Note: Descriptions are shown in the official language in which they were submitted.
- ~ W096/0s364 ~ P~~ r~FI
Bleaching of chemical paper pulp under
sulphonating conditions
TECHNICAL FIELD 2 ~ 9 ~ 2 9 0
The present invention relates to a method of bleaching
chemical paper pulp in a bleaching se~uence which
comprises at least one acidic, s~lphite-containing,
reducing bleaching stage and at least one alkaline,
oxidizing bleaching stage in arbitrary order At least
one of the said sulphite-c~nt~;~;ng stages is carried
out under conditions which are in the main sulphonat-
ing. Thi=s is brought about by opt;mi7ing the system
parameters pressure, temperature, time, pH and sulphur
charse in s~ch a manner that a maximum proportion of
water-soluble lignin is formed. The bleaching sequence
is preferably entirely free of chlorine, but one or
more supplementary chlorine-~nt~ining stages can also
be ;n~ln~
STATE OF T~E ART AN~D PROBLEMS
Due to the ircreasing interest in the environment, and
comprehension of the ecological cycle in nature, a
great desire exists both among consumers and among
producers to decrease discharges~of pollutants result-
ing from human activities.
Producers of pulp and paper have often been portrayedas villains in relation to the environment. However, in
~ recent years we have besn working feverishly to
decrease the discharges from our pulp and paper mills,
and much progress has also been made. The demand of the
market for paper which has been bleached without using
chlorine or chlorine dioxide has led to a start being
made in the use of alternative bleaching chemicals such
as ozone (Z), persulphuric acid (Caro's acid, Ca),
peracetic acid (Pa) and hydrogen peroxide (P). However,
one of the disadvantages of these bleaching chemicals
is that they either require g~n~r~t;ng equipment having
high capital costs or else entail large costs for
purchasing the bleaching chemicals. Other disadvantages
W096/0~364 2 2 1 9 4 2 9 0 r~ l/D~' _. t ~ I
are, for example, that ozone and peracetic acid have a
deyrading effect on the :pulp. The disadvantage when
bleaching with hydrogen peroxide alone is that the
finally bleached puIp will contain a~residual lignin
content correspondiny to 2 - 4 kappa units, which more
than doubles the colour reversion induced by heat and
W light. Pulp which is bleacked to full brightnes6,~88
- 90 ISO, using only alkaline peroxide also tends~to
require higher refining energy and yield inferior
strength properties as compared wlth sequences
including ozone or peracids.
A method of bleaching using hydrogen peroxide has been
developed by the Swedish company EKA Nobel AB. The
method, which goes under ~ the name LIGNOX (see
SE 8902058), involves the urbleached pulp first being
delignified with oxygçn and then, after washing, being
treated with EDTA or other suitable complex-forming
agent ~Q) in order to remove transition metals which
are bound in the pulp. Sulphuric acid is used as an
acidifier in the~ complex-forming stage. The EDTA stage
is followed by an intensiv~peroxide-bleaching stage.
The quantity of hydrogen peroxide re~uired for this
method is relatively high, being 15 - 35 kg per ton of
pulp, depending on t~e brightness desired and on the
bleachability of the pulp. The time required is long,
being 4 hours or more, and the quantity of residual
peroxide is significant. Only 40 - 60 ~ of the added
peroxide is utilized. This method only achieves a
limited increase in brightness to 80 - 82 ISO.
A similar technique has also been described by Interox.
In this technique, the pH in the complex-forming stage
is adjusted using sulphur dioxide or bisulphite instead
of sulphuric acid, see Troughton N. "The efficient use
of hydrogen peroxide as a pulp delignification agent":
The Macrox Process; 1992 TAPPI Pulping Conference in
Poston, MA.
~ W096l0s364 2 1 9 ~2qO P~
5 EKA Nobel As has also proposed se~uences :of t~e type
QPZ or QPZP, using which briyhtne~sses in the range 82 -
87 ISO and 87 - 89 ISO, respectlvely, can be achieved,
depending on the pulp type, see ~Non-Chlorine
Bleaching", J. Basta, ~. Andersson, W. Hermansson;
Proceedings March 2 - 5, 1992 - Westin Resort - Hilton
Head - South Carolina; Copyright by Miller Freeman Inc.
EK~ Nobel' 8 patent application SE 9101300 (SE 468 355)
additionally describes a bleaching method in which
complex-forming agents are used prior to an ozone ar
peroxide stage. This application principally relates to
020ne being used directly after a complex-forming
stage.
