21.1 Introduction 858
21.1.1 Background 858
21.1.2 Characterization of the Pulp and Paper Industry 859
21.1.3 Industry Size and Geographic Distribution 859
21.1.4 Economic Trends 861
21.2 Process Description 862
21.2.1 Processes in the Pulp and Paper Industry 862
21.2.2 Raw Material Inputs and Pollution Outputs in the Production Line 873
21.2.3 Management of Chemicals in Wastestreams 876
21.3 Pollution Prevention Opportunities 876
21.3.1 Extended Delignification 879
21.3.2 Oxygen Delignification 880
21.3.3 Ozone Delignification 880
21.3.4 Anthraquinone Catalysis 880
21.3.5 Black Liquor Spill Control and Prevention 881
21.3.6 Enzyme Treatment of Pulp 881
21.3.7 Improved Brownstock and Bleaching Stage Washing 881
21.3.8 Improved Chipping and Screening 882
21.3.9 Oxygen-Reinforced/Peroxide Extraction 882
21.3.10 Improved Chemical Controls and Mixing 882
21.4 Applicable Federal Statutes and Regulations 882
21.4.2 Resource Conservation and Recovery Act (RCRA) 884
21.4.3 Emergency Planning and Community Right-to-Know Act (EPCRA) 884
21.4.5 State Statutes 887
21.4.6 Summary of National Regulatory Requirements 888
21.4.7 Summary of World Bank Liquid Effluents Guidelines 889
21.5 Treatment of Wastewater from Pulp and Paper Facilities 890
21.5.1 Pretreatment 891
21.5.2 Primary Treatment 892
21.5.3 Secondary Treatment 893
21.5.4 Tertiary Treatment 895
21.5.5 Biosolids Management Processes 895
21.5.6 Biosolids Disposal Processes 896
21.5.7 Air Pollutant Emissions from Treatment Plants 896
21.5.8 Water Pollutant Discharges from Treatment Plants 896
21.5.9 Biosolids/Hazardous Waste Discharges from Treatment Plants 898
21.5.10 Recovery of Fibers and Titanium Dioxide 898
21.6 Case Studies 899
21.6.1 Case I: International Paper Company, Jay, Maine 899
21.6.2 Case II: Upgraded Treatment Plant at a Paper Mill in Lufkin, Texas, Using a DAF Cell 901
21.6.3 Case III: Deep Shaft Plant at Ohtsu Paper Company in Ohtsu, Japan 902
21.6.4 Case IV: Cotton Fiber Recovery Flotation Cell of
Krofta Engineering Corporation, Lenox, Massachusetts 904
21.6.5 Case V: Fiber and Titanium Dioxide Recovery Facility at Mead Corporation, South Lee, Massachusetts 905
21.6.6 Case VI: Resource Recovery Facility of Lenox Institute of Water Technology (LIWT), Lenox, Massachusetts 905
The paper and allied products industry comprises three types of facilities: pulp mills that process raw wood fiber or processed fiber to make pulp; paper and board mills that manufacture paper or board; and converting facilities that use these primary materials to manufacture more specialized products such as writing paper, napkins, and other tissue products. The process of converting paper is not a source of water or air pollution, as is the case for the first two facilities. This chapter focuses primarily on the greatest areas of environmental concern within the pulp and paper industry: those from pulping processes.
The specific components in the pulp and paper industry include the following1,2:
1. Pulp mills. These separate the fibers of wood or other materials, such as rags, linters, waste-paper, and straw, in order to create pulp. Mills may use chemical, semichemical, or mechanical processes, and may create coproducts such as turpentine and tall oil. Most pulp mills bleach the pulp they produce, and, when wastepaper is converted into secondary fiber, it is deinked. The output of some pulp mills is not used to make paper, but to produce cellulose acetate or to be dissolved and regenerated in the form of viscose fibers or cellophane.
2. Paper mills. These are primarily engaged in manufacturing paper from wood pulp and other fiber pulp, and may also manufacture converted paper products. Establishments primarily engaged in integrated operations of producing pulp and manufacturing paper are included in this industry if primarily shipping paper or paper products.
3. Paperboard mills. These are primarily engaged in manufacturing paperboard, including paperboard coated on a paperboard machine, from wood pulp and other fiber pulp; they may also manufacture converted paperboard products.
4. Paperboard containers and boxes. These establishments are engaged in the manufacture of corrugated and solid fiber boxes and containers from purchased paperboard. The principal commodities of this industry are boxes, pads, partitions, display items, pallets, corrugated sheets, food packaging, and nonfood (e.g., soaps, cosmetics, and medicinal products) packaging.
5. Miscellaneous converted paper products. These establishments produce a range of paper, paperboard, and plastic products with purchased material. Common products include paper and plastic film packaging, specialty paper, paper and plastic bags, manila folders, tissue products, envelopes, stationery, and other products.
One important characteristic of the pulp and paper industry is the interconnection of operations between pulp mills and downstream processing of pulp into paper, paperboard, and building paper. Another important characteristic of the pulp and paper industry is that the range of processes, chemical inputs, and outputs used are used in pulp manufacture. On the whole, pulp mill processes are chemical intensive and have been the focus of past and ongoing pollution prevention rulemaking. There are also numerous manufacturers of finished paper and paperboard products from paper and paperboard stock. Some companies are involved in both the manufacture of primary products and converting, particularly in the production of tissue products, corrugated shipping containers, folding cartons, flexible packaging, and envelopes.
21.1.2 Characterization of the Pulp and Paper Industry
The pulp and paper industry produces primary products—commodity grades of wood pulp, printing and writing papers, sanitary tissue, industrial-type papers, containerboard, and boxboard— using cellulose fiber. The two steps involved are pulping and paper or paperboard manufacturing.
