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

Chapter 55.2.2.2 GrindingDepending on the phosphate rock properties and the process applied, grinding of the phosphaterock can be necessary. This is usually carried out using ball or rod mills. Both types of mills canoperate with wet or dry rock. Some grades of commercial rock do not need grinding, theirparticle size distribution already being acceptable for the dihydrate reaction section. Most otherphosphate rocks require particle size reduction. The energy requirements for grinding dependson the type of phosphate rock and is about 15 – 18 kWh per tonne phosphate rock [9, AustrianUBA, 2002].5.2.2.3 RecrystallisationRecrystallisation is applied in order to improve the overall P 2 O 5 yield (P 2 O 5 efficiency). Fordescriptions of different recrystallisation processes, see Sections 5.4.3, 5.4.4 and 5.4.5.5.2.2.4 EvaporationCurrently, almost all evaporators are of the forced circulation design (see Figure 5.2). Theforced circulation evaporator consists of a heat exchanger, vapour or flash chamber, condenser,vacuum pump, acid circulating pump and associated pipework. A fluosilicic acid scrubber isusually included with the system.All the evaporators in this service are generally of the single-effect design because of thecorrosive nature of phosphoric acid and the very high boiling point of the process. The heatexchangers are fabricated from graphite or stainless steel with the rest of the equipment madefrom rubber-lined steel. All equipment designs need to be made using the best availableengineering practices. A multiple effect evaporation system may be used depending on theconcentration needed.Vacuum system, tailgas treatmentCondenserCoolingwaterSteamHeatexchangerVapoursEvaporator bodyH 2OScrubber(Alternative)BarometricsealH 2O to recycle ordisposal orrecovery of H 2SiF 6Coolingsystem(alternatively)H 2O and condensateto recycle or disposalProduct acidPumpWeak acidFigure 5.2: Forced circulation evaporator for the concentration of H 3 PO 4This figure is based on [15, Ullmanns, 2001, 31, EFMA, 2000]220 Large Volume Inorganic Chemicals – Ammonia, Acids and Fertilisers