CA1116816A - Process for producing pulp - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
The process for producing pulp according to the present invention consists in the subsequent steps of an aqueous treat-ment of a vegetable stock at a temperature within the range of from 110 to 170°C under a pressure of from 2 to 7 atm.g. for a period of from 0.3 to 4 hours at a weight ratio of the vegetable stock to water of 1:2-4 respectively; a simultaneous impregnation and a cooking of said vegetable stock in a solution of nitric acid at a temperature of from 70 to 90°C for 0.5-3 hours; cooking at a temperature of from 100 to 130°C under a pressure of from 1 to 4 atm.g. for 0.3-0.5 hour; an alkali extraction of the vegetable stock; decontamination of the gaseous cooking products by mixing thereof with the air in a volumetric ratio of 1:0.5-1 respectively, and treatment with the spent liquor resulting from the stage of alkali extraction. The process according to the present inven-tion makes it possible to increase the vield of pulp and sub-stantially improve its quality characteristics.

Description

I?ield of ~pplication The present invention relates to the production of fib-rous prefabricated articles and, more specifically, it relates to a process for producing pulp useful in the pulp-paper industry and in chemical industry as the starting material for the manu-facture of man-made fibres.
sackground of the Invention Known in the art are processes for producing pulp which comprise impregnation of vegetable raw materials such as wood chips with a solution of nitric acid, cooking in a vapour-gas or liquid medium, followed by alkali extraction.
Concentration of nitric acid is varied within the range of from 3 to 42~ by weight and the impregnation temperature de-pends on concentration of acid and is varied within the range of from 12C (for a concentrated acid) to the boiling temperature (in the case of diluted solutions). Duration of impregnation is determined by the technology employed and is varied from several minutes to 18 hours and more.

To accelerate the process of impregnation of the vege-~20 table raw materials and ensure a uniform distribution of nitric acid therein, various methods for intensification of this operation have been developed.
These are exemplified by impregnation of the vegetable stock with acid in vacuum, setting wood chips under vacuum prior to supply of nitric acid into the apparatus, followed by ele-vation of pressure up to 3 atm.g; disintegration of the vege-table stock and the use thereof in the form of thin shavings as well as keeping of the impregnated vegetable stock at a temperature within the range of from 40 to 80C for a period of from 3 to 5 hours to equalize the concentrati~on of the initially absorbed acid over the entire volume of the vegetable stock. There is also a method comprising mechanical compression and decomperssion of ~ - 2 -a vegetable stock submerged into a solution of nitric acid thrice elevating and diminishing the pressure in alternation.
However, the use of said operations prior to impregnation of the vegetable stock with a solution of nitric acid does not ensure a required uniformity of distributic;n of acid in the vege-table stock which causes a substantial amount of a non-cooked product (wastes) lowering the yield of the final pulp.
~ Furthermore, the resulting pulp has an insufficient mechanical strength and a low viscosity of its cuprammonium solutions.
In the prior art processes for the production of pulp the vegetable stock impregnated with a solution of nitric acid is subjected to cooking in a liquid or vapour-gas medium. It is advisable to use the following conditions, namely: maintaining the vegatable stock under atmospheric pressure at a temperature of from 96 to 99C for a period of from 0.5 to 1.5 hour. At the temperature of 98-99C there are formed about 503m of gaseous products per l ton of the vegetable stock. The volume of the gaseous mixture formed during the cooking is decreased with de-creasing the process temperature; however, the cooking duration is increased from 7 to lO hours.
The main disadvantage of the prior art processes for the production of pulp upon cooking under the above-described condi-tions is the formation of pulp containing a considerable amount of hemi-cellulose ~hich exerts a detrimental effect on the quality of the final pulp product. Furthermore, in the prior art processes for the production of pulp there is lack of processes of decontamination of the gaseous products of the cooking which results in pollution of the environments.
Also known in the art is a process for producing pulp from a vegetable stock including steaming effected by passing a live steam through a bed of the vegetable stock for 0.3-0.5 hour, 8~

