Electro-winning

In terms of scale of production (around 2 x 10 ton year worldwide) aluminium electrolysis is second in importance only to the chlor-alkali industry. This is because aluminium is both light and strong and therefore suitable for many engineering and construction applications, may readily and cheaply by treated by anodizing (section 8.3.1) to retard corrosion and is the principal alternative to copper as a conductor of electricity. Moreover, the known reserves of aluminium ores are relatively high. 

Aluminium is normally produced from the ore, bauxite, which is a hydrated aluminium oxide containing silica and other metal oxides, particularly iron. It is converted to a pure alumina using the equilibrium  

The ore is first treated with sodium hydroxide under pressure. The aluminium largely dissolves as the aluminate, the iron oxide is insoluble and the silica also remains in the form of a sodium aluminium silicate, which leads to a loss of aluminium. Hence, the best bauxites are those low in silica. After filtration, the hydrated aluminium oxide is reprecipitated by seeding and the sodium hydroxide solution may be reused. The alumina is washed and then heated at 1200° C to remove water. The final step in the production of aluminium metal has to be electrolytic since the reduction of alumina with carbon is only possible at very high temperatures and the reverse reaction occurs on cooling. Moreover, because of the chemistry of aluminium, the electrolysis medium cannot be water in fact, almost all commercial production of aluminium during the last 90 years has used an electrolysis in molten cryolite, Na3AlF6. 

The carbon anode is consumed in a stoichiometric amount but the free energy for this reaction is 340 kJ mol A1 (at lOOO C) compared with 640 kJ mol for the reaction  

A cell for the Hall-Heroult process is shown in Fig. 4.1, and a block diagram of the whole process is shown in Fig. 4.2. Cell design is determined largely by the need to contain molten cryolite at high temperatures and to withstand attack by molten aluminium and also by sodium and fluorine formed as minor products at the cathode and anode respectively. Hence, the cell is a strong steel box lined first with alumina to act as a refractory, thermal insulator and then with carbon. 

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Electrolysis. Electro winning of hafnium, zirconium, and titanium has been proposed as an alternative to the KroU process. Electrolysis of an all chloride hafnium salt system is inefficient because of the stabiHty of lower chlorides in these melts. The presence of fluoride salts in the melt increases the StabiHty of in solution and results in much better current efficiencies. Hafnium is produced by this procedure in Erance (17). 

Oxygen-Evolving Anode. Research efforts to iacorporate the coated metal anode for oxygen-evolving appHcations such as specialty electrochemical synthesis, electro winning, impressed current, electrodialysis, and metal recovery found only limited appHcations for many years. 

Electrometallurgy. A term covering the various electrical processes for the working of metals, eg, electro deposition, electrorefining and electro winning, and operations in electric furnaces. 

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. 

Cementation. Cementation is the precipitation of copper from copper leach solutions by replacement with iron. It was formerly the most commonly used method of recovering copper from leach solutions but has been replaced by solvent extraction—electro winning. The type of iron used ia cementation is important, and the most widely used material is detinned, light-gauge, shredded scrap iron. This operation can be performed by the scrap iron cone (Keimecott Precipitation Cone) or a vibrating cementation mill that combines high copper precipitation efficiency and reduced iron consumption (41). 

Fig. 11. Conceptual flow sheet of solvent extraction—electro winning.

In electro winning, the cathode reaction is the same as for electrorefining (see eq. 31). However, because of the use of insoluble anodes, oxygen is released at the anode. 

To date the material has been used as an electrode in electro-winning, electro-chlorination, batteries and electrostatic precipitators, but only to a very limited extent in cathodic protection. 

To conclude this section, reference may be drawn to what is called the Placid process for recycling lead from batteries. Placid denotes the leaching of lead in warm, slightly acidic, hydrochloric acid brine to form soluble lead chloride. Lead is won from the lead chloride on the cathode of an electro winning cell and is collected. Chloride anions are released simultaneously, but then react immediately with hydrogen ions that have been produced stoichio-metrically from electrolysis of water in the anolyte and passed into the catholyte through a membrane. The hydrochloric acid that is formed is returned as a make-up content to the leaching bath. 

Cuprex [Copper extraction] A process for extracting copper from sulfide ores, combining chloride leaching, electro-winning, and solvent extraction. Piloted by a consortium consisting of ICI, Nerco Minerals Company, and Tecnicas Reunidas. 

A recent and extremely important development lies in the application of the technique of liquid extraction to metallurgical processes. The successful development of methods for the purification of uranium fuel and for the recovery of spent fuel elements in the nuclear power industry by extraction methods, mainly based on packed, including pulsed, columns as discussed in Section 13.5 has led to their application to other metallurgical processes. Of these, the recovery of copper from acid leach liquors and subsequent electro-winning from these liquors is the most extensive, although further applications to nickel and other metals are being developed. In many of these processes, some form of chemical complex is formed between the solute and the solvent so that the kinetics of the process become important. The extraction operation may be either a physical operation, as discussed previously, or a chemical operation. Chemical operations have been classified by Hanson(1) as follows ... 

By analogy with the HaU-Heroult process for electro winning of aluminum, first attempts used molten salt electrolytes at high temperature [1]. Silicon oxide... 

The extracted zinc is transferred to a sulfate medium, which is preferred for the electrowinning of zinc, by stripping with a solution containing sulfuric acid (being spent electrolyte from the electro winning plant) ... 

Systems for evaluating electrolytes for metal electro winning have been developed and are being used commercially in zinc production (96). Computerized mathematical models of zinc electrowinning cells have been developed and validated by comparison with experimental data taken from pilot-plant cells (97). 

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