Pulping is the process of separating wood chips into individual fibers by chemical, semichemical, or mechanical methods. The particular pulping process used affects the strength, appearance, and intended use characteristics of the resultant paper product. Pulping is the major source of environmental impacts from the pulp and paper industry. There are more than a dozen different pulping processes in use in the U.S.; each process has its own set of process inputs, outputs, and resultant environmental concerns.3 Table 21.1 provides an overview of the major pulping processes and the main products that they produce. Kraft pulp, bleached and unbleached, is used to manufacture the majority of paper products. Together, chemical pulping processes account for 84% of the pulp produced in the U.S.1 Figure 21.1 presents the relative outputs of the major pulping processes.
A bleached kraft pulp mill requires 15,140 to 45,420 L (4000 to 12,000 gal) of water and 8.56 to 12.22 million chu (14 to 20 million Btu) of energy per ton of pulp, of which ca. 4.44 to 5.56 million chu (8 to 10 million Btu) are typically derived from biomass-derived fuel from the pulping process itself.4 Across all facilities, the pulp, paper, and allied products industry is the largest consumer of process water and the third largest consumer of energy (after the chemicals and metals industries).5,6 The large amounts of water and energy used, as well as the chemical inputs, lead to a variety of environmental concerns.
The paper or paperboard manufacturing process is similar for all types of pulp. Pulp is spread out as extremely dilute slurry on a moving endless belt of filtering fabric. Water is removed by gravity and vacuum, and the resulting web of fibers is passed through presses to remove more water and consolidate the web. Paper and paperboard manufacturers use nearly identical processes, but paper-board is thicker (more than 0.3 mm).
The pulp and paper industry is characterized by very large facilities; of the 514 pulp and paper mills reported by the Bureau of the Census in 1998, 343 (67%) had 100 or more employees. Across all of
Description of Pulping Processes
Highly bleached and purified kraft process wood pulp suitable for conversion into products such as rayon, viscose, acetate, and cellophane
Bleached or unbleached kraft process wood pulp usually converted into paperboard, coarse papers, tissue papers, and fine papers such as business, writing and printing
Highly bleached and purified sulfite process wood pulp suitable for conversion into products such as rayon, viscose, acetate, and cellophane Sulfite process wood pulp with or without bleaching used for products such as tissue papers, fine papers, and newsprint Pulp is produced by chemical, pressure, and occasionally mechanical forces with or without bleaching used for corrugating medium (cardboard), paper, and paperboard Pulp manufacture by stone groundwood, mechanical refiner, thermo-mechanical, chemi-mechanical, or chemi-thermomechanical means for newsprint, coarse papers, tissue, molded fiber products, and fine papers Pulps from recovered paper or paperboard using a chemical or solvent process to remove contaminants such as inks, coatings, and pigments used to produce fine, tissue, and newsprint papers
Secondary fiber nondeink Pulp production from recovered paper or paperboard without deinking processes to produce tissue, paperboard, molded products, and construction papers Nonwood chemical pulp Production of pulp from textiles (e.g., rags), cotton linters, flax, hemp, tobacco, and abaca to make cigarette wrap papers and other specialty paper products
Source: U.S. EPA, Profile of the Pulp and Paper Industry, 2nd ed., report EPA/310-R-02-002, U.S. EPA, Washington, November 2002.
these facilities, there are 172,000 employees who produced USD 59 billion in shipments (in 1998 dollars). In 2000, the industry employed 182,000 people and produced USD 79 billion in shipments. In contrast, the downstream facilities (container and specialty product manufacturers) tend to be more numerous but smaller. More than 75% of these facilities have fewer than 100 employees. Table 21.2 presents the employment distribution for both pulp and paper facilities and downstream manufacturers
Bleached papergrade kraft and soda Unbleached kraft Dissolving sulfite
Papergrade sulfite Semichemical
Secondary fiber deink
Size of Paper and Allied Products Facilities
Employees per Facility (% of Total)
Pulp mills Paper mills Paperboard mills Paperboard containers and boxes Misc. converted paper products
14 (34%) ó3 (24%) 77 (3ó%) 1311 (4ó%) 111ó (3ó%)
Source: U.S. EPA, Profile of the Pulp and Paper Industry, 2nd ed., report EPA/310-R-02-002, U.S. EPA, Washington, November 2002.
in 1997 (the most recent data available) as reported by the U.S. Census Bureau.7 Because recent years have seen some facility closures, the current number of facilities may be somewhat lower.
The integrated pulp and paper industry is among the top 10 U.S. manufacturing industries in value of shipments. The industry shipments amount to 146 billion USD with an employment of 609,480. Individual pulp and paper mills employ only 28% of the workers in the industry, but produce over 40% of the shipments.8
The geographic distribution of pulp and paper mills varies according to the type of mill. As there are tremendous variations in the scale of individual facilities, tallies of the number of facilities may not represent the level of economic activity (nor possible environmental consequences). Pulp mills are located primarily in regions of the country where trees are harvested from natural stands or tree farms, such as the Southeast, Northwest, Northeast, and North Central regions.9 Pulp mills that process recycled fiber are generally located near sources of waste paper. Paper mills, however, are more widely distributed. They are located near pulping operations or near converting markets. The distribution of paperboard mills reflects the location of manufacturing in general, as such operations are the primary market for paperboard products. Figure 21.2 presents the locations of pulp and paper mills in the U.S.
