Large Volume Inorganic Chemicals - Ammonia ... - ammk-rks.net

Chapter 66.2 Applied processes and techniques6.2.1 OverviewHydrogen fluoride and hydrofluoric acid are produced by the conversion of dried fluorsparusing concentrated sulphuric acid at elevated temperatures in accordance with the followingreaction:CaF 2 + H 2 SO 4 f 2HF + CaSO 4 jH R = 59 kJ/mol (1)There are secondary reactions due to impurities contained in the fluorspar. These occuralongside the main reaction, forming, e.g. silicon tetrafluoride, sulphur dioxide, hydrogensulphide, carbon dioxide, water and elementary sulphur in accordance with the followingreaction equations:SiO 2 + 2CaF 2 + 2H 2 SO 4 f SiF 4 + 2CaSO 4 + 2H 2 O (2)CaCO 3 + H 2 SO 4 f CaSO 4 + CO 2 + H 2 O (3)R 2 O 3 + 3H 2 SO 4 f R 2 (SO 4 ) 3 + 3H 2 O (R = Fe, Al) (4)CH 3 (CH 2 ) n COOH + (2n + 3)H 2 SO 4 f (n + 2)CO 2 + (3n + 5)H 2 O + (2n + 3)SO 2 (5)Fe (Reactor) + 2H 2 SO 4 f FeSO 4 + SO 2 + 2H 2 O (6)MS + H 2 SO 4 f MSO 4 + H 2 S (M = Metals) (7)2H 2 S + SO 2 f 3S + 2H 2 O (8)H 2 S formed by reaction (7) is converted to sulphur according to reaction (8). Water formed bythese reactions is removed from the reaction gases by absorption in sulphuric acid. Oleum isadded, to provide the SO 3 reacting according to equation (9) to maintain the concentration of thesulphuric acid being fed to the reactor, at a constant level.H 2 O + SO 3 f H 2 SO 4 (9)As an alternative, HF can be produced starting from H 2 SiF 6 (see Section 6.4.10).6.2.2 Fluorspar[22, CEFIC, 2000]The raw material for the production of HF is fluorspar, which is mined with a mass content ofbetween 30 and 60 % CaF 2 in the crude ore. The crude ore is ground and the CaF 2 is separatedfrom the other minerals by physical flotation at the mining site. The flotation chemicals incurrent use are mainly saturated and unsaturated vegetable fatty acids. They remain in thefluorspar, which should ideally contain at least 97 % CaF 2 for use as "acid spar".Table 6.2 shows the components in acid spar. Acid spar should not contain any fraction above0.2 mm in diameter. It is transported to the user as a bulk material with a water content ofapproximately 10 % to minimise dust generation.The fluorspar needs to be dried before it is fed to the hydrogen fluoride plant. This is usuallycarried out in directly heated flue-gas driers at approximately 120 ºC. There are also plantswithout spar drying equipment, but which purchase the spar already dried elsewhere.258 Large Volume Inorganic Chemicals – Ammonia, Acids and Fertilisers