Preparation of pulp and paper by industrial process

1. PULP AND PAPER INDUSTRY
SukhRam
EVS
BBAU

2. Content
1. History
2. Wood: Chemical Composition and Types
3. Pulp
4. Pulping
 Mechanical
 SGW
 RMP
 TMP
 CTMP
 Semichemical (NSSC Process)
 Chemical
 Kraft Process
 Sulfite Process

3. History of Paper Making
• 105 A.D. Ts'ai Lun, a Chinese court official,
invented paper
mulberry bark, hemp and rags with water
• Paper was first produced in Egypt around 900
A.D.
• England early in the 14th Century
To this point all paper and
books were produced by hand

4. History of Paper Making
• Gutenberg Press in 1436
• On September 30, 1452 Guttenberg's
Bible was published
The printing press allowed
printed materials to be
produced at a greater rate
thus increasing the demand
for paper

5. History of Paper Making
• First North American paper mill in 1690
near Philadelphia
• America's first writing papers were
produced by collecting, separating, and
cleaning old cloth rags
• The paper was made in single sheets

6. History of Paper Making
• 1798, Nicholas-Louis Robert of France
invented a paper making machine that
produced paper on an endless wire screen
• Further developed in England by Brian Donkin
• Not put into service until 1804

7. History of Paper Making
The Fourdrinier
1st Fourdrinier machine
in the US, 1827

8. History of Paper Making
1866
American Benjamin
Tilghman developed the
sulfite pulping process
Dominant pulping process
until the 1930s
1879
German chemist, C. F. Dahl,
developed the Kraft
pulping process
1930s dominant pulping
process and still is today
Two major advancements in pulping were
developed in the late 1800s

9. Advantages of the Kraft Process
• The majority of the chemicals used in the
pulping process are recoverable for use in
further pulping operations
• Large amounts of energy are produced in the
recovery boilers during the recovery process
• The Kraft process can pulp pine trees

10. The Modern Paper Making Process
Paper Making Process Overview [Chesterton, 2004]

11. The Modern Paper Making Process
Wood Yard [Chesterton, 2004]

12. Chemical Composition and Conversion of wood
Cellulose
Cellulose is a long linear molecule with over 7,000 glucoses linked end to end. Bundles of
cellulose molecules fasciculate laterally by hydrogen bonds to form microfibrils. Microfibrils
are the framework of the cell wall and are responsible for the strength of wood.

13. Chemical Composition of Wood
Hemicelluloses
Hemicelluloses are a group of compounds made up of 60 to 200 units
of different sugars including glucose. Softwood hemicelluloses are
different from hardwood hemicelluloses. Hemicellulose and lignin are
ingredients of natural glue in plant materials and together they make up
the matrix of the cell wall microfibrils are embedded in this matrix.
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Softwood hemicellulose
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Hardwood hemicelluose

14. Chemical Composition of Wood
Lignin
Lignin is a three dimensional molecule, formed by linking 15 to 18
phenolic units called phenylpropanes. Lignin contributes to the
rigidity of wood. Lignin together with hemicelluloses form the matrix
system in the cell wall; they also are the natural glue to hold cells
together.
A
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B
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1
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Lignin monomers: A=coniferyl alcohol；B=sinapyl alcohol；C=p-coumaryl alcohol

16. Chemical Composition of Wood
Extractives
Extractives are defined as compounds in plant materials and wood that
can be extracted with various solvents, such as water and organic
solvents. Extractives include turpentine, fatty acids, tannins, etc. that
give wood colors and odors. Extractive content of wood is extremely
variable depending upon species. Some extractives are responsible
for the durability of wood against fungi decay and insect injuries; other
extractives have medicinal values.

17. Chemical Composition of Wood
• Chemical Composition
Species Cellulose Hemicellulose Lignin
Softwoods 43% 28% 29%**
Hardwoods 45% 34%** 21%
** These differences are significant
Chemical composition of crop residues is similar to hardwoods

18. Hard and soft woods
It is common to classify wood as either softwood or hardwood. The
wood from conifers (e.g. pine) is called softwood, and the wood
from dicotyledons (usually broad-leaved trees, e.g. oak) is called
hardwood. These names are a bit misleading, as hardwoods are not
necessarily hard, and softwoods are not necessarily soft. The well-
known balsa (a hardwood) is actually softer than any commercial
softwood. Conversely, some softwoods (e.g. yew) are harder than
many hardwoods.
One of the densest woods is black ironwood.

