METHOD OF BLEACHING PULP

METHODE DE BLANCHIMENT DE LA PATE

English Abstract

2149332 9411568 PCTABS00032
The present invention relates to a method for bleaching pulp and
especially to novel types of pro-environmental bleaching
processes used in the wood-processing industry, in which methods the
chlorine chemicals have been replaced by other bleaching chemicals
involving discharges less harmful to the environment. It is a
characteristic feature of the invention that bleaching comprises at
least two peroxide stages between which stages the fibers are
subjected to an activating treatment, for example, acidation or hot
alkali extraction.

Claims

CLAIMS
1. Method of bleaching pulp with chlorine-free chemicals,
such as, for example, oxygen, hydrogen peroxide or ozone in
a multistage bleaching sequence, in at least at two points
of which fibers are treated in at least one of the
following treatments: acidification, metal stripping,
entzyme treatment and hot alkali stage, in order to improve
the bleachability thereof, said sequence being
characterized in that
a) pulp is treated in an acid stage,
b) pulp is bleached in a hot alkali stage with
peroxide, to which stage oxygen is added and the
temperature of which stage is raised above 90°C,
and
c) pulp is further bleached with ozone.
2. Method in accordance with claim 1, characterized in
that complex formers, entzyme and/or ozone is/are used as
additional chemicals in stage a).
3. Method in accordance with claim 1, characterized in
that the temperature in stage b) is above 100°C.
4. Method in accordance with claim 1, characterized in
that acid treatment is combine to the ozone bleaching of
stage c).
5. Method in accordance with one of claims 1 - 4,
characterized in that there is yet another peroxide stage
subsequent to the stage c).
6. Method in accordance with claims 1-4, characterized in
that pulp is washed and/or pressed subsequent to stages a),
b) and c).
7. Method in accordance with claim 5 or 6, characterized in
that pulp is washed and/or pressed subsequent to the
peroxide stage following stage c).

8. Method in accordance with claims 1-7, characterized in
that the temperature in stage b) is 120°C.
9. Method in accordance with claims 4 - 8, characterized in
that one ozone stage is used in the sequence to increase
the final brightness above 85 (ISO).
10. Method in accordance with claim 8, characterized in
that the kappa number of pulp is subsequent to bleaching 2
- 4.
11. Method in accordance with claim 5, characterized in
that the peroxide stage is performed at a temperature of 90
- 120 °C.
12. Method in accordance with one of claims 1 - 11,
characterized in that the consumption of peroxide is less
than 30 kg H2O2/adt.

Description

~ Wo94/11568 2149332 PCI'/F193/00362 1 ~
METHOD OF BLEACHING PULP
. .
The present invention relates to a method of bleaching
pulp. It relates especially to novel pro-environmental
bleaching process types used in the wood-processing in-
dustry, in which methods the chlorine chemicals are
replaced by other bleaching chemicals involving dis-
charges less harmful to the environment.
,
Cellulose pulp is traditionally bleached with chlorine-
containing chemicals, such as chlorine gas and chlorine
dioxide. The chlorine compounds gen~erating in bleaching
processes are unpleasent because they are malodorous and
- also because they cause problems in the purification of
the effluents. Partially~for the above mentioned reasons
the environmental requirements on!the cellulose industry
have become more strict, and therefore there is a
tendency to use more and more chlorine-free/non-chlorine
bleaching chemicals, such as peroxide and ozone. Peroxide
and ozone bleaching are both known bleaching processes.
~owever, replacing chlorine chemicals with peroxide or
ozone has not always led to final products of sufficient
quality. By using the processes in accordance wlth the ``
; present invention it is possible to intensify peroxide
and oxygen bleaching in such a way that no or hardly any
difference may be seen in the quality of the final
product, at least~any negative, compared with chlorine
; bIeaching.
The present invention relates to a~ method of blea~hing
pulp in such;~a~way~that peroxide and ozone bleaching is
`~ intensifled with metal stripping,-acidat~ion ~and enzyme
treatment.
Fo~llowing~abbreviations are used of the bleaching stages
in thF enclosed text~
O = oxygen stage
p~= peroxide stage ; ~ ~
CONRRM~ION C41~ 1

. . ~ .. ~ ',
WO94t11568 ` PCT/F193/00362 ~ ,
21~'J`33`~ ` 2
E = alkali extraction stage
HAE = hot alkali extraction `
Z = ozone stage
A = acidation stage
~ ~ ~
It is possible to use addition`al chemicals in different ~ `-
stages, and the following abbreviat1ons are used to re~er
to them: `:
~ e = entzyme ~ ~:
Q = complex former
The above symbols are used in such~ a ~ay that, for
example, AQ refers~to acid;stage, in which a complex
- former,~such as EDTA, is used~for r~e~oving metal ions. -
-Respectively AeQ refers to acidation stage,~in both the i
~ complex former and entzymes are used.
: .
,
~;~ Complex formers `are substances, which are used for
removing metal ions from the~pu~lp stock~because the metal
ions decompose peroxide and ozone and thus decrease the
~; bleaching efficiency. ; ~ ~
, ~
Entzymes are used for treating t~e fibers in such a way
that the bleaching chemicals may more~readily af~ect the;
lignin. Ent~z~mes partly Gause the;fiber walls to swell ` ~ ;
and partly open pores.
HAE, i.e. hot a~lkali extraction~stage is an alkali stage,
in which the temperature is above 90C, preferably~above
100C. In said ~hot alkali stage~the pores of the~fiber ~ i~
swell in such a way that the bleach~ng chemical is better
able to react and also the residual lignin may be more~
readily~exttacted f rom e~e f1be1 waLls.
It~is`~a characteri~ing f~ature~of~the~present bleachlng
method ~that~in ~multlstage~b1ea~ching ln at lea~st~two
:~ point~s~of the~ sequence~the~lbers ar~e~subjected to a
treatment, which improves~the b1eachab11ity thereof.~ ~

