Electrolytic metal extraction

Many metals are extracted from their compounds, as found in ores, by electrolytic processes. By far the most important is the Hall-Heroult process, invented in 1886, for producing aluminium from alumina, itself refined from bauxite ore. Alumina is dissolved in molten cryolite, Na3Alp6, and electrolysed, using carbon anodes and the aluminium itself as cathode. While various details are being steadily improved, the basic process is still the same today. 

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Electroslag Also called ESR. A general term for any electrolytic metal extraction process in which the metal is produced in the molten state beneath a layer of molten slag. Used mainly for ferrous alloys. 

In addition, solvent extraction is appHed to the processing of other metals for the nuclear industry and to the reprocessing of spent fuels (see Nuclearreactors). It is commercially used for the cobalt—nickel separation prior to electrowinning in chloride electrolyte. Both extraction columns and mixer-settlers are in use. 

Figure 6.22 Schematic diagrams of electrolytic cells for metal extraction from moltent salts (a-f) monopolar cells.

There are many applications in chemical engineering where diffusion of charged species is involved. Examples include ion exchange, metals extraction, electrochemical reactors, and membrane separations. There is an excellent textbook in this area (Newman, 1991). Here we will be content to show that the treatment of electrolyte diffusion follows naturally from the generalized treatment of diffusion given in Section 2.3. 

In some electrolytic solutions, especially those strong in halides, many metal eations are complexed by anions to the extent that they exist primarily as neutral ion pairs or anionic species. The formation of anionic complexes may retard metal extraction by solvation or cation exchange, but it can be exploited by use of anion-exchanging extractants. 

Chem. Descrip. Propylene carbonate CAS m32-7 EINECS/ELINCS 20S 572-1 Uses Solvent, reactant, plastidzer for fibers, textiles, dyeing, plastics and resins, gas treating, aromatic hydrocarbon extraction, metal extraction, surf, coatings (paints, varnishes, adhesives, plastics, epoxies, mastics), foundry sand binder, lubricants, electrolytes, cosmetics (polar additive for montmorillonite or bentonite clay gellants) fire extinguishing compds. antifoam for antifreeze hydraulic brake fluids as plating medium... 

Copper and zinc are the principal metals extracted by electrolysis in aqueous solution the total world production of both approaches 10 ton yr although the electrolytic route accounts for only 10% of the copper and 50% of the zinc produced. Moreover the large electrolytic plants are limited to sites in Africa, Australia and Canada where hydroelectric power is available close to the mines. Cobalt, nickel, chromium, manganese, cadmium, gallium, thallium, indium, silver and gold have also been reported to have been extracted by a hydrometallurgical process but, since these metals are only produced in a low tonnage, the electrolytic processes are on a small scale. 

The only electrolytic processes of any commercial importance for rare metal extraction are based upon the use of molten salt baths. Chlorides or fluorides of the rare metal are invariably electrolysed from solutions in similar but inert salts, usually of the alkali or alkaline earth metals. 

Oxygen evolution is the counter electrode reaction in several electrolytic processes (e.g. metal extraction, chromium plating, several organic electrosyntheses) while oxygen reduction is the desired cathode reactions in fuel cells and metalair batteries, a potential cathode reaction in improved chlor-alkali cells and a component reaction in corrosion. 

The matte can be treated in different ways, depending on the copper content and on the desired product. In some cases, the copper content of the Bessemer matte is low enough to allow the material to be cast directly into sulfide anodes for electrolytic refining. Usually it is necessary first to separate the nickel and copper sulfides. The copper—nickel matte is cooled slowly for ca 4 d to faciUtate grain growth of mineral crystals of copper sulfide, nickel—sulfide, and a nickel—copper alloy. This matte is pulverized, the nickel and copper sulfides isolated by flotation, and the alloy extracted magnetically and refined electrolyticaHy. The nickel sulfide is cast into anodes for electrolysis or, more commonly, is roasted to nickel oxide and further reduced to metal for refining by electrolysis or by the carbonyl method. Alternatively, the nickel sulfide may be roasted to provide a nickel oxide sinter that is suitable for direct use by the steel industry. 

Copper. Domestic mine production of copper metal in 1994 was over 1,800,000 t. Whereas U.S. copper production increased in the 1980s and 1990s, world supply declined in 1994. There are eight primary and five secondary smelters, nine electrolytic and six fire refiners, and fifteen solvent extraction—electro winning (SX—EW) plants. Almost 540,000 t/yr of old scrap copper and alloy are recycled in the United States accounting for - 24% of total U.S. consumption (11). New scrap accounted for 825,000 t of contained copper. Almost 80% of the new scrap was consumed by brass mills. The ratio of new-to-old scrap is about 60 40% representing 38% of U.S. supply. 

J. E. Hoffmann, "Recovery of Selenium from Electrolytic Copper Refinery Slimes," in V. Kudryk, D. A. Corrigan, and W. W. Liang, eds.. Precious Metals Mining Extraction and Processing H, TMS, Warrendale, Pa., 1983. 

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