Cryolite complexes with

Aluminum occurs widely in nature in silicates such as micas and feldspars, complexed with sodium and fluorine as cryolite, and in bauxite rock, which is composed of hydrous aluminum oxides, aluminum hydroxides, and impurities such as free silica (Cotton and Wilkinson 1988). Because of its reactivity, aluminum is not found as a free metal in nature (Bodek et al. 1988). Aluminum exhibits only one oxidation state (+3) in its compounds and its behavior in the environment is strongly influenced by its coordination chemistry. Aluminum partitions between solid and liquid phases by reacting and complexing with water molecules and anions such as chloride, fluoride, sulfate, nitrate, phosphate, and negatively charged functional groups on humic materials and clay. 

Concentration overvoltage is caused by slow diffusion of the electroactive species. In many cases ions which react at the electrode are not the most mobile ions which transport the current. For example, in the case of electrolysis of cryolite-alumina melts, the Na+ ions transport the current, but at the cathode the Al(in) ions are discharged. In the melt, aluminum form complexes with oxygen and fluoride ions and it is transferred by diffusion. 

Complexes with all the halide ions have been made, though the fluorides and chlorides are the most studied. Fluorides ALnp4, A2LnFs and AsLnFe exist (A = alkali metal) most have structures with 8- or 9-coordinate metals, though a few AsLnFe have the six-coordinate cryolite structure. 

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... 

A series of complex silico-arsenides has been obtained 6 by melting metals with silicon and an excess of arsenic under a layer of molten cryolite and sodium chloride. The following have thus been prepared copper silico-arsenide, a grey crystalline brittle mass zinc silico-arsenide, which behaved as above with hydrochloric acid iron, cobalt and nickel siMco-arsenides, of composition M2SisAs4, similar in appearance to the copper compound. When platinum was treated in the same way, a hard white product of indefinite composition was obtained, almost insoluble in nitric acid. 

Sterten [33] used activity data and calculated the concentration of complex ions in cryolitic melts saturated with alumina and the distribution of anions as a function of the molar cryolite ratio (NaF/AlF3), as shown in Figure 5. Julsrud [34] and Kvande [35] suggested the existence of some ions of Al2OF84 for electrolytes with cryolitic ratio = 3, while the A1202F42 ions were in majority in... 

Fluorine never occurs as a free element in nature. The most common fluorine minerals are fluorspar, fluorapatite, and cryolite. Apatite is a complex mineral containing primarily calcium, phosphorus, and oxygen, usually with fluorine. Cryolite is also known as Greenland spar. (The country of Greenland is the only commercial source of this mineral.) It consists primarily of sodium aluminum fluoride (Na3ALF6). The major sources of fluorspar are China, Mexico, Mongolia, and South Africa. In 2008 in the United States, fluorspar was produced as a by-product of limestone quarrying in Illinois. The United States imports most of the fluorspar it needs from China and Mexico. 

Assuming that Fe203 reacts with cryolite under the formation of fluoride and oxofluoride aluminum complexes according to the equation... 

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