Pulp mill

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A pulp mill in Rauma, Finland Rauman sellutehdas.jpeg
A pulp mill in Rauma, Finland
Woodchips for paper production Woodchips for paper production.jpg
Woodchips for paper production

A pulp mill is a manufacturing facility that converts wood chips or other plant fiber sources into a thick fiber board which can be shipped to a paper mill for further processing. Pulp can be manufactured using mechanical, semi-chemical, or fully chemical methods (kraft and sulfite processes). [1] The finished product may be either bleached or non-bleached, depending on the customer requirements.

Contents

Wood and other plant materials used to make pulp contain three main components (apart from water): cellulose fibres (desired for papermaking), lignin (a three-dimensional polymer that binds the cellulose fibres together) and hemicelluloses, (shorter branched carbohydrate polymers). The aim of pulping is to break down the bulk structure of the fiber source, be it chips, stems or other plant parts, into the constituent fibers.

Chemical pulping achieves this by degrading the lignin and hemicellulose into small, water-soluble molecules that can be washed away from the cellulose fibers without depolymerizing the cellulose fibers (chemically depolymerizing the cellulose weakens the fibers). The various mechanical pulping methods, such as groundwood (GW) and refiner mechanical (RMP) pulping, physically tear the cellulose fibers one from another. Much of the lignin remains adhering to the fibers. Strength is impaired because the fibers may be cut. Related hybrid pulping methods use a combination of chemical and thermal treatment to begin an abbreviated chemical pulping process, followed immediately by a mechanical treatment to separate the fibers. These hybrid methods include thermomechanical pulping (TMP) and Chemi-thermomechanical pulping (CTMP). The chemical and thermal treatments reduce the amount of energy subsequently required by the mechanical treatment, and also reduce the amount of strength loss suffered by the fibers.

The earliest known methods for preparing pulp for paper making were water-powered, in 8th-century Samarkand, Abbasid Caliphate. [2]

The mill

International Paper Company, pulp mill InternationalPaper6413.jpg
International Paper Company, pulp mill

Much of the information about the technology in following subsections is from the book by C.J. Biermann. [3] The chemistry of the various pulping processes can be found in Sjöström's book. [4]

Preparation of fibre source

The most common fiber source for pulp mills is pulpwood. Other common sources are bagasse [5] and fibre crops. The first step in all mills using wood (trees) as the fiber source is to remove the bark. Bark contains relatively few usable fibers and darkens the pulp. The removed bark is burned, along with other unusable plant material, to generate steam to run the mill. Almost all wood is then chipped before being processed further in order to free the fibers.

Removal of the bark is done in a barker (or debarker). The bark adhesion is about 3–5 kg/cm2 in the growing season (summer) and 2-3 times higher in the dormant season (winter). The bark of frozen logs is even more difficult to remove.

In chemical pulp mills, the bark introduces unwanted contaminants such as calcium, silica, and aluminum that cause scaling and give an extra loading for the chemical recovery system. Birchbark contains betulin, a terpenoid that easily creates deposits in a pulp mill.

Mechanical pulp mills

The earliest mills used sandstone grinding rollers to break up small wood logs called "bolts", but the use of natural stone ended in the 1940s with the introduction of manufactured stones with embedded silicon carbide or aluminum oxide. The pulp made by this process is known as "stone groundwood" pulp (SGW). If the wood is ground in a pressurized, sealed grinder the pulp is classified as "pressure groundwood" (PGW) pulp. Most modern mills use chips rather than logs and ridged metal discs called refiner plates instead of grindstones. If the chips are just ground up with the plates, the pulp is called "refiner mechanical" pulp (RMP), if the chips are steamed while being refined the pulp is called "thermomechanical" pulp (TMP). Steam treatment significantly reduces the total energy needed to make the pulp and decreases the damage (cutting) to fibers. Mechanical pulp mills use large amounts of energy, mostly electricity to power motors which turn the grinders. A rough estimate of the electrical energy needed is 10,000 megajoules (MJ) per tonne of pulp (2,750  kWh per tonne)

