Removal of Dissolved Chlorine and Chlorate

The discussion of brine purification so far has dealt with the removal of impiuities that enter the plant accidentally or with the salt, the process water, or auxiliary materials. In mercury- and membrane-cell plants, the partly exhausted brine, or depleted brine, that leaves the cells must be recovered and resaturated for recycle to the cells. With those technologies, therefore, impurities that form or accumulate in the cells or the brine recycle loop are also important. 

Depleted brine will be physically saturated with chlorine, and some chlorine wUl react to form hypochlorite (Section 7.5.9.1). This chlorine value represents an economic asset to be recovered and, particularly in the case of membrane cells, an intolerable contaminant in the brine treatment system. There are several approaches to this problem [208], and we cover these below. We divide them into methods aimed at recovery of the bulk of the chlorine in a useful form (primary dechlorination Section 7.5.9.2) and those whose purpose is to reduce the active chlorine to chloride and safeguard the environment or other parts of the process (secondary dechlorination Section 7.5.9.3). Some of the hypochlorite that forms in the anolyte will continue to react to form chlorate. This is a much less harmful impurity in the cells, and higher concentrations are tolerable. Many plants keep the chlorate concentration under control by natural or deliberate purges from the brine system (Section 7.5.7.2A). In others, it is necessary to reduce some of the chlorate ion to chloride in order to maintain control (Section 7.5.9.4). 

Solubility of Chlorine in Aqueous Systems. Before discussing dechlorination, we consider the solubility of chlorine in water and brine. Even though the solubility of chlorine in water is not great, the system does not follow Henry s law. The reason for this is the hydrolysis of chlorine to hydrochloric and hypochlorous acids  

This is a reversible reaction, and the HCl formed will be essentially completely ionized. At the low pH of the resulting solution, the hypochlorous acid, with a pK of about 7.5, will remain molecular. The complex ion CI3 also exists  

This reaction is not favored in water, where the low concentration of the chloride ion allows the equilibrium to lie far to the left [209]. In brine, CI3 is more of a factor. More chlorine can enter into combination with the ion, at least up to CI5, but only in very small amounts. 

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