Reference Manual

8−4
totally enclosed pressure diffusion washers
following O
2
delignification to further reduce toxic
effluent.
Another change involves increased substitution of
chlorine dioxide for chlorine gas. Chlorine dioxide
does not release the chlorine ions responsible for
forming dioxin. Although chlorine dioxide is more
expensive to produce, it requires 2.5 - 3 times less
to bleach the same amount of pulp. Some
processes which use O
2
delignification prior to
bleaching have achieved 100%.
Though evolution has caused dioxin emissions to
decrease overtime, changes such as this will
continue to take place in the future. Federal and
state regulator agencies continue to disagree on
allowable emission limits. Future technology will
continue to move toward zero discharge limits for
dioxins and other by-products of the bleaching
process.
Stock Preparation
Pulp, as produced in the pulp mill, is not suitable
for manufacture of most grades of paper.
Properties must be added to the pulp which will
aid in uniform sheet distribution and bonding of
fibers. Two major steps used to impart desired
properties are stock proportioning and mechanical
treatment by beating and refining pulp fibers.
Stock proportioning involves the addition of other
types of pulp and chemical additives to achieve a
desired grade of paper. Different pulps, along with
water, are added to achieve proper consistency.
Consistency is defined as the percentage by
weight of dry pulp fiber in a combination of pulp
and water. Typical consistencies in the paper
machine range from 1/2 - 3%. “Broke” pulp may
also be added to the mixture. “Broke” pulp
consists of paper breaks and trim ends from the
paper machine which have been beaten in a broke
repulper.
Various chemical additives are required to aid in
proper sheet formation and drainage of water.
Some additives and their effects are:
D Starch — improves paper strength and
surface “feel” at the dry-end of the paper machine
D Alum — pH control and chemical retention
onto pulp fibers
D Fillers — common fillers are clay, calcium
carbonate, or titanium dioxide. These particles
serve to fill gaps between fibers to produce a
smoother, brighter sheet.
These are a few of the many additives that may
be used. The chemical additives and various pulp
are mixed in a blending chest and the mixture is
commonly referred to as furnish.
Another step in pulp treatment involves
mechanical action. The two most common
treatments are referred to as beating and refining.
This action tends to separate and shear pulp
fibers which increases paper strength and allows
fibers to more easily absorb water and additives.
Two basic types of refiners are conical and disk
refiners. Both types consist of rotating elements
and a stationary housing to provide shearing
action. Refining is often done in two stages. One
stage involves treatment of virgin pulp fiber only
and a second stage for treatment of virgin fiber,
broke, and chemical additives.
Finally, the mixture of pulp and chemicals from the
blending chest is pumped through refiners to the
machine chest. The machine chest is also
supplied by the save-all. The save-all screens
fibers from white water drained from the paper
sheet into the wire pit.
Paper Machine
Following stock preparation, the furnish is sent to
the machine room for final sheet forming. Even
though different types of paper machines exist for
manufacture of various grades of paper, most all
perform the same basic functions which can be
divided into two broad categories. The “wet end” is
where the pulp and water solution is spread onto a
moving wire and dewatered to form sheet. The
sheet then moves to the “dry end” for further
evaporation of water and smoothing of the sheet.
The type of machine used most today is the
Fourdrinier paper machine.
The portion of the wet end that supplies stock to
the machine is referred to as the approach
system. Primary components in this system are
the wire pit, machine chest, fan pump, basis
weight valve, screens, and cleaners. The system
involves the fan pump accepting a mixture of white
water from the wire pit and stock from the
machine chest. The basis weight valve controls
the flow of stock from the machine chest to the fan
pump. The mixture of stock and white water is