Cryolite, electrolysis

Aluminium is obtained on a large scale by the electrolysis of the oxide, dissolved in fused cryolite The oxide, occurring naturally as bauxite, AI2O3.2H2O, usually contains silica and iron(III) oxide as impurities. These must be removed first, since aluminium, once prepared, cannot be freed of other metals (which will be deposited on electrolysis) by refining it. The crude oxide is dissolved under pressure in caustic soda solution the aluminium oxide and silica dissolve and the ironflll) oxide is left ... 

The method of obtaining aluminum metal by the electrolysis of alumina dissolved in cryolite was discovered in 1886 by Hall in the U.S. and at about the same time by Heroult in France. Cryolite, a natural ore found in Greenland, is no longer widely used in commercial production, but has been replaced by an artificial mixture of sodium, aluminum, and calcium fluorides. 

Molten cryohte dissolves many salts and oxides, forming solutions of melting point lower than the components. Figure 1 combines the melting point diagrams for cryolite—A1F. and for cryohte—NaF. Cryohte systems ate of great importance in the HaH-Heroult electrolysis process for the manufacture of aluminum (see Aluminumand ALUMINUM alloys). Table 5 Hsts the additional examples of cryohte as a component in minimum melting compositions. 

Production of A1 metal involves two stages (a) the extraction, purification and dehydration of bauxite, and (b) the electrolysis of AI2O3 dissolved in molten cryolite Na3AlF6. Bauxite is now almost universally treated by the Bayer process this involves dissolution in aqueous NaOH, separation from insoluble impurities (red muds), partial precipitation of the trihydrate... 

Fluorides are used in many materials processes, and can poison the environment when they are discarded. Examples are ciyolite (sodium aluminofluo-ride, Na,AlF ) used to dissolve aluminum oxides for electrolysis, and hydrofluoric acid (HE) used in etching lamp bulbs and semi-conducting circuits. Today lamp bulbs are etched much less than they used to be to reduce fluoride disposal not much has been done to reduce the amount of cryolite for aluminum production. 

Aluminum, though the third most abundant element, was quite expensive until about 1886, when a practical commercial electrolysis process was developed by a young American chemist, C. M. Hall. Bauxite, A1203-jcH20, is dissolved at about 1000°C in molten cryolite, Na3AlF6, and electrolyzed. 

Aluminum is produced by electrolysis of its oxide dissolved in molten cryolite (Na AIFJ. Find the mass of aluminum that can be produced in 1.00 day (d) in an electrolytic cell operating continuously ai 1.00 X 105 A. The cryolite does nor react. 

Hall process (Hall-Herault process) The production of aluminum by the electrolysis of aluminum oxide dissolved in molten cryolite, haloalkane An alkane with a halogen substituent. 

Fluorine comes from the minerals fluorspar, CaF, cryolite, Na3AlF6 and the fluorapatites, Ca,F(P04)3. The free element is prepared from HF and KF by electrolysis, but the HF and KF needed for the electrolysis are prepared in the laboratory. Chlorine primarily comes from the mineral rock salt, NaCl. The pure element is obtained by electrolysis of liquid NaCl. Bromine is found in seawater and brine wells as the Br ion it ts also found as a component of saline deposits the pure element is obtained by oxidation of Br (aq) by Cl,(g). Iodine is found in seawater, seaweed, and brine wells as the I" ion the pure element is obtained by oxidation of I (aq) by Cl,(g). 

Aluminum metal is produced from aluminum oxide by electrolysis using the Hall-Heroult process, whose story is detailed in our Chemical Milestones Box. The melting point of AI2 O3 is too high (2015 °C) and its electrical conductivity too low to make direct electrolysis commercially viable. Instead, AI2 O3 is mixed with cryolite (Na3 AlFfi) containing about 10% CaF2. This mixture has a melting point of 1000 °C, still a high temperature but not prohibitively so. Aluminum forms several complex ions with fluoride and oxide, so the molten mixture... 

