CA1036304A

CA1036304A - Process for producing paper-making pulps from grasses

Info

Publication number: CA1036304A
Application number: CA198,550A
Authority: CA
Inventors: Sadayoshi Watanabe
Original assignee: Individual
Current assignee: Individual
Priority date: 1973-05-01
Filing date: 1974-04-30
Publication date: 1978-08-15
Also published as: AR199615A1; GB1446817A; AU6847474A; BR7403522D0; JPS5622991B2; JPS49134901A

Abstract

A B S T R A C T
There is disclosed a process for producing paper-making pulps form grasses. The process comprises impregnating a grass in the form of chips with a 0.5 to 6% by weight aqueous solution of alkali at a temperature of 10 to 100°C., compressing the grass to remove an 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 temperature of from about 60°C to about 130°C., and then washing the resulting pulp with water and drying it.

Description

~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, ~. ~
~.. . . . . . . . .
, ,. ~ , ; - .

.. - . . .

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:~ :

.. , .-~` ~

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., ..... - ~ ~ .

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 .''' ' ' ":

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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 .
- .

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 , : : . : ' 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 .
, ~ . .
.. . . . .
.. .. . . . . .
.

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.

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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 .. .. - . .. . ~

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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.

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~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~

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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.

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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:-

1. A pulping process for producing paper-making pulps 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 from the group consisting of sodium hydroxide, potassium hydroxide, sodium carbonate and potassium carbonate at a temperature of from 10 to 100°C., com-pressing 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 temperature of from about 60 to about 130°C., and then washing 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 of the alkaline solution and tempera-ture are such as to dissolve a minimum amount of lignin and hemicellulose.

2. The process of claim 1 wherein said grass is selected from the group consisting of rice straw, cereal straws, bagasse, esparto, timothy, chaff, cornstalks, and bamboos.

3. The process of claim 1 wherein said grass is selected from the group consisting of rice straw, cereal straws, bagasse, timothy and esparto.

4. The process of claim 1 or 2 wherein the alkali concentration of said aqueous solution of alkali is from 2 to 4% by weight.

5. The process of claim 1 or 2 wherein the temperature of the impreg-nating step is from 20 to 80°C.

6. The process of claim 1 wherein the temperature of treating the porous mass is from 80 to 120°C.