Bleaching 101: the basics of bleaching.
Bleaching processes consume significant capital and operating expenses, often more than other processes. Also, many regulatory constraints are related to the environmental impact of bleaching. For these reasons, bleaching should receive a higher share of optimization and management focus than other parts of the pulping process. Ignoring the bleaching process always leads to higher operating costs, sometimes to unexpected maintenance surprises, and even regulatory difficulties. Small projects within bleaching systems can sometimes allow mills to avoid major environmental capital spending.
FIBER AND LIGNIN
Papermaking fibers are derived primarily from wood, which contains two main components: fibers (cellulose and hemicellulose) and dark-colored lignin, which is like glue or cement that holds the fibers together. Removing lignin brightens the pulp. Bleaching continues the process of removing lignin remaining after the cooking process. In the case of mechanical pulp, other high-yield pulps, and recycled fibers, bleaching brightens the lignin that remains in the fiber.
Chemicals used for bleaching fall into two major chemical categories--oxidative and reductive. Oxidative reactions typically remove lignin. Reductive chemicals brighten the remaining materials. Alkali is also used to remove oxidized lignin. The box below lists the major chemicals and their symbols (used to summarize bleaching sequences).
OXIDATIVE REDUCTIVE * Chlorine (C) * Hydrosulfite (Y), usually * Chlorine dioxide (D) sodium based * Oxygen (O) * FAS (formamidine * Ozone (Z) sulfinic acid) * Hypochlorite (H) * Hydrogen peroxide (P)
Mills are using more enzymes in bleaching. They are catalysts of highly specific reactions and are considered "environmentally friendly." Enzymes are chemicals made by microorganism fermentation. The enzymes are not biologically active, but their complex structures are unique in attacking specific organic structures in the fiber and can be cost effective. For example, a good application for enzymes is pitch control in mechanical pulps.
Most pulp bleaching systems use chemical combinations. The equipment is arranged in stages. Each stage starts with chemical mixing, goes into a tower to provide retention time (minutes to hours), and ends with a washer.
For a specific chemical reaction to be effective, certain conditions must be optimized. These include the temperature, concentration of the chemical to the pulp (that is affected by both pulp consistency and chemical dosage), and pH. The pH is a measure of the acid or base (also referred to as alkaline) concentration. Sulfuric acid ([H.sub.2]S[O.sub.4]) is typically added to lower the pH, and sodium hydroxide (NaOH--commonly called caustic) is added to raise the pH; however other chemicals are sometimes used.
Pulp consistency measurement is vital for dosage rate control of bleaching chemicals and an internal accounting function. The impact of this key issue on overall process control is often overlooked.
Mixing of chemicals with pulp is a critical step. Uniform mixing of gases with pulp requires more effort and attention than mixing of liquids with pulp. The reaction time is limited by the volume available (tower size) after mixing and before the next process, pulp washing. Steam is typically added if the temperature must be raised to drive the desired reaction within the available time. Filtrates are recycled to conserve both heat energy and residual chemicals. The recycled filtrates can have a negative impact on purging trace elements which can cause scale, and decomposition of bleaching chemicals, and other problems.
In nearly all chemical pulp bleaching sequences, the first acid oxidative stage is followed by an alkaline extraction (E) stage. The hot, high pH stage uses caustic to dissolve the lignin attacked, but not removed, in the prior stage. This darkens the pulp as more unoxidized lignin is exposed. Almost all modern stages operate continuously at about 10%-12% consistency (medium consistency).
A typical bleaching sequence may consist of four stages: OD(EOP)D. This means that following cooking, washing and screening of the brown fiber, oxygen delignification (O) is used to further reduce the lignin content by 20%-70%, depending on the wood type and system configuration. Then chlorine dioxide (Cl[O.sub.2]) is used in two stages with an oxygen- and peroxide-reinforced alkaline extraction (E) stage between them, hence the designation EOP. This increases lignin removal and adds brightening between the two acid D stages.
Mechanical pulps are usually bleached with an alkaline peroxide stage, a reductive stage, or both. Fiber brightening is usually the primary objective rather than lignin removal. Some recovered fiber operations have similar bleaching processes to brighten the fiber and any remaining ink or contaminants. The raw material must be controlled and matched with the bleaching process to provide stable brightness and final pulp properties.
Typical fiber quality parameters are brightness (ISO standard is most widely accepted), a speck or dirt count measurement (parts per million from a two-dimensional area measurement), and some aspect of the pulp fiber's strength.
Viscosity is a common fiber strength test for virgin fiber. This test involves dissolving a small amount of fiber into its individual chemical strands. A liquid viscosity test relates to final paper making properties, within certain limits.
As the fiber is bleached, some of the organic material is dissolved. The loss of weight is expressed as a yield loss which almost always exits with the effluent.
Residuals (unconsumed chemicals at the end of the reaction) are usually a waste and can often interfere with downstream process reactions. In some cases they will cause the brightness to revert or reduce, rather than continue to improve.
Dissolved metal ions in trace amounts can be a serious bleaching challenge. Some sequences use chelants to capture and contain these metal atoms. Because of differences in metal ion contents in the wood and water from place-to-place, or even season-to-season, chelant optimization must be tailored to the specific situation.
Increased environmental regulation has caused changes to pulp bleaching. Elemental chlorine and hypochlorite are heavily regulated by most countries and permitted only in certain applications. Air emissions are also regulated, usually from D stages.
The volume and quality of effluent--water borrowed from and returned to the environment--are significant issues for bleaching operations. Both environmental pressures and demand for fresh water from growing populations will continue to drive change in bleaching. Effluent reduction can change internal process efficiencies, mainly through variations in trace chemical elements coming in with the wood.
Low cost projects can often have a large positive impact to improve effluent issues. Operating parameters should be optimized for the mill's unique situation, including chemical cost, infrastructure limitations, washing constraints, and regulations.
* The following texts are available from TAPPI PRESS online at www.tappi.org, or for more information on ordering, contact TAPPI at 1 800 332-8686 (US); 1 (800) 446-9431 (Canada); +1 770 446-1400 (Worldwide); +1 770 446-6947 (Fax); Email: email@example.com
* Pulp Bleaching: Principles and Practice, edited by Carlton W. Dence and Douglas W. Reeve. This text covers fundamentals and processes from chemical composition of pulp to technology, production, and the environmental impact of bleaching.
* Elemental Chlorine Free Bleaching: A TAPPI PRESS Anthology of Published Papers, edited by Katherine A. Kulas, Ph.D. This anthology details actual mill conversions to ECF Bleaching.
* Mechanical Pulps: From Wood to Bleached Pulp, by Johannes Kappel. This volume provides a comprehensive summary of the entire mechanical pulping process.
* Organochlorine Compounds in Bleach Plant Effluents--Genesis and Control, by P. Bajpai. This publication discusses the generation and environmental impact of organochlorine compounds and the various approaches used to minimize their generation, both at the source and end-of-pipe.
Editor's Note: This is the first article in a continuing series on the "basics" of pulping and papermaking. The series provides practical, easy to understand information on key processes.
ABOUT THE AUTHOR
Wayne Bucher is process consultant for WB Consulting Inc., Birmingham, Alabama, USA. Contact him at +1 205 408-1874 or by email at firstname.lastname@example.org.
WAYNE BUCHER. WB CONSULTING
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|Publication:||Solutions - for People, Processes and Paper|
|Date:||Feb 1, 2004|
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