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{{Short description|Process of converting wood into wood pulp}}
[[File:InternationalPaper6413.jpg|thumb|right|[[International Paper]]: Kraft paper mill]]
[[File:Woodchips for paper production.jpg|thumb|Woodchips for paper production]]

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]] (Na<sub>2</sub>S), 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 [[waste]]s.<ref>Hoffman, E., Lyons, J., Boxall, J., Robertson, C., Lake, C. B., & Walker, T. R. (2017). Spatiotemporal assessment (quarter century) of pulp mill metal (loid) contaminated sediment to inform remediation decisions. Environmental Monitoring and Assessment, 189(6), 257.</ref><ref>Hoffman, E., Bernier, M., Blotnicky, B., Golden, P. G., Janes, J., Kader, A., ... & Walker, T. R. (2015). Assessment of public perception and environmental compliance at a pulp and paper facility: a Canadian case study. Environmental Monitoring and Assessment, 187(12), 766.</ref><ref>Rudolf Patt et al. "Paper and Pulp" in Ullmann's Encyclopedia of Industrial Chemistry 2002 Wiley-VCH, Weinheim. {{doi|10.1002/14356007.a18_545.pub4}}</ref>

The process name is derived from the German word ''[[wikt:Kraft#German|Kraft]]'', meaning "strength" in this context, due to the strength of the [[kraft paper]] produced using this process.<ref>Both capitalized and lowercase spelling ("Kraft process" and "kraft process") appear in the literature, but "kraft" is most commonly used in the [[pulp and paper industry]].</ref>

==History==
[[File:Papelparamolde.jpg|thumb|A roll of kraft paper]]
A precursor of the kraft process was used during the [[Napoleonic Wars]] in England.<ref>{{Cite book|url=https://books.google.com/books?id=fIdtAAAAIAAJ&q=England+Sulphur|title = Pulp and Paper Manufacture, vol. 1 : The Pulping of Wood |date = 1969|publisher = [[McGraw Hill]]|edition = 2nd|volume = 1|editor-first1 = Ronald Gordon|editor-last1 = Macdonald|editor-first2 = John N.|editor-last2 = Franklin|author = Joint Textbook Committee of the Paper Industry |oclc = 867778880}}</ref> The kraft process was invented by [[Carl F. Dahl]] in 1879 in [[Gdańsk|Danzig]], [[Prussia]], [[German Empire|Germany]]. {{US patent|296935}} was issued in 1884, and a pulp mill using this technology began in [[Sweden]] in 1890.<ref name="bier">{{cite book |last=Biermann |first=Christopher J. |title=Essentials of Pulping and Papermaking |year=1993 |publisher=Academic Press, Inc. |location=San Diego |isbn=0-12-097360-X |url-access=registration |url=https://archive.org/details/essentialsofpulp0000bier }}</ref> The invention of the [[recovery boiler]] by G. H. Tomlinson in the early 1930s was a milestone in the advancement of the kraft process.<ref name="eero">{{cite book |author= E. Sjöström |title= Wood Chemistry: Fundamentals and Applications |publisher= [[Academic Press]]|year= 1993 |isbn= 0-12-647480-X}}</ref> It enabled the recovery and reuse of the inorganic pulping chemicals such that a kraft mill is a nearly closed-cycle process with respect to inorganic chemicals, apart from those used in the bleaching process. For this reason, in the 1940s, the kraft process superseded the [[sulfite process]] as the dominant method for producing wood pulp.<ref name="bier"/>

==The process==

===Impregnation===
Common [[wood chips]] used in pulp production are {{convert|12|-|25|mm|in}} long and {{convert|2|-|10|mm|in}} thick. The chips are first wetted and preheated with [[steam]]. Cavities inside fresh wood chips are partly filled with liquid and partly with air. The steam treatment causes the air to expand and about 25% of the air to be expelled from the chips. The next step is to saturate the chips with [[Black liquor|black]] and [[white liquor]]. Air remaining in chips at the beginning of liquor impregnation is trapped within the chips. The impregnation can be done before or after the chips enter the digester and is normally done below {{convert|100|°C|°F}}. During impregnation, cooking liquors penetrate into the capillary structure of the chips and low temperature chemical reactions with the wood begin. A good impregnation is important to get a homogeneous cook and low rejects. About 40–60% of all alkali consumption, in the continuous process, occurs in the impregnation zone.

