Pulp and Paper Canada

Conference 2002: International Pulp Bleaching

July 1, 2002  By Pulp & Paper Canada

The 2002 IPBC, held in Portland, OR, during the third week of May, attracted over 170 participants from 16 countries. This represents a dramatic drop as compared to over 500 in Helsinki in 1998 and 35…

The 2002 IPBC, held in Portland, OR, during the third week of May, attracted over 170 participants from 16 countries. This represents a dramatic drop as compared to over 500 in Helsinki in 1998 and 350 in Halifax in 2000. Many pointed to the present financial situation in our industry and to side effects of September 11 as explanations for the reduced participation, but nobody questioned the quality of the conference program.

The conference was a turning point. The excitement about TCF bleaching that has characterized the 90s with its panoply of esoteric chemicals and exotic bleaching sequence is over. In Portland, most of the 31 oral presentations and 15 posters addressed the various problems of the existing ECF operations proposing chemicals, technologies and control strategies that are ready for implementation. It has been a while since this conference has been so focussed on today’s problems. Ashok Jain and Richard Berry, program and conference chairman respectively, and the remainder of the program committee should be thanked for such an industrially relevant program. As mentioned by Berry in his closing remarks, the level of participation and debate during the question periods was of very high quality and made up for the low level of attendance.


One of the challenges for the future was already set in the keynote lecture of George H. Weyerhaeuser, where he identified the transformation of the industry from companies competing against each other toward Competing Value Chains. These chains start with the pulp mill through intermediate customers (like printers) to ultimately include the final customers. The challenge for scientists, researchers and mill engineers is to learn how to create value through application of technology along that chain instead of restricting themselves to the initial step of the chain (the pulp mill).

Within the session addressing the future of bleaching, Jurgen Kramer (Windrose Consulting, USA) presented a very original historical perspective of the 17 IPBC conferences starting in 1955. By considering the time of the first implementation of a technology to the time of its first presentation at IPBC, he identified whether the conference was a mover or a mirror of technology development. The game ended with a tight score in favor of IPBC being a mirror of technology implementation. Another interesting point coming out of this study was that, of all countries involved, Canada was the most prolific contributor of scientific papers. Doug Reeve (University of Toronto) followed with a presentation on the next generation of environmental driving forces and how our industry should prepare itself to pursue the minimization of the impacts of effluent, solid waste and air emission on the environment.

The first three papers of the Pulp Washing Session were dedicated to lignin precipitation and adsorption onto fibres. Agneta Fuhrmann (KCL, Finland) demonstrated that acidification of an unwashed oxygen-delignified softwood kraft pulp to a pH below 6 precipitated lignin and extractives and that only partial removal of that lignin was possible by washing the pulp. Kecheng Li (University of Toronto) presented a study of the lignin adsorption on pulp fibre surface under kraft pulping and washing conditions. He postulated the formation of a monolayer of adsorbed lignin but when calcium was present, precipitation of lignin aggregates also occured. These results were reinforced by the results of Natalia Maximova (University of Helsinki) on calcium- and aluminum-induced lignin precipitation. According to her, lignin adsorption always impaired brightness but not always sheet strength. The session closed with a paper by Olli Joutsimo (KCL, Finland) showing that a good post-oxygen delignification washing system efficiently removed most of the soluble lignin. A 30-minute leaching stage only provided a 1-1.5 kappa unit drop by removal of the “easily leachable lignin”.

Mari Nuopponen (University of Helsinki) opened the Session on Analytical Issues by presenting the spectroscopic technique they have developed to show the inhomogeneity of lignin distribution in kraft pulp (from half to twice the average kappa number!). However, they were not able to correlate the inhomogeneity with pulp bleachability. George Pan (Alberta Research) showed how Raman Spectroscopy can be used to measure dissolved organic substances from peroxide bleaching of aspen CTMP and use it as a predictive tool to monitor pulp quality. The ultimate goal is to develop an on-line sensor. Kamilah Smith (McGill-Paprican) has been successful in her attempt to quantify both hydroxyl and superoxide radicals generated under peroxide bleaching conditions using a 31P-NMR approach. In alkaline solution, they found a much higher concentration of superoxide than hydroxyl radical, which is an insight that will expand our view of peroxide bleaching chemistry.

Arthur Ragauskas (IPST, USA) opened the Oxygen Delignification Session by presenting a study on the impact of pulping conditions (A.A., H factor, brownstock kappa number) on the efficiency of O2 delignification. As an example, they found that high alkalinity enhanced the efficiency of oxygen while low alkalinity improved yield and viscosity. They related the observed differences to residual lignin functional groups. Richard Berry (Paprican) presented the development of a dynamic model for the optimization of oxygen delignification tower. The model was based on both lignin removal and cellulose degradation. Furthermore, various constraints specific to a mill could be included with their associated costs to provide a cost minimization model. Steven Violette (University of Maine) studied the adsorption of carbohydrate polymers onto browstock fibre surface as radical scavengers during oxygen delignification. They found a significant selectivity improvement using less than 1% of either guar or glucomannan; xylan however was not effective. Yonghao Ni (University of New Brunswick) presented an alternative method for the production of peracetic acid based on the oxidation of penta-acetyl glucose by hydrogen peroxide. The results showed that the peracetic acid prepared by this method was equally effective in an OxO type of sequence as the commercial one.

