Reference Manual

8−3
Burning
Black liquor from the evaporators at 50 - 55%
solids cannot be burned in the recovery boiler.
Further evaporation to 65 - 70% solids must be
attained prior to combustion. This is accomplished
by evaporator-like vessels called concentrators or
by direct contact evaporators (cascade or cyclone
type) which use boiler flue gas for evaporation. If
direct contact evaporators are used (older
designs), air is mixed with the black liquor in the
black liquor oxidation system prior to direct
heating. This helps prevent the release of odorous
gases due to direct heat contact. Most new boilers
use concentrators for final evaporation since
indirect steam heating emits fewer odors. This is
commonly referred to as low odor design.
The recovery boiler is one of the largest and most
expensive pieces of hardware in the mill. It is the
heart of the chemical recovery process. The heavy
black liquor is sprayed into the furnace for
combustion of organic solids. Heat liberated from
burning serves to produce steam in the water
circuit and reduce sulfur compounds to sulfide.
The molten sodium compounds accumulate to
form a smelt bed on the furnace floor. The molten
smelt, consisting primarily of sodium carbonate
(Na
2
CO
3)
and sodium sulfide (Na
2
S), flows by
gravity to the dissolving tank. The dissolving tank
is filled with a water solution (weak wash) to cool
the smelt. This solution is called green liquor and
is transferred to the causticizing area.
Causticizing
Green liquor is sent to the causticizing area for
transformation to white liquor for cooking. The
process begins with clarification of the green liquor
to remove impurities called dregs. Clarified green
liquor is then mixed with lime in the slaker to form
white liquor. The lime (CaO) activates the
conversion of Na
2
CO
3
in the green liquor to form
sodium hydroxide (NaOH) for white liquor. To
allow time for a complete reaction, the white liquor
passes to a series of agitated tanks called
causticizers.
A second chemical reaction resulting from the
addition of CaO is the precipitation of lime mud
(CaCO
3
). The lime mud is removed from the white
liquor by filtration or gravity settling and the
clarified white liquor is stored for digester chip
cooking.
Lime mud filtered from the white liquor is washed
to remove residual cooking chemicals. The wash
water, or weak wash, is sent to the recovery boiler
dissolving tank. The washed lime mud is sent to
the lime kiln where heat is added for conversion to
lime. This calcined lime, along with purchased
make-up lime, is used to supply the slaker.
Although many variations exist, this completes a
typical Kraft recovery cycle as illustrated in figure
2. The next step is the preparation of the pulp for
paper making.
Bleaching
The primary objective of bleaching is to achieve a
whiter or brighter pulp. If a mill produces brown
paper such as linerboard, a bleaching sequence is
not required. However, if white paper such as
writing or magazine paper is produced, bleaching
is required. Bleaching removes the lignin which
remains following digester cooking. Lignin is the
source of color and odor for pulp.
The bleach plant has recently evolved to the most
controversial area of pulp and paper production
due to the formation of dioxin from chlorine
bleaching. Environmentalists claim dioxin in pulp
mill effluent is contaminating rivers while other
studies indicate levels of dioxin in effluent are too
low to pose any danger. Nevertheless, many
technology changes have occurred in the past
decade that have significantly reduced dioxin
emissions.
Bleaching practices prior to 1980 used large
amounts of chlorine to achieve the desired level of
brightness. Although other stages using sodium
hydroxide, chlorine dioxide, and hypochlorite were
used, chlorine was the prime bleaching agent.
Following each stage, washing was required to
remove residuals. Large quantities of water were
used following the chlorination stage where the
majority of toxic byproducts are formed. Various
methods of post bleaching treatment of effluent
were used with mixed success.
Beginning in the mid-1980’s, mills began making
significant changes due to increasing
environmental awareness. One change is the
increased use of oxygen (O
2
) delignification prior
to bleaching with chlorine and chlorine dioxide.
This provides lignin removal with the benefit of
chlorine-free effluent. This also allows for less
chemical use in subsequent bleaching stages.
A second change is extensive reuse of washer
filtrate to reduce fresh water usage. This reduces
the amount of effluent to be treated prior to
discharge from the mill. Some modern plants use