We at Kvaerner Pulping Technologies AB (formerly Kamyr
AB) have developed a subst~nt;~lly improved method for
peroxide bleaching (see SE 9301960) to full brightness
with a surpri~ingly high degree of utilization of added
peroxide.-This method, which has aroused a great deal
of interest, involves the peroxide bleaching being
carried out at elevated pressure and temperature.
Nevertheless, despite the progress which has been
achieved using replacement chemicals for chlorine,
there still remain the disadvantages of, for example,
high process and investment costs and a more or less
extensive degradation of the cellulose.
SO~UTION AND ADV~TAGES
By means o~ the present invention, a method is
developed in which ;nt~rn~lly generated sulphur
compounds, which are available in the pulp mill, are
utilized in order to provide, preferably in combination
with hydrogen peroxide, pulps which have preferably
been bleached entirely without using chlorine-
containing chemicals, which method is characterized by
the low chemical cost of ozone bleaching, by the
21 942~0 ~ ~
W O 96/05364 PCT/S195100486
relatively high quality pulp of peracetic acid/chlorine
dioxide bleaching, and by the lower capital cost of
pure peroxide bleaching. The fact that the bleaching
sequence is preferably intended to provlde pulp which
has been bleached entirely without using chlorine
implies that it is preferred for the i~vention to be
carried out as a part of a total bleaching sequence
entirely without any chlorine-containing chemical.
However, it i~ conceivable that, in certain cases, it
may be desirable for one or more supplementary
chlorine-containing stages, for example chlorine gas or
chlorine dioxide, to be included in the total bleaching
sequence.
In the method according to ~the invention, the total
bleaching sequence, or a part of the total bleaching
sequence, consists of a combination of acidic,
sulphite-containing, reducing bleaching stages and
~lk~l ;n~, oxidizing bleaching stages. The ~ ;7ing
stage(s) is/are (a) preferably pressurized peroxide
stage(s) (P0) in accordance with SE 9301960, which
means that it/they contain(s) peroxide and is~are
carried out at a pressure at the top of the bleaching
vessel exceeding l bar, preferably exceeding 1.2 bar,
and more preferably exceeding 1.5 bar. At the bottom of
the bleaching vessel, the pressure should exceed 3 bar,
preferably exceed 4 bar, and more preferably exceed
5 bar. The temperature should exceed 90~C, preferably
be ecual to or greater than 100~C, and more preferably
be between lO0 and l20~C.
The method is characterized by at least one of the
acidic, sulphite-containing, reducing bleaching stages
being carried out under conditions which are in the
main sulphonating. Sulphonation in this context means
that S02 is consumed while HSO3- groups enter as sub-
stituents into the lignin fragments, whereupon water-
soluble compounds of residual lignin arise in the pulp.
~ W09~053~ 2 1 9 4 2 q O rL~ 5 r ~r
This results in a s~bsta~tial kappa reduction directly
after washing. ' ~
That sulphonation is taking place can readily be
est~h~; ~hP~ by the fact that the p~ falls from an
initial 4 - 6 to 3 - 4, and by the fact that SO2 is
consumed and that the kappa number falls.
Expressed as water-soluble lignin, the degree of
sulphonation is defined for a given bleaching stage as
(kappa1 - kappa2)/kappal, where kappa1 is the kappa
number entering the stage and~ kappa2 is the kappa
number leaving the stage.
In the method according to the invention, the
sulphonating, acidic, sulphite-crnt~;n;ng, reducing
bleaching stages are carried out to a degree of
sulphonation exceeding lO ~, preferably exceeding 20 ~,
and still more preferably to ~o - 80 %. The reaction
time for such a stage is 30 seconds - 120 minutes,
preferably 5 - 120 minutes, and still more preferably
30 - 120 minutes. The consumption of sulphur, calcula-
ted as sulphur dioxide, during the sulphonating,
acidic, sulphite-rrnt~1ning, reducing bleaching stages
i8 1 - 40 kg per ton of pulp, preferably 10 - 30 kg per
ton of pulp.
The sulphonating conditions are~accomplished in one or
more sulphite stages by a pressure in the top of the
bleaching vessel exceeding 1 bar~absolute), preferably
~;ng 1.2 bart and more preferably exceeding
1.5 bar, and a pressure in the bottom of the bleaching
vessel exceeding 2 bar, preferably between 3 and
15 bar, and more preferably between 5 - 10 bar, and
also by a temperature p~rp~;ng 90~C, preferably e~ual
to or greater than 100~C, and more preferably between
100 and 120~C. Such a stage is~~deslgna~ed (AR~) and
should also contain complex-forming agents for metal
elimination, for example EDTA, in accordance with the
scheme (SO2 + Q, E) or (SO2 + Q). In this context, E
designates alkaline extraction.
