KRAFT PROCESS

• 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

PulpingThe problem

Wood (this is what we have) Fibers (this is what we want)

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.

Kraft Pulp Making Process

RAW MATERIAL

• Impregnation• Cooking• Recovery process• Blowing• Screening• Washing• Bleaching• Process chemicals

STEPS INVOLVED IN PROCESS

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

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.

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Thermochemical sulfate reduction1. Na2SO4 + 2 C → Na2S + 2 CO2

Calcium carbonate precipitates from white liquor2. Na2S + Na2CO3 + Ca(OH)2 ←→ Na2S + 2 NaOH + CaCO3

Calcination

3. CaCO3 → CaO + CO2

Slaking

4. CaO + H2O → Ca(OH)2

RECOVERY PROCESS

Blowing

Screening

Washing

Screening of the pulp after pulping is a process whereby the pulp is separated from large shives, knots, dirt and other debris

• Pressure diffusers• Atmospheric diffusers• Vacuum drum washers• Drum displacers• Wash presses

cooked wood chips are blown by reducing the pressure to atmospheric by release of steam

In bleaching tower the pulp are bleached to a high brightness

Bleaching

Process chemicalsProcess 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