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. Wood pulp is a dry fibrous material prepared by chemically or mechanically separating the fibers which make up Wood. Cellulose is an Organic compound with the formula, a Polysaccharide consisting of a linear chain of several hundred to over ten thousand β(1→4 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. Sodium hydroxide ( Na[[hydroxide OH]]) also known as Lye, caustic soda and (incorrectly according to IUPAC nomenclature Sodium sulfide is the name used to refer to the Chemical compound Na2S but more commonly its hydrate Lignin or lignen is a complex Chemical compound most commonly derived from Wood and an integral part of the secondary Cell walls of Plants 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.

History

The kraft process was developed by Carl F. Dahl in 1879, and a pulp mill using this technology started (in Sweden) in 1890. "Sverige" redirects here For other uses see Sweden (disambiguation and Sverige (disambiguation. ^[1] The invention of the recovery boiler by G. Recovery boiler is the part of Kraft process of pulping where chemicals for White liquor are recovered and reformed from Black liquor. H. Tomlinson in the early 1930s, was a milestone in the advancement of the kraft process. ^[2] 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. The sulfite process produces Wood pulp which is almost pure Cellulose fibers by using various Salts of Sulfurous acid to extract the Lignin ^[1]

The process

Wood chips are fed into vessels called digesters that are capable of withstanding high pressures. A digester is a device used in Papermaking which takes small Wood chips and cooks them for a number of hours to remove Lignin. 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. ^[3]

International Paper Company: Kraft paper mill

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. International Paper ( is an American pulp and paper company, the largest pulp and paper company in the world Sodium hydroxide ( Na[[hydroxide OH]]) also known as Lye, caustic soda and (incorrectly according to IUPAC nomenclature Sodium sulfide is the name used to refer to the Chemical compound Na2S but more commonly its hydrate 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. A hemicellulose can be any of several Heteropolymers (matrix polysaccharides present in almost all plant cell walls along with Cellulose. 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. Black liquor is a Byproduct of the Kraft process, (also known as Kraft pulping or sulfate process) during the production of paper pulp Carbohydrates (from ' Hydrates of Carbon ' or saccharides ( Greek σάκχαρον meaning " Sugar " are the most Sodium carbonate (also known as washing soda or soda ash), is a Sodium Salt of Carbonic acid. Sodium sulfate is the Sodium salt of Sulfuric acid

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. In Chemistry, a nucleophile (literally nucleus lover as in nucleus and phile) is a Reagent that forms a Chemical bond to The term sulfide ( sulphide in British English) refers to several types of Chemical compounds containing Sulfur in its lowest Oxidation The Bisulfide ion also called hydrosulfide, is the Anion with the formula − (commonly written − ^[2]

net reaction in depolymerization of lignin by SH^- (Ar = aryl, R = alkyl groups). An alkyl is a Univalent radical consisting of Carbon and Hydrogen atoms arranged in a chain

Recovery process

The black liquor is concentrated in multiple effect evaporator to 60% or even 80% solids ("heavy black liquor"^[4]) and burned in the recovery boiler to recover the inorganic chemicals for reuse in the pulping process. Within a Downstream processing system several stages are used to further isolate and purify the desired product Recovery boiler is the part of Kraft process of pulping where chemicals for White liquor are recovered and reformed from Black liquor. 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). ^[5][6] 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. Calcium hydroxide, traditionally called slaked lime, hydrated lime, or pickling lime, is a Chemical compound with the chemical formula 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). Calcium carbonate is a Chemical compound with the Chemical formula Ca[[Carbon C]] O 3 Kilns are thermally insulated chambers or Ovens in which controlled temperature regimes are produced Calcium oxide ( CaO) commonly known as burnt lime, lime or quicklime, is a widely used Chemical compound.

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. Sodium carbonate (also known as washing soda or soda ash), is a Sodium Salt of Carbonic acid. Sodium hydroxide ( Na[[hydroxide OH]]) also known as Lye, caustic soda and (incorrectly according to IUPAC nomenclature

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. ^[7] 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. Wood pulp is a dry fibrous material prepared by chemically or mechanically separating the fibers which make up Wood. Acidic sulfite processes degrade cellulose more than the kraft process, which leads to weaker fibers. The sulfite process produces Wood pulp which is almost pure Cellulose fibers by using various Salts of Sulfurous acid to extract the Lignin 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^[8] interferes with the formation of the hydrogen bonds between cellulose (and hemicellulose) in the fibers needed for the strength of paper^[1] (strength refers to tensile strength and resistance to tearing). In Chemistry, hydrophobicity (from the combining form of water in Attic Greek hydro- and for fear phobos) refers to the physical property of A hydrogen bond results from a Dipole-dipole force between an Electronegative atom and a Hydrogen atom bonded to Nitrogen, Oxygen Tensile strength \sigma_{UTS} or S_U is the Stress at which a material breaks or permanently deforms

