Occurrence and extraction

Key point: The Group 1 elements can be extracted by electrolysis. The name lithium comes from the Greek lithos for stone. The natural abundance of lithium is low, the most abundant minerals being spodumene, LiAlSi₂O₆, from which lithium is most commonly extracted, and lepidolite, which has the approximate formula K₂Li₃Al₄Si₇O₂₁ (F,OH)₃. Spodumene is first converted to LiCl and then electrolysed to produce lithium metal. Sodium occurs as the mineral rock salt (NaCl) and in salt lakes and seawater. Sodium chloride makes up 2.6 per cent by mass of the biosphere with the oceans containing 4X10¹⁹ kg of the salt. Sodium chloride occurs naturally as the mineral rock salt, the deposits of ancient dried saline lakes. Many of these deposits are underground and are mined in the conventional way. Alternatively, water may be pumped underground to dissolve the rock salt, which is then pumped out as saturated brine solution. The metal is extracted by Down’s process, the electrolysis of molten sodium chloride: 2NaCl(l)→2Na(l)+Cl₂(g) The sodium chloride is kept molten at 600°C, a temperature considerably below its melting point of 808°C, by the addition of calcium chloride. A high potential difference, typically between 4 and 8 V, is applied between a carbon anode and an iron cathode immersed in the molten salt. The electrolysis liberates liquid sodium metal at the cathode, which rises to the surface of the cell where it is collected under an inert atmosphere. This process is also used for the industrial production of chlorine, which is generated at the anode. Potassium occurs naturally as potash (K₂CO₃) and carnallite (KClMgCl₂ .6H₂O). Natural potassium contains 0.012 per cent of the radioactive isotope 40K, which undergoes decay, with a half-life of 1.25 Ga, to 40Ca and electron capture to 40Ar. The ratio of 40K and 40Ar can be used for dating rocks, specifically the time when the rock solidified, at which point it traps any 40Ar formed. In principle, potassium could be extracted electrolytically but the high reactivity of the element makes this far too hazardous. In stead, molten sodium and molten potassium chloride are heated together and potassium and sodium chloride are formed.

At the temperature of operation, potassium is a vapour and removing it from the system drives the equilibrium to the right. Rubidium (from the Latin rubidus for deep red) and caesium (caesius for sky blue) were discovered by Robert Bunsen in 1861 and named from the colour their salts impart to a flame. Both elements occur as minor constituents of the mineral lepidolite, from which they are obtained as byproducts of the extraction of lithium. Prolonged treatment of lepidolite with sulfuric acid forms the alums of the alkali metals, M₂SO₄.Al₂ (SO₄)₃. nH₂O. The alums are separated by multiple fractional crystallizations and then converted to the hydroxide by reaction with Ba (OH)₂ and then to the chloride by ion exchange. The metals are obtained from the molten chloride by reduction with calcium or barium.

Caesium also occurs as the mineral pollucite, Cs₄Al₄Si₉O₂₆.H₂O. The element is extracted from the mineral by leaching with sulfuric acid to form the alum Cs₂SO₄Al₂ (SO₄)₃.24H₂O, which is then converted to the sulfate by roasting with carbon. The chloride is formed by ion exchange and is then reduced with calcium or barium as described above. Caesium metal can also be obtained by the electrolysis of molten CsCN.

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مواضيع ذات صلة

Partially crystalline carbon

Fullerene metal complexes

Carbon clusters

Diamond and graphite

Simple compounds

Thermodynamic considerations

The atom and its ions

Oxidation states

Occurrence

Enthalpies of atomization

Electronegativity

Ionization energies and electron affinities