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CHAPTER No. 3 PULP AND PAPER INDUSTRIES

3.1. Introduction:
Cellulose is probably the most versatile raw material known to man, and its conversion to a
group of products that are very important to modern civilization is the everyday function of the
pulp and paper industry.
The pulp industry also supplies purified cellulose for explosives, rayon, and plastics.
Pulp: pulp is a coarse sheet, dry enough to fold into bundles. Pulp is cellulosic fibrous in nature.
However it lacks certain characteristics like proper surface, strength and feel.
Paper: Paper is a web of intermeshed cellulose fiber. These fibers are in fetted layers.
Most papers also contain non fibrous materials like china clay, resin, etc to develop
characteristics like proper surface, opacity, strength and feel.
3.2. MANUFACTURE OF PULP and PAPER
There are two distinct phases in the conversion of raw wood into the finished paper. These are
(1) The manufacture of pulp from the raw wood and,
(2) The conversion of the pulp to paper itself.
In first section we will discuss the manufacturing of pulp. Papermaking will be presented in a
later section.
3.3 Manufacture of Pulp:
Before the manufacture of paper from wood, cellulose must be freed from the matrix of lignin
which cements cellulose fibers together. The fibers may be separated by mechanical methods or
by solution of the various chemicals. The pulp thus formed has its fibers re-cemented together
to form paper when suitable additives are used.
3.4 Raw Material: The raw material employed in paper and pulp industries are of two types:
Fibrous raw material and Non fibrous raw material.
1) Fibrous Raw materials: Some of the fibrous yielding (Paper making fibers) materials are as
follows:
i) Wood: Both hard wood and soft wood are used to make pulp. Birch is an example of hard
wood and all coniferous trees are example of soft wood. But soft wood is preferred over hard
wood.
Bark cannot be used because it is not fibrous and is difficult to bleach. Bulk is removed at the
pulp mill by many debarking methods.
Polymers present in the wood: There are three types of polymers present in the wood.
- Cellulose: 45% of dry weight of wood is cellulose. It is high molecular weight glucose polymer
chain. It is most valuable as fiber.
- Hemicellulose: 20-25%. It has disordered array of several sugar polymer. It has no
economical use but as a fuel.
- Lignin 20-25%. It acts as a binder for cellulose. It is a complex amorphous polyphenol
polymer.
- Extractives: (5-25%).
ii) Cotton: It is pure cellulose in nature and is used for paper industry.
ii) Flax: Linen, hemp, Jute
iii) Grass Fibers: straw, bamboo, bagasse.
iv) Leaf Fibers: Esparto ( a grass).
v) Reused pulp: Paper products such as newspaper.
vi) Waste paper:
2) Non Fibrous Raw Material:
Paper industry is a good consumer of chemical Industry. In addition to chemicals used to
produce pulp, a large number of materials of filter, coatings, dying are required. These materials
are of two types:
i) Inorganic Raw materials: Sulfur, NaOH and soda ash, Lime, Mineral substances and filling
agents,
ii) Organic Raw materials: Resin, Paper fibers, glue, waxes, glycerol.
3.5 Pulping Process:
All the process used for pulping have the same goal to release the fibrous cellulose form its
surrounding lignin while keeping the hemicellulose and celluloses intact, hereby increasing the
yield of useful fibers. The fibers thus obtained are naturally colored and must be bleached before
they can be used for paper.
There are many processes and variation for making pulp form wood.
3.4. Kinds of wood pulp:
There are four different kinds of wood pulp:
1) Mechanical pulp,
2) Sulfite pulp,
3) Sulfate pulp, and

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4) Soda pulp as shown in Tables 1 to 3.

