Pulp and Paper Mills
Pulp and paper mills
Pulp and paper mills take wood and transform the raw product into paper. Hardwood logs (beech, birch, and maple) and softwoods (pine, spruce, and fir) are harvested from managed forestlands or purchased from local farms and timberlands across the world and are transported to mills for processing. Hardwoods are more dense, shorter fibered, and slower growing. Softwoods are less dense, longer fibered, and faster growing.
Today, the process is mainly done with high tech, sophisticated machinery. Wood products, which consist of lignin (30 percent), fiber (50 percent), and other materialscarbohydrates, proteins, fats, turpentine, resins, etc., (20 percent) are transformed into paper consisting of fiber, and additives--clay, titanium dioxide, calcium carbonate, water, rosin, alum, starches, gums, dyes, synthetic polymers, and pigments. Wood is about 50 percent cellulose fiber. The structure of paper is a tightly bonded web of cellulose fibers. About 80 percent of a typical printing paper by weight is cellulose fiber. First in the process, the standard eight-foot (2.4-m) logs are debarked by tumbling them in a giant barking drum and then chipped by a machine that reduces them to half-inch chips. The chips are cooked, after being screened and steamed, in a digester using sodium bisulfite cooking liquor to remove most of the lignin, the sticky matter in a tree that bonds the cellulose fibers together. This is the pulping process.
Then the chips are washed, refined, and cleaned to separate the cellulose fibers and create the watery suspension called pulp. The pulp is bleached in a two-stage process with a number of possible chemicals . Those companies that choose to avoid chlorine bleach will use hydrogen peroxide and sodium hydrosulfite which yields a northern high-yield hardwood sulfite pulp. This pulp is blended with additional softwood kraft pulp after refining as part of the stock preparation process, which involves adding such materials as dyes, pigments, clay fillers, internal sizing, additional brighteners, and opacifiers.
Late in the process, the stock is further refined to adjust fiber length and drainage characteristics for good formation and bonding strength. The consistency of the stock is reduced by adding more water and the stock is cleaned again to remove foreign particles. The product is then pumped to the paper machine headbox.
From here, the dilute stock (99.5 percent water) flows out in a uniformly thin slice onto a Fourdrinier wire--an endless moving screen that drains water from the stock to form a self-supporting web of paper. The web moves off the wire into the press section which squeezes out more water between two press felts, then into the first drier section where more moisture is removed by evaporation as the paper web winds forward around an array of steam-heated drums. At the size press, a water-resistant surface sizing is added in an immersion bath.
From there the sheet enters a second drier section where the sheet is redried to the final desired moisture level before passing through the computer scanner. The scanner is part of a system for automatically monitoring and regulating basis weight and moisture. The paper enters the calendar stack, where massive steel polishing rolls give the sheet its final machine finish and bulking properties.
The web of paper is then wound up in a single long reel, which is cut and moved off the paper machine to a slitter/winder machine which slices the reel into rolls of the desired width and rewinds them onto the appropriate cores. The rolls are then conveyed to the finishing room where they are weighed, wrapped, labeled, and shipped.
In practice, all papers, even newsprint, are pulp blends, but they are placed in one of two categories for convenient description: groundwood and free sheet. And in practice, other pulp varieties enter into the picture. They may be reclaimed pulps such as de-inked or post-consumer waste; recycled pulps which included scraps, trim, and unprinted waste; cotton fiber pulps; synthetic fibers; and pulps from plants other than trees: bagasse, esparto, bamboo, hemp, water hyacinth ; and banana, or rice. But the dominant raw material remains wood pulp. Paper makers choose and blend from the spectrum of pulps according to the demands on their grades for strength, cleanliness, brightness, opacity, printing, and converting requirements, aesthetics, and market price.
