Note: Descriptions are shown in the official language in which they were submitted.
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I'he present invention rela-tes to a multistage, process
for the full bleaching of pulp having at least three-stages.
The bleaching of pulp with peroxides is disclosed in
laid-open German Specifications Nos. 2,327,900 and 2,219,504. In
the process according to the laid-open German Specification
2,219,504 which comprises a stage sequence of P-D-P substantial
corrosion problems may be encountered.
The present invention is a multistage process for the
full bleaching of pulp having at least three-stages. In the
process the pulp, if required after a pre-treatment with magnesium
salts and/or organic complexing agents, is treated with a per
compound in a first stage, with chlorite in a second stage and
bleached with chlorite in a third stage. For difficultly bleach-
able pulps or for attaining a desired degree of polymerization
further known bleaching stages can be added. These bleaching
stages are described in Ullmann's Enzyklopadie der technischen
Chemie, third edition, volume 18, page 778 (1967). The process
according to the invention can be carried out, for example, in
bleaching devices such as those described in Ullmann's Enzyklo-
padie der technischen Chemie, third edition, volume 18, page 755.
Washing with water can be carried out between the
individual bleaching stages in a conventional manner. However,
in a preferred embodiment the washing between the second and
third stage can be dispensed with without disadvantage to the
quality of the product.
The process according to the invention can be used for
all the conventional pulp types. These pulps can be from any
conifer or they can be hardwood-sulphite pulps, as for example,
spruce-sulphite pulp or beechwood-sulphite pulp. The process
according to the invention can be used with special advantage for
the bleaching of spruce, paper and viscose sulphite pulp or of
beechwood sulphite-viscose pulp. The process can also be used
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for the bleaching of any long- or short-fibered pulps such as
those of poplar and birch.
The pre-treatment of the pulp may be carried out in
aqueous solution at a pulp consistency of 2 -to 15% by weight,
(relative to atro pulp), preferably from 3 to 5O by weight. The
temperature may be between 5 and 80C, preferably between 5 and
20C. The time of pre--treatment may be between 5 and 10 minutes.
The concentration of magnesium salt may be from 0.005 to 0.6% by
weight, preferably from 0.1 to 0.4% by weight (relative to atro
pulp and MgSO4). MgSO4, MgC12, Mg(NO3)2, Mg EDTA (ethylene
diamine tetraacetic acid) or magnesium salts of polyoxy-carboxylic
acid are suitable as magnesium salts. These polyoxy-carboxylic
acids are described, for example, in laid-open German Specifica-
tions 1,904,940, 1,904,941 and 1,942,556.
The first bleaching stage may be carried out with a per
compound at a pulp consistency of 5 to 10% by weight, preferably
8 to 12~ by weight (relative to atro pulp) in an aqueous solution.
An organic or inorganic peroxide, such as hydrogen peroxide,
sodium peroxide or t-butyl hydro-peroxide, can be used as the per
compound. In a preferred embodiment hydrogen peroxide is used.
In the first bleaching stage the temperature may be between 40 and
95C, preferably between 50 and 75C. The time of the first
bleaching stage may be between 30 and 240 minutes, preferably
between 60 and 180 minutes. The concentration of the per compound
may be from 0.5 to 5~ by weight, preferably from 0.8 to 1.2% by
weight (relative to atro pulp and 100~ of H2O2). The concentra-
tion of alkali, preferably sodium hydroxide, may be 1.5 to 10%
by weight, preferably 2.0 to 4.0% by weight (relative to atro
pulp and 100% of NaOH).
In the second stage an alkali chlorite, preferably
sodium chlorite, can be used as chlorite in the aqueous solution.
The pulp consistency in the second stage may be 5 to 15o by weight
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preferably 8 to 12% by weight (relative to atro pulp). The temp-
erature may be from 20 to 80C, preferably from 25 to 60C. The
time of the second stage may be between 30 and 240 minu-tes, pre-
ferably between 60 and 180 minutes. The concentration ~f the
alkali chlorite may be from 0.05 to 1.0% by weight, preferably
0.1 to 0.3% by weight(relative to atro pulp and 100% of sodium
chlorite). The pH value of the second stage may be 2.5 to 5.0,
preferably 3.0 to 4. The pH value can be adjusted by adding an
organic acid, for example, formic acid or acetic acid or a mineral
acid. In a preferred embodiment nitric acid (NHO3) is used.
The third stage can be carried out with a per compound
in an aqueous solution at a pulp consistency of 5 to 15% by weight,
preferably 8 to 12% by weight (relative to atro pulp). Inorganic
or organic peroxides, as for example, hydrogen peroxide, sodium
peroxide or t-butyl hydro-peroxide, can be used as the per com-
pound, hydrogen peroxide being the preferred per compound. The
temperature may be from 40 to 95C, preferably from 50 to 65C.
