Note: Descriptions are shown in the official language in which they were submitted.
W093/14262 PCT/SE93/00012
212~88
Bleachin~ of chemical pulp ~:
This invention relates to the bleaching of sulphate pulp
without using bleaching agents containing chlorine. The
technique o~ bleaching chemical pulps is now developing
rapidly, bec~use the bleaching chemicals hereto~ore mainly
-used, Cl2, C102, and hypochl~rite, are fearsd to give
ri~e to products being a risk ~or ~be environment and
health.
Until now, the discussions have been concentrated on the
emissions o* chlori~at~d organic subs~ance to ~he recipient.
Tha ma;ority o~ the cou~tries in th~ world ha~e now establ~
ished limit ~alueR for these emis~ions 9' which normally are
measured as AOX (Adsorbable organ:ic halogens). As a new
trPnd, the customers require t~ an ever increasing degree
paper with a low cont~nt of chlor:inated organic su~stance~
Conventio~al bl~ach~ng ~ith Cl2 dL~appear~ now more and -~
more, and Cl2 is replacsd primari:Ly by ClO2~ Bleaching
with GlO2 c&uses substa~t~all~ lower AOX emissio~s to the
recipient, but the amoun~ of chlorinated ~ubstance in t~e
pulp is a~cted only insigni~icantly.
In recent years, the use o~ hydrop~roxide t the bleaching
of chemical pulps h~s increased considerably. Hydr~peroxide
can replace part o~ the chlorine chemicals.
In patent application SE 89 02058 O, a method is described
at which the pulp is pretreated with complexing agent under
neutral conditionsO B~ this process, coniferou~ sulphate
pulp can ~e bleached with peroxide-containing bleaching
agents to 70 7~% ISO a~ about 10~ pulp consistency. In
ord~r to achieve fully bleached pulp qualities with higher
ISO-brightness (above 83~ ISO~, th s p~ocess can be combined
with final bleaching ~ith ClO2. As a result, t~is final
bleaching then yiel~s emission of AOX from the bleach plant
and chlorinated organic susbstance in the bleached pulp. ~;
W0~3/14262 PCT/SE93/00012
2124~
.~,.
Other known methods of reducing emissions of chlorinated
organic substance to the recipient are to extend the
delignification further in the closed part Or the mill,
i . e . in the digester house and/or oxygen step. ;~
Extended diges~on with maintained pulp quality can be
achieved by impregnating the chips with black liquor in
the initial phase of the cooX, as described in Finni~h
patent ~pplication 90 0663. This method renders it possible
to digest pulp down to a kappa number of about 10 and
obtain str~ngth prop~rtie s equal to or better than with
pulp digested to convention~l level ~ kappa number 25-30.
Combined w~th oxygen de7igni~ication, kappa numbers as
low as 5-6 can be obtained ~o~.further bleachingO
By combining con~rentional digesti~n ~ oxy~en delignificat-
ion and ozon~ bleac~i~g in acid environment, kappa numbers
lower than 10 can be obtained~
It has~ however, no~ been po~sible to produce fully bleach- ~`
ed pulp qualities without final bleaching with ClO2.
The present in~ention implies 9 that fully bleached pulp
with a bright~ess above 83% I50 can be made without the
use of chlorine-containing bleaching agents.
At the s.tart of the bleaching, the pulp shall have a kappa
number below 9 . 5 . This cE~n be achieved by conventional or
extended digestion followed by 02-delignification, altern-
atively by conventional or extended digestion fQllowed by
2- and 03~-deligni~ication~ The bleaching i~ initiated by
a pretreatrnent step with ~::omplexing agent followed by
tr~atment with hydropero~ide~ in al~aline en~rironment in
the presence o~ silicate ~r an organic complexing agent :~
of the type phosph~nic or carboxyllc acid.
W0~3/14262 PCT/SE93/00012
3 212~
The characterizing features of the in~ention are appar-
ent ~rom the attached claims.
The pulp digested and delignified with oxyg~n and possibly
ozone according to above shall ha~e a kappa number below
9.~. This can be achieved by known methods o~, digestion and
delignific~tion. The pulp ~hall therea~ter be pretreated
with complexing agent, for example EDTA or DTPA, at a pulp ~;
consistency of 1-15~ ~or 1-60 minutes, preferably S-30
minutes. The temperature shall be 70-100C, preferably
80-100C, and the pH-value 5-7.
The pulp thus pr~treated is washed and dewatered to a con-
sistency o~ 18-40%, preferably 20-30%. The pH-v~lue is
increased to alkaline level, and hydrDparoxide is added
together with silicate, for example ~odium silicate, alt-
erna~vel~ together ~ith an organic complexing agent of the
~yp~ phosphonic or carboxylic acid. The dwell time o~ the
pulp in thi~ hydrop~roxide ~tep shall be ~/2 - S hours, ::
preferably 1-4 hours, and th~ tempexature 70-1 oo5c, pre~-
erably 8~-100C. The silica~e charge shall be 2-30 kg/ton
pulp, prefe~ably 5-15 kg/ton pulp, expre~sed as SiO~. -
Al~ernatively, the charge of the organic complexing agent
shall be 0.~-5 kg/ton pulp.
