Note: Descriptions are shown in the official language in which they were submitted.
057~08
The present invention relates to a method of sulphite
pulping of wood.
In particular, the present invention relates to a
method of sulphite pulping in which the wood is impregnated
with the cooking liquid to which has been added a small quantity
of certain esters of formic acid and also desirably formic
acid per se.
In the conventional sulphite pulping of wood the lignin
and the greater part the the hemicellulose are dissolved, and
sometimes also a minor part of the alpha cellulose is dissolved
in the cooking liquid. A substantially pure alpha cellulose
product may be desired, when the pulp is to be used in the
manufacture of viscose cellulose. However, for paper pulp
the removal of the hemicellulose only involves, as a rule, a
loss of yield.
It has now been found that the yield as well as the
strength of the pulp is increased when the cooking liquid
includes a small quantity of at least one formic acid ester
(formate) of hexyl alcohol or of an unsubstituted or methyl-
substituted norbornyl alochol. The formates have the generalstructural formula:
R - O --~ H
where R is a hexyl group or an unsubstituted or methyl-substituted
norbornyl group.
In the process of the present invention at least one
of the aforesaid formates is added before the impregnation of
the wood with cooking liquid, to said liquid in an amount of at
least 0.02 mol, preferably at least 0.1 mol, as calculated per
ton (1000 kg) absolutely dry wood. Generally, the addition of
one of said formates is sufficient, but two or more of them
- 1 - ~
.
105~008
may be added. Preferably, the addition tak~s place immediately
before the introduction of the cooking liquid into the digester.
Before the addition the formate or formates may be dispersed in
water by a non-ionic dispersing agent, and, if desired, the
formates may previously be dissolved in a suitable solvent such
as turpentine, cymene or diethyl benzene.
As mentioned above, the formate or formates must be
added in an amount of at least 0.02 mol. This value may be
regarded as a lower limit, below which no appreciable effect t
can be observed. Among the agents useful as additives according
to the invention endo-fenchyl formate and exo-norbornyl formate
have proved to be especially active.
The method of sulphite cooking according to the present
invention is particularly applicable when using spruce wood as
raw material, but also other kinds of coniferous wood, such as
pine wood, for instance, may be used. Also hardwood may be
used in mixture with coniferous wood.
As suitable formates defined by the aforesaid
structural formula the following may be mentioned:
H O
CH3-(CH2)4-C-O-C-H
hexyl formate ~ I ~ 33
H O
exo-norbornyl formate ~ endo-fenchyl formate
--~ ~CH3
H CH3
exo-isofenchyl formate
~57~08
The way these formates have their favourable influence
on the sulphite cooking has not yet been entirely explained, but
observations hitherto made seem to indicate that the formates
counteract a condensation or polymerization of the lignin. In
the pulping, the cooking liquid primarily brings the lignin of the
wood into solution by sulphonation. In the process the carbon-
oxygen bonds are broken, and reactive and rapidly sulphonatable
intermediate substances of benzylium ion type or with a chinon
methid structure are formed. It is known that the sulphonation
of these substances occurs in competition with lignin condensa-
tion reactions, in which benzylium ions or chinon methids
(according to the conditions) will, with an aromatic substitution,
react with another phenyl-propane unit. In this way a new and
stable bond carbon-to-carbon is formed, and thus the process
results in an increase of the size of the lignin molecules.
Consequently, this lignin condensation counteracts the desirable
dissolving action of the sulphonating cooking liquid.
By thorough investigations it has been found that,
during the conditions prevailing at the sulphite cooking,
the formates are more or less rapidly hydrolyzed into correspond-
ing alcohol and formic acid. It is assumed that the formic
acid obstructs the iignincondensation reaction mentioned above,
and such an influence probably occurs in two different ways.
