The Electrolytic Cell

Sodium hydroxide is manufactured by electrolysis of concentrated aqueous sodium chloride the other product of the electrolysis, chlorine, is equally important and hence separation of anode and cathode products is necessary. This is achieved either by a diaphragm (for example in the Hooker electrolytic cell) or by using a mercury cathode which takes up the sodium formed at the cathode as an amalgam (the Kellner-Solvay ceW). The amalgam, after removal from the electrolyte cell, is treated with water to give sodium hydroxide and mercury. The mercury cell is more costly to operate but gives a purer product. 

The ammonium hydrogensulphate is returned to the electrolytic cell. A process such as this yields an aqueous solution containing about 30% hydrogen peroxide. The solution can be further concentrated, yielding ultimately pure hydrogen peroxide, by fractional distillation but the heating of concentrated hydrogen peroxide solutions requires care (see below). 

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). 

The spray dried MgCl2 powder is melted ia large reactors and further purified with chlorine and other reactants to remove magnesium oxide, water, bromine [7726-95-6], residual sulfate, and heavy metals (27,28). The molten MgCl2 is then fed to the electrolytic cells which are essentially a modification of the LG. Farben cell. Only a part of the chlorine produced is required for chlorination, leaving up to 1 kg of chlorine per kg of magnesium produced. This by-product chlorine is available for sale. 

Liquid magnesium is removed from the electrolytic cells under vacuum and transferred to the cast house where it is refined, purified, and cast iato a wide variety of shapes, sizes, and alloys. 

Russian production may be going to a flow line cell concept (35). In this process, dehydrated camaOite is fed to a chamber where it is mixed with spent electrolyte coming from the electrolytic cells. The spent electrolyte first enters a metal collection chamber, where the molten magnesium is separated. The electrolyte is then enriched with camaOite and any iasoluble impurities are allowed to settle. The enriched electrolyte is then returned to the electrolytic cells. The result is that most of the remaining impurities are removed ia the first electrolytic cell. 

Dead Sea Works Process. The Dead Sea Works, a subsidiary of Israel Chemicals Ltd., aimounced plans ia 1992 to constmct a 25,000 t/yr magnesium plant at Beer-Sheva, Israel. The plant, to be based on Russian camaHite technology, is designed to use an existing potash plant as the source of camaHte. The chlorine by-product can be either Hquefted and sold, or used ia an existing bromine plant. Waste streams from the camaHite process, as well as spent electrolyte from the electrolytic cells, can be returned to the potash plant. 

When magnesium oxide is chlorinated in the presence of powdered coke or coal (qv), anhydrous magnesium chloride is formed. In the production of magnesium metal, briquettes containing CaCl2, KCl, NaCl, MgO, and carbon are chlorinated at a temperature such that the electrolyte or cell melt collects at the bottom of the chlorinator, enabling the Hquid to be transferred directly to the electrolytic cells. 

Miniature zinc—mercuric oxide batteries may be made with either KOH or NaOH as the electrolyte. Cells having KOH operate more efficiently than those having NaOH at high current drains (Eig. 12) because of the higher conductivity of KOH. On the other hand, batteries with KOH are more difficult to seal, cells with NaOH are more resistant to leakage. 

Air pollution problems and labor costs have led to the closing of older pyrometaHurgical plants, and to increased electrolytic production. On a worldwide basis, 77% of total 2inc production in 1985 was by the electrolytic process (4). In electrolytic 2inc plants, the calcined material is dissolved in aqueous sulfuric acid, usually spent electrolyte from the electrolytic cells. Residual soHds are generally separated from the leach solution by decantation and the clarified solution is then treated with 2inc dust to remove cadmium and other impurities. 

Almost 40 years later the Lummus Co. patented an integrated process involving the addition of chlorine along with the sodium chloride and sodium hydroxide from the cathode side of an electrolytic cell to a tertiary alcohol such as tertiary butanol to produce the tertiary alkyl hypochlorite. The hypochlorite phase separates, and the aqueous brine solution is returned to the electrolytic cells. The alkyl hypochlorite reacts with an olefin in the presence of water to produce a chlorohydrin and the tertiary alcohol, which is returned to the chlorinator. With propylene, a selectivity to the chlorohydrin of better than 96% is reported (52). A series of other patents covering this technology appeared during the 1980s (53—56). 

Anode impurities either dissolve in the electrolyte or fall to the bottom of the electrolytic cell as anode slime. These slimes contain silver, gold, selenium, and tellurium and represent a very significant value. Thus, the recovery of by-products from the anode slime is an important operation. 