W096~05364 2 1 9 4290
ccording to one aspect of the invention it can have a
beneficial effect on the pulp quality not to allow the
pH to fall below 3, preferably not below 3.5, in the
sulphonating stage. This can be effected by adding
alkali, preferably NaOH, as~the pH falls during the
sulphonating stage.
Sulphite solutions or bisulphite solutions, such as
Na2S03 or NaHS03, can, for= example, be used as the
sulphite source in addition to S02. Another ~ossibility
is to utilize sulphur-rnnt~ining process stream frQm a
plant for gasification and combustion of black liquor,
for example a so-called Chemrec reactor (see SE-C-443
173) as the sulphur source.
Yet another pnqc;h;l;ty is to generate sulphur dioxide
;nt~rn~lly by burning sulphur gases, which~have been
driven off from black liquor, to, in the main, sulphur
dioxide. This sulphur dioxide can then be absorbed in
bleaching-plant liquid which is then used in the
sulphite stage or~, alternat~ively, as acidifier, for
~s example in connection with a complex-forming stage.
Anthraquinone is also expediently utilized in the
sulphite stages in order to improve bleaching selec-
tivity still further. Addition of sodium borohydride
represents another option for increasing the reducing
capability of the sulphite solution.
A sulphite stage which is only acidic~and reducing,
without being pressurized, and is consequently not
significantly sulphr,n~t;ng, is designated (AR).
The equipment for carrying out the bleaching sequence
according to the invention expediently consists of a
combination of pressurized reactors with intervening
pressure diffusers for washing. In certain cases,
partial degasification can be required after one or
more pressurized stages, for example after a (PO)
stage, to enable satisfactory washing to take place in
a subsequent diffuser.
21 94290
~ W096l0s364 r~ r -
While it i8 true that it i8 previously known from
WO 92/07139 to carry out a reducing stage of the
sulphite or bisulphite type between two peroxide
stages, the first peroxide stage in this case i9 acidic
and it is only the second which is ~lk~l ;n~. Further-
~ l0 more, the sulphite stage is not carried out under
snlrhnn~t;ng conditions.
SE 461 991 discloses a method of improving subsequentoxygen-delignification by means ~of alkaline sulphite
treatment. This sulphite treatment thus has another
purpose and is furthermore not carried out under
conditions which are to any appreciable extent
sulphonating.
It is known from EP 433 138 to have, in the bleaching
sequence, two alkaline stages, for example peroxide,
with an intervening acidic stage, for example SO2. The
acidic stage is not carried out under sulphonating
conditions in this instance either _In the examples,
2s the temperature i9 stated to be between 20~C and 80~C
and the pressure is presumed to be atmospheric.
That s~l3rhnn~t; ng conditions are not achieved by this
method is evident from the exp~mPntq which are
reported below.
EXPERIMENTS
In a series of experiments, an isothermally cooked pulp
was bleached which, after oxygen delignification, had a
kappa number of 12.1, a viscosity of 1020 dm3/kg, and a
washing carry-over corresponding to 5 kg of COD per ton
of pulp. Three bleaching sequences were ~m;n~
1. Q(Po)(PO): elimination of transition metals followed
by two pressurized peroxide stages, entirely in
accordance with known technique ~SE 9301960). The pulp
was washed between the three given stages.
21'~42~0 ~
W096/0~64 p~ r _'rr r-
2. Q(PO)(AR)(PO): as in 1, but with an acidic So2 ~tage
at 90~C and atmospheric pressure between the two
pressurized (PO~ stages, that is similarly to EP 433
138. The pulp was washed between the four given stages.
3a. Q(PO)(AR*)(PO): a preferred bleaching sequence in
accordance with the invention, which sequence is
carried out in the same way as for experiment 2 but
with the conditions for the=sulphite treatment having
been chosen so that residual lignin is sulphonated,
that is pressure elevated to 6 bar:=(absolute) a~d a
temperature of 105~C. The pulp was washed between the
four stages. ~ =
3b. Q(PO)(AR*) nFo): the same sequence -as in 3a, but
with somewhat different conditions.
Experimental conditions and results are shown in
Table l
The two initial stages Q~PO) were common to the three
experimental sequences. The Q stage was carried out in
accordance with the LIGNOX procedure (SE 8902058).
Thus, H2S04 was used in the~laboratory experiment for
acidifying in the complex-forming stage. However, in
the factory, it is preferred to use sulphur dioxide or
returned H+ ions, liberated in association with
sulphonating reactions in subsequent stages, for
acidifying incoming pulp. Alternatively, the acidifi-
cation stage can be carried out without complex-forming
agents.
