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Xhmka PACTHTE^tHoro CMPba. 2007. №4. C. 31-36.
THE PULPING OF ESPARTO GRASS BY SODIUM MONOSULFITE
© Akchiche Omar , MessaoudBoureghda Khadra
Laboratoire des mineraux et des materiaux composites, Universite de Boumerdes,
35000 (Algeria) E-mail: [email protected]
Traditionally, the esparto’s limbs, in the manufacturing process of the cellulosic pulps, are delignified according to the process chlorinates - alkaline which alternate the action of caustic’s soda and chlorine; the reason is that these sheets cannot be free from the silica which they contain that only in strongly alkaline medium.
In spite of the relative simplicity of implementation of this process, it does not remain free from grievance about it: low yield, an excessive consumption of bleaching agents (5,4% of total chlorine) for a whiteness not exceeding 80 points, a significant time of pulping (3,5 h), the pulp’s quality is low, difficulties of regenerating the chemical agents, which generates the pollution of the environment, due to the rejections of the effluents in nature.
The study that we present constitutes a contribution in the search of the pulping process likely to replace that current. To this end, we chose the process with the neutral sulphite (NSSC pulping) for its simplicity of application, the yield and the quality of the pulp obtained in the treatment of wood.
Following the conclusive preliminary tests that we carried out, the pulping parameters were selected of such kind, which the pulp have of the kappa number ranging between 20-30, i.e., of the pulp easy to bleach, intended for the manufacture of papers for impression and writing.
Mechanical tests carried out on hand sheets supplement the study on the research of the optimization of pulping process.
The cellulosic pulps, obtained with optimized parameters, were bleached according to the traditional bleaching stages.
Bibliographical review
The manufactured papers containing the esparto cellulosic pulps are famous for their excellent quality of printing. The attempts aiming to improve the properties carried all on the processes of pulping of the plant. The application at esparto of the new variants of the alkaline process, clean «without sulfur», «respecting» the environment, whose reagents can be regenerated by the usual methods, seems to monopolize the attention.
Thus, in the study devoted to the manufacture of the cellulosic pulps based on esparto, the authors [2] exposed the possibilities offered by the process soda-oxygen and NaOH + Na2CO3 - oxygen, which seems to supplant in the future the process Kraft in paper industry.
They announce that operation in two stages of the pulping process of Na2CO3 - oxygen improves delignifica-tion: the pulps which result there are equivalent in quality to those intended for the manufacture of the Kraft papers; the yield, obviously, being more raised, (47%) in average.
In work of thesis, the author [3] exposed the optimized pulping parameters to the esparto soda in the presence of the anthraquinone and extracted polyphenols of the eucalyptus; the addition of anthraquinone, in a rate of 0,1% compared to dry esparto absolutely makes the soda pulping process as selective as that kraft.: The characteristics of the obtained pulps , except the tear factor, compete with those of the kraft pulps. The use in the capacity as pulping catalyst the extracts of eucalyptus in quantity of 1,5% compared to the absolute dry esparto, contributes to the improvement of the yield in pulp from 2,0 to 2,5% compared to the process with soda. After studies of pulping of the esparto and eucalyptus mixture, the authors [4] note that the permanganate number of the pulps decreases relative to esparto increases in the mixture. The obtained pulps have: higher qualities of clearness (Cielab colorimetry), a very high aptitude for bleaching, a high yield (46,0%) and a weak permanganate number (13,6), they are thus the conclusions of this study relating to the analysis of the paper characteristics of mixture esparto with eucalyptus, treated in kraft process.
Method and equipment employed
Beforehand, the grass is cut out in brushwood of length 30 at 40 mm, dusted under real conditions of use in the factory and let dry at the ambient air so that moisture is uniform and given for all the series of tests. The brushwood is stored in a ventilated case.
* Author for correspondence.
Equipement description. Pulping tests are carried out in cylindrical stainless steel digesters of 0,5-1 of volume, immersed in glycerine, which has been used as fluid heater filling the laboratory's cooker of a volume of 15 litres. The rises in temperature are controlled by an automatic regulator. The homogenisation of the temperature of the bath is ensured by a propeller of mixing of the fluid heater.
