Electrochemical process

(physical chemistry) A chemical change accompanying the passage of an electric current, especially as used in the preparation of commercially important quantities of certain chemical substances. The reverse change, in which a chemical reaction is used as the source of energy to produce an electric current, as in a battery.

The principles of electrochemistry may be adapted for use in the preparation of commercially important quantities of certain substances, both inorganic and organic in nature. See also Electrochemistry.

Inorganic processes

Inorganic chemical processes can be classified as electrolytic, electrothermic, and miscellaneous processes including electric discharge through gases and separation by electrical means. In electrolytic processes, chemical and electrical energy are interchanged. Current passed through an electrolytic cell causes chemical reactions at the electrodes. Voltaic cells convert chemicals into electricity. Electrothermic processes use electricity to attain the necessary temperature for reaction. See also Electrochemistry; Electrolysis; Electrolytic conductance; Electromotive force (cells).

Electrolysis in aqueous solutions

The electrolysis of water to form hydrogen and oxygen, according to the reaction 2H₂O → 2H₂ + O₂, may be considered as the simplest process for aqueous electrolytes. It does not compete with hydrogen from propane or from natural gas and with oxygen from liquid air, except in small installations. While simplicity, high hydrogen purity requirement, and lower capital cost (in small plants) have justified electrolytic plants, severely rising energy costs have limited such applications. Heavy water, or deuterium oxide, used in moderating nuclear reactors is also a by-product of the electrolysis of water. See also Deuterium; Heavy water; Hydrogen; Oxygen.

Metallurgical applications

Protective or decorative coatings on a base metal such as steel are obtained by electroplating. Plating may also be used to replace worn metal or to provide a wear-resistant surface. Electrogalvanizing is preferred over hot dipping for applying zinc to steel. Tin plate for containers is electrolytic.

Electroforming is a method of forming or reproducing articles by electrodeposition. In contrast to electroplating, the product is removed from the base surface or mold. Electrodeposition of metal powders is used to produce particles in the 1- to 1000-micrometer range for use in powder metallurgy and metallic pigments. Electrolytic polishing of metals is accomplished by making the article anodic in an electrolyte of mixed acids. Electrolytic machining of metals is accomplished by making the metal part anodic in a suitable electrolyte. Electrorefining is a process for purifying metals and recovering their impurities, which at times are more valuable than the original metal. Electrowinning, sometimes termed aqueous electrometallurgy, involves processing of metallic ores by leaching solutions to obtain metal-containing electrolytes which can be processed with insoluble anodes and metal cathodes.

Alkali-chlorine processes

Electrolysis of alkali halides is the basis of the alkali-chlorine and chlorate industries. Chlorine, Cl₂, and caustic soda, NaOH (or caustic potash, KOH), are made by electrolysis of brine, a solution of sodium chloride, NaCl, in water. Hydrochloric acid electrolysis is of interest for recovery of chlorine from HCl resulting as a by-product from organic chlorinations. See also Chlorine.

Oxidations and reductions

These reactions occur in all cells, but in a narrower sense oxidation reactions are those in which oxygen or chlorine at the anode oxidizes some material to form a new compound; reduction reactions are those in which hydrogen, liberated at the cathode, reduces a material to a new product. There are no commercial applications of inorganic electrochemical reductions by this narrow definition.

Ion-permeable membrane cells

These utilize diaphragms made of ion-exchange resins. Cation-permeable membranes permit cations to pass through but not anions, whereas the reverse holds for anion-permeable membranes. Purification of sea water is the most important application. Salt has been recovered from sea water which has been concentrated in this way. See also Ion-selective membranes and electrodes.

Fused-salt electrolysis

Aluminum, barium, beryllium, cerium and misch metal, fluorine, lithium, magnesium, sodium, molybdenum, thorium, titanium, uranium, and zirconium are obtained by electrolysis of fused salts, because water interferes with the desired reaction. Raw materials must all be purified before addition to fused-salt cells, because purification of the electrolyte is not economical as in aqueous electrolytes. Metallizing is a process of depositing a metal as an alloy on a substrate from a fused complex metal salt.

