Ind. Eng. Chem Prod Res. Dev. 1983, 22, 506-508
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Dissolving Pulps from Zea Maize by Alkaline Sulfide and Sulfite Pulping Mohamed A. Abou-State," Fouad F. Abd El-Megeld, and Refaat I . Nesseem Department of Chemistry, Faculty of Science, University of Cairo, Giza, A.R.E.
Dissolving pulps are obtained by subjecting prehydroylzed Egyptian corn stalks to alkaline sulfide or sulfite pulping liquors. Lowering the sodium hydroxide concentration by increasing the sulfidity or the amount of sodium used as sulfite at a constant total sodium concentration improves the chemical, physical, and submicroscopic characteristics of the pulp, its degree of delignifition, and results in better chemical reactivii (xanthation). The optimum condition is to use 30% sulfidity, or 30% of the total sodium as sodium sulfite. The latter results in the highest a-cellulose content and the best reactivity toward xanthation. The pulps obtained compared very well with viscose pulps from bagasse and softwood.
The presence of sodium sulfide or sodium sulfite in alkaline pulping liquors results in highly favorable effects. The former increases the rate of delignification and produces marked change in the yield as well as the physical and chemical characteristics of the pulp (Casey, 1960), while the latter accelerates the rate of pulping and improves the strength and bleaching qualities of the pulp (Bray and Springer, 1947). Most of the previous studies about alkaline sulfide and sulfite pulping were concerned with pulp and paper from pulpwood. Very little work was concerned with dissolving pulps and in particular from agricultural residues. The aim of the present investigation was to produce dissolving pulps from Zea Maize by alkaline sulfide and sulfite pulping which are reactive toward xanthation. The a-cellulose content of commercial viscose pulps is about 90,95, and 98% in the case of low a wood pulps, high a wood pulps, and cotton linters pulps, respectively. The degree of whiteness should not be less than 85% as an assurance of the essential freedom from noncellulosic materials. The ash content should be less than 0.15%. A high pentosan content is objectionable since it indicates that the morphological structure is not sufficiently altered to obtain the desired reactivity (Heuser, 1950). Prehydrolyzed corn stalks were subjected to akaline sulfide and sulfite pulping liquors and the effects of these treatments on the lignin content of the unbleached pulp, and its bleachability as well as the chemical, physical, and submicroscopic characteristics of the bleached pulp and in particular its reactivity toward xanthation were investigated. The results were compared with those obtained from cotton linters pulp, dissolving pulp from bagasse, and commercial softwood viscose pulp prepared by conventional pulping methods. Experimental Section
Raw Material. The raw material used in this work was Egyptian corn stalks (Zea Maize). It was obtained from the farms of the Faculty of Agriculture, Cairo University. Prehydrolysis, Pulping, and Bleaching. These were carried out as before (Abou-State, 1974). The raw material was treated with boiling water for 1 h at a liquor ratio of 20:l before prehydrolysis since this reduces the ash content and pentosans and raises the a-cellulose as well as the degree of whiteness. It also lowers the lignin content and improves the bleachability of the pulp (Nesseem, 1982). Prehyclrolysis was carried out with 0.75% sulfuric acid solution for 6 h E t 100 '1' and a liquor ratio of 20:l. This 0196-4321/83/1222-0506$01 50/0
was followed by alkaline pulping for 5 h at 100 "C. The total sodium was 18% (based on prehydrolysed material). In experiment 1 pulping was carried out by the action of sodium hydroxide alone. In experiment 2,10% of the total sodium was as sodium sulfide and the rest as sodium hydroxide, while in experiments 3,4, and 5 the sulfidity was 20,30, and 40%, respectively. In experiments 6, 7,8, and 9 the total sodium was the same as in experiment 1,but 10,20, 30, and 40%, respectively, of the total sodium was as sodium sulfite and the rest as sodium hydroxide. Chemical Analysis of Pulp. The ash, lignin, pentosans and a-cellulose were determined according to the American Tappi Standards T 211 os-58, T 233 ts-63 and T 203 OS-61(Technical Association of the Pulp and Paper Industry). The natural chlorine requirement of the