impregnation of the vegetable stock with nitric acid having con-centration of from 6 to 15% by wei~ht at a temperature wi-thin the range of from 40 to 50C under atmospheric or an overatmos~
pheric pressure; cooking in a li~uid or vapour-gas mixture at a temperature of 96-99C accompanied by liberation of toxic products and an alkali extraction. The akali extraction is effected by means of aqueous solutions of caustic soda, potash, soda, am-monia, calcium hydroxide or a mixture thereof. This process makes it possible to produce pulp with a yield of up to 45% by weight and containing up to 89~ of alpha-cellulose and down to 12% of hemi-celluloses. This pulp, however, does not posses the required quality parameters.
This prior art process has the following disadvantages:
a long duration of the impregnation stage, non-uniformity of im-pregnation of the vegetable stock with nitric acid resulting in a large amount of under-cooking, a high content of hemi-cellu-loses, absence of decontamination of the gaseous cooking products.
Object of the Invention It is an object of the present invention to increase the yield of pulp.
It is another object of the present invention to improve quality characteristcs of pulp.
srief Description of the Invention These objects are accomplished by that in a process for producing pulp from a vegetable stock by impregnation there-of with a solution of nitric acid, cooking in a liquid or vapour-gas medium and an alkali extraction ofthe thus-treated vege-table stock, in accordance with the present invention, the vege-table stock is first pretreated with water at a weight ratio of the vegetable stock to water of 1:2-4 respectively and at a temperature of from 110 to 170C under a pressure of from 2 to 7 atm.g for a period of from 0.3 to 4.0 hours followed by a simul-~ _ 4 _ taneous impregnation and cooking of the veyetable s-tock in a solution of nitric acid at a temperature within the range of from 70 to 90C for a period of from 0.5 to 3.0 hours with a sub-sequently continued cooking at a temperature within the range of from 100 to 130C under a pressure of from 1 to 4 atm.g. for 0.3-0.5 hour; the gaseous products resulting from cooking of the vege-table stock are decontaminated by intermixing with air in a volu-metric ratio of 1:0.5-1.0 respectively with a subsequent treat-ment with the spent liquor resulting fromthe alkali extraction stage.
The above-mentioned aqueous treatment contributes to the formation of a more porous structure of the vegetable stock which facilitates a more complete and uniform impregnation thereof with a solution of nitric acid and lowers the amount of ; under-cooking, i.e., increases the yield of pulp. At the same time, the content of hemi-celluloses is reduced and the content of alpha-celluloses is increased which ameliorates quality charac-teristics of the resulting pulp.
The use of the above-mentioned aqueous treatment of the vegetable stock makes it possible to perform simultaneously the process of impregnation and cooking of the vegetable stock in a solution of nitric acid, thus substantially improving quality characteristics of the resulting pulp produce and substantially reducing the total duration of the process for the manufacture of pulp.
The subsequent elevation of the cooking temperature ensures a rapid completion of the residual lignine with nitric acid, elimination of the maximal amount of hemi-culluloses and the achievement of the required quality characteristics of pulp.
At a temperature above 130C the cooking is inexpedient, since a noticeable destruction of pulp is observed.
Decontamination of the gaseous products resulting from cooking of the vegetable stock makes it possible to eliminate ~ pollution of the environment with toxic gaseous compounds with-¦ out any additional comsumption of chemicals for purification~ At the same time, there is ensured an increased content of combined , nitrogen in the spent liquor which should increase its value as an agricultural fertilizer.
In the process according tothe present invention it~s ad-visable-toperform simultaneouscooking andimpregnation ofthe vege-~ table stockin asolution ofnitric acïd under a continuous vibroagitation .~ -at-afrequency ofmechanical vibrations of about 3,000 perminute. Owing to vibroagitationnitric acidpenetra~es into the vegetablestock at a ~ubstantially higherspeed than in aconventionaI impregnationprocess without vibroagi~tation. The use of vibroagitation in theprocesb ac-~, cording tothe present invention makesi,tpossible to subs~cantially in-tensifv thesame while increasingthe output from the process equipm~nt as well.
It is advisable to subject the spent liquor resulting from the alkali extraction stage to oxidation with an oxygen-containing gas at a temperature within the range of from 200 to 250C under a pressure of from 10 to 30 atm.g for a period of from 0.5 to 2 hours and to use it repeatedly in said stage of alkali extraetion.
It is preferable, in the stage of simultaneous impre-nation and cook'ing of the vegetable stock, to use a 3-10% solu-tion of nitrie'acid.
Detailed Description of the Invention ;, The process for producing pulp from a vegetable stock according to the present invention is perfomed in the following ~' manner.
, ,' As a vegetable stock use can be made of deciduous and ', 30 coniferous wood, cotton stems, sugar cane wastes, straw.

