Note: Descriptions are shown in the official language in which they were submitted.
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3332 ~28
PROCEDURE FOR DELIGNIFICATION OF SULPHATE PULP
The present invention relates to a procedure for the
manufacture of unbleached or bleached sulphat:e pulp, whereby a
chemical cellulose pulp after digestion but hefore any subsequent
bleaching is delignified with an alkaline polysulphide solution.
Delignification of pulp with an alkaline polysulphide
solution can take place in milder conditions than prevail in a
normal sulphate digestion and imply that the carbohydrate yield
and pulp viscosity at a certain lignin content will be higher than
if a corresponcling delignification had been achieved through Eur
ther cooking. To att~in thi~ eE~ect it is necessar~ either Eor the
pulp to have been treated prior to delignification with polysul-
phide in order to remove metal ions (e.g. washing with acid) or
for the polysulphide solution to contain a stabilizer for polysul-
phide.
In the manufacture of bleached sulphate pulp it is
desirable for reasons of environmental conservation to perform the
delignification in the closed part of the plant as ar as possible
so as to limit emissions from the plant. The closed part of the
~ 20 plant comprises digestion and, where provided, oxygen bleaching.
;~ The waste liquors from these stages containing consumed chemicals
and liberated wood substance are returned via the washing stages
to evaporation and chemical recovery.
The quality requirements imposed on the pulp set a
limit, however, to how far the delignification can be taken in
- cooking and oxygen bleaching. The final delignification in the
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~ manufacture of bleached pulp i5 normally perEormed with the aid of
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chlorine chemicals and results in emission of liberated wood sub-
stance and residual chemicals with the effluentO
Increased delignification through so-called modified
digestion implying a controlled variation of the contents of chem-
icals during the digestion process is a recently introduced method
for reduction of environmental impact in the manufacture of
bleached sulphate pulp. According to the invention the polysul-
phide treatment is carried out after sulphate digestion regardless
of how this is elaborated and at lower temperatures than used for
digestion. Moreover, in accordance with the proceclure according
to the invention actions are taken to stabilize polysulphide.
S. Rydholm, Pulping Processes, Interscience Publishers,
New York, 1967, p. 642 and G.C. Smith, S.E. Knowles, R.D. Green,
Paper Trade Journal 38 (1975), p. 13, 159, indicates that the
addition of polysulphide to sulphate cooking liquid ~white liquor)
results in a higher pulp yield in sulphate digestion, thanks to
stabiliYation of the carbohydrates. If polysulphide is added
during cooking, the carbohydrate stabilizing reactions take place
during the initial phase or impregnation phase of the cook, i.e.
before the maximum cooking temperature has been reached. In conse-
quence of the instability of the polysulphides, the sulphide in
the cooking liquid will no longer appear in the form of polysul-
phide at the end of the cook but as monosulphide, i.e. sulphide
and hydrogen sulphide ions.
According to the present invention polysulphide is used
to accomplish further delignification of sulphate-cooked pulp and
in a manner that counteracts reversion of polysulphide to monosul-
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phide. The object of using polysulphide in this way is in fact
to achieve an increased carbohydrate yield.
U.S. Patent 3,716,448 discloses that polysulphide can be
used as an additive in the actual oxygen reactor in oxygen bleach~
ing in order to reduce depolymerization of the cellulose. This,
however, has proved not to function satisfactorily in practice.
Normally, white liquor is used as the source of alkali
in oxygen bleaching. According to standard procedure, the white
liquor is oxidized before being added to the pulp in oxygen
bleaching so as to transform sulphide into thiosulphate. It is
well known that the presence oE sulphide ions during oxygen
bleaching affects the ratio between lignin and carbohydrate
di~solution negativel~. This i9 expressed in a lowered viscosity
of the pulp.
It has also been proposed by A.G. Kirkman, E.K. Andrews,
H.M. Chang, AIChE Annual Meeting, Los Angeles, ~ovember 1982, to
render the oxygen bleaching more efficient through pretreatment
of the sulphate pulp with green liquor. The principal ingredients
of green liquor are sodium sulphide and sodium carbonate and the
content of polysulphide is negligible.
The procedure according to the present invention seeks
to attain a lower lignin content than normal, with retained high
pulp ~iscosity, prior to bleaching with chlorine chemicals. In
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this procedure, after boiling at a temperature between lOO~C and;
160C the pulp is treated with a solution containing 0.2-0.6 mol/l
` effective alkali, 0.5-5 per cent by weight sulphur of the weight
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of the pulp in the form of polysulphide, i.e. ~ia2Sx, where x is
2 or higher, and in the presence of a stabilizer for polysul-
phide.
The treatment can be combined advantageously with oxygen
bleachin~, resul-ting in a beneficially low lignin content before
chlorine bleaching. The process of this invention makes it possi~
ble when manufacturing bleached pulp, to lower the lignin content
of the pulp before bleaching with chlorine chemicals to a lower
level than is possible with the traditional sulphate digestion
technique without the viscosity becoming too low. Treatment of
sulphate pulp with alkaline polysulphide according to the process
of the invention before oxygen bleaching makes it possible to
attain a lower lignin content after oxygen bleaching than normal,
with retained pulp quality. The polysulphide treatment results in
a lowering of the lignin content of the pulp by about 25 per cent.
In subsequent oxygen bleaching, the lignin content can be further
lowered by around 50 per cent.
By carrying out the polysulphide treatment in accordance
with the invention in a new stage of the manufacturing process for
sulphate pulp, namely after digestion but before oxygen bleaching,
and through addition of a stabilizer for the polysulphide, it has
; proved possible to attain a selective delignification, i.e. a good
dissolution of lignin with a significantly reduced dissolution and
degradation of carbohydrates.
