Note: Descriptions are shown in the official language in which they were submitted.
~28~ 7
Process for preparing bleached pulp out of lignocellulosic
raw material
The present invention relates to a process for preparing
bleached pulp out of lignocellulosic raw material. According
to said process, the raw-material is first treated with a
pulping liquor containg oxidizing components, whereupon the
pulp is bleached.
The preparation of fully bleached pulp i5 nowadays carried
out by means of methods which call for the use of sulphur-
and chlorine-containing chemicals. These have a detrimental
environmental influence, the minimization of which forms an
important cost factor in industry. The environmental re-
quirements of ever increasing stringency are obviously
going to raise said costs even further in the future. It is
conceivable that the efforts to provide less contaminating
pulping processes during the last few years have been the
objects for constant interest.
Non-polluting pulping has been aimed at by using closed
processes. Furthermore, there have been efforts to find
chemicals, the use of which is accompanied with a diminished
adverse environmental influence, when compared with conven-
tional pulping chemicals, and which would allow the process
to be closed even better.
The closing of the process in the first stage of conventional
pulping, i.e. during cooking, has to rather a high degree
already been achieved for the present pulping processes.
This is true especially for the chemical circulation of the
main process, i.e. the kraft process. In this context, the
formation of malodorous sulphur-containing volatile compounds
which exhibit an adverse environmental influence is, however,
still a problem.
The closing of the process of the second stage of pulping,
i.e. bleaching, is difficult. The present bleaching processes
are mainly based on the use of chlorine and compounds there-
( of, but lO0 % recirculation of bleaching waste-liqours
containing chlorine compounds in the process has proved to
be very difficult mainly due to corrosion problems. The
various detrimental chlorine compounds formed during
bleaching are therefore to rather a large extent emitted
to the environment.
In pulping, the chemicals which cause a minimum of pollution
and which are preferable when it comes to the closing of the
process, have met an ever increasing interest, as the
possibilities of the present pulping processes for meeting
the stricter environmental requirements have been shown to
be limited. Industrial applications have not yet been
reached by means of this way, although the matter is largely
investigated around the world today. The new methods are
expensive and this has frequently been mentioned as the rea-
son for their not being widely used.
Pulping which causes a minimum of environmental pollution is
best reached when the chemicals used therein contain carbon,
hydrogen, and oxygen only. Many of the new methods tested
do not, however, fulfil this requirement; they may contain
e.g. nitrogen which is an unpredictable element when it
comes to the environmental influence thereof. It may even
prove to be very troublesome.
2s ~he first stage of pulping, i.e. cooking, using chemicals
containing carbon, hydrogen, and oxygen only, has mainly
been investigated in the connection with so-called organo-
solv pulping methods. Said methods are based on the use of
organic solvents. These usually contain some compound
which is called catalyst, which enhances the degradation of
lignin and the conversion thereof into a dissolving form.
The drawback of said organosolv-methods lies in their rela-
tively low delignification capacity and the difficulty of
pulping softwood. Furthermore, the catalysts oftentimes con-
tain undesirable elements, such as chlorine or sulphur.
... i
This makes the non-polluting character of the method
questionable when larger amounts of catalysts are employed.
However, it has been discovered that the dissolving of lignin
in certain organosolv-solvents, the carboxyli~ acids, may be
significantly increased when a lignin-oxidizing chemical,
e.g. hydrogen peroxide, is used instead of said catalyst. If
hydrogen peroxide is added to a liquid carboxylic acid, such
as acetic or formic acid, lignin is oxidized to dissolve in
carboxylic acid. Hydrogen peroxide forms together with
carboxylic acids peroxyacids, which in an acid system have a
strong influence on lignin but much less influence on the
other components of wood. A selective delignification is
thus achieved by means of which the wood fibres may be
separated from each other, i.e. pulp is obtained.
The use of a mixture of carboxylic acid/peroxide in the
preparation of paper and paperboard pulp has been disclosed
in the SU Patent Specification 761647 published in 1980.
