Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~036304
~his invention relates to a novel process for pro-
ducing paper-makin~s pulps from grasses.
PUlp8 for paper making have previously been pro-
duced mainly from wood a~ a raw material by a two-step process
which comprises a digestion step of boiling chips of wood
toE~;et~r with aqueous solutions containing chemicals at a high
teD~perature and under a high pressur~ to remove a greater
part of lignin, and a refinin~3 step consisting Or chlorina-
tion, alkali extraction and bleaohing of the digested product
in order to remove the residual lignin and coloring substances.
~Iowever, this process is uneconomical because of the con-
su~ption of great energy in the digesting step and the high
cost Or the chemicals used, and there has been a demand for
replacing it by a more economical process in which the di-
gestion step i~ omitted~ On the other hand, it has been
desired to develop a process for producillg pulp for paper
making, using inexpensive raw materials othor than wood (the
most important rew mater1Als being grAssos)~ in order to cope
with the shortage and high cost of wood in recent years.
Since lignin undergoes oxidation with ox~gen re-
latively easily and thus c~n be removed, attempts have been
j made to produce pulp by directly oxidizing wood chips with
1':
ox;rgen. A~ one example, there was proposed a method which
comprises pre-treating wood chips with an aqueous solution
25 of alkali to increase their reactivit;r to oxidatio~ with
oxygen. However, the wood chips permit only very poor per-
meation of the alkali solution there-through, ~d this resu~tG
in extremely non-uniform oxidation of the wood chip~. In
aotual operation, therefore, it i8 necessar~ to render the
~036304
wood chips of about the same size as match stick~. As this
requires much labor and cost, this process is not feasible
on an ~ndustrial scale, although it may be perfor~ed on a
laboratory ~cale. Accordiugly. this process has never gone
into commercial operation,
Another prior method for oxidizing wood chips with
oxygen comprise~ dlgesting ordinary wood chips with a ~odium
hydroxide or sodium carbonate solution at a temperature of
u~ually as high as more than 160C. under a high pressure to
soften the wood chips to a considerable extent, mechanically
loosening the chips to make them into a bundle of fibers, and
oxidizing the fiber bundle. This method is not substantially
different from the two-step method oomprising alkali diges-
ti~ and refining by oxidation with oxygen, a~d there i~
aotually no omi~sion of the digesting step. Thust in a
strict sense of bhe word? this method i8 not a direct oxida-
tiQn method,
Alkali oooking iq most commonl~ u~ed for pulping
grasses. For example, ~. I. Aronovsky et al. (Paper ~rade
Jaurnal, Vol. 117, No. 25, 1943), and ~. F. Clark et al.
(~appi, ~ol. 43, No. 11, 1960) proposed alkali pulping of
grasses. ~hese processes in~olve using ~raa~ses ~uch as wheat
straw or rice straw as raw materials, and digesting them with
an aqueous solution of an alkali such as sodium hydroxide at
a temperature of as high aæ 16~ to 170C. under a high pres-
sure to obtain pulps. ~hese processes? howevert cannot be
free from the subsequent refining step, and thus from economic
disadvantages in that they comprise a digestion tep and a
re~ining step. Furthermore, there is the ~rious defect that
. . . . . .
~03S304
papers obtained from these pulps have inferior strength to those
obtained from wood pulps in general. For this reason, these
processes have neither proved satisfactory.
It has now been found that pulps which give papers
of superior quality having high brightness and strength comparable
to that of paper from wood pulps can be obtained at low cost by
employing a process which comprises using grasses as raw material,
pre-treatlng the grasses wlth an aqueous solutlon of alkall of
relatlvely low concentratlon at a relatlvely low temperature to
10 lncrease their reactivity to the subsequent oxidation wlth oxygen,
and then oxidizing the pre-treated grasses with oxygen, whereby the
treatlng effect achleved ln the conventional processes by a two-
step process consisting of digestion and refining can be realized
in thls lnventlon only by thls oxldation step. Since the pre-
treatment with the alkali aqueous solution in this process is
carried out at a relatively low temperature within the range of
10 to 100C., it is essentlally different from the alkali diges-
tlon, and thls process is completely free from the dlsadvantage of
the usual dige~tlon step in consu~ing 8reat energy and requlrlng a
2Q costly apparatus that can endure hlgh pre~sures. Furthermore, whlle
the conventional process for producing pulps for paper making
require two steps of digestion and refining in order to remove
lignin and refine the pulps, the process of the present invention ~-~
makes it possible to achieve this ob~ect by a single step of oxida-
tion with oxygen.
