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Pulp & Paper

INTRODUCTION

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The Indian pulp and paper industry is over a hundred

years old. First mill in the country was commissioned

in 1812 in Serampur (West Bengal).

Over the years, the installed capacity has grown from a

insignificant 0.15 million tones in the early fifties to

the present level of 8.3 million tones.

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Based on the raw material utilized, the paper units can be classified

into three broad categories as:

Wood based (Bamboo, hardwood etc.)

Agro-based (Bagasse, jute, rice & wheat straw)

Waste paper based

Manufacturing Process

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A variety of processes are in use in the paper industry depending on the type of

raw material used and the end product desired.

Kraft (Sulphate) process, Semi-Mechanical process and Sulphite process are the

most popular ones.

In the Indian pulp and paper industry, the Kraft process dominates the

wood/bamboo pulping. Paper making essentially consists of following stages:

Preparation of pulp

Stock preparation

Sheet formation & water removal

Introduction

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India was also the first country in the world to use bamboo as a basic

raw material for making paper.

Due to limited forest resources, other raw materials like bagasse,

straw, jute, etc. were identified and are now extensively used.

Waste paper is also being widely used for paper making. Almost all

varieties of paper are produced using there materials.

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The Indian paper industry can be classified into the following six

groups of manufacturing units according to Chemical Weekly

magazine:

1. Large integrated paper and paperboard units based primarily on

conventional raw materials like bamboo, wood, etc., with inhouse

pulping facilities.

2. Small paper units based either exclusively on non-conventional

raw materials like bagasse, wheat and rice straws, alone or in

combination with imported pulp.

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3. Small paper units based on waste paper .

4. Paper units set up as a part of large integrated sugar

complexes for exploiting the waste products like bagasse.

5. The large integrated newsprint manufacturing units.

6. About 400 hand paper producing units, using cotton rags,

jute waste, cotton linters, etc. These units are under control

of Khadi & Village Industries Commission (KVIC) and

produced about 4,000 tons of paper in 1989-90.

PULP

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Pulp is a commercial cellulose derived from bamboo, bagasse, wood,

etc. by mechanical or chemical methods.

METHODS OF PRODUCTION

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1. Groundwood method:

debarked wood is mechanically shredded to form fibers suitable for

the production of newsprint, toweling, toilet tissues, and cheap

paperback books where strength and ease of bleaching are not

important.

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2. Chemical-the cellulose from wood is freed from lignins and other non-

cellulose ingredients by reaction with chemical reagents .

The two important processes are sulfate (Kraft), and sulfite.

The non-cellulosic fraction is solubilized with the insoluble pulp consisting of

strong fibers of soft texture. These can be bleached to a white or near

white .

The yield is only about half that from the mechanicalgroundwood process

(40-65 %), but the pulp is the only type suitable for chemical grade

cellulose (rayon andcellulose derivatives) and for paper of high strength

and/or fine texture

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3. Semichemical-wood chips are given a mild chemical treatment with

dilute mixture: sulfite, sulfate, caustic soda, and/or soda ash reagents .

The wood is softened sufficiently to allow mechanical separation of fibers

without excesspower .

A high yield of 65-90% with a somewhat better quality than groundwood

pulp is the reason for theincreasing interest in semichemical pulp as a

substitute for groundwood pulp .

The lower yield results from moredrastic chemical treatment, but a better

grade of fiber is produced

PRODUCTION PATTERN

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The production pattern for India is not certain as plants are

being changed or modified to yield various types of pulp,

depending on the raw material and finished production

situation .

The lack of sufficient raw material is forcing the production

towards a pattern of higher yield, poorer quality pulp via

mechanical or semichemical.

SULFATE (KRAFT) PULP PROCESS

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Process Description

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Digestion of wood-base materials: The newer continuous process will be

presented. Logs with bark are debarkedby tumbling and rubbing action, then

conveyed to chippers where large rotary disks with many heavy knives reduce

the wood to 2-5 cm flat chips.

The chips are metered via star valve to a deaerator-preheater.

After several minutes, the chips are discharged through a rotating tapered plug

into the lift line where recirculating digestion liquor at 12 atms transfers chips

to the upper soaking zone of the 25-30 m tall digestor tower.

Chips flow down past a series of circumferential screen plates .

Cooking liquor is withdrawn as side streams and circulated through external heat

exchangers to reheat and control the digestion temperature within the tower.

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The digestion time and temperature is adjusted so that maximum lignin removal is

accomplished with a minimum cellulose hydrolysis and consequent loss of bulk yield.

The digested chips are cooled at the base of the tower by injection of cold black liquor.