According to said patent, cooking is carried out at normal
pressure in a temperature below 100C. No data as to the
species of wood nor to the bleaching of the pulp are given.
Due to the huge amount of peroxide required in the pulping
in accordance with said patent, a later patent tSU Patent
Specification 821614, published in 1981) arrived at a
process in two stages, wherein the wood chips are initially
treated with carboxylic acid without peroxide at a tempera-
ture of 90C to 95 C, whereupon the acid/peroxide treatment
is conducted. No more detailed data as to the wood species
used, or of the bleaching of the pulp are given. Another
pulping process in two stages, wherein the raw material is
treated with carboxylic acid, is disclosed in US Patent
Specification No. 3 458 394. A pulping liquor containg
acetic acid and chlorine dioxide is used in said process.
The lignin separated and reacted due to the action of the
peracetic acid formed in the liquor is removed from the pulp
during the second stage of the pulping by means of a dilute
alkali solution. According to the publication, the yield
is from 75,6~% to 86,0 %.
According to the processes disclosed in said patents it is
possible to obtain chemical defibration of the wood, i.e.
to obtain pulp. However, the product as such is not suitable
for the preparation of most of the paper and paperboard
qualities due to the unsufficient degree of brightness. The
brightness of the pulp has to be raised to a required level
by means of conventional bleaching methods employing
chlorine chemicals. This results, however, in the formation
of polluting chlorine compounds, whereby the advantage over
conventional pulping offered by the carboxylic acid/
peroxide-process is lost.
Chlorine-free bleaching methods are known per se. Thus,
during the recent years great expectations have been focused
on ozon bleaching employing as its main bleaching chemical
an o~on containing gas dissolved in a suitable medium.
Hydrogen peroxide has been used in the bleaching of cotton,
linen and wool based textiles for long. It has also to some
extent been used in the bleaching of mechanical pulp and
for the bleaching of pulp from sulphite and sulphate proces-
ses. In last mentioned applications, it has not been used toany larger extent, since it is not possible to obtain as
high a brightness or viscosity as in multistage chlorine
bleaching. Said methods are exemplified by the solution
mentioned in the FI Patent Specification No. 6868~. Said
publication discloses a bleaching process in two stages for
bleaching of sulphite pulp, in the first stage of which
hydrogen peroxide (0,2 - 3,0 % by weight) and peracid pre-
pared out of an organic carboxylic acid (0,1 - 5,0 ~ by
weight) is employed, and in the second stage of which an
alkaline peroxide treatment is conducted by adding aqueous
alkali to the liquor obtained from the first stage.
One ob~ect of the present invention is to offset the draw-
backs related to the known pulping and bleaching processes
and to provide an entirely novel process for preparing pulp
directly out of lignocellulosic raw material essentially in
two stages. The invention is based upon using in a first
stage a pulping liquor containing peroxyacids derived from
carboxylic acids and subsequently treating the defibrated
pulp thus obtained with an alkaline solution of peroxyacids
in a second stage.
Accordingly, the process in accordance with the comprises
the combination of defibering the cellulosic raw material by
means of a cooking liquor containing as active agent organic
peroxyacids, and bleaching the defibred pulp with an alkal~i-
ne solution containing hydrogen peroxide, the pH of which is
at lea5t lO in the beginning of the treatment, and which is
provided by adding hydrogen peroxide to an alkali solution.
The amount of hydrogen peroxide (calculated as per cents of
the dry weight of the matter coming to the treatment) cor-
responds to from 0.20 to 0.80, preferably from 0.25 to 0.70
and most preferably from 0.45 to 0.65 times the kappa number
of the pulp obtained from the first stage of the process.
The liquor employed in the first stage containing
peroxyacids is preferably provided by adding hydrogen per-
oxide to an organic carboxylic acid, e.g. formic, acetic,
propionic, or butyric acid.
