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1 Products: Sub-Products THE ROLE OF SUGARCANE STALK FIBERS IN BAGASSE CHEMICAL PULPING J. Femhdezl & H. P6rez2 Institute de Investigaciones de Pulpa y Papel CUBA-9, PO Box 8, 33500, Quivich, Havana ICIDCA, Via Blanca, No. 804 y Canetera Central 4026, Havana ABSTRACT Bagasse is fundamentally composed of three fibrous elements from the sugarcane stalk: the rind, fibrovascular bundles and pith. The ratio of these three elements to each other will depend chiefly on the cane variety. The main characteristics and contribution of each element to the strength properties of chemical pulp were investigated. Rind fibers contributed positively to strength properties of bagasse pulp, especially tear resistance. The presence of pith accentuated pulping and papermaking problems. The Monda and Tilby technologies for producing sugar separate the fiber fractions, permitting the production of higher quality bagasse. Keywords: Sugarcane, fiber, bagasse, soda pulping, strength properties, Cuba.

2 INTRODUCTION Interest in the use of fiber from annual plants-specifically bagasse from sugarcane-for papermaking has increased in recent years. Almost 100 paper mills worldwide are producing different kinds of papers and boards from bagasse (Espinosa 1986); however, despite the great availability of bagasse in some developing countries, pulp and paper production from this raw material is extremely low. This could be due, on the one hand, to a lack of lcnowledge about the chemical, physical and morphological characteristics of bagasse fibers and on the other hand, to the lack of suitable pulping methods for small mill operations, typical of this kind of raw material. Bagasse, which is the fibrous residue remaining after juice extraction in the sugar mill, is composed fundamentally of three elements from the cane stalk: the rind, fibrovascular bundles and pith. In addition there are some fragments of green leaves, tops, attached dead leaves and dirt.

3 The proportion of the three main elements will vary depending mainly on the cane variety (Lathrop & Aronovsky 1954). The structure of the cane stalk consists of two parts: the rind (the outside part) comprises a relatively tough ring having a layer of thick-walled cells protected by an outer cuticle and the pith or parenchymatous tissue (inside part). Lengthwise, through this tissue are located numerous fibrovascular bundles (Fig. 1). The smallest are packed near the outside; the largest are distributed toward the center of the stalk. The pith groups around and adheres both to the fibrovascular bundles and rind fibers. The sugar in the stalk is concentrated in the pith, while the fibers are concentrated in the rind. In the rind there are basically thick-walled fibers with narrow lumen, which are relatively long, thin- walled fibers, with wide lumen having a shorter length. The fibrovascular bundles consist of vessels.

4 And sieve tubes, surrounded by thin-walled fibers. From a papermaking standpoint, the pith is com posed of delicate thin-walled cells denominated nonfibrous elements for their short length and large diameter. When bagasse is compared with the tradi- tional source of papermaking fibers, it has the disad- vantages of having a greater variety of cell types and - a higher percent of nonfibrous elements (Femdez et mgure a a1 1995, Triana et a1 1990). When cane is crushed, cross-section of cane stalk. fibers are damaged, resulting in a low-quality bagasse J. Feddez & H. Perez in terms of the strength properties of the pulp produced from this material (Gartside et a1 1983). Development of nonconventional procedures in sugar making, which do not cause fiber damage, would be expected to increase the overall fiber length and improve the strength obtainable with bagasse pulps. Two new processes for producing cane sugar by means of nonconventional techniques are: * 'Tilby (Keith 1978) or cane separator process (Bourmschky 1988).

5 The separator process is based on separation of the hard outer rind of the cane stalk from the soft pithy tissue containing the sucrose. This process yields wax and a pith-free fiber fraction, which has optimal morphological and chemical characteristics for producing paper and board (JCeith 1978). * Monda technology (ABTECNIA 1988). Based on the principle of separating the cane stalk in its two basic components: the fibers and pith. High-efficiency techniques are applied for juice extraction in both components. The fibrous fraction so obtained is of high quality for pulping and board. It has a low content of pith, solubles and fines (ABTECNIA 1988). The purposes of this research were to: Investigate the characteristics and contribution of cane stalk fibrous elements when chemical pulp is produced Compare the bagasse chemical pulping with pulping from other raw materials supplied by new sugar-making processes that permit the separation of the fiber fraction from the cane stalk MATERIALS AND METHODS The cane stalks selected for this study were cut manually in the field; and the leaves, tops and trash carefully After removing the epidermis, the stalks were crushed in three regular sets of milling rolls used for juice extraction.