According to this invention, there is provided a pulping
process for producing paper-making pulp9 from grasses, which comprises
impregnating a grass in the form of chips with a 0.5 to 6% by weight
of an aqueous solution of at least one alkaline substance selected
30 from the groups consisting of sodium hydroxide, potassium hydroxide,
- 4 -
~. ~
~.. . . . . . . . .
, ,. ~ , ; - .
.. - . . .
1036304
sodium carbonate and potassium carbonate at a temperature of from
10 to 100C., compressing the grass to a squeeze ratio of 1.5 to 5 ~ -
to remove excess aqueous solution so as to provide a wet mass of
grass, loosening the wet mass to provide a porous mass of grass,
treating the porous mass with an oxygen-containing gas at a temp-
erature of from about 60 to about 130C., and then washlng the
resulting pulp with water, whereby said impregnation with the
aqueous alkaline solution is effected in order to ensure a smooth
oxidation at a later stage, and the concentration oP the alkaline
solution and temperature are such as to dissolve a minimum amount
of lignin and hemicellulose.
Figures 1 to 4 of the accompanying drawings are graphic
representations of the experimental results obtained in Example 1.
The process of this invention will be described in
greater detail below.
The grasses used as a raw material in the present in- ~;
vention typically include, for example, rice straw, cereal straws,
bagasse, timothy, esparto, chaf~, cornstalks, and bambooes.
The grasses are made into the chip form, then impregnated
with an aqueous solution of alkali, If soils, sand~ or other
foreign matters adhere to the grasses, they are removed by suit-
able methods before or after making them into chips. U6ually, the
grasses are m~de into chips by cutting them lengthwise to a size of
not more than 100 cm, preferably about 1 to 20 cm. Since the grasses
are generally in the form of slender rods of small cross sectional
area, this cutting operation can give chips in the slender form.
Generally, the grasses are very coarse in organization as compared
with wQod, and therefore, permit very good permeation of the alkali
solution simply by being made into the chip form. However, when
bagasse, corn stalks or bambooes are used as a raw material, it is
-
t:~ :
.. , .-
~` ~
1036304
preferred to crush them further mechanically in order to better
the permeation of the alkali solution. Since the grasses are far
softer than wood, the energy required for chipping or crushing is
far lower than that required for making wood into chips.
Rice straw, cereal straws, timothy, esparto, and
bagasse are especially preferred for use in the present invention.
According to this invention the grasses are impregnated
with the alkali aqueous solutlon at a temperature of 10 to 100C.
Most preferably, the impregnation i9 accomplishet by immersing the
grasses in an aqueous solution of alkali. But other suitable methods
can also be utilized.
The main ~urpose of impregnating the grasses with the
aqueous alkali solution is to facilitate the smooth performance
of the subsequent oxidation with oxygen, and therefore, this step
is esaentially different from the alkali treatment in the conven-
tional processes which i8 intended to remove lignin present in
grasses. Thus, the concentration of alkali in the impregnating step
of the process of this invention is of utmost importance, and should
be maintained at a relatively low level, i.e. 0.5 to 6% by weight,
preferably 2 to 4% by weight. The impregnating temperature is also
very important, and should be maintained at 10 to 100C.,
..... - ~ ~ .
6304
perferably 20 to 80C, which is far lower than the tempera- ;~
ture of the alkali digestion in the conventional processes.
Generally, in the production of paper-making pulps,
it is advantageous, for increasing the strength of paper, to
inhibit the dissolving of pentosans and other hemicelluloses
contained in the raw material to the greatest possible extent
and to leave as much hemicelluloses as possible in the pulps
as final product. This also results in an increased yield of
pulps because the hemicelluloses can also be utilized as the
final pulp product. In this regard, paper-making pulps -
differ greatly from pulps for producing viscose rayon whose
hemicellulose content is minimized because a high hemicellulose
content will cause a reduction in the strength characteristics
of the resulting rayon fibers.
