Industrial Chemistry

1. CHAPTER 7. PAPER INDUSTRY
7.1.Introduction
 Paper is an aqueous deposit of any vegetable fibre in
sheet form.
 Fibers are very small in size (not much more than a
millimeter in length) and there are millions of them in any
piece of paper.
 Cellulose is the most important organic substance
available in nature.
 Pulp (soft tissue) and paper industries make extensive use
of Cellulose in manufacture of paper and other related
products, and wood are the principal source of cellulose
for making paper.

2. 7.2. Manufacture of Pulp by Mechanical and Chemical
Process
This is the first step in Manufacture of paper in which
which wood and other materials are converted to pulp.
pulp.
 Pulp is Manufactured either by mechanical or chemical

3. The debarked wood is then subjected to
mechanical grinding in presence of water in
grinders to remove the heat of fraction and to
float the extreme small pieces of fibers away.
The pulp and water mixture thus obtained
contain all the lignocelluloses of the original
wood.
The mixture from the grinders is withdrawn into
a stock sewer, below the grinders and then
conveyed to the silver screen.
The fine particles of the material are collected in
the screened stock pit, from where these are
pumped to the fine screen.

4. The finest particles, thus screened by these
screens are then concentrated in thickeners to
get commercial mechanical pulp.
 The larger particles retained by silver screen
and the fine screen are treated in refiners and
then returned to the screens again to recover
more mechanical pulp.
Chemical process: this process is used to obtain
high grade paper. Various types of pulps such
as sulphate pulp, soda pulp, sulphite pulp
etc.are prepared by chemical process. Let us see
one of them
Sulphate pulp- this pulp is prepared by an

5. The logs are slashed, debarked and then
disintegrated into small chips by means of chippers.
The chippers are large rotating discs containing
four or more long heavy knives.
 The disintegrated chips are screened on either
rotary or vibrating screens.
The over sized chips retained by the screens are
passes through crushers or chippers to reduce them
to proper size.
 The desired sized chips are then digested under
pressure, with sodium sulphide and caustic soda.
The digestion or cooking takes about three hours.
These treatments dissolve incrusting lignin of
lignocelluloses leaving behind more or less

6. The fibrous material, after separation from the
cooking liquor, is washed with water.
The washed pulp is allowed to pass through
knotters, riffles and screens to separate any small
silvers or uncooked wood and finally to filters and
thickeners.
The thickened pulp is bleached with chlorine and
washed again. It is then beaten into pulp with
water in a pulp beater.
The washed pulp (after bleaching) may be
rethickened to convert into sheets dry enough to
fold into a bundle, called a lap.
The lapping is done on a wet thickener which
consists of cylinder immersed into a vat, filled with

7. The spent cooking liquor, called black liquor, is
removed from the pulp in the pulp washer and
pumped to storage for recover of chemical charged
to digester.
It contains organic sulphur compounds, sodium
sulphide, sodium carbonate and small amount of
sodium sulphate, salt, silica and traces of lime,
alumina, potash and iron oxide.
The black liquor is concentrated, burned, smelted
and limed. As a result of smelting, organic
compounds are broken down, the carbon is burned
away and inorganic compound are melted.
Moreover, sodium sulphate is converted into
sulphide, as follows

8. The molten smelt is then dissolved in the dissolving
liquor to obtain characteristic green liquor.
It is allowed to stand for few hours, as a result of
which, insoluble impurities settle down at the
bottom.
 Slaked lime prepared from the recovered CaCO3 is
then added to the green liquor to causticize any
sodium carbonate still present in it.
Na2CO3 + 2Ca(OH)2 NaOH + 2CaCO3
The CaCO3 slurry is separated in settlers and rotary
contentious filters, and passed to a lime kiln to get
CaO for re use in the process.

9. •The filter or white liquor containing caustic soda,
sodium sulphide, and trace of sodium carbonate,
sodium sulphate and sodium sulphide etc. is used
again in the cooking of the fibers.
•Soda pulp is also prepared by the alkaline process
similar to sulphate pulp.
7.3. Refining of Pulp
The bales of dry pulp are broken up in a
hydrapulper (a large circular metal tank in which
a mixture of pulp and water circulates).
The liquid pulp, looking a little like porridge, is
then pumped to machines known as refiners.
These modify the fibres by passing them between
sets of fixed and rapidly moving metal bars.

10. As the fibres undergo this treatment they are chopped or
bruised to varying extents.
This operation, sometimes referred to by its older name of
‘beating’, is extremely important as it mainly controls the
type of paper being made.
The pulp is now known as stock. It is at this point that the
size and colours are added.
 Size consists of starch or resin whose particles settle on the
fibres, producing a surface that does not blur when inked.
Dyes are added to colour the paper.
Even if the paper is to be white, some colouring is added, as
without it the paper has a slightly yellowish look.
Quite frequently mineral matter such as china clay is
added to the stock, to make the paper less transparent.
Such materials are known as fillers.

11. 7.4. Manufacture of Paper
 The pulp mixed with filling, sizing and coloring material
obtained is called soup.
 The latter is first sent through screens into a stock box
having a thin slit at the bottom.
 It is forced through the slit to fall on an endless band of
exceedingly fine bronze wire screen in motion.
 During the forward movement of screen, the pulp fibers
remain on the screen while a great portion of water drains
drains from the soup.
 As the screen moves forward, it has a sidewise shaking
motion which acts to orient some of the fibers and provide
provide better felting action and more strength to the sheet.
sheet.
 It causes the minute shreds to mat together.
 The removal of water is further facilitated by vacuum pumps

12. •Pressure of the roller removes more water and makes
the paper more compact.
•Leaving the first felt, the paper pass through steel
smooth rolls, where it is subjected to more pressure
under rollers and then picked up by the second felt,
which carries it to a series of drying rolls, which are
steam heated drying cylinder.
•The paper enters the steam heated drying cylinders
with moisture content of 60-70% water and leaves
them with 90-94% dry.
• If the sizing material has not been added in the
heater, it can be applied in the form of paste on one or
both sides of the sheet, before and after drying.
•The paper obtained from this is rough and irregular.
They become compact, smooth and receive fine glaze
when they pass through a series of hot and polished

13. A separate process is the coating of the paper with
china clay and then polishing it through the calender.
This makes art paper, the glossy kind used for
pictures. There are 3 main kinds of modern coating
processes:
 Blade (cutting edge) coating
air knife coating
Roll (turn over) coating.
Blade coating is normally done off the paper
machine, as is air knife coating.
But roll coating is done on the paper machine and
can either be a complete coating or a first coat
followed by ‘offmachine’ coating by the blade or air
knife processes.