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International Pulp Bleaching

If you produce market kraft pulp from eucalyptus and use ozone in your bleaching sequence, you would have felt very much at home at the 2005 International Pulp Bleaching Conference, June 14-16 in Stoc…

If you produce market kraft pulp from eucalyptus and use ozone in your bleaching sequence, you would have felt very much at home at the 2005 International Pulp Bleaching Conference, June 14-16 in Stockholm.

Clearly, eucalyptus market pulp is the ever-rising star in temperate and tropical zones, and gargantuan new kraft mills to process it are in construction or have recently started up (e.g., Hainan Jinhai, at 3000 tpd, in China).

If, on the other hand, you had been hoping for serious discussion of mill process water closure, or enzymes in bleaching, you might have been disappointed. Although they were hot topics the last time the IPBC show came to Stockholm (1991), in this millennium closure was hardly mentioned (except in the business sense) and enzymes were relegated to the poster session.

Most of the world has adopted a common bleaching religion called ECF, with only a few heretical TCF sects in Europe. Diligent searching of the horizon for some new bleaching chemical or process seems fruitless. Chlorate producers should be very happy.

Following the Pak

One of the most intriguing presentations in Stockholm wasn’t about bleaching at all. Given by two of the three Swedish winners of the 2005 Wallenberg Award on the opening morning, it concerned the development of Tetra Recart, a new Tetra Pak which is designed to replace the 200-year-old technology we call canned food. Able to protect the contents from light, oxidation, and microorganisms for up to two years, these new packs are just beginning to enter the marketplace. Imagine the challenge of developing a six-layer package whose contents may be sterilized for up to three hours at 130C, followed by automatic sealing! Outward from the innermost layer of polyethylene are aluminum foil, polypropylene, special paperboard for stiffness and wall strength, an outside layer of polypropylene to print on, and a lacquer surface coat. It took more than ten years to figure out the package and the equipment for Recart.

IPBC 2005 ran co-currently with the annual SPCI conference, a one-day SPCI PRIMA marketing conference, and an extensive equipment and services exhibition (it covered 14,000 m2 — to Canadians, that’s almost nine hockey rinks). Stockholmsmssan was the world’s pulp and paper capital for the week. PAPTAC, the Finnish Paper Engineers’ Association, and TAPPI were co-sponsors. People came from three dozen countries on six continents.

First D stage a hot topic

Although D is now the (almost) ubiquitous first stage of ECF sequences, that doesn’t mean that it is identical everywhere. In fact, the push to optimize every bleaching stage seems the major focus of both research and industrial practice these days. So, should you use DHT instead? For eucalyptus pulps, some proponents suggested that the hot version, at 85-90C, was a good idea, especially for efficient removal of both residual lignin and hexenuronic acids. Others were not so sure — an acid stage prior to Do could be entirely satisfactory.

One thing certainly emerged during the three days — the species from which a eucalyptus kraft pulp is made is an important variable in the bleaching equation, and cannot go unidentified. We’ve long known that pines, for example, are not all alike within the genus, so there’s no obvious rationale for treating eucalypts as identical either.

Ascendency of eucalyptus

It should surprise no one that eucalyptus kraft pulp is in a significantly ascendant phase. It has propelled Brazil to fourth place globally in bleached kraft pulp exports (Canada is first, with more than three times the tonnage). For more than two decades, Japan — the world’s third biggest woodpulp producer — has been a leading producer of eucalyptus kraft pulp, all for papermaking domestically, even though it has no commercial eucalyptus forests (the wood is imported from Australia and South America). It’s not unusual when ocean freighters unload eucalyptus chips at Nordic mills, including some whose previous reputations rested solely on top-quality softwood pulps. And those new 3000 tpd state-of-the-art kraft mills — most of them run on eucalyptus, whether for market pulp or integrated to fine paper production. Eucalyptus has serious momentum.

Rapid expansion for ozone?

