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

Chapter 77.2.2 Phosphate rock digestionPhosphate rock digestion is carried out with nitric acid (nitrophosphate route) or with acombination of nitric, sulphuric and/or phosphoric acids (mixed acid route).For digestion of phosphate rock, see• production of phosphoric acid, Section 5.2.2 “Wet process”• Section 7.2.2.1 “Nitrophosphate route”• production of SSP/TSP, Section 10.2 ”Applied processes and techniques”.Because of the required starting materials and the creation of co-products, the nitrophosphateroute (also called the “ODDA” process) is usually realised integrated with ammonia, HNO 3 andCAN production. Additionally, liquid ammonia can be economically used to cool thenitrophoshporic acid section (crystallisation of CNTH).7.2.2.1 Nitrophosphate routeFor an overview of the nitrophosphate route, see Figure 7.2. The nitrophosphate process forproducing NPK grades is characterised by the use of nitric acid for rock phosphate digestion andthe subsequent cooling of the digestion solution in order to separate out most of the calciumions from the solution as calcium nitrate crystals. The main difference to processes based onsulphuric acid is that it avoids the formation of gypsum. After filtration of the calcium nitratecrystals, the resultant NP solution is used for neutralisation and particulation as in other NPKprocesses. The final cooling temperature affects the range of NPK formulations possible withthis process. The by-product calcium nitrate is converted into ammonium nitrate and calciumcarbonate for CAN production or, after purification, marketed as CN fertiliser. Because of therequired starting materials, the nitrophosphate route is usually realised integrated with ammonia,nitric acid and CAN production.Phosphate rockT = 60 – 70 °C60 % HNO 3 ReactionH 3PO 4Filtration(insolubles)Insoluble matterT = 0 –20 °CCooling andH 2O and HNO 3Ca(NO 3 ) 2crystallisationFiltrationCa(NO 3) 2 . 4H 2OslurryReturn acidto NPK fertiliserproductionFigure 7.2: Overview of the nitrophosphoric acid processBased on [29, RIZA, 2000].286 Large Volume Inorganic Chemicals – Ammonia, Acids and Fertilisers