Chemical pulp mills

Pulp mill at Blankenstein (Germany) Zellstoffwerk Rosenthal 4.jpg
Pulp mill at Blankenstein (Germany)

Chemical pulping processes such as the kraft (or sulfate) process and the sulfite process remove much of the hemicelluloses and lignin. The kraft process does less damage to the cellulose fibers than the sulfite process, thereby producing stronger fibers, but the sulfite process makes pulp that is easier to bleach. The chemical pulping processes use a combination of high temperature and alkaline (kraft) or acidic (sulfite) chemicals to break the chemical bonds of the lignin. However, atmospheric pollution and contaminants in wastewater effluent in the kraft pulping process has been widely documented [6] [7] [8]

The material fed into the digester must be small enough to allow the pulping liquor to penetrate the pieces completely. In the case of wood, the logs are chipped and the chips screened to ensure that the feed to the digester is of a uniform size. Oversized chips are either used as fuel or run through the chipper again, while sawdust may be burned or collected for sale. The screened chips or cut plant material (bamboo, kenaf, etc.) goes to the digester where it is mixed with an aqueous solution of the pulping chemicals, then heated with steam. In the kraft process the pulping chemicals are sodium hydroxide and sodium sulfide and the solution is known as white liquor. In the sulfite process the pulping chemical is a mixture of metal (sodium, magnesium, potassium, or calcium) and ammonium sulfite or sulfite.

After several hours in the digester, the chips or cut plant material breaks down into a thick porridge-like consistency and is "blown" or squeezed from the outlet of the digester through an airlock. The sudden change in pressure results in a rapid expansion of the fibers, separating the fibers even more. The resulting fiber suspension in water solution is called "brown stock".

Brown stock washers, using countercurrent flow, remove the spent cooking chemicals and degraded lignin and hemicellulose. The extracted liquid, known as black liquor in the kraft process, and red or brown liquor in the sulfite processes, is concentrated, burned and the sodium and sulfur compounds recycled in the recovery process. Lignosulphonates is a useful byproduct recovered from the spent liquor in the sulfite process. [9] The clean pulp (stock) can be bleached in the bleach plant or left unbleached, depending on the end-use. The stock is sprayed onto the pulp machine wire, water drains off, more water is removed by pressing the sheet of fibers, and the sheet is then dried. At this point the sheets of pulp are several millimeters thick and have a coarse surface: it is not yet paper. The dried pulp is cut, stacked, bailed and shipped to another facility for whatever further process is needed.

Bleached kraft pulp and bleached sulfite pulp are used to make high quality, white printing paper. One of the most visible uses for unbleached kraft pulp is to make brown paper shopping bags and wrapping paper where strength is particularly important. A special grade of bleached sulfite pulp, known as dissolving pulp, is used to make cellulose derivatives such as methylcellulose which are used in a wide range of everyday products from laxatives to baked goods to wallpaper paste.

Chemi-mechanical pulp mills

Some mills pretreat wood chips or other plant material like straw with sodium carbonate, sodium hydroxide, sodium sulfite, and other chemical prior to refining with equipment similar to a mechanical mill. The conditions of the chemical treatment are much less vigorous (lower temperature, shorter time, less extreme pH) than in a chemical pulping process, since the goal is to make the fibers easier to refine, not to remove lignin as in a fully chemical process. Pulps made using these hybrid processes are known as Chemi-thermomechanical pulps (CTMP). Sometimes a CTMP mill is located on the same site as a kraft mill so that the effluent from the CTMP mill can be treated in the kraft recovery process to regenerate the inorganic pulping chemicals.

Scheduling

The pulping process involves many production stages, usually coupled with intermediate storage tanks. As each stage has a different reliability and bottlenecks may vary from day to day, scheduling a pulp mill needs to take into account these bottlenecks and the probability of a disturbance or breakdown. [10] Each stage also may have different decision variables, such as steam / water / chemical input, etc. Finally, scheduling needs to consider fuel optimization and CO2 emissions, because part of the energy requirements may be met from fossil-fuel boilers. [11] The overall aim is to maximize production at minimum cost.

