Reference Manual

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Chapter 13
Recausticizing and Lime Recovery
The recausticizing and lime recovery plant is the
final step in the Kraft recovery process. It serves
as a link between the Kraft recovery boiler and the
digester. The function of the recausticizing area is
to convert the inorganic chemicals in the green
liquor from the recovery boiler dissolving tank to
white liquor for cooking chips in the digester. This
process consumes lime and produces lime mud.
The purpose of lime recovery is to convert the lime
mud back into lime for the recausticizing process.
Proper control of the recovery and reclaim of the
cooking chemicals is essential in the economic
success of a Kraft recovery mill.
Recausticizing
As mentioned earlier, the recausticizing process
involves reclaiming the cooking chemicals
contained in the green liquor and converting or
regenerating them to produce white liquor. The
white liquor is then used to cook wood chips in the
digester. A typical flow sheet is shown in figure
13-1.
Dissolving Tank
Green liquor is produced in the recovery boiler
dissolving tank by mixing weak wash and smelt.
Smelt, consisting primarily of Na
2
CO
3
and Na
2
S,
is produced by burning black liquor in the recovery
boiler furnace. Weak wash, which is basically
water, is a product of lime mud washing. In
addition to the chemical components, the green
liquor contains impurities known as dregs which
consist of unburned carbon and inorganic
impurities such as calcium and iron.
Valve: FV-9 Weak wash to dissolving
tank
D Typical process conditions:
— Fluid: Secondary condensate
— T = 176°F
— P = 70 - 75 psig
— ΔP = 40 - 45 psig
D Typical valve selection:
— NPS 6 valves with alloy 6 scraper seats
due to concerns with scaling. A SST
valve with an alloy 6 HD seal and alloy 6
bearings should be used in this
application. Depending upon process
conditions, the Control-Disk could act as
a great alternative.
Green Liquor Clarifier
The dregs, which cause the green color, are
impurities that must be removed from the green
liquor. These fine particles are removed by
pumping the green liquor to a sedimentation tank
or clarifier. Since the density of the dregs is
greater than the green liquor, settling of the dregs
by gravity occurs. A slow moving rake pulls the
material to a discharge cone in the center of the
clarifier where it is concentrated and removed.
The clarified liquor exits via overflow piping to a
storage tank.
Dregs Washing
Before disposing of the dregs, they must be
washed to recover any residual cooking
chemicals. This is important from an economical
and environmental aspect. Most mills employ a
pre-coat filter to wash the dregs. The system
consists of a rotating cylindrical filter in a vat. Lime