Mercury brine circulation system

The aspects of the operation of mercury cells that typically differ from those of the other processes are the brine circulation system, the cell room, treatment of the products, measurement and control, and reduction of mercury emissions. 

A typical brine circulation system for the mercury cell process is shown in Figure 24. In the ceils the sodium chloride concentration of the brine is reduced by 35 - 60 g/L to 260 - 280 g/L at 70 - 85 °C. To avoid mercury emissions into the air, the resaturators are generally closed vessels. The mercury cathode is very sensitive to poisoning by heavy metals therefore, a test [91] has been developed that allows rapid determination of the suitability of any particular salt or brine. 

Figure 24. Schematic diagram of a brine circulation system in the mercury cell process a) Electrolysis cell b) Anolyte tank c) Vacuum column dechlorinator d) Cooler e) Demister f) Vacuum pump g) Seal tank h) Final dechlorination i) Saturator k) Sodium carbonate tank I) Barium chloride tank m) Brine reactor n) Brine filter o) Slurry agitation tank p) Rotary vacuum filter q) Vacuum pump r) Brine storage tank s) Brine supply tank...

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... 

The circulating brine contains mercury concentrations of 2-20mg/L. Mercury emissions from the brine system can occur through losses of brine into the wastewater, by brine vaporization in the resaturators, or by disposal of the residues from the brine purification filter. These emissions are minimal at a chlorine concentration < 30 mg/L, giving a redox potential > 500 mV vs. NHE. Under these conditions mercury remains dissolved in the brine as a mercury chloride complex even if the brine is alkaline. 

In Western Europe, an EC directive has been issued on the subject of the mercury content of wastewater from chlor-alkali plants, following various earlier agreements such as the Rhine protection agreement, the EC guidelines concerning the protection of natural waters, and the Paris Convention (106). This directive requires plants with circulating brine systems to have a hmit of 1.0 g of Hg per tonne of chlorine produced. 

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