The U.S. produces roughly 30% of the world's paper and paperboard. The pulp and paper industry is one of the most important industries for the balance of trade in the U.S. This trade balance increased through most of the 1990s. In 1999, exports were USD 8.5 billion. In recent years, however, exports have been declining and imports have been increasing. Between 1997 and 2000, exports declined 5.5% and imports increased by more than 20%. The declining exports and increasing imports are partly due to a strong dollar in this period and the recent slow down of the U.S. economy.1
The U.S. industry has several advantages over the rest of the world market: modern mills, a highly skilled work force, a large domestic market, and an efficient transportation infrastructure. Major export markets for pulp are Japan, Italy, Germany, Mexico, and France. The U.S. Department of Commerce anticipates exports to grow faster than production for domestic markets through 2004. World Trade Organization (WTO) efforts to reduce tariffs include those on pulp and paper products; if these are successful, the U.S. industry expects pulp and paper export rates to increase even further.
However, pulp and paper are commodities and therefore prices are vulnerable to global competition. Countries such as Brazil, Chile, and Indonesia have built modern, advanced pulp facilities. These countries have faster-growing trees and lower labor costs. Latin American and European countries are also adding papermaking capacity. Because of this increased foreign competition, imports of paper to the U.S. market are expected to increase 3% annually through 2004.10 In order
to compensate for this increasingly competitive market, pulp and paper companies have undertaken a considerable number of mergers and acquisitions between 1997 and 2002.
Historically, U.S. pulp and paper companies have invested heavily in capital improvements to their facilities. Capital investments in recent years, however, are well below historic levels due to the difficult market conditions. For the first time, industry capacity actually declined in 2001.1 Because few new mills are being built, most capital expenditures are for plant expansions, upgrades, and environmental protection initiatives at existing facilities. Throughout the time period 1985-1999, capital improvements related to environmental protection claimed from 4% to 22% of the total investments, with significant increases in the early and late 1990s.1
A major movement within the pulp and paper industry has been an increased focus on the use of recovered paper. Nearly 50% of paper is now recovered and used either as recycled paper or as products such as home insulation. Furthermore, recovered paper contributes to U.S. exports; roughly ten million tons of recovered paper were exported in 2000.1
21.2 PROCESS DESCRIPTION
Simply put, paper is manufactured by applying a watery suspension of cellulose fibers to a screen that allows the water to drain and leaves the fibrous particles behind in a web. Most modern paper products contain nonfibrous additives, but otherwise they fall within this general definition. Only a few paper products for specialized uses are created without the use of water, using dry forming techniques. The production of pulp is the major source of environmental impacts from the pulp and paper industry.
Processes in the manufacture of paper and paperboard can, in general terms, be divided into three steps:
1. Pulp making
2. Pulp processing
3. Paper/paperboard production
Wood yard and shipping
FIGURE 21.3 Simplified flow diagram of an integrated mill. (Taken from U.S. EPA, Profile of the Pulp and Paper Industry, 2nd ed., report EPA/310-R-02-002, U.S. EPA, Washington, November 2002.)
Paper and paperboard production processes are similar. After the fibers are separated and impurities have been removed, the pulp may be bleached to improve brightness and processed to a form suitable for paper-making. At the paper-making stage, the pulp can be combined with dyes, strength-building resins, or texture-adding filler materials, depending on its intended end product. Afterwards, the mixture is dewatered, leaving the fibrous constituents and pulp additives on an endless fabric belt. The fibers bond together as the web passes through a series of presses and around heated drum driers. Additional additives may be applied to the moving web. The final paper product is usually spooled on large rolls for storage (see Figure 21.3). If more information on paper making processes is desired, reference 3 is recommended.
Table 21.3 presents an overview of wood pulping types by the method of fiber separation, resultant fiber quality, and percent of 1998 U.S. pulp production.11,12 Many mills perform multiple pulping processes at the same site, most frequently nondeink secondary fiber pulping and paper-grade kraft
General Classification of Wood Pulping Processes
Fiber Separation Method
Combination of chemical and mechanical treatments Chemicals and heat
Short, weak, unstable, impure fibers
"Intermediate" pulp properties (some unique properties)
Long, strong, stable fibers Kraft, sulfite, soda
Stone groundwood, refiner mechanical pulp High-yield kraft, high-yield sulfite
% of Total 1998 U.S. Wood Pulp Production
pulping.3 The following three basic types of wood-pulping processes are detailed below, followed by a discussion of secondary fiber pulping techniques:
1. Chemical pulping
2. Semichemical pulping
Various technologies and chemicals are used to manufacture pulp, but most pulp manufacturing systems contain the process sequence shown in Table 21.4. Overall, most of the pollutant releases associated with pulp and paper mills occur at the pulping and bleaching stages where the majority of chemical inputs occur.
According to the National Census,13 wood is used in some form by approximately 95% of pulp and paper manufacturers. Wood can be in a variety of forms and types. Wood logs, chips, and sawdust are used to make pulp. Due to different physical and chemical properties, however, certain pulping processes are more efficient when used on specific wood types. The species of wood used has a profound influence on the characteristics of the pulp. In general, softwood fibers are longer than those from hardwood and have thinner cell walls. The longer fibers of softwood produce papers of greater strength, particularly tear strength.
Secondary fibers comprise the next most common furnish constituent. Secondary fibers consist of preconsumer fibers (e.g., mill waste fibers, which were always recycled internally) and postconsumer fiber, which is what is generally referred to as recycled paper. Postconsumer fiber sources are diverse, but the most common are newsprint and corrugated boxes. Although secondary fibers are not used in as great a proportion as wood furnish, ca. 70% of pulp and paper manufacturers use some secondary fibers in their pulp production and ca. 200 mills (40% of the total number of mills) rely exclusively on secondary fibers for their pulp furnish.11,14 Secondary fibers must be processed to remove contaminants such as glues, coatings, or bindings, and, depending on the end product, may or may not be processed to remove ink or brighten the pulp.
