Sulfite process

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The sulfite process produces wood pulp that is almost pure cellulose fibers by treating wood chips with solutions of sulfite and bisulfite ions. These chemicals cleave the bonds between the cellulose and lignin components of the lignocellulose. A variety of sulfite/bisulfite salts are used, including sodium (Na+), calcium (Ca2+), potassium (K+), magnesium (Mg2+), and ammonium (NH4+). The lignin is converted to lignosulfonates, which are soluble and can be separated from the cellulose fibers. For the production of cellulose, the sulfite process competes with the Kraft process which produces stronger fibers and is less environmentally costly.

Contents

idealized scheme for lignin depolymerization by the Sulfite process. SulfiteProcessSimple.png
idealized scheme for lignin depolymerization by the Sulfite process.

History

The use of wood to make pulp for paper began with the development of mechanical pulping in the 1840s by Charles Fenerty in Nova Scotia [1] and by F. G. Keller [2] in Germany. Chemical processes quickly followed, first with Julius Roth's use of sulfurous acid to treat wood in 1857, followed by Benjamin Chew Tilghman's US patent on the use of calcium bisulfite, Ca(HSO3)2, to pulp wood in 1867. [3] Almost a decade later in 1874 the first commercial sulfite pulp mill was built in Sweden. It used magnesium as the counter ion and was based on work by Carl Daniel Ekman.

By 1900 sulfite pulping had become the dominant means of producing wood pulp, surpassing mechanical pulping methods. The competing chemical pulping process, the sulfate or kraft process was developed by Carl F. Dahl in 1879 and the first kraft mill started (in Sweden) in 1890. [3] The first sulphite mill in the United States was the Richmond Paper Company in Rumford, Rhode Island in the mid-1880s. The invention of the recovery boiler by G. H. Tomlinson in the early 1930s [2] allowed kraft mills to recycle almost all of their pulping chemicals. This, along with the ability of the kraft process to accept a wider variety of types of wood and produce stronger fibers [4] made the kraft process the dominant pulping process starting in the 1940s. [3] Sulfite pulps now account for less than 10% of the total chemical pulp production [3] and the number of sulfite mills continues to decrease. [5] [6] [7]

Magnesium was the standard counter ion until calcium replaced it in the 1950s.

Pulping liquor preparation

The pulping liquor for most sulfite mills is generated by treating various bases (alkali metal or alkaline earth hydroxides) with sulfur dioxide:

SO2 + MOH → MHSO3
MHSO3 + MOH → M2SO3 + H2O

Similar reactions are effected with divalent cations (Mg2+, Ca2+) and using carbonates in place of hydroxide.

The ratio of sulfite to bisulfite depends on pH; above pH=7, sulfite predominates.

Calcium-based

The earliest process used calcium, obtained as inexpensive calcium carbonate, and there was little incentive to recover the inorganic materials. At least in Sweden the brown liquor from this process was previously frequently used for producing ethanol, while with other brown liquors the fermentable hexose sugars are left to contribute to the energy needed in the recovery process. Calcium sulfite, which is poorly soluble, converts to calcium bisulfite only at low pH. Therefore calcium-based sulfite processes require acidic conditions.

Ammonia-based

Ammonia-based processes do not allow recovery of the pulping chemicals since ammonia or ammonium salts are oxidized to nitrogen and nitrogen oxides when burned.

Magnesium-based

The recovery process used in magnesium-based sulfite pulping the "Magnefite" process is well developed. [8] The concentrated brown liquor is burned in a recovery boiler, producing magnesium oxide and sulfur dioxide, both of which are recovered from the flue gases. Magnesium oxide is recovered in a wet scrubber to give a slurry of magnesium hydroxide.

MgO + H2O → Mg(OH)2

This magnesium hydroxide slurry is then used in another scrubber to absorb sulfur dioxide from the flue gases producing a magnesium bisulfite solution that is clarified, filtered and used as the pulping liquor.

Mg(OH)2 + 2 SO2 → Mg(HSO3)2
Sodium-based

Sodium-based processes use a recovery system similar to that used in the kraft recovery process, except that there is no "lime cycle".

Processes involved in sulfite pulping

The process is conducted in large pressure vessels called digesters. Sulfite pulping is carried out between pH 1.5 and 5. The pulp is in contact with the pulping chemicals for 4 to 14 hours and at temperatures ranging from 130 to 160 °C (266 to 320 °F), again depending on the chemicals used.

