Electrolytic processes

Caustic soda is produced as a coproduct of chlorine in the sodium chloride brine electrolytic process employing diaphragm cells, mercury cells, and membrane cells. 

The diaphragm cell produces caustic soda containing 11% caustic soda and 15% salt, with a low concentration of sodium chlorate. This solution, called cell liquor, is evaporated to produce 50% caustic soda. During evaporation, salt crystallizes as the caustic concentration increases. About 1 % salt is present in the 50% caustic soda solution. 

The mercury cell produces 50% caustic, and it can also produce up to 70% caustic soda. This is a highly pure caustic with a few ppm of salt and less than 5 ppm of sodium chlorate. However, it carries trace amounts of mercury, and hence, many customers do not accept the caustic soda from mercury cells. 

The membrane cell produces about 35% caustic soda, which is concentrated by evaporation to 50%. Membrane cell caustic soda is the preferred product compared to the diaphragm and mercury-cell caustic soda. 

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The current efficiency of an electrolytic process ( current) ratio of the amount of material produced to the theoreticaUy expected quantities. 

The principal use of AIF. is as a makeup ingredient in the molten cryoflte, Na.. AIF AI2O2, bath used in aluminum reduction cells in the HaH-Haroult process and in the electrolytic process for refining of aluminum metal in the Hoopes cell. A typical composition of the molten salt bath is 80—85%... 

Manufacture and Economics. Nitrogen tritiuoride can be formed from a wide variety of chemical reactions. Only two processes have been technically and economically feasible for large-scale production the electrolysis of molten ammonium acid fluoride and the direct fluorination of the ammonia in the presence of molten ammonium fluoride. In the electrolytic process, NF is produced at the anode and H2 is produced at the cathode. In a divided cell of 4 kA having nickel anodes, extensive dilution of the gas streams with N2 was used to prevent explosive reactions between NF and H2 (17). 

The electrolytic processes for commercial production of hydrogen peroxide are based on (/) the oxidation of sulfuric acid or sulfates to peroxydisulfuric acid [13445-49-3] (peroxydisulfates) with the formation of hydrogen and (2) the double hydrolysis of the peroxydisulfuric acid (peroxydisulfates) to Caro s acid and then hydrogen peroxide. To avoid electrolysis of water, smooth platinum electrodes are used because of the high oxygen overvoltage. The overall reaction is... 

This electrolytic process technology is no longer used because of the extensive and continuous electrolyte purification needs, the high capital and power requirements, and economic inabiHty to compete with large-scale anthrahydroquinone autoxidation processes. 

Inorganic Methods. Before the development of electrolytic processes, hydrogen peroxide was manufactured solely from metal peroxides. Eady methods based on barium peroxide, obtained by air-roasting barium oxide, used dilute sulfuric or phosphoric acid to form hydrogen peroxide in 3—8% concentration and the corresponding insoluble barium salt. Mote recent patents propose acidification with carbon dioxide and calcination of the by-product barium carbonate to the oxide for recycle. 

Alkali AletalIodides. Potassium iodide [7681-11-0] KI, mol wt 166.02, mp 686°C, 76.45% I, forms colorless cubic crystals, which are soluble in water, ethanol, methanol, and acetone. KI is used in animal feeds, catalysts, photographic chemicals, for sanitation, and for radiation treatment of radiation poisoning resulting from nuclear accidents. Potassium iodide is prepared by reaction of potassium hydroxide and iodine, from HI and KHCO, or by electrolytic processes (107,108). The product is purified by crystallization from water (see also Feeds and feed additives Photography). 

The cathode material is stainless steel. The lead produced by this method analyzes 99.99 + %. The overall power consumption is less than 1 kWh/kg of lead, so that the electrolytic process for treating spent batteries has much less of an environmental impact than the conventional pyrometaUurgical process. 

M. V. Ginatta, "G.S. Electrolytic Process for the Recovery of Lead from Spent Electric Storage Batteries," paper presented at the Mnnual MIME Meeting 1975, New York. 

Alkalies. In the 1960s, 3.2-34 x 10 t /yr of lime was captively produced by the U.S. alkaH industry for manufacturing soda ash and sodium bicarbonate via the Solvay process. Electrolytic process caustic soda and natural soda ash (trona) from Wyoming have largely replaced the Solvay process. Three of the trona producers in Wyoming now purchase quicklime for producing caustic soda. 

Electrolysis of Aqueous Solutions. The electrolytic process for manganese metal, pioneered by the U.S. Bureau of Mines, is used in the Repubhc of South Africa, the United States, Japan, and beginning in 1989, Bra2il, in decreasing order of production capacity. Electrolytic manganese metal is also produced in China and Georgia. 

In electrolytic processes, the anode is the positive terminal through which electrons pass from the electrolyte. Anode design and selection of anode materials of constmction have traditionally been the result of an optimisation of anode cost and operating economics, in addition to being dependent on the requirements of the process. Most materials used in metal anode fabrication are characteristically expensive use has, however, been justified by enhanced performance and reduced operating cost. An additional consideration that has had increasing influence on selection of the appropriate anode is concern for the environment (see Electrochemical processing). 

Another approach for the production of phosphine is an aqueous electrolytic process, whereby nascent hydrogen reacts with elemental phosphoms (70). Phosphine is produced at the cathode. 

