Simultaneous neutral sulphite semichemical pulping of hardwood and softwood.
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Date
2000
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Abstract
The work described in this thesis was aimed at obtaining a better understanding of the neutral sulphite
semichemical pulping process employed by Mondi Kraft's Piet Retief mill, and to investigate ways to
improve this process. The unique feature of the process in this mill is that hardwood and softwood
species are pulped simultaneously in a continuous digester. The pulping trials described were carried
out in a laboratory batch digester which was build as a part of this project. Pulps were evaluated for
yield, Hypo number as an indication of the residual lignin content and strength properties. The first
part of the experimental work focused on the effect that different pulping variables have on the process
and the resulting pulp. Variables investigated were the chemical charge, pulping temperature, chip
composition and anthraquinone dosage. The second part of the work was to investigate ways in which
the process can be improved. In particular it was investigated whether it would be advantageous to
pulp hardwood and softwood separately and mix the two pulps together after pulping. The effect of
changing to an alkaline sulphite process was also briefly investigated.
It was found that the current process is optimized as far as the chemical charge and pulping temperature
is concerned. Increasing the softwood percentage used to 50 % (from current value of 41 %) increases
the tear strength, whilst decreasing it to 30 % increases the tensile strength of the resulting pulp. It was
also determined that increasing the AQ dosage from 0.1 % to 0.5 % might bring savings in chemical
costs. It is suggested that this is investigated in a mill trial.
It was further found that pulping the two species separately improves the tear strength of the pulp by
about 20 % compared with that which was pulped simultaneously. The results indicate that no benefits
concerning the chemical costs, pulping temperature, pulp yield, burst strength or tensile strength are to
be gained from separate pulping.
Preliminary results indicated that significant strength increases and possible chemical cost savings are
to be gained by changing from a neutral sulphite to an alkaline sulphite process. Further work to
determine the reproducibility of these results, as well as the effect of different chemical charge and
ratios is suggested.
Description
Thesis (M.Sc.Eng.)-University of Natal, Durban, 2000.
Keywords
Wood-pulp--Chemistry., Sulphite pulping process., Theses--Chemical engineering.