TREATMENT OF CHEMICAL PULP WITH A CHELATING AGENT AND AN OXIDIZING AGENT IN CONNECTION WITH BLEACHING

TRAITEMENT D'UNE PATE CHIMIQUE AVEC UN AGENT CHELATANT ET UN AGENT OXYDANT EN VUE D'UN BLANCHIMENT

English Abstract

A method of chlorine free bleaching of pulp where the metals in the pulp are
removed by means of a chelating agent. The pulp is treated with the chelating
agent at a pH 2-7 for a period of 5 minutes to 2 hours. The chelating agent is
activated by the addition of an oxidizing agent. After the treatment the pulp
is washed.

French Abstract

On décrit un procédé de blanchiment sans chlore de la pâte à papier, les métaux présents dans cette pâte étant éliminés par un agent chélatant. La pâte est traitée avec cet agent chélatant à un pH allant de 2 à 7, pendant une période de 5 minutes à 2 heures. L'agent chélatant est activé par l'adjonction d'un agent oxydant. Après ce traitement, la pâte est lavée.

Claims

Claims
1. A method at chlorine free bleaching of pulp where
metals in the pulp are removed by means of a chelating agent,
c h a r a c t e r i z e d in that the pulp is treated with
the chelating agent at a pH 2-7 for a period of 5 minutes to
2 hours, and the chelating agent is activated by the addition
of an oxidizing agent, and that the pulp is thereafter washed.
2. A method as defined in claim 1,
c h a r a c t e r i z e d in that the oxidizing agent is an
oxygen-containing gas in an amount of 1-10 kg per ton pulp,
counted as pure oxygen gas.
3. A method as defined in claim 1,
c h a r a c t e r i z e d in that the oxidizing agent is
hydrogen peroxide in an amount of 0,5-3 kg per ton pulp.
4. A method as defined in claim 1,
c h a r a c t e r i z e d in that the oxidizing agent is
ozone in an amount of 0,5-3 kg per ton pulp.
5. A method as defined in any one of the preceding claims,
c h a r a c t e r i z e d in that the treatment with
chelating agent is carried out at a pressure of 1,5-10 bar and
a temperature of 100-130°C.
6. A method as defined in any one of the preceding claims,
c h a r a c t e r i z e d in that the washing is carried out
at a temperature of above 80°C.

Description

W 0 96/11298 ~1 a 1 ~ 9 2 5 7 PCrlSE95/01135
Treatment of chemical pulp with a chelating agent
and an oxidizing agent in connection with bleaching
This invention relates to the bleaching of chemical
pulp with chlorine free bleaching chemicals. The invention,
more precisely, refers to a method of treating the pulp with
chelating agent in connection with the bleaching.
Chlorine free bleaching of chemical papermaking pulp is
carried out to a large extent with hydrogen peroxide in
sequences which normally also include the bleaching chemicals
oxygen gas and at times ozone. In future probably also so-
called peracids, such as peracetic acid and persulphuric acid,
will be used. The lignin, which must be removed from the paper-
making pulp in order to achieve high ISO-brightness of the
pulp, is firmly bound, and for performing the bleaching in a
lenient way and with economic use of chemicals it is favourable
to use a combination of several bleaching chemicals. The
individual chemicals are added in steps, and the pulp is washed
between the steps.
Hydrogen peroxide is a highly effective bleaching
chemical, if the bleaching conditions are right ones, and by
using only oxygen gas and hydrogen peroxide brightness levels
of almost full brightness, i.e. almonst 90~ ISO, have been
achieved on mill scale. This presupposes, however, very special
conditions, such as that the papermaking pulp is digested and
oxygen bleached to a far-reaching extent, i.e. below kappa
number lO, and that the pulp prior to the peroxide step
substantially entirely has been freed of disturbing metals,
such as manganese, copper, iron and others. The presence of
such metals causes parts of the hydrogen peroxide to be
degraded without any bleaching effect, which tendency increases
with increased metal content and increased charging of
peroxide. As the peroxide step is driven very far, i.e.
precisely to the limit at which the chemical is capable to
bring about a maximum brightness level, the peroxide steps of
today are highly sensitive even to low metal contents. The
manganese content in pulp entering the peroxide step, for
example, must not exceed l g/ton pulp when highest brightness
is to be achieved. This level, however, is difficult to attain
on mill scale. A high brightness of the pulp is desired,
because such pulp can be sold at a higher price.