Materials of construction

Stainless steels is used extensively in the pulp and paper industry [12] for two primary reasons, to avoid iron contamination of the product and their corrosion resistance to the various chemicals used in the papermaking process. [12]

A wide range of stainless steels are used throughout the pulp making process. For example, duplex stainless steels are being used in digesters to convert wood chips into wood pulp and 6% Mo super austenitic stainless steels are used in the bleach plant.

See also

Related Research Articles

<span class="mw-page-title-main">Hemicellulose</span> Class of plant cell wall polysaccharides

A hemicellulose is one of a number of heteropolymers, such as arabinoxylans, present along with cellulose in almost all terrestrial plant cell walls. Cellulose is crystalline, strong, and resistant to hydrolysis. Hemicelluloses are branched, shorter in length than cellulose, and also show a propensity to crystallize. They can be hydrolyzed by dilute acid or base as well as a myriad of hemicellulase enzymes.

<span class="mw-page-title-main">Fiber</span> Natural or synthetic substance made of long, thin filaments

Fiber or fibre is a natural or artificial substance that is significantly longer than it is wide. Fibers are often used in the manufacture of other materials. The strongest engineering materials often incorporate fibers, for example carbon fiber and ultra-high-molecular-weight polyethylene.

<span class="mw-page-title-main">Pulp (paper)</span> Fibrous material used notably in papermaking

Pulp is a fibrous lignocellulosic material prepared by chemically, semi-chemically or mechanically producing cellulosic fibers from wood, fiber crops, waste paper, or rags. Mixed with water and other chemicals or plant-based additives, pulp is the major raw material used in papermaking and the industrial production of other paper products.

<span class="mw-page-title-main">Paper engineering</span>

Paper engineering is a branch of engineering that deals with the usage of physical science and life sciences in conjunction with mathematics as applied to the converting of raw materials into useful paper products and co-products. The field applies various principles in process engineering and unit operations to the manufacture of paper, chemicals, energy and related materials. The following timeline shows some of the key steps in the development of the science of chemical and bioprocess engineering:

<span class="mw-page-title-main">Pulpwood</span> Timber intended for processing into wood pulp for paper production

Pulpwood can be defined as timber that is ground and processed into a fibrous pulp. It is a versatile natural resource commonly used for paper-making but also made into low-grade wood and used for chips, energy, pellets, and engineered products.

<span class="mw-page-title-main">Lyocell</span> Regenerated cellulose fiber made from dissolving pulp

Lyocell is a semi-synthetic fiber used to make textiles for clothing and other purposes. It is a form of regenerated cellulose made by dissolving pulp and dry jet-wet spinning. Unlike rayon; which is made by the more common viscose processes, Lyocell production does not use carbon disulfide, which is toxic to workers and the environment. Lyocell was originally trademarked as Tencel in 1982.

<span class="mw-page-title-main">Kraft process</span> Process of converting wood into wood pulp

The kraft process (also known as kraft pulping or sulfate process) is a process for conversion of wood into wood pulp, which consists of almost pure cellulose fibres, the main component of paper. The kraft process involves treatment of wood chips with a hot mixture of water, sodium hydroxide (NaOH), and sodium sulfide (Na2S), known as white liquor, that breaks the bonds that link lignin, hemicellulose, and cellulose. The technology entails several steps, both mechanical and chemical. It is the dominant method for producing paper. In some situations, the process has been controversial because kraft plants can release odorous products and in some situations produce substantial liquid wastes.

<span class="mw-page-title-main">Black liquor</span> Industrial by-product

In industrial chemistry, black liquor is the by-product from the kraft process when digesting pulpwood into paper pulp removing lignin, hemicelluloses and other extractives from the wood to free the cellulose fibers.

<span class="mw-page-title-main">Kraft paper</span> Paper or paperboard produced from chemical pulp produced in the kraft process

Kraft paper or kraft is paper or paperboard (cardboard) produced from chemical pulp produced in the kraft process.

Bleaching of wood pulp is the chemical processing of wood pulp to lighten its color and whiten the pulp. The primary product of wood pulp is paper, for which whiteness is an important characteristic. These processes and chemistry are also applicable to the bleaching of non-wood pulps, such as those made from bamboo or kenaf.

