- Sulfite process
The sulfite process produces
wood pulpwhich is almost pure cellulosefibers by using various salts of sulfurous acidto extract the ligninfrom wood chips in large pressure vessels called digesters. The salts used in the pulping process are either sulfites (SO32−), or bisulfites (HSO3−), depending on the pH. The counter ioncan be sodium(Na+), calcium(Ca2+), potassium(K+), magnesium(Mg2+) or ammonium(NH4+).
pulp millusing the sulfite process was built in Swedenin 1874 and used magnesium as the counter ion.cite book |last=Biermann |first=Christopher J. |authorlink= |coauthors= |title=Essentials of Pulping and Papermaking |year=1993 |publisher=Academic Press, Inc. |location=San Diego |isbn=0-12-097360-X] Calcium became the standard counter ion until the 1950s. Sulfite pulping was the dominant process for making wood pulp intil it was surpassed by the kraft processin the 1940s. Sulfite pulps now account for less than 10% of the total chemical pulp production.
The sulfite process is
acidicand 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 yieldof pulp (based on wood used) is higher than for kraft pulping and sulfite pulp is easier to bleach. Apart from printing and specialty papers, a special grade of sulfite pulp, known as " dissolving pulp" is used to make cellulose derivatives.cite web|url=http://www.npchem.co.jp/english/product/dp/index.html|title=Dissolving pulp by the sulfite process|accessdate=2007-10-12] Lignosulfonatesare an important byproduct of sulfite bleaching.cite web|url=http://www.lignin.org/whatis.html|title=Uses of lignosulfonates|accessdate=2007-10-07] These materials are used in making concrete, drilling mud, drywalland so on.
The use of wood to make pulp for paper began with the development of
mechanical pulpingin Germanyby F.G. Kellerin the 1840s. Chemical processes quickly followed, first with J. Roth's use of sulfurous acidto treat wood, followed by B. Tilghman's US patenton the use of calcium bisulfite, Ca(HSO3)2, to pulp wood in 1867. Almost a decade later the first commercial sulfite pulp millwas 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 processwas developed by Carl F. Dahlin 1879 and the first kraft millstarted (in Sweden) in 1890.cite book |last=Biermann |first=Christopher J. |authorlink= |coauthors= |title=Essentials of Pulping and Papermaking |year=1993 |publisher=Academic Press, Inc. |location=San Diego |isbn=0-12-097360-X] The invention of the recovery boilerby G.H. Tomlinsonin the early 1930s cite book |author= E. Sjöström |title= Wood Chemistry: Fundamentals and Applications |publisher= Academic Press|year= 1993] 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 [cite web|url=http://www.indiapapermarket.com/history1.asp|title= History of Paper|accessdate=2007-10-08] made the kraft process the dominant pulping process starting in the 1940s. Sulfite pulps now account for less than 10% of the total chemical pulp productionand the number of sulfite mills continues to decrease. [cite journal |year=1997 |month=January |title=Swedish, German mills phase out sulfite |journal=Pulp and Paper |volume= |issue= |pages= |id= |url=http://findarticles.com/p/articles/mi_qa3636/is_199701/ai_n8747040 |accessdate= 2007-10-08 |quote= ] [cite web|url=http://www.secinfo.com/drvzf.z35.htm|title=Wisconsin sulfite mill shuts down 2005|accessdate=2007-10-07] [cite news |first=Steven |last=Friederich |title=Living on borrowed time its whole life (Weyerhauser sulfite mill) |url=http://www.thedailyworld.com/articles/2006/09/25/local_news/01news.txt |work= |publisher=The Daily World |date=September 25, 2006 |accessdate=2007-10-08 ]
Sulfite pulp remains an important
commodity, especially for specialty papers and as a source of cellulose for non-paper applications. Sulfite pulp is used to make fine paper, tissue, glassine. [cite web|url=http://www.paperonweb.com/grade11.htm|title=Grades and uses of paper|accessdate=2007-10-12] and to add strength to newsprint. A special grade of bleached sulfite pulp is known as " dissolving pulp" which is the raw material for a wide variety of cellulose derivatives, for example rayon, cellophane, cellulose acetateand 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 adhesivesto baked goodsto pharmaceuticals. [cite web|url=http://www.dow.com/methocel/index.htm|title=Applications for Methocel cellulose ethers from Dow Chemical|accessdate=2007-10-12]
Processes involved in sulfite pulping
Pulping liquor preparation
The pulping liquor for most sulfite mills is made by burning
sulfurwith the correct amount of oxygento give sulfur dioxide, which is then absorbed into water to give sulfurous acid.
