Note: Descriptions are shown in the official language in which they were submitted.
2Q83319
The invention relates to a process for improving the degree
of whiteness obtained by peroxide bleaching of wood pulp by
the addition of cyanates. Stopping the use of chlorine-
containing bleaches has led to an altered level of whiteness
during the bleaching of wood pulps. Previously, the degree
of whiteness was 88 to 90% ISO, but now, when bleaching
exclusively with oxygen and hydrogen peroxide, normally only
83 to 85~ ISO is achieved with softwood pulps or 85 to 87%
with hardwood pulps. These lower degrees of whiteness may
some~imes be acceptable because wood pulp bleached with
hydrogen peroxide has a much lower tendency to turn yellow
during thermal ageing. Basically, however, it would still be
desirable to achieve the same degree of whiteness using
chlorine-~ree bleaching processes as with chlorine bleaching.
However, the thresholds mentioned above represent
economically reasonable final levels with the exclusive use
of oxygen and hydrogen peroxide for delignifying and
bleaching. Higher degrees of whiteness cannot be reached
reliably even with the use of extremely high amounts of
hydrogen peroxide. only the use of supplementary bleaches,
such as for example ozone, allows the production of higher
final levels of whiteness. This additional investment, for
example in an EOP-Z-P bleaching sequence, may be avoided if
another way to improve the results of peroxide bleaching
could be found.
-- 1 --
2083319
This invention is a process to improve the degree of
whiteness of wood pulp which has been predelignified and
prebleached using oxygen, peroxide (hydrogen peroxide) and/or
ozone, characterised in that an alkali metal cyanate is added
with hydrogen peroxide in a further bleaching step.
Pre-bleached and pre-delignified means that the wood pulp has
previously passed through generally well-known bleaching
sequences and extraction steps using oxygen, peroxide and/or
ozone or combinations thereof. The kappa value of the wood
pulp to be bleached is between 2 and 15, particularly 2 and
lO, and its degree of whiteness is generally between 60 and
75% IS0 (for beech wood pulp) and 40.. to 50% IS0 (for spruce
wood pulp and kraft pulp).
The process conditions for the peroxide bleaching step
accordinq to the invention correspond with the known
conditionæ per se for bleaching steps of this type: the pH
is generally between 10 and 12, the pulp density has a value
between, 5 and 30% by weight, preferably 10 to 25% by weight.
The bleaching temperature is between 40C and 90C,
preferably 50C and 80C, depending on the type of wood pulp
to be bleached.
-- 2
2083319
The amount of alkali metal cyanate used, especially sodium
cyanate, is S to 50% by weight, preferably 15 to 40% by
weight, of the amount o~ hydrogen peroxide used, of which 0.5
to 5~0% by weight is used, especially 1.5 to 3.0~ by weight,
relative to oven-dry wood pulp.
It was found that it was possible to greatly improve the
results of bleaching with hydrogen peroxide if cyanates are
added during bleaching. An addit.ional in¢rease in the degree
of whiteness of 1 to 2 points is possible. The stability of
the higher degree of whiteness achieved in this way is
equivalent to the stability of the degree of whiteness
without an additive.
The process is suitable for wood pulps made from softwoods
and hardwoods and one-year old plants, prepared by the kraft
pulp process, or the acid or alkaline sulphite process,
optionally in the presence of alcohols (methanol, ethanol).
Examples
Table 1
-- 3
2083319
Bleaching a beech sulphite wood pulp, pre-delignified to
kappa 5.5 with 2 and H22~ after extraction with oxygen and
peroxide, at 25% pulp density and 75C using hydrogen
peroxide in the presence of cyanate. Constant additive, 2%
waterglass.
Test ~22 NaOH ~aOCN Whiteness ISO
% ~ ~ content ~ aged* after
bleaching
1 1 0.25 - 85.3 82.8
2 1 0.25 0.15 86.6 83.1
0 3 1 0.20 0.25 87.1 84.1
____________________~____________________________________
~ 1.5 0.25 - 86.5 84.4
5 1.5 0.25 Q.15 87.4 85.8
6 1.5 0.20 0.25 88.6 86.0
7 1.5 0.20 0.35 88.8 86.1
_______________________ ___ ______________________________
* Heat-ageing according to Tappi Standard.
Table 2
Bleaching a spruc~ sulphite wood pulp, pre-delignified to
kappa 6.5 and whiteness 75.1~, after extraction with oxygen
and peroxide, using H202at 12~ pulp density and 60C with the
addition of cyanate.
2~83319
Test H202 NaOH NaOCN Whiteness content
~ ~ ~ ISO
1 1.0 1.0 - 81.2
2 1.0 1.0 0.20 82.8
S ----________
3 2.0 1.2 - 84.3
4 2.0 1.2 0.~ 85.8
5 2.0 1.0 0.4 86.5
Bleaching kraft pulp (spruce) delignified to kappa 7.5 and
44.8% ISO whiteness using oxygen and ozone, with hydrogen
peroxide at 25% pulp density and 75C with soda/waterglass
buffering and the addition of cyanate.
Teqt H202 NaO~ Na2CO3 Water NaOCN Whitene~s content
% ~ ~ glass % ~ ~ISO
12.0 0.6 0.5 1.0 - 72~7
22.0 0.6 0.5 1.0 0.2 74.9
33.0 0.8 0.6 1.0 - . 74.8
43.0 0.7 0.6 1.0 0.4 79.1
