Performance Characteristics of Membranes

This is an ideal representation that approximates the real behavior of a membrane cell. A variety of ionic and neutral species besides Na , Cl , and OH exist both in the anolyte and in the catholyte, and these impiuities also affect the performance of the cell. Consequently, the effects of these impiuities on membrane-cell behavior have been investigated extensively in order to determine the allowable limits of impurity concentrations and develop industrial procedures for the removal of these impurities from the electrolysis system. 

Crude salt, whether it be rock or sea salt, contains a number of chemical constituents (see Table 5.1). The saturated NaCl solution prepared from these salts must be treated to remove impurities before going to a cell. Brine specifications depend on the type of cell used (diaphragm, mercury, or membrane) and the operating conditions. Water is transported from the anolyte to the catholyte through the membrane, as stated above, but more is required to maintain the water balance in the cathode compartment. 

Process water for brine treatment and catholyte adjustment may also contain impurities. On the catholyte side, the likely impurities are silica from the water supply and iron and nickel from metallic corrosion. However, the catholyte is removed continuously from the system, and while the presence of impurities may be of concern, they are not permitted to accumulate to high concentrations. On the other side, the anolyte is recirculated between the brine-treatment plant and the electrolyzers, allowing the accumulation of trace impurities to undesirable levels. 

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. 

Before discussing impurity effects, let us examine the pH profile across the membrane during the course of electrolysis. Using experimental data from a laboratory cell, Ogata and coworkers [18,103] and Obanawa and coworkers [104] found the pH in a sulfonate-carboxylate bilayer to be in the range 9-12 over the bulk of the membrane with the exception of narrow regions near the membrane/solution interfaces. On the anode side, the pH decreased steeply to about 3, while on the cathode side, it increased to 14. Thus, the pH of 9-12 in the membrane can indeed force the precipitation of metal hydroxides [105] when the metal ion concentration exceeds the dictates of the solubility product (Fig. 4.8.34). It should be noted that Hg and Fe are electrodeposited on the cathode and oxides of Mn, Pb, and Fe are formed on the anode as a result of oxidation of the relevant ionic species by the active chlorine in the anolyte. 

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Post-formation chemical, thermal and physical treatments are often used to modify the performance characteristics of membranes. Flnken ( ) presents recent results from his studies on thermal and physical modifications of Integrally-skinned cellulosic blend... 

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