The kraft process, also known as the sulfate process, is an industrial process that converts wood into pulp consisting mainly of pure cellulose fibers. It involves treating wood chips with a mixture of sodium hydroxide and sodium sulfide known as liquor. This breaks the bonds linking lignin to the cellulose. The process includes impregnating chips with white liquor, cooking them to remove lignin, recovering chemicals from the spent liquor, screening and washing the pulp, and sometimes bleaching it to produce high brightness pulp. The kraft process was invented in the 1870s and is now the dominant pulping process worldwide.

1. KRAFT
PROCESS

3. • The kraft process also known as kraft
pulping or sulfate process
• Is an industrial process for conversion
of wood into wood pulp consisting of almost
pure cellulose fibers
• Treatment of wood chips with a mixture
of sodium hydroxide and sodium sulfide,
known as liquor,
• That breaks the bonds that link lignin to the
cellulose

4. The problem
Pulping
Wood (this is what we have)
Fibers (this is
what we want)

5. History
• German word Kraft meaning paper
• invented by Carl F. Dahl in 1879
• a pulp mill using this technology started
in Sweden in 1890.
• The invention of the recovery
boiler by G.H. Tomlinson in 1930s.

6. Kraft Pulp Making Process

7. RAW MATERIAL

8. STEPS INVOLVED IN PROCESS
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Impregnation
Cooking
Recovery process
Blowing
Screening
Washing
Bleaching
Process chemicals

10. IMPREGNATION
• Common wood chips used are 12–25 millimeters
long
• Prestreamed and wetted to impregnate the chips
with white liquor
• White liqour contain NaOH+NaSH

11. COOKING
• enters the digester below 100 °C (212 °F).
• Typically delignification requires several
hours at 170 to 176 °C (338 to 349 °F).
• The combined liquids, known as black
liquor
contain
lignin
fragments,
carbohydrates from the breakdown of
hemicellulose, sodium carbonate, sodium
sulfate and other inorganic salts.

13. 13

14. RECOVERY PROCESS
Thermochemical sulfate reduction
1. Na2SO4 + 2 C → Na2S + 2 CO2
Calcium carbonate precipitates from white liquor
2. Na2S + Na2CO3 + Ca(OH)2 ←→ Na2S + 2 NaOH + CaCO3
Calcination
3. CaCO3 → CaO + CO2
Slaking
4. CaO + H2O → Ca(OH)2

16. Blowing
cooked wood chips are blown by
reducing the pressure to
atmospheric by release of steam
Screening
Screening of the pulp after pulping
is a process whereby the pulp is
separated from large shives, knots,
dirt and other debris
Washing
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Pressure diffusers
Atmospheric diffusers
Vacuum drum washers
Drum displacers
Wash presses

18. Bleaching
In bleaching tower the
pulp are bleached to a
high brightness

20. Process chemicals
Process chemicals are added to improve the production
process:
• Surfactants may be used to improve impregnation
• Anthraquinone is used as a digester
• An emulsion breaker can be added in the soap separation
• Defoamers remove foam
• Dispersing agents, detackifiers and complexing agents are
keeping the system cleaner
• Fixation agents are fixating finely dispersed potential
deposits to the fibers