricted to the embodiments set forth above, but can be varied within the scope of the invention idea.

Claims: Claims 1. A method of bleaching sulphate pulp without using chlorine-containing bleaching agents, comprising oxygen delignification and possibly ozone delignification of the pulp tr> a kappa number below 9-5 and subsequent peroxide bleaching, c h a r a c t e r i z e d i n that~the pulp directly after the delignification is pretreated with complexing agent for 1-60 minutes, the pulp consistency being 1-15%, the temperature 70-110°C and pH 5-7, and that the pulp thus pretreated is washed and dewatered to a consistency of 18-40% and thereafter peroxide bleached in alkaline environment in the pres¬ ence of silicate or an organic. complexing agent of the type phosphonic or carboxylic acid. 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 peroxide bleaching is carried out for 1/2-5 hours at the temperature 70-110°C. 3- A method as defined in claim 1 or 2, c a r a c t¬ e r i z e d i n that as complexing agent at the pretreatment EDTA or DTPA is used. 4« A method as defined in any one of the preceding claims, c h a r a c t e r i z e d i n that at the peroxide bleaching silicate is added in an amount of 2-30 kg/ton pulp, expressed as SiU2. 5. A method as defined in any one of the claims 1-3, c h a r a c t e r i z e d i n that at the peroxide bleaching an organic complexing agent of the type phos¬ phonic acid orcarboxylic acid is added in an amount of 0.5-5 kg/ton puϊp. ' :6. A method as defined in any one of the preceding claims, c h a r a c t e r i z e d i n that the blea.ching is carried out to a brightness exceeding 83% ISO.

Inventors: DAHL MAARTEN, LINDSTROEM LARS-AAKE, NORDEN SOLVEIG, SJOEDIN LARS, DAHL, MAARTEN, LINDSTROEM, LARS-AAKE, NORDEN, SOLVEIG, SJOEDIN, LARS
Applicants: SUNDS DEFIBRATOR IND AB, SUNDS DEFIBRATOR INDUSTRIES AKTIEBOLAG
Application year: 1993
Main classification: D21C9
IPC codes: D21C9