19. PULP
Pulp is a lignocellulosic fibrous material prepared by
chemically or mechanically separating cellulose fibres
from wood, fiber crops or waste paper. The wood fiber
sources required for pulping are "45% sawmill residue,
21% logs and chips, and 34% recycled paper" (Canada,
2014). Many kinds of paper are made from wood with
nothing else mixed into them. This includes newspaper,
magazines and even toilet paper. Pulp is one of the most
abundant raw materials worldwide.

20. Structure of fibres of pulp

21. Manufacture of wood pulp
A pulp mill is a manufacturing facility that converts wood chips or
other plant fibre source into a thick fiberboard which can be shipped
to a paper mill for further processing. Pulp can be manufactured
using mechanical, semi-chemical or fully chemical methods (kraft
and sulfite processes). The finished product may be either bleached
or non-bleached, depending on the customer requirements.
Wood and other plant materials used to make pulp contain three
main components (apart from water): cellulose fibers (desired for
papermaking), lignin (a three-dimensional polymer that binds the
cellulose fibres together) and hemicelluloses, (shorter branched
carbohydrate polymers). The aim of pulping is to break down the
bulk structure of the fibre source, be it chips, stems or other plant
parts, into the constituent fibres.

22. Chemical pulping achieves this by degrading the lignin and
hemicellulose into small, water-soluble molecules which can be washed
away from the cellulose fibres without depolymerizing the cellulose
fibres (chemically depolymerizing the cellulose weakens the fibres). The
various mechanical pulping methods, such as groundwood (GW) and
refiner mechanical (RMP) pulping, physically tear the cellulose fibres
one from another. Much of the lignin remains adhering to the fibres.
Strength is impaired because the fibres may be cut. There are a number
of related hybrid pulping methods that use a combination of chemical
and thermal treatment to begin an abbreviated chemical pulping process,
followed immediately by a mechanical treatment to separate the fibres.
These hybrid methods include thermomechanical pulping, also known as
TMP, and chemithermomechanical pulping, also known as CTMP. The
chemical and thermal treatments reduce the amount of energy
subsequently required by the mechanical treatment, and also reduce the
amount of strength loss suffered by the fibres.

23. Mechanical Pulping
The primary goals of pulping are to free fibers in wood from the lignin that
binds these fibers together, and then to suspend the fibers in water into a
slurry suitable for paper making. As the oldest form of pulping, mechanical
pulping uses mechanical energy to weaken and separate fibers from wood
via a grinding action. The advantage to mechanical pulping is that it
produces much higher yields than chemical pulping processes (up to 97% -
However, because this process does not dissolve lignin and because
mechanical grinding produces shorter fibers, the fiber strength and age
resistance of the resulting pulp are low. Consequently, most mechanical
pulp is used for lower grade papers such as newsprint, magazines, and
catalogues. Mechanical pulping also requires more raw materials screening
to remove contaminants such as dirt, and knots than chemical pulping
processes.

24. Stone groundwood (SGW) pulping: In the SGW process, small logs are
ground against artificial bonded stones made of silicon carbide or aluminum
oxide grits. The process gives a high yield, but the fibers produced can be very
short and often must be combined with expensive chemical fibers to be strong
enough to pass through the paper machine and subsequent coating and printing
processes.
Refiner mechanical pulping (RMP): In RMP wood feedstock is ground
between two grooved discs. The process keeps the high yield advantages of the
SGW process, while producing somewhat longer fibers with greater strength.
This permits lighter weight paper to be used for printing and result in more
print media per ton of feedstock. The RMP process can use wood feedstock
other than logs, such as wood scraps and sawdust from lumber mills.
Mechanical Pulping : Four Types

25. Thermomechanical pulping (TMP): In TMP wood chips are first steamed to
soften them before being ground in the same manner as the RMP process. The
TMP process generates the highest grade mechanical pulp but is also a high
energy intensity process due to its steam use. This process can also produce a
darker pulp that is more costly to bleach. Despite these drawbacks, TMP is the
most common mechanical process in use today.
Chemi-thermomechanical pulping (CTMP): CTMP involves the application of
chemicals to wood chips prior to refiner pulping. The chemical pre-treatment of
wood chips allows for less destructive separation of fibers from the feedstock,
resulting in longer fibers, higher fiber content, and far fewer shives. The CTMP
process also produces more flexible fibers (which provide higher sheet density,
burst strength, and tensile strength) and higher pulp brightness than the TMP
process. Its primary drawback, like TMP, is that it is a high energy intensity
process