Most of the intermediates involved in delignification in sulfite pulping are resonance-stabilized carbocations formed either by protonation of carbon-carbon double bonds or acidic cleavage of ether bonds which connect many of the constituents of lignin. It is the latter reaction which is responsible for most lignin degradation in the sulfite process. [2] The electrophilic carbocations react with bisulfite ions (HSO3−)to give sulfonates.

R-O-R' + H+ → R+ + R'OH
R+ + HSO3− → R-SO3H

The sulfite process does not degrade lignin to the same extent that the kraft process does and the lignosulfonates from the sulfite process are useful byproducts.

Chemical recovery

The spent cooking liquor from sulfite pulping is usually called brown liquor, but the terms red liquor, thick liquor and sulfite liquor are also used (compared to black liquor in the kraft process). Pulp washers, using countercurrent flow, remove the spent cooking chemicals and degraded lignin and hemicellulose. The extracted brown liquor is concentrated, in multiple effect evaporators. The concentrated brown liquor can be burned in the recovery boiler to generate steam and recover the inorganic chemicals for reuse in the pulping process or it can be neutralized to recover the useful byproducts of pulping. Recent developments in Chemrec's black liquor gasification process, adapting the technology to use in the sulfite pulping process, could make second generation biofuels production an alternative to the conventional recovery boiler technology. [9] Around 1906 Gösta Ekström a Swedish engineer patented a process of ethanol generation from the residual 2-2.5% fermentable hexose sugars in the spent liquor. [10]

The sulfite process can use calcium, ammonium, magnesium or sodium as a base.

Applications

The sulfite process is acidic and one of the drawbacks is that the acidic conditions hydrolyze some of the cellulose, which means that sulfite pulp fibers are not as strong as kraft pulp fibers. The yield of pulp (based on wood used) is higher than for kraft pulping and sulfite pulp is easier to bleach.

Commodity

Sulfite pulp remains an important commodity, especially for specialty papers and as a source of cellulose for non-paper applications. It is used to make fine paper, tissue, glassine, [11] and to add strength to newsprint.

Dissolving pulp

A special grade of bleached sulfite pulp is known as dissolving pulp [12] which is the raw material for a wide variety of cellulose derivatives, for example rayon, cellophane, cellulose acetate and methylcellulose.

Rayon is a reconstituted cellulose fiber used to make many fabrics.

Cellophane is a clear reconstituted cellulose film used in wrapping and windows in envelopes.

Cellulose acetate was used to make flexible films for photographic use, computer tapes and so on and also to make fibers.

Methylcellulose and other cellulose ether derivatives are used in a wide range of everyday products from adhesives to baked goods to pharmaceuticals. [13]

Byproducts

Sulfite pulping is generally less destructive than kraft pulping, so there are more usable byproducts.

Lignosulfonates

Chief among sulfite process byproducts are lignosulfonates, which find a wide variety of uses where a relatively inexpensive agent is needed to make a water dispersion of a water-insoluble material. Lignosulfonates are used in tanning leather, making concrete, drilling mud, drywall and so on. [14]

Oxidation of lignosulfonates was used to produce vanillin (artificial vanilla), and this process is still used by one supplier (Borregaard, Norway) while all North American production by this route ceased in the 1990s. [15]

Other byproducts

Acid hydrolysis of hemicelluloses during sulfite pulping produces monosaccharides, predominantly mannose for softwoods and xylose for hardwoods, [2] which can be fermented to produce ethanol.

See also

Related Research Articles

<span class="mw-page-title-main">Sodium carbonate</span> Chemical compound

Sodium carbonate refers to a family of inorganic compounds with the formula Na2CO3(H2O)x, where x can range from 0 to 10. All forms are white, odourless, water-soluble salts that yield alkaline solutions in water. Historically, it was extracted from the ashes of plants grown in sodium-rich soils. Because the ashes of these sodium-rich plants were noticeably different from ashes of wood, sodium carbonate became known as "soda ash". It is produced in large quantities from sodium chloride and limestone by the Solvay process.

<span class="mw-page-title-main">Lignin</span> Structural phenolic polymer in plant cell walls

Lignin is a class of complex organic polymers that form key structural materials in the support tissues of most plants. Lignins are particularly important in the formation of cell walls, especially in wood and bark, because they lend rigidity and do not rot easily. Chemically, lignins are polymers made by cross-linking phenolic precursors.

<span class="mw-page-title-main">Pulp (paper)</span> Fibrous material used notably in papermaking

Pulp is a lignocellulosic fibrous material prepared by chemically or mechanically separating cellulose fibers from wood, fiber crops, waste paper, or rags. Mixed with water and other chemical or plant-based additives, pulp is the major raw material used in papermaking and the industrial production of other paper products.

