Diaphragm cell

FIGURE 2.5. Schematic of historical diaphragm cells. The symbols are the same as in Fig. 2.4. 

Following the invention of deposited asbestos diaphragms, Stuart developed the Hooker S cell, which had vertical cathode fingers with deposited diaphragms. These S cells were very popular after World War II and were phased into operation throughout the world. In 1960, almost 60% of the chlorine production is the United States was in Hooker cells. 

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In 1988 diaphragm cells accounted for 76% of all U.S. chlorine production, mercury cells for 17%, membrane cells for 5%, and all other production methods for 2%. Corresponding statistics for Canadian production are diaphragm cells, 81% mercury cells, 15% and membrane cells, 4% (5). for a number of reasons, including concerns over mercury pollution, recent trends are away from mercury cell production toward the more environmentally acceptable membrane cells, which also produce higher quality product and have favorable economics. 

Conversion of aqueous NaCl to Cl and NaOH is achieved in three types of electrolytic cells the diaphragm cell, the membrane cell, and the mercury cell. The distinguishing feature of these cells is the manner by which the electrolysis products are prevented from mixing with each other, thus ensuring generation of products having proper purity. 

Separation of the anode and cathode products in diaphragm cells is achieved by using asbestos [1332-21 -4] or polymer-modified asbestos composite, or Polyramix deposited on a foraminous cathode. In membrane cells, on the other hand, an ion-exchange membrane is used as a separator. Anolyte—catholyte separation is realized in the diaphragm and membrane cells using separators and ion-exchange membranes, respectively. The mercury cells contain no diaphragm the mercury [7439-97-6] itself acts as a separator. 

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. 

Component Mercury cell Diaphragm cell Membrane cell... 

For estimating CI2 efficiency (Tlcb) term Cl2(a) in equation 14 should be dropped. The corresponding expression for caustic efficiency for diaphragm cells is... 

Equation 16 is the correct material balance expression for calculating the chlorine efficiency of diaphragm cells. Whereas many approximate versions are used (8), the one closest to equation 16 is the "six equation" ... 

Fig. 8. Anode for monopolar diaphragm cells a, activated (coated) expanded metal b, expanding spring c, titanium-clad copper bar d, copper thread to fix...

The toxicological problems associated with asbestos have been widely pubHshed and asbestos has been banned from most uses by the EPA. However, modem diaphragm cell chlorine plants have not had difficulty meeting the required exposure limits for asbestos fibers, and, as of 1990, the chlorine industry had an exemption allowing the continued use of asbestos as a diaphragm material. 

In the United States, 76% of the chlorine produced is from diaphragm cells. Production is equally divided between bipolar and monopolar electroly2ers. 

Fig. 9. Dow diaphragm cell, section view a, perforated steel back plate b, cathode pocket c, asbestos diaphragm d, DSA anode e, copper back plate f,...

Fig. 11. Cut view of OxyTecli H-4 diaphragm cell operatiag at a nominal load of 150 kA.

Table 10. Operating Capacities and Characteristics of OxyTech Diaphragm Cells...

Table 11. Design and Operating Characteristics of HU Series Diaphragm Cells...

The brine feed to the electroly2ers of all the processes is usually acidified with hydrochloric acid to reduce oxygen and chlorate formation in the anolyte. Table 14 gives the specifications of the feed brines requited for the membrane and diaphragm cell process to reali2e optimal performance. 

Three forms of caustic soda are produced to meet customer needs purified diaphragm caustic (50% Rayon grade), 73% caustic, and anhydrous caustic. Regular 50% caustic from the diaphragm cell process is suitable for most appHcations and accounts for about 85% of the NaOH consumed in the United States. However, it caimot be used in operations such as the manufacture of rayon, the synthesis of alkyl aryl sulfonates, or the production of anhydrous caustic because of the presence of salt, sodium chlorate, and heavy metals. Membrane and mercury cell caustic, on the other hand, is of superior quaUty and... 

Caustic Soda. Diaphragm cell caustic is commercially purified by the DH process or the ammonia extraction method offered by PPG and OxyTech (see Fig. 38), essentially involving Hquid—Hquid extraction to reduce the salt and sodium chlorate content (86). Thus 50% caustic comes in contact with ammonia in a countercurrent fashion at 60°C and up to 2500 kPa (25 atm) pressure, the Hquid NH absorbing salt, chlorate, carbonate, water, and some caustic. The overflow from the reactor is stripped of NH, which is then concentrated and returned to the extraction process. The product, about 62% NaOH and devoid of impurities, is stripped free of NH, which is concentrated and recirculated. MetaUic impurities can be reduced to low concentrations by electrolysis employing porous cathodes. The caustic is then freed of Fe, Ni, Pb, and Cu ions, which are deposited on the cathode. 

E. H. Cook and M. P. Grotheer, Energy S avingDevelopments for Diaphragm Cells and Caustic Evaporators, 23rd Chlorine Plant Manager s Seminar, New Orleans, The Chlorine Institute, Inc., Feb. 6, 1980. 

De Nora Glanor Diaphragm Cells, De Nora Permalec SpA, Milan, Italy. 

Diaphragm Cells, OxyTech Systems, Inc., Chardon, Ohio, 1988. 

Japan s Chlor—Alkali Producers Save Energy by Retrofiting Diaphragm Cells (Case History), E. I. du Pont de Nemours Co., Inc., Wilmington, Del. 

Nickel. Most nickel is also refined by electrolysis. Both copper and nickel dissolve at the potential required for anodic dissolution. To prevent plating of the dissolved copper at the cathode, a diaphragm cell is used, and the anolyte is circulated through a purification circuit before entering the cathodic compartment (see Nickel and nickel alloys). 

There are three main technologies available for carrying out this process diaphragm cells, mercury cells, and membrane cells. Membrane cells are the most recent development, and are generally chosen for new production capacity. 

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