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The State of Bleaching in Canadian Kraft Mills

So what has happened lately?

Not much, is the truthful answer. But not nothing. To assess the state of bleaching in the kraft sector, Pulp & Paper Canada talked with three people closely associated with the subject: Paul Earl, a consultant on industrial bleaching, based in Mississauga, ON; Fred Munro, innovator and pulp mill guru at Domtar’s Espanola, ON, mill; and Chad Bennington, NSERC/Paprican professor of chemical engineering at the University of British Columbia in Vancouver, BC. Here’s what they had to say.

PPC: Pulp bleaching is now overwhelmingly ECF. Can you see this changing anytime soon?

Earl: It is an ECF world, whether you look in North America, Europe, South America, and now Japan is catching up too. Only in Scandinavia can you find a few TCF producers, all tightly linked to TCF customers. And even there, I don’t see much likelihood of more bleach plant conversions, or that the TCF market will grow. There’s nothing on the horizon that suggests a significant change away from ECF.

Front end: Oxygen delignification

PPC: Let’s consider process sequences, starting with oxygen delignification. Will its on-going development simply be a continuation of two-stage systems or something different? Will people be buying the same thing 5-10 years from now?

Bennington: Well, you have to look at the whole context of a fibreline and what you want to do with it. Certainly, oxygen delignification will continue to play a big role. But, because of the large cost difference among the options (one-stage, two-stage, mini, or double-O after Do), all of the fibreline components have to be integrated in the decision, including pulping and bleaching systems. Overall, the more delignification you get at this point in a fibreline, the better for process economics. But a well-operated single-stage system can get you almost to where a two-stage system can go.

PPC: How about adding a second reactor to a one-stage system?

Bennington: Absolutely, that’s another interesting option.

Earl: There’s an almost universal use of oxygen inside bleach plants now to reinforce first extraction stages prior to final bleaching. Putting a double-O system after Do inside a bleach plant is another new option operating in a couple of mills in Canada.

But naturally, that doesn’t contribute nearly the same environmental benefits as true oxygen delignification up front, with the filtrate going to chemical recovery.

PPC: Why don’t mills with efficient oxygen delignification systems push their digesters to higher in-going brownstock kappa numbers to gain a yield advantage?

Earl: I really don’t know. I’m a big fan of higher-kappa softwood pulp, whether or not oxygen delignification precedes bleaching. With wood costs escalating, everyone should be looking at better fibre economy this way. Of course, you have to be able to produce high-quality pulp, and then screen and wash it effectively.

PPC: How high is “high” for this argument? Is 40 kappa too high? Should 35 be common?

Earl: With polysulphide pulping, you could probably go to 40-kappa brownstock. Some kraft mills operate in the mid-30s. I believe that the higher-kappa pulps are more easily bleached — for example, 10% higher kappa doesn’t require 10% higher charges of bleaching chemicals.

Ozone a contender?

PPC: At Espanola, you have a hardwood bleach plant that’s unique in Canada — it leads with ozone. Do you think there’ll ever be another one in a Canadian mill?

Munro: Good question. There are two main aspects to consider, technical versus financial. From a technical standpoint, I could imagine more installations like ours, but probably no Canadian mill is currently in a financial position to do so. Most mills will continue to run the ECF bleach plants they already have.

PPC: What do you like about ozone?

Munro: It’s a particularly good fit with hardwood pulp because of the way it reacts with hexeneuronic acids. Our Z/D stage is very flexible; if there’s a problem with the ozone supply, for instance, the chlorine dioxide can pick up the difference, so we don’t lose quality or productivity. That’s one of the reasons we chose that configuration over ozone alone.

PPC: Have you ever crossed softwood pulp over to the hardwood bleach plant?

Munro: We haven’t to date, but we’ve thought about it.

Making bleach plants better

PPC: Moving farther into bleach plant operations, are there opportunities which don’t need big outlays of capital? Are better sensors required, for example?

Munro: I can’t imagine many sensors we don’t already have, but there might be some that could work better! Consider something as simple as pH measurement. It appears to be simple to do, but in our experience requires quite a bit of effort by instrument mechanics. The normal variability of process conditions in a bleach plant makes it that much harder. For residual chemical sensing and brightness, the situation is reasonable, I’d say. Likewise with consistency — our newer mechanical-design ones are pretty good. Because we run multiple hardwood species, we’d like to have better tracking of fibre length to see the transitions accurately. And adding some modeling which accounts for changes in flows and consistencies would be nice. Everything is geared to maintaining quality, reducing variation, and minimizing cost.

Earl: One of the things we learned in the PAPTAC Bleaching Committee’s 2003 survey was that many mills had installed automatic kappa analyzers but often hadn’t linked them to their process control systems. It’s great to have valuable information in a timely fashion, but you need to use it. And there are other things you need to track closely in bleaching control strategy — filtrate carryover, chlorine dioxide concentration, temperature, pH, consistency, and so on.

Bennington: To operate a mill well, you have to know how efficient it is, because mills tend to fall out of their windows of maximum efficiency fairly readily. And it’s not just equipment — it’s also a matter of having the right people to monitor the system and tune it up when needed. You could probably gain 10% efficiency in that sense alone.

Earl: Even doing simple things well can have big payoffs. Take temperature, for instance. You want the extraction stages as hot as possible and the chlorine dioxide stages just hot enough to complete the reaction. Getting accurate temperature sensing all the time is important. Another is measurement and control of alkaline pH — if the probes continually go out of calibration, the caustic charge won’t be right.

PPC: Are there enough people available to do the monitoring and tuning?