unbleached pulp is the maximum amount of chlorine which can be absorbed by the pulp from a chlorine solution in which the remaining chlorine will be at its minimum. It gives a quantitative measure of bleachability. The natural chlorine requirement was determined as before (AbouState, 1974). Physical Properties. The average degree of polymerization (D. P.) and the degree of whiteness were estimated as before (Abou-State, 1974). Submicroscopic Structure. There are no absolute methods for measuring the submicroscopic properties of the pulp. However, some properties of cellulose give a clear comparative indication of the submicroscopic structure. The degree of swelling in water or in sodium hydroxide liquor are the best common examples. The submicroscopic structure also includes the ratio of crystalline to amorphous cellulose. In this work the water retention value (W.R.V.) was estimated according to Jayme (1958). The liquor retention value (L.R.V.) and sodium hydroxide retention value (NaOH R.V.) were determined by allowing the pulp to swell in sodium hydroxide solution of mercerising strength at 20 "C followed by centrifuging to eliminate the excess alkali. The centrifuged pulp was weighed, washed with distilled water to neutrality, dried to constant weight, and weighed again. The washings were titrated against standard acid. Thus L.R.V. and NaOH R.V. were determined. The reactivity toward xanthation was estimated by carrying out emulsion xanthation using 50 mL of 8% sodium hydroxide solution and 1mL of carbon disulfide. The dissolved cellulose in the viscose was determined volumetrically and was substracted from the original 1983 American Chemical Society
Ind. Eng. Chem. Prod. Res. Dev., Vol. 22, No. 3, 1983 507
Table I. Effect of Sulfidity experiment no. total sodium sulfidity, % liquor ratio maximum temperature, "C time at max temp, h concn of NaOH/100 g of material concn of NaOH soln, % concn of Na,S/100 g of material concn of Na,S soln, % yield, % ash, % lignin in unbl. pulp, % nat. C1, req. of unbl. pulp, % D.P. a-cellulose, % pentosans, % degree of whiteness, % W.R.V., % L.R.V., % NaOH R.V., % crystallinity, % reactivity (% insol. cellulose)
1 18
__
20:l 100 5 31.14 1.56
__ __
36.6 0.06 6.6 3.8 1120 95.1 5.1 89 105.0 275.8 78.9 79.0 70.5
2 18 10 20:l 100 5 28.03 1.40 3.06 0.15 34.5 0.08 5.9 3.5 1140 95.2 5.0 89 106.0 275.1 78.8 80.0 70.7
3 18 20 20:l 100 5 24.91 1.25 6.12 0.31 33.4 0.07 4.8 3.0 1100 96.3 4.8 90 108.2 275.9 78.6 79.0 70.0
4 18 30 20:l 100 5 21.8 1.09 9.18 0.46 32.8 0.09 4.3 2.8 1000 96.5 4.5 91 115.1 282.5 79.8 78.2 69.0
5 18 40 20:l 100 5 18.68 0.93 12.24 0.61 35.7 0.10 5.4 3.3 1080 95.2 5.1 92 104.2 272.4 78.0 81.6 71.9
Table 11. Pulping withAlkaline Sulfite Liquor experiment no. total sodium sodium as Na,SO,, % sodium as NaOH, % liquor ratio maximum temperature, "C time at max temp, h concn of Na,SO, (g/100 g of material) concn of Na,SO, soln, % concn of NaOH g/100 g of material concn of NaOH soln, % yield, % ash, % lignin in unbl. pulp, % nat. C1 req. of unbl. pulp, % D.P. a-cellulose, % pentosans, % degree of whiteness, % W.R.V., % L.R.V., % NaOH R.V., % crystallinity, % reactivity (% insol. cellulose)
1 18
__
100 20:l 100 5
__-
31.14 1.56 36.6 0.06 6.6 3.8 1120 95.1 5.1 89 105.0 275.8 78.9 79.0 70.5
amount of cellulose according to the method of Fock (1959). The degree of crystallinity was determined by the iodine adsorption method of Hessler and Power (1954). Results and Discussion Effect of Sulphidity (Table I). It is clear from Table I that increasing the sulfidity improved the degree of delignification and the bleachability. It also resulted in an increase in the a-cellulose content and a decrease in pentosans. The degree of whiteness was improved and the pulp obtained was characterized by more open and accessible inner structure as indicated by higher W.R.V., L.R.V., and NaOH R.V., lower crystallinity, and better reactivity toward xanthation. These improvements in pulp characteristics increased with sulfidity up to 30% (experiment 4). Further increase in sulfidity to 40% (experiment 5) increased the yield and improved the degree of whiteness, but increased the lignin content and the bleachability, and resulted in a less accessible fine structure. It was generally observed that the increase in sulfidity was accompanied by a slight increase in the ash content. This was probably due to the decrease in the alkalinity of the pulping liquor.