Secondary fiber use is increasing in the pulp and paper industry due to the increasing prices of virgin pulp and the continuing improvement in deinking technology. Environmental concerns have led to consumer acceptance of lower brightness of products made from recycled paper, and government specifications set a minimum level of product quality. Recovered fiber accounted for 75% of the industry's increase in fiber consumption between 1990 and 2000.15 The utilization of secondary
Pulp Manufacturing Process Sequence
Fiber furnish preparation and handling Pulping
Pulp processing Bleaching
Pulp drying and baling (nonintegrated mills) Stock preparation
Debarking, slashing, chipping of wood logs and then screening of wood chips/
secondary fibers (some pulp mills purchase chips and skip this step) Chemical, semichemical, or mechanical breakdown of pulping material into fibers Removal of pulp impurities, cleaning and thickening of pulp fiber mixture Addition of chemicals in a staged process of reaction and washing increases whiteness and brightness of pulp, if necessary At nonintegrated pulp mills, pulp is dried and bundled into bales for transport to a paper mill
Mixing, refining, and addition of wet additives to add strength, gloss, texture to paper product, if necessary fibers, expressed as the ratio of recovered paper consumption to the total production of paper and paperboard, is ca. 39% and is climbing slowly.1 In a resource-deficient country such as Japan, the secondary fiber utilization rate is ca. 50%, whereas the average utilization rate in Europe is ca. 40%.16 Due to losses of fiber substance and strength during the recycling process, a 50% utilization rate is considered the present maximum overall utilization rate for fiber recycling.12
Until recently, secondary fiber was not used for higher quality paper products. Contaminants (e.g., inks, paper colors) are present, so production of low-purity products is often the most cost-effective use of secondary fibers. Approximately 68% of all secondary fiber in the U.S. is presently used for multi-ply paperboard or the corrugating paper used to manufacture corrugated cardboard.15 Recently, continuing improvement of deinking processes together with the demand created by environmental concerns have resulted in an increasing use of deinked fiber for newsprint or higher-quality uses, such as office copier paper.
Other sources of fibers include cotton rags and linters, flax, hemp, bagasse, tobacco, and synthetic fibers such as polypropylene. These substances are not used widely, however, as they are typically for low-volume, specialty grades of paper.
The types of furnish used by a pulp and paper mill depend on the type of product produced and what is readily available. Urban mills use a larger proportion of secondary fibers due to the postconsumer feedstock being close at hand. More rurally located mills are usually close to timber sources and thus may use virgin fibers in a greater proportion.
Wood is prepared for pulp production by a process designed to supply a homogeneous pulping feedstock. In the case of roundwood furnish (logs), the logs are cut to manageable size and then debarked. At pulp mills integrated with lumbering facilities, acceptable lumber wood is removed at this stage. At these facilities, any residual or waste wood from lumber processing is returned to the chipping process; in-house lumbering rejects can be a significant source of wood furnish at a facility. The bark of those logs not fit for lumber is usually either stripped mechanically or hydraulically with high pressure water jets in order to prevent contamination of pulping operations. Depending on the moisture content of the bark, it may then be burned for energy production. If not burned for energy production, bark can be used for mulch, ground cover, or to make charcoal.
Hydraulic debarking methods may require a drying step before burning. Usually, hydraulically removed bark is collected in a water flume, dewatered, and pressed before burning. Treatment of wastewater from this process is difficult and costly, however, whereas dry debarking methods can channel the removed bark directly into a furnace.12 In part because of these challenges, hydraulic debarking has decreased in significance within the industry.1
Debarked logs are cut into chips of equal size by chipping machines. Chippers usually produce uniform wood pieces 20 mm long in the grain direction and 4 mm thick. The chips are then put on a set of vibrating screens to remove those that are too large or small. Large chips stay on the top screens and are sent to be recut, while the smallest chips are usually burned with the bark. Certain mechanical pulping processes, such as stone groundwood pulping, use roundwood; however, the majority of pulping operations require wood chips. Nonwood fibers are handled in ways specific to their composition. Steps are always taken to maintain fiber composition and thus pulp yield.
Chemical pulps are typically manufactured into products that have high quality standards or require special properties. Chemical pulping separates the fibers of wood by dissolving the lignin bond holding the wood together. Generally, this process involves the cooking/digesting of wood chips in aqueous chemical solutions at elevated temperatures and pressures. There are two major types of chemical pulping used in the U.S., which differ in the chemicals employed and in the waste produced:
1. Kraft/soda pulping
Kraft pulping processes produced approximately 83% of all U.S. pulp tonnage during 2000 according to the American Forest and Paper Association.1 The success of the process and its widespread adoption are due to several factors. First, because the kraft cooking chemicals are selective in their attack on wood constituents, the pulps produced are notably stronger than those from other processes (kraft is German for "strength"). The kraft process is also flexible, in so far as it can be applied to many different types of raw materials (i.e., hard or soft woods) and can tolerate contaminants frequently found in wood (e.g., resins). Lignin removal rates are high in the kraft process—up to 90%—allowing high levels of bleaching without pulp degradation. Finally, the chemicals used in kraft pulping are readily recovered within the process, making it very economical and reducing potential environmental releases.
The kraft process uses a sodium-based alkaline pulping solution (liquor) consisting of sodium sulfide (Na2S) and sodium hydroxide (NaOH) in 10% solution. This liquor (white liquor) is mixed with the wood chips in a reaction vessel (digester). The output products are separated wood fibers (pulp) and a liquid that contains the dissolved lignin solids in a solution of reacted and unreacted pulping chemicals (black liquor). The black liquor undergoes a chemical recovery process to regenerate white liquor for the first pulping step. Overall, the kraft process converts ca. 50% of input furnish into pulp.
The kraft process evolved from the soda process. The soda process uses an alkaline liquor of only sodium hydroxide (NaOH). The kraft process has virtually replaced the soda process due to the economic benefits of chemical recovery and improved reaction rates (the soda process has a lower yield of pulp per pound of wood furnish than the kraft process).