The first is prepared by purely mechanical means, the other three by chemical means. The
mechanical pulp contains all of the wood except the bark and that lost during storage and
transportation. Chemical pulps, however, are essentially pure cellulose, the unwanted and
unstable lignin and the other non-cellulosic components of the wood having been dissolved away
by the treatment. Because of this, chemical pulps are much superior to mechanical (or ground-
wood pulp) for fine papermaking.
However, owing to the special processing required, they are too expensive for the cheaper
grades of paper, such as newsprint.
3.5 Mechanical Pulp:
This process involves no chemical treatment of the pulp whatsoever. The chief woods employed
are from coniferous species.
1) Debarking
It is usually done in a:
i. Drum barker where bark is removed by the rubbing action of logs against each other in a
large rotating drum.
ii. Hydraulic barkers using high-pressure water jets are excellent for large logs.
iii. Mechanical knife barkers are becoming more common and are extensively used in
small operations because of their lower capital cost.
2) Pulping Process
1) Debarked logs are forced against the face of a cylindrical abrasive stone rotating at relatively
high speeds.
2) The logs are positioned so that their axes are parallel to the axis of the rotating stone. (The
fibers will be torn apart rather than broken).
3) Sufficient water must be added to the stone to serve as a coolant and to carry the pulp away.
4) The freed fibers are dropped into a container known as the stock sewer and passed along to a
screen.
5) The fine fibers pass into the stock pit, the coarse material are separated and refined, then
returned to the screens.
6) The only chemical change occurring in mechanical pulp is a slight hydration of the cellulose by
long contact with water.
7) The fine fibers are concentrated in thickeners, yielding mechanical pulp.
3.6 Chemical Pulping
There are 4 processes principally used in chemical pulping which are:
1. Sulphate/ Kraft
2. Soda.
3. Sulphite
3.6.1. Sulfate or (Kraft / Alkaline) Pulp
Kraft or sulfate pulping is an alkaline process. This process is responsible for the major part of
the pulp manufactured at the present time. Almost any kind of wood, hard and soft, may be
used. However coniferous woods are mostly employed. The process was developed especially to
remove the large amounts of" oil and resins in these woods.
It is an outgrowth of soda process. Soda process uses 12 % solution of NaOH and Na2CO3. Soda
process gives low yield and was limited to short fibered hard wood.
In Sulfate process, Na2SO4 is added to cooking liquor so named as Sulphate process. Na2SO4
itself is not a cooking liquor.
Cooking liquor solution contains NaS, NaOH, and Na2CO3 and this liquor is obtained from Na2SO4
and recovery of the cooking liquor.
All sorts of wood can be cooked by kraft process and fibers obtained are bleachable and strong.
The chemicals used can be recycled and regenerated.
Coniferous materials released during, are air pollutants and are difficult to control.
Most Kraft process use coniferous woods and the process deals with large amount of oil and
resins.
Cooking process causes chemical reactions involving the hydrolysis of lignin and solubilization of
lignin.
The turpentine is volatilized and sodium soaps are formed from the resin acids.
Hydrolysis of free mercaptans and organic sulfides which are the source of foul odor associated
with Kraft mills.
Fig. 3.1 given below shows the overall Kraft process including the important black liquor
(processing chemical) recovery system.

When using a continuous digester, the manufacture of Sulphate pulp involves the
following sequences;
1) Debarking: removal of bark is done as described above.

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2) Chips formation. Debarked wood is shifted in chipper where they are chipped into suitable
size by the help of rotating disc having blades.
3) Chips screening: Chips are screened on either rotating or vibrating screens to separate the
oversized chips, desired product and saw dust. The oversized chips are sent again to chipper to
reduce them to proper size.
4) Continuous Digester: Chips enter the continuous digester and pre steamed at
approximately 100 Kpa. Wood chips are fed into a solution of Sodium Hydroxide and Sodium
Sulfide (By addition of Na2SO4 and chemicals coming from recycling) at high temperature in a
digester which breaks the lignin chemical bonds which bind the cellulose fibers together.
i) Above part: In this part chips are added from chips bin. Function of this part is to volatilize
the turpentine and non-condensable gases.
ii) Lower Part: In this lower part of the digesting tank the conditions are:
Pressure: 900 Kpa, Time: 1.5 h, Temp: 170 oC.
After the completion of digestion i.e., the separation of cellulose part from non-cellulosic part
takes place.
5) Steaming: The pressure is then reduced producing flash steam which is used for the-pre
steaming step on the entering chips.
6) Brown Stock: The chips along with their adhering liquor is called brown stock.
7) Pulp Washer: The brown stock is washed with hot water to remove any adhering substance.
In pulp washer there is also separation operation. As a result of this, pulp and black liquor (the
spent cooking liquor) is separated.
8) Screens: The washed pulp is passed over screens to remove knots, unreacted chips, slivers,
trash etc, and then sent to thickness and filtration.
9) Thickening: Thickening is to remove water from pulp. This is obtained by press rolled.
10) Bleaching: The thickened pulp is next bleached. Bleaching is often done in dilute solutions
followed by pulp concentrated by dewatering and may use much water contaminating all of it.
There are two types of bleaching:
i) Oxidizing Bleaching
ii) Reducing Bleaching
i) Oxidative Bleaches are Ozone, Na2O2, H2O2, ClO2 and Cl2. These are used to oxidize and to
destroy the dyes and tannins of the wood leave. Chloride residues in the wash water and harm
the cellulose fibers.
ii) Reducing Bleaching agents are Sodium thionate, Sodium Borohydride or Bisulphite.
11) Concentration of the pulp: After the bleaching, the pulp is washed and re-thickened for
making it into coarse sheets dry enough to fold into a bundle, store and ship. These are called
laps. The pulp may also be used directly to make papers.
12) Laps Formation: Laps are made on a wet thickener consisting of a suction cylinder dipping
into a vat filled with fiber suspension. The cylinder discharges its load onto an endless felt belt
which carries the pulp through squeeze rolls, then a series of press rolls. To remove moisture, it
is passed through a hydraulic press and pressing them at 20 MPa. The laps emerge with 50-60%
air dry fiber.
13) Production of Fibers: Kraft pulp made from coniferous wood has the longest fibers of all
the pulps. Making production of very strong paper.
14) Recovery of Black liquor: An essential factor in the Kraft process has been the recovery
of the spent liquor from the cooking process.
The black liquor removed from the pulp in the pulp washer or diffuser contains 95-98% of the
total chemical charged to the digester.
The chemical present in the black liquor are: Organic sulfur containing compounds in
combination with : Na2S, Na2CO3, Traces of lime, Na2SO4, Potash, Iron Oxide, Salt, Silica,
Alumina.
Steps for the recovery of the liquor: There are three steps for the recovery of Black liquor:
Concentrated, Burned and Limed.
Total solids usually about 20%. This black liquor is concentrated, burned and limed as shown in
Fig. below. In the smelting furnace any remaining Organic compounds are broken down. The
carbon burned away and inorganic chemicals melted. The reaction takes place as below:
Na2SO4 + C ---------Na2S + 2CO2

