Pulping/Bleaching

No longer viewed as an unpredictable bleaching agent, ozone is becoming a more common element in bleach plants worldwide

BY KIRK J. Finchem, Technical Editor

Ozone, Chlorine Dioxide Combination Gains Appeal in Bleaching Sequences

And now, with the final promulgation of the Cluster Rule, mills that are developing strategies to convert to full-time elemental chlorine-free (ECF) operations-without spending capital to expand their chlorine dioxide generating capacity-are taking a second look at ozone (O3). For many, the least expensive option for adding O3 to their bleaching sequence will be to add it to the first chlorine dioxide bleaching stage, creating either a "DZ" or "ZD" stage, depending on which chemical is added first.

"Of course, for mills with sufficient ClO2 generator capacity-or effluent discharge flexibility-there is little need for DZ or ZD bleaching," says Norman Liebergott of Liebergott & Associates Consulting Inc. in Laval, Que. "But, with the new laws and regulations that are coming into force, if a mill does not have an oxygen delignification stage and/or does not have sufficient ClO2 generator capacity, a combined bleaching stage offers a low capital cost option." And a combined DZ or ZD bleaching stage can also be less expensive to operate than a D100 stage (Figures 1 and 2).

"Also," says Liebergott, "Europeans are getting very strict about organic halide (OX) concentrations in pulp. U.S. and Canadian softwood pulps typically have concentrations of OX in the range of 130 to 150 ppm. Europeans typically want concentrations below 100 ppm. For DZ bleaching, the O3 seems to reduce the OX concentrations in the pulp by first reacting with the OX, and, secondly, by increasing its propensity to transfer into the effluent. As mills close up their effluent streams-and more filtrates are recycled-this may become a more important issue."

PLACEMENT DEBATE. Traditionally, when O3 has been used in its own bleaching (Z) stage, the accepted wisdom has been that the Z stage is best positioned at the beginning of a bleaching sequence to exploit O3's delignification efficiency. The relatively high lignin content at the beginning of the sequence also helps to retard cellulose degradation. When ozone is added later in a bleaching sequence, its charge is usually limited to very low amounts (lower than 0.5%), also to avoid degradation.1

According to Liebergott, "Ozone bleaching is not new. There is a lot of technical literature on the subject. Beginning in the early 1970s, most of the investigation was of a D stage followed by a Z stage with washing between the stages. It's the idea of combining the stages-without washing-that is relatively new."

He explains, "Interest in combining stages is continuing. There will be five or six DZ or ZD bleaching stages operating in the world by the end of 1998, and perhaps more, as people see the merits of combining the stages."

Investigating the combined stage, researchers have reported that replacing a portion of the ClO2 charge in the first bleaching stage with O3 has reduced the formation of AOX for both hardwood and softwood bleaching.1 In obtaining similar final brightnesses, replacement ratios (of O3 for ClO2) for hardwood pulps exceeded those for softwood pulps. For hardwood pulps, 0.6% O3 replaced 1.31% ClO2. For softwood pulps, 1.2% O3 replaced 1.05% ClO2.1

The same study reported that "full replacement of the D1 stage by a Z stage required the use of significant amounts of O3, which can explain the lower final pulp viscosities." The authors reported that when D and Z are combined into a single stage, synergies can be exploited. They identified several advantages of combining chlorine dioxide and ozone into a single bleaching stage, including the following:

One washing stage can be eliminated, saving electrical energy and, depending on the filtrate recycling scheme, shower water.

Chlorine dioxide stages are acidic, eliminating the need to acidify specifically for the ozone stage.

Ozone is capable of reacting with (i.e., "destroying") some AOX in bleaching effluents.

The reaction chemistries of chlorine dioxide and ozone with residual lignin complement one another, making the combined stage particularly efficient as a delignifying stage.

According to the authors, the residual lignin in pulp is more depolymerized after a D stage than after a Z stage at the same lignin content. They point out that that there are more carboxyl groups formed on lignin during the ozonation reaction than during a typical first D-stage reaction. They proposed these phenomena as an explanation for the relative ease with which lignin can be solubilized during the Z-stage. O3 is also capable of reacting with almost all types of structures in the lignin, whereas ClO2 reacts only with the free phenolic groups.

The authors recommended adding ClO2 first so it can react with the free phenolic groups before the highly reactive ozone is introduced, concluding that reaction kinetics clearly favored the DZ approach relative to the ZD approach.1 They reported that 1 kg could replace from 2 kg to 4 kg of chlorine dioxide when added to the first stage of a DEDED bleaching sequence.

Studies have continued to investigate the reaction kinetics and to compare the results of DZ and ZD bleaching.2,3

Liebergott says that while the DZ vs ZD debate is continuing, the choice should hinge on the existence of an upstream oxygen delignification process. "For oxygen-treated pulps, the initial ClO2 of a DZ-stage does not seem to give the same result as the initial charge of O3 in a ZD-stage. The reason is that the ClO2 is trying to react with the same structural groups that the oxygen reacted with. On the other hand, O3 reacts with different structural groups."