An intriguing graph on Day three showed the chronology of worldwide bleached kraft pulping capacity, that of oxygen delignification, and the more recent history of high-consistency ozone bleaching. Where the third of these is going is conjecture, but an optimist might imagine it becoming like the oxygen curve — a slow induction period (ten years already), then a leap upward. A single new 3000 tpd mill with ozone bleaching will add one million tonnes to the curve.

The driving reason to use ozone is the chlorine dioxide savings it offers relative to a Do or DHT stage. With — you guessed it — eucalyptus pulp, what doesn’t change is also good — pulp strength, beatability, and yield. Examples are the ZeDD bleach plants at the Oji/Nichinan and VCP/Jacarei mills (the latter with two high-consistency Z stages in parallel). Oji’s installation is the first in Japan, on hardwood; best operating conditions include a pH of 3.0 and 38% consistency. BOD and AOX are well down, but bleaching cost increased by 20% (the electricity for ozone generation is not cheap in Japan).

The best position for ozone in a bleaching sequence? The consensus seems to be at the front, and higher incoming kappa is better than lower.

Pulp yield going up?

Gone is the wild enthusiasm of 15 years ago for just how low kappa targets could be pushed in modified kraft pulping. Once again, normal values abound: ~30 with softwoods, and ~15 with hardwoods. Then, assume that oxygen delignification will drop those numbers by about half. What’s left to say about yields in bleaching?

Quite a bit, actually. Several presentations reinforced the wisdom of bringing brownstock kappas back to higher values. And one speaker proposed that yield values across bleach plants could be determined by detailed chemical analysis of filtrates centrifuged from the pulps before and after the stages. Curiously, dissolved material derived from lignin comprised ~60% of the total in hardwood cases versus ~80% with softwoods.

Odds and ends

And now, a brief compilation of some presentations which seemed to escape the bounds of the program categories:

You can model a variety of oxygen delignification systems accurately if you account for all the main conditions of chemical reaction and mass transport.

The same is true for non-process elements in kraft mills, establishing concentration profiles along complete fibrelines in both pulp and process water. The ghost of Howard Rapson might have commented, “When that was done 15 years ago in Canada…”

Using 0.25% formaldehyde on softwood kraft pulp saved ~11 kg/t of ClO2 in Do without penalties in effluent properties, pulp strength, or human exposure.

The world’s first industrial peracetic acid bleaching stage went into operation in 2004 on softwood Magnefite pulp at Stora-Enso’s Nymlla mill, adding two points to final brightness (92%); the sequence is OQ(PaaP).

A 20-year compendium of pulp strength delivery studies along two dozen fibrelines showed that digester systems remain the major reason for the strength that’s missing in many softwood kraft mills.

Radically new

As mainstream as most of the conference was, one off-the-wall idea did emerge – the use of carbonate radicals for bleaching. Produced from peroxynitrite, they were tried with lignin and carbohydrate model compounds, cotton linters, and softwood kraft pulp. Unlike hydroxyl radicals, carbonate ones provide a direct route to lignin fragmentation. With pulp, it’s possible to get a 10-kappa decrease in a few minutes at room temperature. With the (almost) complete departure of chlorine from bleaching, might this reagent foster a C ch
ange in the nomenclature?

A sign of things to come?

At this IPBC, it wasn’t just the temperate zone pulp manufacturers talking about what they’re doing (or hoping to do). Nordic R&D people were in the majority; collectively they’re doing lots of research on bleaching, more often than not on eucalyptus pulps, whether working for equipment suppliers, at research institutes or company labs, or at universities. Everywhere, eucalyptus is clearly the “pulp du jour.”

Another few years, and perhaps we’ll be going to the IEucPBC…

Martin MacLeod is a freelance writer living in Beaconsfield, QC.

INTERVIEW WITH PETER AXEGARD, PROGRAM CHAIRMAN OF IPBC 2005

P&PC: Where do you think bleaching is going in the next 5-10 years? Do you see the start of something new?

