Cooling water
Condenser
Flue gas
Steam
Flash Steam
Stack
Smothering
steam
NCG
Ejector
Steam
Water
6th
effect
5th
effect
4th
effect
3rd
effect
2nd
effect
Black liquor
Flash
tank
Heating section
Clean condensate
Electrostatic
precipitator
(ESP)
1st
effect
Induced
draft
fan
Weak
black liquor
feed
Direct contact
evaporator (DCE)
to steam
stripping
system
Soap
to 2nd or 3rd effect
Soap
skimming
tank
Flash steam
to 2nd or 3rd effect
Air
blower
Black liquor
oxidation tank
(blox)
Flash
tank
Emergency liquor
divert system
Figure 4. Evaporators
Figure 5. Recovery
atmosphere. This cooking action dissolves the bond between the cellulose
fibers and the glue-like material called
lignin that cements the fibers together.
After cooking, the pulp is blown
through a blow valve into a blow tank
where the shock of the material hitting
the tank wall separates the lignin and
fibers.
There is also an enhanced cooking
batch digester, where the liquor goes
through heat exchangers to help cook
the chips without using all the steam in
a standard batch digester, which means
more energy efficiency and economy.
The next area of the mill that uses
metal-seated valves is the recovery area.
The recovery area is where chemicals
from the spent cooking liquors are
recovered. The reconstitution of these
chemicals can form fresh cooking liquor
at the same time incinerating organic
residuals creates energy.
While the main focus of pulp and
paper mills is on the fibers that make
the paper, the mills use every part of the
tree. For example, the bark goes into
the bark boiler to make steam. The
lignin has high BTU value so mills try to
recover the lignin to burn in the recov-
ery boiler, which also recovers many of
the chemicals used in the cooking
process. The recovery process is
achieved through a well-defined series
of steps, starting with the weak black
liquor coming from the brown stock
washers. The process steps are then:
; Concentration of liquor in multiple evaporators and concentrators
to get to heavy black liquor
; Addition of salt cake to make up
for soda loss
; Incineration of heavy black liquor
in the recovery boiler
; Dissolving smelt from the boiler
to form green liquor (new liquor)
; Burning of lime mud to recover
lime
; Causticizing of green liquor with
lime to form white liquor.
EVAPORATORS
After the blow tank, the stock is washed
in a brown stock washer (Figure 4). The
fibers are then transferred to the stock
prep area, and the wash is recovered
and sent to the evaporators. Evapora-
tors take the water out of the liquor to
create high-density black liquor to burn
in the recovery boiler. The evaporator
island may include as many as six evap-
orators which are called effects—for
example, the vapor in one effect
becomes the steam supply in the next
unit. There are four types of evapora-
tors: rising film, falling film, cascade
and cyclone.
RECOVERY BOILER
Heavy black liquor is pumped to a black
liquor storage tank at the recovery boiler (Figure 5) where the liquor is mixed
with salt cake—sodium sulphate—as a
make-up chemical to replace the chemicals lost during washing and evaporation. The black liquor is then pumped by
the nozzle pump into the boiler, where
the liquor is vaporized and burned, the
organics in the liquor burn as fuel, while
the chemicals fall to the bottom of the
boiler and flow out as smelt. The smelt
flows into a dissolving tank filled with
weak wash liquor from the causticizing
area. The smelt is agitated and recycled
to break up the molten smelt and prevent an explosion. The liquor in the dissolving tank is called green liquor.
CAUSTICIZING
Green liquor from the dissolving tank is
pumped to the causticizing area (Figure
5) where it is treated with milk of lime,
calcium hydroxide, to form white liquor.
Because green liquor contains impurities
called dregs, it must be filtered in a clarifier. The clarified green liquor is then
pumped to the slaker where it is mixed
with burnt lime, calcium oxide. The lime-
