FONCTIONNEMENT D'UNE INSTALLATION DE BLANCHIMENT

BLEACH PLANT OPERATION

Abrégé anglais

BLEACH PLANT OPERATION
ABSTRACT OF THE DISCLOSURE
The first bleaching stage of a bleach plant
sequence is effected using mixtures of chlorine dioxide and
chlorine at elevated temperatures above 35°C up to 70°C.
Bleaching at these temperatures may be carried out without
pulp viscosity lass and permits fresh water use in a counter-
current washing bleach plant to be decreased.

Revendications

The embodiments of the invention in which an exclusive
property or privilege is claimed are defined as follows:
1. In a multistage bleaching process for bleaching
wood pulp in aqueous medium by bleaching the wood pulp in a
first stage with a mixture of chlorine and chlorine dioxide
in which from about 20 to about 95% of the available chlorine
is provided by the chlorine dioxide, caustic extracting the
wood pulp, and subjecting the wood pulp to a subsequent
bleaching with chlorine dioxide, the improvement which
comprises effecting said first stage bleaching at a tempera-
ture of greater than about 35°C up to about 70°C.
2. The process of claim 1 wherein chlorine dioxide
provides about 40 to about 90% of the available chlorine of
the mixture of chlorine dioxide and chlorine.
3. The process of claim 1 wherein chlorine dioxide
provides about 70% of the available chlorine of the mixture
of chlorine dioxide and chlorine.
4. The process of claim 1, 2 or 3, wherein said first
stage bleaching is effected at a temperature of about 50°
to about 60°C.
5. In a multistage bleaching process wherein wash water
flows countercurrently to pulp through the multistage process,
the improvement which comprises effecting a first stage
bleaching at a temperature of greater than about 35°C up to
about 70°C using a mixture of chlorine dioxide and chlorine
in which from about 20 to about 95% of the available chlorine
is provided by chlorine dioxide.
6. The process of claim 5 wherein said multistage
process employs a D/CEDED sequence.

7. The process of claim 6, wherein the chemical stages
involved in said D/CEDED sequence are effected at temperatures
such that the filtrate from the washing of pulp from the first
stage bleaching has a temperature of about 50° to about 60°C.
8. The process of claim 5, 6 or 7, wherein filtrate
from the washing of pulp from the first stage bleaching is
used to dilute pulp stock to the consistency required for
the first stage bleaching.
9. The process of claim 5, 6 or 7 wherein filtrate
from the washing of pulp from the first stage bleaching is
used to dissolve chlorine gas for use in said first stage
bleaching.
10. The process of claim 1 or 5 wherein the dosage of
chemical applied to the pulp in said first stage bleaching
is controlled to avoid the presence of residual bleaching
chemicals at the end of said first stage bleaching.

Description

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The present invention relates to bleach plan;
operations.
In our Canadian Patent No. 731,0~5, we describe an
improved pulp bleaching operation to produce pulp of high
brightness and color stability. The sequence involves a
first bleaching stage using a mixture of chlorine dioxide
and chlorine, a caustic extraction stage and a subsequent
bleaching stage using chlorine dioxide. The mixture of
chlorine dioxide and chlorine used in the first stage contains
chlorine dioxide in the proportion o~ about 20 to about 95%
of the available chlorine of the mixture. The sequence may
be combined with additional bleaching and/or caustic
extraction steps, and a preferred sequence is the D/CEDED
sequence, wherein D/C refers to bleaching using mixtures of
chlorine dioxide and chlorine, D refers to bleaching with
chlorine dioxide and E refers to caustic extraction with
aqueous sodium hydroxide solution. ~ ;
In the disclosure of that patent, it i5 indicated
that the first stage bleaching is generally effected at a
temperature of from about 0 to about 35C. In accordance
with this invention, the first stage bleaching may be effected
at temperatures in excess of 35C up to about 70~C without
adversely affecting the properties of the pulp.
It is well known that with increasing temperature,
damage to the cellulose increases sharply, resulting in loss
of pulp viscosity and it is, therefore, surprising that the
first stage bleaching may be effected at elevated temperatures
without pulp damage with a mixture of chlorine and chlorine
dioxide. It would appear that the chlorine dioxide in some
way protects the pulp from high temperature chlorination
damage.
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Although the chlorine dioxide protects the pulp
from ove~chlorination, it is preferred to control the
chlorine dosage on the pulp to minimize pulp damage. This
control may be achieved by using a sensor which senses the
kappa number of the ~ulp to determine the chlorine require-
ment to result in suitable chlorination and no residual
; chlorine in the pulp. An optical sensor or an oxidation-
reduction potential sensor may be used.
- Since the chlorine dosage which is applied to the
pulp results in no residual chlorine in the bleached pulp,
efficient chemical usage is achieved, as opposed to the
conventional practice where residual chlorine remains in the
pulp to achieve shives bleaching. It has been found that a
cleaner pulp is obtained using the mixture oE chlorine dioxide
and chlorine at high temperature, inaicating that the
shives have been bleached more efficiently than in the pri~r
art chlorination and this improvement is achieved without
; the necessity for residual chlorine in the pulp.
The ability to use temperatures in the range of
above about 35C to about 70C in the first bleaching stage
is beneficial, especially where countercurrent washing oper-
ations are effected in the bleach plant. Since elevated
temperatures generally are used in caustic extraction and in
subsequent chlorine dioxide bleaching stages, when spent
liquor is countercurrently flowed in the bleach plant with
respect to pulp flow and part is ultimately used for washing
pulp from the first bleaching stage, the filtrate from that
washing has an elevated temperature, for example, about 50
to about 60C. Such a countercurrent operation is described,
for example, in copending Canadian application Serial No.
279,885 filed June 6, 1977 and assigned to ERCO Envirotech Ltd.