Sulfite pulping was used for approximately 2% of U.S. pulp production in 2000.1 Softwood is the predominant furnish used in sulfite pulping processes. However, only nonresinous species are generally pulped, particularly when a light colored pulp is required. This process is used, for example, almost exclusively for the manufacture of viscose.17 To manufacture sulfite pulp, wood chips are boiled under pressure in large digesters with calcium sulfite, ammonium sulfite, magnesium sulfite, or sodium sulfite. The sulfite pulping process relies on acid solutions of sulfurous acid (H2SO3) and bisulfite ion (HSO3- ) to degrade the lignin bonds between wood fibers. In sulfite pulping most water pollution arises from spent liquor, condensates, bleach plant effluents, and accidental discharges.
Sulfite pulps have less color than kraft pulps and can be bleached more easily; however, they are not as strong. The efficiency and effectiveness of the sulfite process is also dependent on the type of wood furnish and the absence of bark. For these reasons, the use of sulfite pulping has declined in comparison to kraft pulping over time.
Semichemical pulping comprised 6% of U.S. pulp production in 1993.1 Semichemical pulp is often very stiff, making this process common in corrugated container manufacture. This process primarily uses hardwood as furnish.
The major process difference between chemical pulping and semichemical pulping is that semichemical pulping uses lower temperatures, more dilute cooking liquor or shorter cooking times, and mechanical disintegration for fiber separation. At most, the digestion step in the semi-chemical pulping process consists of heating pulp in sodium sulfite (Na2SO3) and sodium carbonate (Na2CO3). Other semichemical processes include the Permachem process and the two-stage vapor process. The yield of semichemical pulping ranges from 55 to 90%, depending on the process used, but pulp residual lignin content is also high so bleaching is more difficult.
Mechanical pulping accounted for 9% of U.S. pulp production in 2000.1 Mechanically produced pulp is of low strength and quality. Such pulps are used principally for newsprint and other nonpermanent paper goods. Mechanical pulping uses physical pressures instead of chemicals to separate furnish fibers. The processes include the following:
1. Stone groundwood
2. Refiner mechanical
The stone groundwood process simply involves mechanical grinding of wood in several high-energy refining systems. The refiner mechanical process involves refining wood chips at atmospheric pressure. The thermo-mechanical process uses steam and pressure to soften the chips before mechanical refining. In the chemi-mechanical process, chemicals can be added throughout the process to aid the mechanical refining. The chemi-thermo-mechanical process involves the treatment of chips with chemicals for softening followed by mechanical pulping under heat and pressure. Mechanical pulping typically results in high pulp yields, up to 95% when compared to chemical pulping yields of45-50%, but energy usage is also high. To offset its structural weakness, mechanical pulp is often blended with chemical pulp.
Secondary fiber pulping accounted for 39% of domestic pulp production in 2000.1 Nearly 200 mills rely exclusively on recovered paper for pulp furnish, and ca. 80% of U.S. paper mills use recovered paper in some way.14 In addition, consumption of fiber from recovered paper is growing more than twice as fast as overall fiber consumption. Secondary fibers are usually presorted before they are sold to a pulp and paper mill. If not, secondary fibers are processed to remove contaminants before pulping occurs. Common contaminants consist of adhesives, coatings, polystyrene foam, dense plastic chips, polyethylene films, wet strength resins, and synthetic fibers. In some cases, contaminants of greater density than the desired secondary fibers are removed by centrifugal force while light contaminants are removed by flotation systems. Centri cleaners are also used to remove material less dense than fibers (wax and plastic particles).18
Inks, another contaminant of secondary fibers, may be removed by heating a mixture of secondary fibers with surfactants. The removed inks are then dispersed in an aqueous medium to prevent redeposition on the fibers. Continuous solvent extraction has also been used to recover fibers from paper and board coated with plastics or waxes.
Secondary fiber pulping is a relatively simple process. The most common pulper design consists of a large container filled with water, which is sometimes heated, and the recycled pulp. Pulping chemicals (e.g., sodium hydroxide, NaOH) are often added to promote dissolution of the paper or board matrix. The source fiber (corrugated containers, mill waste, and so on) is dropped into the pulper and mixed by a rotor. Debris and impurities are removed by two mechanisms: a ragger and a junker. The ragger withdraws strings, wires, and rags from the stock secondary fiber mixture. A typical ragger consists of a few "primer wires" that are rotated in the secondary fiber slurry. Debris accumulates on the primer wires, eventually forming a "debris rope," which is then removed. Heavier debris is separated from the mixture by centrifugal force and falls into a pocket on the side of the pulper. The junker consists of a grappling hook or elevator bucket. Heat, dissolution of chemical bonds, shear forces created by stirring and mixing, and grinding by mechanical equipment may serve to dissociate fibers and produce a pulp of desired uniformity.
Contaminant removal processes depend on the type and source of secondary fiber to be pulped. Mill paper waste can be easily repulped with minimal contaminant removal. Recycled postconsumer newspaper, on the other hand, may require extensive contaminant removal, including deinking, prior to reuse. Secondary fiber is typically used in lower-quality applications such as multiply paper-board or corrugating paper.
After pulp production, pulp processing removes impurities12 such as uncooked chips, and recycles any residual cooking liquor via the washing process (Figure 21.4). Pulps are processed in a wide variety of ways, depending on the method that generated them (e.g., chemical, semichemical). Some pulp processing steps that remove pulp impurities include screening, defibering, and deknotting. Pulp may also be thickened by removing a portion of the water. At additional cost, pulp may be
blended to ensure product uniformity. If pulp is to be stored for long periods of time, drying steps are necessary to prevent fungal or bacterial growth.1
Residual spent cooking liquor from chemical pulping is washed from the pulp using brown stock washers. Efficient washing is critical to maximize the return of cooking liquor to chemical recovery and to minimize the carryover of cooking liquor (known as brown stock washing loss) into the bleach plant, because excess cooking liquor increases consumption of bleaching chemicals. Specifically, the dissolved organic compounds (lignins and hemicelluloses) contained in the liquor will bind to the bleaching chemicals and thus increase bleach chemical consumption. In addition, these organic compounds are the precursors to chlorinated organic compounds (e.g., dioxins, furans). The most common washing technology is rotary vacuum washing, carried out sequentially in two or four washing units. Other washing technologies include diffusion washers, rotary pressure washers, horizontal belt filters, wash presses, and dilution/extraction washers.
