How a kraft mill works: the chemical loop that made modern pulp

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In 1980, a pulp engineering firm put the limit for one pulping line at 1,000 to 1,400 tons a day. In 2021, a Brazilian mill ordered a line of more than 8,000. This post explains the process that makes such a line possible: five steps from log to bale, and a loop that recovers the cooking chemicals.

Published October 27, 2026 · Facts as of October 27, 2026 · 9 min read · Sources listed at the end

Facts as of October 27, 2026. This page states the facts as they stood on that date. Figures, rules and company plans may have changed since publication. We review this guide every six months and update the pages.

April 15, 1884Date Carl F. Dahl was granted US patent 296,935 for the sulfate process that became kraft ⁸

40 to 45%Typical yield of bleached chemical pulp from dry wood, per a standard pulping textbook ⁷

1,000 to 1,400Tons a day: the 1980 view of the limit for one pulping line ¹³

Over 8,000Air-dry tons a day: the fiberline Andritz announced for Suzano's Cerrado mill in 2021 ¹²

The 60-second version

What is it?

Kraft pulping cooks wood chips in a hot chemical solution. The solution dissolves lignin, the natural glue in wood, and frees the fibers ². The spent solution is then burned and rebuilt into fresh solution, so the chemicals run in a loop ¹.

Why does it matter?

A USDA Forest Service review lists three reasons for kraft's lead: stronger paper, an efficient recovery process, and the ability to pulp any wood species ¹⁰. The loop also gives the mill heat for steam ¹.

How does it work?

Five steps turn logs into bales: chip and cook, wash, oxygen stage, bleach, dry and bale ². A second circuit recovers and rebuilds the cooking chemicals ^1,2.

Myth or fact?

"A pulp mill uses up a fresh batch of cooking chemicals every day."

See the answer

Mostly myth. In a kraft mill the cooking chemicals go around a loop. The spent liquor is burned, and its chemicals are rebuilt and used again ^1,3. A mill still has to add some chemical to replace losses. Dahl's 1884 patent named sulphate of soda for this ⁸. The sources we found give no figure for how much a modern mill adds.

In this post

1. Why did kraft come to lead chemical pulping?
2. What are the five steps from log to bale?
3. How does a kraft mill reuse its chemicals?
4. How big can one line be?
5. What should a buyer or lender take from the loop?

01Why did kraft come to lead chemical pulping?

Chemical pulp is wood fiber freed by cooking wood in chemicals. Kraft is one such method. Carl F. Dahl of Danzig was granted US patent 296,935 on April 15, 1884. It describes boiling wood under pressure in a sodium solution, and it names sulphate of soda as the make-up chemical, the chemical a mill adds to replace what it loses ⁸.

The first commercial sulphate pulp came from Munksjö in Sweden in the second half of the 1880s. The Swedish government's 1914 handbook gives 1885, and some textbooks give 1890 ⁹.

A USDA Forest Service review gives three reasons kraft came to lead. "Paper produced from kraft pulp is stronger than paper produced from other pulps." It "has an efficient recovery process." And "All wood species can be pulped" ¹⁰. In 1990 the US Environmental Protection Agency (EPA) said kraft made over 80 percent of the chemical pulp produced in the United States ¹. No source we found gives a current world share. The other methods, and the full timeline, are in From rags to eucalyptus: 180 years of pulp.

02What are the five steps from log to bale?

A mill runs these steps as one chain of machines called a fiberline. UPM's Fray Bentos mill in Uruguay lists the order. Logs are washed and cut into chips. The chips are cooked. The pulp is washed in drums. Oxygen removes more lignin. Bleaching raises the brightness. The pulp is then dried and baled ².

Cooking happens in a tall steel vessel called a digester. In a continuous digester, chips and cooking liquor enter a pressurized vessel and the chips "move down through successive cooking zones within the digester" and leave at the bottom as pulp ⁴. UPM says cooking dissolves the lignin to free the fibers ².

Oxygen delignification sits between cooking and the bleach plant. Valmet, an equipment maker, says it removes up to 65 to 70 percent of the lignin left after cooking, which lowers the bleaching chemicals needed ⁵. UPM says its bleaching uses mainly chlorine dioxide, with hydrogen peroxide and caustic soda ². At the end, a drying plant screens, dewaters, dries and bales the sheet. Andritz, another equipment maker, says a full-size drying line can handle a web width of over 10 meters ⁶.

