Reference Manual

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Chapter 9
Pulping
Pulping is the process of converting wood material
to separate pulp fibers for paper making.
Processes range from purely mechanical, in which
the wood is ground into fibers by refiners or
grindstones, to chemical processes, in which the
fibers are separated by chemically degrading and
dissolving the lignin that binds fibers together. In
many cases, mills will produce various grades of
paper having both mechanical and chemical
pulping processes.
Mechanical Pulping
Stone Groundwood
Process:
The most basic type of mechanical pulping is
known as stone groundwood (SGW), and has
been virtually unchanged since its development in
the 1840s. This process involves rotating
manufactured grindstones to be pressed against
small wood logs that are oriented parallel to the
axis of the stone where a typical modern SGW
plant will consist of only four to six grinders to
supply a large paper machine.
The quality of the produced pulp (strength and
drainage properties) depends primarily on the
surface characteristics of the stone. Water is
added to wash away the torn fibers. Virtually all
stones are artificially manufactured using a hard
grit material, typically embedded with silicon
carbide or aluminum oxide.
This process is a large consumer of electric power
due to the rotating grindstone, and the pulp
produced is typically used for the production of
newsprint grade paper.
Refiner Mechanical Pulp
Process:
Commercial production of refiner mechanical pulp
(RMP) began in 1960. It is produced in most
modern mills using chips rather than logs and
rigged metal discs used for shredding and grinding
of the wood chips. The chips are ground between
the rotating discs in a refiner, producing RMP.
This process is typically done in two separate
stages operating in series, and produces a
longer-fibered pulp than SGW. As a result, the
pulp is stronger, freer, bulkier, and usually
somewhat darker in color.
Thermomechanical Pulping
Process:
The first major modification to RMP was the
addition of steam before the refiner. This is known
as thermomechanical pulping (TMP). The
steaming serves to soften the chips, resulting in
the pulp having longer fibers and fewer shives
than RMP. These longer fibers produce a stronger
pulp than either SGW or RMP, making for a
stronger final sheet of paper. This process is still
employed on a large scale to produce high-tier
pulps for newsprint and board.
Referring to figure 1, chips are fed by a feeding
plug and screw feeder to a presteamer, which is
heated by PCV-1 to typically 15 to 30 psig and
265 to 285°F. After a retention time of a couple of
minutes, the pressurized chips are fed to the