<span class="mw-page-title-main">Paper engineering</span>

Paper engineering is a branch of engineering that deals with the usage of physical science and life sciences in conjunction with mathematics as applied to the converting of raw materials into useful paper products and co-products. The field applies various principles in process engineering and unit operations to the manufacture of paper, chemicals, energy and related materials. The following timeline shows some of the key steps in the development of the science of chemical and bioprocess engineering:

<span class="mw-page-title-main">Kraft process</span> Process of converting wood into wood pulp

The kraft process (also known as kraft pulping or sulfate process) is a process for conversion of wood into wood pulp, which consists of almost pure cellulose fibres, the main component of paper. The kraft process involves treatment of wood chips with a hot mixture of water, sodium hydroxide (NaOH), and sodium sulfide (Na2S), known as white liquor, that breaks the bonds that link lignin, hemicellulose, and cellulose. The technology entails several steps, both mechanical and chemical. It is the dominant method for producing paper. In some situations, the process has been controversial because kraft plants can release odorous products and in some situations produce substantial liquid wastes.

<span class="mw-page-title-main">Sulfonic acid</span> Organic compounds with the structure R−S(=O)2−OH

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<span class="mw-page-title-main">Pulp mill</span>

A pulp mill is a manufacturing facility that converts wood chips or other plant fiber sources into a thick fiber board which can be shipped to a paper mill for further processing. Pulp can be manufactured using mechanical, semi-chemical, or fully chemical methods. The finished product may be either bleached or non-bleached, depending on the customer requirements.

<span class="mw-page-title-main">Black liquor</span> Industrial by-product

In industrial chemistry, black liquor is the by-product from the kraft process when digesting pulpwood into paper pulp removing lignin, hemicelluloses and other extractives from the wood to free the cellulose fibers.

Lignosulfonates (LS) are water-soluble anionic polyelectrolyte polymers: they are byproducts from the production of wood pulp using sulfite pulping. Most delignification in sulfite pulping involves acidic cleavage of ether bonds, which connect many of the constituents of lignin. Sulfonated lignin (SL) refers to other forms of lignin by-product, such as those derived from the much more popular Kraft process, that have been processed to add sulfonic acid groups. The two have similar uses and are commonly confused with each other, with SL being much cheaper. LS and SL both appear as free-flowing powders; the former is light brown while the latter is dark brown.

<span class="mw-page-title-main">Kraft paper</span> Paper or paperboard produced from chemical pulp produced in the kraft process

Kraft paper or kraft is paper or paperboard (cardboard) produced from chemical pulp produced in the kraft process.

Bleaching of wood pulp is the chemical processing of wood pulp to lighten its color and whiten the pulp. The primary product of wood pulp is paper, for which whiteness is an important characteristic. These processes and chemistry are also applicable to the bleaching of non-wood pulps, such as those made from bamboo or kenaf.

<span class="mw-page-title-main">Calcium sulfite</span> Chemical compound

Calcium sulfite, or calcium sulphite, is a chemical compound, the calcium salt of sulfite with the formula CaSO3·x(H2O). Two crystalline forms are known, the hemihydrate and the tetrahydrate, respectively CaSO3·½(H2O) and CaSO3·4(H2O). All forms are white solids. It is most notable as the product of flue-gas desulfurization.

<span class="mw-page-title-main">Paper</span> Thin material for writing, printing, etc.

Paper is a thin sheet material produced by mechanically or chemically processing cellulose fibres derived from wood, rags, grasses or other vegetable sources in water, draining the water through fine mesh leaving the fibre evenly distributed on the surface, followed by pressing and drying. Although paper was originally made in single sheets by hand, almost all is now made on large machines—some making reels 10 metres wide, running at 2,000 metres per minute and up to 600,000 tonnes a year. It is a versatile material with many uses, including printing, painting, graphics, signage, design, packaging, decorating, writing, and cleaning. It may also be used as filter paper, wallpaper, book endpaper, conservation paper, laminated worktops, toilet tissue, or currency and security paper, or in a number of industrial and construction processes.

Dissolving pulp, also called dissolving cellulose, is bleached wood pulp or cotton linters that has a high cellulose content. It has special properties including a high level of brightness and uniform molecular-weight distribution. This pulp is manufactured for uses that require a high chemical purity, and particularly low hemicellulose content, since the chemically similar hemicellulose can interfere with subsequent processes. Dissolving pulp is so named because it is not made into paper, but dissolved either in a solvent or by derivatization into a homogeneous solution, which makes it completely chemically accessible and removes any remaining fibrous structure. Once dissolved, it can be spun into textile fibers, or chemically reacted to produce derivatized celluloses, such cellulose triacetate, a plastic-like material formed into fibers or films, or cellulose ethers such as methyl cellulose, used as a thickener.

