| Summary: | A relatively dilute (approximately 1 to 2%) water solution of sodium dithionite was produced from sodium-mercury amalgam and aqueous solution of sulfur dioxide in a simple "once through" reactor [proposed process]. The reactor could be run in conjunction with the Castner-Kellner type cell. The manufactured solution could then be used directly for the brightening of groundwood pulp.
The bench scale experiments were carried out in a continuous-flow-stirred-tank reactor where the aqueous and amalgam phases formed an interface. The effects of important process variables on the steady-state concentration of sodium dithionite in the reactor and yields of sodium dithionite on sulfur dioxide in the aqueous feed and on sodium consumed in a single pass were determined.
The above-mentioned yields are important in assessing the economic feasibility of the proposed
process. The steady-state yield of sodium dithionite on sodium in the amalgam entering the reactor and conversion of sodium to different products in the reactor were also determined.
The present investigation showed that the process variables can be controlled to give approximately 2.3% sodium dithionite solution with steady-state Na₂S₂O₄ yields of about 21% on sulfur dioxide in the aqueous feed and about 67% on sodium consumed. The yields obtained depend on the levels of process variables such as:
1. the concentration of sodium in the amalgam entering
the reactor,
2. the concentration of total sulfur dioxide in the aqueous feed solution,
3. the agitation in the aqueous phase,
4. the agitation in the amalgam phase,
5. the residence time in the aqueous phase,
6. the residence time in the amalgam phase,
7. the interfacial-area/aqueous-volume ratio,
8. the temperature of the aqueous phase, and
9. the pH of the aqueous phase.
This experimental study indicates that it may be economically feasible for a pulp mill to change .from zinc dithionite produced in situ to sodium dithionite produced in situ by the proposed; process. Further, the proposed process compared to the manufacture of zinc dithionite in situ avoids the discharge of zinc ions which act as biocidal agents when discharged into the effluent receiving waters.
The models suggested by Ketelaar (44) and Gerritsen (30) were found inadequate to explain the processes occurring in the reacting system sodium-mercury amalgam and aqueous sulfur dioxide. A qualitative model has been suggested on the basis of the experimental work and the information available in the literature.
This work also sheds some light on the type of reactor which would be suitable for the proposed process. === Applied Science, Faculty of === Chemical and Biological Engineering, Department of === Graduate
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