Magnesium salts, e.g. magnesium sulphate, or complexers,
e.g. ethylenediamine tetraacitic acid (EDTA) are used as stabil-
izing substances.
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In the manufacture of sulphate pulp in a continuous
digester it is possibLe to introduce the polysulphide treatment
without having to install a separate reactor. In a continuous
digester with a washing zone the conditions with regard to temper-
ature, dwell time and liquid module prevailing in the washing zone
are such as are favourable for the polysulphide treatment. In such
a digester, the treatment can be performed by adding alkali, poly-
sulphide and stabilizer to the washing liquid which is conducted
into the washing zone.
In the manufacture of sulphate pulp in a batch digester,
the polysulphide treatment according to the process of this inven-
tion can be performed in conjunction with pulp wrashing. In this
case, the process equipment must be supplemented with a reaction
vessel for the polysulphide treatment.
Another possibility is to displace the black liquor at
the end of the cook from the batch digester with polysulphide
solution, and then to use the digester to carry out the polysul~
phide treatment.
The polysulphide treatment can also be utilized to
increase the capacity of an existing digester house. The cook is
then interrupted at a higher kappa number than normal, i.e. after
a shorter cooking time than normally, and
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continued delignification is done with polysulphide
solution. This can be utilized regardless of whether the
pulp is to be bleached or not.
For maximum utilization of the chemicals added for the
polysulphide treatment these can be returned to the ordinary
chemical recovery system via the cookinq, whereby the load
on this system is reduced. This means that the filtrate from
washing of the pulp after the polysulphide treatment is
charged together with white liquor at the start of the cook.
In polysulphide treatment in the washing zone in a
continuous digester it is necessary for the removal of poly-
sulphide solution from the digester to be separated from the
removal of black liquor.
In order to further clarify the invention, a number of
embodiment examples performed on a laboratory scale are
described below. In all the examples, the kappa number and
viscosity determinations have been performed in accordance
with SCAN test.
Example 1
The treatment was performed in an autoclave containing
8 g of dry pine sulphate pulp with a kappa number of 45.3
and 100 ml of an aqueous solution which contained 1.6 g
NaO~, 3.2 g Na2S x 9 ~2~ 39.5 mg MgSO4 and 0.24 g sulphur.
The autoclave was rotated for one hour at a temperature of
135 C. The pulp treated in this manner had a kappa number
of 28.2 and a viscosity of 1208 dm3/kg.
Example 2
A pine sulphate pulp with a kappa number of 32 and a
viscosity of 1177 dm3/kg was treated as follows.
a) Washing with acid followed by polysulphide treatment:
8 g of pulp according to the above was suspended in
- 1.5 1 of water. 40 ml of acetic acid was added while
stirring. After five minutes, the pulp was washed on a
filter with water to a filtrate pH of 5.5. The pulp was then
charged in an autoclave together with 100 ml aqueous
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solution containing 20 g NaOH, 4.0 g Na2S x 9H20 and 0.5 g
sulphur. The autoclave was rotated for 30 minutes at a
temperature of 120 C. The obtained pulp had a kappa number
of 22.7 and a viscosity of 1075 dm3/kg.
b) Polysulphide treatment without prior washing with
acid:
8 g of the pulp with a kappa number of 32 as per the
above was charged in an autoclave together with 100 ml
aqueous solution containing 2.0 g NaOH, 4.0 g Na2S x 9H20
and 0.5 g sulphur. The autoclave was rotated for 30 minutes
at a temperature of 120 C. The obtained pulp had a kappa
number of 26.7 and a viscosity of 1067 dm3/kg.
Example 3
Polysulphide extraction of pine sulphate pulp with a
kappa number of 32 and a viscosity of 1177 dm3/kg.
~ n autoclave containing 8 g of pulp and 100 ml of an
aqueous solution containing 2.~ g NaOH, 4.8 Na2S x 9 H20,
0.4 g sulphur of and 24 mg of ethylenediamine tetraacetic
acid (EDTA) was rotated for one hour at a temperature of
120 C. The obtained pulp had a kappa number fo 23.2 and a
viscosity of 1001 dm3/kg.
Example 4
-Polysulphide extraction of birch sulphate pulp with a
kappa number of 14.5 and a viscosity of 1333 dm3/kg.
An autoclave containing 8 g of pulp and 100 ml of an
aqueous solution containing 2.0 g NaOH, 4.0 g Na2S x 9 H20,
0.32 g sulphur and 39.5 mg MgS04 was rotated for 20 minutes
at a temperature of 120 C. The treated pulp had a kappa
number of 10.9 and a viscosity of 1298 dm3/kg.
Example 5
A pine sulphate pulp with a kappa number of 30.9 and a
viscosity of 1130 dm3/kg was treated as follows:
a) Polysulphide treatment:
50 g of pulp as above was charged in an autoclave
together with 625 ml of an a~ueous solution containing
10.0 g NaOH, 20 g Na2s x 9 H20, 0.25 g MgS04 and 2.0 g
sulphur. The autoclave was rotated for one hour at a
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temperature of 120 C. After the treatment, the kappa number
of the pulp was 23.9 and its viscosity 1124 dm3/kg.
b) Oxygen bleaching:
50 g of pulp treated according to a) above was charged
in an indirectly heated autoclave fitted with an agitator
together with 150 ml of an aqueous solution containing 0.8 g
NaOH and 0~25 g MgSO4. The pulp was treated in the autoclave
for one hour at a temperature of 90 C under an axygen
pressure of 8 bar. The thus treated pulp had a kappa number
of 11.9 and a viscosity of 915 dm3/kg.
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