Pulp screening removes the remaining oversized particles such as bark fragments, oversized chips, and uncooked chips. In open screen rooms, wastewater from the screening process receives wastewater treatment prior to discharge. In closed-loop screen rooms, wastewater from the process is reused in other pulping operations and ultimately enters the mill's chemical recovery system. Centrifugal cleaning (also known as liquid cyclone, hydrocyclone, or centricleaning) is used after screening to remove relatively dense contaminants such as sand and dirt. Rejects from the screening process are either repulped or disposed of as solid waste.
The chemical recovery system is a complex part of a chemical pulp and paper mill and is subject to a variety of environmental regulations. Chemical recovery is a crucial component of the chemical pulping process; it recovers process chemicals from the spent cooking liquor for reuse. The chemical recovery process has important financial and environmental benefits for pulp and paper mills. Economic benefits include savings on chemical purchase costs due to regeneration rates of process chemicals approaching 98%, and energy generation from pulp residue burned in a recovery furnace.12 Environmental benefits include the recycle of process chemicals and lack of resultant discharges to the environment.
Although newer technologies are always under development, the basic kraft chemical recovery process has not been fundamentally changed since the issue of its patent in 1884. The stepwise progression of chemical reactions has been refined; for example, black liquor gasification processes are now in use in an experimental phase. The precise details of the chemical processes at work in the chemical recovery process can be found in Smook's Handbook.12 The kraft chemical recovery process consists of the following general steps:
1. Black liquor concentration. Residual weak black liquor from the pulping process is concentrated by evaporation to form "strong black liquor." After brown stock washing in the pulping process the concentration of solids in the weak black liquor is approximately 15%; after the evaporation process, solids concentration can range from 60 to 80%. In some older facilities, the liquor then undergoes oxidation for odor reduction. The oxidation step is necessary to reduce odor created when hydrogen sulfide is stripped from the liquor during the subsequent recovery boiler burning process. Almost all recovery furnaces installed since 1968 have noncontact evaporation processes that avoid these problems, so oxidation processes are not usually seen in newer mills. Common modern evaporator types include multiple effect evaporators as well as a variety of supplemental evaporators. Odor problems with the kraft process have been the subject of control measures.
2. Recovery boiler. The strong black liquor from the evaporators is burned in a recovery boiler. In this crucial step in the overall kraft chemical recovery process, organic solids are burned for energy and the process chemicals are removed from the mixture in molten form. Molten inorganic process chemicals (smelt) flow through the perforated floor of the boiler to water-cooled spouts and dissolving tanks for recovery in the recausticizing step. Energy generation from the recovery boiler is often insufficient for total plant needs, so facilities augment recovery boilers with fossil-fuel-fired and wood-waste-fired boilers to generate steam and often electricity. Industry wide, the utilization of pulp wastes, bark, and other paper-making residues supplies 58% of the energy requirements of pulp and paper companies.11
3. Recausticizating. Smelt is recausticized to remove impurities left over from the furnace and to convert sodium carbonate (Na2CO3) into active sodium hydroxide (NaOH) and sodium sulfide (Na2S). The recausticization procedure begins with the mixing of smelt with "weak" liquor to form green liquor, named for its characteristic color. Contaminant solids, called dregs, are removed from the green liquor, which is mixed with lime (CaO). After the lime mixing step, the mixture, now called white liquor due to its new coloring, is processed to remove a layer of lime mud (CaCO3) that has precipitated. The primary chemicals recovered are caustic (NaOH) and sodium sulfide (Na2S). The remaining white liquor is then used in the pulp cooking process. The lime mud is treated to regenerate lime in the calcining process.
4. Calcining. In the calcining process, the lime mud removed from the white liquor is burned to regenerate lime for use in the lime mixing step. The vast majority of mills use lime kilns for this process, although a few mills now use newer fluidized bed systems in which the reactants are suspended by upward-blowing air.
Numerous sulfite chemical pulping recovery systems are in use today. Heat and sulfur can be recovered from all liquors generated; however, the base chemical can only be recovered from magnesium and sodium base processes. See Smook's Handbook12 for more information.
Bleaching is defined as any process that chemically alters pulp to increase its brightness. Bleached pulps create papers that are whiter, brighter, softer, and more absorbent than unbleached pulps. Bleached pulps are used for white or light colored paper. Unbleached pulp is typically used to produce boxboard, linerboard, and grocery bags. Of the approximately 65.5 million T (72 million tons) of pulp (including recycled pulp) used in paper production in the U.S. in 2000, about 50% is for bleached pulp.1
Any type of pulp may be bleached, but the type(s) of fiber furnish and pulping processes used, as well as the desired qualities and end use of the final product, greatly affect the type and degree of pulp bleaching possible. Printing and writing papers comprise ca. 60% of bleached paper production. The lignin content of a pulp is the major determinant of its bleaching potential. Pulps with high lignin content (e.g., mechanical or semichemical) are difficult to bleach fully and require heavy chemical inputs. Bleached pulps with high lignin content are subject to color reversion, loss of brightness when exposed to light. Excessive bleaching of mechanical and semichemical pulps results in loss of pulp yield due to fiber destruction. Chemical pulps can be bleached to a greater extent due to their low (10%) lignin content. For more information, the U.S. EPA reference 19 is recommended. Typical bleaching processes for each pulp type are detailed below.
