Optimisation of wheat straw Triticum drum kraft pulping

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Abstract

This paper explains the problems brought about by the raw material and pulping method used in the Afyon-SEKA Pulp Mill producing bleached pulp from mixed wheat straw (Triticum drum L.) (65%) and reed (Phragmites communis) (35%). The use of mixed raw material causes such problems, as heterogeneous pulping, low freeness level, drainage difficulties and wet sheet breaks during the papermaking operations. In addition, silica in raw material results in rather serious difficulties during recovery process. To overcome these problems, first of all, pulping of wheat straw and reed had to be carried out separately. In this case, sulphate pulping characteristics of wheat straw were investigated and optimum parameters were found to be: active alkali charge 14%, sulphidity 20%, maximum temperature 160 °C, time to maximum temperature 55 min, time at maximum temperature 40 min and liquor to straw ratio 4:1. Some properties of wheat straw sulphate pulp produced at optimum cooking conditions were determined to be: screened yield 42%, kappa number 31, SCAN viscosity 1114 ml/g, breaking length 9.8 km, burst index 4.6 kPa m2/g, tear index 5.1 mN m2/g and brightness 29 ISO%. Consequently, it was possible to produce bleachable grade pulp from pure wheat straw in a shorter pulping time and lower active alkali than that of Afyon-SEKA Pulp Mill. Thus, this study showed that production capacity of the mill could be increased and freeness level of pulp improved.

Introduction

In many countries, the wood supplies available will not continue to meet the rising demand for very long; there are many countries wishing to produce pulp and paper where wood is not readily available and in some countries where the market for pulp and paper product is limited. This lack of raw materials has fostered research of potential substitute for wood (Dean, 1993, Edwars, 1993). Wheat straw has long been the subject of much research in this respect (Nardi, 1993, Cheng, 1993). Moreover, the open structure and low lignin content of cereal straw make its chemical processing relatively easy in particular nonsulfur processes.

In Turkey, pulp production in 1999 is 360 000 tonnes/year of which about 31 000 tonnes is non wood based pulp. In addition, the entire pulp industry’s wood demand is expected to be approximately 3 million m3/year. To provide the raw materials needed, short fiber production has been recommended (Tank, 1993). In Turkey, short fibered pulp is produced only with non wood plant fiber, especially from wheat straw. Annual available wheat straw production in Turkey is estimated 41 million tonnes. On the other hand, forest resources are becoming more and more limited (28 million m3 annual growth) and the paper industry of the country is subjected to the competition of other forest industries such as wood based panels industries, fiber board industry and timber production. In paper and board production, 34% of the raw material used comes from wood, 40% from waste paper, 17% from import cellulose and 9% from cereal straw (Özer, 2001).

Annually 36 700 tonnes/year bleached kraft pulp is produced from mixed wheat straw (65%) and reed (Phragmites communis L.) (35%) in state owned Seka-Afyon Pulp Mill. When the amount of available reed is insufficient, wheat straw is used entirely in pulping as fibre material. Because the wet cleaning of the raw material is not carried out in the mill, the sand and impurities plug the screens in 10–15 days after pulping, as a result, about 10–15 tonnes of sand needs to be discharged from the screens. In addition, pulping black liquor with 4–5 g/l dissolved silica is produced and approximately 1300 tonnes/year of lime mud is discharged.

Kraft pulping parameters, pulp properties of wheat straw used in the mill, chemical composition of pulping liquor, black liquor and lime mud are shown in Table 1, Table 2 (Anonymous, 1997, Anonymous, 2002).

Presence of silica in spent pulping liquor leads to several problems in pulping and chemical recovery cycles. The chemical pulping of wheat straw presents two major problems associated with silica. It causes rather serious difficulties during recovery and poor drainage of straw pulp during papermaking (Tandon et al., 1989, Deniz, 1994). The silica is concentrated in the outer parts of cereal straw. The outermost internodal layer is comprised predominantly of epidermal cells and a waxy protective layer, the cuticle. This epidermis is highly lignified and it contains the silica. Also, It has predominantly short fiber or fines. Before pulping mechanical removal of epidermal layer from the internode improves pulping yields, bleaching requirements, drainage and decreases silica content of the black liquor. Removal of the innermost internodal pith layer can also improves wheat straw pulp properties. The pith containing less-lignified, thin-walled parenchyma also plugs the sheet, impairing drainage and is easily crushed during refining perhaps causing further drainage problems. Pith removal reduces the amount of parenchyma cells in the wheat straw pulp (Eroglu and Deniz, 1993, Watson and Garner, 1997). In this regard, Eroǧlu and Deniz (1993) examined predesilication of wheat straw with different chemicals such as NaOH, Na2CO3, Na2S+Na2SO4 and HF. As a result, NaOH was found to be the most convenient condition for desilication with three stage treatment with 1%, resulting in more than 73% silica being removed under relatively mild conditions.