The precipitated copper from this reaction is an important constituent of the slime that collects at the bottom of the electrolytic cells. The accumulation of copper as well as of impurities such as nickel, arsenic, antimony, and bismuth is controlled by periodic bleed-off and treatment in the electrolyte purification section. 

Electrochemical processes require feedstock preparation for the electrolytic cells. Additionally, the electrolysis product usually requires further processing. This often involves additional equipment, as is demonstrated by the flow diagram shown in Figure 1 for a membrane chlor-alkali cell process (see Alkali AND chlorine products). Only the electrolytic cells and components ate discussed herein. 

The electrolytic cells shown ia Figures 2—7 represent both monopolar and bipolar types. The Chemetics chlorate cell (Fig. 2) contains bipolar anode/cathode assembhes. The cathodes are Stahrmet, a registered trademark of Chemetics International Co., and the anodes are titanium [7440-32-6] Ti, coated either with mthenium dioxide [12036-10-17, RUO2, or platinum [7440-06-4] Pt—indium [7439-88-5] Ir (see Metal anodes). Anodes and cathodes are joined to carrier plates of explosion-bonded titanium and Stahrmet, respectively. Several individual cells electrically connected in series are associated with one reaction vessel. 

Aluminum. Aluminum [7429-90-5] Al, is produced worldwide by the Bayer-HaH-Heroult process. This process involves the electrolysis of alumina [1344-28-1J, AI2O2, dissolved in molten cryoHte [15096-52-3] Na AIF (see Aluminumand aluminum alloys). The electrolytic cells or pots operate... 

Electrochemical Process. Several patents claim that ethylene oxide is produced ia good yields ia addition to faradic quantities of substantially pure hydrogen when water and ethylene react ia an electrochemical cell to form ethylene oxide and hydrogen (206—208). The only raw materials that are utilized ia the ethylene oxide formation are ethylene, water, and electrical energy. The electrolyte is regenerated in situ ie, within the electrolytic cell. The addition of oxygen to the ethylene is activated by a catalyst such as elemental silver or its compounds at the anode or its vicinity (206). The common electrolytes used are water-soluble alkah metal phosphates, borates, sulfates, or chromates at ca 22—25°C (207). The process can be either batch or continuous (see Electrochemicalprocessing). 

A Perkin-Elmer 5000 AAS was used, with an electrically heated quartz tube atomizer. The electrolyte is continuously conveyed by peristaltic pump. The sample solution is introduced into the loop and transported to the electrochemical cell. A constant current is applied to the electrolytic cell. The gaseous reaction products, hydrides and hydrogen, fonued at the cathode, are flowed out of the cell with the carrier stream of argon and separated from the solution in a gas-liquid separator. The hydrides are transported to an electrically heated quartz tube with argon and determined under operating conditions for hydride fonuing elements by AAS. 

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. 

Hydrogen fluoride accounts for about 907o of the gaseous fluoride emitted from the electrolytic cell. Other gaseous emissions are SO2, COj, CO, NO2, HjS, COS, CS2, SFg, and various gaseous fluorocarbons. Particulate fluoride is emitted directly from the process and is also formed from condensation and solidification of the gaseous fluorides. 

Comparison of the voltaic/galvanic cell with the electrolytic cell. 

Fig. 20.1 Potential and concentration gradients in the electrolytic cell CU/CUSO4/CU. (a) The electrodes are unpolarised the potential dilference is the equilibrium potential and there is no concentration gradient in the diffusion layer. (f>) The electrodes are polarised Ep of the anode is now more positive than E. whilst E of the cathode is more negative and concentration gradients exist across the diffusion layer c, C), are the concentrations at the electrode...

In the Electrolytic System, corrosion should not be a serious problem. Its principal drawback is design of the electrolytic cell. 

FIGURE 12.12 A schematic representation of the electrolytic cell used in the Dow process for magnesium. The electrolyte is molten magnesium chloride. As the current generated by the external source passes through the cell, magnesium ions are reduced to magnesium metal at the cathode and chloride ions are oxidized to chlorine gas at the anode. 

An example of the electrolytic cell with an LM used for the voltammetric investigation is... 

The electrolytic cells in use in extractive metallurgy mostly operate with liquid electrolytes which may be either aqueous or nonaqueous. In some cases it is possible to use only non-aqueous electrolytes, while in others it is possible to use either aqueous or nonaqueous electrolytes. A nonaqueous electrolyte may be of the organic or of the molten salt varieties. 

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