1) Concentrated: Black liquor is concentrated to about 35% in multiple effect evaporators in

which pre-steamed evaporate water from black liquor which further used as evaporating agent
for next evaporating process.
2) Burning: Concentrated black liquor is burned in THOMLINSON Kraft recovery furnace (the
unit most widely used for burning concentrated black liquor). The solution is sprayed directly
into furnace. Here, with burning, reduction of sulfate to sulfide takes place. Na2SO4-------Na2S.
Steam is made and molten salt mixture or smelt is produced.

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A hazard exists because the molten smelt can cause explosion if it comes in contact with small
amount of water.
The molten chemical smelt is allowed to fall into a weak solution of dissolving liquor in a tank
coming from the causticizing plant.
Green Liquor: The chemicals dissolve immediately to give a characteristic Green Liquor.
Limed: The green liquor and the insoluble impurities are mixed with lime which converts any
soluble carbonate into insoluble CaCO3.
Na2CO3+ Ca(OH)2---------NaOH+ CaCO3
CaCO3 formed is screened by Monel metal screens. Insoluble CaCO3 sludge or mud is sent to
lime Kiln.
i) Lime Kiln: In lime kiln, CaO is recovered for reuse in the process.
CaCO3--------CaO+ CO2
CaO is slacked by dissolving it in water. CaO+H2O------------ Ca(OH)2
White Liquor: The filtrate is the white liquor used in the cooking of the fibers. It contains
caustic soda, sodium sulfide, and small quantities of sodium carbonate, sodium sulfate, sodium
sulfite, and thiosulfate.
Bye Products of Black Liquor: Among the by-products from the black liquor recovery plant is
Tall Oil: tall oil is a black, sticky, viscous liquid composed mainly of resin acids, fatty acids, and
methanol. The tall oil may be separated from the weak black liquor by means of centrifuges (in
America), or obtained by flotation from the concentrated liquors (in Europe). It is used in the
manufacture of soaps and greases and in the preparation of emulsions.
Turpentine: It is a mixture of mono turpenes and is obtained from the digesting tank.

Fig. 3.1 Kraft process including the important black liquor (processing chemical)
recovery system.

3.7 Soda Pulping

It is very similar to the Kraft Pulping process but the chemicals used are NaOH and Na2CO3,
make-up chemical being Na2CO3. This can only be used with short fiber hardwoods.
3.8 Sulfite Process
The wood is debarked, cleaned and chipped. The usual sulfite process consists of digestion of the
wood in an aqueous solution containing calcium/ magnesium/sodium or ammonia bisulfite and
an excess of sulfur dioxide. This process is not used as much because it cases water pollution.
1) The sulfite process involves the two principal types of reactions:
i) The reaction of the lignin with the bisulfite, and

