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

Chapter 55.4.5 Di-hemihydrate recrystallisation process, double-stage filtrationDescriptionFigure 5.7 gives an overview of the di-hemihydrate recrystallisation process with double-stagefiltration (DH/HH or DHH). In this process, although the reaction runs under dihydrateconditions, it is not necessary to obtain a high degree of P 2 O 5 recovery during the separation ofthe acid from the dihydrate. The succeeding dehydration stage yields around 20 – 30 % P 2 O 5and 10 – 20 % sulphuric acid. The strength of the product acid is 32 – 38 % P 2 O 5 .Phosphate rockDihydrateconditionsT = 80 °CReaction andcrystallisationReturn acidH 3PO 4H 2SO 4Filtration32 – 38 % P 2O 5Steam H 2SO 4RecrystallisationHemihydrate conditionsT = 100 °CH 2OFiltrationGypsumCaSO 4 . ½H 2OFigure 5.7: Di-hemihydrate recrystallisation process with double-stage filtration[29, RIZA, 2000, 154, TWG on LVIC-AAF]Achieved environmental benefits• produces a relatively pure hemihydrate• gypsum may be used directly for plasterboard, plaster or as a cement retarder, after theaddition of lime and natural rehydration in a storage pile• high P 2 O 5 efficiency: 98 %.Cross-media effects• although higher strength acid is produced, acid evaporation is required• requires steam for the conversion of dihydrate to hemihydrate• normally requires rock grinding.Operational dataSee description.234 Large Volume Inorganic Chemicals – Ammonia, Acids and Fertilisers