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

Chapter 1010.2 Applied processes and techniques10.2.1 OverviewFor an overview, see Figure 10.1.SSP and TSP are manufactured as follows: phosphate rock is ground very finely and mixed withacid (SSP: 65 – 75 % H 2 SO 4 ; TSP: a H 3 PO 4 with a P 2 O 5 content of 50 – 55 %), giving thefollowing overall reactions:(SSP)(TSP)Ca 10 F 2 (PO 4 ) 6 + 7 H 2 SO 4 + H 2 O f 3 Ca(H 2 PO 4 ) 2·H 2 O + 7 CaSO 4 + 2 HFCa 10 F 2 (PO 4 ) 6 + 14 H 3 PO 4 + 10 H 2 O f 10 Ca(H 2 PO 4 ) 2·H 2 O + 2 HFIn the SSP process, H 3 PO 4 is formed only as an intermediate. The reactions are rapid (more than96 % yield) but can continue for several days as the remaining free acid continues reacting withthe excess phosphate rock. The SSP process retains the calcium sulphate (CaSO 4 ) in the productunlike the similar reaction producing phosphoric acid.Ground phosphate rock and acid are mixed in a reaction vessel. The reaction begins in thereactor, and is exothermic, reaching temperatures of 90 – 100 ºC. The slurry is fed on to a slowmoving conveyor belt (the so-called “den”) or to a holding vessel and held for a period from 10to up to 40 minutes. The superphosphate is crushed and afterwards fed to a granulation line(direct granulation) or to storage (pile) for “curing” from one to up to six weeks. Thesuperphosphate can be fed to the granulation line (indirect granulation) or sold without furtherprocessing from the storage facility.As well as SSP or TSP, partially acidulated phosphate rock (PAPR) can also be produced.PAPR is a mixture of superphosphate and phosphate rock. This product can be made in an SSPor TSP plant using a lower proportion of H 2 SO 4 or H 3 PO 4 to phosphate rock.From the storage pile, the cured superphosphate is ground and granulated. Steam, water or acidmay be added to help granulation. Direct granulation has some advantages in comparison togranulation after storage (pile) for curing; production costs are usually lower and the granulesare denser and stronger [52, infoMil, 2001]. Some disadvantages of direct granulation are theneed to use reactive phosphate rock and the possibility of greater losses of soluble P 2 O 5 due tothe incomplete reactions.Phosphate rock can contain small amounts of organic compounds. During the reaction with theacid, these organic compounds are released. Compounds, such as mercaptanes can cause odournuisance. For phosphate rock selection, see also Section 5.4.9.10.2.2 Raw materialsAn important factor in the production of SSP and TSP is the amount of aluminium (as Al 2 O 3 ),iron (as Fe 2 O 3 ) and magnesium compounds in the phosphate rock; the presence of theseelements reduces the water solubility of the phosphate. Two sources of H 2 SO 4 used insuperphosphate production are virgin acid and spent acid of sufficient quality. Virgin acid isproduced from SO 2 derived from burning elemental sulphur (sulphur-H 2 SO 4 ), pyrite (pyriteacid) or as a by-product acid from non-ferrous metal smelters processing sulphide ores (fatalacid). Spent acid is recycled from various industries (e.g. non-ferrous metal smelters) that uselarge quantities of H 2 SO 4 . For more information on phosphate rock and sulphuric acid, seeSections 5.2.2.1.1 and 5.2.2.1.2.388 Large Volume Inorganic Chemicals – Ammonia, Acids and Fertilisers