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

Chapter 55.4.7 Fluoride recovery and abatementDescriptionFluoride is present in most phosphate rocks at a level of 2 – 4 %, that is 20 – 40 kg/tonne rockor approximately 60 – 120 kg/tonne P 2 O 5 . Fluoride is distributed between the product acid,gypsum, vapours generated in the reactor and the acid concentration section during processing.Initially, fluoride is released in the reaction as hydrogen fluoride (HF), but in the presence ofsilica it reacts readily to form fluosilicic acid (H 2 SiF 6 ) and compounds such as MgSiF 6 andH 3 AlF 6 . The fluosilicic acid may decompose with heating to give volatile silicon tetrafluorideand hydrogen fluoride.In a dihydrate (DH) process the temperature is usually too low to cause the decomposition offluosilicic acid. The majority of fluorine compounds will be evolved with the evaporatorvapours during concentration of the weak phosphoric acid. In the hemihydrate (HH) process,most of the fluoride is released during the reaction. Fluoride leaves the reactor with the vacuumcooler condenser water or with the cooling air, depending on the cooling system used (flashcooler or air circulating cooler). To prevent fluoride emissions with condenser water, theapplication of an indirect condensation system may be considered instead of a direct contactcondenser, thus avoiding that condenser water becoming contaminated with fluoride. A typicalfluoride distribution in the dihydrate process and hemihydrate process is shown in Table 5.9.DH Process (%) HH Process (%)Acid 15 12Gypsum 43 50Reactor off-gas 5 8Flash cooler vapour 3 30Concentrator vapour 35 - xx assuming that no concentration is carried outTable 5.9: Typical fluoride distribution in DH and HH processes[31, EFMA, 2000]Fluoride can be removed by a number of different scrubbing systems (see also Section 6.4.6).Vapours from vacuum flash coolers and vacuum evaporators are usually first led through aseparator for the removal of phosphoric acid droplets that are entrained with the vapours tominimise fluosilicic acid contamination with P 2 O 5 , or something just to minimise contaminationof the scrubber effluent.If the fluorine is not recovered, the scrubber effluent will be discharged. The effluent can beneutralised with lime or limestone to precipitate fluorine as solid calcium fluoride beforedischarge (see also Section 6.4.9).Many companies recover the fluorine as fluosilicic acid (H 2 SiF 6 ), which can be used for theproduction of aluminium fluoride, and other fluorine compounds such as sodium and/orpotassium fluosilicates. In this case, a dilute solution of fluosilicic acid is used as the scrubbingliquid. The reaction to fluosilicic acid results in the formation of free silica. By carefullycontrolling the fluosilic acid strength, silica deposition is controlled. The silica is then removedby filtration. Usually, a product containing 20 – 25 % fluosilicic acid is recovered in the fluoriderecovery system.238 Large Volume Inorganic Chemicals – Ammonia, Acids and Fertilisers