Chemical Engineering March 2022 - 34

CASE STUDY:
A
BRAZIL PULPING MILL
leading integrated producer,
exporter
and recycler of pulp and
paper in Brazil was looking to expand
its pulping operations.
Challenge
The Kraft process used in the production
of pulp enables the efficient recovery of
chemicals and heat in a closed-cycle process.
However, it also allows the undesired
buildup of chloride and potassium in the
recovery cycle, which if left uncontrolled,
can create corrosion and boiler fouling.
This closed circulation
loop maximizes
the economics of
chemical recovery,
reducing chemical
discharge and raw
chemical makeup,
but also creates
challenges for handling
and treating
liquor streams. Nonprocess
elements
(NPEs), including
chloride and potassium
entering
the
mill
through the raw
wood and chemical
makeup, accumulate in the recovery
cycle. Over time, they create conditions
for scaling and plugging in the
boiler, leading to lower energy production.
If left uncontrolled in the
precipitator ash, NPEs can become
a significant problem - causing corrosion
and boiler fouling - ultimately
resulting in a reduction in recovery
boiler capacity and an increase in
operating costs.
By integrating advanced chlorideSolution
After
analyzing the composition and performing
small-scale pilot testing, it was
confirmed that the installation of an enhanced
chloride removal process (ECRP)
as a second stage to the site's existing
system would be able treat the additional
feed of precipitator ash and prevent significant
boiler-capacity losses and increased
operating costs. With the new system, the
site was able to treat up to 10 ton/h of precipitator
ash (equivalent to 240 ton/d).
Creating value from byproducts
In this particular case, the SOP produced
from the ECRP system was used by the
mill to fertilize its forest reserves to enhance
growth rates for the next generation
of trees. In doing so, the facility was
able to close the loop by returning highquality
nutrient compounds back into
the environment.
❑
and steam, which is used elsewhere
within the plant. Contained
in the molten salts (smelt) are most
of the needed pulping chemicals,
which are collected from the recovery
boiler and can be reused in
the digester.
34
and potassium-removal systems to
treat the precipitator ash, mills can
unlock significant operational benefits
and reduced chemical costs,
while simultaneously generating
additional revenue streams from
produced byproducts that also increase
the sustainability of the
production facility.
Chloride removal process
A two-stage crystallization process
can be used for enhanced removal
of NPEs. The first stage consists
of a conventional chloride-removal
process
(CRP)
system (Figure 1)
operating close to
atmospheric pressure,
followed by
a
second-stage
crystallization process
designed to
target potassium
within the chemical
recovery process.
Stage 1 - CRP.
Precipitator ash
is dissolved with
condensate,
resulting
in a slightly
FIGURE 2. Glaserite crystals can form during the chloride-removal
process when the crystallization chemistry exceeds solubility limits for
potassium before reaching the chloride solubility limit
under-saturated solution feed to the
CRP system. The CRP system operates
as a true crystallizer, with proper
crystal generation, growth and washing
to maximize the removal of contaminants
and recovery of sodium
salts. Crystallization occurs through
evaporation of water, causing the
solubility limit of sodium salts to be
exceeded. The resulting sodium salt
crystals are dewatered and washed
in a centrifuge and returned to the recovery
cycle via sluicing with black liquor.
Chloride and potassium remain
in solution and are purged from the
crystallizer in a concentrated stream
with minimal sodium losses. The CRP
system is typically thermally integrated
with a mill's black-liquor evaporation
system in order to achieve low
operational costs.
The conventional CRP system
will achieve a maximum chlorideremoval
efficiency of 98-99% at a
sodium recovery of 85-95%. For
mills with higher potassium levels,
the sodium recovery will be lower.
FIGURE 3. For mills with a particularly high concentration of potassium in
the chemical-recovery unit, a second stage can be added to crystallize additional
sodium salts and potentially produce a sulfate of potash (SOP) stream
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