Firstly, the formic acid may act directly as a reducing agent
thus reducing some of the benzylium ions or chinon methids -
analagousreactions are known. The reaction products then formed
cannot be condensed in the manner as mentioned above with respect
to benzylium ions or chinon methids. The second way, also
dependent on the reducing action of the formic acid, involves
the formation of thiosulphate by reaction between the formic
acid and sulphur compounds in the cooking liquid. Owing to
l~S7(~08
their high nucleophilic capacity the thiosulphate ions then very
rapidly react with the benzylium ions or the chinon methids
which are thus prevented from undergoing the lignin condensation.
Formic acid is normally present in the cooking liquid, t
in varying quantities. Together with the formates added in
accordance with theinvention this formic acid may have a
favourable influence, as long as its concentration is not high
enough to cause a predominance of detrimental secondary effects.
Thus, it may be very suitable to add formic acid to a cooking
10liquid which has a very low initial content thereof. Generally,
however, the supplemental addition of formic acid should be less
than 25% of the weight of the added amount of formate.
It has been found that the best effect of an addition
of the formates is achieved, when the content of total SO2 in
the cooking liquid is at least 1.5 times greater than the con-
tent of combined SO2. This means that the method of the inven-
tion may be applied not only in the old pulping with acid sul-
phite but also in the modern pulping with bisulphite, such as
magnesium bisulphite, for instance.
The invention will be further illustrated by way of the
following Examples, in which all cookings with acid sulphite
have been carried out in a laboratory scale, but the processes
are directly applicable in large-scale production. The examples
of pulping with bisulphite are taken directly from the operation
of a large continuous digester.
Examples 1, 2 and 3
In the laboratory experiments, a laboratory digester
having a volume of 8 litres was charged with dry chips of spruce
wood (about 1 kg absolute dry weight) and 4 litres preheated
cooking acid containing 6.0% total SO2 and 1.7% SO2 bound to
CaO. The initial temperature was 60C. Each cooking began with
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- ~ . - . - . :
10570~8
2 hour impregnation period, during which the temperature was
successively raised to 80C. After the impregnation the supply
of heat was increased such that a final temperature of about
130C was reached 5 hours from the start. The total time of
cooking fluctuated in the various experiments. At the end of
the cooking a common degassing took place, whereupon the cooking
was stopped in the usual way. The yield of pulp from each
experiment was estimated by a thorough weighing of chips having
a known percentage of humidity in combination with a thorough
washing, disintegration, drying and weighing of the quantity
of the pulp obtained.
During the whole series of experiments a single
quality of chips was used, but to control that the results
were not influenced by different times of storing, comparative
cookings without the addition of yield increasing a~ents were
carried out at equal intervals. All these comparative tests
showed a very good correspondence, and the yield of unscreened
pulp was, on an average, 51.4% at a kappa-number of 40. The
brightness was measured to 64% SCAN, and the time of cooking
was, on an average, 9.0 hours.
In all experiments shown in the following Table I
there were mixed 100 parts by weight formate, 3 parts by weight
formic acid and 30 parts by weight non-ionic dispersing agent,
and the mixture was dispersed in water in the ratio 1:20. The
aqueous dispersion was admixed in the cooking acid immediately
before the latter was pumped into the digester.
-- 5 --
Table I
Example No. 1 2 3
_ _
Additive Exo-iso- Endo- Exo-nor-
fenchyl fenchyl bornyl
formate formate formate
Added g/ton abs. dry wood 24 96 abt 10,000
Cooking time in hours 8.5 8.5 9.1
Kappa-number 38 40 35
Brightness SCAN 69.5 70.0 69.5
Yield of unscreened
pulp, % 55.5 57.2 56.2
Examples 4, 5, 6, 7 and 8
The experiments with continuous bisulphite pulping in
large-scale production were carried out in a plant shown in the
accompanying drawing which is a flow sheet of the process, and
the process was as follows:
Chips of spruce wood were fed from a chip bin A via a
chip meter B (formed as a winged wheel meter) and a low-pressure
feeder C (i.e. a rotary vane feeder) into a steaming vessel D
which was formed as a horizontal cylinder containing a feed
screw. Fresh steam was supplied from below through the body of
chips to maintain a pressure of 1.5 atm. gauge in the vessel D.