Successful electrolysis of aluminum requires a liquid medium other than water that can conduct electricity. The key to the Hall-Heroult process is the use of molten cryolite, Na AlFg, as a solvent. Cryolite melts at an accessible temperature, it dissolves AI2 O3, and it is available in good purity. A second important feature is the choice of graphite to serve as the anode. Graphite provides an easy oxidation process, the oxidation of carbon to CO2. ... 

Electrolytic aluminum production is the most important process in both volume and significance. World production is about 15 megatons per year, consuming about 240 billion kilowatthours of electrical energy. Aluminum oxide (alumina), AI2O3, is subjected to electrolysis at a temperature of 950°C to this end it is dissolved in molten cryolite NujAlFg, with which it forms a eutectic melting at about 940°C. Carbon anodes that are anodically oxidized to CO2 in the process are employed. The overall electrolysis reaction can be written as... 

The first production of aluminum was by the chemical reduction of aluminum chloride with sodium. The electrolytic process, based on the fused salt electrolysis of alumina dissolved in cryolite, was independently developed in 1886 by C. M. Hall in America and P. L. Heroult in France. Soon afterwards a chemical process for producing pure alumina from bauxite, the commercial source of aluminum, was developed by Bayer and this led to the commercial production of aluminum by a combination of the Bayer and the Hall-Heroult processes. At present this is the main method which supplies all the world s needs in primary aluminum. However, a few other processes also have been developed for the production of the metal. On account of problems still waiting to be solved none of these alternative methods has seen commercial exploitation. 

Aluminum is produced commercially by the electrolysis of cryolite, Na3AlF6, but bauxite, A1203, is the usual naturally occurring source of the metal. The oxide is a widely used catalyst which has surface sites that function as a Lewis acid. A form of the oxide known as activated alumina has the ability to adsorb gases and effectively remove them. Other uses of the oxide include ceramics, catalysts, polishing compounds, abrasives, and electrical insulators. 

Problem 39 Aluminum metal is produced by the electrolysis of molten cryolite, NajAlFe. How many Faradays of electric charge are needed to produce 1 kilogram of aluminum ... 

Forms fluorides with most elements, except the inert gases. Occurs widely distributed to the extent 0.03% of the earth crust. The chief minerals are fluorapatite cryolite and fluorspar. Can be obtd by electrolysis of molten anhydrous hydrofluoric acid-potassium fluoride melts with special Co-bearing carbon anodes, steel cathodes containers and monel screens. Available in special steel cylinders, both as a liquid and as compressed gas... 

Charles Hall discovered that by mixing the mineral cryolite, Na,AlF6, with alumina he got a mixture that melted at a much more economical temperature, 950°C, instead of the 2050°C of pure alumina. The melt is electrolyzed in a cell that uses graphite (or carbonized petroleum) anodes and a carbonized steel-lined vat that serves as the cathode (Fig. 14.28). The electrolysis half-reactions are... 

ASf° = -68.78 J-mol 1-K 1 (b) Because AGf°(03, g) is positive at all temperatures, the reaction is not spontaneous at any temperature. It is less favored at high temperatures, (c) Because the reaction entropy is negative, the —TAS° term is always positive, so the entropy contribution to AGf° is always positive. The entropy does not favor the spontaneous formation of ozone. 15.29 (a) -219.27 kj-mol1 (b) 77.4°C 15.31 Fluorine comes from the minerals fluorspar (CaF2), cryolite (Na3AlF6), and the fluorapatites (Ca5F(P04)3). The free element is prepared from HF and KF by electrolysis, but the HF and the KF needed are prepared in the laboratory. 

There are no chemical reducing agents strong enough to reduce compounds of the most active metals, so these metals are produced by electrolytic reduction (Section 18.12). Lithium, sodium, and magnesium, for example, are obtained by the electrolysis of their molten chlorides. Aluminum is manufactured by the electrolysis of purified AI2O3 in molten cryolite (NaaAlFg). 

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