The conventional processes for producing pulps from
grasses contemplate the removal of lignin in the grasses
chiefly by the digestion of the grasses with an alkali aqueous
solution to perform pulping~ and therefore~ it is naturally
essential to employ high temperature and high pressure condi-
tions. Owing to such severe reaction conditions, a greater
part of the hemicelluloses are dissolved in the treating
solution together with lignin, and the yield of pulps and the
strength of paper obtained from these pulps decrease. Fur-
thermore, in order to remove coloring substances and increase
the brightness of ~aper, it is essential for these conventional -~ -
processes to include a subsequent refining step including a
bleaching procedure.
In contrast, in the alkali solution impregnating
step of the process of this invention, it is not altogether
.''' ' ' ":
",~, .. ...... . . . ~ - .
.-~. : . '' :: ' ~
1~36304
necessary to remove lignin, because lignin contained in the
grasses can be almost completely removed by subsequent oxida-
tion with oxygen. Rather, in the alkali solution impregnating
step of the process of this invention, it is desirable to
inhibit the dissolution of lignin as much as possible for the
reason to be mentioned. This is accomplished by maintaining
the concentration of alkali at a relatively low level, and
e~pecially by maintainin~ the temperature low, whereby the
dissolving of li~nin and hemicelluloses in the impregnating
step is minimized. By the alkali solution im~re~nating step,
the reactivity of grasses to oxidation with oxygen is in-
creased, and when the grasses containing lignin and hemicel-
luloses left almost undissolved are oxidized with oxygen in
the subsequent step, the liænin is removed almost completely,
At this time, the dissolvin~ of hemicelluloses occur to some
extent, but as compared with the alkali digestion at high
temperatures in the conventional processes, the amount of the
hemicelluloses dissolved is very small, and the amount of the
hemic~llulo6es remaining in the final pulp product very much
increases. Accordingly, the strength of the paper obtained
from the resulting pulp product is very superior, and the
yield of the pulp i8 also good. ~he strength of p~per and
the yield of the pulp are equivalent to, or greater than,
those attained in the case of wood pulps.
One of the reasons for the desirability of the re-
duced dissolution of lignin in the alkali solution impregna-
ting step of the process of this invention is that the low
content of lignin dissolved is convenient when recycling the
excess alkali agueous solution obtained in the subsequent
.
- .
1036304
compressing step and using it again or the impregnation of
grasses. Another reason is that when the pulp waste liquor
resulting from the washing of grasses which have been sub-
?ected to the oxidation step is concentrated and burned so as
to avoid pollution, the burning is easier and the heat energy
recovered is greater if the pulp waste liquor contains as
much soluble ligneous matters as possible.
The tlme period required for the alksli aqueous
solution impregnating step is not particularly critical, and
is usually from 10 minutes to 10 hours. The ratio of the
grasses to the aqueous solution of alkali is neither limited
in particular. Usually, however, it is preferred to use the -~
alkali aqueous solution in an amount 4 to 50 times the weight
of the grasses. If desired, a small amount of a surface ac-
tive agent such as sodium lauryl benzenesulfonate, polyether s
sulfate or a polyoxyethylene alkyl ether may be incorporated
in the aqueous solution of alkali. Usually, the impregnating
step is performed at atmospheric pressure, but if desired,
it can be csrried out at an elevated pressure.
2a The 8rasses impregnated with the aqueous solution
of alkali are then compressed to remove an excess alkali
aqueous solution thereby to obtain a wet mass of grasses.
~he extent of compression is not particularly critical, but
it is convenient to use a squeeze ratio, i.e. a ratio of the
total weight of the wet mass to its absolutely dried weight,
of 1.5 to 5. !~
Generally, grasses have a larger content of hemicel-
lulose than wood. Thus, when the grasses are treated with
alkali at high temperatures as in the conventional processes,
_ ~ _
(;~. '
' ' '- ' ' ~ ' ''- : -
:
10;~6~04
most of the hemicelluloses are dissolved in the treating
solution. Consequently, the tissues of the grasses are des-
troyed, and the grasses become fibrous. As the fibers of the
grasses are intertwined with e~ch other, it ~comes very dif-
ficult to compress the mass of grasses. ~urthermore, it isdifficult to loosen the grasses uniformly, and therefore, to
oxidize the rrasses with oxygen smoothly in the subsequent
~teps. In contrast, in the impr~n~tin~ step of this invention,
the dissolution of the hemicelluloses ~nd lignin is v~ry much
reduced, and most of them remain in the grasses. For this
reason, the grasses ret~in their original form even after
impregnation, and permit easy compression.