Bennington: A good process engineer can do it. But in mills these days, they’re often too busy with other things to keep a close eye on bleaching. One example is chemical mixing, where there’s perhaps another 10% efficiency gain to be had by paying more attention to mixers. Also, the higher the chemical use in a given stage, the greater the return on that 10% improvement in efficiency. So in a Do stage, it could be very effective.

Earl: Another avenue of help is the Bleaching Committee. It can give an engineer a Canada-wide perspective from which to gauge a mill’s performance — where does it sit among two dozen mills, for example, and how much better could it be?

Dream assignment

PPC: Imagine that someone walks up and offers you $50 million in starter money to plan a new bleachable-grade kraft fibreline (note: the most recent one in Canada was ins
talled more than 10 years ago). The pulping side is already taken care of — you will receive high-quality brownstock at whatever kappa number you want. What do you do?

Munro: I’d definitely have oxygen delignification, probably a two-stage system, medium consistency rather than the high-consistency ones we have now, plus modern kappa control. But I must say that our existing systems are pretty simple, they run reliably at 45% delignification, and the maintenance is mostly standard things like seals and bearings. On hardwood pulp, I would certainly look at an acid stage ahead of Z/D, the former for control of metal ions, the latter for good delignification and hexeneuronic acid removal. Then ED to final brightness. So (OO)AZ/DED; perhaps the E would be Ep or Eop. That line-up would provide effective bleaching at minimal operating cost. With softwood pulp, I’d choose the same type of oxygen delignification, then perhaps an A stage to strip metal ions so you could maintain good energy and heat balances and minimize effluent by using tighter filtrate recycle. Without metals control, scaling will be a problem. The softwood bleach plant would be DEoDED to market brightness. Right now, we run DnD in the back ends of our bleach plants, and the lack of separate E2 stages lowers the overall chemical efficiency. I’m actually quite happy with what we have, so I wouldn’t deviate that much from it.

Earl: Friends don’t tell friends to do DnD!

PPC: Okay, but what would you do?

Earl: If you’re prepared to deliver a well-washed, well-screened 40-kappa softwood pulp to me, I’ll use a two-stage oxygen delignification system to drop the kappa number to the low 20s to maximize pulp yield, and with enough money I want a five-stage bleach plant capable of making pulp at 91% ISO brightness. Since you’re giving me a dream assignment, I don’t want to ride the edge of high brightness with only three stages. Call it (OO)DEoDEpD. And while you’re at it, give me some good process engineers to pay attention to the fundamentals — great bleaching practice is probably more people-driven than equipment-driven.

Bennington: First off, I think there’s probably a lot to be done in creating pulp uniformity from the digester. Variability there propagates through the entire fibreline. Many digesters today are running with big swings in quality that can upset operations all the way down the fibreline. I need a narrow standard deviation in kappa, with the kappa actually on target.

PPC: Assume that you’ve got it.

Bennington: Oxygen will have a big part in this fibreline, but so will chlorine dioxide. Some synergy between oxygen and ozone in the first bleaching stage could be good. Capture of filtrates to chemical recovery will be important. The planning needs to integrate all of the main systems — digester, oxygen delignification, and bleaching — to optimize each scenario of interest with respect to highest yield, lowest variability, and the pulp you really want.

So there you have it — modern bleachable-grade kraft mills will probably look much like what we already have in terms of main fibreline components. But they will have significantly better process control, tighter process water loops, greater overall use of oxygen, very hot caustic extraction stages, and high-efficiency washers of a single design all the way down the fibreline. The mainstream sequence — ODEopDED — makes ECF bleaching the gold medal standard.

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

PAPTAC — a font of bleaching information

For many years, PAPTAC’s Bleaching Committee has been a magnet for bleaching practitioners, whether in industry or research. The committee provides a bubbling discussion forum at its meetings and through its website. Its sessions at PAPTAC’s annual meetings bring forth a wealth of useful information on all aspects of bleaching. PAPTAC regional conferences regularly add to this knowledge base. Here are just a few recent examples:

Two pressurized caustic extraction towers prior to an atmospheric retention tower are only marginally better than one. Raising the reaction temperature by 5-10 C in one 15-minute treatment matches what a 30-minute second tower can do [B. van Lierop et al. (Paprican), PAPTAC 2006 Annual Meeting].

Caustic is often overdosed in extraction stages in bleach plants. Simply measuring “final pH” is inaccurate because of buffering effects. A better strategy takes into account the incoming kappa number of the pulp, the intensity of the Do stage, and the carryover from it [H. Suess and D. Davies (Degussa), PAPTAC 2006 Annual Meeting].

Despite the known yield benefit of raising brownstock kappa number, “About half of the mills are cooking to a lower kappa number than five years ago, while another 20-30% are cooking to a higher kappa number now.” [From the Bleaching Committee’s 2003 survey of softwood bleaching practices in Canada].

“Compared with high relative consumption bleach lines, low-consumption conventionally-delignified softwood mills cook to three points higher kappa number” [2003 survey].

A Few Statistics on Bleaching in Canadian Kraft Mills

Number of bleachable-grade mills in operation: 34

Most popular sequences:

ODEopDED (9 mills),

DEopDED (9)

Current production:

softwood ~9 m t/y

hardwood ~2 m t/y

Mills with oxygen delignification: 15 (44% of total)

Some firsts at the Espanola Mill

* First oxygen delignification systems in North America (SW 1977, HW 1980)

* First conversion of a softwood single-vessel hydraulic Kamyr digester to Lo- Solids in Canada

* First ozone bleaching stage in Canada

* First commercial installation of the Paprilox polysulphide process in the world

* First Flakt Biomasster bark-fired boiler in North America