Note: Descriptions are shown in the official language in which they were submitted.
20~78308
--1--
RECOVERING PULP MILL CHLORINE DIOXIDE EMISSION
The invention relates to recovering chlorine
dioxide from the effluent gas from a pulp mill.
Chlorine and chlorine dioxide are the major
chlorine-based chemicals used in bleaching chemical
pulps. The use of chlorine is under environmental
pressures in an attempt to reduce chlorinated or-
ganics in the aqueous effluent, in the air (chloro-
form), and in the pulp. Substitution of chlorine
dioxide for part of the chlorine in a chlorination
stage has been shown to decrease the ~;Rch~rge of
these chlorinated compounds into the environment.
However, chlorine dioxide (ClO2) is considered a
toxic and dangerous chemical. Chlorine dioxide
emissions occur from the various bleaching stages as
well as the generator itself. State and federal
regulations require these off-gases to be scrubbed.
Further, the substitution of chlorine dioxide for
chlorine requires more generator capacity and de-
creases the efficiency of ClO2 bleaching. This tendsto increase the amount of ClO2 emissions in the
bleach plant vent gases.
The ClO2 in pulp mill vent gases are usually
removed by scrubbing with an alkali, preferably white
liquor contA;n;ng sulfides and other reduced sulfur
compounds. It is well known that chlorine dioxide
can be reduced by alkaline hydrogen peroxide to form
sodium chlorite. British patent No. 2,058,602 and
Japanese Kokai No. 55-98965 both disclose that
chlorine dioxide emitted from mills that bleach
textiles with sodium chlorite can be absorbed with
alkaline hydrogen peroxide, and the effluent scrubber
solution cont~;n;ng sodium chlorite can be reused by
the mills as additional sodium chlorite. However,
pulp mills normally do not have a use for sodium
chlorite, and the sodium chlorite generated in such a
207~308
--2--
scrubber must receive further treatment before dis-
posal into the environment.
One proposed solution was to employ "white water"
in a scrubber (Campbell, P.R., "Bleach Plant Vent Gas
Scrubber at Leaf River Forest Products Corp.", TAPPI
1987-Pulping Conference (1987) pages 737-738). While
this process does neutralize the chlorine dioxide,
the costs are excessive because it does not convert
any of the chlorine dioxide scrubbed from the gases
into a useful product, and it consumes both caustic
and sodium sulfide from the white water.
The present invention overcomes the disadvantages
of the prior art process by providing a process for
recovering gaseous chlorine dioxide from pulp mill
emissions comprising:
a. contacting a gaseous pulp mill emission con-
taining chlorine dioxide with an alkaline, aqueous
hydrogen peroxide solution, thereby converting at
least part of the chlorine dioxide into an aqueous
chlorite solution, and
b. incorporating at least part of the aqueous
chlorite solution into a chlorination stage, a
chlorine dioxide stage, or a chlorine dioxide/chlori-
nation stage of the pulp mill, thereby generating
chlorine dioxide therein.
The reaction of alkaline hydrogen peroxide with
chlorine dioxide to form sodium chlorite is well
known. However, until the present invention it was
believed to be necessary to eliminate the sodium
chlorite product with the millwaste according to
Sittig, M., Pulp and Manufacture Enerqy Conservation
and Pollution Prevention, Noyes Data Corp., Park
Ridge, N.J. (1977) page 407.
Not only does the process remove chlorine dioxide
from the vent gases to reduce potential atmospheric
pollution, but the process also decreases the overall
discharge of chemicals into the aqueous effluents
-3- 20783~8
from a pulp mill because the chemical bleaching value
of the chlorine dioxide is recovered and recycled
into the pulp mill as a solution of sodium chlorite
("chlorite"). The chlorite reacts with the chlorine
or acidity of the stage and is converted to Cl02 as a
useful bleaching chemical. This can result in
operational and capital savings as well as environ-
mental improvements. Preferably, the chlorite is
introduced into a stage cont~i n ing chlorine. The
reaction of chlorite with chlorine produces one mole
of Cl02 per mole of chlorite as compared with only
0.8 moles of Cl2 per mole of chlorite when reacted
with an acid.
A particularly desirable source of alkaline,
agueous hydrogen peroxide is readily obtainable as
effluent from stages of pulp mills having either a
peroxygen stage ("P") or a peroxygen assisted oxygen
extraction stage ("EOP"). A particularly desirable
source of alkali alone is available from either an
oxygen extraction stage ("EO") or an extraction stage
("E"). Any of these solutions can be recycled in a
scrubber. Hydrogen peroxide and/or sodium hydroxide
or sodium carbonate may be added as necessary to
provide a solution of sodium chlorite of any conve-
nient concentration. One skilled in the art willreadily recognize that the effluent from the P, EOP,
EO and E stages often contains substantial concentra-
tions of hydrogen peroxide and/or sufficient alkali-
nity to minimize the overall consumption of raw
materials such as hydrogen peroxide and sodium hy-
droxide in the present invention. Thus the present
invention not only eliminates a waste stream, but
also converts waste chemicals into valuable chlorine
dioxide. Further, the process permits generating the
chlorine dioxide in situ in the stages of the mill,
thus eYp~n~ing the overall Cl02 capacity of the mill
without the need of added capital investment.
-4- 20~83~8
Recovering ClO2 off-gases by this process results
in two substantial advantages compared to prior pro-
cesses of scrubbing with an alkaline white liquor
containing reduced sulfur compounds, such as
sulfides:
1. Hazardous or toxic chlorine dioxide emissions
are eliminated without increasing the sulfur load to
the environmental treatment facility and,
2. Chlorine dioxide is generated in a bleach
stage with cost savings in operations (due to credit
of ClO2) and potential decrease in capital for
generator capacity (owing to an increased capacity
for ClO2).
EXAMPLE
The following example is provided to illustrate
one embodiment of the invention and is not intended
as a limitation thereto. The example employs data
that are typical for a mill producing southern pine
kraft at 30 Kappa. It is clear that, in practice,
the optimum concentrations, temperatures will be
dictated by the overall mill operating conditions and
not by this chlorine dioxide emission control/re-
covery step.
The pulp is bleached in a conventional 5 stage
bleach plant; (CD) (EO)DED. The kappa factor (total
available chlorine charge/kappa number) is 0.22 and
the amount of ClO2 substitution is 15%. This results
in a 6.6% total available chlorine charge in the C-
stage, 5.61% C12 and 0.38% Cl02 (0.99% Av. Cl.). The
amount of ClO2 in the D1- and D2- stages is 1 and
0.2%, respectively. Therefore for a 1000 TPD pulp
mill the amount of Cl02 required would be 15.8 TPD.
Data in "Pulp and Paper Manufacture - Energy
Conservation and Pollution Prevention", ed. Marshall
Sitting, Noyes Data Corp. (Park Ridge, NJ), 1977,
pages 406-407 show that 1 pound of ClO2 is emitted
per ton of pulp. Chlorine dioxide emissions from the
-5- 207~308
first D and ceco~ D stages is 0.6 lb/T and 0.4 lb/T
from the ClO2 generator using 20C scrubbing water.
Using the proposed process would result in 1 lb
ClO2/T of pulp scrubbed. The sodium chlorite can be
recovered in the CD-stage or in the D-stage. If
recovered in the CD-stage, this would increase the
ClO2 substitution from 15 to 17.1%. If recovered in
the D-stage, this would decrease the ClO2 charge
required from 1 to 0.95%. For a 1000 TPD mill this
would increase chlorine dioxide availability by 0.5
TPD, resulting in a smaller generator requirement.
