Chlor-alkali brine process

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. 

Removal of brine contaminants accounts for a significant portion of overall chlor—alkali production cost, especially for the membrane process. Moreover, part or all of the depleted brine from mercury and membrane cells must first be dechlorinated to recover the dissolved chlorine and to prevent corrosion during further processing. In a typical membrane plant, HCl is added to Hberate chlorine, then a vacuum is appHed to recover it. A reducing agent such as sodium sulfite is added to remove the final traces because chlorine would adversely react with the ion-exchange resins used later in the process. Dechlorinated brine is then resaturated with soHd salt for further use. 

Electrolytic Preparation of Chlorine and Caustic Soda. The preparation of chlorine [7782-50-5] and caustic soda [1310-73-2] is an important use for mercury metal. Since 1989, chlor—alkali production has been responsible for the largest use for mercury in the United States. In this process, mercury is used as a flowing cathode in an electrolytic cell into which a sodium chloride [7647-14-5] solution (brine) is introduced. This brine is then subjected to an electric current, and the aqueous solution of sodium chloride flows between the anode and the mercury, releasing chlorine gas at the anode. The sodium ions form an amalgam with the mercury cathode. Water is added to the amalgam to remove the sodium [7440-23-5] forming hydrogen [1333-74-0] and sodium hydroxide and relatively pure mercury metal, which is recycled into the cell (see Alkali and chlorine products). 

SAMEX A process for removing traces of mercury from the waste brine from the chlor-alkali process. 

Kvaerner Chemetics have developed a novel, patented process [1] for the removal of multivalent anions from concentrated brine solutions. The prime market for this process is the removal of sodium sulphate from chlor-alkali and sodium chlorate brine systems. The sulphate ion in a brine solution can have a detrimental effect on ion-exchange membranes used in the production of chlorine and sodium hydroxide consequently tight limits are imposed on the concentration of sulphate ions in brine. As brine is continuously recycled from the electrolysers back to the saturation area, progressively more and more sulphate ions are dissolved and build up quickly in concentration to exceed the allowable process limits. A number of processes have been designed to remove sulphate ions from brine. Most of these methods are either high in capital or operating cost [2] or have large effluent flows. 

Figure 12.1 shows a scheme of the brine system for the membrane electrolysis process. The RNDS is installed at the point of depleted brine flow. Figure 12.2 illustrates the principle of the RNDS operation. The required area for the RNDS set-up in a chlor-alkali plant having a capacity of 135 000 tonnes of NaOH per annum is 54 m2. 

When appropriate, the chain is extended further upstream to include production of brine, for example by solution mining. Also, because the chlor-alkali plant is a major user of electric power, it is sometimes convenient to include the power generation plant within the chain. For the purposes of this chapter, however, the definition of the vinyl chain shall be restricted to the five processes listed above (Fig. 21.1). 

You have already seen that chlorine gas can be made by the electrolysis of molten sodium chloride. In industry, some chlorine is produced in this way using the Downs cell described earlier. However, more chlorine is produced in Canada using a different method, called the chlor-alkali process. In this process, brine is electrolyzed in a cell like the one shown in Figure 11.32. Brine is a saturated solution of sodium chloride. 

The largest scale synthesis based on electrolysis is the chlor-alkali process. Sodium ions in a salt brine migrate... 

In the last twenty-five years a new process has been developed in the chlor-alkali industry that uses a membrane to separate the anode and cathode compartments in brine electrolysis cells. The membrane is superior to the diaphragm used in diaphragm cells because the membrane is impermeable to anions. Only cations can flow through the membrane. Because neither Cl- nor OH- ions can... 

Chlor-alkali process the process for producing chlorine and sodium hydroxide by electrolyzing brine in a mercury cell. (11.8)... 

Ion-exchanger regeneration Brine acidification in chlor-alkali industry Wastewater treatment from amino acid processing... 

Brine Starting point of the chlor-alkali process is usually solid NaCl in form of rock salt, solar salt or prepared from solution-mined brine. These materials of course contain impurities. 

The contaminations are harmful for the three chlor-alkali processes in different ways (as will be discussed in the following). So, the brine has to be purified, usually through precipitation by addition of carbonate and hydroxide followed by filtration steps. A more modern possibility... 

Only after viewing the membrane as a thin film semiconductive phase can one begin to seriously evaluate its potentialities. It is a multidimensional problem, and in the chlor-alkali cells the water transport is controlled by brine concentration while caustic strength controls the cathode efficiency. The membrane provides a low energy pathway for the phase change and separation process. 

The principal application of "Nafion" currently is as a membrane separator in chlor-alkali cells, shown schematically in Figure 1. In this process water is decomposed in the cathode compartment to produce caustic and hydrogen, while saturated brine is fed to the anode compartment where the chloride ion is reduced to chlorine gas. The role of the membrane is to separate the two compartments, allow the facile transport of sodium ions from the anode to cathode compartments, and to restrict the flux of hydroxyl ions across the membrane. In the classical picture of ion exchange membranes (14) where the ion exchange sites are... 

Even though the main focus for the chlor-alkali engineers is the cell, and cell room, electrolysis is only one of many equally important operations involved in this process. All electrosynthetic processes require the following ancillary processes reactant feed, (or brine, here), preparation, electrolysis, product recovery, and finally D.C. power. These process units are related as shown in the typical membrane cell plant (6). 

Chlorine gas, CI2, is prepared industrially by the electrolysis of molten NaCl (see Section 19.8) or by the chlor-alkali process, the electrolysis of a concentrated aqueous NaCl solution (called brine). Chlor denotes chlorine and alkali denotes an alkali metal, snch as sodium.) Two of the common cells nsed in the chlor-alkali process are the mercnry cell and the diaphragm cell. In both cells the overall reaction is... 

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

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