The sulfite process produces wood pulp that is almost pure cellulose fibers by treating wood chips with solutions of sulfite and bisulfite ions. These chemicals cleave the bonds between the cellulose and lignin components of the lignocellulose. A variety of sulfite/bisulfite salts are used, including sodium (Na+), calcium (Ca2+), potassium (K+), magnesium (Mg2+), and ammonium (NH4+). The lignin is converted to lignosulfonates, which are soluble and can be separated from the cellulose fibers. For the production of cellulose, the sulfite process competes with the Kraft process which produces stronger fibers and is less environmentally costly.

<span class="mw-page-title-main">Paper</span> Material for writing, printing, etc.

Paper is a thin sheet material produced by mechanically or chemically processing cellulose fibres derived from wood, rags, grasses, or other vegetable sources in water, draining the water through a fine mesh leaving the fibre evenly distributed on the surface, followed by pressing and drying. Although paper was originally made in single sheets by hand, almost all is now made on large machines—some making reels 10 metres wide, running at 2,000 metres per minute and up to 600,000 tonnes a year. It is a versatile material with many uses, including printing, painting, graphics, signage, design, packaging, decorating, writing, and cleaning. It may also be used as filter paper, wallpaper, book endpaper, conservation paper, laminated worktops, toilet tissue, currency, and security paper, or in a number of industrial and construction processes.

Dissolving pulp, also called dissolving cellulose, is bleached wood pulp or cotton linters that has a high cellulose content. It has special properties including a high level of brightness and uniform molecular-weight distribution. This pulp is manufactured for uses that require a high chemical purity, and particularly low hemicellulose content, since the chemically similar hemicellulose can interfere with subsequent processes. Dissolving pulp is so named because it is not made into paper, but dissolved either in a solvent or by derivatization into a homogeneous solution, which makes it completely chemically accessible and removes any remaining fibrous structure. Once dissolved, it can be spun into textile fibers, or chemically reacted to produce derivatized celluloses, such cellulose triacetate, a plastic-like material formed into fibers or films, or cellulose ethers such as methyl cellulose, used as a thickener.

Soda pulping is a chemical process for making wood pulp with sodium hydroxide as the cooking chemical. In the Soda-AQ process, anthraquinone (AQ) may be used as a pulping additive to decrease the carbohydrate degradation. The soda process gives pulp with lower tear strength than other chemical pulping processes, but has still limited use for easily pulped materials like straw and some hardwoods.

<span class="mw-page-title-main">Environmental effects of paper</span> Overview about the environmental effects of the paper production industry

The environmental effects of paper are significant, which has led to changes in industry and behaviour at both business and personal levels. With the use of modern technology such as the printing press and the highly mechanized harvesting of wood, disposable paper became a relatively cheap commodity, which led to a high level of consumption and waste. The rise in global environmental issues such as air and water pollution, climate change, overflowing landfills and clearcutting have all lead to increased government regulations. There is now a trend towards sustainability in the pulp and paper industry as it moves to reduce clear cutting, water use, greenhouse gas emissions, fossil fuel consumption and clean up its influence on local water supplies and air pollution.

<span class="mw-page-title-main">Cellulose fiber</span> Fibers made with ethers or esters of cellulose

Cellulose fibers are fibers made with ethers or esters of cellulose, which can be obtained from the bark, wood or leaves of plants, or from other plant-based material. In addition to cellulose, the fibers may also contain hemicellulose and lignin, with different percentages of these components altering the mechanical properties of the fibers.

White liquor is a strongly alkaline solution mainly of sodium hydroxide and sodium sulfide. It is used in the first stage of the Kraft process in which lignin and hemicellulose are separated from cellulose fiber for the production of pulp. The white liquor breaks the bonds between lignin and cellulose. It is called white liquor due to its white opaque colour.

In industrial paper-making processes, organosolv is a pulping technique that uses an organic solvent to solubilise lignin and hemicellulose. It has been considered in the context of both pulp and paper manufacture and biorefining for subsequent conversion of cellulose to fuel ethanol. The process was invented by Theodor Kleinert in 1968 as an environmentally benign alternative to kraft pulping.