The vegetable stock is poured with water at a weight ratio of 1:2-4 respectively and heated in an autoclave under a , -- 6 --1~16~

pressure of from ~ to 7 atm.y. at a temperature of from 110 to 170C for a period of from 0.3 to 4.0 hours.
This pretreatment with water is accompanied with hy-drolysis and dissolution of a certain amount of readily-hydroly-zable hemi-cellusloses and water-soluble lignine fragments. A
partial dissolution of the vegetable material results in a formation of a more porous and permeable structure of the vege-table stock which further facilitates a more complete and uniform impregnation thereof with nitric acid which makes possible to accelerate the process and reduce the amount of under-cooked material. Furthermore, the thus-produced pulp features a substan-tially lower content of hemicelluloses and a higher content of alpha-celluloses.
On completion of the water-treatment the vegetable stock is mixed with a solution of nitric acid of a concentration of from

2 to 12~ by weight and heated at a temperature within the the range of from 70 to 90C for a period of from 0.5 to 3.0 hours;
therewith, two processes occur simultaneously, namely: impreg-natlon and cookin`g of the vegetable stock. This ensures most optimal conditions for a uniform cooking of the vegetable stock which substantially improves quality of the resulting cellulose.
Then the reaction mass in continued to be heated to the temperature of 130C for a period of from 0.3 to 0.5 hour. It is preferable r to accelerate the processes of impregnation and cooking, improve-ment of uniformity of distribution of nitric acid over the vege-table stock, to perform impregnation with continuously operating vibroagitating means performing dscillating movements at the frequency of 50 Hz. During the vibroagitation there are formed wave-like propagating compression zones which apparently is the main reason of intesification of mass transfer in wood. Owing to the vibroagitation there is achieved a sharp acceleration of diffu-sion of nitric acid inside the vegetable stock.

