Reference Manual

14−5
brightness effect is even reversible when the final
product is exposed to sunlight.
Mechanical pulping bleaching is done with sodium
hydrosulfite (Na
2
S
2
O4) and hydrogen peroxide
(H
2
O
2
). A two-stage alkaline sequence can be
used to raise the brightness of chemi-
thermomechanical pulp to 85% or higher. Both
hydrosulfite and peroxide attack the chemical
groups that can cause darkening of the paper.
Unlike the case in chemical pulp bleaching, lignin
is not removed in brightening.
Caustic (NaOH) Valve Applications
Sodium hydroxide (NaOH) solution is a caustic
chemical used to break down the lignin that binds
cellulosic fibers. This is a highly used chemical but
requires precise control so accurate addition to the
wood chips is provided. Poor control can lead to
economic loss both in NaOH solution and wood
chip degradation.
Chips are fed via a screwfeeder into the top of a
digester where it is mixed with cooking liquor
where it is then cooked to a schedule. In modern
Kraft mills, the lignin is removed by the action of
sodium hydroxide and sodium sulfide under heat
and pressure. This solution is known as white
liquor. As the chips are cooked the lignin and other
components are dissolved, and the cellulose fibers
are released as pulp.
Design Considerations
D Material choice highly temperature
dependent
D Low flows require low flow trims
D Tight shutoff
Typical Specification
Body:
Fisher V150 in CG8M (317SST)
CW2M (Hastelloy-C) premium selection
Trim:
Ball: CG8M chrome coated (Microscratch,
Micronotch, or Macronotch)
Seal: Alloy 6 HD (Alloy 255HD also
acceptable)
Shaft: Nitronic 50
Bearings:
Alloy 6 (PEEK if temperature allows)
Packing:
PTFE
Actuator:
Spring-and-diaphragm
Positioner:
FIELDVUE DVC6200 PD level
Chlorine Dioxide Applications
Due to environmental restrictions many mills are
going to elementally chlorine free (ECF) bleaching
practices. This has pushed the mills of today to a
new chemical compound to bleach and brighten
their pulp rather than the traditional pure,
elemental chlorine. Chlorine dioxide (ClO
2
) has
been the choice by the majority of mills today. This
is because the compound minimizes degradation
to the cellulose fibers while still achieving higher
final brightness to the pulp. However, it is an
expensive chemical to generate and highly
corrosive, so proper care must be given to choose
the correct solution.
Depending on the type of furnish created at each
mill, pulp from the digester can head toward the
bleaching section of the mill. Bleaching is done by
removing the lignin whereas brightening pulps
changes the chemical groups in lignin to forms
that do no darken pulp. Chlorine dioxide is rapidly
becoming an industry standard as a bleaching
agent because of its high selectivity in destroying
lignin without degrading the cellulose fibers, thus
preserving pulp strength while still providing a
stable brightness.
After the oxygen delignification stage (typically
found in modern mills), the medium consistency
stock (10-14% bone dry) heads to the
conventional bleaching sequence which can vary
from four to six separate stages depending on the
end-user’s brightness requirements. A standard
mill would utilize a DE
OP
DED sequence, or an
alternating sequence of chlorine dioxide (D) and
alkaline extraction stages (E) and an oxygen (O)
and peroxide (P) brightening stage.
One will always see chlorine dioxide stages
interspersed with alkaline extraction stages. This
alternating pattern of acidic and alkaline stages
helps to break down the increasingly smaller
amount of residual lignin. But this combination of
chlorine dioxide and alkaline mixture also makes