In iadustry, chemical reductioa is preferred over electrolytic processes for potassium productioa. AppHcatioa of the Dowa s electrolytic sodium process to produce potassium has aot beea successful. Potassium—sodium ahoy is easily prepared by the reactioa of sodium with molten KCl, KOH, or sohd K2CO3 powder (see Sodiumand sodiumalloys). 

Metalliding. MetaUiding, a General Electric Company process (9), is a high temperature electrolytic technique in which an anode and a cathode are suspended in a molten fluoride salt bath. As a direct current is passed from the anode to the cathode, the anode material diffuses into the surface of the cathode, which produces a uniform, pore-free alloy rather than the typical plate usually associated with electrolytic processes. The process is called metalliding because it encompasses the interaction, mostly in the soHd state, of many metals and metalloids ranging from beryUium to uranium. It is operated at 500—1200°C in an inert atmosphere and a metal vessel the coulombic yields are usually quantitative, and processing times are short controUed... 

Refining. The cmde tin obtained from slags and by smelting ore concentrates is refined by further heat treatment or sometimes electrolytic processes. 

An electrolytic process for purifying cmde vanadium has been developed at the U.S. Bureau of Mines (16). It involves the cathodic deposition of vanadium from an electrolyte consisting of a solution of VCI2 in a fused KCl—LiCl eutectic. The vanadium content of the mixture is 2—5 wt % and the operating temperature of the cell is 650—675°C. Metal crystals or flakes of up to 99.995% purity have been obtained by this method. 

Production processes are given in Table 12. Electrolytic processes are dominant because of lower cost and fewer environmental problems. Production of slab zinc in Tennessee commenced in 1978 when Jersey Miniere Zinc Company began operation of its new 90,000 t/yr electrolytic smelter at Clarksville. Today, Tennessee is the leading producer of slab zinc. Although the U.S. demand for zinc metal in the past 16 years has increased by 47%, smelting capacity has declined by almost 50%. Plants closed because they were obsolete and could not meet environmental standards or obtain sufficient concentrate. Consequendy, slab zinc has replaced concentrates as the principal import form. This situation is expected to prevail up to the year 2000 (69-71). 

Electrolytic Processes. The electrolytic procedures for both electrowinning and electrorefining beryUium have primarily involved electrolysis of the beryUium chloride [7787-47-5], BeCl2, in a variety of fused-salt baths. The chloride readUy hydrolyzes making the use of dry methods mandatory for its preparation (see Beryllium compounds). For both ecological and economic reasons there is no electrolyticaUy derived beryUium avaUable in the market-place. 

Betts Electrolytic Process. The Betts process starts with lead bullion, which may carry tin, silver, gold, bismuth, copper, antimony, arsenic, selenium, teUurium, and other impurities, but should contain at least 90% lead (6,7). If more than 0.01% tin is present, it is usually removed from the bullion first by means of a tin-drossing operation (see Tin AND TIN ALLOYS, detinning). The lead bullion is cast as plates or anodes, and numerous anodes are set in parallel in each electrolytic ceU. Between the anodes, thin sheets of pure lead are hung from conductor bars to form the cathodes. Several ceUs are connected in series. 

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. 

Oxidative surface treatment processes can be gaseous, ie, air, carbon dioxide, and ozone Hquid, ie, sodium hypochlorite, and nitric acid or electrolytic with the fiber serving as the anode within an electrolytic bath containing sodium carbonate, nitric acid, ammonium nitrate, ammonium sulfate, or other electrolyte. Examples of electrolytic processes are described in the patent Hterature (39,40)... 

Purification. The metal obtained from both electrolytic processes contains considerable oxygen, which is beheved to cause brittieness at room temperature. For most purposes the metal as plated is satisfactory. However, if ductile metal is desired, the oxygen can be removed by hydrogen reduction, the iodide process, calcium refining, or melting ia a vacuum ia the presence of a small amount of carbon. 

Hydrometallurigcal Processes. In hydrometaHurgical processes, metal values and by-products are recovered from aqueous solution by chemical or electrolytic processes. Values are solubilized by treating waste, ore, or concentrates. Leaching of copper ores in place by rain or natural streams and the subsequent recovery of copper from mnoff mine water as impure cement copper have been practiced since Roman times. Most hydrometaHurgical treatments have been appHed to ores or overburden in which the copper was present as oxide, mixed oxide—sulfide, or native copper. PyrometaHurgical and hydrometaHurgical processes are compared in Reference 34. 

Potassium cyanide is primarily used for fine silver plating but is also used for dyes and specialty products (see Electroplating). Electrolytic refining of platinum is carried out in fused potassium cyanide baths, in which a separation from silver is effected. Potassium cyanide is also a component of the electrolyte for the analytical separation of gold, silver, and copper from platinum. It is used with sodium cyanide for nitriding steel and also in mixtures for metal coloring by chemical or electrolytic processes. 

In the heavy-water plants constmcted at Savannah River and at Dana, these considerations led to designs in which the relatively economical GS process was used to concentrate the deuterium content of natural water to about 15 mol %. Vacuum distillation of water was selected (because there is Httle likelihood of product loss) for the additional concentration of the GS product from 15 to 90% D2O, and an electrolytic process was used to produce the final reactor-grade concentrate of 99.75% D2O. 

Thus, because the standard cell potential for reaction 15 is positive, the reaction proceeds spontaneously as written. Consequendy, to produce chlorine and hydrogen gas, a potential must be appHed to the cell that is greater than the open-circuit value. This then becomes an example of an electrolytic process. 

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