26. Mechanical Pulping
• Groundwood: Drum-debarked bolts are loaded into the machine and pressed against revolving
grinding stone to convert logs into wood pulp. A water shower is applied to cool the
stone, wash the pulp off the stone into a vat and adjust the consistency of the slurry.
• Pulp Quality: Over 90% pulp yield; little discoloration of the pulp; suffer certain degree of fiber
breakage; presence of fiber bundles.
• Species Selection: Light-colored softwoods preferred because the pulp uses less chemicals for
bleaching and long fibers compensates fiber breakage.
• Major Uses: Add about 15% long-fiber chemical pulp to increase tensile strength for newsprint.

27. Thermal Mechanical Pulping (TMP)
• Basic Principle: Wood chips are first softened with steam to over 180 oC,
followed by disk refining softened chips into pulp.
• Major Use: TMP is mainly used to manufacture medium density fiberboard and
hardboard (high density fiberboard)

28. Chemical Thermal Mechanical process (CTMP)
• In this process southern pine wood chips (brightness about 55%
to 57%) are impregnated with 2% sodium bisulfite (NaHSO3)
based on dry weight of chips, followed by processing with the
thermomechanical pulping. Sodium bisulfite is able to dissolve
some lignin and softens the wood chips.
• Because it also is a bleaching agent southern pine pulp
produced in this manner has a brightness about 65%, making
the species an important raw material in U.S. for newsprint
production.

29. Semichemical pulping
• Neutral Sulfite Semichemical Process (NSSC)
– The NSSC process is the most important in the production of corrugating
medium, which is used in the construction of cardboard boxes. Cardboard is
made by sandwiching corrugating medium with linerboards (thick kraft paper).
– The process starts by impregnating wood chips with 12% pulping liquor
containing sodium sulfite (Na2SO3) and sodium carbonate (Na2CO3), cooking the
chips in a screw-digester at 165 oC for 1 hr, after which the softened chips are
refined into pulp with a disk refiner.
--Pulp for corrugating medium typically has a
yield about 80-85%. High lignin content in the pulp
provides the medium with necessary rigidity to
sustain compression strength, while linerboards
provide the tensile strength.
--NSSC pulping also can be used to produce
paper pulp by using more pulping chemicals (~20%)
and cooking at higher temperatures (~180 oC) for
3 hours.

30. NSSC Pulping
• Spent Liquor of NSSC Process
--The spent liquor contains pulping chemicals and dissolved organic
matters from wood chips. The spent liquor is concentrated to about 50%
solids by evaporating water, followed by spraying the concentrated spent
liquor into a boiler. Combustion of organic matters in the furnace
generates energy, and the pulping chemicals melt in the smelt as sodium
sulfate (Na2SO4) and sodium carbonate (Na2CO3).
--Unfortunately, the two chemicals can not be separated from each other
and reused in the NSSC pulping. But the mixture can be shipped to kraft
mills to be converted into pulping chemicals used by the kraft mills.

31. Chemical Pulping
• Chemical pulp is produced by combining wood chips and
chemicals in large vessels known as digesters where heat
and the chemicals break down the lignin, which binds
the cellulose fibres together, without seriously degrading
the cellulose fibres. Chemical pulp is used for materials
that need to be stronger or combined with mechanical
pulps to give a product different characteristics. The kraft
process is the dominant chemical pulping method, with
the sulfite process being second. Historically soda
pulping was the first successful chemical pulping method.

32. Kraf process

33. 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 fibers, the main
component of paper. The kraft process entails treatment of wood
chips with a hot mixture of, water, sodium hydroxide, and
sodium sulfide, 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
smelly products and in some situations produce substantial
liquid wastes
KRAFT PROCESS

34. History
The kraft process (so called because of the superior strength
of the resulting paper, from the German word Kraft for
'strength') was invented by Carl F. Dahl in 1879 in
Danzig,Prussia, Germany. U.S. Patent 296,935 was issued in
1884, 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 a nearly closed-cycle process with respect to inorganic
chemicals, apart from those used in the bleaching process.
For this reason, in the 1940s, the kraft process superseded the
sulfite process as the dominant method for producing wood
pulp.

