The Kraft process
(also known as Kraft pulping
or sulfate process
) 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
that break the bonds that link lignin
to the cellulose. 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.
The kraft process was developed by Carl F. Dahl
in 1879, and a pulp mill using this technology started (in Sweden
) in 1890. The invention of the recovery boiler
by G.H. Tomlinson
in the early 1930s, was a milestone in the advancement of the kraft process. 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. For this reason, in the 1940s, the kraft process surpassed the sulfite process
as the dominant method for producing wood pulp.
Wood chips 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.
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 delignification requires several hours at 130 to 180 °C (265 to 355 °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.
One of the main chemical reactions that underpin the kraft process is the scission of ether bonds by the nucleophilic sulfide (S2-) or bisulfide (HS-) ions.
The black liquor is concentrated in multiple effect evaporator
to 60% or even 80% solids ("heavy black liquor) 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). 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 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 + 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. Additionally, bark and wood residues are often burned in a separate power boiler to generate steam.
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 interferes with the formation of the hydrogen bonds
between cellulose (and hemicellulose) in the fibers needed for the strength of paper (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, and non-wood species like bamboo and kenaf can be used in the kraft process.
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.
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.
Byproducts and emissions
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 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.
The process effluents are treated in a biological effluent treatment plant, which guarantees that the effluents are not toxic in the recipient.