Beyond the Cluster Rule: new bleaching challenges: the Cluster Rule dominated the United States bleaching scene for many years, but new challenges have emerged, including increasing bleaching effectiveness while lowering bleaching costs.

The U.S. Environmental Protection Agency (EPA) first proposed the "Cluster Rule" in December 1993 to regulate air and water discharges, including dioxin from pulp and paper mills. As a coordinated, simultaneous effort under both the Clean Water Act and the Clean Air Act, the Rule allowed mills to select the best combination of pollution prevention and control technologies to achieve pollution reductions.

Today, all but a few bleach plants with special exceptions have implemented their Cluster Rule compliance projects. This does not mean that the bleaching process is no longer under environmental scrutiny. Other challenging environmental issue remain.

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WATER CONSUMPTION

"Looking forward, water consumption is a potential issue for bleached kraft mills," says Wayne Bucher, process engineer for WB consulting Inc., Birmingham, Alabama, USA. "These huge appetites for fresh water regardless of existing permits are facing stiff competition from agriculture and municipalities."

As mills focus on efforts to tighten their use and reuse of water, they will face technical challenges, including corrosion, scale deposits, and process efficiencies. "Again, mills will address some issues by first improving controls to minimize the chemical doses required," Bucher commented.

Tom McDonough, professor emeritus, Institute of Paper Science and Technology, Atlanta, Georgia, suggested that mills pay close attention to local regulations on effluent color when assessing their bleaching practices and bleach plant operation. "Color can be minimized by improvements in brown stock washing, oxygen delignification with good post-oxygen washing, and use of hydrogen peroxide, ozone, or both to supply part of the oxidant demand in pulp bleaching," he said.

Air quality is another important concern for mills and their surrounding communities. "Regulation of gaseous emissions requires good mixing of chlorine dioxide, elimination of channeling in towers, and appropriate use of scrubbers," McDonough advised.

RECYCLED FIBER QUALITY

The quality of the recycled fiber a mill takes in can also be a cause for concern. Many paper mills are using higher levels of recycled fiber, often in a more contaminated form as recyclers tap lower quality fiber sources.

Adjustments in bleaching strategy can help a mill deal with higher levels of contaminants Bucher said. "Recycled fiber pulping systems may have to employ oxidative bleaching with reductive bleaching agents to meet desired pulp quality. An integrated kraft mill may focus on maximizing the strength while maintaining certain brightness standards, including minimizing brightness reversion. This will provide the papermaker with a wider fiber mix that can achieve the desired paper properties."

COST VS. PERFORMANCE

Cost is an ever present factor in the pulp and paper industry as sluggish paper markets and low paper pricing continue to put pressure on mills to reduce operating costs. Approaches to increasing bleaching performance while lowering costs center on better control, fine tuning operations, and new treatments.

Douglas C. Pryke, a consultant based in Erin, Ontario, Canada, is an advocate of better control. "Promising opportunities are improved process stability and control through advanced sensor techniques that can allow more accurate application of bleaching chemicals," he said. "This allows operators to make grade and species changes with more precision and less transitional products."

McDonough noted several "cost effective" approaches. He advocates the following:

* Ensure that residual alkali after cooking does not fall too low. Low residual alkali adversely affects pulp bleachability.

* Optimize brown stock washing to minimize carryover of dissolved organic material to the bleach plant.

* In hardwood bleach plants, evaluate possibilities for hot acid pretreatment to decrease unbleached kappa number by removing hexenuronic acid. A long, hot DO stage has a similar effect.

* Try using enzyme (xylanase) pretreatment to decrease chlorine dioxide consumption in subsequent stages. Also examine the possibility of applying the enzyme in a low-pH first alkaline extraction stage. This is an alternative that offers the possibility of NaOH savings and chlorine dioxide savings.

* Look to effectively increase the number of bleaching stages with low capital outlay by adding a rapid D0 stage and use the existing D0 stage later in the sequence. Chemical consumption decreases when the number of stages increases.

* Use oxygen and peroxide aggressively in the first extraction stage.

* Run trials in the mill or laboratory to optimize the distribution of chlorine dioxide across the three D stages of the DEDED sequence.

Bucher cited several approaches to reducing bleaching costs that are currently available--improved controls, oxygen, and enzymes--and two-special catalysis and lower temperature bleaching with future potential.

"Bleaching costs can be reduced with improved controls starting with maximizing control of pulp consistency into the bleaching system," said Bucher. "This allows for more stable operation and options for ratebased chemical application control." He also noted that improved kappa, brightness, and residual chemical sensors have also made it practical to reduce over/under bleaching at the front of the bleach plant.

"Especially with short threestage sequences, improved controls will allow reducing the extra chemicals typically consumed to avoid the risks of off-quality pulp." If mills are considering cost, Bucher noted that oxygen is less expensive than chlorine dioxide or peroxide. "Each chemical has its place in a balanced sequence, and every mill is a little unique in the relative cost of these chemicals and their efficiency," he stated. "Oxygen is more effective as the pressure is increased. This opens some opportunities for mills to do more work with less expensive oxygen."

Finally, while enzymes are finding use by a growing number of mills, their success is often very mill and wood specific according to Bucher. He noted that enzymes may see further development.

Ozone might be another growth area. "Ozone may be more popular as a bleaching chemical in the future," said Bucher. "Some mills are using it primarily due to effluent limitations."

Reducing or eliminating side reactions may reduce costs further. "A significant portion of the bleaching chemical is consumed by side reactions that may be limited or prevented with the use of special catalysis," said Bucher. "When this chemistry becomes practical, significant cost reductions should be achieved by saving some chemicals presently consumed by the unbeneficial reactions. There is also the potential to bleach at lower temperatures. This could further reduce operating costs."

ADDITIONAL RESOURCES:

* Note: All the articles below are available for download on www.tappi.org

* "A new insight into the bleachability of kraft pulps," by Z.H. Jiang, B. Van Lierop, A. Nolin, R. Berry, Journal of Pulp and Paper Science, February 2003.

* "Bleaching and the environment in the 21st century," by Don Meadows, Solutions!, June 2002.

* Bleaching: Determining optimum EO stage pressure," by Wayne Bucher, Solutions!, November 2002.

RELATED ARTICLE: IN THIS ARTICLE, YOU WILL LEARN:

* Post-Cluster Rule environmental challenges affecting pulp bleaching

* Promising approaches to increasing bleaching performance while lowering costs.

ALAN ROOKS, Editorial Director