===Cooking===
The wood chips are then cooked in pressurized digesters. Some digesters operate in a batch manner and some in a continuous process. Digesters producing 1,000 tonnes or more of pulp per day are common, with the largest producing more than 3,500 tonnes per day.<ref name="epa">{{cite web |url = https://nepis.epa.gov/Exe/ZyPURL.cgi?Dockey=9100PTP2.txt |title=Pollution Prevention Technologies for the Bleached Kraft Segment of the U.S. Pulp and Paper Industry|access-date=2022-02-06 |last=Woodman |first=Jocelyn |year=1993 |publisher=U.S. Environmental Protection Agency|page = 66}}</ref> Typically, delignification requires around two hours<ref>{{cite web
|url=http://biorefinery.utk.edu/technical_reviews/Basics%20of%20Kraft%20Pulping.pdf
|title=Basics of Kraft Pulping
|publisher=University of Tennessee Knoxville - Department of Chemical and Biomolecular Engineering
|access-date=2021-10-23
|archive-date=2021-10-23
|archive-url=https://web.archive.org/web/20211023194838/http://biorefinery.utk.edu/technical_reviews/Basics%20of%20Kraft%20Pulping.pdf
|url-status=dead
}}</ref> at {{convert|170|to|176|°C|°F}}. Under digesting conditions, lignin and [[hemicellulose]] degrade to give fragments that are soluble in the strongly basic liquid. The solid pulp (about 50% by weight of the dry wood chips) is collected and washed. At this point the pulp is known as ''brown stock'' because of its color. The combined liquids, known as [[black liquor]] (because of its color), contain lignin fragments, [[carbohydrate]]s from the breakdown of hemicellulose, [[sodium carbonate]], [[sodium sulfate]] and other inorganic salts.

[[Image:KraftEq.png|250px|thumb|right|net reaction in depolymerization of lignin by SH<sup>−</sup> (Ar = [[aromaticity|aryl]], R = [[alkyl]] groups).]]
One of the main chemical reactions that underpin the kraft process is the scission of ether bonds by the [[nucleophilic]] [[sulfide]] (S<sup>2−</sup>) or [[bisulfide]] (HS<sup>−</sup>) ions.<ref name="eero"/>

===Recovery process===
The excess black liquor contains about 15% solids and is concentrated in a [[Multiple-effect evaporator|multiple effect evaporator]]. After the first step the black liquor has about 20–30% solids. At this concentration the [[rosin]] soap rises to the surface and is [[Skimmer (machine)|skimmed]] off. The collected soap is further processed to [[tall oil]]. Removal of the soap improves the evaporation operation of the later effects.

The weak black liquor is further evaporated to 65% or even 80% solids ("heavy black liquor"<ref>{{cite web|url=http://www.sealspecialists.net/black%20liquor.htm |archive-url=https://web.archive.org/web/20050420032421/http://www.sealspecialists.net/black%20liquor.htm |url-status=dead |archive-date=2005-04-20 |title=Equipment to handle heavy black liquor |access-date=2007-10-09 }}</ref>) and burned in the [[recovery boiler]] to recover the inorganic chemicals for reuse in the pulping process. Higher solids in the concentrated black liquor increases the energy and chemical efficiency of the recovery cycle, but also gives higher viscosity and precipitation of solids (plugging and fouling of equipment).<ref>{{cite web |url=http://thor.lib.chalmers.se/inst_fack/kurser/keml/TK/materials/dc220.pdf |title=Second Critical Solids Black Liquor Scaling |access-date=2007-10-09 |last=Hsieh |first=Jeffery S. |author2=Smith, Jason B. |publisher=Pulp and Paper Engineering, School of Chemical Engineering, Georgia Institute of Technology |url-status=dead |archive-url=https://web.archive.org/web/20110831033707/http://thor.lib.chalmers.se/inst_fack/kurser/keml/TK/materials/dc220.pdf |archive-date=2011-08-31 }}</ref><ref>{{Cite patent
 |country=US |number=5527427 |status=granted |title=High solids black liquor of reduced viscosity and viscosity reduction method for high solids black liquor |gdate=1996-06-18 |invent1=Mualla Berksoy |invent2=Yaman Boluk |assign1=Optima Specialty Chemicals & Technology Inc}}</ref> During combustion, sodium sulfate is [[redox|reduced]] to sodium sulfide by the organic carbon in the mixture:

:1. Na<sub>2</sub>SO<sub>4</sub> + 2 C → Na<sub>2</sub>S + 2 CO<sub>2</sub>

This reaction is similar to [[thermochemical sulfate reduction]] in geochemistry.

