Studies on Hemp Stalk and Shive Pulping

by V.S. Krotov

Ukrainian Pulp and Paper Research Institute

Kiev, Ukraine

Dear Colleagues,

     I hope that here my allies are assembled who consider that cutting forest to use only a third of wood for papermaking and to turn its two-thirds into environmentally hazardous wastes is barbarity. It is more correct to call modern pulp industry a production of generation and partial neutralization of its own wastes where pulp may be regarded as a by-product of the latter.

     Ideally our concept consists in that the countries abounding not in forests but in agricultural resources would become major pulp producers in the world and that over half of raw material mass would be converted into pulp and the rest would be returned to the land as valuable organomineral fertilizers restituting soil fertility. The time will come when massive paper and board grades are mainly produced from agricultural fibrous wastes and high-grade papers and boards and their specialties are made of plantation-grown bast-fibre plants.

     Hemp is a bast-fibre plant most suitable for the countries with moderate climate. I would not speak about the advantages of this plant; you know them quite well. I would only note that until recently the hemp selection has mainly been directed to meeting the requirements of the textile industry. If one work a little on the selection of hemp to meet the requirements of the pulp and paper industry, this plant could become beyond compare among all conventional raw materials for pulp, paper and board production, including fast-growing wood species. The directions of selection are known and such work is carried by the Institute of Bast Fibre Plants (UkrNIILK) in a town of Glukhov, Ukraine. I would like to tell you, using hemp as an example, about our work in the field of pulp production and the steps which should be done for annual plants and agricultural fibrous wastes to compete with wood raw materials.

     Modern technology and equipment for pulp production are aimed at pulping large lots of raw materials concentrated at harvesting sites and having uniform composition. It is not economically feasible to use existing technology for processing low capacity, distributed over a vast territory, difficultly transportable and stored raw materials with a seasonal cycle of harvesting. The imperfection of industrial technologies, their hazard for the nature increase in the case of non-wood raw materials. Additional complications arise when hemp and other bast-fibre plants are pulped. From the point of view of the pulp production, the hemp stalk consisting of bast (20-25%) and woody (75-80%) fibres is a mixture of two different raw materials which differ from each other to a greater extent than, e.g., softwoods and hardwoods. It is impracticable to pulp such mixture in one process by known industrial methods, since pulping conditions optimum for one raw material are not suitable enough for the other. As a result, the quality is deteriorated and the pulp yield is reduced followed by inevitable impairment of environmental characteristics of the production. A more progressive technology of bast-fibre plant pulping is based on mechanical separation of stalks into bast fibres and woody part (shive). These components can be pulped, washed and refined in two different lines with their own optimum conditions and, if necessary, the two pulps can be mixed to make a furnish necessary for papermaking. But the cost of higher yield and improved quality of pulp and paper is higher capital investments and operating costs, including energy consumption. Besides, the advantages of higher pulp yield are reduced to a minimum due to the loss of raw materials during mechanical stalk separation. Here collection and disposal of dust emissions also require additional energy consumption. From our point of view, the technology which tries to solve environmental problems by increasing energy consumption, even if for the disposal of emissions, cannot be considered a progressive method. In this case only transformation of some pollutants to the others and their redistribution from the location of the production of a final product to the location of fuel production and energy generation usually take place.

     In this connection we posed the problem of developing such technology which could account for all specific features of non-wood plants and hemp in particular, the technology which could surpass existing wood pulping technologies in all parameters, viz. capital investments, energy intensity, water consumption, environmental safety, and costs of pulp production.

     This technology is based on our method with aqueous-organic ammonia and sulfur dioxide solutions developed earlier for wood pulping. For this purpose one can use any organic solvents which mix with water and do not degrade or form insoluble condensation products with lignin at pulping temperatures.

     From the practical point of view, it is most expedient to use ethanol or methanol as a primary organic solvent for initiating the process, therefore we called this method ammonia sulfite-alcohol (ASA) pulping. The pulping is carried out at a weight ratio of NH₃ : SO₂ from 1:0.2 to 1:2.5, a volume ratio of water to organic solvent from 0.4 :1 to 4:1, ammonia consumption of 5-25% on raw material basis, and temperature of 150-185° C. A fresh organic solvent (ethanol) is necessary only for starting the process. Further a recovered solvent is used. Since during the pulping organic liquids (methanol, acetone, ethyl acetate and others) are additionally formed, the solvent composition will gradually change until it is stabilizes. The process can be organized in such a way that solvent losses from the system would be made up by its formation from lignin and hemicelluloses during the pulping (due to reactions of elimination of Methoxy, ethoxy, acetyl groups and other reactions). In this case one may say for fun that plant fibres are stewed in their own juice.

     The use of mild reagents, weakly alkaline medium and organic solvents ensures very soft pulping conditions. It favours the conservation of carbohydrate components of the raw material. Thus, our method is characterized by exclusively high selectivity and extensive delignification and is unrivalled in these parameters among all known pulping methods, including new organosolvent developments: Alcell, Organocell, ASAM, etc. Depending on a raw material and pulping conditions, the yield of ASA pulp is 20-40% higher than that of kraft or soda-anthraquinone pulps at equal Kappa numbers.

     I stop further describing our method. Its advantages as well as the technology itself, schematic diagram and design of the unit are detailed elsewhere (World Pulp and Paper Technology 1994/95. The International Review for the Pulp and Paper Industry, Sterling Publishing Group PLC, London, 1994, pp. 67-70).

     I should only say that the technology we develop allows to combine impregnation and pulping of a raw material, pulp washing and dewatering, recovery of an organic solvent and reagents which were not consumed in the reactions, collection and use of dirty condensates, utilization of heat of secondary steam and condensates in one continuous unit. The technology does not require fresh water from a natural source and therefore does not generate effluents. We may say that one unit would combine practically all major equipment of an unbleached pulp plant. This offers scope for the development of highly profitable low-capacity plants and even mobile units which can be used in the regions of growing and processing agricultural products.