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(ii) The hydrolytic splitting of the cellulose-lignin complex. The hemicelluloses are also
hydrolyzed to simpler compounds, and the extraneous wood components acted on.
2) More than half of the raw material entering the process appears at the end as dissolved
organic solids so disposal of water liquor creates a serious water pollution problem.
3) Sodium and ammonia have also been substituted for calcium as a pulping base and are used
in a limited number of commercial plants. So far costly and complicated chemical recovery limits
their widespread use.
Reactions involved in the process of cooking liquor:
Following reactions are involved:
S + 02~ S02
2S02 + H20 + CaCO3 ~ Ca(HSO3)2 + CO2
2S02 + H20 + MgCO3 ~ Mg(HSO3)2 + CO2
Sequence of Process:
1) Melting of sulfur: Sulfur is melted in the tank heated by the rotary burner and then fed to
this burner for oxidation. Any sulfur that is vaporized in the burner enters a combustion
chamber, where it is oxidized to sulfur dioxide.
S + 02~ S02
The quantity of O2 is controlled to prevent the formation of SO3.
2) Quickly cooling of SO2:
The sulfur dioxide obtained is cooled quickly in a horizontal, vertical, or pond cooler consisting
essentially of a system of pipes surrounded by water.
3) Absorption of the gas (SO2):
The next step in the process is the absorption of the gas in water, in the presence of calcium and
magnesium compounds. This is accomplished in a series of two or more absorption towers
packed with limestone. A fine spray of water passes down through the tower system
countercurrent to the sulfur dioxide gas, which is blown up through the tower.
The final liquor as charged to the digesters is a solution of calcium and magnesium bisulfites,
analyzing about 4.5 per cent" total" sulfur dioxide and about 3.5 per cent " free" sulfur dioxide.
4) Construction of Digester: The digester is filled with chips and the acid cooking liquor is
pumped in at the bottom. The digesters are cylindrical steel vessels with a capacity of from 1 to
23 tons of fiber and 3,000 to 51,000 gal. of "acid." A special lining of cement, crushed quartz,
and acid-resisting brick is used to avoid the corrosive action of the cooking liquor. The digester is
heated with direct steam. The pressure varies from 70 to 160 lb. depending upon the
construction of the plant.
5) Washing: The stock is washed, thickened, and sent to the machine chest.
6) Laps Formation: Pulp from the chest is formed into laps of about 35 per cent dry fiber
content and the laps are dried with steam-heated rolls to a product which is 80 to 90 per cent
dry fiber.
Sulfite pulp is a high-grade type of pulp and serves for the manufacture of some of the finest
papers. It is used either alone or with some rag pulp to make writing paper and high grade book
paper.
Waste Sulfite Liquor: The disposition of the waste liquors formed in this process has been the
subject of much research. Until very recently it was common practice to dump this liquor into a
near-by stream. Unfortunately, a great deal of work still remains to be done on this problem-
either the disposal or the utilization of the by-product.

PAPER MAKING PROCESSES

The pulp manufactured in the form of coarse sheet still lacks those properties desirable for
finished paper. These properties are:
i) Paper surface ii) Opacity iii) Strength iv) Feel
The above mentioned properties are achieved by following process:
i) Beating ii) Refining
There is no sharp distinction between these two operations. Mills use either one or the other
alone, or both together.
1) Hollander Beater: Hollander beater is the most important type of beater.
Construction: It consists of a wooden or metal tank having rounded ends and a partition part
way down the middle. This partition provides a channel around which the pulp circulates
continuously.
On one side is a roll, equipped with knives or bars, and directly below this, a bedplate consisting
of stationary bars.

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Working: In operation, the circulating pulp is forced between the bars on the revolving roll and
the stationary bars of the bedplate. The roll itself may be raised or lowered to achieve the
results desired.
Out Come:
1) Beating the fibers makes the paper stronger, more uniform, more dense, more opaque, and
less porous.
2) Bonding between fibers is increased by beating.
2) Conical Refiner or Jorden Engine
The standard practice in making the finer grades of paper, however, is to follow the beaters
with the refiners, which are continuous machines.
Construction:
1) The Jordan engine (Fig. below) is the standard refiner and consists essentially of a conical
shell, on the inside of which are set stationary bars.
2) Revolving inside the shell is a core, also set with bars. It is the action between these two sets
of bars that produces the desired effect on the pulp.
Working: Pulp enters the small end of the cone and passes out at the other end.

Out Come: Pulp is deformed, de-fibered and dispersed but not cut by the machine.
In addition to Fiber, paper also contains filler, sizing and coloring. These materials are gradually
added during refining.
Various types of pulp are blended to give the desired properties. Then filler and colors are added
to the mixture and beaten to uniformity. Alum is added to coat the fibers and coagulate the
materials present.
All papers except the absorbent one (Tissue, toweling, filter) require a filler to give a smoother
surface, a more brilliant whiteness, improved smoothness and printability.
Filler is always a finely ground inorganic material usually naturally occurring substance such as
talc or special clay or TiO2 etc.
Sizing is added to paper to impart resistance to penetration by liquids. The common sizing
agent is either a soap made from the saponification of rosin with alkali or a wax emulsion. This
treatment gives a gelatinous film on the fiber, which loses water of hydration and produces a
hardened surface.
PAPER MACHINE:
The machines used for the actual formation of the sheet are of two general types:
1) the Fourdrinier machine and
2) the cylinder machine.
The basic principles of operation are essentially the same for both machines. First of all pulp is
mixed with Water to form very dilute solution with 96 % water in mixture. The sheet is formed
on a traveling wire or a cylinder, dewatered under rollers, dried by heated rolls, and finished by
calender rolls.

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Paper and Pulp manufacture: