Kraft process

International Paper: Kraft paper mill

The kraft process (also known as kraft pulping or sulfate process)^[1] describes a technology for conversion of wood into wood pulp consisting of almost pure cellulose fibers. The process entails treatment of wood chips with a mixture of sodium hydroxide and sodium sulfide, known as white liquor, that break the bonds that link lignin to the cellulose.

History

The kraft process was developed by Carl F. Dahl in 1879, and a pulp mill using this technology 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 a nearly closed-cycle with respect to inorganic chemicals, apart from those used in the bleaching process. For this reason, in the 1940s, the kraft process surpassed the sulfite process as the dominant method for producing wood pulp.^[2]

The process

Continuous Kraft pulp mill

Woodchips are fed into vessels called digesters that are capable of withstanding high pressures. 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 with the largest producing more than 3,500 tonnes of pulp per day.^[4] Wood chips are impregnated with the cooking liquors. The cooking liquors consist of warm black liquor and white liquor. The warm black liquor is the spent cooking liquor coming from the blowing. White liquor is a mixture of sodium hydroxide and sodium sulfide, produced in the recovery process. In a continuous digester the materials are fed at a rate which allows the pulping reaction to be complete by the time the materials exit the reactor. Typically delignification requires several hours at 130 to 180 °C (266 to 356 °F). Under these conditions lignin and some hemicellulose degrade to give fragments that are 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.

net reaction in depolymerization of lignin by SH^- (Ar = aryl, R = alkyl groups).

One of the main chemical reactions that underpin the kraft process is the scission of ether bonds by the nucleophilic sulfide (S^2-) or bisulfide (HS^-) ions.^[3]

Recovery process

The excess black liquor is concentrated in a multiple effect evaporator to 60% or even 80% solids ("heavy black liquor"^[5]) 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).^[6][7] 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

This reaction is similar to Thermo Chemical Sulfato Reduction (TSR) in geochemistry.

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 resulting solution of sodium carbonate and sodium sulfide 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 + 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 so-called recausticizing process where sodium carbonate is reacted to regenerate sodium hydroxide.

The recovery boiler also generates high pressure steam which is fed 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.^[8] Additionally, bark and wood residues are often burned in a separate power boiler to generate steam.

Blowing

The finished cooked wood chips are blown by reducing the pressure to atmospheric pressure. This releases a lot of steam and volatiles. The steam produced can then be used to heat the pulp mill and any excess used in district heating schemes or to drive a steam turbine to generate electrical power. 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 is separated from large shives, knots, dirt and other debris. The accept is the pulp. The material separated from the pulp is 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 is separated from the rest of the reject and reprocessed either in a refiner and/or is sent back to the digester. The content of knots are typically 0.5 - 3.0% of the digester output, while the shives content are about 0.1- 1.0%.

Washing

The brown stock 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 counter current 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:

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

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.^[9]

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.

Process chemicals

Process chemicals are added to improve the production process:

• Impregnation aids. Surfactants may be used to improve impregnation of the wood chips with the cooking liquors.
• Anthraquinone is used as a digester additive. It works as a redox catalyst by oxidizing cellulose and reducing lignin. This is protecting the cellulose from degradation and makes the lignin more watersoluble.^[10]
• An emulsion breaker can be added in the soap separation to speed up and improve the separation of soap from the used cooking liquors by flocculation.^[11]
• Defoamers remove foam and speed up the production process. Drainage of washing equipment is improved and gives cleaner pulp.
• Dispersing agents and complexing agents are keeping the system cleaner and reduce the need for maintenance stops.
• Fixation agents are fixating finely dispersed potential deposits to the fibers and thereby transporting it out of the process.

Comparison with other pulping processes

Pulp produced by the kraft process is stronger than that made by other pulping processes. Acidic sulfite processes degrade cellulose more than the kraft process, which leads to weaker fibers. Kraft pulping removes most of the lignin present originally in the wood whereas mechanical pulping processes leave most of the lignin in the fibers. The hydrophobic nature of lignin^[12] 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,^[13] and non-wood species like bamboo and kenaf can be used in the kraft process.

Byproducts and emissions

The main byproducts of kraft pulping are crude sulfate turpentine and tall oil soap. The availability of these are strongly dependent on wood species, growth conditions, storage time of logs and chips and the mills process.^[14] Pines are the most extractive rich woods. The raw turpentine is volatile and is distilled of 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.^[14]

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 sulfur dioxide emissions of the kraft pulp mills are much lower than sulfur dioxide emissions from sulfite mills. In modern mills where high dry solids are burned in the recovery boiler hardly any sulfur 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 sulfur dioxide forming sodium sulfate.

Pulp mills are almost always located near large bodies of water due to their former substantial demands. Delignification of chemical pulps released 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. Reducing the environmental impact of this effluent is accomplished by closing the loop and recycling the effluent where possible, as well as employing less damaging agents in the pulping and bleaching processes. The process effluents are treated in a biological effluent treatment plant, which guarantees that the effluents are not toxic in the recipient.

See also

• Sulfite process
• Soda pulping
• Pulp mill
• Wood pulp
• Kappa number
• H-factor
• Bleaching of wood pulp
• Paper

Further reading

• Gullichsen, Johan; Carl-Johan Fogelholm (2000). Papermaking science and Technology: 6. Chemical Pulping. Finland: Tappi Press.. ISBN 952-5216-06-3. 

References

External links

• Pulp and Paper Circle provides industry-related information (especially Kraft Mills) to pulp and paper engineers and technologists.
• US EPA article on Kraft pulping
• Reference Document on Best Available Techniques in Pulp and Paper Industry by European Commission, 2001