This cooker can receive three cylindrical digesters, thus we have the possibility to carry out three tests of cooking each time under the identical conditions.
Employed Methods. The cooking's tests method led to the laboratory is inspired of that of micro-cooking [5]. It is a technique used in laboratory to reproduce at reduced scale the industrial cooking processes of the plants for the manufacture of the paper pulp.
Preparation of the cooking's liquor. The sodium sulfite cooking's liquors used are in the form of titrated solutions, which makes possible to use a volume containing for each test the exact quantity of reagent indicated by the experiment conditions. The reagents are pure chemically. During all our tests, we maintained the liquid esparto constant ratio equal to 5, this for reasons of expansion (increase in the chopped esparto volume) for a better homogeneity of cooking; since the digester are fixed. Consumption of the reagents compared to the absolute dry plant is expressed respectively in their own unit.
Pulp analyses. After pulping, the pulp is washed in a laboratory spin-dryer until to neutral pH (tested with phe-nolphthalein), disintegrated up to fibres in a disintegrator of the SPROUT WALDRON type, and are classified.
- Evaluation of the rate of rejects.
- Kappa number NF. 12018.
- Determination of the yield of the screened pulp.
- Determination of remaining lignin. TAPPI T 222 bones-74
The principal mechanical characteristics of the unbleached pulp, after refining until ° SR 40, were determined on sheets of weight 75g/m2 approximately, made on RAPID KOTHEN formet.
■ The Schopper-Riegler °SR. is given in accordance with standard NF Q 50 003.
■ Breaking length: NF Q 03-004.
■ Burst factor: NF Q 03-053 and NF Q03-054.
■ Tear factor. NF Q 03-011.
The unbleached pulps are bleached according to the different sequences of conventional bleaching C E H1 H2, C E1 D1 E2 D2, C/D E1 D1 E2 D2.
Realization of the sodium monosulfite pulping.
The sodium monosulfite has been among the first chemical agents suggested for the manufacture of the cellu-losic pulps. The process belongs to sulfite at soluble bases. The cooking agent is a solution of Na2S03 in the presence of alkalis to maintain the pH of medium of neutral cooking.
A neutral reactional medium or slightly alkaline have a double advantage: the protection of hemicelluloses of a possible degradation; in addition, to protect the equipment of corrosion
For the description of delignification effectiveness of esparto by the neutral sodium sulfite we were brought to examine several conditions of the process for an optimization of a cooking mode Optimization has been carried out on three accessible parameters in experiments: the temperature, duration of cooking and the consumption of the agents of delignifica-tion; it consists in varying one of the parameters by keeping two other. The quantity of vegetable matter to treat is fixed at an absolute dry weight of 60 g for each test. In the glycerin bath used, one can immerse three cookers cylinders simultaneously; what enables us to realize under the identical conditions, three cooking tests for each operation.
Experimental conditions, first series
Variable parameters. Sodium sulfite rate at 9 to 30% with a step of 3,0%, compared to the absolute dry plant.
Fixed parameters. Maximum cooking temperature. 170 °C; Cooking time 60 mn min; The rise time of temperature: 30 mn min.
The results and discussion
The average values of the characteristics of the pulps, summarized in table 1, indicate that the increase in the rate of Na2S03 from 12 to 21% in weight of the chopped dry esparto induces an improvement: of delignification of yield and pulps mechanical properties.
The presence of rejects would be at origin the insufficiency in liquid of cooking of delignification agent. At the
rate of Na2S03 equal to 21% corresponds a pulp whose the ratio Kappca 102 = 0,75 [5], testifies of the better bleach-
yield
ing aptitude.
