METHOD FOR CHLORINE-FREE BLEACHING OF PULP WITH ACETIC ACID AS ACIDIFYING AGENT

METHODE DE BLANCHIMENT SANS CHLORE DE LA PATE AU MOYEN D'ACIDE ACETIQUE COMME AGENT D'ACIDIFICATION

English Abstract

A method of bleaching pulp of cellulosic fibre material without the use of chlorine when manufacturing chemical pulp is described,
wherein the pulp entering is fed continuously in a bleaching line and is treated with a sequestering agent in the presence of an acid in order
to adjust the pH to a value below 7 and is thereafter bleached in a sequence of several stages consisting of peroxide, at least one organic
per-acid and peroxide. According to the invention it is suggested that in the treatment with the sequestering agent the pulp is acidified with
acetic acid.

Claims

C L A I M S
1. A method of bleaching pulp of cellulosic fibre
material without the use of chlorine when manufacturing
chemical pulp, wherein the pulp entering is fed
continuously in a bleaching line and is treated with a
sequestering agent in the presence of at least one acid
in order to adjust the pH to a value below 7 and is
thereafter bleached in a sequence of several stages
consisting of peroxide, one or more organic per-acids and
peroxide, characterized in that in the treatment with the
sequestering agent the pulp is acidified with acetic
acid.
2. A method as claimed in claim 1, characterized in
that the peroxide used is hydrogen peroxide.
3. A method as claimed in claims 1 or 2, characterized
in that the one or more organic per-acids used is/are
performic acid, peracetic acid or perpropionic acid,
preferably peracetic acid or a mixture of at least two of
said per-acids.
4. A method as claimed in any of claims 1-3,
characterized in that acetic acid, is added in such an
amount that the pH value is below 6, preferably below 5.
5. A method as claimed in any of claims 1-4,
characterized in that each peroxide stage is performed in
alkaline solution.
6. A method as claimed in any of claims 1-5,
characterized in that the consistency of the pulp during
the treatment with sequestering agent and during said
bleaching sequence is about 6-16%, preferably 9-11%.

7. A method as claimed in any of claims 1-6,
characterized in that the pulp entering has been treated
with oxygen gas and has a Kappa number of about 14 or
less, preferably 12 or less and most preferably 10 or
less.
8. The use of acetic acid, as acidifying agent when
treating a chemical pulp with a sequestering agent in the
bleaching of pulp without the use of chlorine, wherein
the pulp entering is fed continuously in a bleaching line
and is treated with a sequestering agent in the presence
of acetic acid, in order to adjust the pH value to a
value below 7, and is thereafter bleached in a sequence
of several stages consisting of peroxide, one or more
organic per-acids and peroxide.

Description

W094/17~9 21~ 4 n 7 6 PCT/SE94/00054
-
Method for chlorine-free bleaching of pulp with
acetic acid as acidifying agent.
The present invention relates to a method of bleaching
pulp of cellulosic fibre material without the use of
chlorine when manufacturing chemical pulp, wherein the
pulp entering is fed continuously in a bleaching line and
is treated with a sequestering agent in the presence of
at least one acid in order to adjust the pH to a value
below 7 and is thereafter bleached in a sequence of
several stages consisting of peroxide, one or more
organic per-acids and peroxide.
The strict environmental requirements concerning the use
of bleaching agents containing chlorine for bleaching
pulp have resulted in extensive research work to find
other bleaching agents that are not detrimental to the
environment in the way the chlorine-containing bleaching
agents are which form chlorinated organic compounds.
Thus, it has been proposed to bleach pulp using ozone and
peroxide in various sequences preceded by treatment with
sequestering agent such as those described in SE-A-
9101300-3, for instance. However, the use of ozone
involves a number of drawbacks. Ozone has a decomposing
effect on the cellulose, and the equipment for producing
and supplying ozone in connection to the bleaching
process is vast and expensive. US-A-3,867,246
corresponding to SE-B-405130 describes another bleaching
process without the use of chlorine, including the
sequence peroxide, peracetic acid and peroxide, the first
peroxide stage being preceded by treatment with
sequestering agent in the presence of an acid in order to
adjust a low pH value. The acid used is generally
sulphuric acid. However, like ozone, also peracetic acid
has a strongly decomposing effect on the cellulose.
The object of the present invention is to solve the
drawback in bleaching pulp without the use of chlorine,

WO94/17~9 PCTISE94/00054
21S~07~i 2
using peracetic acid in combination with peroxide in the
bleaching sequence PPaP preceded by treatment with a
sequestering agent (Q) in tha presence of an acid, so
that a ready-bleached pulp is obtained that has
acceptable viscosity and strength properties and a
brightness of 85-90% ISO requested by the market.
The method according to the invention is characterised in
that during the treatment with the sequestering agent the
pulp is acidified with acetic acid.
The pulp entering is preferably produced by means of a
digestion process giving a low Kappa number, and is
thereafter subjected to delignification with oxygen gas
to that the pulp obtains a Kappa number of about 12 or
less and a viscosity of at least about lOOO dm3/kg.
The peroxide used is preferably hydrogen peroxide.
The second bleaching stage, following the first peroxide
stage, is performed using performic acid, peracetic acid
or perpropionic acid, preferably peracetic acid or a
mixture of at least two of said per-acids.
According to a suitable embodiment acetic acid is added
in such an amount that the pH value is below 6,
preferably below 5.
The temperature during all bleaching stages should be
30 80C or above, preferably 9OC.
Each peroxide stage is suitably performed in alkaline
solution.
The peracetic acid stage is preferably performed in acid
solution.