The molten salts ("smelt") from the recovery boiler are dissolved in a process water known as "weak wash". This process water, also known as "weak white liquor" is composed of all liquors used to wash lime mud and [[green liquor]] precipitates. The resulting solution of sodium carbonate and sodium sulfide is known as "green liquor". The green liquor's eponymous green colour arises from the presence of colloidal iron sulfide.<ref name="Pump Handbook">{{cite book|title=Pump Handbook|year=2008|publisher=McGraw Hill|location=New York|isbn=9780071460446|url=http://accessengineeringlibrary.com/browse/pump-handbook-fourth-edition|edition=4th|author1=Giddings, J.F.|author2=Roll, D.R.|author3=Cappellino, C.A.|author4=Day, M.|author5=Nardone, R.A.|editor=Karassik, I.|editor-link=Igor Karassik|editor2=Messina, J.|editor3=Cooper, P.|editor4=Heald, C.|accessdate=February 21, 2013|chapter=12.12.4.|archive-date=March 4, 2013|archive-url=https://web.archive.org/web/20130304154803/http://accessengineeringlibrary.com/browse/pump-handbook-fourth-edition|url-status=dead}}</ref> This liquid is then mixed with [[calcium oxide]], which becomes [[calcium hydroxide]] in solution, to regenerate the white liquor used in the pulping process through an equilibrium reaction (Na<sub>2</sub>S is shown since it is part of the green liquor, but does not participate in the reaction):

:2. Na<sub>2</sub>CO<sub>3</sub> + Ca(OH)<sub>2</sub> ←→ 2 NaOH + CaCO<sub>3</sub>

[[Calcium carbonate]] precipitates from the white liquor and is recovered and heated in a [[lime kiln]] where it is converted to [[calcium oxide]] (lime).

:3. CaCO<sub>3</sub> → CaO + CO<sub>2</sub>

Calcium oxide (lime) is reacted with water to regenerate the calcium hydroxide used in Reaction 2:

:4. CaO + H<sub>2</sub>O → Ca(OH)<sub>2</sub>

The combination of reactions 1 through 4 form a closed cycle with respect to sodium, sulfur and calcium and is the main concept of the so-called recausticizing process where [[sodium carbonate]] is reacted to regenerate [[sodium hydroxide]].

The recovery boiler also generates high pressure steam which is fed to turbogenerators, reducing the steam pressure for the mill use and generating [[electricity]]. A modern kraft pulp mill is more than self-sufficient in its electrical generation and normally will provide a net flow of energy which can be used by an associated paper mill or sold to neighboring industries or communities through to the local electrical grid.<ref>{{cite web
 |url=http://www2.biotech.wisc.edu/jeffries/bioprocessing/pulping.html 
 |title=Kraft pulping: Energy consumption and production 
 |access-date=2007-10-21 
 |last=Jeffries 
 |first=Tom 
 |date=March 27, 1997 
 |publisher=University of Wisconsin Biotech Center
 |url-status=dead 
 |archive-url=https://web.archive.org/web/20110928021005/http://www2.biotech.wisc.edu/jeffries/bioprocessing/pulping.html 
 |archive-date=September 28, 2011 
}}</ref> Additionally, bark and wood residues are often burned in a separate power boiler to generate steam.

Although recovery boilers using G.H. Tomlinson's invention have been in general use since the early 1930s, attempts have been made to find a more efficient process for the recovery of cooking chemicals. [[Weyerhaeuser]] has operated a [[Chemrec]] first generation [[black liquor]] [[gasification|entrained flow gasifier]] successfully at its [[New Bern]] plant in [[North Carolina]], while a second generation plant is run in pilot scale at [[Smurfit Kappa|Smurfit Kappa's]] plant in [[Piteå]], [[Sweden]].<ref>{{Cite web |url=http://chemrec.se/Chemrec-plants.aspx |title=Chemrec web site |access-date=2011-01-07 |archive-url=https://archive.today/20120709130105/http://chemrec.se/Chemrec-plants.aspx |archive-date=2012-07-09 |url-status=dead }}</ref>

