A kraft paper mill in The Pas, Manitoba
Woodchips for paper production

The kraft pulping process, or simply kraft or sulfate process (spelling, sulphate in the U.K. and older literature) is a preeminent industrial process for conversion of wood into wood pulp,[1] a product that consists of almost pure cellulose fibres, the main component of paper.[not verified in body] The process name is derived from the Swedish and German word Kraft, meaning "strength" in this context, due to the characteristic strength of the kraft paper produced using the process.[1][2]

The kraft process involves treatment of wood chips with a heated "white liquor"—a mixture that includes sodium hydroxide and sodium sulfide dissolved in water—that breaks the chemical bonds that link the branched wood heteropolymers lignin and hemicellulose, and homopolymeric cellulose. The process entails several steps, both chemical and mechanical,[not verified in body] and it is the predominant process globally and within the U.S. for producing the wood pulp used in the manufacture of paper.[3][verification needed] In some locales, manufacturing using the process has been controversial because kraft plants release odorous volatiles and produce and store substantial liquid chemical intermediates and waste.[4][5][6][non-primary source needed]

Process overview

The Kraft pulping process is a wood pulp-producing process used in paper manufacture that is amenable to all wood species, and that produces high strength brown pulps (unless bleached).[3]: p.59  The process is highly alkaline (pH 13-14), a consequence of use of a combination of sodium hydroxide (NaOH) and sodium sulfide (Na2S), in a general proportion of 15-25% by weight "on wood" (see following).[3]: p.59  The process is performed in an unlined industrial reaction vessel, the digester, and is associated with a "high recovery of pulping chemicals", but also with sulfurous odors.[3]: p.59  The process is used, unbleached, to produce paper for bags, wrapping, and linerboard, and bleached for various types of white paper products.[3]: p.59  Process yields are more modest than for mechanical and chemi-mechanical pulping methods, but achieve on the order of 65-70% for unbleached paper (closer to 50% when the pulp is destined for bleaching), and after bleaching, overall yields may be on the order of 45% or less.[3]: p.59 

History

A roll of kraft paper

The naming of this process dates to the first production of "kraft paper", in Jõnkoping, Sweden, at the Munksjõ mill, and their manufacturing pulp according to a process patented by C. F. Dahl, of Danzig, in 1885.[1][7] (Sometimes dated to 1890 rather than 1885, the Swedish location of the manufacturing has been attributed to the German paper industry not accepting the new process.[3]: p.86 )

Dahl had been experimenting with the substitution of sodium sulfate ("sulphate"), supplied from "salt cake", for sodium carbonate (supplied from soda ash), as a replacement for "alkali lost in cooking" to produce straw pulp, the sulfate being reduced to the corresponding sulfide ("sulphide"), "in the recovery furnace."[1][3]: p.86  He had begun to experiment with this substitution process in 1879, following U.S. Patents from others, from 1870-1871 (see following), at first with unsatisfactory results, but with eventual greater success and extension of the process to include coniferous woods.[1] The successful mill production was reported to be the result of an insightful response of the Munksjõ mill manager to operator error—a sulfate digester run having been terminated ("blown") prematurely—to which he responded by processing rather than discarding the chips perceived incompletely digested, to discover that the resulting pulp gave a paper "dark in color [that] was far stronger than any paper hitherto known".[1] Its strength led to the Swedish-German word for that characteristic, kraft, being applied to the process (and so the sulfate/sulfide addition to the earlier alkali process became known as the kraft or sulfate/sulphate process).[1]

The generated mixture of sodiums hydroxide and sulfide to pulp the wood, with the sulfide "accelerat[ing] the delignification", led to shortened chip exposures to hot alkali (relative to the earlier soda process), with the greater observed strength of kraft pulp over soda pulp (and less carbohydrate degradation) being attributed to this reduced alkali exposure.[1][3]: p.86  The crediting of Dahl as discoverer is complicated by the existence of two earlier patents on sulfide use in pulping wood from A. K. Eaton, in August 1870 and September 1871, more than a decade before Dahl's patent.[1] Moreover, there is discussion in the primary and secondary manufacturing literatures in England, in the 1940s-1960s, suggesting that the addition of "sulphur and sulphides" to procedures for the pulping of straw using alkali dated to "experiments in England during the Napoleonic Wars (i.e., 1805-1814)".[1] Even so, Dahl is, in particular, credited with establishing the recovery process for the chemicals used in the kraft process, which is considered by at least one scholar as "perhaps more important than the kraft cooking process" itself.[3]: p.86 