~he residence time in the steaming vessel D was 2-4 minutes.
Steam mixed with air expelled from the chips was vented through
the chip bin due to leakage through the vane feeder and the
chip meter, but it could also be discharged from the steaming
vessel in a controlled amount through a specific venting
conduit.
From the steaming vessel D the chips dropped down the
chip chute E down into a high pressure feeder F, where, by means
-- 6 --
. . . .
.
l~D57~QB
of circulating cooking acid, they were stuffed in the valve
chamber initially in vertical position. When this valve chamber,
on the rotation of the cock, had been turned to a horizontal
position, more cooking acid was supplied to convey the chips to
the top separator G which is an inclined cylinder containing a
concentric screw adapted to convey chips and acid. In its lower
part said cylinder has a strainer to withdraw the acid circulating
through the high pressure feeder. On their way from the high
pressure feeder to the outlet end of the top separator the
steamed chips were impregnated during about 3 minutes at 110C
and 12 atm. gauge. Cooking acid heated to about 90C by means
of vapours from lye evaporation was supplied in the top separator
in such an amount that the ratio wood:cooking acid was adjusted
to 1:2. The acid contained about 8.5~ by weight total SO2 and
4.0% by weight MgO-combined SO2.
From the top separator the chips and the acid were
tilted down into the steam space of the digester H which con-
sisted of a vertical cylinder having a diameter of 4 metres, a
height of 48 metres and a volume of 600 m3. The outputer per day
would be 350 ton pulp. The upper half of the digester was used
for the proper cooking, while the lower half was used for a
counter-current high heat washing.
Live steam was supplied to the steam space in such a
way that the contents of the digester was instantaneously heated
to full cooking temperature, in this case 158-160C. At the end
of the cooking zone J the waste liquor was discharged through a
pipe K and led to a recovery unit. In the lower zone, i.e. the
washing zone L, the pulp was washed with water supplied through a
conduit M, and the washed pulp was withdrawn through an outlet N
at the bottom of the digester.
10,57V08
,, ,
The following Table II includes three examples. One
of them was carried out without additives to serve as a compara-
tive test, while in the other examples 50 g endofenchyl formate
(EFF) were added per ton absolutely dry wood. In the same way as
in the laboratory experiments described above, the formate was
dispersed in water (in one example together with 3% by weight
formic acid) by means of a non-ionic dispersing agent, and the
dispersion was supplied to the cooking acid immediately before
the high pressure feeder.
Table II
_ i
Example No. 4 5 6
Additive g/ton dry wood No 50 g pure 50 g pure ~;
addition EFF EFF + 3~
formic acid
% total SO2 in the acid 8.5 8.5 8.5
% bound SO2 in the acid 4.0 4.0 4.0
Kappa-number 45 45 45
Breaking length at
40 SR, m 8500 8700 9600
Breaking length in m.
after beating 20 min. 6600 6800 8500
Tearing strength in g. at
8000 m breaking length 85 90 101
Brightness SCAN 69 72 75
Yield o unscreened
pulp, % 53.0 56.0 57.5
Experiments in large-scale production have also been
carried out while adding hexyl formate together with 3% formic
acid, calculated on the weight of the formate. The results
30 appear from the following Table III,where the comparative test
is the same as in Table II.
S7008
Table III
Example No. 4 7 8
: '
Additive g/ton dry wood No 50 g HF + 100 g HF +
addition 3% formic 3% formic
acid acid
total SO2 in the acid 8.5 8.5 8.5
~ bound SO2 in the acid 4.0 4.0 4.0
Kappa-number 45 45 45
Breaking length at
40 SR, m 8500 8600 9300
Breaking length in m.
after beating 20 min. 6600 6800 8300
Tearing strength in g. at
8000 m. breaking length 85 87 99
Brightness SCAN 69 71 74
Yield of unscreened
pulp, ~ 53.0 55.8 57.3
