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

4.2.4. Non-conventional pulping processes
The non-conventional pulping processes include well-known pulping principles, which are, however,
only rarely applied commercially. Their industrial application is often limited by high costs of
chemicals and special equipment requirements. However, they posses the advantage of more
environmental friendliness techniques. Appendix 4B gives a survey of different non-conventional
pulping procedures. Pulping via sodium-xylene sulphonate, aqueous ethanol, ketone-ammonia
mixture, ethanol amine, peracetic acid, oxygen-alkali, etc., are some of the efforts to develop pollution
free pulping processes. Among the various processes, some commonly referred methods are described
below :
4.2.4.1. Ethanol-water pulping
Aqueous ethanol is a powerful delignifying agent. Retention of lignin is strongly dependent on pH of
the cooking liquor. After distilling off the alcohol from ethanol-water black liquor, a major fraction of
the solubilized lignin separates as a quasi-molten phase.
4.2.4.2. Hydrotropic pulping
Certain substances which are only slightly soluble in water become more soluble in the presence of
certain salts known as hydrotropic salts. These are salts of organic acids which have a large organic
group and are themselves soluble in water. It has been shown that a near-saturated aqueous solution of
sodium xylen sulphonate at approximately 70 0 C, will dissolve large quantities of lignin. These salts
appear to act as catalysts. As they are not consumed, they can be recovered unchanged for reuse. Also
they hydrolyse wood or straw at a much faster rate than the chemicals used in other processes of pulp
making. In pulping with sodium xylene sulphonate, cooking could be accomplished in shorter time
and at lower temperature than with other standard processes. For most woods the composition of the
hydrotropic cooking solution is approximately one third salt and two thirds water; the solution is
adjusted to a slightly acid pH (3.5). A typical cook, for paper bags takes 2-3 hrs. at 145 0 C and 4 bar
pressure. At the end of the cook, the solution is filtered from the pulp and reused half a dozen times
for other cooks. The solution is then diluted with warm water (40- 60 0 C) to a salt concentration of 8-
10 per cent for precipitating the lignin. The lignin is removed by filtration and the solution is
evaporated until the concentration of sodium xylenesulphonate is nearly 30 per cent; at this stage the
solution can once again be used for cooking. The solution is nonscaling and noncorrosive and free
from objectionable odour. The pulp is pressed to a consistency of about 40 per cent, washed free of
the remaining hydrotropic salt. The properties of the pulp fall between those of an acid sulphite and an
alkaline kraft pulp. The yield is 10 per cent higher and gives a high white colour in the usual
bleaching processes.
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