He points out that one of the major drawbacks to the ZD approach is the pH shift that is required. "You have to raise the pH a bit between the addition of O3-and its reaction-and the addition of ClO2. If you do not, you will have a very low pH (about 2.0) before you add the ClO2. The low pH favors the formation of chlorate, and chemical efficiency is negatively affected. Typically, you have to add a little caustic to raise the pH from 2.0 for the Z stage to a pH of 3.5 to 4.0 for the D-stage to achieve maximum efficiencies for both stages.

"On the other hand, in a DZ sequence the pulp typically exits the D stage at a sufficiently low pH for an effective Z stage. And for both approaches-DZ and ZD-the pulp should be degassed between chemical additions."

LOW CAPITAL. According to Liebergott, most D stages in the U.S. run at fairly low consistencies (3.5%), which provides a challenge when implementing ozone. "This low-consistency approach is an artifact of the D-stage origin. Most of the existing D stages are running in place of their predecessor chlorination stages and use much of the same equipment. Ozone, of course, runs much better at consistencies of 10%."

He adds that this has meant that early adopters of Z-stage bleaching have had to raise the consistency of the pulp. Thus, there is a lot of work directed at running a Z stage at consistencies closer to 4%. The primary issue is mixing, because at low consistencies, considerable mixing is necessary.

According to David Scheeff, group director of pulp and paper at ozone supplier Air Liquide, "The real benefit of adding O3 to the first D stage is that a mill can 'beef up' its bleaching sequence without increasing its ClO2 capacity." And the addition of O3 to a typical low-consistency D stage can be uncomplicated (Figure 3).

"Most mills' O3 needs will be satisfied by third-party chemical specialty firms that build and operate the O3 generator system, allowing the mill to buy their ozone 'over the fence' (Figure 4).The big attraction is that no capital is needed. Presumably, the specialty firms enjoy lower costs of capital, as well as greater operating focus and expertise. Also, by integrating the oxygen plant with the ozone plant, some operating efficiencies can be gained," Scheeff says.

According to Jorge Colodette, professor of wood and bleaching chemistry at the Federal University of Viscosa in Viscosa, Brazil, "In terms of investment, DZ is the cheapest way to get O3 into the process. It's much cheaper than adding a "full" Z-stage. If the chloride concentration of the effluent is not an issue, the success of DZ (or ZD) bleaching will hinge on the replacement ratio."

For mills that are concerned about the chloride concentration in their effluents, DZ (or ZD) bleaching has an added advantage. "As mills replace ClO2 with O3, they also reduce the chloride concentrations in their bleach plant filtrates," Scheeff says. "This makes effluent closure a less complicated issue (Figure 5)."

He adds "Our experience is that the replacement ratios in a mill setting are typically in the range of 2 to 2.5 kg of ClO2 to 1 kg of O3," says Scheeff. "In many cases, we are able to reduce a mill's chemical cost compared with the fully absorbed operating cost of adding a new ClO2 plant or expanding the capacity of the old one, either physically or with a more efficient chemistry."

"Of course, the size of the O3 plant is a critical issue," Scheeff adds. "Plants in the 3 tpd to 5 tpd (i.e., replacing 6 to 12 tpd of ClO2) are very cost-effective. The cost of smaller ozone plants-less than 1 tpd-rises, making them uneconomical."

RECENT MILL INSTALLATIONS. Brazilian pulp producers have been aggressive in implementing ozone bleaching sequences. According to Colodette, "Of the four ozone installations in Brazil, two are Z stages within TCF bleaching sequences, one is a Z stage which is part of an ECF sequence, and one is a DZ stage at Votorantim. After trying to start up its DZ stage in March 1997, Votorantim has not yet been able to achieve sustained operation. There have been several problems, beginning with metallurgy-their titanium mixers were incompatible with O3. At this point, Votorantim continues to be a very important test case. Many mills are watching the progress."

E.B. Eddy's Espanola Mill in Ontario has announced plans to modify its bleach plant to add a ZD stage. The mill opted for the ZD order since its fiberline includes oxygen delignification. According to Liebergott, the mill's D0 stage currently runs near 10% consistency. "When they add the O3, they will be able to run the combined stage at 10% consistency," Liebergott says.

References 1. C. Chirat and D. Lachenal, Other Ways to Use Ozone in a Bleaching Sequence, 1995 TAPPI Pulping Conference, Chicago, pp.415-419.

2. C. Chirat, D. Lachenal, R. Angelier, and M. Viardin, (DZ) and (ZD) Bleaching: Fundamentals and Application, 1996 International Pulp Bleaching Conference, Washington, pp.197-211.

3. G. Homer, S. Johnson, M. Epiney, and M. Sundar, State of the Art ECF Bleaching Part 3: Effects of Ozone and Chlorine Dioxide on Bleaching and Effluent Chemistry, 1997 Emerging Technologies Conference, Orlando, Fla.