The time of the third bleaching stage may be from 30 to 240 min-
utes, preferably from 60 to 120 minutes. The concentration of
per compound, preferably hydrogen peroxide, may be from 0.2 to
2.0% by weight, preferably 0.4 to 0.8% by weight (relative to
atro pulp and 100% of H2O2). The concentration of alkali can be
from 0.5 to 3.0% by weight, preferably 0.8 to 1.2% by weight
(relative to atro pulp and 100% of NaOH). Sodium hydroxide is
preferably used as alkali.
The process according to the invention has the advantage
that because of the combination and sequence of the bleaching
` stages per compound, chlorite and per compound and by operating
the chlorite bleaching stage at a pH value between 3 and 5 the
amount of chlorite used can be maintained so low that the amounts
of chlorine getting into the waste water disposal system are
insignificant. This means that the process according to the
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invention operates alr,lost chlorine--free.
The white conten-t of the final product as compa-ed
with that of the final products of the conventional processes
is improved. More over the yield can also be increased by 2 to
3%. Furthermore, the washing between the second and third l~leach-
ing stage can be dispensed with, whereby an improved hcat cconomy
is attained.
The temperature in the second stage can be reduced to
20C, whereby the corrosion problems are also substantially
reduced; these problems can be encountered, for example, in the
chlorine dioxide stage (D) in the process according to the German
Patent No. 2,219,504.
The process according to the invention is illustrated
and described in greater detail by means of the following Examples.
Example 1
Bleaching of a beechwood-sulphite-rayon pulp according to the
prior art
first stage = 2.2~ of C12 1 hour
pulp consistency 3% 15C
washing
second stage = 3.6% of NaOH 3 hours
pulp consistency 10% 75C
washing
third stage = 0.4% of active chlorine 4 hours
pulp consistency 10% 40C
washing (Ca hypochlorite)
Example 2
Bleaching of a beechwood-sulphite-rayon pulp according to the
invention
first stage = 1.0% of H2O2 3 hours
pulp consistency 10% 75C
3% of NaOH
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washing
second stage = 0.15% of NaClO2 12 hours
pulp consistency 10% 30C
0.4% of HNO3
no washing
third stage = 0.4% of H2O2 2 hours
pulp consistency 10% 65C
1.0% of NaOH
Results
Example 1 Example 2
white content bleached (Elrepho) 92.0 94.6
yield % 93.3 96.6
~-cellulose % 90.6 90.4
Rl9 % 94.2 93.8
viscosity (SKV) 178 214
For a beechwood-sulphite-rayon pulp the two examples
show that better bleaching can be attained with the process
according to the invention. The pulp yield, which is higher by
3.3%, is obtained with an almost unchanged content of ~-cellulose.
The Rl9 value, which has only slightly decreased, indicates that
the gain in yield is largely maintained with the pulp alkaliza-
tion and viscose production. The higher viscosity indicates
that the process is extremely easy on the fibres.
In the known bleaching process according to Example 1
the total consumption of chlorine was 2.6%, relative to pulp,
and in the bleaching operation according to the invention 0.06r.
Example 3
Bleaching of a spruce-sulphite pulp according to the prior art
~: first stage = 4% of C12 1 hour
pulp consistency 3% 15C
washing
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second stage = 3.5~ of NaOi~ 1 hour
pulp consistency 10-~ 70C
washing
third stage = 1.0% of active chlorine 3 hours
: pulp consistency 10% 40C
washing
fourth stage = 1.0 of active chlorine 3 hours
pulp consistency 10% 40C
Example 4
Bleaching of a spruce-sulphite pulp for paper according to the
invention
first stage = 1.5% of H2O2
3.0% of NaOH 1 hour
pulp consistency 10% 70C
washing
second stage = 0.4~ of NaClO2
0.5% of HNO3 3 hours
pulp consistency 10% 50C
no washing
third stage = 1.0~ of H2O2
1.5% of NaOH 3 hours
pulp consistency 10% 60C
` washing
Results
Example 3Example ~
(invention)
white content (bleached) 90.3 90.7
yield ~ 92.5 93.1
At a slightly higher white content and a slightly better
yield the chlorine content in the effluent is reduced by the pro-
cess according to the invention from 6~o to O.16% relative to pulp
applied.
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Example 5
Bleaching of a spruce-sulphite pulp according to the invention
pre-treatment = 0.3% of MgSO4 10 minutes
pulp consistency 5% 15C
concentrating to a pulp consistency of 10%
first stage = 1.3% of H2O2
3.0% of NaOH l hour
pulp consistency 10% 70C
washing
second stage = 0.4% of NaClO2 3 hours
pulp consistency 10% 30C
third stage = 1% of H2O2
1.5% of NaOH 3 hours
pulp consistency 10% 60C
The bleached pulp attained a white content of 91.0%.
By pre-treatment with MgSO4 the use of peroxide in the first
stage could be reduced to 1.3% of H2O2.
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