Reference Manual

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Chapter 8
Process Overview
The modern pulp and paper mill is a complex
manufacturing process involving many varied
types of operation. Many factors influence the type
of process used by a specific mill. Some of these
factors include: type of wood available (hardwood
or softwood), type of paper or paperboard
produced, age of mill, and availability of an
abundant supply of water. This sourcebook will
focus primarily upon the Kraft or sulfate pulping
process as illustrated in figure 1.
Wood Preparation
Wood preparation is a series of steps that
converts logs to a suitable form for use in the pulp
mill. This area of the mill is commonly referred to
as the woodyard.
Logs from the forest are usually received from a
truck, rail car, or barge. Large overhead cranes
are used to unload and sort the logs into piles for
long or short logs. Logs may pass through a
slasher, which cuts the logs into segments, if a
certain length is required.
The next step involves debarking, which removes
both dirt and bark from the logs. The most
common method employed is mechanical
debarking via a barking drum. Logs are fed into
the rotating cylinder and the rotating/tumbling
action rubs the bark from the logs. The bark falls
out of the cylinder via slots and debarked logs exit
the opposite end of the cylinder. Bark is used as
fuel for the power boiler or log boiler.
Following debarking, the logs are fed to the
chipper. The chipper uses high speed rotating
blades to reduce the logs to chips of a suitable
size for pulping. Chips are then screened for
acceptable sizes by passing them over a set of
vibratory screens. The rejects are returned for
further chipping and acceptable chips are stored in
large outdoor piles or silos for pulp mill use.
Pulping
Pulping is the process of separating the wood
chips into fibers for paper manufacture. This is
accomplished primarily by mechanical, chemical,
or combined mechanical/chemical processes.
Some mills that produce various grades of paper
have both mechanical and chemical pulping
processes.
Mechanical pulping, or the groundwood process,
involves pressing logs against a rotating
grindstone and washing away the torn fibers with
water. This process is a large consumer of electric
power due mainly to the grindstone motor. This
type of pulp is used primarily for the production of
newsprint grade paper.
More modern methods of mechanical pulping
involve shredding and grinding of wood chips
between the rotating disk of a refiner. The product
is referred to as refiner mechanical pulp (RMP).
Variations of this process involve pretreating of
wood chips with steam and/or chemicals. This is
commonly referred to as thermo-mechanical pulp
(TMP) or chemithermomechanical pulp (CTMP).
The majority of pulping processes in North
America are chemical processes. The most
common are the sulfate and sulfite processes. Of
these two, the sulfate or Kraft process is the
dominant process. The Kraft cooking process is
part of a larger process called the Kraft recovery
cycle. A typical Kraft recovery cycle is illustrated in
figure 2.
The Kraft process involves cooking the wood
chips under pressure in an alkaline solution of