Note: Descriptions are shown in the official language in which they were submitted.
2~750
r ~O gl/O~9lO PCT/US90/05826
- -1- .,
Descri~n
CHLORINE DIOXIDE PULP BLEACHING PROCESS USING `~`
SEQUENTIAL C~LORINE ADDITION
Technical Field
The present.invention relates to the bleaching of
pulp and more particularly to an improved proces~ for ~-~
bleaching wood pulp with chlorine dioxide and chlorine in
a manner whereby the wood ~ulp is subjected to a two-step ~ :
high pH/low pH bleaching sta9O where!in chlorine dioxide
and chlorine are used in tl-a first step and chlorine is
used in the second step.
.
round ~rt.
~ As is well known in the wood pulp bleaching art, the ..
: main objectives of wood pulp bleaching are to increase
the brightness oE the pulp and to make it suitable for
the manufacture of printing and tissue grade papers by
removal o~ modification of some of the constituents of
. the unbleached pulp, including the lignin and its
degradation products, resins, metal ions, non-cellulosic .
20 carbohydrate components, and various types o~ flecks. .~
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2~77~ :
-~WO91/05910 PCT/US90/05826
-2-
The bleaching of chemical wood pulp is normally carried
out in multiple processing stages utilizing elemental
chlorine, caustic soda, hypochlorites, oxygen, hydrogen - -
peroxide, and chlorine dioxide. The number of stages
required in a particular bleaching process is dependent
upon the nature of the unbleached pulp as well as the end
use to which the pulp will be put. A sul~ate or kraft
pulp is today most typically bleached in a five-stage
sequence which is designated as (CD)(EO)DED. In the
10 ~CD)~EO)DED designation, D denotes chlorine dioxide, C -
denotes elemental chlorine, E denotes caustic extraction,
.:
and 0 denotes oxygen gas. The multi--stage process in
essence comprises a chlorination step (CD), a first
oxidative extraction stage (EO), a ~irst bleaching stage
~Dl), a second caustic extraction stage (E2), and a
second and final bleaching stage tD2~.
In the conventional (CD)(EO)DED multi~stage
bleaching process, each o~ the two chlorine dioxide
bleaching stages is carried out in a one-5tep process at
a ~inal p~ of about 3.8 for three hours at 70
c~ntigrade. It is commonly known that p~l has an
important bearing on brigh~ness and strength properties -
as well as the clemical species present in the wood pulp
mixture, and this particular pll has heretofore been
considered optimal for eacll of the two chlorine dioxide
bleaching stages in the (CD~(EO~DED sequence. It should
. .
also be appreciated that although the (CD)(EO)DED
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2~77~0
-WO91/0~910 PCT/US90/05826
-3-
sequence has been specifically addressed, the one-step
chlorine dioxide bleaching stage can be used in any D-
stage for mo~t other three, four, fivej or six-sta~e
bleaching processes known to those familiar with the art
of wood pulp bleaching.
Chlorine dioxide used at the mill site normally
contains only about 10% chlGrine in the one-step chlorine
dioxide bleaching stage pr~sently u~ed in the pulp and
paper industry. Existing ~echnology does not allow for
more than about 20% chlorine substitution without
detrimentally affecting wood pulp quality. This is a
s~gnific~nt concern in v~ew of the hi~h c06t of chlorlne
dioxide in contrast to the relatively less expensive
chlorlne. The present inv~ntion solves this well-known
deficiency in state of the art chlorine dioxide bleaching
by making it possible to perform higher levels of
chlorine substitution through a two-step sequential
addit$on of chlorine at controlled high/low pH levels
durlng the ~hlorine dioxide bleaching proce6s. The
advantages are a reduced use of chlorine dioxide and a
slgniEicant reduction in chemical costs for the wood pulp
bleachlng process.