This is to avoid mechanical weakening of fibers from steam explosion of the hot liquor when

passed through the blow-down valve .

The pulp liquor slurry is passed through the valve to a blow tank where residual heat is

recovered in the form of steam which passes overhead with turpentine vapor to the chip

preheater.

The pulp is filtered to separate black liquor and screened toremove wood knots and other

undigested residue. The brown pulp goes either to product finishing operations or to the

bleaching plant.

Modified Process For Bagasse

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The sulfate process described for wood-base materials must be modified for

bagasse raw material which contains dirt and pith. This latter is the thin-

walled, short cells which make poor paper fiber and must be removed.

Depithing methods are based on the fact that the fibrous portion of bagasse

is much more difficult to break up by mechanical action than the pith.

Thus, exposure of bagasse to strong mechanical shredding-grinding action

reduces pith to a fine powder while the desired fiber bundles are reduced

in size .

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Wet grinding in a hammer mill, with the water washing pith through

the screens, is the preferred process. The other major modification

is in chemical pulping .

The finely divided structure and chemical make-up of bagasse

allows rapid penetration and reaction by alkaline pulping

materials.

Thus, 5 mins in the digestor are sufficient, providing a basis for

high throughput continuous digestors.

Bleaching of Pulp

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Removal of color residue or bleaching may be accomplished by use of one of various

oxidizing agents which must be cheap and have a minimum degrading action on

cellulose. Traditionally, chlorine-type oxidizing agents were used in a stage-wise

operation .

However, chlorine bleaching has been found to produce dioxins and other undesirable

components of the bleacher effluent .

The use of chlorine as a paper mill bleach has steadily been decreasing. Chlorine has been

replaced to a substantial extent by hydrogen peroxide.

In the modem and improved bleaching stage, hydrogen peroxide is added together with

sodium hydroxide, which activates the peroxide .

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A stabilizer is necessary, however, to maintain the peroxide concentration at

effective levels, and sodium silicate is currently the most commonly used agent.

A resulting scale or thin crust that forms on the internal surfaces of the pulp mill

equipment, especially on heat transfer surfaces must be removed in periodic

downtime.

Replacing the sodium silicate with an organic agent developed by Dow Chemical

Co., can eliminate the scale formation and the resultant downtime for cleaning .

It has been estimated that the pulping industry will use about 240 million pounds

per year of hydrogen peroxide for bleaching by the year 2001.

Finishing Operations of Pulp

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If the plant is a captive paper type, the wet pulp is conveyed to the beater

operations which is the first step in the production of paper.

Dewatering is necessary if the pulp is shipped any distance for use in paper or as

chemical cellulose. This can be done in several ways :

1. Hydraulically pressing pulp at 200-300 atms. to form wet lap sheets which

can be dried further.

2. Vacuum flash drying of mechanically dewatered pulp to produce a dry fluffy

material which can be dried further.

3. Extrusion in form of easily-handled noodles or pellets containing 30-40%

solids for conveying short distances or dried to 90% solids for longer range

shipping .

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Recovery of chemicals

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Black liquor from the blow tank contains 98-99 % of the digestion

chemicals which must be recovered to avoid water and air

pollution problems and to provide a balanced economy of

operation.

Multiple effect evaporation using 5-6 stages of calandria equipment

followed by disk evaporators concentrates the liquor from 15- 18%

solids to the point where combustion can be sustained in a

smelting waste heat boiler .

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This concentration is around 60% solids. Organic carbon burns in the

smelting furnace, supplying the necessary heat and CO2 to produce

an inorganic molten slag or smelt. Make-up alkali is supplied via

Na2SO4 chemical reaction .

The molten smelt falls into a dissolver where it contacts cold H2O to

yield green liquor solution .

The insoluble impurities, such as unburned carbon, are settled out and

the clear liquor causticized by adding lime. Filtration removes the

calcium carbonate sludge while the filtrate (white liquor) is returned to

the digestor. The carbonate sludge is calcined to lime for recycle .

Paper Products:

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Definition

Paper is defined as matted or felted sheets of fibers, usually

cellulosic and generally formed on a fine wire screen from a

water suspension .

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Types of Paper Products

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Paper products are classified by use requirements or by characteristics of

the paper. Examples are:

Wrapping paper-bag paper, grease-proof paper

Tissue paper-cigarette, carbon, toilet, towel, napkin papers

Book paper-coated or uncoated, lithograph offset ,textbook paper

Writing paper-bond weight, linen papers

Groundwood printing paper-catalog, newsprint, tablet, poster, wall

papers

Paperboard-heavier, less flexible, laminated paper stock

Raw Materials

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Fibro s Raw Materials

Paper pulp-groundwood, bleached and unbleached sulfite and

sulfate, semichemical pulps. Choice depends on end uses and

blending of various pulps is frequently required to impart proper

specifications to end-products with maximum yield from pulping

materials .