The process according to the invention forms, in spite of
the provision of two stages, an integrated delignification
system, the stages of which function synergetically. In
this context it should be mentioned that it is not possible
to raise the brightness of, e.g. unbleached pulp from the
kraft process to as high a level as that of the carboxylic
acid/peroxide-pulp by separately employing the peroxide
bleaching process of the present invention. As shown in the
examples lA and lB hereinafter, the brightness of the pulp
in accordance with the invention is about twice as high as
that of the unbleached kraft pulp after treatment with an
alkaline peroxide` solution.
By means of the invention, significant advantages are
achieved. Thus, fully bleached pulp is obtained by using
39L5~
delignification chemicals containing carbon, hydrogen and
( oxygen only. The sodium or corresponding alkali or earth
alkali metal present in the alkali added in the second
stage is the only inorganic material to be added. Environ-
mentally polluting chemicals are not used at all. The pulps
obtained exhibit high brightness and sufficiently high
viscosity. The yield is comparable to that of bleached kraft
pulp .
The apparatus required by the process is simple. The acid
stage as well as the alkaline one may be conducted at normal
pressure, since there is no need for raising the reaction
temperatures above the boiling point of the mixture. The low
temperatures result in huge savings in energy consumption.
The present delignification process in two stages may be
used for delignification of all kinds of lignocellulosic
materials. It is particulary well suited for chips of
hardwood and softwood commonly employed in pulping. De-
lignification of hardwood proceeds somewhat better than that
of spruce and pine chips. Annual plants, grass, straw and
bagasse may also be used as raw materials.
The first stage of the process, i.e. the peroxyacid stage,
is preferably conducted by using a mixture of some liquid
carboxylic acid and hydrogen peroxide. As known, due to the
influence of hydrogen peroxide, the carboxylic acids are
oxidized to peroxyacids which form the active agents of the
cooking liquor.
The peroxyacid reacts with lignin, and the oxidization
products formed from this reaction dissolve in the solution
of carboxylic acids. The various carboxylic acid have a
different capability of forming peroxyacids, i.e. their
oxidative capacity differs. In principle, no carboxylic acid
may be excluded from the process. Aliphatic as well as
aromatic acids are suitable. In practice, formic and acetic
acids are particulary preferred, of which the first mentio-
~l~8~S~
ned acid more easily forms peroxyacid, and it is thereforein this sense preferred. The pulp obtained when using formic
acid in the first stage is darker than that obtained by
using acetic acid, on the other hand, the viscosity of the
pulp is lower when the last-mentioned acid has been employed.
In addition to aforementioned acids, propionic and butyric
acids may also be used.
The formation of peroxyacids may be enhanced by using some
known catalysts, e.g. sulphuric, phosphoric or boric acids,
but this is neither necessary nor always even preferable.
It has been discovered that, eOg., sulphuric acid may degrade
the carbohydrates during the cooking, which leads to a
decreased viscosity of the pulp.
In the process, the concentration of the carboxylic acid
lS employed is in the range of 40...100 %, preferably in the
range of 70...100 %, the liquid-to-wood ratio being from 2:1
up to 10:1. The ratio 8:1 normally used in pulping may be
used, it is, however, preferable to use a smaller liquid-
ratio, e.g. 4:1, because said ratio makes it possible to
decrease the required amount of hydrogen peroxide and
enhance the progress of the delignification reaction. Pulp
yield and brightness of the pulp are also improved by a
smaller liquid-to-wood ratio. On one hand, when it comes to
recovery of the formic acid, it would be preferable to use
an azeotropic formic acid-water solution, containing 80 % of
formic acid, but on the other hand, when it comes to de-
lignification, it is preferable to use an acid containing
as small an amount of water as possible.
Alternatively, the peroxyacids may be formed in the solution
in some other known manner, e.g. by reacting the
corresponding aldehydes with molecular oxygen or an oxygen-
ozone mixture. In this case, the carboxylic acids needed for
the dissolution of the reaction products are formed as
decomposition products of the peroxyacids. The peroxyacids
may also be formed in the cooking liquor prior to the
addition of the cellulosic raw material.