An additional technology is employed to lower the use of lime.  In "partial borate autocausticizing" (PBAC), boric acid is added which produces sodium borate in place of sodium carbonate.<ref name=UllBO>{{cite book |doi=10.1002/14356007.a04_263.pub2 |chapter=Boric Oxide, Boric Acid, and Borates |title=Ullmann's Encyclopedia of Industrial Chemistry |date=2015 |last1=Schubert |first1=David M. |pages=1–32 |isbn=978-3-527-30385-4 }}</ref>

===Blowing===
The finished cooked wood chips are blown to a collection tank called a blow tank that operates at atmospheric pressure. This releases a lot of steam and volatiles. The volatiles are condensed and collected; in the case of northern [[softwood]]s this consists mainly of raw [[turpentine]].

===Screening===
Screening of the pulp after pulping is a process whereby the pulp is separated from large [[Shives (papermaking)|shives]], [[Knots (papermaking)|knots]], dirt and other debris. The ''accept'' is the pulp. The material separated from the pulp is called ''reject''.

The screening section consists of different types of [[sieve]]s (screens) and centrifugal cleaning. The sieves are normally set up in a multistage cascade operation because considerable amounts of good fibres can go to the reject stream when trying to achieve maximum purity in the accept flow.

The fiber containing shives and knots are separated from the rest of the reject and reprocessed either in a refiner or is sent back to the digester. The content of knots is typically 0.5–3.0% of the digester output, while the shives content is about 0.1–1.0%.

===Washing===
The brownstock from the blowing goes to the washing stages where the used cooking liquors are separated from the cellulose fibers. Normally a pulp mill has 3-5 washing stages in series. Washing stages are also placed after oxygen delignification and between the bleaching stages as well. Pulp washers use [[countercurrent flow]] between the stages such that the pulp moves in the opposite direction to the flow of washing waters. Several processes are involved: [[thickening]] / [[Concentration|dilution]], [[displacement (fluid)|displacement]] and [[diffusion]]. The '''dilution factor''' is the measure of the amount of water used in washing compared with the theoretical amount required to displace the liquor from the thickened pulp. Lower dilution factor reduces energy consumption, while higher dilution factor normally gives cleaner pulp. Thorough washing of the pulp reduces the chemical oxygen demand ([[chemical oxygen demand|COD]]).

Several types of washing equipment are in use:
* Pressure diffusers
* Atmospheric diffusers
* Vacuum drum washers
* Drum displacers
* Wash presses

===Bleaching===
{{Main|Bleaching of wood pulp}}

In a modern mill, brownstock (cellulose fibers containing approximately 5% residual lignin) produced by the pulping is first washed to remove some of the dissolved organic material and then further delignified by a variety of [[bleaching of wood pulp|bleaching]] stages.<ref>{{cite web|url=http://www.environmentaldefense.org/pdf.cfm?ContentID=1626&FileName=WP5.pdf|title=Environmental Comparison of Bleached Kraft Pulp ManufacturingTechnologies|access-date=2007-09-28|url-status=dead|archive-url=https://web.archive.org/web/20041218171736/http://www.environmentaldefense.org/pdf.cfm?ContentID=1626&FileName=WP5.pdf|archive-date=2004-12-18}}</ref>

In the case of a plant designed to produce pulp to make brown sack paper or linerboard for boxes and packaging, the pulp does not always need to be bleached to a high brightness. Bleaching decreases the mass of pulp produced by about 5%, decreases the strength of the fibers and adds to the cost of manufacture.