`'', ' ~ -
Disclosure of the Inve
In accordance with the present invention, applicants
25 provide an improved process for bleaching wood pulp in ;
agueou9 suspencion using chlorine ùioxide and rhlorine in
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~W091/05910 2~77~0 PCTtUS90/OS826
-4-
a two-step high p~/low pH bleaching stage wherein -~
chlorine dioxide and chlorine are used in the first step ~-
and chlorine is used in the second step. The chlorine
dioxide and chlorine comprise between about 15-80%~`
chlorine on an available basis. The novel process
comprises rirst subjecting the aqueous wood pulp
suspens~on to a first bleaching step by mixing it with
chlorine dioxide and chlorine and maintaining the mixture
at a pB between about 7-lO for about 5-40 minutes. Next, ~;~
the mixture is subjected to a ~econd bleaching step by
mixing it with the remaining chlorine and maintaining the -~
m~xture at a pH of 4 or less for at least 30 minutes.
This novel process can be used in any D stage of the
(CD)(EO)DED bleaching sequ~nce a~ well as in any D
bleaching stage oE other three, four, five, ~ix, and
seven-stage bleaching sequencesO ALl o~ the chlorine ~`
dioxide and approximately 0-40% of the chlorine charged
are added initially in the first step of the bleaching
process~ and th~ remainder of the chlorine is added at
the second stage o~ the bleaching proce~s. The novel
bleaching process provides for increasing chlorine
substitution up to 60~ with nominal effect on wood pulp
quality.
It is therefore an object of the present invention
to pro~ide a more efficient wood pu1p b1eaching process.
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~WO91/05910 2 ~ ~ ~ 7 ~ ~ PCT/US90/0~826
It is another object of the present inverltion to
increase chlorine substitution in the wood pulp bleaching
process
It is still another object of the present invention
to reduce the chlorine dioxide requirements of tlle wood .
pulp bleaching process and to thereby reduce chemical
costs.
It is yet another object of the present invention to
allow for chlorine substitution in the D bleaching stage .
at a level greater than the conventionally accepted 20%
limit without a detrimental effect on wood pulp guality. .;
Description of the Drawings
Some of the objects o~ the present invention having
been stated, other objects will become evident as the
lS description proc~eds, when taken in connection with the
acco~panying drawings, ln which~
Fi~ure l is a graph of Dl brightness versus percent
available chlorine on pulp as chlorine dioxide for one~
fitep D-stage ~leaching, conventional one-step chlorine~
chlorine dioxide mixture bleaching at maintain~d pl
,. ;~.
levels o~ 4 and 2 (as taught by the Rapson and Reeve ~ .
patents referenced herein), and the novel two-step ..
chlorine-chlorine dioxide method of the present invention ~ : :
- : :
~chlorine-chlorine dioxide mixtures contain 60% chlorine .;
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-~WO9l/05910 2 ~ ~ 7 7 ~ ~ PCT/US90/05826
:~ ' " "' .
-6
on an aYailable chlorine basis, (CD)E kappa equals 5.1, :
and temper~ature is 70 centigrade); ~ -~
Figure 2 is a graph of D2 brightness versus D
charge for the subsequent bleaching of the pulps of .
Figure 1 through E2 and D2 stages tD2 charge of 0.3%
chlorine dioxide on pulp and temperature oP 70
centigrade);
Eigure 3 is a graph o~ the final D2 viscosity
measurement6 for the pulps of E'igures 1 and 2 versus D
10 charge illustrating higher brightness and viscosity for .
the method of the invention when compared to conventional .-
bleaching methods used in the Dl stage~
Figure 4 is a graph of Dl brightness versus D
charge ~percent available chlorine on pulp as chlorine
15 dioxide) for conventional one-step D stage bleaching and .
the method of the invention ((CD)hl is 60% chlorine, 40~ :
chlorine diox~de on an available basis and (CD)E kappa
equals 5.2. Reaction temperatures are 70 centi~rade and ;~
50 cantigrade for (CD)hl and 70 centigrade for .
