Large Volume Inorganic Chemicals - Ammonia ... - ammk-rks.net
Large Volume Inorganic Chemicals - Ammonia ... - ammk-rks.net 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
Chapter 55.2.2.5 Co-product phosphogypsumGypsum (calcium sulphate) is an unavoidable co-product in wet phosphoric acid production(“phosphogypsum”); for every tonne of phosphoric acid (P 2 O 5 ) around 4 – 5 tonnes of gypsumis produced. Phosphate rock contains a range of impurities which are distributed between theproduct acid and the calcium sulphate. Because of the volume of the gypsum production and thetype and level of impurities in the gypsum, this co-product constitutes an environmentalchallenge.5.2.2.6 Co-product fluosilicic acidMost phosphate rocks contain fluoride between 2 – 4 % w/w. This fluoride is released duringacidulation as hydrogen fluoride, but reacts readily with excess silica forming fluosilicic acid(H 2 SiF 6 ). Magnesium and aluminium compounds also react with HF forming MgSiF 6 andH 3 AlF 6 . A proportion of the fluoride is released in the vapour, the amount depending on thereaction conditions, with the rest remaining in the acid solution. Some of this residual amountmay combine with other impurities at a sufficient rate for removal by filtration. The remainingamount will appear as sludge in the product acid. Volatile fluorine compounds can also bepresent in the release from the evaporator system.Large Volume Inorganic Chemicals – Ammonia, Acids and Fertilisers 221
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Chapter 55.2.2.5 Co-product phosphogypsumGypsum (calcium sulphate) is an unavoidable co-product in wet phosphoric acid production(“phosphogypsum”); for every tonne of phosphoric acid (P 2 O 5 ) around 4 – 5 tonnes of gypsumis produced. Phosphate rock contains a range of impurities which are distributed between theproduct acid and the calcium sulphate. Because of the volume of the gypsum production and thetype and level of impurities in the gypsum, this co-product constitutes an environmentalchallenge.5.2.2.6 Co-product fluosilicic acidMost phosphate rocks contain fluoride between 2 – 4 % w/w. This fluoride is released duringacidulation as hydrogen fluoride, but reacts readily with excess silica forming fluosilicic acid(H 2 SiF 6 ). Magnesium and aluminium compounds also react with HF forming MgSiF 6 andH 3 AlF 6 . A proportion of the fluoride is released in the vapour, the amount depending on thereaction conditions, with the rest remaining in the acid solution. Some of this residual amountmay combine with other impurities at a sufficient rate for removal by filtration. The remainingamount will appear as sludge in the product acid. Volatile fluorine compounds can also bepresent in the release from the evaporator system.<strong>Large</strong> <strong>Volume</strong> <strong>Inorganic</strong> <strong>Chemicals</strong> – <strong>Ammonia</strong>, Acids and Fertilisers 221