Extraction of the group 17 elements

   Most fluorine-containing compounds are made using HF, the latter being prepared from fluorite by reaction 1.1; in 2001, ≈ 80% of CaF₂ consumed in the US was converted into HF. Hydrogen fluoride is also recycled from Al manufacturing processes and from petroleum alkylation processes, and re-enters the supply chain. Difluorine is strongly oxidizing and must be prepared industrially by electrolytic oxidation of F^- ion. The electrolyte is a mixture of anhydrous molten KF and HF, and the electrolysis cell contains a steel or copper cathode, ungraphitized carbon anode, and a Monel metal (Cu/Ni) diaphragm which is perforated below the surface of the electrolyte, but not above it, thus preventing the H₂ and F₂ products from recombining. As electrolysis proceeds, the HF content of the melt is renewed by adding dry gas from cylinders.

                     (1.1)

   We have already described the Downs process for extracting Na from NaCl and this is also the method of manufacturing Cl₂  one of the most important industrial chemicals in the US. The manufacture of Br₂ involves oxidation of Br^- by Cl₂, with air being swept through the system to remove Br₂. Similarly, I^- in brines is oxidized to I₂. The extraction of I₂ from NaIO₃ involves controlled reduction by SO₂; complete reduction yields NaI.

Fig. 1.1 (a) Industrial uses of Cl₂ in Western Europe in 1994 [data: Chemistry & Industry (1995) p. 832]. (b) The trends in uses of bleaching agents in the pulp industry between 1990 and 2001; ClO₂ has replaced Cl₂. Both elemental chlorine-free and totally chlorine-free agents comply with environmental legislations [data: Alliance for Environmental Technology, 2001 International Survey].

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