35. Full Chemical Pulping—The Kraft Process
• Kraft Process (Sulfate Process): This is the main full chemical process to
produce paper pulps for the following reasons:
-- All materials including all woody and non-woody species can be pulped
-- Produces strong paper pulp
-- Tolerate about 10% bark in wood chips, allowing the use of whole-tree chips
-- 98% pulping chemicals can be recovered
• The process uses Sodium hydroxide (3 parts, NaOH) and sodium sulfide (1 part,
Na2S) to cook chips. Wood chips are charged into a continuous digester,
impregnated with the pulping liquor and heated 170 oC (1.5 hours), followed by
cooking the chips at that temperature for additional 1.5 hr.
• During washing stages dissolved hemicelluloses precipitate and deposit onto
fiber surfaces, which acting as an adhesive to bond fibers resulting in strong
paper products.

36. Full Chemical Pulping—The Kraft Process

37. The process
1. Impregnation
2. Cooking
3. Recovery process
4. Blowing
5. Screening
6. Washing
7. Bleaching
8. Process chemicals

38. Impregnation
Common wood chips used in pulp production are 12–25 millimetres (0.47–0.98 in)
long and 2–10 millimetres (0.079–0.394 in) thick. The chips normally first enter the
presteaming where they are wetted and preheated with steam. Cavities inside fresh
wood chips are partly filled with liquid and partly with air. The steam treatment
causes the air to expand and about 25% of the air to be expelled from the chips. The
next step is to saturate the chips with black and white liquor. Air remaining in chips
at the beginning of liquor impregnation is trapped within the chips. The
impregnation can be done before or after the chips enters the digester and is
normally done below 100 °C (212 °F). The cooking liquors consist of a mixture of
white liquor, water in chips, condensed steam and weak black liquor. In the
impregnation, cooking liquor penetrates into the capillary structure of the chips and
low temperature chemical reactions with the wood begin. A good impregnation is
important to get a homogeneous cook and low rejects. About 40–60% of all alkali
consumption in the continuous process occurs in the impregnation zone

39. Cooking
The wood chips are then cooked in pressurized vessels called digesters. Some
digesters operate in a batch manner and some in a continuous process. There
are several variations of the cooking processes both for the batch and the
continuous digesters. Digesters producing 1,000 tonnes or more of pulp per
day are common, with the largest producing more than 3,500 tonnes per day.
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 170 to 176 °C (338 to 349
°F). Under these conditions lignin and hemicellulose degrade to give
fragments that are soluble in the strongly basic liquid. The solid pulp (about
50% by weight of the dry wood chips) is collected and washed. At this point
the pulp is known as brown stock because of its color. The combined liquids,
known as black liquor (because of its color), contain lignin fragments,
carbohydrates from the breakdown of hemicellulose, sodium carbonate,
sodium sulfate and other inorganic salts.

40. 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 (S2−)
orbisulfide (HS−) ions

41. Recovery process
The excess black liquor contains about 15% solids and is
concentrated in a multiple effect evaporator. After the first
step the black liquor has about 20 - 30% solids. At this
concentration the rosin soap rises to the surface and is
skimmed off. The collected soap is further processed to tall
oil. Removal of the soap improves the evaporation operation
of the later effects.
The weak black liquor is further evaporated to 65% 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.
During combustion sodium sulfate is reduced to sodium
sulfide by the organic carbon in the mixture:

42. 1. Na2SO4 + 2 C → Na2S + 2 CO2
This reaction is similar to thermochemical sulfate reduction 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. The resulting solution of sodium
carbonate and sodium sulfide is known as "green liquor", although it is
not known exactly what causes the liquor to be green. This liquid is
mixed with calcium oxide, which becomes calcium hydroxide in
solution, 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):

43. 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 .

44. 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 which can be used by an associated paper mill or
sold to neighboring industries or communities through to the local
electrical grid. Additionally, bark and wood residues are often
burned in a separate power boiler to generate steam.

46. Chemical Recovery
• Evaporation: 15% solids spent liquor concentrated to 50%
• Incineration: Concentrated spent liquor sprayed into furnace;
Organic matters burned producing energy:
Inorganic = Na2CO3 + Na2S
Dissolved into water = green liquor
• Causticizing: CaO + H2O Ca(OH)2
Ca(OH)2 + Na2CO3 + Na2S 2NaOH + Na2S + CaCO3
2NaOH + Na2S = White liquor recovered
• Lime Recovery: CaCO3 heat 100oC CaO + CO2

47. Blowing
The finished cooked wood chips
are blown to a collection tank
called a blow tank that operates at
atmospheric pressure. This releases
a lot of steam and volatiles. The
volatiles are condensed and
collected; in the case of northern
softwoods this consists mainly of
raw turpentine.