Kraft pulp is darker than other wood pulps, but it can be bleached to make very white pulp. Bleaching of wood pulp is the Chemical processing carried out on various types of Wood pulp to decrease the color of the pulp so that it becomes whiter 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^[9] and non-wood species like bamboo and kenaf can be used in the kraft process. This article is about the tree For other uses of the term "pine" see Pine (disambiguation. Bamboo is a group of Woody perennial Evergreen Plants in the True grass family Poaceae, subfamily Kenaf Persian Hibiscus cannabinus, is a plant in the Malvaceae family

Bleaching

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. Bleaching of wood pulp is the Chemical processing carried out on various types of Wood pulp to decrease the color of the pulp so that it becomes whiter Bleaching of wood pulp is the Chemical processing carried out on various types of Wood pulp to decrease the color of the pulp so that it becomes whiter ^[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.

Byproducts and emissions

In the case of softwood (conifer) pulping, a soaplike substance is collected from the liquor during evaporation. Softwood is a generic term used in Woodworking and the Lumber industries for Wood from Conifers (needle-bearing trees from the order Pinales The soap is acidified to produce tall oil, a source of resin acids, fatty acids and other chemicals. SOAP (see below for name and origins is a protocol for exchanging XML -based messages over Computer networks normally using Tall oil, also called liquid Rosin or tallol, is a viscous yellow-black odorous liquid obtained as a byproduct of the Kraft process of Wood Resin acids are Protectants and Wood preservatives that are produced by Parenchymatous epithelial cells that surround the Resin ducts In Chemistry, especially Biochemistry, a fatty acid is a Carboxylic acid often with a long unbranched Aliphatic tail ( chain) which Also turpentine originates from softwood. Turpentine (also called spirit of turpentine oil of turpentine wood turpentine gum turpentine is a fluid obtained by the Distillation of Resin obtained from trees

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. Hydrogen sulfide (or hydrogen sulphide) is the Chemical compound with the formula H 2 S. Methanethiol (also known as methyl mercaptan) is a colorless gas with a smell like rotten Cabbage. Dimethyl sulfide (DMS or methylthiomethane is an organosulfur compound with the formula (CH32S Sulfur or sulphur (ˈsʌlfɚ see spelling below) is the Chemical element that has the Atomic number 16 A pulp mill is a manufacturing facility that converts wood chips or other plant fiber source into a thick fiber board which can be shipped to a Paper mill for further 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. Sodium sulfate is the Sodium salt of Sulfuric acid

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

References

1. ^ ^{a b c} Biermann, Christopher J. The sulfite process produces Wood pulp which is almost pure Cellulose fibers by using various Salts of Sulfurous acid to extract the Lignin A pulp mill is a manufacturing facility that converts wood chips or other plant fiber source into a thick fiber board which can be shipped to a Paper mill for further Wood pulp is a dry fibrous material prepared by chemically or mechanically separating the fibers which make up Wood. Bleaching of wood pulp is the Chemical processing carried out on various types of Wood pulp to decrease the color of the pulp so that it becomes whiter Paper is thin material mainly used for writing upon printing upon or packaging (1993). Essentials of Pulping and Papermaking. San Diego: Academic Press, Inc. . ISBN 0-12-097360-X.  
2. ^ ^{a b} E. Sjöström (1993). Wood Chemistry: Fundamentals and Applications. Academic Press. Academic Press ( London, New York and San Diego) was an Academic Book Publisher that is now part of Elsevier.  
3. ^ 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. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 9 - The Battle of the Teutoburg Forest ends 506 - The Bishops of Visigothic Gaul
4. ^ Equipment to handle heavy black liquor. Retrieved on 2007-10-09. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 768 - Carloman I and Charlemagne are crowned Kings of The Franks.
5. ^ 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. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 768 - Carloman I and Charlemagne are crowned Kings of The Franks.
6. ^ 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 
7. ^ Jeffries, Tom (March 27, 1997). Year 1996 ( MCMXCVI) was a Leap year starting on Monday (link will display full 1996 Gregorian calendar) Events 618 - Coronation of the Chinese governor Li Yuan as Emperor Gaozu of Tang, the new Emperor of China, initiating three centuries Kraft pulping: Energy consumption and production. University of Wisconsin Biotech Center [2]. Retrieved on 2007-10-21. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 1512 - Martin Luther joins the theological faculty of the University of Wittenberg.
8. ^ Hubbe, Martin a. ; Lucian A. Lucia (2007). "The "Love-Hate" Relationship Present in Lignocellulosic Materials" (). BioResources 2 (4): 534-535.  
9. ^ The Southern Pines. US Department of Agriculture (1985). Retrieved on 2007-09-13. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 509 BC - The Temple of Jupiter on Rome 's Capitoline Hill is dedicated on the ides of September
10. ^ Environmental Comparison of Bleached Kraft Pulp ManufacturingTechnologies. Retrieved on 2007-09-28. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 48 BC - Pompey the Great is assassinated on orders of King Ptolemy of Egypt after landing in Egypt.

External links

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