The expanding use of the process in the United States, especially in southern states where the pulping of chips from "resinous pine species" was not well suited to the earlier sulfite process (in particular, using a calcium base), was marked in the period from 1915-1930.[3]: p.86  The invention of the recovery boiler by G. H. Tomlinson in the early 1930s,[where?] was a further milestone in the development and expanded use of the kraft process.[8][page needed] 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.[citation needed] For this reason, by 1993, the kraft process was being described as "the dominant process" for producing wood pulp.[3]: p.86 [needs update] (The kraft process superseded the sulfite process as the dominant process in the 1940s.[citation needed])

Detailed process

Impregnation

Common wood chips used in pulp production are 12–25 millimetres (0.47–0.98 in) long and 2–10 millimetres (0.079–0.394 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 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 100 °C (212 °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.[citation needed] Some digesters operate in a batch manner and some in a continuous process.[citation needed]

As of the work of McLeod in 1992, reported by the EPA in 1993,[needs update] the mean capacity of "extended cooking installations", globally, was >1,100 tons per day, with the largest facility tabulated, the 1992-launched Union Camp in Savannah, Georgia, itself producing 2,450 tons per day.[9] Typically, the conditions for the stage of delignification involving this caustic white liquor treatment taking place in the large pressure vessel termed the digester include heating to 170°C, for a period of time dependent upon "the degree of delignification desired" (perhaps on the order of two hours).[10] One of the main chemical reactions that underpins the kraft process is the scission of ether bonds by the nucleophilic sulfide (S2−) or bisulfide (HS−) ions.[8][page needed]

At 170 to 176 °C (338 to 349 °F),[citation needed] under these white liquor "cooking" conditions, lignin and hemicellulose degrade to give fragments that are soluble in the strongly alkaline liquid.[citation needed] The solid pulp—about 50% by weight of the dry wood chips—is collected and washed.[citation needed] The pulp is known as "brown stock" at this point, and the combined liquids as "black liquor", both because of their colors.[citation needed] The mixture contains lignin fragments, carbohydrates from the breakdown of hemicellulose, sodium carbonate, sodium sulfate, and other inorganic salts.[citation needed]

Recovery process

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

The weak black liquor is further evaporated to 65% or even 80% solids ("heavy black liquor"[11][full citation needed]) 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).[12][13] During combustion, sodium sulfate is reduced to sodium sulfide by the organic carbon in the mixture:[citation needed]

1. Na2SO4 + 2 C → Na2S + 2 CO2

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

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", owing its eponymous green colour to the presence of colloidal iron sulfide.[14] 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; Na2S is shown, since it is part of the green liquor (but does not participate in the reaction):[citation needed]

2. Na2CO3 + Ca(OH)2 ←→ 2 NaOH + CaCO3

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

3. CaCO3 → CaO + CO2

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

4. CaO + H2O → Ca(OH)2

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.[citation needed]

The recovery boiler also generates high pressure steam which is fed to turbogenerators, reducing the steam pressure for the mill use and generating electricity.[citation needed]

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.[15] Additionally, bark and wood residues are often burned in a separate power boiler to generate steam.[citation needed]

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 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's plant in Piteå, Sweden.[16][full citation needed]

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.[17]

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 softwoods this consists mainly of raw turpentine.

Screening

Screening of the pulp after pulping is a process whereby the pulp (called accept) is separated from large shives, knots, dirt and other debris (called reject).

The screening section consists of different types of sieves (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 / dilution, 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 (COD).

Several types of washing equipment are in use, including pressure and atmospheric diffusers;[clarification needed] vacuum drum washers;[clarification needed] drum displacers;[clarification needed] and wash presses.[clarification needed][citation needed]

Bleaching

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 stages.[18][full citation needed]

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.[citation needed] Bleaching decreases the mass of pulp produced by about 5%, decreases the strength of the fibers and adds to the cost of manufacture.[citation needed]

Process chemicals

Process chemicals are added to improve the production process:

Comparison with other pulping processes

Pulp produced by the kraft process is stronger than that made by other pulping processes[citation needed]—it maintains a high effective sulfur ratio (sulfidity), an important determiner of the strength of the paper.[citation needed] Acidic sulfite processes degrade cellulose more than the kraft process, leading to weaker fibers.[citation needed]

As well, Kraft pulping removes most of the lignin originally present in the wood, whereas mechanical pulping processes leave most of the lignin in the fibers.[citation needed] Lignin is hydrophobic in nature,[21] and that hydrophobicity interferes with the formation of the hydrogen bonds between cellulose (and hemicellulose) in the fibers, which is needed for the strength of paper.[3][verification needed] (Strength refers to tensile strength and resistance to tearing.[citation needed])

Kraft pulp is darker than other wood pulps, but it can be bleached to make very white pulp.[citation needed] Fully bleached kraft pulp is used to make high-quality paper where strength, whiteness, and resistance to yellowing are important.[citation needed] Kraft pulp's high cellulose and low lignin content is suited for paper for printing and writing,[22][full citation needed] In contrast, mechanical pulp is produced by grinding wood without removing lignin.[23][full citation needed] The lignin retained in mechanical pulp provides the opacity and bulk necessary for newsprint, but it results in lower tensile strength and limits the pulp's use in higher-grade applications.[24]

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,[25] and non-wood species like bamboo and kenaf can be used in the kraft process.

