Brine circuits

Figure 12.6 Brine circuit for rooms at different temperatures...

If brine circuits are open to the atmosphere, air maybe entrained, with consequent oxidation, and the solution will become acid. This will promote corrosion and should be prevented as far as possible by ensuring that return pipes discharge below the tank surface. 

The preferred brine circuit is that shown in Figure 12.5, and having the feed and expansion tank out of the circuit, which is otherwise closed. This avoids entrainment of air and too much surface exposure. The same arrangement can be used with the divided storage tank shown in Figure 12.6, except that the tank will be enclosed, with a separate feed and expansion tank. 

The decision to retrofit or install new filters is influenced by plant-specific factors such as the condition of the existing filters, compatibility of the filters to retrofit, availability of space within the plant, etc. Bayer s analysis of the costs and benefits of the retrofit compared to the investment in new filters did not show a clear favourite either decision could be justified. With regard to the rubber lining, gaskets, hydraulic system etc., the Kellys were in very good condition. Because of this, the decision was made to retrofit two of the filters and to use them in a separate brine circuit for the new electrolysers, with a second circuit feeding the amalgam plant. 

Mercury droplets, mercuric chloride, and mercury vapor are present in the hot moist chlorine gas produced by the electrolyzer. Carryover of entrained mercury in the chlorine is decreased by passage through a demister, a labyrinth filter of titanium ribbon. Any mercury vapor present is condensed along with water vapor in an indirect, process water-cooled heat exchanger. The condensate, which contains 0.1-0.2pg/g mercury, may be recycled to the brine circuit or charged to a waste brine treatment system prior to discharge [25]. 

Sludges result from the pretreatment of resaturated brine for removal of impurities, and from brine to be discharged, which was occasionally necessary because of water buildup in the brine circuit. These sludges contain 8-15 mg/g (dry basis) mercury as a complex mixture of compounds. To recover the mercury, most of the water is removed, and then the sludge is resuspended in aqueous sodium hypochlorite. The hypochlorite oxidizes the sulfide and any elemental mercury present (Eqs. 8.46 and 8.51) in order to produce a concentrated aqueous stream of dissolved mercury salts. Insoluble components are then removed by filtration, and the solution is then returned to the brine circuit. When this reaches the electrolyzer, electrolytic reduction recovers the dissolved mercury present (Eq. 8.52). 

Since the synthetic membrane of membrane cells replaces the older asbestos fiber diaphragm cells, no control precautions are necessary. Graphite (or later, titanium) cathodes avoid the use of mercury eliminating the need for mercury control. However, if membrane cells are operated on the same site as mercury cells, they do require a separate brine circuit from the mercury cells to maintain their mercury-free status. 

PLACID/PLINT process [13,14], The PLACID process (see also Chapter 16) is based on the electrowinning of lead from a chloride solution in a cell where the cathode and anode are separated by a semipermeable membrane. Lead oxide and lead dioxide are converted to soluble lead chloride, whilst lead sulfate is converted in a hot brine circuit. The sulfate ion is removed as gypsum after the addition of lime. The FLINT process is an advanced adaptation of the PLACID process and operates in parallel with a pyrometallurgical route that smelts the metallic lead fractions from the battery. 

Operation of separate brine circuits treatment of brine to remove dissolved mercury... 

Impurities in Brine. Control of brine impurity levels is achieved by a combination of chemical treatment plus ion exchange and an effective purge from the brine circuit. The purge may be made selective by some of the techniques discussed in Chapter 7. The qualitative effects of some of the major impurities are discussed in some detail in Chapter 4, and Table 7.10 sununarizes effects and remedies. Still, a brief summary as part of this discussion does not seem out of place. 

Mercury also acts as a poison for precious metal-coated cathodes. If present in a brine circuit as a result of conversion from mercury to membrane technology, it would pass through the membranes and contaminate the cathodes. For this reason, a brine circuit... 

The water that evaporates from the dechlorinated brine is condensed in a cooler. The condensate, which may be chemically dechlorinated, is returned to the brine circulation system if necessary to maintain the volume of the brine circuit. If necessary, the remaining chlorine content can be further reduced by blowing with compressed air, by a second vacuum treatment, by treatment with activated carbon (63], or by chemical treatment with hydrogen sulfite, thiosulfate, sulfur dioxide, or sodium hydrogensulfide. 

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