~ - 7 -On completion of cooking, the vegetable stock is washed and subjected to an alkali extraction with a solution of, for exam-ple, caustic soda, soda or ammonia. It is preferable, to reduce the consumption of fresh water, the base and to eliminate the formation of harmful waste waters, that the spent liquor resulting f~om the alkali extraction stage be subejcted to oxidation with an oxygen-containing gas at a temperature within the range of from 200 to 250C under a pressure of from 10 to 30 atm.g. for a period of from 0.5 to 2.0 hours and repeatedly used in said alkali ex-traction stage.
The organiccompounds contained in the spent aklaline liquor are oxidized to carbon dioxide or to simple carboxylic aeid remaining in the liquor in the form of salts. All coloured compounds in the liquor are also destroyed. The resulting colourless or light-yellow solution contains carbonates, bicar-bonates and salts of trivial carboxyllc acids, i.e. oxalates, formiates, acetates and the like. The resulting solution possesses an alkaline reaction and can be repeatedly used for the alkaline extraction. -The gaseous products resulting from cooking of the vegetable stock are deeontaminated by mixing same with air in a volumetrie ratio of 1:0.5-1.0 respeetively and treated with the spent liquor resulting from the alkali extraction stage.
The process for producing pulp according to the present invention has the following advantages over the prior art processes namely: lncreased yield of pulp by 3-5% by weight due to a reduced amount of the non-cooked material; improved quality charaeteristies of pulp due to a redueed amount of hemi-eelluloses from 7.6% by weight to 3.4% by weight and an increased content of alpha-cellulose from 88.6% by weight to 94% by weight.
Owing to the use of vibroagitation in the proeess of simultaneous impregnation and eooking of the vegetable stoek in a ~ - 8 -13 1~8;3!~i solution of nitric acid, duration of these operations is reduced by 5-7 times. The process for producing pulp with decontamination of the gaseous cooking products makes it possible to eliminate air pollution with toxic gaseous compounds without consumption ~; of any additional chemicais for purification.
- For a better understanding of the present inventi~n `~ some specific Examples illustrating the process for producing pulp are given hereinbelow.
Example 1 Asp chips containing 45 g of absolutely dry wood are ~- subjected to water-treatment at the temperature of 120C under the pressure of 2 atm for 1 hour at the weight ratio between the vege-~- table stock and water of 1:5 respectively. Then the vegetable stock.~ .
is intermixed with a 8% solution of nitric acid and impregnation ~- and cooking are conducted simultaneously at the temperature of 80C for 2 hours! followed by further cooking at the temperature of 120C for 0.5 hour under the pressure of 2 atm.g. The extrac-tion is performed wi~h a 2% solution of caustic soda at a tem-~-~ - perature of from 95 to 100C for 1 hour, followed be decontami-nation of the gaseous cooking products. The resulting pulp - has the following characteristics:
yield of pulp 44.5%
~-~ yield of the under-cooked product 0.6%
content of alpha-cellulose91.3%
content of pentosans 5.8%
viscosity of a 1% cuproa-mmonium solution, ml 848 average degree of polyme-rization 1,343.
- Example 2 Asp chips containing 45 g of absoiutely dry wood are _ g _ subjected to water-treatment at the temperature of 170C under the pressure of 7 atm.g. for 0.3 hour at the weight ratio between the vegetable stock and water of 1:2 respectively. Then the ~ege-table stock is mixed with a 10% solution of nitric acid and im-pregnation is conducted simultaneously with cooking under contin-uous vibroagitation (with the frequency of mechanical oscillations of about 3,000 per minute) at the temperature of 70C for 0.5 hour, followed by further cooking at the temperature of 100C for 0.5 hour under the pressure of 1 atm.g. The alkali extraction is conducted with a 1.5% solution of caustic soda at 90-95C for one hour, followed by d~contamination of the gaseous cooking pro-ducts. The resulting pulp has the following characteristics, per cent by weight:
yield of pulp 46.3%
yield of the non-cooked product 0.1~
content of alpha-cellulose 93.4%
content of pentosans 4.1~
content of lignine 0.15%
Example 3 Birch chips containing 50 g of absolutely dry wood are subjected to water-treatment at the temperature of 150C under the pressure of 5 atm.g. for 0.5 hour at the weight ratio between the ve~etable stock and water of 1:3 respectively. Then the vege-table stock is mixed with a 6% solution of nitric acid and impregnation is-effected simultaneously with cooking at the tem-perature of 85C for 3 hours, followed by continuation of cooking at the temperature of 130C for 0.3 hour under the pressure of 4 atm.g.
The alkali extraction is carried out using a 1.5%
solution of caustic soda for 1 hour, followed by decontamination of gaseous products resulting from the cooking.
The resulting pulp has the following characteristics, ;
weight per cent:
yield of pulp 48.1%
yield of the under-cooked product 0.3~
content of alpha-cellulose90.1%
COntent of lignine 0.~%
content of pentosans 5.4%
Example 4 Birch chips containing 70 grams of absolutely dry wood are subjected to water-treatment at the temperature of 110C
; 10 under the pressure of 2 atm.g. for the period of 4 hours at the weight ratio between the ~egetable stock and water of 1:4 rés-pectively. Then the vegetable stock is mixed with a 4~ solution of nitric acid and impregnation is conducted simultaneously with 711' ~' cooking at the temperature of 90C for 2.5 hours, followed by continuation of cooking at the temperature of llDC for 0.5 hour under the pressure of 2 atm.g.
Tha alkali extraction is conducted by means of a 1.5%
, solution of caustic soda at a temperature of from 90 to 9~5C
,~i for one hour,! followed by decontamination of the gaseous cooking J~:
products. The resulting pulp has the following characteristics, weight per cent:
~! yield of pulp 47.3~
yield of the under-cooked product -0.5%
content of alpha-cellulose92.8%
content of lignine 0.6%
; content of pentosans 5.6%
Example 5 Asp chips containing- 244 g of absolutely dry wood are subjected to water-treatment at the temperature of 110C under the pressure of 2 atm.g. for 0.5 hour at the weight ratio between the ~egetable stock and water of 1:4 respectively. Then the vegetable stock is mixed with a 3% solution of nitric acid and impregnation is conducted simultaneously with cooking at the temperature of 90C for 3 hours followed by further cooking at the temperature of 120C for 0.3 hour under the pressure of 3 atm.g.
The thus-treated chips are divided into four equal portions which are then used for the alkali extraction performed in the following manner:
(a) The first portion of the resulting asp chips is treated with 250 ml of a 1% soda solution at a temperature of 95-98C for 1 hour. Then the spent alkali liquor is separated in the amount of 190 ml. The mass is washed with water and the first portion of the washings (60 ml) is combined with the spent liquor which is then subjected to oxidation at the temperature of 200C
for 2 hours under the pressure of 30 atm. and added with 0.5 g of sodium carbonate to replenish the losses;
(b) The second portion of the asp chips is extracted with the spent alkali liquor oxidized at the temperature of 240C for 0.5 hour under the pressure of oxygen of 20 atm.g. The extraction is carried out at a temperature of 95-98C for one hour. Then the chips are washed and the first portion of the washings is combined with the spent liquor. The latter is again subjected to oxidation for 1.5 hour at the temperature of 200C and then added with 0.5 g of sodium carbonate;
(c) The third portion of the asp chips is subjected to extraction with the repeatedly oxidized spent alkali liquor described in item tb) hereinabove at a temperature of 95-98C
for one hour. The washings are again combined with the spent alkali liquor which is then oxidized for the third time;
(d) The fourth portion of the chips is extracted with the spent alkali liquor preliminary oxidized at the temperature of 250C under the pressure of 10 atm.g. for one hour. Extraction is conducted at 95-98C for one hour.
Quality characteristics of the resulting pulp are shown in Table 1 hereinbelow Table 1 . _ . . .
Quality cha-Example 5Example S Example 5 Example 5 racteristics(a) (b) (c) (d) , Yield of pulp 51.8 52.8 53.6 54 Yield of the under-cooked product 2.5 2.1 2.5 1.8 Breaking length, m 8,200 8,350 Bending stren-10 gth, number of double inflec-tions 1,360 2,770 Tear strength,g 38 65 - -Punching strength, kg/cm 3.5 3,9 Example 6 The method for producing pulp is effected in a manner similar to that described in Example 1 hereinbefore.
(a) The gaseous products resulting from cooking are mixed with the air in the volumetric ratio of 1:0.5 and passed through the spent liquor resulting from extraction of asp chips with a solution of caustic soda;
` (b) The gaseous products resulting from the cooking are mixed with the air in the volumetric ratio of 1:1 and passed through the spent liquor resulting from extraction of asp chips with a solution ofsoda;
(c) The gaseous products resulting from cooking are mixed withthe air in the volumetric ratio of 1:1 and passed through the spent liquor resulting from extraction of asp chips with a solution ofammonia. The test results are shown in the following Table 2.

