Brine Specifications and Treatment Techniques

This section discusses the processes used to bring brine within the specifications imposed by the various cell technologies, as summarized in Section 4.8.6. Brine specifications depend strongly on the type of cell being used, because the effects of the various impurities, discussed in Section 7.5.2, often are specific to the type of cell. This situation is reflected in differences in the approach to brine treatment. The sections that follow describe the steps in a typical process, and these are shown in the simple diagram in Fig. 7.33, where the process steps are referred to the relevant section of the text. 

Major impurities (calcium, magnesium, and other metals) are removed from solution by precipitation (Section 7.5.2). The solids are separated from the treated brine by settling (Section 7.5.3) and one or two stages of filtration (Section 7.5.4). The precipitated solids are removed from the settler for disposal, and the residual brine contains a few ppm of hardness. This is not acceptable in membrane cells. An ion-exchange process therefore follows in that case (Section 7.5.5). The fully treated brine then is ready for use in the cells but is alkaline and contains carbonate. Most plants add acid to the brine in order to improve cell operation and chlorine quality, and this is the subject of Section 7.5.6. 

Other contaminants found in brine as produced include sulfate and a group of less common impurities that require treatment in some plants. Finally, brine treatment must deal with certain byproducts of cell operation that must be removed from brine recycle systems. These include dissolved chlorine (hypochlorite) and chlorate. The latter forms 

FIGURE 7.33. Guide to brine treatment processes. Basis membrane cells numbers in boxes correspond to sections in text. 

Each type of cell has its own requirements. Diaphragm cells are perhaps the most forgiving of impurities. While mercury cells can tolerate a higher total concentration because of their lesser sensitivity to the major impurities, they are susceptible to damage by small quantities of certain heavy metals. Membrane cells have the most exacting brine specification and are distinguished front the others by their very low hardness specification. 

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