A pulping textbook chapter says bleached chemical pulp yields are typically 40 to 45 percent ⁷. A mill therefore starts with about 2.2 to 2.5 tons of dry wood for each ton of bleached pulp (our calculation: 100 divided by 45 and by 40).

1. 1. Chip and cookLogs are washed and cut into chips. Chips are cooked in white liquor, which dissolves lignin and frees the fibers ².
2. 2. WashDrum washers rinse the cooked pulp. The liquor goes to recovery ².
3. 3. Oxygen stageOxygen, alkali and heat remove more lignin. Up to 65 to 70 percent of what is left after cooking, Valmet says ^2,5.
4. 4. BleachChlorine dioxide, hydrogen peroxide and caustic soda raise the brightness, UPM says ².
5. 5. Dry and baleThe pulp is screened, dewatered, dried, cut and baled ⁶.

03How does a kraft mill reuse its chemicals?

The cooking solution is called white liquor. The EPA describes it as "a water solution of sodium sulfide and sodium hydroxide" ¹. After cooking, the spent liquor plus wash water is called weak black liquor. Evaporators boil off water until the liquor is strong enough to burn. In 1990 the EPA gave about 55 percent solids for multiple-effect evaporators and 65 percent after a further stage ¹.

The strong black liquor is fired in a recovery furnace, also called a recovery boiler. The chemicals in it collect at the bottom as molten smelt. Burning the organic matter gives heat for process steam and turns sodium sulfate into sodium sulfide ¹. Smelt dissolved in water is called green liquor. Lime then turns green liquor back into white liquor, and a solid called lime mud settles out. The lime kiln heats the mud, a step called calcining, to make quicklime again ^1,3.

Babcock and Wilcox says it developed the world's first kraft recovery boiler with G. H. Tomlinson in 1929 ¹¹. Brazil's industry association Ibá credits black liquor for the fact that 90 percent of the energy its sector uses is renewable ¹⁶. Mills that make their own power covers the energy and water side of the boiler.

The loop

The chemical recovery loop, with the size of each unit at Cerrado

Loop steps from the EPA ¹ and Valmet ³. Unit sizes from Andritz for Suzano's Cerrado mill ¹². tds/d means tons of dry solids a day. The solids share is an EPA figure from 1990.

Unit	What happens	What comes out	Size at Cerrado
Digester	Chips are cooked in white liquor, a water solution of sodium sulfide and sodium hydroxide 1.	Pulp and weak black liquor	Fiberline of more than 8,000 adt/d 12
Evaporators	Water is boiled off the weak black liquor 1.	Strong black liquor, about 55 to 65 percent solids in 1990 EPA data 1	Black liquor evaporation of 2,650 t/h 12
Recovery boiler	Strong black liquor is fired. Chemicals collect as molten smelt, and heat makes steam 1.	Smelt and steam	11,150 tds/d 12
Dissolving tank	Smelt is dissolved in water and clarified 3.	Green liquor	No figure in the maker's release 12
Causticizing	Lime is mixed with green liquor. The mud is removed 3.	White liquor and lime mud	White liquor, 25,100 m3 a day 12
Lime kiln	The kiln heats the lime mud to regenerate quicklime 1.	Quicklime, sent back to causticizing	Two kilns, 940 t/d each 12

The recovery boiler fires strong black liquor, and the molten smelt that collects at its bottom is turned back into cooking liquor, the EPA says ¹.

04How big can one line be?

In 1980, Jaakko Poyry wrote for the United Nations Industrial Development Organization (UNIDO) that "the physical limitations on essentially single line operation appear to be between 1,000 and 1,400 tons per day" ¹³. A single line means one fiberline doing the whole job.

In 2021, Andritz announced the fiberline for Suzano's Cerrado mill in Brazil. It is over 8,000 air-dry tons a day, and Andritz calls it the world's largest single-line fiberline ¹². An air-dry ton (adt) is a ton of pulp dried to the standard dryness at which it is sold. The 1980 paper does not state its ton basis as air-dry ¹³. Against the two ends of the 1980 range, 8,000 is about 6 to 8 times larger (our calculation: 8,000 divided by 1,400 and by 1,000).