~ he wet mass of grasses obtained in the compressing
step is then loosened by suitable methods to render it entirely
into a porous mass of grasses. This makes it possible to
perform the subsequent oxidation step smoothly and uniformly.
The resulting porous mass of ~rasses is then oxidized
with an oxygen-containing gas. This oxidation causes the
lignin to be converted to solubl~ oxidized li~nin and to be
able to be removed almost completely by subse~uent washin~
with water. At the same time, coloring substances contained
in the grasses are oxidized and decomposed. ~his contributes
to a very much increased brightness of the paper, and even
when a bleaching step is omitted, it is possible to obtain
2~ paper hsving fully satisfactory brightness for ordinary uses.
It h~s been known to apply oxygen oxidation to wood,
but the application of the oxygen oxidation method to grasses
has never been known before. Especially, -the application of
a combination of alkali treatment at low temperatures using
-- 10 --
, : : . : '
1036304
al~ali solutions of low concentration and a subsequent step
of oxidation with oxygen to ~rasses has completely been un-
known prior to the present invention. As described in detail
above, the various advantages of the present invention which
cannot be expected at all from the conventional processes can
be obtained only by this specific combination of the two
steps.
~ he oxygen-containing ~as to be used for the oxida-
tion with oxy~en in accordance with thi~ invention may, for
example t be air, a ~as comprising a higher oxygen content
than air, or 10~/o pure O~Jgen. ~he suitable temperature for
the oxidation is 60 to 130C., preferably 80 to 120C. The
preferred oxidation time is usually from lO minutes to 2 hours.
~he pressure for the oxidation may be normal atmospheric
pressure, but generally, it is preferred to per~orm the oxida-
tion at an elevated pressure.
~ he grasses which have been subjected to the oxida-
tion with oxygen are then washed with water, and dried by
customary methods. As a result, pap~r-making pulps as the
intended final product are obtained. Since the pulps ob-
tained have alr~ady been bleached to sufficient brightness
by the oxidation treatment as described above, an additional ~-
bleaching step is usually not required. When it is desired
to increase the brightne~s further, the grasses are further
bleached by a customary method.
The pulp waste liquor resulting from the washing
of the oxidized grasses has a very low alkali content as
compared with pulp waste liquors resulting in the conventional
soda process, i.e. about l/3 to l/12 of the conventional
.
, ~ . .
.. . . . .
.. .. . . . . .
.
1036304
processes. Therefore, even if the pulp waste liquor is dis-
carded as such, environmental pollution resulting from i-t is
far reduced. But in order to avoid pollution, the pulp
waste liquor may be concentrated and burned, and then sodium
carbonate or potassium carbonate contained in it ma~ be con-
verted to hy~roxide using lime and recovered.
The steps of the process of this invention described
above can be performed either continuously or batchwise.
In the process of this invention, both the impregna-
ting step and the oxidation step can be performed at relatively
low temperatures. ~her~fore, the process of this invention
has the a~vantage that in comparison with the conventional
processes which involve digestion at high temp~ratures and
pressures, the amount of fuel to be consumed is very much
reduced. ~urthermore, since the impregnating step is usually
carried out at atmospheric pressure, a high pressure apparatus
usually required for digestion can be omitted in the present
invention. For these reason~ and also by the reason that in-
expensive gra9se~ are used a5 raw materials, the process of
this invent~iOn has the advantage that the cost of product is
very low. A further advantage of the process of this invention
is that that the resulting pulp can give paper having bright-
ness and strength comparable to those of paper from wood pulp,
and that the costly bleaching step can be omitted.
The following Examples il~ustrate the present inven-
tion specifically. ~he various properties described in the
Examples were measured by the following methods.
- 12 -
~., .... . . , . :-
- ;:
1036304
Brightness: JIS~ P~123-61
Breaking length: JIS P~113-~2
Burst factor: JIS P8112-63
Tear factor: JIS }'8116-63
~JIS stands for Japanese Industrial ~tandards.
