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The Kraft process (also known as Kraft pulping or sulfate process) produces wood pulp which is almost pure cellulose fibers by using sodium hydroxide and sodium sulfide to extract the lignin from wood chips in large pressure vessels called digesters. Some digesters operate in a batch manner and some in a continuous process such as the Kamyr digester. Digesters producing 1,000 tonnes of pulp per day and more are common.^[1]

The process name is derived from German kraft, meaning strength/power; both capitalized and lowercase names (Kraft process and kraft process) appear in the literature, but "kraft" is most commonly used in the pulp and paper industry. It was developed by Carl Dahl in 1879 and the first kraft mill started (in Sweden) in 1890.^[2] The invention of the recovery boiler by G.H. Tomlinson in the early 1930s, was a milestone in the advancement of the kraft process.^[3] It enabled the recovery and reuse of the inorganic pulping chemicals such that a kraft mill is almost closed-cycle with respect to inorganic chemicals, apart from those used in the bleaching process. This was one of the reasons that in the 1940s the kraft process took over from the sulfite process as the dominant method for producing wood pulp.^[2]

Kraft pulp makes paper that is stronger than that made from any of the other pulping processes. The sulfite process degrades cellulose more than the kraft process and degraded cellulose makes weaker fibers. Kraft pulping removes most of the lignin present originally in the wood while the mechanical pulping processes leave most of the lignin in the fibers. The hydrophobic nature of lignin^[4] interferes with the formation of the hydrogen bonds between cellulose (and hemicellulose) in the fibers needed for the strength of paper^[2] (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^[5] and non-wood species like bamboo and kenaf can be used in the kraft process.

Contents 1 Pulping process 2 Recovery process 3 Bleaching 4 Byproducts and emissions 5 See also 6 References 7 External links

Wood chips and white liquor, a mixture of sodium hydroxide and sodium sulfide, produced in the recovery process, are added to the top of the digester. In a continuous digester the materials are fed at a rate which allows the pulping reaction to be complete by the time the material exit the reactor. Typically this takes several hours and is done at high temperatures (130 to 180 °C (265 to 355 °F). These conditions break down lignin and some hemicellulose and the fragments become soluble in the strongly basic liquid. The solid pulp (about 50% by weight based on the dry wood chips) is collected and washed. At this point the pulp is quite brown and is known as "brown stock". The combined liquids, known as black liquor (so called because of its color), contain lignin fragments, carbohydrates from the breakdown of hemicellulose, sodium carbonate, sodium sulfate and other inorganic salts.

The black liquor is concentrated in multiple effect evaporator to 60% or even 80% solids ("heavy black liquor"^[6]) 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).^[7][8] The combustion is carried out such that sodium sulfate is reduced to sodium sulfide by the organic carbon in the mixture:

1. Na₂SO₄ + 2 C → Na₂S + 2 CO₂

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 and is kept in a tank called weak wash storage tank. The solution of sodium carbonate and sodium sulfide resulted is known as "green liquor". This liquid is mixed with calcium hydroxide 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):

2. Na₂S + 2 Na₂CO₃ + Ca(OH)₂ ←→ Na₂S + 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).

3. CaCO₃ → CaO + CO₂

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

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 called recausticizing process where sodium carbonate is reacted to regenerate sodium hydroxide.

The recovery boiler also generates high pressure steam which is led 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 to the local electrical grid.^[9] Additionally, bark and wood residues are often burned in a separate power boiler to generate steam.

Main article: 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 stages.^[10]

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.

In the case of softwood (conifer) pulping, a soaplike substance is collected from the liquor during evaporation. The soap is acidified to produce tall oil, a source of resin acids, fatty acids and other chemicals. Also turpentine originates from softwood.

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. Outside the modern mills the odour is perceivable only during disturbance situations, for example when shutting the mill down for maintenance break. This is due to practiced collection and burning of these odorous gases in the recovery boiler along with black liquor. The sulphur dioxide emissions of the kraft pulp mills are much lower than sulphur dioxide emissions from sulfite mills. In modern mills where high dry solids are burned in the recovery boiler hardly any sulphur dioxide leaves the boiler. This is mainly due to higher lower furnace temperature which leads to higher sodium release from the black liquor droplets that can react with sulphur dioxide forming sodium sulfate.

The process effluents are treated in a biological effluent treatment plant, which guarantees that the effluents are not toxic in the recipient.

• Sulfite process
• Pulp mill
• Wood pulp
• Bleaching of wood pulp
• Paper

1. ^ Woodman, Jocelyn (1993). [1] Pollution Prevention Technologies for the Bleached Kraft Segment of the U.S. Pulp and Paper Industry (see p 66)]. U.S. Environmental Protection Agency. Retrieved on 2007-09-11.
2. ^ ^{a b c} Biermann, Christopher J. (1993). Essentials of Pulping and Papermaking. San Diego: Academic Press, Inc.. ISBN 0-12-097360-X. 
3. ^ E. Sjöström (1993). Wood Chemistry: Fundamentals and Applications. Academic Press. 
4. ^ Hubbe, Martin a.; Lucian A. Lucia (2007). "The "Love-Hate" Relationship Present in Lignocellulosic Materials". BioResources 2 (4): 534-535. Retrieved on 2007-09-15. 
5. ^ The Southern Pines. US Department of Agriculture (1985). Retrieved on 2007-09-13.
6. ^ Equipment to handle heavy black liquor. Retrieved on 2007-10-09.
7. ^ Hsieh, Jeffery S.; Smith, Jason B.. Second Critical Solids Black Liquor Scaling. Pulp and Paper Engineering, School of Chemical Engineering, Georgia Institute of Technology. Retrieved on 2007-10-09.
8. ^ US patent 5527427, "High solids black liquor of reduced viscosity and viscosity reduction method for high solids black liquor", granted 1996-06-18, assigned to Optima Specialty Chemicals & Technology Inc 
9. ^ Jeffries, Tom (March 27, 1997). Kraft pulping: Energy consumption and production. University of Wisconsin Biotech Center [2]. Retrieved on 2007-10-21.
10. ^ Environmental Comparison of Bleached Kraft Pulp ManufacturingTechnologies. Retrieved on 2007-09-28.

• US EPA article on Kraft pulping
• Reference Document on Best Available Techniques in Pulp and Paper Industry by European Commission, 2001