The peroxide used may comprise, e.g., an industrially
prepared hydrogen peroxide solution, the concentration of
which is 50 %. The concentration of the peroxide may, how-
ever, be higher or lower, i.e. from about 30 ~ up to 90 % by
weight. The amount used varies within a large range depending
on the extent to which the delignification is to be conducted
during the acid stage. It is preferable to achieve a de-
fibration of the chips before the second stage. The amount
of peroxide used in the examples has been varied from 5 ~ up
to 60 % based on the oven dry (o.d.) weight of the chips,
but said values are no absolute limits. It has been ascer-
tained that, especially for birch raw material, delignifica-
tion is achieved when using a liquor containing l % by
weight of hydrogen peroxide. Due to the relatively high
price of the hydrogen, it is preferable to use as small
amount thereof as possible. The usefulness of the process
is demonstrated by the fact that by employing 20 % of hydro-
gen peroxide a birch pulp is obtained, the kappa number of
which is on the order of about 20. By carboxylic acid pre-
treatment of the raw material as described hereinafter, itis possible to further decrease the consumption of the hyd-
rogen peroxide without letting the kappa number of the pulp
rise to a hlgher level.
Water present in the carboxylic acid, in the peroxide as
well as in the chips, is introduced into the cooking liquor.
As mentioned above, the water content of the acid influences
especially the dissolution of the oxidized lignin. Thus, it
should be kept as low as possible.
The acid peroxy-treatment may be conducted at any tempera-
ture between ambient temperature and the boiling point of
the system used. However, high temperatures are not suitable
due to the fact that the peroxyacids are decomposed when the
temperature is raised, whereby the deligni~ic tion capacity
of the cooking liquor is lost. Thus, the oxidizing capacity
of a pure formic acid/peroxide mixture disappears altogether
within an hour when the temperature is 80C. The oxidizing
capacity of the mixture of acetic acid and peroxide is
conserved rather a long time even at 95C, but this is due
to the fact that the rate of peroxyacid formation is slower
for acetic acid than for formic acid. Accordingly, the
cooking temperature to be selected depends on the acid used.
For formic acid the temperature is preferably in the range
of about 70- 90C, and for acetic acid somewhat higher.
For propionic and butyric acids still higher temperatures
may be used.
It is also possible to carry out the cooking such that the
cooking liquor is heated for some time at a higher tempera-
ture, e.g. at about 90C, whereupon the actual cooking is
conducted at a lower temperature of, e.g., from 70C up to
75C.
Cooking time may also vary within a large range depending on
the temperature. At ambient temperature the treatment lasts
for several days, whereas it may be as short as half an hour
near to the boiling point of water. Too long a treatment
time at high temperature lowers the viscosity of the pulp.
Depending on the optional pretreatment, the acid used and
the temperature of the cooking, the cooking time is from 2
hours up to 10 hours, preferably from 4 hours up to 6 hours.
It is preferable to impregnate the chips with the treatment
liquor either ln vacuo or by means of pressure before
starting the actual treatment. The impregnation may be
conducted at ambient temperature. It has been discovered
that a delignifying pretreatment of the chips decreases
considerably the amount of hydrogen peroxide needed in the
peroxyacid treatment stage, thus reducing the actual cooking
time. This is true especially for birch raw material. The
pretreatment may be conducted at ambient temperature or
preferably at increased temperature by means of some chemi-
cal, e.g. an alkali solution or preferably formic acid as
taught by the SU Patent Specification No. 821614. The acid
treatment is carried out, e.g., at the boiling point of the
acid, whereat prolonging of the pretreatment time lowers
the kappa number. As pretreatment acid spent liquor of the
peroxyacid cooking may be used. In an alkaline pretreatment
the alkali solution used may be the same as in the second
phase of the process. By means of the alkali treatment it is
possible to reach an extremely high brightness.
A preferred embodiment of the first stage of the invention
comprises pulping the chips first in peroxyformic acid,
thereafter in formic acid and subsequently again in peroxy-
formic acid. Delignified pulp (kappa number on the order oflO) is easily obtained by said procedure. Peroxyacetic acid
pulping may also be conducted in a similar manner.
