Extraction and manufacture of group 14 elements

   Sources of natural graphite are supplemented by manufactured material formed by heating powdered coke (hightemperature carbonized coal) with silica at ≈ 2800 K. Approximately 30% of diamonds for industrial use in the US are synthetic (see Box 1.5). Diamond films may be grown using a chemical vapour deposition method (see Section 27.6), and hydrothermal processes are currently being investigated.† The manufacture of amorphous carbon (carbon black, used in synthetic rubbers) involves burning oil in a limited supply of air. Silicon (not of high purity) is extracted from silica, SiO₂,

by heating with C or CaC₂ in an electric furnace. Impure Ge can be obtained from flue dusts collected during the extraction of zinc from its ores, or by reducing GeO₂ with H₂ or C. For use in the electronic and semiconductor industries, ultrapure Si and Ge are required, and both can be obtained by zone-melting techniques (see Box 5.3 and Section 27.6). Tin is obtained from cassiterite (SnO₂ ) by reduction with C in a furnace (see Section 7.8), but a similar process cannot be applied to extract Pb from its sulfide ore since Δ_fG^o (CS₂,g) is ‏67 kJ mol^-1; thermodynamically viable processes involve reactions 1.1 or 1.2 at high temperatures. Both Sn and Pb are refined electrolytically.

                (1.1)

                        (1.2)