conventional D stage bleaching);
Figure 5 is a graph o~ D2 brightness versus D
charge (percent available chlorine on pulp as chlorine
dioxide) for the pulps of Eigure 4 after E2 and D2 stage
bleachin~ (D2 charge is O.~S C1O2 on pulp, and
25 temperature is 70 centigrade); -
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-7-
Figure 6 is a graph of Dl brightness versus
charge (percent available chlorine on pulp as chlorine
dioxide) for conventional one-step D stage ClO2 bleaching
and the method of the invention ((CD)E kappa eguals 4.6,
(CD)hl is 60% chlorine, 40% chlorine dioxide on an
available basis, and temperature is 70 centigrade);
Figure 7 is a graph oP Dl and D2 brightness versus
percent available chlorine on pulp as chlorine dioxide
for conventional one-step D stage ClO2 bleaching and the
lO novel two-step (CD)hl bleaching process of the present ~ :
invention wherein tCD) comprises 60% Cl2 and 40% ClO2 on
An ~vallable basi~, temperature is 70 centlgraae, and
(CD)E kappa equals 4.9;
Figure 8 is a graph of Dl brightne~s versu~ D
charge for conventional one-step D stage bleaching and
the novel two-step (CD)hl bleaching process of the ~ -
present invention at 70 centigrade for 3 hours with ; ::
5CD)E kappa equals 5.0 and wherein ~CD) is 60% Cl2 and
40% ClO2 on an available basis;
Figure 9 is a graph of D2 brightness versus D
charge or the pulps of Fi~ure 7 wherein the D2 charge i6 ';, -
0.3% ClO2 on the pulp and temperature is 70 centigrade; ,;
and . .
Figure lO is a schematic representation o~ two ~ ~:
different process systems for a wood pulp bleaching plant
which incorporate the chlorine dioxide with the ::
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sequential chlorine addition wood pulp bleaching process
of ~he present inventionr
~est_Mode for Carr~_n~ Out the Invention
It has preYiously been suggested in U.S. Patent No.
3,536,577 to Rapson to bleach wood pulp using an aqueous ~ ~
solution of chlorine dioxide and chlorine, treating the -
wood pulp with alkali, and then treating the wood pulp ;
with chlorine dioxide. The process of Rapson provides
for chlorine dioxide to constitute about 20-95% of the
10 total available chlorine of the solution in the first
(CD) stage.
Also of interest, Canadian Patent No. 1,120,660 to
Reeve discloses a bleachin~ sequence which~comprises
subjecting the wood pulp to a ~irst bleach~ng step with a
chlorine dioxide and chlorine solution and, after a
period o~ ~ime from about 5 seCOnd~ to about 10 minutes,
applying an aqueous chlorine solution to the pulp :
suspension in a second bleaching step which is allowed to
,
proceed for about 10-60 minutes. Reeve dificloses that
20 the first bleaching step is used usually effected at a ~:
pulp suspension pH of about 1-6, and the second bleaching ~ .:
step is usually effected at a pulp suspension pH of about
0.7-3.
Applicants' process provides for the use of -
chlorine-chlorine dioxide bleaching in stages other than
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- wo gl/o~g~o 2 ~ ~ 7 ~ ~ ~ PCT/~S90/0~826
_g_
the ~ir~t (CD) stage, preferably the D stage designation
~n any 3-7 stage bleachinc3 saquence, whereas Rapson and
Reeve both refer specifically to the fir~t (CD) stage of
bleachlng.
With reference now to Figures 1-10 of the drawings,
it ca~ be seen that applicants have improved upon the
processes taught by Rapson and Reeve as well as upon the
con~entional one-step bleaching stage used in the ;~ -
majority of commercial bleaching mills. More .
10 specifically, applicants have invented a new and useful .
two-stage process for bleaching wood pulp slurries in a - :
chlorine-chlori~e dloxlde ~C12-~1O2) mlxture whlch gives ; :~
high~r brightness and pulp quality ~characterized by .
v~scosity) when compared to the conventional one-step :
