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

Chapter 55.2 Applied processes and techniques5.2.1 OverviewTwo different processes using different raw materials can be used in the manufacture ofphosphoric acid:1. the thermal process uses elemental phosphorus as the raw material. Here, elementalphosphorus is produced from phosphate rock, coke and silica in an electrical resistancefurnace (see Section 5.4.15)2. wet processes, using phosphate minerals, which are decomposed with an acid.The share of wet phosphoric acid amounts to about 95 % of the total phosphoric acid in the EU.Part of the wet process acid is purified for the manufacture of technical and food gradephosphate salts, usually employing a solvent extraction process. In the EU, the production ofphosphoric acid using the solvent extraction purifying route is preferred over the thermal routebecause of the energy costs.5.2.2 Wet processThere are three possible subgroups of wet processes depending on which acid is used for theacidulation, i.e. HNO 3 , HCl or H 2 SO 4 . The wet digestion of phosphate rock with H 2 SO 4 is thepreferred process in terms of volume. For descriptions of specific wet processes using H 2 SO 4 ,see:• Section 5.4.1, “Dihydrate process”• Section 5.4.2, “Hemihydrate process”• Section 5.4.3, “Hemi-dihydrate recrystallisation process, single-stage filtration”• Section 5.4.4, “Hemi-dihydrate recrystallisation process, double-stage filtration”• Section 5.4.5, “Di-hemihydrate recrystallisation process, double-stage filtration”.For a description of the “Nitrophosphate route”, using HNO 3 , see Section 7.2.2.1.Figure 5.1 gives an overview of the production of H 3 PO 4 by wet processes. The tri-calciumphosphate from the phosphate rock reacts with concentrated H 2 SO 4 to produce H 3 PO 4 and theinsoluble salt calcium sulphate.Ca 3 (PO 4 ) 2 + 3 H 2 SO 4 2 H 3 PO 4 + 3 CaSO 4The insoluble calcium sulphate is filtered from the H 3 PO 4 . The reaction between phosphate rockand H 2 SO 4 is restricted by an insoluble layer of calcium sulphate which forms on the surfacearea of the rock. This restriction is minimised by contacting the phosphate rock withrecirculated H 3 PO 4 , thereby converting as much of it as possible to the soluble mono calciumphosphate, followed by precipitation as calcium sulphate with H 2 SO 4 .Ca 3 (PO 4 ) 2 + 4 H 3 PO 4 3 Ca(H 2 PO 4 ) 23 Ca(H 2 PO 4 ) 2 + 3 H 2 SO 4 3 CaSO 4 + 6 H 3 PO 4Calcium sulphate exists in a number of different crystal forms depending particularly on theprevailing conditions of temperature, P 2 O 5 concentration and free sulphate content.214 Large Volume Inorganic Chemicals – Ammonia, Acids and Fertilisers