6 The separation of the hard outer rind of the crushed stalks from the soft inner tissue was done by hand. These two fractions were cut to approx. 5 cm length and soaked in water overnight. Afterwards they were treated in a lab refiner (Sprout Waldron Model 105-A) equipped with 305-mm diameter plates @2A-507) in order to detach the fiber bundles in a wet process. Pith and fiber fractions, to which water was added, were separated mechanically using a vibrator screen. Monda and Tilby process fibers were supplied bj; pilot plant facilities and bagasse was obtained after a two-stage commercial depithing. Chemical cooking was camed out in a rotating digester (18-1 capacity), heated indirectly by electrical resistances. different dosages of NaOH (12, 15 and 18 %) were applied. Cooking parameters were kept constants (max. temperature: 160 C; liquor-to-fiber ratio, 7:l; time to temperature, 30 min; time at temperature, 30 min). Beating of pulps was performed in a PFI mill at 10% consistency.

7 Biometric and strength properties were determined accordhg to TAPPI and SCAN standards, resp. RESULTS AND DISCUSSION Table 1 compares the properties of cane fibers for bagasse, Monda and Tilby fibers. Fibers from the rind and the Tilby process present greater avg fiber length than those from the other process. No desirable differences were found in fiber diameter or wall thickness. The influence of alkali dosage and Qp of fibrous elements on cooking yield is shown in Figure 2. The higher the alkali charge, the lower the cooking yield. As reactions proceed faster in those materials having a greater contact surface, the chemical would be expected to act Table 1. Fiber dimensions of different fibrous sources. much more rapidly on Dimensions Fibrovascular Monda Tilby pith and fibrovascular (mm) Rind bundles Pith Bagasse fiber fiber bundles than on rind; Avg length (L) therefore( the decrease in Avg diw. @) yield would be similar.

8 Lumen width - As can be seen in Wall bidth - Figure 3, the pulp made LID 56 from only pith has the highest beating degree. Products: Sub-products Table 2. Strength properties of pulp from cane stalk elements. Properties Unit --------Rind ---------- ---Vascular bundles-- -------Pi& ------ -------- Bagasse------- PFI revolu- 0 1 0 1 0 0 1 tions ('000) This fact is of technical importance because this type of pulp will present poor drainage; whereas pulp from rind has better Cookcng yield (%) drainage. Obviously, in the case of bagasse, the drainage of 65 the pulp will depend on the amount of pith. Rid fibers are the longest in the cane; for this reason they [ will produce stronger papers, particularly with respect to tear 55 resistance pig. 4). As expected, pulp from pith makes a I Beating 13 21 33 43 16 21 37 43 33 45 57 16 24 36 47 degree (SR) sheet of low tear index independent of the alkali charge.]

9 Pulp made from rind fibers raised the tear index by 23 % compared 45 with that of bagasse. The strength properties of pulp from cane stalk fibrous Density 500 600 720 740 530 610 710 750 660 730 770 500 590 700 750 (kg/m3) Burstindex (Wam'Vg) Tearindex 2 (mNm2/g) Pfth -* - Breaking length (km) elements after beating in a PFI mill are shown in Table 2. t Elongation Fold (no.) 9 23 200 290 16 30 97 130 - 15 58 180 270 35 - 12 15 la Cooking parameters were kept constant: temperature (160" C), NaOH (%I NaOH (16 %), reaction time (60 min). The analyzed Figure 2. Influence of alkali charge and type properties-except tear index-increased their values when of fibrous elements on cooking yield. beating degree or no. of revolutions was increased in the PPI mill. As expected, pith decreased drainage, making the paper denser with a lower tear index; but contrary to our expectations, breaking length was increased.)

10 The strength properties of pulp from rind are superior to those from the other fibrous elements when compared at the same beating degree. According to the degree of depithing, the strength development for bagasse pulp will vary so that pulp from pith-free bagasse will tend to approach the strength of rind pulp. As can be seen in Table 3 the strength properties of traditional softwood pulp are superior to the corresponding values for pulp from any sugarcane fibrous elements. Pulps from the Monda and Tilby processes are better in drainage, density, tear index and breaking length than pulp made from bagasse. Table 3. Strength properties of pulp from different raw materials. Properties unit Monda fiber Tilby fiber Depithed bagasse Softwood kraft PFIrevolutions('000) 0 0 0 0 Beatingdegree(SR) 12 18 30 43 12 17 28 40 16 24 36 47 14 25 35 45 Density (kg/m3) 520 630 710 730 520 590 620 710 500 590 700 750 430 600 635 660 Burstindex(kPamz/g) Tearindex(mNm2/g) Breakinglength(km) Elongation (%) Fold (no.)