From cotton fiber-based sheets to the less expensive papers made from groundwood, to recycled grades manufactured with various percentages of wastepaper content, papermakers have consistently responded to the need of the marketplace. In today's increasingly environmentally conscious marketplace, papermakers are being called on to produce pulp that is environmentally friendly. Eliminating chlorine from the bleaching process is a major step in eliminating unwanted toxins . The changeover costs money and is the source of controversy here in the United States.
However, the chlorine-free trend has taken a firm hold in Europe. All of Sweden requires its printing and writing paper mills to be chlorine-free by the year 2010. France, Germany, and several other countries have several mills that are reported to be chlorine-free and the trend is moving across Canada.
While there are growing exceptions, most North American mills still use a chlorine bleaching process to create a bright, white pulp. Why the need to eliminate the chemical?
In the bleaching process, chlorine, chlorine dioxide, and other chlorine compounds create toxic byproducts. These byproducts consist of over 1,000 chemicals, some of which are the most toxic known to man. The list includes: dioxin and other organochlorines compounds such as PCBs, DDT, chlordane , aldrin, dieldrin, toxaphene , chloroform, heptachlor and furans . These are formed by the reaction of lignin in the pulp with chlorine or chlorine-based compounds used in the bleaching sequence of all kraft pulps.
These unwanted chemicals byproducts must be discharged and end up in the effluent . The effluent is released into our rivers, lakes and streams and is threatening the groundwater and our drinking water, as well as the food chain through fish and birds. Epidemic health effects among thirteen species of fish and wildlife near the top of the Great Lakes food web have been identified. Not only are these toxic chemicals causing cancer and birth deformities in humans and wildlife, but they are very persistent, building up in our waterways and eventually into our bodies.
Dioxin traces have been found in papers and even in coffee from chlorine-bleached coffee filters and milk from chlorine-bleached milk cartons, as well as in women's hygiene products. Most of the paper being sold in the United States today as "dioxin free" is actually "dioxin undetectable." That is because dioxin can be measured in parts per trillion or parts per quintillion, but beyond that level, there are no scientific measurements sophisticated enough. Or if measurable, the process becomes very expensive. (If exposed often enough, even these minuscule quantities build up in the environment and in humans.) To be truly dioxin free, the paper must be made from pulps that have been bleached without chlorine or chlorine-based compounds.
The newer trend is to eliminate chlorine from the bleaching process completely. Some North American mills are turning to hydrogen peroxide, oxygen brightening, or ozone brightening. These compounds do not produce dioxin or other organochlorine compounds and are considered "environmentally-sound." The United States pulp and paper industry has sharply reduced its use of the chemical and plans to curtail use further during the next few years.
In part, this reduction can be traced to the increased sophistication of pulp and paper plants during the past decade. The cooking and bleaching operations have been fine-tuned. Wood chips are cooked more before they go to the bleach plant, so less bleaching is required. Also, in some plants, industry is trying chlorine dioxide as a substitute; it produces less dioxin, but still contains chlorine.
The result of these changes means an 80 percent reduction in the amount of dioxin associated with bleaching. Between 1988-1989, a total of 2.5 lb (11.1 kg) of dioxin was produced. As of 1993, that number fell below 8 ounces (22.7 g) per year. Eight ounces sound like a small amount, but scientists measure dioxin in parts per million , billion, trillion and quintillion, so eight ounces is still too high.
A number of lawsuits have been filed by residents living near or downstream of dioxin-contaminated pulp mills because of the health threats. The more the plaintiffs win, the sooner will the use of chlorine be eliminated completely. It is estimated that the amount of chlorine used in pulp and paper bleaching will fall from 1.4 million tons in 1990 to 920,000 tons by 1995.The eventual goal is zero discharge .
[Liane Clorfene Casten ]
Ferguson, K. Environmental Solutions for the Pulp and Paper Industry. San Francisco: Miller Freeman, 1991.
Jenish, D. "Cleaning Up a Chemical Soup." Maclean's 103 (29 January 1990): 32–4.