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

Chapter 55.4.15 Thermal processDescriptionAbout the thermal process, see also [155, European Commission, 2006].The production of thermal phosphoric acid is carried out in two stages. First, elementalphosphorus is produced from phosphate rock. Then the elemental phosphorus is oxidised withair to P 2 O 5 , which is subsequently hydrated to produce phosphoric acid.Elemental phosphorus is obtained from phosphate rock. The example plant uses a mixture ofsedimentary rock and igneous rock. The process starts with the grinding of the phosphate rock.The ground phosphate rock is mixed with a slurry consisting of water, clay and various wastestreams containing phosphorus, to produce pellets in a granulator. The pellets are sintered in afurnace at a temperature of about 800 °C. Phosphorus is released from the sintered pellets byheating the pellets to about 1500 °C in an electric resistance furnace together with cokes (toprovide a reducing environment) and gravel (slag formation). The overall reaction can besummarised as follows:2 Ca 3 (PO 4 ) 2 + 6 SiO 2 + 10 C f P 4 + 10 CO + 6 CaSiO 3The process mainly produces gaseous phosphorus, carbon monoxide and a liquid slag. The gasphase is first passed through an electro filter to remove dust (Cottrell dust). Subsequentlygaseous phosphorus is recovered completely through condensation. The remaining gas phasemainly consists of carbon monoxide. This gas is used as fuel gas on-site (e.g. in the sinterfurnaces) and is sold to the nearby power plant. The leftover gas, if any, is flared. The liquidslag is tapped from the furnace in batches, and yields a phosphorus slag (the major part) and aferrophosphorus slag (a minor amount) by-product. After further processing, the former is usedas foundation material in large construction works, while the latter is used as a steel additive inthe iron and steel industry. The P-recovery efficiency of the phosphorus process is about 94 %.The remaining phosphorus will mainly end up in the furnace slag (as unreacted phosphate).Minor amounts end up in ferrophosphorus (as an alloy) and the Cottrell dust.To produce phosphoric acid from elemental phosphorus, first the phosphorus is brought into areaction vessel together with air, after which the phosphorus oxidises to P 2 O 5 . The heat evolvingfrom this reaction is used for the generation of high pressure steam. Subsequently, the P 2 O 5 iscontacted with diluted phosphoric acid, and reacts with the water present in the acid to formphosphoric acid. Two configurations for this process are used. In one case, the absorption ofP 2 O 5 by diluted phosphoric acid is carried out in the same reaction unit as where oxidation ofthe phosphorus takes place. In the other, preferential, case, the reaction of P 2 O 5 to phosphoricacid is carried out in a separate absorption tower, allowing energy recovery as high pressuresteam. The production of phosphoric acid from elemental phosphorus is represented by thefollowing reactions:P 4 + 5 O 2 f 2 P 2 O 5P 2 O 5 + 3 H 2 O f 2 H 3 PO 4Achieved environmental benefits• production of purer acid.Large Volume Inorganic Chemicals – Ammonia, Acids and Fertilisers 251