A.Manufacturing process of “Bagasse and Rice
Straw” Pulp:
i) Raw Material Preparation:
Bagasse:
Bagasse is received both in bales and
in loose form, which is fed to debaler through the underground belt conveyor, where the bales are disintegrated.
After disintegration it is sent to depither by belt conveyor. Raw
bagasse contains about 28% to 30% pith. The depither removes most of
the pith and the accepts will go to wet
cleaning system for washing. The wet cleaned bagasse is fed to an
Aqua Separator to remove the wash water and the left over pith and
dirt from it. The cleaned bagasse is fed through Screw Press for
further removal of water and then fed to Continuous Digester system
for cooking. The outlet pulp of the Continuous Digester system is
taken to a Blow Tank for Storage. In Continuous Digester system,
Caustic and Steam are injected at 7 kg/cm2 (g) as cooking media
chemicals.
Rice Straw:
The Rice Straw is mixed with Caustic Soda thoroughly in Caustic
Lye Mixer and digested in Rotary Digesters. It is cooked with steam
of 7 kg/cm2(g) at 165ºC for about 2 hours
and stored in blow tank.
ii) Washing & Screening:
The Bagasse Pulp received from Blow Tank along
with Rice Straw Pulp is washed in 4 stages Brown Stock Washing System
in a counter current vacuum filter system and the filtrate known as
Black Liquor from first stage brown stock washing system is sent to Chamical Recovery Plant for the recovery of
Chemicals. The 4 stage washed pulp is subjected to cleaning in
Centrifugal Screen, Centricleaners are then thickened in a thickener.
Now the pulp is ready for bleaching.
iii) Bleach
Plant:
Washed and cleaned pulp is subjected to bleaching in C-E-H-H (Chlorination,
Alkali Extraction Hypo-I & Hypo-II) sequence. The unbleached pulp
is mixed with Chlorine water in Chlorine Mixer and then passed
through upflow retention tower. It is then
washed in Chlorine washer and at the outlet of Chlorine washer alkali
is mixed and then passed to down flow alkali retention tower through
heater mixer. After alkali extraction, the pulp is washed in alkali
washer. At the outlet of alkali washer, Calcium hypo chlorite is
added and then passed through down flow Hypo-I retention tower. After
the hypo-I retention tower, the pulp is washed in Hypo-I washer. At
the outlet of Hypo-I washer, second stage calcium hypo chlorite is
added and then passed through Hypo-II washer. Now the pulp is ready
to make white variety paper.
B. Process
of Wood Pulp:
Bleached hard wood pulp/soft wood pulp is purchased from various industries
in the form of sheets or cakes. The sheets are pulped in a pulper in
water medium and then stored in a chest. Then it is refined through a
refiner batch wise in a chest, and then stored in wood pulp storage
chest.
C. Paper
Machine Section:
The pulp i.e., Bagasse and Wood are taken in required proportions to
stock preparation section and blended. The required sizing chemicals, colours etc., are added depending upon the
quality of the paper. The blended pulp is forwarded to head box
through S.R box after dilution with white water. From Head box, the
stock is distributed evenly and uniformly across the wire section.
The major part of the water is removed in wire part and then the
paper web is passed to press section where further removal of water
is effected and the web is passed to drier section. In the drier
section the paper is heated indirectly by steam and the water content
in the paper is evaporated and final sheet of paper is reeled on POPE
Reel after passing through calendars. Paper is converted into
required size reels on rewinder and then converted into sheets on
sheet cutters, packed and despatched after
finishing operation.
D. Chemical
Recovery Section:
Chemical recovery system consists of three major units viz.,
Evaporation plant, Chemical Recovery Boiler and Re-causticizing Plant. The details of which are
given below.
i.Evaporation Plant:
The evaporation plant is of tubular falling film type. The evaporator
system consists of six evaporators. Evaporation plant will evaporate
the weak Black Liquor from 8% solids to 48%-52% solids.
ii.
Chemical Recovery Boiler
The chemical Recovery Boiler is a top suspended, single drum furnace
and boiler. The boiler is equipped with a cascade evaporator to
concentrate black liquor from evaporator plant to 60%-63% dry solids.
The boiler has an efficient combustion air system followed by an ESP
for flue gas cleaning. Concentrated black liquor from evaporation
section will be fired in the chemical recovery boiler. Before the
actual firing in the recovery boiler, the black liquor concentration
from Evaporators is further increased by the heat available from flue
gas in direct contact in cascade evaporator to a solid content of
62%-63% and it will enter recovery boiler furnace and there it is
fired initially by auxiliary fuel (natural gas) and subsequently by on
its own firing because of the high temperature in the furnace. During
this process of burning in the recovery boiler, all the organic
contents will be burnt to CO2 producing steam, while the inorganic
consisting of mainly sodium carbonate will collect at the bottom of
recovery boiler furnace (this is called smelt) and this smelt will be
taken into dissolving tank where it will be dissolved in weak white
liquor (low concentration recovered sodium hydroxide solution). This
liquor in the smelt-dissolving tank, known as green liquor, is sent
to causticizing section for converting to
sodium hydroxide solution by reacting with lime solution.
iii.
Re-Causticizing Plant:
The causticizing plant is a conventional uniclarifier type with Rotary slaker, recausticisers and dregs washing system, two-stage mud washing and lime mud filter.
The green liquor from recovery boiler after clarification is treated
with slaked lime by which process the sodium carbonate will be
converted into sodium hydroxide and Calcium carbonate. The causticizing operation consists of no. of large
diameter tankages with slowly moving rake
agitator for producing clear sodium hydroxide solution known as white
liquor clarifiers is washed off in clarifiers known as mud washers
for recovery of residual sodium hydroxide, if any. Relatively pure
lime sludge is taken into vacuum filters for increasing the
concentration to around 50% and then disposed off.
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