Reference Manual

13−2
mud is admitted to precoat the outside of the filter.
A vacuum is maintained on the inside of the filter
with a vacuum pump. The dregs slurry is pulled
through the lime mud and filter media. A knife
blade removes the lime mud as it becomes
saturated with dregs. The lime mud/dregs are
hauled to a landfill and the clear filtrate is recycled
to the green liquor clarifier.
Valve: LV-1 Dregs Precoat Filter Level
D Typical process conditions:
— Fluid: Lime mud
— T = 212°F
— P = 55 - 65 psig
D Typical valve selection:
— These are typically NPS 2 or NPS 3 SST
valves with solid VTC (ceramic) internals
and alloy 6 bearings.
Valve: FV-3 Dregs Slurry Underflow
from Green Liquor Clarifier
Please reference the lime mud underflow
information below as this application closely
resembles its process.
Slaker
The slaker is the heart of the recausticizing
operation. At this point, clarified green liquor is
mixed with lime to produce white liquor. The
reburned lime (CaO) from the lime kiln and
makeup lime are added and react with the water in
the green liquor to form calcium hydroxide
(Ca(OH)
2
). This reacts with Na
2
CO
3
in the green
liquor to form sodium hydroxide (NaOH) or caustic
and precipitate calcium carbonate (CaCO
3
) or lime
mud. A retention time of approximately fifteen
minutes is allowed in the slaker. Recausticizing
efficiency is improved by steam heating the
incoming green liquor to near boiling and adding
an agitator to the slaker.
Valve: FV-5 Green Liquor to Slaker
D Typical process conditions:
— Fluid: Green liquor
— T = 212°F
— P = 80 - 85 psig
— ΔP = 5 - 10 psig
D Typical valve selection:
— NPS 6 valves with alloy 6 scraper seats
due to concerns with green liquor scaling.
A SST valve with an alloy 6 HD seal and
alloy 6 bearings should be used in this
application.
Causticizers
The retention time in the slaker is not enough to
allow a complete reaction between the lime and
green liquor. Causticizers consist of a series of
two or more agitated tanks having a total retention
time of 1-1/2 - 3 hours. The white liquor slurry
usually flows by gravity from the slaker and
through the causticizers.
White Liquor Clarifier and Lime Mud
Washer
The white liquor clarifier is essentially the same as
the green liquor clarifier described earlier. The
white liquor slurry is pumped in from the last
causticizer and the lime mud solids (CaCO
3
) settle
to the bottom of the clarifier due to density
differential. The lime mud, at 35-40% suspended
solids, is raked to a center discharge cone where
it is concentrated and removed. The clarified white
liquor containing sodium hydroxide and sodium
sulfide overflows and is pumped to the digester for
cooking wood chips.
The lime mud underflow from the white liquor
clarifier must be washed to recover residual
cooking chemicals. The lime mud washer is very
similar to a white liquor clarifier with the possible
exception of multiple compartments. The feed to
the washer is from a mix tank, which accepts
filtrate streams from the lime mud filter and lime
kiln scrubber in addition to the lime mud
underflow. The washed lime mud is removed at
45-50% suspended solids and sent to storage.
The overflow, referred to as weak wash, is sent to
weak wash storage and primarily used in the
recovery boiler dissolving tank.
Valves: FV-6/FV-8/FV-16/FV-19 Lime
Mud Underflow
D Design Considerations and Service
Conditions
— Lime mud is extremely erosive and
difficult to handle due to fine particulate
and high solids concentration.
— The underflow valve is throttled to control
mud density which directly impacts the