6 18 10 90 20:l 100 5 4.93 0.25 28.03 1.40 36.0 0.06 5.5 3.4 1080 95.4 4.7 90 112.4 277.9 80.2 78.0 69.6
7 18 20 80 20:l 100 5 9.86 0.49 24.01 1.25 35.0 0.07 5.0 3.0 987 96.5 4.4 91 118.5 280.4 81.8 76.3 68.0
8 18 30 70 20:l 100 5
14.80 0.74 21.8 1.09 33.1 0.09 4.2 2.3 970 96.9 4.0 93 125.4 289.5 83.9 75.1 66.8
9 18 40 60 20:l 100 5 19.73 0.99 18.68 0.93 35.7 0.12 4.8 2.5 1100 95.7 4.5 94 109.1 279.0 82.0 79.3 70.4
Pulping with Alkaline Sulfite Liquor (Table 11). In experiment 1prehydrolyzed corn stalks were subjected to pulping with sodium hydroxide alone. The use of 10% of the total sodium as sodium sulfite (experiment 6), improved the degree of pulping, as indicated by the decrease in the lignin content and the natural chlorine requirement of the unbleached pulp, lowered the yield, D.P., and pentosans, improved the a-cellulose content and the degree of whiteness, and resulted in higher affinties toward water and alkali, lower crystallinity, and better reactivity. Further increase in the amount of sodium used as sodium sulfite to 20 and 30% (experiments 7 and 8, respectively) resulted in further decrease in the yield, D.P., and pentosans. It also increased the a-cellulose content, the degree of whiteness, the accessibility to water and alkali, and the reactivity toward xanthation. However, further increase in the amount of sodium sulfite at the expense of the hydroxide (experiment9) improved the degree of whiteness and yield, but increased the lignin content and the natural chlorine requirement, deteriorated the chemical characteristics, and resulted in a less open and accessible fine structure and lower reactivity toward xanthation. The decrease in alkalinity resulting from increasing the amount
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 22, No. 3, 1983
Table 111. Properties of Other Dissolving Pulps from Wood and Nonwood Fibers
type of pulp a-cellulose, % pentosans, % ash, % degree of whiteness,
cotton linters pulp 99.10
___
0.08 91
dissolving pulp from bagasse 92.56 4.51 0.07 86
commercial softwood viscose Pulp 93.61 3.20 0.04 89
%
D.P. W.R.V., % L.R.V., % NaOH R.V., % crystal1 inity , % reactivity ( % insol. cellulose)
2340 73.9 247.9 50.7 91.7 95.3
920 120.3 315.6 67.0 68.8 70.3
860 121.o 317.5 67.8 72.6 68.5
of sodium sulfite at a constant total sodium concentration increased the ash content. Properties of other Dissolving Pulps from Wood and Nonwood Fibers (Table 111). The cotton linters pulp was characterized by the highest a-cellulose content. However, it was the least reactive toward xanthation due to its highly ordered and compact fine structure as indicated by high crystallinity and low affinities for water and alkali. The softwood pulp was of higher a-cellulose and better reactivity than the bagasse pulp. It is clear that the Zea Maize pulps compared very well in their properties with those of the bagasse and the softwood pulps. They were of higher a-cellulose contents. Pulping with alkaline sulfite liquor in which 30% of the total sodium was as sulfite and 70% as hydroxide (experiment 8) resulted in the highest a-cellulose content and the best reactivity. The next step in product development will be pilot plant ex-
periments on pulps from Zea Maize. Conclusions Increasing the sulfidity up to 30% improves the degree of delignification, the bleachability, the a-cellulose content, the degree of whiteness and results in a more open and accessible fine structure and better reactivity toward xanthation. When pulping is carried out with alkaline sulfite liquors the use of 10% of the total sodium as sulfite improves the degree of pulping and bleachability, lowers the yield, D.P., and pentosans, raises the a-cellulose and the degree of whiteness, and results in higher affmities toward water and alkali, lower crystallinity, and better reactivity. This goes on when the amount of sodium used as sodium sulfite increases up to 30%. The Zea Maize pulps compared very well in their properties with the viscose pulps from bagasse and softwood. They were of higher a-cellulose content. Pulping of Zea Maize with alkaline sulfite liquor in which 30% of the total sodium is as sulfite and 70% as hydroxide results in the highest a-cellulose content and the best reactivity. Registry No. Sodium hydroxide, 1310-73-2.
Literature Cited Abou-State, M. A. faserforsch. TetMech. 1874, 25(12), 552-554. Bray, M. W.; Springer, B. Paper Trade J . 1847, 125(8),49-53. Casey, J. P. "Pulp and Paper. I"; Interscience: New York, 1960; p 238. Fock, W. fapier (Darmstadt) 1858, 73,92-95. Heuser, E. Tappll850, 33, 118-124. Hessler, L. E.; Power, R. E. Text. Res. J . 1854, 2 4 , 822-827. Jayme, 0. Tappi 1850. 4 1 , 180A-183A. Nesseem, R. I . Thesis, Department of Chemistry, Faculty of Science, Cairo University, Glza, ARE., 1982. Tappi Standards, Technical Association of the Pulp and Paper Industry, 1 Dunwoody Park, Atlanta, GA 30341.
Receiued for reuielv September 13, 1982 Accepted April 13, 1983