15 C12~ClO2 mixture technology typically used today in i .
conventional bleaching mills. ~- :
Presently, wood pulp bleachinc~ with chlorine-
chlorine dioxide mixtures 1~ typically carried out at 25 .:
centigrade - 60 centigrade in the fir~t stage of a
~CD)~EOlDED or (CD)(EO)D sequence. The consistency is
usually around 3-5~, and bleaching times range from 15
minutes to 1 hour with an end pH of about 1.6-2.5 The ~:
application of conventional ~CD) bleaching procedures in
thc D-staye of ~he above seyuences or in any other :
- 25 bleaching sequences has little commercial value due to
the high viscosity loss (carboh~drate degradation) and
poor brightness characteristics of the wood pulp. Thus,
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_~WO91/05910 2 ~ ~ 7 7 ~ ~ PCT/US90/0~826
-10-
existing ~CD) technology is typically only used as a
first stage or pre-bleaching stage in the multi-stage
bleaching process on wood pulp. Although others have
proposed chlorine-chlorine dioxide bleaching in either
one or two-step stages, such as disclosed in the Rapson
and Reeve patents discussed above, it can be seen in
Figures 1, 2 and 3 of the drawings that the novel two- ~
step chlorine-chlorine dioxide bleaching process of the .-
present invention provides surprisingly and une~pectedly
10 better brightness and viscosity than has heretofore been . . ~ ;
achieved.
.. .,~. ..
In order to increa~e the effectivene~s of chlorine- .
chlorine dioxide bleaching of wood pulp, applicants have .~
discovered a new two-step chlorine-chlorine dioxide .
process which includes the following steps:
1. Pulp slurry is reacted with a chlorine and
chlorine dioxide mlxture for about 5-40 minutes at a pH
of about 7-10. Normally, caustic is also mixed with the ~ ..
pulp slu~ry, and the caustic most suitably comprise -~
20 sodium hydroxide. The reaction temperature is between : :
about 40-80 centigrade, most suitahly about 70 .
centigrade, and the optimal pll during the first bleaching
step is between about B-9.5; and ..
2. After the ~irst bleaching step, the remaining
25 chlorine is added ~o the pulp mixture, and the pH is ;~
maintained at 4 or less, most suitably a pH of about 2- :
' '.
~WO91/0~91~ PCT/US90/05826
2~677~
3.5 for at least 30 minutes, preferably about 30 minutes
- 2.5 hours. The reaction temperature is between about ~ : -
40-~0 centigrade, most suitably 70 centigrade.
Pulp con~istency fo~ both stepfi ifi between about 10
13.5% w$th the final consistency of the mixture after the ~ -~
second step at most suitably about 10%. The chlorine
. . .
dioxide and chlorine used in the bleaching proce~s . ~ ;
comprise between about 15-80% chlorine on an available
basis, and preferably all of the chlorine dioxide and
10 one-third of the chlorine are added at the first step, ~
and the remaining two-thirds of the chlorine are added at : :
the se~ond bleaching step. It should be appreclated that
to calculate total charge such as percent chlorine on ~ :
pulp, the formula C102 ~ 2.63 C12 i8 used. For example, ..
if a 1% C102 charge is placed on wood pulp, it is
equivalent to 2.63% available chlorine on the wood pulp.
Thus, lf one charges 1% available C102 on wood pulp,
~here is actually charged 2.63~ available chlorine of
which 40% is C102 and 60~ is C12.
The new two-step process developed by applicants has
. .;: .
been used to bleach pulp in the Dl stage of a (CD)(EO)DED
sequence with chlorine-chlorine dioxide mixtures of up to
60% of the chemical charged as chlorine on an available - :
basi~ with brightness and viscosity characteristics
25 comparable to a conventional chlorine-chlorine dioxide ~- :
stage (~ee Figures 1, 2 and ~-9). .~ .
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W~91/05910 P~T/US90/05826
2~67~
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Applicants' process is particularly attractive since ~-
the new two-step chlorine-chlorine dioxide bleaching
process allows for substituting chlorine for chlorine
dioxide and thus results in substantial chemical cost
reduction ~ince chlorine is a significantly less -
expensive chemical. With the use of the novel two-step
bleaching process of the present invention, it i8 now
commercially possible to bleach wood pulp to a comparable ~
brightness as can be achieved in conventional one-step --
lo chlorine aioxide bleaching ~t a significantly reduced
commercial cost due to savings achieved by substituting -`
cheaper chlorine for more expensive chlorine dioxide in
the wood pulp bleaching process.
Method of the Invention
To prove the efficacy oE the new process generally
described above, detailed bleaching experiments were
carried out by applicants on souther~ pine k~aft pulp.
.: . . .