:S + O2 → SO2:SO2 + H2O unicode| H2SO3
Care must be taken to avoid the formation of
sulfur trioxidesince it gives undesired sulfuric acidwhen it is dissolved in water.
:2 SO2 + O2 → 2SO3:SO3 + H2O unicode| H2SO4
Sulfuric acid is undesirable since it promotes hydrolysis of cellulose without contributing to delignification.
The cooking liquor is prepared by adding the counter ions as hydroxides or carbonates. The relative amounts of each species present in the liquid depned largely on the relative amounts of sulfurous used. For monovalent (Na+, K+ and NH4+) hydroxides, MOH:
:H2SO3 + MOH → MHSO3 + H2O:MHSO3 + MOH → M2SO3 + H2O
For divalent (Ca2+, Mg2+) carbonates, MCO3:
:MCO3 + 2H2SO3 → M(HSO3)2 + CO2 + H2O:M(HSO3)2 + MCO3 → 2 MSO3 + CO2 + H2O
Sulfite pulping is carried out between pH 1.5 and 5, depending on the counterion to sulfite (bisulfite) and the ratio of base to sulfurous acid. 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
carbocationsformed 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. The electrophiliccarbocations 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
ligninto the same extent that the kraft processdoes and the lignosulfonates from the sulfite process are useful byproducts.
The spent cooking liquor from sulfite pulping is called brown or
red liquor(compared to black liquorin the kraft process). Pulp washers, using countercurrent flow, remove the spent cooking chemicals and degraded lignin and hemicelulose. The extracted brown liquor is concentrated, in multiple effect evaporators. The concentrated brown liquor can be burned in the recovery boilerto 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.
Ammonia-based processes do not allow recovery of the pulping chemicals since ammonia or ammonium salts are oxidized to
nitrogenand nitrogen oxideswhen burned. The earliest process used calcium, obtained as inexpensive calcium carbonateand there was little incentive to recover the inorganic materials. Sodium-based processes use a recovery system similar to that used in the kraft recovery process, except that there is no "lime cycle".
The recovery process used in magnesium-based sulfite pulping the "Magnefite" process is well developed. [cite web|url=http://www.p2pays.org/ref/10/09463.htm|title=Magnefite porcess|accessdate=2007-10-11] The concentrated brown liquor is burned in a recovery boiler, producing
magnesium oxideand sulfur dioxide, both of which are recovered from the flue gases. Magnesium oxide is recovered in a wet scrubberto give a of magnesium hydroxide.
:MgO + H2O → Mg(OH)2
This magnesium hydroxide slurry is then used in another scrubber to absorb
sulfur dioxidefrom 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
Sulfite pulping is generally less destructive than kraft pulping, so there are more usable byproducts. Chief among these are
lignosulfonates, which find a wide variety of uses whereas relatively inexpensive agent is needed to make a water dispersion of a water-insoluble material. Lignosulfonates are used in tanningleather, making concrete, drilling mud, drywalland so on.
Oxidation of lignosulfonates was used to produce
vanillin(artificial vanilla), but this process is no longer used. [cite journal
last = Hocking| first = Martin B.| year = 1997| month = September
title = Vanillin: Synthetic Flavoring from Spent Sulfite Liquor
journal = Journal of Chemical Education| volume = 74| issue = 9| pages = 1055| url = http://jchemed.chem.wisc.edu/hs/Journal/Issues/1997/Sep/abs1055.html
format = PDF| accessdate = 2006-09-09]
Bleaching of wood pulp
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