Table 1. Characteristics of the cellulosic esparto grass pulps obtained by sodium monosulfite pulping
Sulfite Screened Rejects, Ash rate, Remaining Kappa Breaking Burst fac- Tear factor, An average
rate, pulp yield, % % lignin , number length, tor, mN-m2/g Brightness,
% % % m kPa-m2/g %
9 52,7 17,4 3,84 5,92 38,1 4460 3,70 1,90
12 54,6 14,5 3,56 5,86 36,50 4450 3,66 2,50
15 57,1 9,6 3,97 5,32 33,84 4620 3,84 3,42
18 58,8 3,2 3,41 4,92 29,70 4850 4,02 4,10 52
21 57,8 2,9 3,81 5,06 26,34 4985 4,10 4,00
24 57,2 1,1 3,88 4,88 23,22 4830 4,20 4,30
27 56,8 1,5 4,12 4,72 21,60 4708 4,10 4,30
30 56,3 - 4,20 4,80 22,12 4850 3,80 3,10
With the expenditure in Na2SO3 higher than 21%, the rejects miss radically in the pulp, the yield, and the kappa number thus that the mechanical characteristics don't present any notable change what corroborates the study relating to the pulping of the corn straw with the same process [6].
Indeed, at these rates, the yield oscillates around 56...57%, the Kappa number is on average equal to 22,3. With the increase of rate in the sodium sulfite, one notes the increase in the quantity of ashes up to 4,20% in the pulp after incineration. Contrary to the esparto pulp obtained by the Kraft process, this one is about 2,0%.
Experimental conditions, second series
Variable parameters. Sodium sulfite rate. 15, 18, 21% compared to absolute dry esparto; Pulping temperatures to the stage 155, 160, 165 et 170 °C (and); Cooking times: 30, 60, 90 min.
Fixed parameters. The rise time of temperature 30 min.
The results and discussion
The figures 1 and 2 representing the graphics reflect the results of the study on the kinetics of the esparto delig-nification by the neutral sodium sulfite. This study relates the interaction and the influence reciprocal and simultaneous of the various technological parameters on the yield and the kappa number of the pulps. It follows from the data of the experiments.
That with the simultaneous increases in the cooking temperature and the sulfite rate in the intervals chosen, the yields in induced pulps grow in a decreasing way, it follows from there an improvement of the delignification which has as a result a fall of the kappa number and a regular and consequent reduction of the rejects
That is in the temperatures interval of 155...165 °C and for the sulfite rates equal to 15 and 18% that the profits in yields and the reductions in the kappa number and the rejects are significant practically, we have an increase in yield of 1% for a rise in temperature of 5 °C.
The maximum variations of the increase in the yield and the kappa fall of 3,2 and 4,9% are respectively raised for a rate of sulfite of 18%. At the temperatures of 165...170 °C and for the sulfite rates of 18 and 21%, the yields converge towards a maximum with a total absence of rejects, at the same time to that regularly, the kappa number drops but the curves declivity tends to reduce towards 165 °C, the intensity of delignification arrives at its limits.
The analysis of the influence of the increase in the cooking duration on the yield and the kappa number of the pulps, indicates: that for a sulfite rate given (15%), the pulp yields increase approximately to 1,1% with each increase in 30 min, while the kappa number decrease by 2 points. The characteristic curves are almost parallel. As and when the sulfite rate and temperature increase, the curves converge towards a value of the yield close to 58%.
Conclusion
Thus, if theoretically the delignification depends on the expenditure in reagents of cooking, however, in fact, the dissolution of lignin for the sulfite rates chosen depend more temperature and duration of cooking; these two factors appear determining on the course of the reactions.
If at the beginning, the duration and the cooking temperature are essential factors during the process of delignifi-cation, once the final temperature (165... 170 °C) is reached, the influence of the cooking time is reduced.
The sodium monosulfite pulping in presence of alkalis
During cooking, the delignification reactions involve the formation of the organic acids, the pH of medium, drop and become then favorable to the hydrolysis and the dissolution of the low molecular weight carbohydrates, especially at the end of the cooking. This affects the yield and mechanical and optical pulp properties.
