Brine purification

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. 

The contaminants can be brought into the brine tystem by salt, by chemicals used in brine purification steps, by water for dissolving the salt, firom materials of tanks, pipework, and ceU components, or by the process itself [142]. The impurities in the salt depend upon the origin of the raw material. Rock salt, vacuum salt, sea salt, brine from well mining, or salt from waste incinerators serve as supplies of NaCl. The more varied the sources are, the more diverse the impurities. 

Membrane and electrode damage effect cell performance, i.e., cause lower current efficiency, increased cell voltage, and, as a result, increased power consumption [143]. Some impurities affect the anode or cathode coating and cause an increase in overvoltage or simply deposit in the membrane, increasing its resistance and thus the cell voltage. The increase in voltage may in some cases be partially reversible when the impurity concentration drops to the recommended limits. 

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 

CO salt, precipitation with N32C03 or BaCO j 0.4 g/l NaaCO, CO H2O formation of CO2 -H-  

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. 

Membrane cells require ultrapure brine since the Ca and Mg hydroxides deposited on the membrane not only affect the mechanical integrity of the membrane19 but also cause a decrease in cell performance. Hence, the membrane manufacturers recommend that the Ca and Mg levels in the feed brine be reduced to less than 0.04 and 0.08 ppm, respectively, depending on the membrane type and operating conditions to realize long life and good performance. 

In mercury cells, impurities play a significant role during the formation of sodium amalgam and, hence, quality control of brine purity is an important unit operation. 

It may be noted that combination of two or more impurities is more harmful even though the impurity by itself has no effect on the amalgam formation, a typical example being Mg + Fe. Although Al has no effect, the colloidal A1(OH)3 occludes other impurities and forms amalgam butter, which is undesirable. While 

Ti and Ta are classified as harmful impurities, these metals are corrosion resistant in brine and, hence, have no significant influence on the primary cathodic process. 

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Chlorine Plant Auxiliaries. Flow diagrams for the three electrolytic chlor—alkali processes are given in Figures 28 and 29. Although they differ somewhat in operation, auxiUary processes such as brine purification and chlorine recovery are common to each. 

Advances during the past 20 years in membrane, electrolyser, electrode, and brine purification technologies have substantially raised the performance levels and efficiency of chlor-alkali production by ion-exchange membrane electrolysis, bringing commercial operations with a unit power consumption of 2000-2050 kWh per ton of NaOH or lower at 4 kA m-2 current density with a membrane life of four years or longer. 

The soluble ions of iron and aluminium are usually reduced to a minimum by adjusting the electrolyte pH. For the removal of solid iron hydroxide and aluminium hydroxide Bayer decided to use a new pre-coat-free brine purification technology -back-pulse filtration using GORE-TEX membrane filter cloths. 

Brine Purification by Ion Exchange with Water Elution... 

Brine purification. The system sodium bicarbonate-sodium chloride-magnesium carbonate-water. Ind. Eng. Chem., 26 1099-1104. 

Influence of Electrolysis Conditions. Among the various electrolysis conditions, brine purity has the most significant effect on the life of the membranes. The presence of a small amount of multivalent cations leads to formation of metal hydroxide deposits in the membrane, and thus causes a decrease in current efficiency, an increase in cell voltage, and damage to the polymer structure of the membrane. With perfluorocarboxylic acid membrane, the presence of more than 1 ppm of calcium ion will begin to cause these problems in a very short period (1 - 8). To obtain stable current efficiency and cell voltage, it is therefore essential to establish effective brine purification methods. 

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

K07I. . Brine purification muds from the mercury cell process in chlorine production where separately prepurified brine is not used. 

The typical layout of a mercury cell plant for producing Cl2 and NaOH is presented in Fig. 6. The brine purification procedure is... 

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... 

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