Membrane process brine purification

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. 

Other steps in these processes are purification of the sodium chloride brine before electrolysis, evaporation of water to further concentrate the caustic soda, removal of oxygen from the chlorine. The hydrogen obtained in partitioned cells is very pure and can be used, for example, in the food industry. There is now a general changeover to membrane cells worldwide. 

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

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. 

Finally, we consider the membrane cells in Fig. 6.5. The electrode processes are the same as those in the diaphragm cells (Eqs. 1 and 2). Anolyte processing is quite similar to that practiced with mercury cells. We saw above in the discussion on brine treatment that membrane cells had stricter requirements. The same is true regarding dechlorination of the depleted brine. After vacuum dechlorination, the residual active chlorine content is high enough to damage the ion-exchange resin in the brine purification... 

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. 

A closed-loop anolyte circuit with solid salt as feed for the brine saturation is necessary. The required, extremely high purity of the brine is elucidated in section Brine Purification for the Membrane Process. ... 

In the diaphragm process, the removal of sulfate is not always necessary because 504 can be removed from the cell liquor as pure Na2S04 during the concentration process. In the membrane process, the brine must be purified to a much higher degree to avoid the deterioration of the membrane. The Ca and Mg " concentration must be < 0.02 ppm (20 ppb), so a second, fine purification step is required (see Section 7.2.1). 

Brine System. The brine system for the diaphragm process is the simplest of the three — there is neither dechlorination nor sulfate precipitation, except in some very specific cases — and makes up only 3 - 4 % of the capital investment. The brine system is the most complex for the membrane process, for fine purification by ion exchange is necessary. However, the two- or three-fold greater depletion of the brine in the membrane process allows the brine system to be smaller than that for the mercury process. Therefore, the cost of the brine system for either process is approximately the same, 4 - 7 % of the total. 

The fixed costs for operators and other personnel, taxes, insurance, repairs, and maintenance are about the same for all three processes. The 20 % lower depreciation of the membrane process is offset by the additional expense for purchase and replacement of the membranes and for the more elaborate brine purification. 

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. 

As in the mercury process, the brine is dechlorinated and recirculated, which requires solid salt to resaturate the brine. The life of the expensive membrane depends on the purity of the brine. Therefore, after purification by precipitation-filtration, the brine is also purified with an ion exchanger. 

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