Chlor-alkali process brine purification

Chlorine Plant Auxiliaries. Flow diagrams for the three electrolytic chlor—alkali processes are given in Figures 28 and 29. Although they differ somewhat in operation, auxiUary processes such as brine purification and chlorine recovery are common to each. 

H. Aikawa, Brine purification for ion exchange membrane chlor-alkali process, Nippon Kaisui Gakkaishi (Bull. Soc. Sea Water Sci.), 1994, 48, 439—450. 

Impurities in brine affect electrode reaction kinetics, cell performance, the condition of some cell components, and product quality. Treatment of brine to remove these impurities has always been an essential and economically significant part of chlor-alkali technology. The brine system typically has accounted for 15% or more of the total capital cost of a plant and 5-7% of its operating cost. The adoption of membrane cells has made brine specifications more stringent and increased the complexity and eost of the treatment process. Brine purification therefore is vital to good electrolyzer performance. This section considers the effects of various impurities in all types of electrolyzer and the fundamentals of the techniques used for their control. Section 7.5 covers the practical details of the various brine purification operations. 

Water, after the preliminary treatment methods of Section 12.4.1, can be called purified. Here, we use the term to refer to the higher levels of purification in Table 12.1 or to those processes which remove dissolved contaminants. In the chlor-alkali process, the major uses of purified water are dilution of catholyte, processing of membrane-cell caustic liquor, preparation of ion-exchange system regenerants, manufacture of hydrochloric acid, acidification of brine, and, sometimes, dissolving of salt. It also serves as utility and seal water in the membrane preparation area and in certain parts of the process. 

The unit operations in a commercial chlor-alkali plant can be generally classified as follows (1) brine purification, (2) electrolytic cells, (3) H2 and Cl2 collection, and (4) caustic concentration and salt removal. In this section, the general process flowsheets for diaphragm, membrane, and mercury cell technologies are discussed with emphasis on the need for brine purification and the manner in which it is carried out. 

The unit processes in a commercial chlor-alkali plant can be broadly classified as follows (1) Brine preparation and purification, (2) Electrolytic cells, (3) CI2 processing, (4) Caustic concentration, and (5) H2 processing. The materials of construction in these various processes are discussed in the following sections [22-24] and in Chapters 7 and 9. 

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