===Process chemicals===
Process chemicals are added to improve the production process:
* Impregnation aids. [[Surfactant]]s may be used to improve impregnation of the wood chips with the cooking liquors.
* [[Anthraquinone]] is used as a digester additive. It works as a [[redox]] [[catalyst]] by oxidizing [[cellulose]] and reducing [[lignin]]. This protects the cellulose from degradation and makes the lignin more water-soluble.<ref>{{cite book |last=Goyal|first=Gopal C. |title=Anthraquinone Pulping. A TAPPI Press Anthology of Published Papers, 1977-1996 |year=1997 |publisher=TAPPI Press |location=Atlanta |isbn=0-89852-340-0}}</ref>
* An [[flocculant|emulsion breaker]] can be added in the soap separation to speed up and improve the separation of soap from the used cooking liquors by flocculation.<ref>{{Cite web |url=http://wcm.paprican.ca/wcmpaprican/publishing.nsf/AttachmentsByTitle/BO_Tall_Oil_Soap_PDF_Eng/$FILE/0225-E-TallOilSoapRecovery.pdf |title=Tall Oil and Tall Oil Soap Recovery |access-date=2008-12-19 |archive-url=https://web.archive.org/web/20120305151739/http://wcm.paprican.ca/wcmpaprican/publishing.nsf/AttachmentsByTitle/BO_Tall_Oil_Soap_PDF_Eng/$FILE/0225-E-TallOilSoapRecovery.pdf |archive-date=2012-03-05 |url-status=dead }}</ref>
* [[Defoamer]]s remove foam and speed up the production process. Drainage of washing equipment is improved and gives cleaner pulp.
* [[Dispersing agent]]s, [[detackifier]]s and [[complexing agent]]s keep the system cleaner and reduce the need for maintenance stops.
* Fixation agents fix finely dispersed potential deposits to the fibers and thereby transport them out of the process.

==Comparison with other pulping processes==
Pulp produced by the kraft process is stronger than that made by other [[Pulp (paper)#Pulping|pulping processes]] and maintains a high effective sulfur ratio (sulfidity), an important determiner of the strength of the paper. Acidic [[sulfite process]]es degrade cellulose more than the kraft process, which leads to weaker fibers. Kraft pulping removes most of the lignin present originally in the wood whereas mechanical pulping processes leave most of the [[lignin]] in the fibers. The [[hydrophobic]] nature of lignin<ref>{{cite journal |last=Hubbe |first=Martin a. |author2=Lucian A. Lucia  |year=2007 |title=The "Love-Hate" Relationship Present in Lignocellulosic Materials |journal=BioResources |volume=2 |issue=4 |pages=534–535 |doi=10.15376/biores.2.4.534-535 |url=http://ojs.cnr.ncsu.edu/index.php/BioRes/article/view/BioRes_2_4_534_535_Hubbe_Lucia_Love_and_Hate_Lignocellulosics |access-date= 2015-02-03 |doi-access=free }}</ref> interferes with the formation of the [[hydrogen bonds]] between cellulose (and hemicellulose) in the fibers needed for the strength of paper<ref name="bier"/> (strength refers to [[tensile strength]] and resistance to tearing).

Kraft pulp is darker than other wood pulps, but it can be bleached to make very white pulp. Fully bleached kraft pulp is used to make high-quality paper where strength, whiteness, and resistance to yellowing are important.

The kraft process can use a wider range of fiber sources than most other pulping processes. All types of wood, including very resinous types like southern [[pine]],<ref>{{cite web |url=http://www.fpl.fs.fed.us/documnts/usda/amwood/256spine.pdf |title=The Southern Pines |access-date=2007-09-13 |year=1985 |publisher=US Department of Agriculture |archive-date=2007-07-09 |archive-url=https://web.archive.org/web/20070709235422/http://www.fpl.fs.fed.us/documnts/usda/amwood/256spine.pdf |url-status=dead }}</ref> and non-wood species like [[bamboo]] and [[kenaf]] can be used in the kraft process.

==Byproducts and emissions==
{{Further|Environmental impact of paper}}
[[File:Forchem Rauma 2.jpg|thumb|Forchem tall oil refinery in [[Rauma, Finland|Rauma]], Finland.]]
The main byproducts of kraft pulping are crude sulfate [[turpentine]] and [[tall oil]] soap. The availability of these is strongly dependent on wood species, growth conditions, storage time of logs and chips, and the mill's process.<ref name="woodchem">{{cite book |editor1-first= Per |editor1-last= Stenius |title= Forest Products Chemistry |series= Papermaking Science and Technology |volume= 3|year= 2000 |publisher= Fapet OY |location= Helsinki, Finland |isbn= 952-5216-03-9 |pages= 73–76|chapter= 2 }}.</ref> [[Pine]]s are the most extractive-rich woods. The raw turpentine is [[volatility (chemistry)|volatile]] and is distilled off the digester, while the raw [[soap]] is separated from the spent [[black liquor]] by [[decantation]] of the soap layer formed on top of the liquor storage tanks. From pines the average yield of turpentine is 5–10&nbsp;kg/t pulp and of crude tall oil is 30–50&nbsp;kg/t pulp.<ref name="woodchem" />

