A very beneficial and cost
effective way to use ozone in the delignification
of pulp is in conjunction with chlorine dioxide
and no intermediate washing. The process is
referred to as D/Z or Z/D for production of ECF
pulp. Ozone can replace between 2 to 4 kg ClO2
per kg of ozone applied in the D0
In TCF or, totally chlorine free bleaching,
ozone is a key stage to achieve a high brightness,
high quality pulp without the use of chorine
Ozone can be added to pulp at
either high consistency (HC), medium consistency (M.C.)
or low consistency. All are operated under acidic
conditions. HC systems require a fluffer and a
press prior to the stage to thicken the pulp to
Medium consistency ozonation is
used for those mills already under 11-14%
consistency. Ozone at 10-14% weight concentration
is injected into the pulp slurry at high pressures
in high shear mixers. Higher ozone concentrations
are required in M.C. systems to minimize the gas
fraction in the mixer and maximize ozone usage. A
small transition tube with a few seconds retention
time follows the last mixer for separation of gas
from pulp. and some additional kappa reduction.
The same M.C. system is considered for low
consistency D0 stages, but with the
possibility of using only one mixer.
Following the rapid reaction of
ozone with pulp, the ozone stage vent gas (mainly
oxygen) is separated from the pulp and sent to an
ozone destruct unit to destroy any residual
The table below summarizes the
operating conditions for each system.
With a high consistency reactor,
lower ozone concentration and atmospheric pressure
leads to lower cost ozone since there is no need
for ozone compression. Mill modifications include a pulp press,
a fluffer and a high consistency reactor which can
lead to high capital expenses estimated at
With medium or low consistency
systems, a higher ozone concentration is
recommended and an ozone compressor is needed. The
ozone cost is slightly higher, but the mill
modifications cost in terms of piping, high
shear mixer, transition tube, scrubber and M.C.
pump are reduced to around US$3,500,000 installed.
For a low consistency system, the
capital cost installed is around US$2,500,000.
As of December 2000, there were
25 ozone installations in operation worldwide and
one under construction. Of these, three mills in
North America utilize ozone for bleaching of virgin pulps (Union
Camp in Franklin, VA, Consolidated Papers in
Wisconsin Rapids, WI, Domtar [formerly E.B.
Forest Products] in Espanola, ON).
click image to enlarge
EB EDDY Forest Products
click image to enlarge
Recently there have been new
start-ups at Votorantim in Brazil and Domtar
(formerly E.B.Eddy Forest Products) in Canada. The
systems are described below.
Recent comments made by Domtar at
the CPPA Bleaching committee meeting were:
"Ozone has operated since April
1999 and is a very effective bleaching agent. The
replacement ratios of kg of ClO2
replaced by kgs of ozone have been up to 4 kg/kg."
For a while many mills looked
at ozone bleaching to produce TCF pulps because of
tightening environmental regulations. Ozone today
is seen as a complement to produce ECF at a lower
cost than standard ECF pulp without ozone. With
replacement ratios of ClO2 by O3
in the range of 2.0 to 3.5, savings of 6-8 US$/mT
of pulp net can be achieved.
Whenever chlorine bleaching
chemicals are replaced by non-chlorine chemicals, a
reduction in AOX is seen. When ozone completely
substitutes the first chlorination stage (C/D),
then the extract from the washers following the
ozone stage can be recycled to the chemical
recovery system leading them closer to closing the
mill water loop. If an extraction stage follows
the ozone stage the extract from this washer can
also be recycled. This is greatly desired since
most of the COD and BOD, are contained in this