ISSN ………… - International Network for Bamboo and Rattan

the desired pulp type and quality. Defibration is carried out by means of single or multi-stage
refinement process using disk refiners.
The lignin content of the NSSC pulps is high as compared to those from chemical pulps, and ranges
between 10 and 15 per cent. Due to the high lignin and polyoses content the NSSC pulp has low
conventional strength properties. The typical NSSC pulp is normally much more rigid and stiff than
kraft pulp. Therefore, it is the most typical and suitable fibre material for the production of
corrugating medium.
The caustic soda in the soda semichemical process reacts with the lignin-carbohydrate complex to
form soluble sodium lignate, and the carbohydrates are solubilized by hydrolysis. However, this lignin
reaction occurs only after a major portion of the caustic soda has been consumed in neutralizing the
readily available acetyl and methoxyl groups and in hemicellulose dissolution. Therefore, lignin
removal is the least in alkaline semichemical pulping.
The cold-soda or cold caustic process, which is less important than the NSSC process, involves in
principle, the treatment of chips with a sodium hydroxide solution at temperatures generally between
20 and 30 0 C and a final refiner defibration. In the cold-soda chemimechanical pulping, the caustic
soda attacks the fibre bond mainly by reaction with the acetyl and other acid groups, which are
reactive even at room temperature. The most important step in the cold-soda pulping is the
impregnation with alkaline liquor to reach a very fast but total penetration of the chips, causing the
necessary swelling of fibres and avoiding considerable losses of polyoses. Impregnation times are
between 15 and 120 minutes with generally short reaction times of 15-30 minutes in pressurized and
continuous systems. The concentrations of NaOH are generally low (0.25–2.5%), but up to 10 per
cent in the case of some roller mill impregnation systems. Cold-soda pulping requires little installation
capital, and despite the cost of the chemicals, the processing costs are actually lower than in stone
grinding, because of reduced energy consumption. In some process modifications the liquor is reused
up to 20 times.
The cold-soda pulp yield ranges between 85 and 92 per cent, whereby the selectivity of lignin and
polyoses dissolution is much lower than NSSC pulping. The main disadvantage of cold-soda pulps is
a generally a low brightness level (40-50%), which can be effectively increased by a two-stage
peroxide-hypochlorite bleaching.
As the cold-soda pulps have properties comparable to NSSC pulps, these are used as unbleached
coarse grades for corrugating medium production, and as bleached grades for printing papers and
newsprint in combination with groundwood pulp and chemical pulp.
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