W 096/05364 r~/D~
Table 1 = ~ 3'-= = - '
1. Ac~ition3 Seo,uence 1 Sequence 2 Sequence.3a Seguence 3b
2. Condition~ Q(PO~(PO~ Q(PO~(AR~(PO~ Q~PO~(AR~(PO~ Q(PO~(AR~)(PO~
3. P~esult~
, .
Q ~tage (common to 1-3~
1- H25O4, kg ptpl 5
EDTA, kg ptp 2
2. Conc., ~ 10
Time, min 60
Temp., ~C 70
3. Final pH 5.2
Po ~tage (common to 1-3~
1. H2O2, kg ptp 18
DTPA, kg ptp 2
MgSO4, kg ptp 3
~aOH, kg ptp 19
2. Conc., ~ 10
Time, hr 2
Temp., ~C 120
8ressure, bar(abs~ 6
3. Consumption of
H2O2, kg ptp lq
~appa 4.0
Viscosity,
dm3/kg 851
~rightness,
ISO 82
21 942qO ~
W 096/05364 ~ 7~ e -
(AR) or, ~ltern~tively, ~A~ ) ~t~ge
1. 5~2~ kg ptp 18 18 27
DTPA, kg ptp 2 2 2
Mysog/ kg ptp 3 3 3
2. Conc., ~ 10 10 10
Time, hr
Temp., C 90 105 105
Pressure, bar(abs) 1 6 6
Initial pH 5.2 5.2 4.0
3. Final pH 5.0 3.q 2.7
Consumption of 5~2~ kg ptp 10 16 24
Kappa 3.7 1.8 1.9
Viscosity, dm3/kg 829 767 73~
Brightness, ~ ISO ~ 82.5 82.3 82.4
PO atage
1. H2O2, kg ptp 23 14 14 18
DTPA, kg ptp 2 2 Z 2
MgSO4, kg ptp 3 3 3 3 ~
NaOH, kg ptp 22 13 13 18
2. Conc., ~ 10 10 10 10.
Time, hr 2 2 2 2
Temp, 'C 115 115 115 115
Pressure, bar(abs) 6 6 6 6 ~
3. Final pH 11.~ 10.5 8 10.0 10.3
Consumption of
H2O2, kg ptp 17 8 8 14
Rappa 3.3 3.4 1.4 l.q
Viscosity,
dm3/kg 686 773 ~ 723 ~~ ~0
Brightness,
~ ISO 88.0 87.9 88.0 90.0
Colour rever- =
sion, ~ ISO2 84.4 84.1 86.3 87.7
Brightness
loss, ~ ISO 3.6 ~ ~.8 1.7 2.3
Total consr~=ption of
B2O2, kg ptp 31 22 22 28
1) kg ptp = kilograms per ton of pulp 2~ after 3 hours at 185~C, 0 ~ relative humidity
~ W096/0s364 2 l 9 4 2 9 0 P~ c ,- -
It is evident from the experimental results that, after the
prPl ; mi n~ry bleaching Q(P0), the pulp has obtained a kappa
number of 4.0, a viscosity of 851 dm3/kg and a brightness of
82 ~ IS0.
After sulphite treatment under atmospheric conditions (AR)
in accordance with sequence 2, the lignin content of the
pulp, expressed as kappa number, has to a large extent been
left intact with a value of 3.7. In accordance with the
previous definition, the degree o~ sulphonation in this case
is (4.0 - 3.7)/4.0 = 8 ~. By contrast, the sulphonating
1~ conditions (AR*) in accordance with sequence 3a have reduced
the kappa number to leGs than half, with a value at 1.8.
That sulphonation has taken place is also evident from the
fall in the pH to 3.4 as compared with sequence 2, pH 5Ø
As can be seen, the degree of sulphonation:(4.0 - 1.8)/4.0 =
55 ~ is on a completely different level.
After the rn~rln~;ng pressurized peroxide stage (P0) for the
three sequences, it is clear that the sulphonating bleaching
according to the invention (sequence 3a) not only reduces
the peroxide consumption by about 10 kg of H2~2 per ton of
pulp but also decreases the 1088 of brightrLess due to aging
and also decreases the content of residual lignin to less
than half after full bleaching.
The sequence 3b is the same as 3a except that the
Gulphonation has been carried out somewhat more vigorously
and at a lower pH. This experiment demonstrates that very
high brightn~sP~ (90.0 IS0) can be achieved using the
method according to the invention.
~ood results (not shown), especially with high brightnesses
has also been achieved when substituting the initial Q stage
with an (AR*) stage in any of the sequences described in
Table 1. For example full brightness can be achieved by a
(AR*)(PO)(P0) sequence.