Chemical pulp bleaching has undergone significant process changes since around 1990. Until that time, nearly every chemical pulp mill that had used bleaching had incorporated elemental chlorine (Cy into some of its processes. Because of environmental and health concerns about dioxins, U.S. pulp mills now use elemental chlorine free (ECF) and total chlorine free (TCF) bleaching technologies. The most common types of ECF and TCF are shown in Table 21.5. The difference
Common Chemicals Used in Elemental Chlorine Free (ECF) and Total Chlorine Free (TCF) Bleaching Processes
Sodium hydroxide Chlorine dioxide Hypochlorite Oxygen Ozone
Hydrogen peroxide Sulfur dioxide Sulfuric acid
HClO, NaOCl, Ca(OCl)2
ECF and TCF
ECF and TCF ECF and TCF ECF and TCF ECF and TCF ECF and TCF
FIGURE 21.5 Typical bleach plant source. (Taken from U.S. EPA, Profile of the Pulp and Paper Industry, 2nd ed., report EPA/310-R-02-002, U.S. EPA, Washington, November 2002.)
FIGURE 21.5 Typical bleach plant source. (Taken from U.S. EPA, Profile of the Pulp and Paper Industry, 2nd ed., report EPA/310-R-02-002, U.S. EPA, Washington, November 2002.)
between ECF and TCF is that ECF may include chlorine dioxide (ClO2) and hypochlorite [HClO, NaOCl, and Ca(OCl)2] based technologies. In 2001, ECF technologies were used for about 95% of bleached pulp production, TCF technologies were used for about 1% of bleached pulp production, and elemental chlorine was used for about 4% of production.20
Chemical pulp is bleached in traditional bleach plants (see Figure 21.5), where the pulp is processed through three to five stages of chemical bleaching and water washing. The desired whiteness, the brightness of the initial stock pulp, and the plant design determine the number of cycles needed.
Bleaching stages generally alternate between acid and alkaline conditions. Chemical reactions with lignin during the acid stage of the bleaching process increase the whiteness of the pulp. The alkaline extraction stages dissolve the lignin/acid reaction products. At the washing stage, both solutions and reaction products are removed. Chemicals used to perform the bleaching process must have high lignin reactivity and selectivity to be efficient. Typically, 4 to 8% of pulp is lost due to bleaching agent reactions with the wood constituents cellulose and hemicellulose, but these losses can be as high as 18%.12 Semichemical pulps are typically bleached with hydrogen peroxide (H2O2) in a bleach tower. Mechanical pulps are bleached with hydrogen peroxide (H2O2) or sodium hydrosulfite (NaHSO3). Bleaching chemicals are either applied without separate equipment during the pulp processing stage (i.e., in-line bleaching), or in bleaching towers. Full bleaching of mechanical pulps is generally not practical due to bleaching chemical cost and the negative impact on pulp yield.
Deinked secondary fibers are usually bleached in a bleach tower, but may be bleached during the repulping process. Bleach chemicals may be added directly into the pulper. The following are examples of chemicals used to bleach deinked secondary fibers: hypochlorite [HClO, NaOCl, Ca(OCl)2], hydrogen peroxide (H2O2), and hydrosulphite (NaHSO3).
At this final stage, the pulp is processed into the stock used for paper manufacture. Market pulp, which is to be shipped off-site to paper or paperboard mills, is processed little, if at all at this stage. Processing includes pulp blending specific to the desired paper product desired, dispersion in water, beating and refining to add density and strength, and addition of any necessary wet additives. Wet additives are used to create paper products with special properties or to facilitate the paper-making process. Wet additives include resins and waxes for water repellency, fillers such as clays, silicas, talc, inorganic/organic dyes for coloring, and certain inorganic chemicals (calcium sulfate, zinc sulfide, and titanium dioxide) for improved texture, print quality, opacity, and brightness.
22.214.171.124 Processes in Paper Manufacture
The paper and paperboard making process consists of the following general steps:
1. Wet end operations: formation of paper sheet from wet pulp
2. Dry end operations: drying of paper product, application of surface treatments, and spooling for storage
The processed pulp is converted into a paper product via a paper production machine, the most common of which is the Fourdrinier paper machine (see Figure 21.6). In the Fourdrinier system,3 the pulp slurry is deposited on a moving belt (made from polyester forming fabrics) that carries it through the first stages of the process. Water is removed by gravity, vacuum chambers, and vacuum rolls. This waste water is recycled to the slurry deposition step of the process due to its high fiber content. The continuous sheet is then pressed between a series of rollers to remove more water and compress the fibers.
After pressing, the sheet enters a drying section, where the sheet passes around a series of steam-heated drums. It then may be calendared. In the calendar process the sheet is pressed between heavy rolls to reduce paper thickness and produce a smooth surface. Coatings can be applied to the paper at this point to improve gloss, color, printing detail, and brilliance. Lighter coatings are applied on-machine, and heavy coatings are performed off-machine. The paper product is then spooled for storage.
Pulp and paper mill energy generation is provided in part from the burning of liquor waste solids in the recovery boiler, but other energy sources are needed to make up the remainder of mill energy needs. Over the last 25 years the pulp and paper industry has changed its energy generation methods from fossil fuels to a greater utilization of processes or process wastes. The increase in use of wood wastes from the wood handling and chipping processes (Table 21.6) is one example of this industry-wide movement. During the period 1972 to 1999, the proportion of total industry power generation from the combination of woodroom wastes, spent liquor solids, and other self-generation methods increased from ca. 41% to ca. 58%, while coal, fuel oil, and natural gas use decreased from ca. 54% to ca. 36%.12,21
Estimated Energy Sources for the U.S. Pulp and Paper Industry
Energy Source 1972 1979 1990 1999
Waste wood and wood chips 6.6% 9.2% 15.4% 13.5% (hogged fuel) and bark
Source: U.S. EPA, Profile of the Pulp and Paper Industry, 2nd ed., report EPA/310-R-02-002, U.S. EPA, Washington, November 2002.