Another problems in Seka-Afyon Mill is the low freeness level of the pulp discharged from digester due to heterogeneous raw material used in pulping. Indeed, freeness level of the pulp produced is about 32°SR after pulping and increases to 36–37°SR on fourdrinier machine. Deniz (1994) showed that, about 32.5% of the cells in straw are fibres, remaining 67.5% consists of nonfibrous cells e.c. parenchyma, epidermis cells, vessels and spirals. The majority of these nonfibrous cells are short in length and causes pour drainage in wheat straw pulp (Jacobs, 1999). Besides, wheat straw pulp contains 50% tracheids, 15% epidermis cells, 5% vessels and 30% parenchyma cells (Mansour, 1985). Owing to heterogeneous pulping at high temperature and long pulping time, wheat straw fibers having lower density and porosity than those of reed fibers, undergo more penetration, diffusion and chemical attack.

Strength development from refining wheat straw pulp is often accompanied by a rapid drop in freeness (Watson and Garner, 1997). Wheat straw primary fines are predominantly thin-walled parenchyma cells. These parenchyma are easily deformed and when present, also reduce drainage (Cheng et al., 1994). Even though wheat straw fines reduce freeness and increase the water retention, they improve the tensile strength of unbeaten pulp. The thin-walled parenchyma cells must collapse thereby aiding in bonding and contributing to the tensile strength (Watson and Garner, 1997).

Primary fines are the small (pass 200 mesh), non-tracheidal cells in an unrefined pulp (Laivins and Scallan, 1996). Both hardwoods and wheat straw often have parenchyma cells in their primary fines fraction. Wheat straw primary fines also contain epidermal cells from outer layer of plant. The second type of fines, secondary fines, is the result of refining and is pieces and fibrils from the outer layers of fibers that are broken off during refining.

To solve these problems in Afyon-Seka Pulp Mill, first, the pulping of wheat straw and reed should be considered separately and the improvements to the non-sulphur pulping methods needed to minimise environmental pollution should be realised. Optimisation of soda–oxygen pulping conditions of reed has been reported recently (Kırcı, 1986). In this study, it is aimed to optimise sulphate pulping parameters used in pulp mill. Strength properties of produced pulp are also studied.

Section snippets

Experimental

Raw material was Triticum drum Desf.V. hordeiforme (Klk K Çdb) which is the main wheat species planted in Northern Anatolian region (Kün, 1988). Before cooking, foreign materials and grains were removed and then wheat straw was chopped to a length of 6–8 cm.

Chemical analyses of wheat straw were made according to Tappi Standart Methods with the exception of holocellulose determined by Wises’s sodium chlorite method, cellulose according to Kurscher and Hoffner’s nitric acid method, viscosity of

Chemical composition of wheat straw

The results of chemical analyses and solubility values of wheat straw are given in Table 3.

As can be easily seen from Table 3, average holocellulose content of wheat straw was found as 74.5%. Approximately, similar values were found as 74.9 and 73.9% for Triticum aestivum L. (Eroglu and Deniz, 1998, Eroglu, 1983), 73.7 and 72.9% for European and Denmark wheat straws respectively. Cellulose, α-cellulose and lignin fractions of T. drum are approximately similar to those of T. aestivum L. in

Conclusion

The most important factor for pulp properties and strength properties of paper sheets was found to be alkali charge. Optimum alkali charge is 14%. There is no need to increase alkali charge beyond 14% on the basis of oven dry straw weight. On increasing alkali charge from 14 to 16% screened yield decreased from 42.1 to 40.6% and viscosity of pulp decreased from 1114 to 872 significantly. However, increasing the pulping time from 20 to 40 min while keeping other parameters constant, has slightly

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