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Reuse pulp-paper products, such as newspapers and paperboard,

are repulped and mixed with new pulp for paper mill feedstock.

This source accounts for 46% of fibrous starting materials. In

view of raw material sources, increased attention should be given

to collection and reuse of waste paper .

Miscellaneous cellulose pulp-straw, linen, cotton and rags .

Specialty pulp-inorganic fibers such as asbestos and glass .

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Non-fibrous Raw Materials

The paper industry is a good customer of the chemical industry. In

addition to chemicals used in producing the pulp, a large variety

of materials for fillers, sizing, and coatings are required .

Inorganic raw materials-clay, talc, titanium dioxide, zinc sulfide,

calcium carbonate, calcium sulfate, barium sulfate, alum

Organic raw materials-rosin, glue, casein, waxes, glycerol,

dyestuffs

Methods of Production

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The commercial method for paper making is to prepare a

suitable fiber suspension in water which is fed to a paper

machine where the fibers are matted and dried .

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Preparation of Fiber Suspensions

Pulps are water slurried to - % fiber content by mechanical disintegrators

of various designs .

These usually consist of rotating drums with knife attachments or rotating

stationary disks to produce viscous shear. Such operations are called beating

and refining .

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Fillers are added to the slurry to increase brightness, bulk,

flexibility, softness, and weight of finished paper .

Coloring agents are also mixed into the slurry before

processing. These are generally synthetic organic dyestuffs,

but some coloring is done by water-insoluble pigments .

Formation of Paper

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Conversion of fiber suspension into paper sheet incorporates

three principal steps :

Random arrangement of fibers into a wet web

Removal of free water from wet web by wet pressing and

compaction of the sheet

Progressive removal of additional water by heated rolls

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The Fourd,rinier process was developed to

accomplish all three steps and the extremely

complex machine used in the process is termed the

Fourdrinier machine. Modifications of this machine

produce laminated paperboard stock and fine tissue.

Capacity of a single machine is as high as 400-50

tons of paper per day .

Forming a wet web

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A wet sheet is formed by running 99 % water-fiber slurry evenly

onto a moving endless belt of wire cloth at speeds of 50 m/minute for

fine paper to 500 m/minute for newsprint. Water drains by gravity, a

part is next removed by a pressure roll and then by a suction roll. The

screen also has a side-wise shaking motion to give better interlocking

of fibers on the mat .

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The water collected in this section of the machine is known as white water

and is reused to obtain maximum recovery of fiber, to conserve water and

additives, and to prevent stream pollution .

Pressing the wet sheet

The wet paper sheet containing about 80% water is fed via felt roll to the

press section where water is removed by mild pressure to reduce content to

60-65% water. Bond or water mark, if needed, is formed on the sheet

during the pressing .

Drying the sheet

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The sheet from the press section has sufficient strength to carry its own

weight .

It is passed through smoothing rolls, then a series of steam-heated metal

cylinders where heat and moisture are transferred to a felting or canvas

belt running on top of the paper. Paperboard is dried directly without a

felt .

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As the sheet leaves the last drying roll with 5-6% H20, it is passed

through a final series of pressure or calendering rolls to produce a

smooth, well-finished paper. It is wound on large rolls and

transferred to the finishing department where it may be cut, coated,

and packaged .

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In designing the roll speed in dryer section, allowance must be made for

a 510% progressive stretching as the paper is pulled through the dryer.

The tendency of paper to shrink on drying, coupled with the machine

tendency to stretch, creates a delicate force balance in machine design .

Recent Improvements

In Paper Making

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Increase in Wet-strength of Paper Products

The sizing of paper with dialdehyde starch and the use of newer types

of resin and plastics such as polyethylene, particularly in Kraft-type

paperboard, is responsible for the increased use of paper products

under varying moisture conditions.

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Modifications in Paper Machines to Improve Properties

of Paper

Research is constantly being done to improve the tensile strength of paper. The

KIupack process developed by West Virginia Pulp and Paper Company in the

USA is one such improvement. It incorporates a rubber belt before the pressing

rolls which can be varied in degree of elongation. By changing to a less-

stretched condition at the end of the operation, the fibers on the wet web can

be compressed and intertwined to give vastly improved strength on the final

paper .