Various byproducts containing [[hydrogen sulfide]], [[methyl mercaptan]], [[dimethyl sulfide]], [[dimethyl disulfide]], and other volatile [[sulfur]] compounds are the cause of the malodorous air emissions characteristic for [[pulp mill]]s utilizing the kraft process.<ref>{{cite journal |doi=10.1080/00022470.1962.10468107 |title=Odor ''and'' Fallout Control ''in a Kraft Pulp Mill'' |date=1962 |last1=Hansen |first1=G. A. |journal=Journal of the Air Pollution Control Association |volume=12 |issue=9 |pages=409–436 |pmid=13904415 }}</ref><ref>Hoffman, E., Guernsey, J. R., Walker, T. R., Kim, J. S., Sherren, K., & Andreou, P. (2017). 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.</ref> The [[sulfur dioxide]] emissions of kraft-pulp mills are much lower than those from sulfite mills. In the ambient air outside a typical modern kraft-pulp mill, the sulfur-dioxide odour is perceivable only during disturbance situations, for example when the mill is shut down for a maintenance break, or when an extended power outage occurs. Control of odours is achieved through the collection and burning of these odorous gases in the recovery boiler alongside the black liquor. In modern mills, where well-dried solids are burned in the recovery boiler, hardly any [[sulfur dioxide]] leaves the boiler. At high boiler temperatures, the sodium released from the black liquor droplets reacts with sulfur dioxide, thereby effectively scavenging it by forming odourless [[sodium sulfate]] crystals.
  
Pulp mills are almost always located near large bodies of water due to their substantial demand for water. Delignification of chemical pulps releases considerable amounts of organic material into the environment, particularly into rivers or lakes. The wastewater effluent can also be a major source of pollution, containing lignins from the trees, high [[biochemical oxygen demand|biological oxygen demand]] (BOD) and [[dissolved organic carbon]] (DOC), along with [[Alcohol (chemistry)|alcohol]]s, [[chlorate]]s, heavy metals, and [[Chelation|chelating]] agents. The process effluents can be treated in a biological [[Industrial wastewater treatment|effluent treatment plant]], which can substantially reduce their toxicity.<ref>Hoffman, E., Bernier, M., Blotnicky, B., Golden, P. G., Janes, J., Kader, A., ... & Walker, T. R. (2015). Assessment of public perception and environmental compliance at a pulp and paper facility: a Canadian case study. Environmental monitoring and assessment, 187(12), 766.</ref><ref>Hoffman, E., Lyons, J., Boxall, J., Robertson, C., Lake, C. B., & Walker, T. R. (2017). Spatiotemporal assessment (quarter century) of pulp mill metal (loid) contaminated sediment to inform remediation decisions. Environmental monitoring and assessment, 189(6), 257.</ref>

==See also==
{{div col|colwidth=28em}}
* [[H-factor]]
* [[Johan Richter (inventor)|Johan Richter]] – Inventor of the continuous process for digesting wood pulp
* [[Kappa number]]
* [[Kraft paper]]
* [[Organosolv]]
* [[Paper]]
* [[Paper chemicals]]
* [[Pulp mill]]
* [[Soda pulping]]
* [[Wood pulp]]
{{div col end}}

==References==
{{Reflist|30em}}

==Further reading==
* {{cite book | last = Gullichsen | first = Johan |author2=Carl-Johan Fogelholm  | title = Papermaking science and Technology: 6. Chemical Pulping | publisher = Tappi Press.| year = 2000 | location = Finland | isbn = 952-5216-06-3}}

==External links==
{{Commons category}}
* [https://web.archive.org/web/20090706192109/http://pulpandpapercircle.com/ Pulp and Paper Circle provides industry-related information (especially kraft mills) to pulp and paper engineers and technologists.]
* [http://www.epa.gov/ttn/chief/ap42/ch10/final/c10s02.pdf US EPA article on kraft pulping]
* [https://web.archive.org/web/20070621152420/http://eippcb.jrc.es/pages/FActivities.htm Reference Document on Best Available Techniques in Pulp and Paper Industry by European Commission, 2001]

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