As is e.yident; the degree of sulphonation affects the
quality properties of the bleached pulp in a decisive manner
and makes the pulp comparable with chlorine-free pulp
W096/05364 2 ~ q 4290 ~ c -- -
produced using ozone or a pero~y acid ~peracetic acid,
persulphuric acid). This is illustrated in more detail in
diagrams 1 - 4 in which data from bleaching sequences
cn~t~;n;ng ozone (4 kg of 03 ptp), Q 0 (ZQ)(P0), and
peracetic acid (6 kg as H202 ptp), Q(OP)(PaQ)(P0), are
compared with the reference- Q~PO)(P0) and a numberh of
sequences in accordance with the invention
(AR*)(Po)(AR*)(Po), Q(PO~(AR*)(P0). In diagrams 1 - 2 there
is also shown an alternative sequence in accordance with the
invention,(AR~)O(APO)(P0), where (AP0) denotes a pressurized
acetic acid peroxide stage. This type of sequence is shown
to be almost as efficient and selective as the ozone
sequence although it is very cost efficient.
Diagram 1 demonstrates that 16 - 24 kg of S02 consumed per
ton of pulp saves approximately 10 kg of hydrogen peroxide
as compared with the reference bleached only with alkaline
hydrogen peroxide As regards the total consumption of
hydrogen peroxide, the sulphonating sequences according to
the invention by large give the same result as the peracetic
acid sequence. Comparison of the sulphonating sequence and
the ozone sequence ~hows that about 10 kg of peroxide:are
required in order to replace 4 kg of ozo~e.-The sequence
with a pressurized acetic acid peroxide stage,
(AR~)O(APO)(P0), is even better in that only about 5 kg of
peroxide are required in order to replace 4 Eg of ozone
It can be seen from diagram 2 that the viscosity of;pulp
which has been bleached in ~cc~ n~P with the invention is
approximately the same as that of the ozone-bleached or
peracetic acid-h]~a~hpd pulp but considerably higher~than
the reference at high brightness (approximately 90 ~ IS0).
Diagrams 3 and 4 show that the peracetic acid sequences and
snlph~n~ting sequences give approxima~ely the same result as
21 9~290
~ W096/05364 .~ 'crr~
13
far as colour reversion is r~ n~ and that ~his result is
superior to that for the ozone sequence.
-
Diagram ~ shQw the effect on visc~sity versus kappa numberwhen using a sulphonating stage in c~nn~t ~ ~n with an oxygen
delignification stage. The beneficiary effect of preceeding
an oxygen stage :with a complex-forming stage is previously
known. It has now been found that when replacing the
complex-forming stage with a sulphonating stage, even lower
kappa numbers can be reached, with acceptable viscosity If
an additional sulphonating stage is added after the oxygen
stage, the kappa number can be even more lowered.
In Table 2 is shown a comparison of physical properties of
ECF (Elementary Chlorine Free) and TCF (Totally Chlorine
Free) pulps. The property values have been interpolated to
correspond to tensile index 80 kNm/kg.
Table 2
Sequence: D(EOP)DIED) QlP0) QOIZQ)(PO) Q(PO)(AR~)(F
~appa no: 3.7 1.5 2.2
Vicco~ity, d~/kg: 860 B20 740 765
~ ' , %IS0: 90 86 89 89.1
Yield, % on pulp kappa 12: 96 - 95 g6
2ero sp~n, knm/kg: - - 94 98
Evolutions, PFI: 1300 1600 1300 1300
Slownes~, ~S~: 17.0 17.0 19.5 16.0
Den~ity, kg/dm3: 660 680 670 650
Air ~ P, sec/100~1: 3.8 3.8 5.0 4.0
P,ur~t index, MN/kg: 5.9 6.4 6.0 5.5
T-ar index, Nm /kg: 14.8 12.8 14.2 15.0
~llphnnAting ~equence - charge S02: 10 kg ptp, ~nn _-;nn 5~2: 4 kg
ptp, ~1l~ ;nn H2~2: 21 kg ptp
W096/0~64 2 1 9 42~O . ~ f '
As can be seen in Table 2, the sulphonating sequence gives
yield and strength properties similar to those of the ECF
sequence and better than those of the ozone sequence.
The invention is not limited by the embodiment in the
experiments, but can be varied within the scope of the
subsequent patent claims. Thus, it will be readily evident
to the pe~son skilled in the art that there are very many
options for varying the construction of the bleaching
1~ sequence, especially since the alkaline, oxidizing stage(s)
do(es) not need to consist of ~a) peroxide stage(s) but can
be constituted by any oxidizing bleaching chemical
whatsoever
.