~ WO94/llS~ 2 1 4 9 3 3 2 PCT/FI93/00362
The invention will be described more in detail below with
reference to the examples.
. .
Example 1 `
It is known that by using bleaching se~uence O-AQ-P-P and
O~Ae~P P it is possible to bleach pulp to the kappa number `
12 - 8 when the brightness is with birch about 75 - 80
and with pine 65 - 75.
1 0 ` ` ~ ,,
The experiments have surprisingly shown that the addition
of entzymes prior to the first bleaching stage when
bleaching blrch ~pulp affects the bleaching result only
very little,~but instead when used between two peroxide
stages the entzymes significantly intensify the
bleaching process. Thus the sequence O-A~-P-Ae-P results
in high brightness and low consumption o~ peroxide. It ~ ;
;~became apparent in research that~a two-stage peroxide ~ i~
treatment with birch pulp including entzyme act`ivation
between the two P-stages, produces with the same peroxide
consumption approximately 5 % higher brightness in the
` final product. Respectively, the same final brightness is
achieved by abou~t~lO kg H2O2/adt~lower peroxide doslng.
~.
;~ 25 The phenomenon illustrated in the~above example l may be ;
` utilized in many~ways. In the peroxide bleaching the
brightness may further be lncreased~by using ozone. An
ozone stage is preferably added to ~ollow the~acid~s~age,
~` because ozone ~requires ~acid conditions to act.
Appropriate points for ozone bleaching are pointed out~in~
the following sequence with ~Z)~. Normally it is
reasonable to add one or two ozone~stages at most in a
bleachi~ng~ sequence. The~ sequences using ozone ~as~an
addit~ional chemical for peroxlde bleaching, are thus:~
~ ~ ~ ~AQ~(Z~)~P-A, (~Z)~ P (Z)~
It ~is seldom~u~eful~ to~us;~uch~ a long sequen~ce, and~
therefore~applicable~sequen~ies~are, for ex~mple:
O-AQ-P-Aeg-P~or~

WO94/11568 PCr/FI93/00362 ~ ~
2 1 4 J 3 3 2 0-A -Z-P-A -P or ~,
O-AQ-P-Ae-P- Z . ''``"
The combination AèZ means that there is no washing between
the Ae~ and Z- stages. The dash "-"~means~that there is
either a washing or pressing stage between the stages.
The possibility to use;a HAE-stage between the~peroxide ,~
stages has not yet been~tested more accurately in the
laboratory. Since the peroxide stages are hot, it might
be possible to position a HAE-stage between the P-stages.
The purpose of the HAE-stage would correspond to~that of
Ae-stage. The temperature of the~HAE-stage might be, for~
examplej 120C. A suitable~ bleaching sequence would
therefore be, for~example,
0-AQ-Z-P-HAE-P or
0-AQ-P-HAE-P-Z. ~ ~ :
An alternative to~replace the~second P-stage is;~to~add~ `
peroxide to the HAE-stage (HAEp),~wher~eby the sequence is
simplified considerably. The sequence might then be 0-AQ-
P-HAEp-Z or 0-AQ-HAEp-P-Z, although the ozone stage may, if
1~ ~ required, also be positioned su~sequent~to the~AQ-stage
either to replace the ozone~stage~in~the above described ~ !~
sequence or to add~a second ozone stage to the sequence.
Example 2
The pulp was digested to a kappa number 20-25 prior to
the~oxyge~n`bleaching, in whlch;the~kappa number~decreased ~ ~
to 12 - 16~. ;The bleachlng ~sequence subsequent to the `
digestion was 0-Aea-EP0-Ae-P;. EP0 means peroxide stage~, in~
which also~oxygen~is~used.
~ ~n tbe~tes~runs~the~fina;l~p}odu~ct~ob=aloed was~pu_D,~the
brightness of which was~almost~80 and~the kappa number~at
the~ end of~ t;he;te~st run 6. ~The~consumptlon of peroxide~
was~on~ly ~30~ kg~H20z.~;By~adding~an ~o~zone~ stage~;to~ the ;~

214933~ 1,```
WO94/11568 PCT/F193/00362 t
sequence the brightness might readily be raised above 85
and respectively the kappa number to about 2 - 4.
As may be seen from~ the examples and a number of
different bl~aching sequences described therewith, it is
possible to~ obtain quality pulps~ with very high
brightness~and very low kappa number. The above given
examples as well as the sequences described therewith ~`
must be considered only examples ~o~f the innumerous
variations, which the process arrangement in accordance
wlth the present invention enables.~Therefore, the above
examples and~example sequences~ are by no means given to
limit the invention from what- is described in the
attached claims ~which ~alone~; deflne~ the scope of
.invention. For example,~although both in the description
and in the claims a sequence~seems to end `at the ozone
stage,~ it has no~significance, since;it is~ obvious that,~ E
desired~ it is possible to~extend the sequence by
adding a treatment stage, which,~however, has no s~peciflc
20 ~ meaning in view of~the present in~ention.
-
t