48. 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 are 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 is typically 0.5 - 3.0% of the digester output, while
the shives content is about 0.1- 1.0%.

49. Washing
The brownstock 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.

50. 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.
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

51. 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 protects the cellulose from
degradation and makes the lignin more water-soluble
•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.
•Defoamers remove foam and speed up the production process. Drainage of
washing equipment is improved and gives cleaner pulp.
•Dispersing agents, 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.

52. Sulfite Pulping
The sulfite process produces wood pulp which is
almost pure cellulose fibers by using various salts
of sulfurous acid to extract the lignin from wood
chips in large pressure vessels called digesters. The
salts used in the pulping process are either sulfites
(SO3
2−), or bisulfites (HSO3−), depending on the
pH. The counter ion can be sodium (Na+),calcium
(Ca2+), potassium (K+), magnesium (Mg2+) or
ammonium (NH4+).

53. 2. Processes involved in sulfite pulping
2.1 Pulping liquor preparation
2.2 Pulping
2.3 Chemical recovery
2.3.1 Calcium-based
2.3.2 Ammonia-based
2.3.3 Magnesium-based
2.3.4 Sodium-based

54. Pulping liquor preparation
The pulping liquor for most sulfite mills is made by burning sulfur with the correct
amount of oxygen to give sulfur dioxide, which is then absorbed into water to
give sulfurous acid.
S + O2 → SO2 SO2 + H2O ⇌ H2SO3
Care must be taken to avoid the formation of sulfur trioxide since it gives
undesired sulfuric acid when it is dissolved in water.
2 SO2 + O2 → 2SO3
SO3 + H2O ⇌ 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 depend 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

55. Pulping
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 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.[1] 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.

56. 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.[7]
The sulfite process can use calcium, ammonium, magnesium or
sodium as a base.

57. The sulfite process can use calcium, ammonium,
magnesium or sodium as a base.
Calcium-based:
The earliest process used calcium, obtained as inexpensive calcium carbonate
and there was little incentive to recover the inorganic materials.
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.

58. 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".

59. 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.

60. Pulp Bleaching
• Brightness and Whiteness
--Brightness is physical measurement of how much light is
reflected from an object (pulp or paper)
--Whiteness is physiological perception of how bright an object is
• Importance of initial brightness of pulp
--The brighter the initial brightness the less is the effort to bleach to
certain brightness level
--Protect pulp logs from blue stain
--Pretreatment of water to remove metallic ions

61. Methods of Bleaching
• Lignin preserving methods
for groundwood (newsprints) and other high-yield pulps such
as low-yield NSSC pulp for printing purposes
• Ligin removing methods for chemical pulps (kraft)
By removing lignin paper is stable in brightness

62. Lignin Preserving Bleaching
• Lignin and extractives in wood are responsible for color of
mechanical pulps
• Peroxide bleaching has a low impact on the environment
• Oxidation reactions by hydrogen peroxide (H 2O2) remove color
• Procedures
--Treat water with EDTA (0.2-0.5%) to remove heavy-metal ions
--Adjust pH to 10.5-11.0 with NaOH
--Add 1.0 to 3% hydrogen peroxide to 15% consistency pulp
--Bleach 1 to 3 hours at 30 to 50 oC
--Brightness increased from 5 to 12% depending upon conditions

63. Lignin Removing Bleaching
• Most lignin in chemical pulps must be removed so that brightness
of paper can be maintained for a long time
• Bleach is done in multi-stages
--Chlorination (C): pulp is treated with chlorine (Cl) to breakup
lignin molecules
--Extraction (E): Use NaOH to extract lignin fragments
--Oxidative bleaching
Hypochlorite (NaOCl) bleaching (H)
Chlorine dioxide bleaching (D)
Peroxide bleaching (P)
--

64. Results of Multi-stage Bleaching
• CEH bleaching increases brightness of kraft pulp from 30%
brightness to 75%
• CEHEH increase brightness from 30% to 80%
• CEHEDP increase brightness from 30% to 90%
• Because chlorine has a great impact on the environment
research is underway to find chlorine-free bleaching methods