The session on Bleaching Fundamentals started with a presentation by Sari Panula-Ontto (KCL, Finland) on the effect of cell wall thickness on bleaching. Results showed that oxygen delignification and ozone bleaching efficiency increased with cell wall thickness. However a thicker cell wall increased hydrogen peroxide consumption. No correlation was found between chlorine dioxide consumption and cell wall thickness. Chad Bennington (University of British Columbia) presented the results on gas-liquid mass transfer measured in a laboratory-scale retention tower. They found that the mass transfer rate varied with consistency through a minimum which coincides with a change in gas hydrodynamics from bubble column behavior (consistency < 3%) to porous solid behavior (consistency >6%). Brian Brogdon (Kemira Chem.) explained the cause of lignin reactivation toward ClO2 during alkaline extraction. The quinone and muconic acids formed during a D stage were respectively reduced to polyphenols and saponified to acid methyl esters during the subsequent extraction stage (more soluble).

Marcelo Costa opened the ECF Bleaching Session with a study on bleachability of oxygen delignified Eucalyptus kraft pulp. Residual lignin and hexenuronic acids were found to be the main contributors to ClO2 consumption even if HexA’s consumed less ClO2 per kappa unit drop. Removal of HexA’s with a hot acid wash significantly decreased ClO2 charge, OX in pulp and AOX in filtrates. Luc Lapierre (Paprican) followed with a demonstration that hydrogen peroxide decomposition took place through interaction with transition metal located in the pulp rather than with the ones in the aqueous bulk phase. He also showed that the most efficient form of magnesium for peroxide protection was Mg complexed either with the pulp or a chelant. Zhi-Hua Jiang (Paprican) showed that the rate and effic
iency of ClO2 delignification are improved by adding a small charge of aldehydes (particularly formaldehyde). Higher delignification was obtained because of the reaction between formaldehyde and chlorite which regenerates ClO2. This approach saves ClO2 with minimal capital investment. The final paper of the session, given by Martin Ragnar (Kvaerner Pulping, Sweden) dealt with the application of a 2-stage bleaching sequence for oxygen delignified Eucalyptus kraft pulp. In order to avoid the yellowing problem typical of 2-stages bleaching, they proposed a first hot ClO2 treatment combined with chelation followed by washing and a pressurized hydrogen peroxide stage.

Within the Ozone Session, Christine Chirat (EFPG, France) proposed to use ozonated water for improving the final brightness of bleached pulp. She was able to get a couple of brightness points by applying less than 0.05% of dissolved ozone. On the topic of (DZ) bleaching, Charles Courchene (IPST) demonstrated the efficiency of (DZ) stages of less than 2 minutes on delignification of southern pine kraft pulp. Maria Wennerstrm (Metso Paper, Sweden) showed that ozone was more efficient for HexA’s removal than ClO2 and improved the brightness reversion properties of Eucalyptus kraft pulp.

In the Mills Experiences Session, Stan Heimburger described the optimization of an ECF sequence (from laboratory to mill implementation) in order to meet the U.S. Cluster Rule regulations. For mills which are ClO2 limited, Jane Mathews (Warwick International, U.K.) presented mill trial results about in-situ peracetic acid addition at the end of the oxygen stage to decrease lignin content and ClO2 requirements. Marcelo Rodrigues da Silva (Votorantim Celulose e Papel, Brazil) described the implementation of a hot acid stage for Hexa’s removal. The process was beneficial for chemical consumption, brightness reversion and oxalate scaling. However, the pulp was harder to refine and a 5% drop in tear strength was observed.

The final session addressed the bleach plant closure topic. Paulo Barata (Portucel, Portugal) explained the change they made on the washing circuit of their bleach plant. The replacement of counter-current acidic line for a jump from D2 to D0 stage has solved calcium carbonate and barium sulphate scaling problems coupled with a reduction in chemical consumption. Minna Viirimaa (University of Oulu, Finland) has demonstrated that black liquor condensates could be used as process water in D0. The only compounds that impair the bleaching results are DMS and DMDS and they should be minimized. Karin Lindgren (STFI, Sweden) closed the session with an original approach to remove the NPE and improve bleach plant closure. An acidic leaching treatment of the chips seemed to remove a large part of the metal ions and therefore may allow higher level of closure.

Jean Bouchard of Paprican is also the vice-chairman, PAPTAC Bleaching Committee

For those interested in bleaching, have you considered:

PAPTAC Bleaching Committee

The PAPTAC Bleaching Committee is celebrating its 25th year. The goals of the committee are:

To discuss bleaching practices, technical problems and needs

To promote co-operation between those involved in research, development and production

To gather and exchange information on new and existing technology related to bleaching

To promote writing and presentation of papers and roundtable sessions on all aspects of bleaching technology

To actively monitor new developments in the area of bleaching

To provide training sessions and courses

The committee has been successful over the years in the sponsorship of bleaching courses, promoting information exchange and providing a forum where technical problems and technology can be discussed. Meetings are held twice a year and routinely have 40-50 attendees. The next meeting is tentatively scheduled for Skookumchuck, BC, in the fall.

For more information, please e-mail Bleaching Committee Chairman, F. Donald McCabe, P. Eng., at fwdmc@aol.com

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