W096/11298 0 2 ~ ~ 9 2 5 7 rCT/SE95/01135
A low metal content in the pulp prior to the peroxide
step is achieved by treating the pulp with a chelating agent,
for example EDTA or DTPA and thereafter washing the pulp to
eliminate the released metal ions prior to the peroxide step.
Such a method is reported, for example, in EP-A-402335, the so-
called Lignox-method. It is there stated that the chelating
agent treatment shall be carried out between 10C and 100C,
preferably 40-90C, for a period of up to 6 hours and at a pH
between 3.1 and 9Ø
The pulp industry manufactures today chlorine free
papermaking pulp with high brightness by a far-reaching
digestion and oxygen bleaching of the pulp, by using existing
bleach plants and towers so that a very long stay-time in the
chelating step and peroxide step is achieved, and by washing
the pulp very carefully between different bleaching steps. Such
washing, however, requires large amounts of water and,
therefore, cannot be used when the bleach plants have to be
closed, which probably will be the case when various
environment commitments must be fulfilled. At the construction
of new plants, or the reconstruction of existing ones, it is
also desired that the stay-times in different process steps are
as short as possible, in order thereby to minimize the building
volume and, thus, also the investment costs. It is, finally,
important to minimize the amount of added chelating agent,
because it is an expensive chemical and, so far, it has not
been possible to dismiss its possible damage to the
environment.
The present invention offers a solution for the
aforesaid problems. The invention, thus, results in a more
effective chelating agent treatment which is carried out prior
to the peroxide step, in that the chelating agent is activated
by treating the pulp with an oxidation agent. By activation is
meant that the efficiency of the chelating agent is increased
by oxidation or that the metals, which should be removed from
the pulp, are oxidated. In both cases the metal binding is
facilitated and the metal content in the pulp is reduced.
According to the invention, it is possible, for example, to
reduce the manganese content to below 1 g per ton pulp.

WO96111298 ~ 2 ~ ~ 9 2 5 7 PCT/SE95101135
The characterizing features of the invention are
apparent from the attached claims.
The invention is described in greater detail in the
following with reference to some embodiments thereof.
The pulp from a preceding process step, for example
oxygen delignificaiton is wahsed in a pulp washer room.
Thereafter the pH value is adjusted by the addition of acid or
liquor. The acid/liquor shall be admixed in a homogenous way,
for example in connection with a dilution of the pulp or by
means of a pump or mixer. When a mixer is used in this
position, a very intense and uniform admixing is ensured, which
in certain cases can be of decisive importance. The pH value
shall there be adjusted to between 2 and 7, preferably 4-6.
Subsequent to the pH adjustment, the chelating agent,
for example EDTA or DTPA, is added in a separate mixer which
causes an effective admixing of the chelating agent to the
pulp. The amount of chelating agent suitably is 1-5 kg per ton
pulp, preferably 1.5-2.5 kg/t.
An oxidizing agent is also added to the pulp. As
oxidizing agent oxygen-containing gases such as air or oxygen
gas can be used. Alternatively, hydrogen peroxide or ozone can
be used. The charging of oxidizing agent, which contains oxygen
gas, shall be 1-10 kg per ton pulp, preferably 3-7 kg/t,
counted as pure oxygen gas. When hydrogen peroxide is added,
the amount shall be 0.5-3 kg, preferably 1-2 kg/t. At the use
of ozone, an ozone/air mixture with 0.5-3 kg ozone per ton
pulp, preferably 1-2 kg/t, is added.
The pulp is thereafter pumped to a reactor vessel, in
which a temperature of 40-130C, preferably 100-130C, and a
pressure of 1-10 bar, suitable 1.5-10 bar, preferably 3-8 bar,
are maintained. The stay-time of the pulp in the vessel shall
be between 5 minutes and 2 hours, preferably 10-60 minutes.
The pulp treated with chelating agent is directed from
the reactor vessel to a pulp washer room where the temperature
is held above 80C whereby the metal ions, which are bound by
chelating agent and thereby can be washed out, are washed out
effectively.

WO96/11298 PCTISE95/01135
a 2~1 99 257~ 4
This pulp washer can be a pressurized filter or
preferably a roll press, which is very effective and has low
water consumption (4-5 m3/ton pulp).
At the chelating agent treatment according to the
invention, the oxidizing agent has the object to activate the
added chelating agent so that it binds undesired metal ions,
such as manganese, copper and iron, more actively. The
oxidizing, besides, facilitates the separation of metals by
releasing them more easily from the pulp. The metals thereby
can be bound better by the chelating agent and be washed out
from the pulp in the subsequent washing step. The pulp will
thereby be purer from a metal aspect.
The pulp is directed after the washing to a subsequent
bleaching step where chlorine free bleaching is carried out
with peroxide-containing bleaching agent. The low metal content
in the pulp renders it possible to bleach more effectively to
maximum brightness.
EXAMPLE
An oxygen bleached coniferous wood pulp with kappa
number 12.9 and brightness 40.0~ ISO was treated with a
chelating agent (Q), washed and bleached with peroxide (P).
The Q-step was performed at lO~ pulp consistency in
l hour and final pH 5.0-5.2. The Q-step was performed with and,
respectively, without addition of oxygen (Q2). Then the pulp
was washed and bleached in a pressurized P-step to which oxygen
was added, a so called (PO)-step. The conditions in the (PO)-
step was 12~ pulp consistency, 2 hours retention time,
lO0-110C and final pH lO.0-lO.4. The results is shown in the
table below.

WO96/11298 ~ a 1 9 9 2 5 7 PCT/SEg5/01135
Temp, C Q2 addition kg H2O2 % ISO
per ton pulp
lO0 No 40 83.3
lO0 Yes 40 83.9
llO No 20 78.4
llO Yes 20 80.7
llO No 40 83.5
- llO Yes 40 85.5
This result shows that the final brightness of the pulp
is increased if the chelating step is activated by addition of
oxygen while the peroxide bleaching conditions are unchanged.
The invention, of course, is not restricted to the
embodiments shown, but can be varied within the scope of the
invention.