Note: Descriptions are shown in the official language in which they were submitted.
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~ I The present invention relates to the production of a
doli~uificd chemical cellulose pulp.
Cellulose or pulp for the manufacture of paper or for
chemical purposes is manufactured from fibrous raw materials
primarily wood. In such processes of manufacture the binding
substance between the fibres i.e. the lignin for the paper
pulp is dissolved with as little as possible extraction or
degradation of the holo-cellulose components of the raw material.
Conventional processes for the production of such
pulps have heretofore primarily been the sulphite process and
the sulphate process. In the sulphite process fibrous raw
material is delignified by cooking with a bisulphite solution
containing calcium, magnesium, sodium or ammonium. In the -
sulphate process, the fibrous raw material is cooked with a
solution of caustic soda and sodium sulphide. The sodium
c~m~ D r~ents
valu~ are subsequently recovered, and to make up for losses,
- sulphate is added in a recovery process. The deliquification
can also be carried out in the soda process without sulphide,
using caustic soda for make-up, giving however, under conven-
tional conditions, a lower yield of pulp with inferior physical
properties.
The most common fibrous raw material used is wood
which is normally chipped to pieces of 15-25 mm in length, 10-
20 mm in width and 3-8 mm in thickness. The chips are impreg- -
nated with the cooking liquor to various degrees of penetration -
and then cooked at elevated temperature and pressure until a
desired amount of lignin has been dissolved. In the alkaline
processes i.e. the sulphate and soda processes the impregnation -
of the cooking liquor takes place in both the fiber direction
and perpendicular to this direction at essentially ~fi~ the
same speed. When the cooking liquor passes through the chip
the wood absorbs alkali and the impregnated liquor becomes
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gradually weaker. Depending on the thickness of the chips,
there is an accentuated difference in alkali concentration
in the liquor between the outer and inner parts of the chips
up to the extreme condition of zero alkali in the center of
the chips. The consequence of this is that the outer parts
of the chi~s will~ e overcooked and the lnner parts undercooked
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' or even h~draoli~od, yielding less pulp than expected at a
certain degree of delignification and with reduced strength - ~ -
properties.
In the process disclosed in U.S. Patent No. 2,904,460 -
William J. Nolan partly removed this disadvantage partly by
shredding the chips to splinters before cooking, the splinters
having a cross-sectional dimension of about 2.3 to about 4.7 ~
mm. By doing so, he also reduced substantially the cooking time -
from the conventional 1-3 hours to 15-25 minutes. In order to - -
further improve the process the applicants Companhia Suzano de
; Papel e Celulose, in Brazilian No. 205,374 proposed a method to
delignify a defibered fiberous raw material in which in order
to achieve defibration of the raw material without damaging
the fiber walls, the raw material e.g. wood chips is reduced
to match stick size, which sticks are completely and homogeneous-
ly impregnated with cooking li~uor at a temperature of 120-130C.
The temperature is then raised to a cooking temperature of 160-
180C, at which temperature the lignin is perfectly thermoplastic.
In a subsequent mechanical defibration, the separation of the
fibers occur in the binding lignin. Cooking of the defibered
raw material is then carried out rapidly in 2-5 minutes. The
advantages of the process include (a) a very fast process
compared to conventional processes. (b) An absolutely homo-
geneous end product. (c) An increased yield, essentially dueto the short reaction time.
In U.S. Patent No. 3,773,610 to Joseph C. Chouvlin et. al.
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a very similar process is disclosed in which the inventors
however failed to recognize the importance of the use of small
sticks of thin chips rather than conventional mill sized chips.
This is fundamental not only to obtain a homogeneous impregna-
tion and thus a homogeneous end product but also to achieve a
homogeneous heating of the impregnated chips before defibrating.
If the chips are not hot enough in the inner parts, the fiber
walls of these parts will be damaged during the mechanical
- defibration. If, however, heating is carried out slowly enough
~ 10 to permit perfect penetration of the heat to the center of the
- chips, substantlal delignification will take place before de-
fibering and the fiber walls will be damaged because the fibers
will not have the necessary protection of an intact lignin layer
during the defibering process.
- Laboratory tests carried out on Eucalyptus wood using
a cooking liquor with 25~ sulphidity have qiven the following
comparative results:
T~ LE -
20 ~ Time to Time at Screened Strength -
170C170 C yield at at l~C.S.
min min 15 PN Fre~
. .. _ . ....... ... ...