Byproducts and emissions

Forchem tall oil refinery in 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.[26] Pines are the most extractive-rich woods. The raw turpentine is 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 kg/t pulp and of crude tall oil is 30–50 kg/t pulp.[26]

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 mills utilizing the kraft process.[27][28]

The sulfur dioxide emissions of kraft-pulp mills are much lower than those from sulfite mills.[citation needed] In the ambient air outside a typical modern kraft-pulp mill, the sulfur-dioxide odour is perceivable only during disturbance situations,[citation needed] for example when the mill is shut down for a maintenance break, or when an extended power outage occurs.[citation needed] Control of odours is achieved through the collection and burning of these odorous gases in the recovery boiler alongside the black liquor.[dubious – discuss][citation needed] In modern mills, where well-dried solids are burned in the recovery boiler, hardly any sulfur dioxide leaves the boiler.[citation needed] 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.[citation needed][contradictory]

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 biological oxygen demand (BOD) and dissolved organic carbon (DOC), along with alcohols, chlorates, heavy metals, and chelating agents. The process effluents can be treated in a biological effluent treatment plant, which can substantially reduce their toxicity.[4][6][non-primary source needed][5][non-primary source needed]

See also

Further reading

  • Ragnar, M; Henriksson, G; Lindström, ME; Wimby, M; Blechschmidt, J & Heinemann, S (30 May 2014). "Pulp". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim, Germany: Wiley-VCH. doi:10.1002/14356007.a18_545.pub4.{{cite book}}: CS1 maint: multiple names: authors list (link)
  • Blechschmidt, J; Heinemann, S; Putz, H-J; Laufmann, M; Kogler†, W; Gliese, T & Auhorn, WJ (15 January 2012). "Paper and Board, 2. Raw Materials for Paper and Board Manufacture". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim, Germany: Wiley-VCH. doi:10.1002/14356007.o18_o10.{{cite book}}: CS1 maint: multiple names: authors list (link)
  • Patt, R; Kordsachia, O; Süttinger, R (15 October 2011). "Pulp". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim, Germany: Wiley-VCH. doi:10.1002/14356007.a18_545.pub2.{{cite book}}: CS1 maint: multiple names: authors list (link)
  • Gustafsson, J; Alén, R; Engström, J; Korpinen, R; Kuusisto, P; Leavitt, A; Olsson, K; Piira, J; Samuelsson, A & Sundquist, J (2011). "Pulping". In Fardim, Pedro (ed.). Book 6, Chemical pulping. Part 1, Fibre Chemistry and Technology (2nd ed.). Espoo Finland, Peachtree Corners, GA: Paperi ja Puu Oy-FFPEA. pp. 187–381. ISBN 9789525216417. Retrieved 22 June 2026.{{cite book}}: CS1 maint: multiple names: authors list (link) FFPEA abbreviates the English translation of the Finnish Puunjalostus-Insinöörit as Finnish Forest Products Engineers' Association.
  • Paulapuro, H; Gullichsen, J & Fogelholm, C-J (1999). Fardim, Pedro (ed.). Book 6A. Chemical Pulping. Papermaking Science and Technology (1st ed.). Helsinki, Finland: Fapet Oy-TAPPI. ISBN 9525216063.{{cite book}}: CS1 maint: multiple names: authors list (link) See also ISBN 9525216004 at Google Books.