Fig. 1. Influence of the variation of the temperature and the cooking times on the pulp yield. The sodium sulfite rates are respectively a=15%, b=18%, c= 21%
155 160 165 170
(f) Temperature,°C
Fig. 2. Influence of the variation of the temperature and the cooking times on the kappa number. The sodium sulfite rates are respectively: d=15%, e=18%, f= 21%
To mitigate this process insufficiency, one adds an alkali to the cooking liquor. The advantage would not be either exaggeratedly to increase the alkalinity which can involve a decomposition of polysaccharides and a pulp with very marked color with a less yield, but to create a buffer solution.
With this intention, we carried out a cooking series to the sodium sulfite, first in the presence of soda then of the soda bicarbonate.
Experimental conditions of sodium monosulfite pulping in presence of soda and soda bicarbonate
Variables parameters. Soda rates are 1,5; 3,0% compared to chopped absolute dry esparto; Soda bicarbonate rates are 1,5; 3,0% compared to chopped absolute dry esparto.
Fixed parameters. Sodium sulfite rate 15% compared to chopped absolute dry esparto; Maximal temperature of cooking. 170 °C; Duration of cooking: 60 min.
The results of cooking are exposed in table 2.
Interpretation of the results
If one refer to the characteristics of the sulfite pulp, the addition of soda to the cooking liquor contributes to the rise in the pH of the reactional medium at the end of cooking; what has as a consequence: an improvement of the output of 1% (variant I) followed by a fall of almost as much (variant II); delignification, even if it seems to improve, the fall of the kappa number is only 1,6.
The refining faculty of the pulp increases in the presence of soda like additive, which confirm °SR degrees reached after twenty minutes of refining of the various pulps.
The tear factor and the brightness of the pulps are the characteristics most sensitive to the variations of the content of soda. The first progressively increases with increase in this content, while whiteness, on the contrary, it regressed.
From the start, the variant with 1,5% of soda addition seems the best. The yield (58,2%) and Kappa of (21,5), translate a better report of delignification contained in polysaccharides than the other variant, the tear factor is improved, the clearness of the pulp weakened, the other characteristics did not vary much compared to those of the pulp with reference neutral sulfite.
The yields, the refining degree and the brightness of the pulp are improved in cooking pulp in the presence of bicarbonate of sodium, while the other characteristics remain rather close to those obtained by associating sulfite and soda.
However, more extended study on the kinetic of pulping according to the composition of the cooking liquor could make the most of our results.
Bleaching tests
In order to test the aptitude for bleaching, three cellulosic pulp samples (3x50g each one of absolutely dry fibers), resulting from cooking with optimized modes, are subjected to the following bleaching of the conventional sequences preached under the industrial conditions of manufacture of papers writing and printing, [1, 7, 8]. The first sequence of bleaching is in conformity with the practice in force in the plant. The two other diagrams are inspired by the new developed technique lately ECp (ECF) (Elemental Chlorine Free) which reduce the environmental impacts of bleaching. The sequence of bleaching ECF uses as well ClO2 like principal reagent for delignification, at the beginning, as for bleaching with several phases, thereafter.
The experimental conditions, the characteristics of the unbleached pulp and those after bleaching are consigned in tables 4 and 5.