Cerrado's drying plant has two lines of 4,000 adt/d, each with a working width of 10,005 mm, plus six baling lines. Annual output is 2.55 million tons ¹². Fastmarkets reported that the mill started on July 21, 2024 ¹⁵. Arauco's Sucuriu mill in Brazil is BEING BUILT. It plans 3.5 million tons a year on two fiberlines, with the first bale in the fourth quarter of 2027 ¹⁷.

FAO authors wrote in 1980 that the minimum economic mill size "has therefore increased tremendously" ¹⁴. No source we found explains which single change let one line grow from 1,400 to 8,000 tons a day. For the reasons companies give, see Why new pulp mills are so big.

Chart

One pulping line: the 1980 limit and Cerrado

Tons a day for one line (1980: tons, basis not stated; Cerrado: air-dry tons). Sources: 1980 limit, Jaakko Poyry for UNIDO ¹³. Cerrado fiberline, Andritz press release of November 9, 2021 ¹². Cerrado is shown at its stated minimum of 8,000.

05What should a buyer or lender take from the loop?

A fiberline, a recovery boiler and a lime kiln are capital goods: machines and plant that a mill buys once and uses for years. Fastmarkets reported that Cerrado is part of a 22 billion reais investment, with 15.9 billion reais for the plant and 6.3 billion reais for planting and logistics ¹⁵. Andritz says Cerrado is expected to sell about 180 MW of surplus energy ¹².

Arauco said in September 2024 that it was evaluating financing options for Sucuriu such as export credit agencies, multilaterals and bond issues ¹⁷. An export credit agency is a state-backed body that supports exports and the loans behind them. For how such loans work, see financing industrial goods trade and ECA-backed finance.

The mill's output is a bale that travels to a buyer, usually far away. Working capital sits in that journey. Follow one bale traces it from a Brazilian tree to a tissue roll in China, and working capital and trade finance explains how that money is funded. A pulp exporter sells what this chain makes.

What this means for traders

• Read a mill's size by its line: one fiberline of more than 8,000 air-dry tons a day is the 2021 design, against a 1980 view of 1,000 to 1,400.
• The recovery loop returns the cooking chemicals and gives the mill heat, so a kraft mill design includes a recovery boiler and a lime kiln, and makers publish their sizes unit by unit.
• Equipment cost is large: Cerrado's plant alone is 15.9 billion reais, so check who financed it and on what terms.

Questions traders ask

What is the kraft process?

The kraft process cooks wood chips in white liquor, a water solution of sodium sulfide and sodium hydroxide. The liquor dissolves lignin and frees the wood fibers. Carl F. Dahl was granted a US patent for the sulfate process in 1884.

What is black liquor?

Black liquor is the spent cooking liquor after the cook, mixed with wash water. It holds the dissolved lignin and the cooking chemicals. A mill boils off water and burns it in the recovery boiler.

What does a recovery boiler do?

It burns strong black liquor. The chemicals collect as molten smelt, which the mill turns back into cooking liquor. The burning also gives heat for process steam.

How much pulp does a ton of wood make?

A standard pulping textbook gives a typical yield of 40 to 45 percent for bleached chemical pulp. That is about 2.2 to 2.5 tons of dry wood for each ton of bleached pulp.

How big is the largest single pulp line?

Andritz announced a fiberline of more than 8,000 air-dry tons a day for Suzano's Cerrado mill in 2021 and calls it the world's largest. A 1980 paper put the limit for one line at 1,000 to 1,400 tons a day.

Read next

• Inside the millWhy new pulp mills are so big
• Inside the millMills that make their own power: energy, water and the recovery boiler
• StoriesFrom rags to eucalyptus: 180 years of pulp
• Start hereFollow one bale: from a Brazilian tree to a tissue roll in China
• Inside the millWho builds the mills, and who finances them
• InnovationMore than paper: the biorefinery
• Ossiano Trade Finance GuideHow each trade finance instrument works
• GlossaryTrade finance terms in plain English

Financing the equipment or the cargo of a pulp mill? Ossiano Capital works on trade finance for pulp and paper trades.