WO94/17~9 21 5 4 n 7 6 PCT/SE94/00054
Acetic acid and peracetic acid, or other suitable per-
-acids are preferably manufactured on site.
The consistency of the pulp during the various treatment
stages is about 8-16%, preferably 9-11%.
The following example illustrates the invention further
and shows its unexpected results.
Example
A number of experiments were performed using a sulphate
pulp of softwood which had been delignified with oxygen
gas and washed. The delignified pulp had a Kappa number
of 12.1, brightness 33.7% ISO and a viscosity of
1020 dm3/kg. It was supplied into the bleaching line in a
consistency of lO~ and this was maintained during the
treatment with the sequestering agent EDTA and all the
bleaching stages. In experiments 1 and 2 sulphuric acid
was used as acidifying agent in the Q-stage, whereas
acetic acid was used in experiments 3 and 4 in accordance
with the present invention.

WO94117~9 PCT/SE94100054
215~0~
Experiments 1-4
l. Additives Bleaching sequence
2. Conditions OPPaP OPPaP
3. Result l 2 3 4
O-stage
lO l. EDTA, kg/BDMT 2.0 2.0
H2SO4, kg/BDMT 6.2
Acetic acid, kg/BDMT - 15.0
2. Initial pH value 4.7 4.8
Time, min. 60 60
Temperature, C 70 70
P-sta~e
l. H22, kg/BDMT 35 35
MgSO4, kg/BDMT 3.0 3.0
NaOH, kg/BDMT 25 25
2. Time, min 240 240
Temperature,C 90 90
Final pH value lO.9 ll.O
3. Consumed H2O2, kg/BDMT 18 16
Kappa number 5.1 4.9
Viscosity, dm3/kg 9lO 9ll
Brightness, % ISO77. 3 77.0
Pa-stage
l. Peracetic acid calculated as
total H2O2, kg/BDMT 20 30 20 30
2. pH, buffer solution
(sodium acetate) 4.3 4.3 4.3 4.3
Time, min. 240 240 240 240
Temperature, C 90 90 90 90
Cont.

_ WO94tl7~9 21 S 4 n 7 G PCT/SE94/00054
1. Additives Bleaching sequence
2. Conditions OPPaP OPPaP
~ 5 3. Result 1 2 3 4
3. Consumed peracetic acid
as H2O2, kg/BDMT 11.6 17.9 9.9 14.2
Kappa number 1.1 0.7 1.2 0.9
Viscosity, dm3/kg 842 814 852 841
Brightness, % ISO84.4 87.4 84.4 86.8
P-sta~e
1. H202, kg/BDMT 4.0 4.0 4.0 4.0
MgS04, kg/BDMT 3.0 3.0 3.0 3.0
NaOH, kg/BDMT 7.0 7.0 7.0 7.0
2. Time, min. 180 180 180 180
Temperature, C 90 90 90 90
Final pH value 10.3 10.4 10.6 10.5
3. Consumed
H202, kg/BDMT1.5 1.3 0.9 1.1
Kappa number 0.8 0.5 1.1 0.7
Viscosity, dm3/kg 808 778 834 828
Brightness, % ISO88,1 89,3 87,8 89,2
The results show that surprisingly the pulps pre-treated
and bleached in accordance with the invention have
significantly improved viscosity, which in turn indicates
improved strength properties as compared with the pulps
pre-treated and bleached according to Examples 1 and 2.
Using equal quantities of peracetic acid (Pa) in the
second bleaching stage, 20 kg in Examples l and 3 and
30 kg in Examples 2 and 4, the use of acetic acid instead
of sulphuric acid as acidifying agent in the Q-stage
results in an increase in viscosity by 26 and 50 units,
respectively.

WO94/17~9 PCT/SE94/00054
215407~ 6
As mentioned above, acetic acid and peracetic acid or
other suitable per-acids are ~referably produced on site.
Acetic acid, for instance, can be recovered from the
black liquor in quantities up to 50 kg per ton of pulp
when manufacturing pulp. Peracetic acid, for instance,
can be produced in known manner from the acetic acid
recovered. Consequently the method according to the
invention can be performed in an extremely advantageous
manner from the economical point of view.