Reference Manual

14−3
Figure 14-2. Conventional Bleaching Process
Drawing is from TAPPI’s Making Pulp and
Paper Series and is used with permission.
alternating pattern of acidic and alkaline stages
helps to break down the increasingly smaller
amounts of residual lignin, ultimately dissolving the
majority of the lignin so it can be washed out of
the pulp.
Pulp brightness only increases modestly in oxygen
delignification and does not increase uniformly
across the bleaching sequence. This is due to the
action that each chemical has on the lignin.
Brightness increases substantially in the first and
second chlorine dioxide stages and modestly
again in the final chlorine dioxide stage (see figure
14-2). The alkaline extraction stages do not
chemically whiten the pulp, they actually darken.
The alkaline stages are there to dissolve and
remove the lignin which has already been broken
down by the chlorine dioxide. Kraft market pulps
are normally bleached to a final brightness of 90%
or higher; however, the final brightness is based
solely on customer’s standards and needs for the
marketplace.
The filtrate flow in a bleach plant is counter-
current, or opposite that of the pulp flow. In this
case, there are actually two filtrate flows (see
figure 14-3). Filtrate from the final chlorine dioxide
stage washer is used as shower water on the third
chlorine dioxide stage washer, and its filtrate is
used on the first chlorine dioxide stage washer.
Some of the acidic filtrate from the first chlorine
dioxide stage washer is used to dilute and control
the consistency of the pulp effluent treatment.
Chlorine dioxide is always used with other chlorine
dioxide stages and not mixed with the alkaline
stages.
The filtrate flows of the two alkaline stages are
connected in a similar fashion (see figure 14-4).
Filtrate flow from the second extraction stage is
used as shower water on the first extraction stage
and the filtrate from the first extraction stage
washer goes to the effluent treatment. Unlike the
yellowish acidic filtrate stream, the effluent from
this process is brown due to the lignin.
Fiberline
The fiberline refers to the equipment and
processes that the pulp travels through as it is
being processed from chips to final bleached pulp
(figure 14-1). Kraft pulp’s strength declines along a
fiberline. This loss is due to the degradation of the
cellulose fibers during the pulping and bleaching
process. Chip quality, appropriate pulping
equipment, and good operating practices are key
aspects to produce strong kraft pulps.
Other quality aspects of bleaching includes pulp
cleanliness, i.e. no dark particles caused by bark,
pitch, small stones, and a minimal amount of
extractives such as resins, which cause problems
in the papermaking process. Fiber bundles, known
as shives, have to be removed and reconverted
into good pulp fibers. Metal ions must be removed
via chelation, or the binding of ions similar to
calcium being removed by a water softener.
Totally Chlorine Free
As previously mentioned, a few paper mills are
TCF. This process eliminates organo-chloride
compounds from bleach plant filtrates and makes
it possible to recycle these filtrates back to