Measurements of Chlorine Dioxide Bleaching in Pulp and Paper Mills0 pages

Application Data Sheet
ADS 2600-06/rev.B
May 2008

Pulp and Paper Industry

Measurements in Chlorine Dioxide (ClO2)
Bleaching Used in Pulp Mills
BACKGROUND

THE MEASUREMENTS

Bleaching is a whitening process that is used in the
paper industry to produce paper with high brightness.
Chemicals—such as chlorine gas (C), sodium
hypochlorite (H), oxygen (O), hydrogen peroxide (X),
ozone (Z), and chlorine dioxide (D)— are used in
various combinations to produce pulp with the desired
properties. Recent limitations on the use of chlorine
(and hypochlorite) have resulted in more extensive
use of chlorine dioxide as an alternate bleaching
chemical. Chlorine dioxide bleaching is referred to as
Elemental Chlorine Free (ECF) and is the process
used in over 50% of U.S. mills.

pH is measured in the D tower to control the bleaching
reaction conditions. pH is also measured in the E
stages to control addition of the caustic used for
extraction. ORP can be used in the washer between
the D and E stages to verify that rinsing of the chlorine
dioxide has been completed. Attempts to use ORP to
control chlorine dioxide dosage in the D tower have
been mostly unsuccessful due to the nonlinear character
of ORP. See Application Data Sheet #43-014 for more
information on ORP.

THE PROCESS
The chlorine dioxide is mixed with the pulp and contacted in each D (chlorine dioxide) tower (see Figure 1)
for about 1 hour per stage. The primary purpose of
bleaching is to maximize removal of the lignin binding
agents in the pulp without degradation of carbohydrates, which weakens the final product (paper). The
pH is maintained at relatively low levels (around 4) to
foster rapid chemical reaction. Higher pH levels require
more chlorine dioxide for equivalent bleaching action.
Following bleaching in each D tower, the reacted lignin
is removed by dissolving the lignin under high pH
conditions (typically over 11). Lower pH levels will not
dissolve all of the lignin. This caustic extraction (E)
stage is preceded by a washing step to reclaim the
chlorine dioxide and to minimize carryover of the acidic
chemicals. The amount of caustic needed is proportional to the amount of bleach added in the D stage.
Bleach plant sequences vary widely from mill to mill;
however, the D-E-D-E stage sequence is quite common. At least 2 bleaching stages are generally
required to produce the specified brightness, with
additional stages for more specialized uses.
Consistency (% solids) is typically maintained at a
high level (up to 10%) to minimize water use.
Temperature is controlled around 140°F (60°C), and
doses of 5% chlorine dioxide are common.

THE PRODUCTS
pH measurements in the D and E stages have historically been among the most difficult applications in the
pulp mill. The high pulp consistency of the process
stream and harsh bleaching chemicals degrade the
performance of most general purpose sensors. Those
sensors tend to require cleaning regularly and frequently, and they may not be able to withstand the
strongly oxidizing environment.
The TUpH Model 398R pH and ORP sensors are
well-suited for this kind of application. The wide-area
junction provides resistance to coating, while the
patented helical pathway prevents process intrusion
into the inner reference chamber. Construction materials
of Tefzel and Kalrez are used to withstand the strongly
oxidizing environment.
The Models 54e pH/ORP and 5081 pH/ORP are ideal
instruments for monitoring and controlling pH and
ORP in pulp and paper mills. All configuration and
calibration can be conducted remotely using the HART
protocol and the AMS PC interface. The combination
of the Model 398R with either the AC-powered Model
54e or the DC-powered Model 5081 provides ease of
installation, ease of operation, and the lowest maintenance possible.