Fig~ 1 is a diagram, in which the brightness o~ the pulp
is plo~ted as a function of the pulp consi~tency at the ~::
peroxide bleaching,partly at conventional peroxide bleach-
i~g and par~ly at bleaching according to the inv ntion~
The starting pulp was oxygen-delignified coniferous wood
sulphate pulp with kappa number 6.5 (10.5 a~ter digestion)~
Curve 1 show~.conventional peroxide bleaching, with a con-
~umption of H20z of 2~ kg/ton pulp. Cur~e 2 and t respect- ~
ivel~, 3 ref~P to bleaching according to the invention 1~ th ~:
silicate addition and a consumption of H202 of 20 and,
respectiYely, 30 kg/ton pulp.
WO93/14262 PCT/SE93/00012
21~408~
It appears from the diagram how the brightness, after
bleaching according to the invention a~d, respe.~tively,
after conventional bleaching~ depends on the pulp con-
sistency. At about 12% pulp consistency, the brightness
obtained i5 equal in both cases, but at increased pulp
consistency in the peroxid0 step an increased brightness
is obtained according to the invention~ ~hile convention-
al peroxids bleachi~g results in a deteriorated bright ,
ness.
The ~ollowing ex~mples ha~e the object to additionally
elucidate th~ in~ention and i~s adv~ntages.
Example 1
An oxygen-deligni~ied co~iferous wood sulpha~e pulp
(Scandina~ian fir/pine) with kappa number 6.4 ~10.5
after digestio~ according to a~ove Finn~sh paten~ applic-
ation 90 0663)~ brightne3s 43~ IS0, intrinsic ~i~cosity
605 dm3/kg, wa~ pr~treat~d with complexing agent and
peroxide bleached according to the i~vention.
Pretreatment
_ ,, ,
Pulp consistency % 5
Time min 15
Temperature ~C 90
EDTA ~g/ton 2 -:~
Final pH 6.1
~x~l~ ~0, ....
Pulp consistency % 25
Time min 240
Temperature C 90
SiO2 kg/ton 10 :
H22 consumption kg/ton 13.5 32.0
Final pH_ 10.2 10.3
Brightness % IS0 79.6 84,2
Intrinsic viscosity dm3/kg 520 447
WO93/l4262 PCT/SE93/00012
212~88
Example 2
A coniferous w~od sulphate pulp with kappa number 13.3
(digested of Scandina~ian fir/pine according to ~innish
patent application 90 0663), oxygen delignlfied to kappa ~:
number 8.3:and ozone bleached in acid environment to kap- :
pa number 3.2, brightness 61% IS0, intrinsic viscosity
582 dm3/Xg, was bleached according to the inventio~
Pretreatment a~ in Exampls 1
Peroxide bleachin,~,
Pulp cons.istency % 2
Time min . 240
Temperature G 90
SiO2 kg/ton 10
H22 consumption kg/ton 7.3 14.2 3091
Final pH 8.9 9.5 10
Bri~htnesq % ISQ 85~3 88r~ 90
Intrinsic viscosity dm3/~g 536 ~98 420 ~-
Example
.
An oxygen deligni~isd co~ rous wood sulphate pulp
(digested of Pinus taeda according to Finnish patent
application 90 0663) with kappa numb~r 9.9, ozone bleach~
ed to kappa number 4.8, brightne~ S4.$~ IS0, intrins~
ic ~iscosity 607 dm3lkg was pretreated and bleached
according to the invention.
Pretreatment as in Example 1
, ............................................................ ..
' Perox_de b~
~,
Pulp consi~tency % 25
Time min 2~0
Temperature C 90
SiO2 kg/ton 10 ,~
H22 consumption kg/ton 7.9 14.6 29.6
Final pH 10.2 10.4 10.6
~'
WO~3/14262 PCT/SE~3/000l2
2~2 ~o~ ~ 6
Brightness % IS0 81.3 85.~ 87.8
Intrinsic viscosity dm3/kg 570 534 459.
E~ample ~
An oxygen delignified birch sulphate pulp with kappa
number 9.0, brightness 59.4~ IS0 and intrinsic ~iscos-
ity 996 dm3/kg was bleached according to the invention..
Pretroatment as in Example 1 ::
Peroxide b~ eg
Pulp consi~i~enky ~ . 25
Time min ~0
Temperature ~C .90
SiO2 kg/ton 10
H22 conSumptiOn kg/ton 8.~ 17~5 35
Final pH ~0.~ 10.~ 10~5
Brightn~s~ % IS0 82.2 85.5 86~5 ;~
Intrinsic YiSCosit~ dm3/~g 903 975 &22 .;
," ..~
The in~elltion i5 not reætricted to the embodimen~s set -.
~orth abov~, but can b~ varied within the scope o~ the
invention idoa. I :