Power boilers at pulp and paper mills are sources of particulate emissions, SO2, and NOx. Pollutants emitted from chemical recovery boilers include SO2 and total reduced sulfur compounds (TRS).
21.2.2 Raw Material Inputs and Pollution Outputs in the Production Line Pulp and paper mills use and generate materials that may be harmful to the air, water, and land:
1. Pulp and paper processes generate large volumes of wastewaters that might adversely affect freshwater or marine ecosystems.
2. Residual wastes from wastewater treatment processes may contribute to existing local and regional disposal problems.
3. Air emissions from pulping processes and power generation facilities may release odors, particulates, or other pollutants.
The major sources of pollutant releases in pulp and paper manufacture occur at the pulping and bleaching stages, respectively. As such, nonintegrated mills (i.e., those mills without pulping facilities on site) are not significant environmental concerns when compared to integrated mills or pulp mills.
The pulp and paper industry is the largest industrial process water user in the U.S.5 In 2000, a typical pulp and paper mill used between 15,140 and 45,420 L (4000 to 12,000 gal) of water per ton of pulp produced.4 General water pollution concerns for pulp and paper mills are effluent solids, biochemical oxygen demand (BOD), and color. Toxicity concerns historically occurred from the potential presence of chlorinated organic compounds such as dioxins, furans, and others (collectively referred to as adsorbable organic halides, or AOX) in wastewaters after the chlorination/ extraction sequence. With the substitution of chlorine dioxide for chlorine, discharges of the chlorinated compounds have decreased dramatically.
Due to the large volumes of water used in pulp and paper processes, virtually all U.S. mills have primary and secondary wastewater treatment systems to remove particulates and BOD. These systems also provide significant removals (e.g., 30 to 70%) of other important parameters such as AOX and chemical oxygen demand (COD).
Common Water Pollutants from Pulp and Paper Processes
Water used in wood handling/debarking and chip washing
Chip digester and liquor evaporator condensate
"White waters" from pulp screening, thickening, and cleaning
Bleach plant washer filtrates Paper machine-water flows Fiber and liquor spills
Solids, BOD, color
Concentrated BOD; can contain reduced sulfur Large volume of water with suspended solids; can have significant BOD BOD, color, chlorinated organic compounds Solids, often precipitated for reuse Solids, BOD, color
Source: U.S. EPA, Profile of the Pulp and Paper Industry, 2nd ed., report EPA/310-R-02-002, U.S. EPA, Washington,
November 2002. BOD, biochemical oxygen demand.
The major sources of pollutants from pulp and paper mills12 are presented in Table 21.7.
Wood processing operations in pulp mills use water for a variety of purposes. The resulting wastewaters contain BOD, suspended solids, and some color. The condensates from chip digesters and chemical recovery evaporators are sources of BOD and reduced sulfur compounds. Wastewaters containing BOD, color, and suspended solids may be generated from pulp screening operations in mills using "atmospheric" systems, although most mills have modern pressure screens that virtually eliminate such wastewaters. Kraft bleaching generates large volumes of wastewater containing BOD, suspended solids, color, and chlorinated organic compounds. From paper machines, excess white water (named for its characteristic color) contains suspended solids and BOD. Fiber and liquor spills can also be a source of mill effluent. Typically, spills are captured and pumped to holding areas to reduce chemical usage through spill reuse and to avoid loadings on facility wastewater treatment systems.
Wastewater treatment systems can be a significant source of cross-media pollutant transfer. For example, waterborne particulates and some chlorinated compounds settle or absorb onto treatment sludge and other compounds may volatilize during the wastewater treatment process.
126.96.36.199 Air Pollutants
Table 21.8 is an overview of the major types and sources of air pollutant releases from various pulp and paper processes.12
Water vapors are the most visible air emission from a pulp and paper mill, but are not usually regulated unless they form a significant obscurement or are a climate modifier.
Pulp and paper mill power boilers and chip digesters are generic pulp and paper mill sources of air pollutants such as particulates and nitrogen oxides. Chip digesters and chemical recovery evaporators are the most concentrated sources of volatile organic compounds. The chemical recovery furnace is a source of fine particulate emissions and sulfur oxides. In the kraft process, sulfur oxides are a minor issue in comparison to the odor problems created by four reduced sulfur gases, known collectively as total reduced sulfur (TRS): hydrogen sulfide, methyl mercaptan, dimethyl sulfide, and dimethyl disulfide. The TRS emissions are primarily released from wood chip digestion, black liquor evaporation, and chemical recovery boiler processes. TRS compounds create odor nuisance problems at lower concentrations than sulfur oxides; odor thresholds for TRS compounds are approximately 1000 times lower than that for sulfur dioxide. Humans can detect some TRS compounds in the air as a "rotten egg" odor at a level as low as 1 ^g/L.
Common Air Pollutants from Pulp and Paper Processes
Kraft recovery furnace
Fly ash from hog fuel and coal-fired burners
Sulfite mill operations
Kraft pulping and recovery processes
Chip digesters and liquor evaporation
All combustion processes
Fine particulates Coarse particulates Sulfur oxides Reduced sulfur gases Volatile organic compounds Nitrogen oxides
Pulp and paper mills have made significant investments in pollution control technologies and processes. According to industry sources, the pulp and paper industry spent more than USD
1 billion/yr from 1991 to 1997 on environmental capital expenditures. In 1991 an
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