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Sources and notes

1. US Environmental Protection Agency, AP-42 Compilation of Air Pollutant Emission Factors, Chapter 10.2 Chemical Wood Pulping, September 1990 (reformatted January 1995). https://www.epa.gov/sites/default/files/2020-10/documents/c10s02.pdf
2. UPM Uruguay, How the mill works (Fiberline and operational system of the mill), undated company document. https://www.upm.uy/siteassets/documents/howmillworks.pdf
3. Valmet, Green liquor production and place in liquor cycle (webinar), article page, undated. https://www.valmet.com/insights/articles/up-and-running/performance/GreenLiquorWebinar/
4. Valmet, Application Report 2611_02_03en, Continuous digester ANDRITZ Continuous Pulp Cooking System Digester Cooking Zone, June 2022. https://www.valmet.com/globalassets/sharepoint/imported/2611_02_03en.pdf
5. Valmet, Oxygen delignification, product page, undated. https://valmet.com/pulp/cooking-and-fiberline/oxygen-delignification
6. Andritz, Technology for pulp sheet drying plants, product page, undated. https://www.andritz.com/products-en/pulp-and-paper/pulp-production/kraft-pulp/pulp-drying-finishing/pulp-sheet-drying
7. Chapter 3, Pulping Fundamentals, in a pulping and papermaking textbook hosted by the University of Cincinnati College of Engineering (author and edition not shown on the page). https://www.eng.uc.edu/~beaucag/WoodPapers/August%20Papers/Paper%20Making%20Book/Pulping%20part%201%2073620_03a.pdf
8. United States Patent Office, US Patent 296,935, Process of Manufacturing Cellulose from Wood, C. F. Dahl, granted April 15, 1884. https://patents.google.com/patent/US296935A/en
9. Guinchard, Sweden: Historical and Statistical Handbook, 1914 (Swedish government), page 395, on sulphate pulp at Munksjö; and National Archives of Australia, Chronology of Papermaking. https://runeberg.org/sweden14/2/0409.html
10. USDA Forest Service, Forest Products Laboratory, Carl Houtman, Lessons Learned from 150 Years of Pulping Wood, in Lignin Valorization: Emerging Approaches, Royal Society of Chemistry, 2018. https://research.fs.usda.gov/download/treesearch/56522.pdf
11. Babcock and Wilcox, Process Recovery Boilers, brochure E101-3194, 2021. https://www.babcock.com/assets/PDF-Downloads/Steam-Generation/E101-3194-PR-Boilers-Babcock-Wilcox.pdf
12. Andritz, press release, Suzano, Brazil orders a pulp production line from ANDRITZ, November 9, 2021. https://www.andritz.com/newsroom-en/pulp-paper/2021-11-09-suzano-cerrado
13. J. C. Easton (Jaakko Poyry) for UNIDO, Criteria for Selection of Small or Large-Scale Pulp and Paper Mills for the Developing Countries, Manila, October 30, 1980. https://downloads.unido.org/ot/47/91/4791662/10001-15000_10928.pdf
14. A. J. Leslie and Borje Kyrklund, Small-scale mills for developing countries, FAO Unasylva No. 128 (paper presented June 1980). https://www.fao.org/4/N8595E/n8595e02.htm
15. Fastmarkets, Suzano starts up Cerrado project in Brazil, July 24, 2024. https://fastmarkets.com/insights/suzano-cerrado-project-brazil-pulp-capacity-h2-24
16. Ibá (Indústria Brasileira de Árvores), Manual do setor 2026, page 17, Autossuficiência energética. https://iba.org/wp-content/uploads/2026/07/IBA2026_Manual_do_setor-v2-1.pdf
17. Arauco, Conference Call Sucuriu, presentation, September 2024. https://arauco.com/wp-content/uploads/2024/09/Conference-Call-Sucuriu-VF4.pdf

Trade figures come from UN Comtrade (2015 to 2025, as reported by 10 countries) and FAOSTAT (1961 to 2024), checked and recalculated by us; each chart names whose data it uses. All tons are metric tons. All facts on this page are as of October 27, 2026, the date of publication, and may have changed since. For rules and tariffs, check the pulp watch before you act.

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