Agriculture Reference
In-Depth Information
system maintained under pressure. The raw
material is impregnated with the cooking liquor
before feeding the digester. The process is illus-
trated in Fig. 13.10.
After cooking, the hemp pulp needs to be
washed, shortened and bleached. Washing is
an important operation to eliminate residual
dissolved material that could consume the
chemicals in the bleaching stages.
Shortening the fibre is necessary to make
it usable in the next part of the paper
process.
Bleaching can be carried out when the
previous steps are completed.
Washing and shortening are usually car-
ried out simultaneously in a Hollander beater
(Fig. 13.11). The Hollander beater is a cham-
ber into which the pulp is introduced. The
pulp is moved in this equipment by the rota-
tion of a cylinder equipped with special
blades. The cutting action is regulated by the
pressure applied on this cylinder. The
Hollander beater is usually fitted with a drum
filter. Washing is performed during the cut-
ting time.
The time spent for cutting can be 6-8 h,
allowing for the bleaching stage using sodium
hypochlorite in some applications. This proc-
ess is described in Fig. 13.12.
The pulp preparation is made batch by batch.
13.5.3 Bleaching
After cooking and pulp preparation, bleaching
stages are necessary to obtain pulps with a
high level of brightness.
The brown pulp obtained after cooking
contains residual lignin and diverse chromo-
phoric compounds. The aim of bleaching is to
make these compounds soluble.
The principal chemicals used are:
Chlorine
Sodium hypochlorite
Caustic soda (itself for alkaline extraction or
in the presence of oxygen and/or hydro-
gen peroxide)
Chlorine dioxide
Oxygen
Ozone
Hydrogen peroxide
Peracetic acid
Enzymes
WHITE
LIQUOR
BLACK
LIQUOR
FEED CONVEYOR
FROM METER
LIQUOR-MIXING
TA N K
CHUTE
PUMP
STEAM
VENT
HOPPER
MIXER
IMPREGNATOR
F RESH WATER
SCREW
FEEDER
HOT WATER
TA N K
BLOW
TA N K
HOT WATER
HORIZONTAL-TUBE
CONTINUOUS DIGESTER
PUMP
ORIFICE
DISCHARGER
TO WASHING SYSTEM
MAGNETIC
TRAP
TO
SEWER
STOCK
PUMP
Fig. 13.10. Hemp continuous digester process.
 
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