Table 2. Influence of additions, soda and soda bicarbonate on the papermaking characteristics of the esparto pulp
General conditions and characterization of pulps Reference sulfite pulp Sulfite pulp with NaOH I Sulfite pulp with NaOH II Sulfite pulp with NaHCO3 I Sulfite pulp with NaHCO3 II
Add of NaOH and 0 1,5 3,0 1,5 3,0
NaHCO3, %
pH initial 7,8 11,78 11,92 10,26 10,82
pH final 6,4 9,64 9,82 7,68 8,72
Ash, % 3,58 3,50 3,48 3,74 3,63
Kappa number 22,3 21,5 19,9 23,00 21,30
Yield, % 57,2 58,2 57,4 58,9 58,4
SR to 0, 10, 20 min of 22 34 46 21 38 54 26 36 58 26 44 56 25 48 58
refining time
Breaking length, m 4680 4690 4710 4660 4710
Burst factor, KPa-m2/g 3,60 3,68 3,86 3,72 3,84
Tear factor, mN-m2/g 3,6 3,8 4,20 4,12 4,10
Brightness, % 52 50 46 54 54
Table 4. Experimental conditions of bleaching
parameters/stage Chlorine, C Chlorine/dioxide, C/D Extraction, E NaOH Hypochlorite, H NaClO Chlorine dioxide, D ClO2
pulp consistency, (%) 3,5 3,5 10 7 10
Time, (min) 60 б0 60 90 180
Temp. °C 20 20-25 60 36-40 80
Chemical addition (one pulp) 50 for stage se- 25/25 1%NaOH/ 35 et 15(H1, H2) 35 et 15(D1, D2)
%Cl2 compared to the total quence I, 40, 40 for absolutely and and
necessary with bleaching sequence II et III dry pulp
pH 2,2 2,8 - 5,5
The overall consumption of active Cl 2 compared to the absolutely dry esparto pulp is 3%
Table 5. Characteristics of bleached pulps
Bleaching stage Nature of pulp Yield after bleaching Brightness, % Breaking length, m Burst factor, kPa-m2/g Tear factor, mN-m2/g Double fold
Sulfite I 88,2 7б,0 4200 4,60 4,12 26
C E1 H1 H2 Sulfite Soda II 85,8 72,0 4700 4,50 4,36 30
Sulfite bicarbonate III 87,7 78,0 4550 4,64 4,26 42
Sulfite I 91,8 82,0 4500 4,54 3,92 36
C E1 D1 E2 D2 Sulfite Soda II 92 78,4 5000 4,62 4,24 70
Sulfite bicarbonate III 92,2 81,2 4860 4,78 4,42 64
Sulfitel 93,9 84,3 4680 4,46 4,08 68
C/D E1 D1 E2 D2 Sulfite Soda II 91,7 82,б 5220 4,68 4,56 78
Sulfite bicarbonate III 94,4 85,8 4870 4,62 4,54 74
Results and discussions
Relatively to soda pulp, the neutral sulfite pulps only or sulfite pulp obtained in the presence of an alkali buffer are much easier to bleach, they require less derivatives chlorinated to reach higher brightness. Of the three bleaching sequences suggested, the third seems to adapt best. The losses in yield in bleached pulp are approximately 6%, the maximum degree of whiteness reached is of 87% and good mechanical characteristics, in particular the length of rupture to 5220 m; this confirms one of the advantages of the use of the chlorine dioxide in a sequential bleaching, which is an oxidizer soft, which causes one very light decomposition of cellulose. This study would deserve to be supplemented by: a measurement of viscosity, determinations of the copper index and alkaline solubility of the pulp to confirm the validity of our assertions.
Conclusion
On the basis of these result, we can affirm that the sodium monosulfite process of cooking can ensure, after a choice of a suitable mode of cooking, a good esparto grass delignification and so it can be recommended to manufacture cellulosic pulps easy to be bleached and intended to various uses and by the same one, to replace the alkaline process advantageously.
Bibliography
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2. Houacine A., Pla F., And Robert A. Pulping of Esparto (Stipa tenacissima L) by Alkaline Processes. Cooking with Na2CO3, Na2CO3 + O2 and NaOH - Na2CO3 + O2.Sonderdruck aus // Holzforschung und Holzverwertung. 1988. V. 40. Heft 2, S. 26-33.
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5. Janin G. Technique des micros cuissons papetieres. Perfectionnement et precision des rendements en pate / Annales des sciences forestiers. 1981. P. 107-125.
6. Rue J., Monson O. Pulping of straw with sodium sulfite // Pulp and Paper. 1985. №6.
7. Hamzen Y. Evaluation des nouveaux procedes de delignification et de blanchiment en reacteur a deplacement de liqueur en comparaison avec les procedes traditionnels. These de Doctorat, INPG; EFPAG Grenoble 2005.
8. Mortha G., D' Aveni A., Renaud M. Etude du blanchiment par ClO2 des pates cellulosiques dans un reacteur a alimentation progressive // Revue ATIP. 1990. V. 44. №7.
Поступило в редакцию 29 мая 2007 г.
После переработки 25 июня 2007 г