<span class="mw-page-title-main">Paper chemicals</span> Chemicals used in paper manufacturing

Paper chemicals designate a group of chemicals that are used for paper manufacturing, or modify the properties of paper. These chemicals can be used to alter the paper in many ways, including changing its color and brightness, or by increasing its strength and resistance to water. The chemicals can be defined on basis of their usage in the process.

Mechanical pulping is the process in which wood is separated or defibrated mechanically into pulp for the paper industry.

References

  1. ↑ Quanz, Meaghan E.; Walker, Tony R.; Oakes, Ken; Willis, Rob (2021-04-01). "Contaminant characterization in wetland media surrounding a pulp mill industrial effluent treatment facility". Wetlands Ecology and Management. 29 (2): 209–229. Bibcode:2021WetEM..29..209Q. doi: 10.1007/s11273-020-09779-0 . ISSN   1572-9834.
  2. ↑ Lucas, Adam (2006), Wind, Water, Work: Ancient and Medieval Milling Technology, Brill Publishers, pp. 65 & 84, ISBN   978-90-04-14649-5
  3. ↑ Biermann, Christopher J. (1993). Essentials of Pulping and Papermaking . San Diego: Academic Press, Inc. ISBN   978-0-12-097360-6.
  4. ↑ Eero Sjöström (1993). Wood Chemistry: Fundamentals and Applications. San Diego: Academic Press. ISBN   978-0-12-647481-7.
  5. ↑ Rainey, Thomas; Covey, Geoff (16 May 2016). Sugarcane-based Biofuels and Bioproducts: Pulp and Paper Production from Sugarcane Bagasse. John Wiley & Sons. p. 259. ISBN   978-1118719916.
  6. ↑ Hoffman, Emma; Guernsey, Judith R.; Walker, Tony R.; Kim, Jong Sung; Sherren, Kate; Andreou, Pantelis (2017-09-01). "Pilot study investigating ambient air toxics emissions near a Canadian kraft pulp and paper facility in Pictou County, Nova Scotia". Environmental Science and Pollution Research. 24 (25): 20685–20698. Bibcode:2017ESPR...2420685H. doi:10.1007/s11356-017-9719-5. ISSN   1614-7499. PMID   28712086.
  7. ↑ Hoffman, Emma; Lyons, James; Boxall, James; Robertson, Cam; Lake, Craig B.; Walker, Tony R. (2017-05-06). "Spatiotemporal assessment (quarter century) of pulp mill metal(loid) contaminated sediment to inform remediation decisions". Environmental Monitoring and Assessment. 189 (6): 257. Bibcode:2017EMnAs.189..257H. doi:10.1007/s10661-017-5952-0. ISSN   1573-2959. PMID   28478542.
  8. ↑ Hoffman, Emma; Alimohammadi, Masi; Lyons, James; Davis, Emily; Walker, Tony R.; Lake, Craig B. (2019-08-23). "Characterization and spatial distribution of organic-contaminated sediment derived from historical industrial effluents". Environmental Monitoring and Assessment. 191 (9): 590. Bibcode:2019EMnAs.191..590H. doi:10.1007/s10661-019-7763-y. ISSN   1573-2959. PMID   31444645.
  9. ↑ "Uses of lignosulphonates". Archived from the original on 2007-10-09. Retrieved 2007-10-07.
  10. ↑ Gunnarsson, H., Rönnqvist, M. (2008). Solving a multi-period supply chain problem for a pulp company using heuristics - an application to Södra Cell ab. International Journal of Production Economics 116 (1): 75-94
  11. ↑ Klugman, S., Karlsson, M., Moshfegh, B. (2009). A Swedish integrated pulp and paper mill energy optimization and local heat cooperation. Energy Policy 37 (7): 2514-2524
  12. 1 2 A. H. Tuthill (2002). "Stainless Steels and Specialty Alloys for Modern Pulp and Paper Mills". Nickel Institute. Archived from the original on 2018-01-16. Retrieved 2018-01-17.
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