Tensil Tear
Conventional cooking 60 30 51.0 9190 80
Nolan 0 20 54.8 12000 91
Chouvlin 10 3 49.0 8500 82
¦3 15S.0 ~ llE00
All these methods, however, have a common disadvantage
namely the cooking liquor used for impregnation penetrates into
the lumen i.e. the hollow parts of the fibers where the cellulose
components of the wood are not protected by a lignin layer and
at temperatures between 120C and 170C the dissolution of
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cellulose components, especially the pentosanes, is already
substantial, and at 170C or more, serious. This explains why
the yield of a raw material, containing 30~ or less lignin, is
only about 50~.
It has now been found that the disadvantage may be
substantially avoided when the impregnation of the fibrous raw
material is effected with any alkali agent of reduced or ~
weakened chemical activity and preferably sodium carbonate with ~ -
or without sodium sulphide, available in sulphate mills in the
form of "green liquor~, mixed with used cooking liquor, black
liquor, containing normally about S to 10 g/l active alkali,
most of which in form of sodium sulphide.
This liquor dissolves little or no cellulose sub-
stance during the impregnation and heating periods but prevents
r ~ the formation of acid conditions and thus hydrolysis during ~ C
impregnation and defibration periods of the process.
- Agents with high chemical activity such as caustic
soda with or without sodium sulphide, e.g. white liquor in the
sulphate mills, is added during the defibration or immediately
thereafter. This liquor, when added, will dissolve the lignin
layer on the outside of the fibers, exposed by the defibering
process, but the hollow inside parts of the fibers remain pro-
tected by the impregnation liquor during the short period of
2-5 minutes the cooking will take.
According to the present invention therefore there is
provided a method of manufacturing a chemical pulp from a fibrous
raw material which comprises homogeneously impregnating the fib-
rous raw material with an alkali solution of weaker chemical
activity whereby to minimize delignification of said raw material,
subjecting said alkali impregnated raw material to mechanical
defibration and treating said defibrated raw material with an
alkali solution of stronger chemical activity to effect deligni-
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fication thereof.
The method of the present invention has given sur-
prisingly good results. Thus, impregnating shredded chips
of Eucalyptus of about 2 by 2 mm section at 125C during 5
minutes with a mixture of green liquor and black liquor with a
total alkali content of 8~ based on dry wood weight, raising
the temperature in 15 secor.ds by direct injection of steam to
170C, defibrating the material at this temperature, adding in
the defibration white liquor with 25% sulphidity containing 10%
active alkali based on dry wood and cooking the defibered mat-
erial during 3 minutes, the following results have been obtained: -
Screened yield: 60.1%
Tensil strength: 12,500 -
Tear strength: 118
Tests also indicate that essentially the same results
may be achieved without adding sodium sulphide to the process, -
thus allowing for a sulphur-free high yield, high quality pulping
process. This process adapts itself perfectly to the deligni-
fication of fiberous raw material with oxygen in an alkali
solution. Laboratory tests have been carried out with Eucalyptus
wood and it i8 anticipated corresponding improvements will be
achieved using any wood or other fiberous raw material as for
example sugar cane bagasse and bamboo.
Due to the fact that the delignification i8 effected
on defibered raw material, the process is independent of the
- ~ density of this raw material. Thus, a heterogeneous raw material
such as mixed hardwoods or a mixture of softwood and hardwood
or a mixture of bamboo and wood of any kind will give a homogen-
eous end product. The pieces of the raw material must not nec-
essarily be in the form of normal chips, reduced to sticks of
about 2 to 3 mm section. The chips of constant thickness as
produced in a drum chipper are very well suited for the process.
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The ideal thickness of the chips is about 2 mm. Length and width
can vary within any practical limit. Normal sized chips as used
in pulp mills may be used but with the disadvantage of not
achieving a homogeneous impregnation and heating through the
chips, resulting in increased reaction time and decreased yield
and strength properties of the pulp.
Impregnation of the raw material with green liquor
or any other slow acting alkali agent followed by cooking with a
B fast acting alkali agent is/little advantage if not used in
combination with defibering the raw material between impregnation
and cooking, because only in this way the selective dissolving
of lignin by the white liquor can take place as described above.
In addition to the advantages already described the
process of the present invention results in less organic sub-
stance per ton of pulp to be treated in the recovery system and
thus higher capacity in terms of ton of pulp/day in existing
plants or smaller and thus cheaper units in new plants. Only
part - about 50% - of the green liquor will be transformed into
white liquor in the causticizing plant, giving correspondingly
higher capacity in existing installations or smaller new in-
stallations. This also applies to the reburning plant for lime
sludge .
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