Example 1
rrhis Example was performed in order to clarify the
influences of the alkali concentration (% by weight) and the
temperature in the impregnating step of the process of this
invention.
Well-selected rice straw was cut to a length of
about 4 cm, and 30 g of the cut straw was immersed for 1 hour
at a temperature of 10, 20, 40, 80, -100 and 120C. respectively
in 1 liter of an aqueous solution containing sodium hydroxide
in a concentration of 1%, ~/o, 3%, 4/~, and 5% respectively
and 0.1% by weight of s~dium lauryl benzenesulfonate as a
surfactant. ~he straw was then squeezed to a squeeze ratio
of 2, and then well loosened for use in a subsequent oxidation
step.
Water was placed at the bottom of a 4-liter auto-
clave, and the straw wetted with the alkali as obtained above
waæ put on a plate inside the autoclave. The atmospheric
air inside the autoclave was substituted by oxygen. Oxygen
was introduced to a partial pressure of 5 Kg/cm2, and the
~:: 25 temperature was raised to 120C. in the course of 30 minutes.
- The straw was oxidized at this temperature for 1 hour.
; After the oxidation, the pulp was well washed with
water, and the yield of the pulp was measured. Under any of
the reaction conditions, the resulting pulp had a freeness of
- 13 -
.. .. - . .. . ~
. :.: . ~- - ~ - , - - . -
:: :: . ..~ .
' .: .,:
1~36304
330 + 20. Therefore, without subjecting to a beater, the
pulp was directly made into paper. The strength and bright-
ness of the paper were measured. The experimental results
obtained are shown in Figures 1 to 4. Figures 1 and 2 are
graphic representations showing the influence of the immersing
temperature when the alkali concentration is 2k and 3%, res-
pectively. Fi~ures 3 and 4 are ~raphic representations show-
ing the influence of the a~kali concentration when the im-
mersing temperature is 20C. and 80C. respectively.
It is seen from these experimental results that
when the temperature is 0C0, the yield of the pulp is high,
but the strength and brightness of the paper are markedly
reduced, and that when the temperature is 120C., the yield
of the pulp is markedly reduced and also the strength of the
paper decreases. Furthermore, there is noted a general
tendency that when the temperature is low, good results are
obtained by increasin~ the alkali concentration, and when the
temperature is high, good results are obtaine~ by decreasing
the alkali concentration.
ExamPle 2
Pulps were prepared in the same way as in Example 1
except that the various alkaline substances shown in Table 1
were used instead of the sodium hydroxide in Example 1. The
properties of the pulps and papers made therefrom were measured,
and the results are shown in Table 1.
- 14 -
. - - - -
6304
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-- 15 --
~xample 3 ~04
Pulps were prepared in the same way as in Example 1
except that the concentration of the sodium hydroxide was
changed to 2% by weight, and the immersion temperature was
changed to 40C., and furthermore, the oxidation reaction
conditions were changed as shown in Table 2 below. ~he
properties of the resulting pulps were mea~ured, and the
results are shown in Table 2~
- 16 -
.. .. . . .. .. . . ... .
1~304
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-- 17 --
... : ` . :
10a6304
~xamPle 4
This Example shows the pro~uction of pulps in
accord~nCe with the process of this invention using other
grasses as raw materials.
30 g of each of the various grasses shown in ~able
3 was immersed for 1 hour at the various temperatures shown
in ~able 3 in one liter of an a~ueous solution containing
alkaline substances in various concentrations as shown in
~able ~ and 0.1% by weight of sodium lauryl benzenesulfonate
as a surfactant, and squeezed to a squeeze ratio of 2. The
grass was well loosened.
Water was placed at the bottom of a 4-liter auto-
clave, and the grass wetted with the alkali as obtained
above was placed on a plate disposed within the autoclave.
The ~tmospheric air inside the autoclave was substituted by
oxygen. ~he oxygen was introduced to a partial pressure
of 5 Kg/cm2~ and the temperature was raised to 120C~ over
the course of ~0 minutes. The grass was oxidized at this
temperature for 1 hour~
After the oxidation, the resulting pulp was well
washed with water, and the yield of the pulp was measured~
~he pulp was subjected to a beater to a freeness of 400
20, and then made into paper. The strength of the paper
was tested. The results are shown in Table 3.
,
c
- 18 -
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~036304
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-- 19 --
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