Brine impurities

J. T. Keating and K. J. Behhng, Brine, Impurities, and Membrane Chhr—Alkali Cell Pe ormance, presented at the London International Chlorine Symposium, 1988. 

Modern data acquisition and evaluation help to optimise the plant under review within a short period of time, to eradicate faults in plant operation and to determine the best materials for the operation of the chlorine electrolysis plant being examined. In this way, inter-relationships are examined between the energy consumption and variables such as membrane types, anode and cathode coatings, temperature, pressure, and concentrations as well as plant shutdowns, brine impurities, materials of construction and manufacturers. It is conceivable that other inter-relationships will come to light that have so far not been considered. 

A wide range of operating conditions and design philosophies affect mercury cell efficiency. For example, the fundamental distinction between a resaturation and a waste brine process influences the temperature and brine strength profile along the length of the cell and hence the overall efficiency. Another important factor is the quality of the brine. Impurities in the brine can cause base-plate deposits, which tend to reduce the anode/cathode gap. This gradual reduction in gap requires either manual or automatic adjustment and, eventually, the cell must be taken off-line and the thick mercury removed. 

There are two steps in the basic process the upstroke and the downstroke (see Fig. 24.1). During the upstroke, feed brine solution bearing impure salt is pumped into the bottom of the ion-exchange resin bed. The impurity (MX) is sorbed by the resin particles according to Equation 24.1 and a purified brine solution is collected from the top of the bed. Next, during the downstroke, water is pumped into the top of the bed, desorbing the brine impurity from the resin according to Equation 24.2 so that a solution of the brine impurity is collected from the bottom of the bed. The total cycle typically takes about 2-10 min to complete and repeats successively. 

Several new kinds of ion exchangers have been developed in recent years that give more specific and selective removal of divalent brine impurities such as calcium. One such resin was developed by Dow, Rohm and Haas and Mitsubishi, as a crosslinked styrene - divinylbenzene copolymer having iminodiace-tate groups for joining fixed functional group sites to the metals by a chelate bond, (42) as ... 

R.C. Carlson, The Effect of Brine Impurities on DSA Electrodes, 14th Annual Chlorine/Chlorate Seminar, ELTECH Systems Corp., Cleveland,OH (1998). 

Brine impurities originate from various sources and their effects on cell performance differ. We can classify them into several groups cationic impurities such as Mg " " and Ca " ", anionic impurities such as sulfate and halides other than chloride, and nonionic impurities such as alumina and silica. The effects of the interactions of certain combinations of impurities must also be considered. 

Brine impurities Soluble impurities in the anolyte can diffuse into the membrane, assisted by the electric field or water transport through the membrane. The impurities may pass harmlessly through the membrane, selectively displace sodium, reducing the... 

The composite membranes used in membrane cells are prone to damage for a variety of reasons [123,124], some of which are discussed in Section 4.8.5. Obvious forms of physical damage are tears, pinholes, and blisters. However, physical damage can also arise from brine impurity precipitation and void damage from low conductivity operation. 

TABLE 4.8.9 Adv se Effects of Brine Impurities on Membrane Performance (From Ref. [127] Summary of Findings by Japan Soda Industry Association, as Modified by Asahi Kasei-Based Experience and InvestigaticMis)... 

J.T. Keating and K.J. Behling, Brine Impurities and Membrane Cell Perfcxmance. In NAt. Prout and J.S. Moorhouse (eds) Modem Chlor-Alkali Technology, Vol. 4, Elsevier Applied Science, London (1990), p. 125. 

K. Yamaguchi, T. Ichisaka, and I. Kumagai, The Control of Brine Impurities in the Membrane Process in the Chlor-Alkali Industry. In Chlorine Institute s 29th Plant Operation Seminar, Tampa, FL (1986). 

H. Obanawa, Effect of Brine Impurities and Blisters and Membrane Service Life. In ELTECH s 16th Annual Chlorine/Chlorate Seminar, ELTECH Systems Corporation, Cleveland, OH (2000). 

NaCl. In addition, all the feed brine impurities and any chlorate produced in the anode compartments are carried through by bulk flow of the anolyte into the cell liquor. Unless special steps are taken to remove them, they appear in the final product. 

Membrane Cells. Membrane cells are not subject to the electrode poisoning suffered by mercury cells. They are in this respect similar to diaphragm cells, but the membranes themselves are exceptionally sensitive to brine impurities [77], and brine specifications for membrane cells are more onerous. Section 4.8 discussed the structure and performance of membranes and explained the reasons for this sensitivity. Certain impurities can affect cell performance and the service life of the membranes even when present at ppb levels. Their concentrations in the brine must be rigidly controlled. When this is done successfully and ultra-pure brine is consistently available, service life can be quite long, and test cells have operated well for up to 9 years [78]. 

Brine impurities can also affect the anode coating. Barium sulfate, for example, precipitates on or in the coating and can destroy its catalytic action. Other ions harmful to the anode coating include lead, manganese, and fluoride. 

TABLE 7.10. Brine Impurities—Sources, Effects, and Mitigation... 

Section 4.8 and its appendix discuss the action of iodine in more detail. It interacts with other elements to form other precipitates in either NaCl or KO service. These precipitates include barium paraperiodate, and so there may be a synergistic effect when both barium and iodine are present in the brine. Table 4.8.8 lists commercial brine specifications for some of the common membranes [202]. The allowable concentrations of barium and iodine may be related to each other or to operating current density. Table 4.8.9 lists the adverse effeets of various brine impurities. There are reports of physically distinct forms of Ba-I precipitates, with some very fine particles that form in regions away from the main current paths through the membrane [203]. These tend to have relatively little effect on membrane performance [204], and Section 4.8.S.3 also discusses the development of membranes with enhanced resistance to the effects of iodine. 

Z.J. Col6n, Brine Impurity Effect in Mercury Cells, Seventh Annual Electrode Corporation Chlorine/Chlorate Industry Seminar, Cleveland, OH (1991). 

Impurities in Brine. Control of brine impurity levels is achieved by a combination of chemical treatment plus ion exchange and an effective purge from the brine circuit. The purge may be made selective by some of the techniques discussed in Chapter 7. The qualitative effects of some of the major impurities are discussed in some detail in Chapter 4, and Table 7.10 sununarizes effects and remedies. Still, a brief summary as part of this discussion does not seem out of place. 

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