Chlor-alkali process brine dechlorination

Removal of brine contaminants accounts for a significant portion of overall chlor—alkali production cost, especially for the membrane process. Moreover, part or all of the depleted brine from mercury and membrane cells must first be dechlorinated to recover the dissolved chlorine and to prevent corrosion during further processing. In a typical membrane plant, HCl is added to Hberate chlorine, then a vacuum is appHed to recover it. A reducing agent such as sodium sulfite is added to remove the final traces because chlorine would adversely react with the ion-exchange resins used later in the process. Dechlorinated brine is then resaturated with soHd salt for further use. 

Table 7.24 shows where nanofiltration fits in the spectrum of specialized membrane filtration processes. The process has application regardless of the type of chlor-alkali cell in use, and it also can remove sulfate from chlorate plant liquors. The membranes used in the filters are sensitive to free chlorine. In a membrane-cell plant, therefore, the filter usually will treat a sidestream of fully dechlorinated brine. After standard filtration and. 

Mannig and Scherer [225] have discussed the role of hydrogen peroxide in the chlor-alkali industry, including its use in brine dechlorination. They cover handling of the peroxide, certain other applications such as the decolorizing of caustic soda, and the requirements of a dechlorination system. In particular, they comment on process control this is the subject of the following subsection. 

In any of these cases, prior removal of dissolved chlorine from the spent acid is desirable, and this is the subject of Section 9.1.4.4E. Method (4) appears on the list above as well as method (1) because the amount of acid generated may exceed the local demand for neutralization of alkaline waste. This situation depends on a plantwide balance and is not constrained to the battery limits of the chlor-alkali unit. Use of the acid as a dechlorinating agent, as in number (2), is limited to situations in which the treated condensate is not returned to the brine process (e.g., diaphragm-cell plants). The presence of sulfates in the stripped product makes it unsuitable for recycling. Many producers favor option number (3), when it is available. The supplier s ability to handle the material may dictate the concentration of the spent acid. 

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See also in sourсe #XX -- [ ]

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