, . ,~

Table 2 .
Components of Content of nitrogen oxide and dioxide in the the gas mixtu- gas mixture (vol. per cent) re resulting Before de- After decontamination .~ from cooklng of contamina- Example 6 Example 6 Example 6 !~, vegetable stocktion (a) (b) (c) ;
.
Nitrogen oxide 1.6 0.15 0,15 0.1 Nitrogen dioxide 7.8below 0.1 below 0.1 below 0.1 ~- 10 ~s -:

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. ~ .

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, .

Claims (4)

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

1. A process for producing pulp from vegetable stock consisting in the subsequent steps of water-treatment of the vege-table stock at a temperature within the range of from 110 to 170°C
under a pressure of from 2 to 7 atm.g. for a period of from 0.3 to 4 hours at a weight ratio of the vegetable stock to water of 1:2-4 respectively; simultaneous impregnation and cooking of said vegetable stock in a solution of nitric acid at a temperature of from 70 to 90°C for a period of from 0.5 to 3 hours; cooking at a temperature of from 100 to 130°C under pressure of from 1 to 4 atm.g. for 0.3-0.5 hour; an alkali extraction of the vegetable stock; decontamination of the gaseous cooking products by mixing thereof with the air in a volumetric ratio of 1:0.5-1 respectively and treatment with the spent liquor resulting from the alkali extraction stage.

2. A process as claimed in Claim 1, wherein said simul-taneous impregnation and cooking of the vegetable stock in a solu-tion of nitric acid is carried out at vibroagitation with a fre-quency of mechanical vibrations of about 3,000 per minute.

3. A process as claimed in Claim 1, wherein the spent liquor resulting from said alkali extraction is subjected to oxida-tion with an oxygen-containing gas at a temperature of from 200 to 250°C under a pressure of from 10 to 30 atm.g. for 0.5-2 hours and repeatedly used in said alkali extraction stage.

4. A process as claimed in Claim 1, wherein a 3-10%
solution of nitric acid is used for said simultaneous impregnation and cooking of the vegetable stock.