Soda pulping is a chemical process for making wood pulp with sodium hydroxide as the cooking chemical. In the Soda-AQ process, anthraquinone (AQ) may be used as a pulping additive to decrease the carbohydrate degradation. The soda process gives pulp with lower tear strength than other chemical pulping processes, but has still limited use for easily-pulped materials like straw and some hardwoods.

White liquor is a strongly alkaline solution mainly of sodium hydroxide and sodium sulfide. It is used in the first stage of the Kraft process in which lignin and hemicellulose are separated from cellulose fiber for the production of pulp. The white liquor breaks the bonds between lignin and cellulose. It is called white liquor due to its white opaque colour.

In industrial paper-making processes, organosolv is a pulping technique that uses an organic solvent to solubilise lignin and hemicellulose. It has been considered in the context of both pulp and paper manufacture and biorefining for subsequent conversion of cellulose to fuel ethanol. The process was invented by Theodor Kleinert in 1968 as an environmentally benign alternative to kraft pulping.

<span class="mw-page-title-main">Sodium bisulfite</span> Chemical compound

Sodium bisulfite (or sodium bisulphite, sodium hydrogen sulfite) is a chemical mixture with the approximate chemical formula NaHSO3. Sodium bisulfite in fact is not a real compound, but a mixture of salts that dissolve in water to give solutions composed of sodium and bisulfite ions. It appears in form of white or yellowish-white crystals with an odor of sulfur dioxide. For properties of sodium bisulfite, refer to the table located to the right. Regardless of its ill-defined nature, sodium bisulfite is used in many different industries such a food additive with E number E222 in the food industry, a reducing agent in the cosmetic industry, and a decomposer of residual hypochlorite used in the bleaching industry.

<span class="mw-page-title-main">Paper chemicals</span> Chemicals used in paper manufacturing

Paper chemicals designate a group of chemicals that are used for paper manufacturing, or modify the properties of paper. These chemicals can be used to alter the paper in many ways, including changing its color and brightness, or by increasing its strength and resistance to water. The chemicals can be defined on basis of their usage in the process.

Mechanical pulping is the process in which wood is separated or defibrated mechanically into pulp for the paper industry.

References

  1. ↑ Burger, Peter Charles Fenerty and his Paper Invention. Toronto: Peter Burger, 2007. ISBN   978-0-9783318-1-8 pp.25–30
  2. 1 2 3 4 E. Sjöström (1993). Wood Chemistry: Fundamentals and Applications. Academic Press. ISBN   0-12-647480-X.
  3. 1 2 3 4 Biermann, Christopher J. (1993). Essentials of Pulping and Papermaking . San Diego: Academic Press, Inc. ISBN   0-12-097360-X.
  4. ↑ "History of Paper". Archived from the original on 2006-12-08. Retrieved 2007-10-08.
  5. ↑ "Swedish, German mills phase out sulfite". Pulp and Paper. January 1997. Retrieved 2007-10-08.
  6. ↑ "Wisconsin sulfite mill shuts down 2005" . Retrieved 2007-10-07.
  7. ↑ Friederich, Steven (September 25, 2006). "Living on borrowed time its whole life (Weyerhauser sulfite mill)". The Daily World. Retrieved 2007-10-08.[ dead link ]
  8. ↑ "Magnefite process". Archived from the original on 2007-12-17. Retrieved 2007-10-11.
  9. ↑ Chemrec web site
  10. ↑ Kaukoranta, Antti (1981). Sulphite alcohol industry in Finland in 1918-1978. Finland: Paino Polar OY. p. 7. ISBN   951-9479-25-2.
  11. ↑ "Grades and uses of paper". Archived from the original on 2012-09-19. Retrieved 2007-10-12.
  12. ↑ "Dissolving pulp by the sulfite process" . Retrieved 2007-10-12.
  13. ↑ "Applications for Methocel cellulose ethers from Dow Chemical". Archived from the original on 2008-12-24. Retrieved 2007-10-12.
  14. ↑ "Uses of lignosulfonates". Archived from the original on 2007-10-09. Retrieved 2007-10-07.
  15. ↑ Hocking, Martin B. (September 1997). "Vanillin: Synthetic Flavoring from Spent Sulfite Liquor" (PDF). Journal of Chemical Education. 74 (9): 1055. Bibcode:1997JChEd..74.1055H. doi:10.1021/ed074p1055 . Retrieved 2006-09-09.
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