Membrane filtration Mercury cells

Caustic soda is marketed as 50% and 73% solution and as anhydrous beads and flakes. Mercury cells can produce 50% and 70% caustic directly. This is a high-purity caustic compared with the membrane- or diaphragm-cell caustic. The caustic from the decomposer is cooled and passed once or twice through an activated carbon filter. After filtration, the mercury concentration is lowered to ppm levels. This is still unacceptable to some customers, who have switched to using membrane grade caustic soda. The mercury cell caustic soda has a few ppm salt and <5 ppm sodium chlorate. If trace concentrations of mercury are tolerable in the end use of caustic, then the mercury cell caustic will be the highest purity caustic that can be made electrolytically. 

The filtration membranes are sensitive to fouling as well as to free chlorine. This situation is least troublesome in a membrane-cell plant, where the problem components already have been removed from the brine. In mercury-cell plant applications, an installation in the brine recycle loop should include some means of dechlorination. The usual choice is treatment with activated carbon, which is covered in Section 7.5.9.3B. The membranes are in spiral-wound modules placed in cylindrical housings and assembled as on the skid shown in Fig. 7.81. Figure 7.82 shows the construction of a modular element. The low-sulfate permeate flows through the membranes into spacer channels... 

Additionally, if the brine circulating system of an existing mercury cell plant also serves membrane cells, all mercury must be removed in a chemical treatment facility. The brine from the primary filtration is acidified with hydrochloric acid to pH 2.0-2.5 and sodium sulfide is added to precipitate mercury sulfide. Subsequently, the brine is... 

---