After the first phase, the defibrated pulp is preferably
washed with water such that the pulp obtained is at least
approximately neutral. The acid of the spent liquor obtained
from this phase may be re-used after recovery. During reco-
very the acid is separated from the solid substance, e.g.,
by means of distillation. Formic acid and water form an
azeotropic mixture which boils already at 107C. This azeo-
tropic mixture contains about 80 ~ of formic acid and may bere-used as such.
The second phase of the present pulping process, the
alkaline peroxide treatment, may be conducted by using a
water-soluble alkali and hydrogen peroxide. The amount of
the alkali depends on the amount consumed during the treat-
ment. The pH value of the treatment liquor shall initially
exceed lO, the required amount being dependent upon the
amount required for reaching said pH level. It has been
ascertained that acceptable results are reached already at
pH values slightly in excess of lO. Higher pH values may
of course also be employed, but when it comes to the bleach-
ing result or the economy of the chemicals, it is not sen-
sible to increase the alkalinity to a high level. It has
further been ascertained that the amounts of alkali and
hydrogen peroxide are somewhat interdependent. The more
.
ll
hydrogen peroxide is used, the more alkali is needed. During
the treatment the pH of the liquor decreases to some extent,
usually about one pH unit.
The alkaline peroxide phace may also be accomplished such
that the calculated amount of peroxide is used in several
portions, e.g. in 3 to 6 portions. The bleaching liquor is
removed from the pulp after each phase. When the bleaching
is carried out as a conventional bleaching, the pulp is
preferably washed between the phases of treatment, but if
the bleaching is conducted as a so-called displacement
bleaching this is not necessary. ~ multiphase bleaching
has been discovered to have a preferable influence on the
brightness of the pulp. The viscosity of the pulp remains
high at the same time.
The alkali used may be selected from the group consisting of
alkali metal and earth alkali metal hydroxides, carbonates
and bicarbonates. Alkali metal hydroxides and carbonates are
preferred, particulary preferred are sodium hydroxide and
sodium carbonate. The alkali may also comprise mixtures of
said compounds. It is preferable to use sodium hydroxide
because by means of it minor amounts already result in the
required pH value. Sodium carbonate on the other hand is
suitable in the sense that it may be obtained by calcination
of the spent liquor from the alkali phase. Because the pulp
treated in the second phase is almost neutral, no carbon
dioxide is evolved from the carbonate, which might make the
treatment more difficult.
The temperature of the treatment may vary, but e.g. the
common temperature, 80C, used in peroxide bleaching of
conventional pulps may also be used in this context. The
duration of the treatment varies according to the
temperature. At 80C the suitable time is about 1 hour.
Generally, a peroxide residue after this stage on the order
of 0.2 % of the pulp should be aimed at.
i'7
The required amount of peroxide depends on the kappa number,
i.e. the lignin content of the pulp coming to said stage.
Generally, the amount is from 1 % up to 20 % by weight. It
has been discovered empirically that the added amount of
hydrogen calculated as per cent of the dry weigh of the mat-
ter coming to the treatment amounts to about 0.20 to 0.80,
preferably 0.25 to 0.70, most preferably 0.45 to 0.65 times
the kappa number of the pulp from the first phase.
Within the scope of the invention, the second phase may be
accomplished in a different manner also. Thus, the alkali
and the hydrogen peroxide required may be substituted by a
peroxide derivative., e.g. a metal peroxide, preferably
sodium peroxide, which dissolved in water forms hydroxide
and hydrogen peroxide~
To prevent heavy metals possibly introduced into the system
from exhibiting a hydrogen peroxide decomposing effect one
or a few peroxide stabilizers may be added. ~uring the per-
oxyacid stage citric acid may be used, whereas e.g. diethy-
lene triaminepenta acetic acid (DTPA) or/and one magnesium
salt, e.g. Mg sulfate, may be used during the alkaline
peroxide stage. The amount of the stabilizer is preferably
on the order of a few promilles.