Electrothermics

The manufacture of many products requires temperatures higher than can be obtained by combustion methods. Electric heat can usually be developed at, or close to, the point where it is required, so that it is relatively quick. It permits easy control of the atmosphere for oxidizing, reducing, or neutral conditions.

Products of the electric furnace include iron and steel; ferroalloys; nonferrous metals and alloys; the exotic metals titanium, zirconium, hafnium, thorium, and uranium; and nonmetallic products such as calcium carbide, calcium cyanamide, sodium cyanide, silicon carbide, boron carbide, and graphite.

Zone refining of metals for the electronics industry, such as silicon for diodes and transistors, is accomplished by induction melting of the metal in a narrow zone and slow movement of the molten zone in the metal ingot from one end to the other in an evacuated or inert gas–filled enclosure. Impurities move toward the end of the ingot. The operation is repeated until the desired purity is obtained.

Electrodialysis

This is the separation of low-molecular-weight electrolytes from aqueous solutions by migration of the electrolyte through semipermeable membranes in an electric field. It is used on an industrial scale for deashing starch hydrolyzates and whey, and in many municipalities for producing potable water from saline water. Its uses also include the concentration of liquid foods such as dairy products and citrus juices, the recovery of sulfite pulp waste and pickling acid, and the isolation of proteins. See also Colloid; Dialysis.

Electrophoretic deposition

This is the deposition of a non-conductive material in a finely divided state from a suspension in an inert medium. Electrophoresis is the migration of colloidal particles, which acquire positive or negative charges in an electric field. The process is useful in electropainting; for instance, electropainting of automobile bodies and other objects has now been adopted on a large scale. Rubber latex is an example of a negatively charged colloid which can be plated on an anode. Electronic components can be coated with inorganic salts, oxides, and ceramics suspended in organic media. See also Electrophoresis.

Electroendosmosis

This is the movement of a liquid with respect to an immobilized colloid in an electric field. The process is used in the dehydration of peat, dye pastes, and clay. It is also used commercially for dewatering soils in mining, road building construction, and other civil engineering works.

Electrostatic technique

The deposition of charged particles from suspension in gases has many useful applications. The Cottrell electrostatic precipitator removes dusts and mists from gases. See also Electrostatic precipitator.

Spray painting with a high voltage between the spray gun and the work is particularly effective in providing an even coating with an economical use of paint on irregular and open surfaces, such as a screen.

In xerography a sheet of plain paper is electrically sensitized in those areas corresponding to an original so that colored resin particles carrying an opposite charge are attracted and retained only on the sensitized areas, thus producing a visible image corresponding to the original.

Abrasive paper and cloth are coated with an adhesive and abrasive powders attracted to the base material in an electrostatic field. Pile fabrics can be produced in a similar manner, with the short fibers oriented by the electric field.

Organic processes

Organic electrochemistry was once regarded as a tantalizing area with many important laboratory achievements but few successes in commercial practice. This situation is changing, however, in that electroorganic processes are likely to prove commercially advantageous if they can fulfill either of two conditions: (1) performance under conditions of voltage corresponding thermodynamically to the conversion of an organic group to a reduced or oxidized group, with the cell products relatively easy to isolate and purify; (2) performance of a highly selective, specific technique to make an addition at a double bond, or to split a particular bond (for example, between carbon atoms 17 and 18 of a complex molecule having 25 carbon atoms).

Selectivity and specificity are highly important in electroorganic processes for the manufacture of complicated molecules of vitamins and hormones—as well as for the medicinal products whose action on pathogenic organisms is a function of their spatial arrangement, steric forms, and resonance.

The electrolytic approach can also be competitive for some low-cost, tonnage products. Here continuous processing is important, and only a single phase should be present, that is, a solution rather than an emulsion, dispersion, or mechanical mixture. Only for fairly valuable products is it practical to find a conducting solvent and then to engineer around it.

The electrolytic oxidation and reduction of organic compounds differ from the corresponding and more familiar inorganic reactions only in that organic reactions tend to be more complex and have low yields. The electrochemical principles are precisely those of inorganic reactions, while the procedures for handling the chemicals are precisely those of organic chemistry.