Brine Purity Specifications

Keating [119] has estimated the brine flow over a 28-month time span to be about 200,000 kg m of membrane, at a brine flow rate of 200-250 kg day and at a current density of 5 kA m. If the impurity concentration is 1 ppm, then the amount of impurities that are lodged in the membrane can be as much as 200 g. This is 50% of the total membrane weight of 400 gm . If the total weight of the impurities in the membrane is to be restricted to a number of grams, the impurity levels must be in the ppb range. 

One also can estimate the number of ion-exchange sites in a membrane by assuming a cluster diameter of 5 nm and a membrane thickness of 250 p,m. The number of clusters for this case will be 3.6 x 10 m . With 10 exchange sites per cluster, the total number 

The exact specifications depend on the membrane, cell design, and operating conditions [125-127], The brine specifications recommended by membrane suppliers are shown in Table 4.8.8, and Table 4.8.9 describes the impurities and possible mechanisms of membrane damage. The reader is referred to the Appendix, where the effects of impurities are summarized along with the recommended analytical methods suggested by the membrane manufacturers. References [128-131] provide a discussion on brine treatment costs vs membrane costs. 

Electrochemical experiments to measure cell voltage and current efficiency are simple, direct, and usefiil for the investigation of membrane performance. Visual and microscopic observations are indispensable. Visual inspection is the principal method used for determining the soundness of the membrane surface. SEM is quite useful for the inspection of morphology of the surface or cross-sections. It can be used to identify deposits of impurities. XRD and X-ray fluorescence spectroscopy (XRF) are useful for the semiquantitative determination of impurity accumulation and distribution [126]. Table 4.8.10 [ 132] sununarizes the reconunended methods for detecting the physical and chemical damage to membranes. 

Nation Trademark of E.L DnPOntde Nemours Comparer Adplex Tradonarit of Asahi Kaset Cmporatitm Hemion IVademaric of Asahi Glass Compai Ltd. 

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Some older types of membrane were optimized at lower concentrations. There are still plants that operate with these membranes and produce caustic usually in the range 20-25%. While energy efficiency suffers in these plants, brine purity specifications may be less stringent [127],... 

Salt produced in the United States varies in purity from 95% NaCl for rock salt to 99.99% NaCl for mechanically evaporated salt. Mechanically evaporated salt made using purified brine generally has the highest purity rock salt generally has the lowest. Several voluntary standards and mandatory specifications apply to salt to ensure appropriate gradation, quaUty, and purity for particular salt uses (Table 4). 

Related to conductivity are the parameters of salinity and resistivity. Salinity is a measure of a mass of dissolved salts in a given mass of solution, and it is calculated based on conductivity measurements. Salinity is unitless, although it is often reported in percent. The parameter of salinity is used for the characterization of salt content in seawater, estuarine waters, industrial wastewater, and brines. Resistivity and resistance are the reciprocals of the conductivity and conductance. The parameter of resistivity is used in specifications for high purity water, which has a very low conductivity. Resistivity of reagent grade water is expressed in megohms-cm. 

Refractometry is a technique used to detect the concentration of binary mixtures based on differences in their refractive index. Besides concentration, it is also a simple method to quantify purity. Salinity of brine or sucrose concentrations in grapes or syrup are two typical applications. Urine-specific gravity is measured for drug diagnostics and plasma protein is assessed by refractometry in veterinary medicine. 

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