The invention will now be examined in more detail by means
of the following non-limitative practical examples. The
brightness values indicated in the examples have been deter-
mined according to the SCAN C-ll method and the viscosity
values correspondingly according to the SCAN C-15 method.
The kappa numbers have been measured by the SCAN C-1:77
method. All of the indicated per cent amounts are expressed
by weight.
EXAMPLE lA
50 g pine chips (dry matter 92 %) calculated od., was
admixed with 250 ml formic acid, 75 ml water, and 60 ml
50 % hydrogen peroxide solution (corresponding to 60 %
peroxide of the dry weight of the chips). The chips were
impregnated with the mixture in vacuo for 30 minutes,
whereupon the temperature was raised to 70C within 2.5
hours. The cooking was carried~out at a cooking temperature
of 70 C to 75C for 2.5 hours. The cooking liquor was
removed from the soft chips by filtration, the chips were
washed to some extent with water, whereupon it was defibered
with a Waring Blendor laboratory blender. The defibration
time was 30 sec using the smallest effect of the apparatus.
After the defibration the pulp was washed with water and the
shives (0,8 %) were removed. The kappa number of the pulp
was 11.3.
The pulp dried at ambient temperature was treated with an
alkaline hydrogen peroxide solution at 80C for 2 hours, at
10 per cent stock, the added amount of NaOH being 5 % and
the amount of hydrogen peroxide being 7,3 % (the amount of
peroxide = 0.65 x kappa number). The initial pH value was
10.3, and it decreased to a value of 9.4 during the treat-
ment. The stabilizing agent comprised 0.2 % of DTPA. After
the treatment, the pulp was washed and acid was added,
whereupon the pulp was dried. The final brightness of the
pulp was 89.0 %, the viscosity 830 cm3/g, and the pulp yield
in per cent of the raw material 44.4 %.
EXAMPLE lB (control)
Unbleached spruce kraft pulp, the kappa number of which was
31,9, was bleached at 10 per cent stock at 80C for 30 minu-
tes. The amount of NaOH was 3 % of the pulp, the amount of
hydrogen peroxide being 20.7 % (= 0.65 x kappa number), and
the pH value 10.5. In spite of the short treatment time the
peroxide was completely exhausted. The brightness of the
pulp thus obtained was 46.5 % and the viscosity 750 cm3/g.
EXAMPLE 2
As example lA, but birch chips (dry matter 90 %) were used
instead of pine. The cooking was carried out departing from
the foregoing such that the temperature was raised to 70C
during 5 hours, whereinafter the temperature was raised to
80C within one hour and the cooking was finished. The
shives amounted to 3.1 %, and the kappa number of the pulp
was 5.3. The alkaline hydrogen peroxide treatment was con-
ducted at 80C, the duration being one hour. The stock was
10 % and the amount of NaOH was 5 % and of hydrogen peroxide
3 % (peroxide = 0.57 x kappa number). The pH of the solution
was 10.4. Analysis of the pulp: brightness 89.0 %, viscosity
1050 cm3/g.
EXAMPLE 3
As example 1, but spruce chips (dry matter 93 %) was used
instead of pine. The temperature of the cooking was raised
to 80 C within 2.5 hours, and left at said temperature for
2.5 hours. Shives: 11.4 % and the kappa number of the pulp
14Ø The alkaline peroxide treatment was conducted at 10
per cent stock during 2 hours. The added amounts were: 5 %
of NaOH and 4. % of hydrogen peroxide (- 0.32 x kappa
number). The pH of the solution was 10.8 %. Analysis of the
pulp: brightness 84.3 %, viscosity 920 cm3/g and pulp yield
43.1 % (calculated on the wood).