References

  1. 1 2 3 4 5 6 7 8 9 10 Clayton, D.W. & JTCPI (1969). "The Chemistry of Alkaline Pulping". In Macdonald, R.G. & Frankin, J.N. (ed.). Pulp and Paper Manufacture: The Pulping of Wood. Vol. 1 (2nd ed.). Columbus, OH: McGraw Hill-Joint Textbook Committee of the Paper Industry (JTCPI). pp. 347–438, esp. 349. OCLC 867778880. Retrieved 27 June 2026. [P. 349.] The Kraft, or Sulphate, Process / In the kraft process a mixture of sodium sulphide and sodium hydroxide is used to pulp the wood. The sulphide accelerates the delignification; consequently, the chips are exposed to the hot alkali for a shorter time than in the soda process, and this makes it possible to produce a pulp of much greater strength than soda pulp. According to Strachan,[citing ref. 7, Strachan, J., "in discussion of" Grant, J. (1941) Proc. Tech. Sect. Paper Makers' Assoc. Gt. Brit. Ireland 22:9.] experiments in England during the Napoleonic Wars (i.e., 1805-1814) established that the addition of sulphur and sulphides would accelerate the alkaline pulping of straw, but the first patents in the use of sulphides in the pulping of wood are those of Eaton in the United States in 1870 and 1871.[citing ref. 8, A. K. Eaton, U.S. Patents 106,143, Aug. 9, 1870; 119,224, Sept. 26, 1871.] / In spite of these, C. F. Dahl of Danzig, Germany, is usually credited with the development of the kraft, or sulphate, process in 1879. He began to substitute sodium sulphate (salt cake) for the soda ash (sodium carbonate) used to replace the alkali lost in cooking straw. ["The sodium sulphate, had, of course, been reduced to sodium sulphide in the recovery furnace."] The results were at first unsatisfactory, however, and it was not until 1884 that Dahl obtained a patent.[citing ref. 9, C. F. Dahl, U.S. Patents 296,935, Apr. 15, 1884.] The patent was soon applied to coniferous woods, and in 1885 the first kraft paper was produced at the Munksjõ mill in Jõnkoping, Sweden, apparently because, owing to an error, a digester was blown before the chips were fully cooked. Instead of discarding the chips, the mill manager ordered them to be passed through a kollergang to make an inferior grade of paper. The resulting pulp made a paper, which, though dark in color was far stronger than any paper hitherto known. It was given the name kraft, which is Swedish and also German for strength.{{cite book}}: CS1 maint: multiple names: editors list (link)
  2. ↑ 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.[citation needed]
  3. 1 2 3 4 5 6 7 8 9 10 11 12 13 Biermann, Christopher J. (1993). Essentials of Pulping and Papermaking. San Diego, CA: Academic Press. pp. 55–100, esp. 59, 86ff. ISBN 012097360X. Retrieved 27 June 2026.
  4. 1 2 Patt, R; Kordsachia, O; Süttinger, R; Ohtani, Y; Hoesch, JF; Ehrler, P; Eichinger, R; Holik, H; Hamm, U; Rohmann, ME; Mummenhoff, P; Petermann, E; Miller, RF; Frank, D; Wilken, R; Baumgarten, HL & Rentrop, G-H (15 June 2000). "Paper and Pulp". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim, Germany: Wiley-VCH. doi:10.1002/14356007.a18_545.{{cite book}}: CS1 maint: multiple names: authors list (link)
  5. 1 2 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.{{cite journal}}: CS1 maint: multiple names: authors list (link)[non-primary source needed]
  6. 1 2 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.{{cite journal}}: CS1 maint: multiple names: authors list (link)[non-primary source needed]
  7. ↑ Now Gdańsk, Poland, the city-state of Danzig was the a part of Germanic Prussia at that time.[citation needed]
  8. 1 2 E. Sjöström (1993). Wood Chemistry: Fundamentals and Applications. Academic Press. ISBN 0-12-647480-X.[page needed]
  9. ↑ Woodman, Jocelyn (EPA OPPT); Cantin, Jeff (ERG, Inc); EPA & ERG Staffs (August 1993). "Pollution Prevention Technologies for the Bleached Kraft Segment of the U.S. Pulp and Paper Industry" (contract report). Washington, DC-Lexington, MA: Office of Pollution Prevention and Toxics, U.S. Environmental Protection Agency (EPA OPPT)—ERG, Inc. pp. 4-16ff. Contract No. 68-C0-0068. Retrieved 27 June 2026 – via National Environmental Publications Internet Site (NEPIS).{{cite web}}: CS1 maint: multiple names: authors list (link) CS1 maint: year (link) NEPIS/EPA document EPA/600/R-95/110, indicating co-sponsorship by the EPA Office of Environmental Engineering and Technology Demonstration.