Axegrd: Oh, boy — in 5-10 years, it has to be incremental change in the industrial world. My own vision has to do with the colour of kraft pulp. If we could prevent that, then bleaching would require much fewer stages, along with less energy, water, and chemicals. We’ve been stuck with this problem for most of the history of chemical pulping. Ideally, we could have a pulp that leaves the digester at full brightness.

P&PC: Can you picture a process combination where everything occurs in one reaction vessel?

Axegrd: Probably not one, but possibly two. We can already do that in the lab — two stages to produce a strong chemical pulp with a respectable yield.

P&PC: In bleaching, have we finally become the ECF world?

Axegrd: That appears to be the case. Apart from ozone and peroxide in some mills, almost everyone is using ECF. It doesn’t look like anything could change that. But the market is a strong force. And you can never say never!

INTERVIEW WITH GORAN ANNERGREN, JAG CONSULTING, SUNDSVALL

A long and acute perspective on bleaching provides a great platform from which to describe the past and present and peer into the future. Gran Annergren has that background plus the ability to organize a rather long and complicated history into understandable themes. He put his talent to work as the keynote speaker at IPBC 2005 (and also published a new book on bleaching!).

Annergren reached back to such now-historical sequences as CEHDED, then moved on to the environmental push of 1985-1995, when the spectre of chlorinated dioxins forced a significant change in bleaching — oxygen delignification as a standard “pre-treatment,” and 100% chlorine dioxide displacing chlorine. Kraft pulping made great strides forward in the same era.

And today’s path forward? According to Annergren, it’s one of consolidating current bleaching into effective and economical mill practice. Added benefits — some modern bleach plant designs provide the potential flexibility to tailor pulps for particular products, and all of them have far more efficient washing.

P&PC: Where is ozone bleaching going?

Annergren: A lot of mills may use ozone, but in small quantities, so it probably won’t grow the way oxygen delignification has. And it’s not the kind of chemical you can let dominate a bleaching sequence.

P&PC: Are we now in the ECF world for a long time to come?

Annergren: Yes — I hope so. I like chlorine dioxide. I’ve been working with it practically all my life, all the way back to the early 1960s with DEDHH bleaching of hardwood sulphite pulp. ECF-light is a good compromise for today’s mills.

P&PC: What about entirely new approaches to bleaching?

Annergren: Well, I think we’ve come to the point where bleaching is maturing again, so it’s more difficult to push new things. There’s still plenty to do in the mills — cost savings by more balanced bleaching, for example, because there’s lots of scatter in the amounts of chemicals used. These days, most of the development of bleaching has been taken over by the suppliers. It’s hard to say what’s coming next.

INTERVIEW WITH PETER SANDSTROM, SCA GRAPHIC RESEARCH, SUNDSVALL

P&PC: As a TCF pulp producer, what interested you at the conference?

Sandstrm: There was quite a lot about ozone bleaching, which was most interesting for us at SCA; some of the information may be applicable in our mill. Of course, there was plenty about ECF and eucalyptus, and that’s outside our realm, but it brings you some general knowledge which is always good to have.

P&PC: Hardwood kraft pulps — are they still important?

Sandstrm: Well, production of hardwood pulps is growing faster, but softwood production is increasing too.

P&PC: Softwood kraft pulps — are they still important?

Sandstrm: Yes — we need to have softwood kraft pulp even stronger so that there’s a clear benefit in using it. And on the hardwood side, there are lots of questions about brightness reversion, ozone versus hot acid treatment — more research is needed.

P&PC: Closure — is it a dead issue?

Sandstrm: There is no big market today for such technology. Fifteen years ago, 40-50 m3 of bleach plant effluent per tonne of pulp was normal. Now many mills here have come down to around 10, and smaller treatment plants are much cheaper in capital and operating costs. Totally closed? That probably creates more problems than it solves.