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In conventional bleach plant operation, fresh water
having the ambient temperature of the mill ~resh water
supply is used to dilute the puLp from brown stock washing
from its high consistency (typically about 10%) to the lower
consistency required for the fi:rst stage bleaching (typically
about 3%). Further, fresh water is also used to form chlorine
water from gaseous chlorine for use in the first stage.
Since the higher temperature first stage bleaching
at 35to 70C in accordance with this invention does not
significantly impair pulp quantity if overchlorination is
-avoided, the fresh water used in the conventional mill
operations of stock dilution and chlorine dissolving may be
eliminated and the warm filtrate from the first stage
bleaching washer may be used in its place, preferably at
; both locations. Not only does filtrate reuse in this manner
decrease the overall fresh water requirement of the mill but
it also decreases the volume of bleach plant effluent which
must be disposed of.
As mentioned above, chlorine dioxide provides from
about 20 to about 95~ of the available chlorine of the
mixture of chlorine dioxide and chlorine used in the first
stage bleaching, and preferably about 40 to about 90~ of the
available chlorine of the mixture. One particular example
of a suitable mixture is a solution of chlorine dioxide and
chlorine containing about 70% ClO2 and about 30~ C12, expressed
as available chlorine.
The invention is illustrated by the following
Examples:
Example l
In a series of experiments, samples of kraft pulp
were subjected to a D/CEDED sequence of operations and the
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. results compared with subjecting samples of pulp to a CEDED
sequence. The initial bleaching step was effecte~ at 60e
and 3.5% consistency using on the one hand 70% C102 and 30%
C12 of the available ehlorine of the mixture and on the
other hand 10~% C12. The other reaetion conditions were as
follows:
El - 70C, 12~ consistency, 2 hours
Dl - 70C, 6~ consistency, 3 hours
E2 ~ 70C, 12~ consistency, 2 hours
D2 ~ 70C, 6% consistency, 3 hours
; The amount of bleaching chemical used in the first
stage was varied and it was observed that in the case of the
70/30 mixture, there was a moderate deerease in viseosity
as the amount of bleaching chemical increased whereas there
was a 5 times greater decrease in viscosity with inereasing
amount of chemieal in the case of 100% C12.
Example 2
The bleached pulp from the two bleaching sequenees
set forth in Example 1 was examined for purity (shives count)
and these results were compared with the shives count from
samples of pulp subjeeted to sequential bleaehing in the
first stage effeeted at 60C using 70% ClO2 then 30~ C12
followed by EDED.
It was found that the shives eount of the pulp
produced by the D/CEDED sequenee was less than that of the
pulp produeed by the CEDED sequenee whieh in turn was less
than that of the pulp produced by the D ~CEDED sequenee.
The present invention, therefore, provides an
¦ improved bleaehing operation in whieh high temperature
bleaching is effeeted in the first bleaching stage.
Modifieations are posslble within the scope of the invention.
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SUPPLEM:ENTARY DISCLOSURE
The principal disclosure describes the utiliza-
tion of higher-than-normal temperatures for bleaching wood
pulp using mixtures of chlorine dioxide and chlorine in
which about 20 to about 95% of the available chlorine is
provided by chlorine dioxide. The invention is further
illustrated by the following additional Example:
Example 3
In a series of comparative experiments, samples
of a softwood kraft pulp were subjected to a D/CEDED
sequence of bleaching operations, with the initial bleach- ;
ing step being effected in a first group at about 25C
and in a second group at about 50C. Bleaching mixtures
used contained, in one case, 82% ClO2 and 18% C12 of
available chlorine and, in a second case, 50% ClO2 and -
50% C12 of available chlorine.
The other reaction conditions were as follows:
D/C - 3~ consistency, 6% available Clz on pulp, 45 mins.
E - 60C, 12% consistency, 2.5% NaOH, 2 hoùrs
D - 70C, 6% consistency, 1.0% ClO2, 3 hours.~
The brightness after aging and viscosity values
~or the pulp resulting from the series of experiments were
determined ancl compared with each other and also with a
first stage bleaching effected using 100% Cl2.
- The results obtained are reproduced in the follow-
ing Table:
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TABLE
Property 82%C10218%C12- 50%ClO250%C12 1OO~C12
25C 50C 25C 50C 25C 50C
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srightness ~ 77.781.6 82.4 83.8 81.0 81.3
(after aging)
Viscosity, cps 26.0 21.6 23.7 19.6 20.0 12.2
The results of the above Table while there was a
decrease in viscosity of the pulp when the higher bleaching
temperature was used in the case of the mixtures of chlorine
dioxide and chlorine, the decrease was much more severe
when chlorine was us d. Further, a higher brightness was
obtained when the higher temperature was used for the
mixtures of chlorine dioxide and chlorine whereas there
was only a marginal increase in brightness when chlorine
was used. !
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