Brine impurity limits

Du Pont Recommended Brine Impurity Limits for Chloralkali Plants, Aug. 1991. 

The introduction of membrane technology into chlor-alkali electrolysis has dramatically increased the demands on brine purity [141]. The lifetime of chlor-alkali membrane cells is determined by the operating conditions and the quality and purity of the feed into the electrolyzers. Good long-term performance of the cells may be obtained if brine impurities are kept within the limits recommended in Table 14. 

To avoid getting into diffusion-limited regimes arising from the solubility limitations, the amalgam should be stirred sufficiently to reduce the thickness of the diffusion layer. However, impurities such as Mg2+ in the brine can deposit on the amalgam cathode as Mg(OH)2 retarding the convection of the amalgam. Hence, brine purity becomes an important issue in mercury cell operations. 

If capacities are imbalanced toward the diaphragm side, some of the brine from the primary treatment must flow to the resaturator. If the membrane-cell capacity is high, some of the resaturated brine can be sent to those cells. This re-establishes the recycle loop, but the diaphragm cells remain as a gigantic purge from the brine system, and recycle accumulation of impurities still is strongly limited. 

Calcium carbonate also can be decomposed into its components by calcination or into CO2 and CaCl2 by digestion with hydrochloric acid. The CO2 can be absorbed into caustic solution to produce carbonate for brine treatment. If calcium is used to suppress the solubility of sulfates in raw salt, the CaCh also can be recycled. The latter technique can be limited by the presence of other metallic impurities. 

Similarly, lead is a poison that will plate out on the cathode and so should be absent from the brine and the caustic. Table 13.4 gives typical recommended limits for metallic impurities. The water used to dilute the catholyte should meet the specification of Table 13.3. 

Impurity Source Max. limit (w/w) Reagents Solubility 1 brine 11 caustic Mech- anism Damage Negative e Voltage inc An. feet on perl rease Cath. brmance Mem. Ce PQu Methods of control... 

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