EXAMPLE 4
As example 1, but acetic acid was used instead of formic
acid. During the cooking the temperature was raised to
85 C within 1.5 hours, and the cooking liquor was left at
said temperature for 2.5 hours. Shives: 5.5 % and the kappa
number of the pulp 18. The alkaline peroxide treatment was
conducted as in example 3, the amount of hydrogen peroxide
0.25 x kappa number, the pH was 10.9 %. Analysis of the
pulp: brightness 85.2 %, viscosity 740 cm3/g and the pulp
yield 48.8 % (calculated on the wood).
57
EXAMPLE 5
As example 1, but the pine chips was pretreated before the
peroxide cooking for one hour at 90C with 80 ~ formic acid.
The cooking was carried out by raising the temperature with-
in 3 hours to 75C, whereupon the cooking was finished.There were no shives. The kappa number was 7.3. The alkaline
hydrogen peroxide treatment was conducted at 80C, the dura-
tion being two hours at 10 % stock. The amount of NaOH was
5 % and the amount of hydrogen peroxide 4.5 % (= 0.62 x
kappa number). The pH of the solution was 10.1. Analysis of
the pulp: brightness 8~.0 %, viscosity 900 cm3/g and the
pulp yield (calculated on wood) 43.9 %.
EXAMPLE 6
As example 2, but the birch chips were pretreated prior to
the peroxide cooking for one hour at 1~0C with an alkali
solution containing 6 % o~d. wood NaOH. The cooking was car-
ried out using the peroxide application rate 20 % per o.d.
wood. The temperature was raised within 4 hours 20 minutes
to 75C, the cooking being maintained at 75C to 80C for
2 hours. The amount of shives was 12.6 ~, and the kappa
number was 14.5. The alkaline peroxide treatment was con-
ducted at 10 per cent stock during 1 hour. The NaOH applica-
tion rate was 6 %, the amount of hydrogen peroxide being 8 %
(= 0.55 x the kappa number). The pH of the solution was
10.8. Analysis of the pulp gave: brightness 86.5 %, viscosi-
ty 1000 cm3/g and the pulp yield 43.2 % tcalculated on the
wood).
EXAMPLE 7
As example 6, but the temperature of the alkaline peroxide
treatment was 60 C and the duration time 2 hours. Analysis
of the pulp gave: brightness 86.5 ~, viscosity 1080 cm3/g
and the pulp yield 43.5 ~ (calculated on the wood).
5S'7
16
EXAMPLE 8
As example 2, but the liquor-to-wood ratio was lowered and
the amount of hydrogen peroxide was decreased. 50 g birch
chips (dry matter 90 %) calculated o.d., was admixed with
200 ml formic acid. Water was not added at all, and the
amount of 50 % hydrogen peroxide solution was decreased
from 60 ml (in example 2) to 10 ml, which corresponds to
10 % peroxide of the dry weight of the chips. The liquor to
wood ratio was thus 4:1 (instead of 8 1 ) o The reaction mix-
ture was heated to 78C, maintained at said temperature
for 3.5 hours. The kappa number of the pulp was 51.4, the
viscosity 1170 cm3/g, the screened yield being 52 9 % and
the amount of shieves 7.2 %.
The pulp obtained was bleached with an alkaline solution of
hydrogen peroxide such that the calculated amount of hydrogen
peroxide, 30.8 % of the pulp (16.3 % o.d. wood, i.e. 0.60 x
the kappa number), was added in three portions. The tempera-
ture during all stages was 80C~ the reaction time 1 hour,
and the initial pH value 10.5. The stabilizer added compri-
sed 0.2 % DTPA. The brightness of the pulp was 90.3.
The total consumption of hydrogen peroxide was in this
example 10 -~ 16.3 %, i.e. 26.3 % of the o.d. chips.
EXAMPLE 9
50 g pine chips were refluxed in 250 ml of 100 % formic acid
for three hours. The pretreatment liquor was removed, the
peroxyformic acid cooking was subsequently carried out with
a cooking liquor of 200 ml 100 % formic acid to which 10 %
hydrogen peroxide had been added. The temperature of the
cooking was raised to 80C, the total cooking time being
three hours. After the cooking the pulp was washed with
water until the washing water was neutral. The kappa number
was 31.1 at this stage, the viscosity was 1060 cm3/g, the
brightness 19.5 and the yield 43.3. There were scarcely any
shives in the pulp.