  10. ↑ [Ragauskas, Arthur J.] (23 October 2021) [11 March 2016]. "Lignin [Basics of Kraft Pulping]" (PDF). Biorefinery, UTK Dept. of CBE (biorefinery.utk.edu). Knoxville, TN: University of Tennessee Knoxville (UTK), Dept. of Chemical and Biomolecular Engineering (CBE). Archived from the original (PDF) on 23 October 2021. Retrieved 22 June 2026. Originally archived on 11 March 2016, the link at the UTK Biorefinery site "Technical Reviews" collection directs instead to this 23 October 2021 archived document. Its authorship is inferred but unstated; Ragauskas is a faculty member at UTK, has been the prominently featured director of UTK's Biorefinery since 2014, and is the name that appears in the URL of the "Technical Reviews" collection. Note, the title, "Lignin" appears on the work itself; the title "Basics of Kraft Pulping" is the title contained at the review collection, and in its link.
  11. ↑ "Equipment to handle heavy black liquor". Archived from the original on 20 April 2005. Retrieved 9 October 2007.[full citation needed]
  12. ↑ Hsieh, Jeffery S.; Smith, Jason B. "Second Critical Solids Black Liquor Scaling" (PDF). Pulp and Paper Engineering, School of Chemical Engineering, Georgia Institute of Technology. Archived from the original (PDF) on 31 August 2011. Retrieved 9 October 2007.
  13. ↑ US granted 5527427, Mualla Berksoy & Yaman Boluk, "High solids black liquor of reduced viscosity and viscosity reduction method for high solids black liquor", issued 18 June 1996, assigned to Optima Specialty Chemicals & Technology Inc
  14. ↑ Giddings, J.F.; Roll, D.R.; Cappellino, C.A.; Day, M.; Nardone, R.A. (2008). "12.12.4.". In Karassik, I.; Messina, J.; Cooper, P.; Heald, C. (eds.). Pump Handbook (4th ed.). New York: McGraw Hill. ISBN 9780071460446. Archived from the original on 4 March 2013. Retrieved 21 February 2013.
  15. ↑ Jeffries, Tom (27 March 1997). "Kraft pulping: Energy consumption and production". University of Wisconsin Biotech Center. Archived from the original on 28 September 2011. Retrieved 21 October 2007.
  16. ↑ "Chemrec web site". Retrieved 7 January 2011.{{cite web}}: CS1 maint: deprecated archival service (link)[full citation needed]
  17. ↑ Schubert, David M. (2015). "Boric Oxide, Boric Acid, and Borates". Ullmann's Encyclopedia of Industrial Chemistry. pp. 1–32. doi:10.1002/14356007.a04_263.pub2. ISBN 978-3-527-30385-4.
  18. ↑ "Environmental Comparison of Bleached Kraft Pulp ManufacturingTechnologies" (PDF). Archived from the original (PDF) on 18 December 2004. Retrieved 28 September 2007.[full citation needed]
  19. ↑ Goyal, Gopal C. (1997). Anthraquinone Pulping. A TAPPI Press Anthology of Published Papers, 1977-1996. Atlanta: TAPPI Press. ISBN 0-89852-340-0.[page needed]
  20. ↑ "Tall Oil and Tall Oil Soap Recovery" (PDF). Archived from the original (PDF) on 5 March 2012. Retrieved 19 December 2008.[full citation needed]
  21. ↑ Hubbe, Martin a.; Lucian A. Lucia (2007). "The "Love-Hate" Relationship Present in Lignocellulosic Materials". BioResources. 2 (4): 534–535. doi:10.15376/biores.2.4.534-535. Retrieved 3 February 2015.
  22. ↑ "Mechanical Pulping vs. Chemical Pulping: Which one is better?". Yuanhua Paper.[full citation needed]
  23. ↑ "High-Performance Paper Materials: Pulp, Cellulose & Derivatives". Yuanhua Paper.[full citation needed]
  24. ↑ Das, Tapas K.; Houtman, Carl (December 2004). "Evaluating Chemical-, Mechanical-, and Bio-Pulping Processes and Their Sustainability Characterization Using Life-Cycle Assessment" (PDF). Environmental Progress. 23 (4): 347–357. doi:10.1002/ep.10054.
  25. ↑ "The Southern Pines" (PDF). US Department of Agriculture. 1985. Archived from the original (PDF) on 9 July 2007. Retrieved 13 September 2007.
  26. 1 2 Stenius, Per, ed. (2000). "2". Forest Products Chemistry. Papermaking Science and Technology. Vol. 3. Helsinki, Finland: Fapet OY. pp. 73–76. ISBN 952-5216-03-9..
  27. ↑ Hansen, G. A. (1962). "Odor and Fallout Control in a Kraft Pulp Mill". Journal of the Air Pollution Control Association. 12 (9): 409–436. doi:10.1080/00022470.1962.10468107. PMID 13904415.
  28. ↑ 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.
source: https://en.wikipedia.org/wiki/Kraft_process