Cryolite, 2.27

Aluminium is not found free but its compounds are so widespread that it is the most abundant metal in the earth s crust. Aluminosilicates such as clay, kaolin (or china clay), mica and feldspar are well known and widely distributed. The oxide. AI2O3. occurs (anhydrous) as corundum and emery, and (hydrated) as bauxite. Cryolite. Na,AlF. (sodium hexafluoroaluminate). is found extensively in Greenland. 

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 pure oxide is dissolved in molten cryolite in an iron bath lined with graphite which acts as the cathode (see Figure 7.1). The anode... 

Fluorine occurs widely in nature as insoluble fluorides. Calcium fluoride occurs as jluospar or fluorite, for example in Derbyshire where it is coloured blue and called bluejohn . Other important minerals are cryolite NajAlFg (p. 141) and Jluorapatite CaFjSCaj (P04)2. Bones and teeth contain fluorides and some natural water contains traces. 

By far the largest use of hydrogen fluoride is in the manufacture of fluorocarbons which find a wide variety of uses including refrigerants, aerosol propellants and anaesthetics. Hydrogen fluoride is also used in the manufacture of synthetic cryolite, Na3AIFg, and the production of enriched uranium. 

The most common compound is sodium chloride, but it occurs in many other minerals, such as soda niter, cryolite, amphibole, zeolite, etc. 

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 Cryolite, a natural ore found in Greenland, is no longer widely used in commercial production, but has been replaced by an artificial mixture of sodium, aluminum, and calcium fluorides. 

Cryolite, see Sodium hexafluoroaluminate Cryptohalite, see Ammonium hexafluorosilicate Cupric and cuprous, see under Copper Cuprite, see Copper(I) oxide... 

Cryolite. Cryohte constitutes an important raw material for aluminum manufacturing. The natural mineral is accurately depicted as 3NaF AIF., but synthetic cryohte is often deficient in sodium fluoride. Physical properties are given in Table 4. 

Liquid cryolite is an equiUbtium mixture of the products of the dissociation ... 

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. 

Table 5. Minimum Melting Compositions Containing Cryolite...

Cryolite. Cryofite [15096-52-3] Na AlF, is the primary constituent of the HaH-Hfiroult cell electrolyte. High purity, natural cryofite is found in Greenland, but its rarity and cost have caused the aluminum industry to substitute synthetic cryofite. The latter is produced by the reaction of hydrofluoric acid [7664-39-3] HE, with sodium aluminate [1302-42-7] NaA102, from the Bayer process... 

No cryolite is actually needed once the smelting process is in operation because cryolite is produced in the reduction cells by neutralizing the Na20 brought into the cell as an impurity in the alumina using aluminum fluoride. 

Cellulose acetate Silica gel Scoured wool Sawdust Rayon waste Fluorspar Tapioca Breakfast food Asbestos fiber Cotton linters Rayon staple Starch Aluminum hydrate Kaolin Cryolite Lead arsenate Cornstarch Cellulose acetate Dye intermediates Calcium carbonate White lead Lithopone Titanium dioxide Magnesium carbonate Aluminum stearate Zinc stearate Lithopone Zinc yellow Calcium carbonate Magnesium carbonate Soap flakes Soda ash Cornstarch Synthetic rubber... 

Aluminium fluoride (anhydrous) [7784-18-4] M 84.0, m 250°. Technical material may contain up to 15% alumina, with minor impurities such as aluminium sulfate, cryolite, silica and iron oxide. Reagent grade AIF3 (hydrated) contains only traces of impurities but its water content is very variable (may be up to 40%). It can be dried by calcining at 600-800° in a stream of dry air (some hydrolysis occurs), followed by vacuum distn at low pressure in a graphite system, heated to approximately 925° (condenser at 900°) [Henry and Dreisbach J Am Chem Soc 81 5274 1959]. 

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. 

The fluorine industry is intimately related to aluminum production. Aluminum oxide, (AljOj) is electrolyzed to metallic aluminum with a flux of sodium fuoroaluminate (Na AlF,), called cryolite - a rare mineral found in commercial quantities only in Greenland with other uses glass, enamels, and as a filler for resin-bonded grinding wheels. 

Synthetic cryolite solved the supply problem, but synthetic cryolite requires fluorine which is actually more abundant in the Earth s crust than chlorine, but dispersed in small concentrations in rocks. Until the 1960s, fluorspar (CaFj) a mineral long known and used as a flux in various metallurgical operations was the source. A source is phosphate rock that contains fluorine i.s 3% quantity,... 

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... 

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