.
5~7
Test carried out on the pulp showed that it was easily
bleached by means of an alkali hydrogen peroxide solution.
EXAMPLE 10
As example 8, but the amount of peroxide used was only 5 %
of the chips, i.e. 5 ml of 50 % hydrogen peroxide was added.
The kappa number of the pulp was 62.2 %, the viscosity 1070
cm3/g, the screened yield was 45.8 % and the amount of shives
19.7.
The pulp obtained was bleached as described in example 8,
the calculated amount of hydrogen peroxide, 28 ~ of the pulp
(12.8 % o.d. wood, i.e. 0.45 x the kappa number), was also
this time added in three portions. The brightness of the
pulp was 87.0 %.
The total consumption of hydrogen peroxide in this example
was 5 + 12.8 %, i.e. 17.8 % of the o.d. chips.
EXAMPLE 11
As example 6, but the birch chips were pretreated before the
peroxyacid cooking by boiling them for 3 hours in 80 % for-
mic acid under reflux conditions. The cooking liquor was re-
moved, the peroxyformic acid cooking being subsequently car-
ried out employing 85 % formic acid to which 5 % hydrogen
peroxide of o.d. wood only had been added. The temperature
was raised in 50 minutes to 80C and maintained at said
temperature for 1 hour. After the cooking the pulp was was-
hed to neutrality with the required amount of hot water.
The kappa number of the pulp was at this stage 14.7 and the
viscosity 1180 cm3/g and the yield 42.3 %.
The pulp obtained was bleached with an alkaline solution of
hydrogen peroxide such that the calculated amount of hydro-
gen peroxide, 8 % of the pulp (3.4 % o.d. wood, i.e. 0.55 x
the kappa number), was added in four portions: the duration
of the first two stages was 1 hour, the duration of the two
1~
last ones was 2 hours. The temperature during all stages was
80Cr and the initial pH value lQ.7. The stabilizer added
comprised 0.2 % DTPA and 0.5 % (calculated on o.d. wood) Mg
sulfate. The brightness of the pulp obtained was 90.1 %, and
the viscosity 1140 cm3/g.
The total consumption of hydrogen peroxide was in this
example 5 + 3.4 %, i.e. &.4 % of the o.d. chips only.
EXAMPLE 12
50 g spruce chips (dry matter 92,6 %) calculated per o.d.
wood, were heated in a mixture of 200 ml 100 % formic acid,
and 5 ml 50 % hydrogen peroxide. The total time of heating
was 2 hours 45 minutes and the maximum temperature 75C.
The spent liquor was filtered off and the somewhat softened
chips were refluxed for 3 hours in 250 ml of 100 % formic
acid. The cooking li~uor was removed and the chips were
defibred in formic acid. Subsequently, an other peroxyacid
cooking was conducted, comprising as cooking liquor 200 ml
of 100 % formic acid, to which 5 % of hydrogen peroxide had
been added. The mixture was heated for 3 hours 30 minutes,
the maximum temperature being 75C. ~fter the cooking, the
pulp was first washed with formic acid and thereafter with
water. The ~appa number of the pulp was 9.0, the ~iscosity
980, the brightness 35.1 and the screened yield 41.4 %. The
amount of shieves was only 0.2 %.
The pulp obtained was bleached with an alkaline hydrogen
peroxide solution such that the calculated amount of hydro-
gen peroxide, 6 % of the pulp (2.5 % o.d. wood), was added
in three portions. The duration of the first stage was 1
hour, of the second one 2 hours, and of the third one 3
hours. The temperature was 80 C. The stabilizers were
analogous to those employed in example 11. The brightness
of the pulp obtained was 90.5, and the viscosity 940.
The total consumption of hydrogen peroxide was in this
example 10 + 2.5 ~, i.e. 12.5 % of the o.d. chips.
