Caustic electrolytic cells

Rohm and Haas Co.) which is highly effective in chelating divalent metal cations. It removes traces of calcinm from saturated sodium chloride brine, such as the feed streams to chlorine-caustic electrolytic cells (155). This resin also has high affinity for lead and zinc, and has been used for treating industrial-waste streams and coohng-tower waters. 

The catholyte from diaphragm cells typically analyzes as 9—12% NaOH and 14—16% NaCl. This ceUHquor is concentrated to 50% NaOH in a series of steps primarily involving three or four evaporators. Membrane cells, on the other hand, produce 30—35% NaOH which is evaporated in a single stage to produce 50% NaOH. Seventy percent caustic containing very Httie salt is made directiy in mercury cell production by reaction of the sodium amalgam from the electrolytic cells with water in denuders. 

Brine Treatment. The principal use of aqueous HCl is for the acidification of brine prior to feeding it to the electrolytic cells for producing chlorine and caustic soda. Almost all of this HCl comes from captive sources. An estimated 213 thousand metric tons of HCl (100% basis) was used for brine treatment in 1993 (74). 

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). 

Cellophane or its derivatives have been used as the basic separator for the silver—ziac cell siace the 1940s (65,66). Cellophane is hydrated by the caustic electrolyte and expands to approximately three times its dry thickness iaside the cell exerting a small internal pressure ia the cell. This pressure restrains the ziac anode active material within the plate itself and renders the ziac less available for dissolution duriag discharge. The cellophane, however, is also the principal limitation to cell life. Oxidation of the cellophane ia the cell environment degrades the separator and within a relatively short time short circuits may occur ia the cell. In addition, chemical combination of dissolved silver species ia the electrolyte may form a conductive path through the cellophane. 

Hydrogen Liquefaction. Hydrogen can be produced from caustic—chlorine electrolytic cells, by decomposition of ammonia or methanol, or by steam—methane reforming. Hydrogen recovered by these methods must be further purified prior to Hquefaction. This is generally achieved by utilizing pressure swing adsorption methods whereby impurities are adsorbed on a soHd adsorbent. 

Stress corrosion can arise in plain carbon and low-alloy steels if critical conditions of temperature, concentration and potential in hot alkali solutions are present (see Section 2.3.3). The critical potential range for stress corrosion is shown in Fig. 2-18. This potential range corresponds to the active/passive transition. Theoretically, anodic protection as well as cathodic protection would be possible (see Section 2.4) however, in the active condition, noticeable negligible dissolution of the steel occurs due to the formation of FeO ions. Therefore, the anodic protection method was chosen for protecting a water electrolysis plant operating with caustic potash solution against stress corrosion [30]. The protection current was provided by the electrolytic cells of the plant. 

This type of electrolytic cell consists of anodes and cathodes that are separated by a water impermeable ion-conducting membrane. Brine is fed through the anode where chlorine gas is generated and sodium hydroxide solution collects at the cathode. Chloride ions are prevented from migrating from the anode compartment to the cathode compartment by the membrane and this, consequently, leads to the production of sodium hydroxide, free of contaminants like salts. The condition of the membrane during operation requires more care. They must remain stable while being exposed to chlorine and strong caustic solution on either side they must allow, also, the transport of sodium ions and not chloride ions. 

Potassium hydroxide (KOH) is commonly referred to as caustic potash or lye because of its extreme alkahnity (high pH value). It is produced in an electrolytic cell reaction (2KC1 + THfD —> Hj + CIj + 2KOH). It is used to manufacture soaps and as a caustic drain cleaner because it is strong enough to dissolve animal fats that clog household and restaurant drains. 

The electrolysis reaction and types of cells were described adequately under caustic soda. The chlorine gas, contaminated with water from the electrolytic cell, is cooled to 12-14°C to liquefy most of the water, then dried in a tower of sulfuric acid. The pure chlorine gas is compressed to 40 psi and condensed by cooling at -20 to -40 °C to liquefy the gas. 

A new variation of the chlorohydrin process uses t-butyl hypochlorite as chlorinating agent. The waste brine solution can be converted back to chlorine and caustic by a special electrolytic cell to avoid the waste of chlorine. 

The electrolytic cell shown in Figure 2 is the industrial chloralkali cell in which brine (an aqueous sodium chloride solution) is electrolytically converted to chlorine and caustic soda (sodium hydroxide, NaOH). The external power source supplies electric energy to drive the overall reaction. 

Fig. 4. An electrolytic cell for stripping galvanized steel in 90 C caustic electrolyte. (Adapted from [22]).

The invention of new materials and improved engineering methods has truly revolutionized the electrolytic process industries. Electrolytic cells for production of chlorine and caustic had, for example, evolved... 

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