The furnish was obtained from the decker before the
. . .
bleach plant, and to insure maximum mixing ~CD) stage ; ;
bleaching was done in plastic Nalgene bottles which
rolled on a ball-mill type apparatus for the full ~-
reaction time. All other bleaching stages were carried
out in sealed polyester ba9s which were kneaded at ~ -
various times throughout the bleach to insure proper
25 mixing. `~
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Proces~ing parameters used by applicants for the
multiple bleachiny stages are listed in Table 1 below.
Chlorination stage charges were varied to achieve target ~`
(CD)E kappa numbers, and ail charges are on OD brownstock -
pulp. Optimum high/low pH values are 8-9.5 and 2-3.5
respectively. Large batches of (CD3E pulp were made and - -
then divided into individual DED and (CD)hlED runs or
compari~on. All comparisons were made on pulps from the
same (CD)E batch, and all water used in bleaching and
. , . :
washing was distilled. Chlorine dioxide solutions used
in testing were generated on site by acidifying sodium
chlorite solution and absorbing the ClO2 gas in cold ` -
distilled water.
:: '
Proces6ing parameters Eor the bleachin~ experiments
and the analytical methods used in the experiments are as
~ollows~
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--~W091/0~9iO 2 ~ ~ ~ 7 ~ ~ P~T/US90/~5826
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-15-
Thus, applicants' method provides for mixing
chlorine, chlorine dioxide and caustic with an agueou ` `; -
wood pulp slurry for about 5-15 minutes at a pH of about
: .: . .
7-10. The optimum pH is between about 8-9.5 for best
viscosity, and consistency is between about 10-13.5%.
Temperature is between about 40-80 centigrade. ~or the -~
second step in applicants' novel process, chlorine is
then added to the pulp slurry which results in a decrease
ln p~ to about 2-3.5. Reaction time is between about 30 ~ -
10 minutes to 2.5 hours, reaction temperature is between -~
about 40-80 centigrade, and the final consistency of th~
wood pulp is about 10%.
Best brightness and viscosity have been found when
all of the chlorine dioxide and one-third of the chlorine
to be charged are added in the fir~;t step of the novel
bleaching proceæs. The other two-l:hirds of the chlorine
are added at the second step and allowed to react for 1-2
hours at 70 centigrade or 1.5 or more hours at 50
centigrade. Temperature and time dependents for
effici~nt chemical consumption and optimum pulp
brightness are important, and 5uitable combinations can
be developed according to the needs of any given wood
pulp bleaching mill.
Although applicants have disclosed the application
. .
of the two-step chlorine-chlorine dioxide bleaching stage
to the Dl stage of a (CD)(r'O)DED sequence, it should be
appreciated that the process can be used ln any D or lCD) ; ~
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WO91/05910 2 ~ ~ ~ 7 ~ ~ PCT~S90/05826
-16-
stage in any three, four, five, ~ix or seven-stage
bleaching sequence. Also, although applicants disclose
the addition of the remaining chlorine to the pulp in the
second 8tep of the new process, additional chlorine ~ -
dioxide can also be added at this time to the pulp
mixture. -~`
.
Process Apparatus
The two-step high/low p~ bleaching process can be
implemented in both a new plant or an existing pulp `-
bleaching plant. The optimum design schematic is shown
ln Flgur~ 10, where C1O2 and caustic are added to the
first mixer. The pulp flows into a J or U tube (Figure
lOA) or upflow tower (Figure lOB) with a retention time
o~ ~pproximately 5~40 minutes. A second mixer i~
provided to mix chlorine with the pulp for the second
step of the process. The pulp can then be discharged
directly to a down~low tower. The retention time in the
downElow tower is 2 or more hours and most fiuitably
between 2.5-3.9 hours. In an existing bleach plant the
~implest method for implementing the two-step high/low pH
bleaching process technolo9y would be to install a mixer ` ~-
on the discharge from the upflow leg of the tower to the
downflow leg of the tower.
It will be understood that various details of the
invention may be changed without departing from the scope
of the invention. Furthermore, the foregoing description
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~VO91/05910 2 ~ ~ ~ 7 ~ O PCT/US90/05826 -~
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i~ for the purpose of illustration only, and not for the :: ~
purpose of limitation--the invention being defined by the ~ ~:
claims. ;~
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