     Soft conditions and high selectivity or ASA pulping enable bast-fibre plants (hemp, kenaf, flax, etc.) to be effectively processed without their separation into two fractions: long fibre bast with a low lignin content and short-fibre woody part with a very high lignin content. As compared to other pulping methods, woody fibres are pulped more extensively, while the bast ones are not still overcooked and do not lose yield and physico-mechanical properties.

     In support of the above-said, let us examine the results of hemp stalk and shive pulpings. Hashish-free hemp stalks of YuSO-3l species developed by UkrNIILK (Glukhov) were used for Pulping. Hemp stalks were harvested at a stage of industrial ripeness. One of the pulpings was carried out using the whole biomass of green hemp harvested on the 77th day after even seedling emergence. Industrial wastes of the Glukhov hemp mill were used for shive pulpings. Hemp stalk and shive characteristics are given in Table 1. The analysis of raw materials and pulp, preparation and refining of fibrous stock, making of handsheets and determination of their characteristics were performed by Ukrainian standards which mainly conform to ISO standards.

Table l . . . . . . . . Chemical composition of hemp stalks and shive

      In parallel with ASA pulpings we carried out soda and soda-anthraquinone ones. All the pulpings were performed in a lab digester under the conditions close to isothermal ones with heating to final temperature within 3-4 min. ASA pulpings were performed at a 35:65 volume ratio of 95% ethanol regenerate to water. Other pulping parameters and pulp characteristics are summarized in Table 2.

Table 2. . . . . . Conditions and results of hemp stalk and shive pulpings

     As is seen in Table 2, the yield of ASA for hemp stalk pulp is 10.4 - 6.8% higher than that of soda-anthraquinone pulp at about equal Kappa numbers. The yield of ASA hemp shive pulp increases by 17.8% as compared to that of soda pulp and by 8% as compared to that of soda-anthraquinone pulp. A relative increase in yield is 19.5 - 12.8% for stalks and 35.5-17.2% for shive, respectively. The strength properties of ASA pulp characterized by a breaking length-tear strength ratio are also much better. A high breaking length of ASA shive pulp achieving 12 km has engaged our special attention. It is not inferior to the strongest softwood kraft pulps. If we examine the results of soda and soda-anthraquinone pulpings, they can confirm the commonly accepted opinion that hemp shive is of little use in the pulp and paper industry. However, ammonia-sulfite-alcohol pulping allows to make shive pulp superior in yield and not inferior in strength to hardwood kraft pulp. Pulping of the whole green biomass of 77-day hemp has demonstrated the possibility to produce pulp from a green plant having lower characteristics but quite suitable for making certain packing and other massive paper and board grades.

     Fig.1 demonstrates an extremely high refinability of ASA pulps which allows to save energy for papermaking. The Pulps were refined in a standard centrifugal refiner similar to a PFI mill.

     Fig. 2 shows the effect of refining on physico-mechanical characteristics of ASA pulps. In these experiments we used pulps from hemp stalks with a Kappa number of 20.0 and those from shive with a Kappa number of 17.3 presented in Table 2. The breaking length-tear strength ratio for the same pulps is shown in Fig. 3. As is seen in these Figures, it is practicable to restrict the freeness of ASA pulps from hemp stalks to 50° SR and from shive to 60° SR.

     The results obtained show that the ammonia-sulfite-alcohol method can give bleachable pulp from hemp stalks which is not inferior in strength and much superior in yield to softwood kraft pulps .

     All ASA pulps posses higher initial brightness which is dependent on pulping temperature and is within 50-55% ISO for ASA pulp from hemp stalks. Similar soda-anthraquinone pulp exhibits brightness of 34% ISO.

Conclusion

     One of great scientists told that any idea went through the three stages of development: first, when they say: “this is not really,” second: “there is something in it,” third: “the whole world knows it for ages.” We were granted the Author's Certificates for organosolvent pulping in 1974, for the unit in 1980, for ASA pulping in 1984. Our technology is based on these three inventions. The results of investigations of ASA pulping were published and reported at conferences and symposia. However, up to now we could hear from the specialists who first got to know about our technology: “it is too good to be the truth.” The first stage of development lasts already for a decade and this demonstrates how conservative our industry is. Even now I think, especially among lumber industry lobby, there is a good deal of skeptics and opponents of radical reorganization of pulping technology. However, the reserves of improving traditional technology are nearly exhausted, and the environment is in such condition that modern industrial monsters hazardous for the nature cannot survive for a long time. The reduction of anthropogenic hazard for the nature to the level of its self-purification and recovery can be achieved on condition of:

• -- development of fundamentally new pulping technology radically surpassing traditional technologies in conservation of resources, and first of all, in energy intensity;
• -- cessation of logging and use of pulpwood;
• -- change-over from a centralized system of giant plants to the distributed system of small-capacity plants orientated to the use of local raw materials, first of all agricultural resources.

     The development of effective hemp pulping technology is an integral part of this problem. Such technology should be versatile and allow the processing not only of hemp but also of other raw materials, including large-tonnage agricultural and industrial wastes: cereal straw, corn, sunflower, rape, cotton stalks, grasses, flax shive, etc.

     To all being in doubt, I propose to take any nonwood material or hardwood and pulp it in a conventional digester using parameters cited in the presentation and be convinced of possibilities and advantages of the ASA method. The potentials of our technology are far wider than those presented in this paper. We develop it in different directions and, if I have another chance, I would be glad to share with you the results of further developments and achievements.

     Thank you for your attention.