Selecting a potential Swedish fibre crop: fibres and fines in different crops as an indication of their usefulness in pulp and paper production

Abstract

Eleven different crops, suitable for growth on excess farmland in Sweden, have been investigated with regard to their contents of fibres and fines. The usable fractions in agricultural crops are assumed to be those comparable in size to hardwood fractions. The pulps investigated were thus divided into two fractions. The fraction of the pulp retained on a screen of 300 μm mesh was considered to be the fibres. The fraction of the pulp passing the 300 μm mesh screen was considered to be the fines. The fibre fraction of the pulps was assumed to be similar in functional value to hardwood libriform fibres. The fines in pulps of both wood and non-wood origin may give the final sheet of paper a smooth surface and high light scattering which are desired properties of paper used for printing. They were considered though to be of less value than the fibres because of the difficulties they may cause in the pulp and papermaking processes due to their water-retaining and clogging tendencies. Particles smaller than the fines were considered to be of no value. Pulps from the different crops and, for comparison, and industrial unbleached birch kraft pulp were investigated by a standard BauerMcNett fractionation procedure. The pulps from the crops all had a fines fraction larger than the fines fraction in the birch pulp, which was about 17% of the pulp. The pulps from grass species had smaller amounts of fines than pulps from leguminous plants. The amounts of fibres varied in the crops from 30 to 55% of the pulp. The fibre content of leguminous plants was generally so small that their usefulness as a potential fibre source is very doubtful. The total production of fibres (and fines) per hectare per year determines the economy of a crop as a potential fibre source. It was found that the reed canary grass ( Phalaris arundinacea) had the largest harvested yield (9800 kg per hectare per year) and a yield of fibres (2200 kg per hectare per year) that was larger than that of other crops investigated, and indeed even larger than that of birch ( Betula verrucosa), which was about 2000 kg per hectare per year. The harvested yields of most of the leguminous plants were lower than of the gasses and the yield of fibres were less than 1000 kg per hectare per year for all the leguminous plants investigated. The influence of fines on the drainage time was investigated for reed canary grass by a standard procedure. The longer drainage time of the grass pulp (6.2 s) than of the birch pulp (3.7 s) was considered to be due to the larger amount of fines in the grass pulp. The content of fibres and fines in leaves and straw of reed canary grass was also investigated. It was found that the leaves contain an about 60% more fines than the straw and are therefore less suitable for pulp production.