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

Chapter 55.4.9 Phosphate rock selection (1)DescriptionIdeally, the best possible phosphate source rock would consist entirely of tricalcium phosphateCa 3 (PO 4 ) 2 . Impurities in phosphate rock are natural but generally undesirable for an assortmentof reasons – economic, technical and environmental.Rock phosphate can be of igneous/volcanic origin or can be sedimentary ores. Whereas igneousrocks (South Africa, Russia) generally show higher P 2 O 5 contents, these rocks are generally notavailable. Sedimentary ores (US, Morocco, Algeria) possess higher amounts of calciumcompounds, thus lowering the P 2 O 5 content and increasing the CaO/P 2 O 5 ratio. In many cases,sedimentary ores are concentrated and beneficiated at the mine in order to increase thephosphate content and to remove impurities like insoluble sand and stones. For beneficiation byfloating, usually organic additives are used which remain partially in the rock phosphate. Thenumber of countries supplying rock phosphate is limited. Some countries no longer export rockphosphates (US) or only in limited quantities (Russia). This affects the rock phosphate costs.For selection of the rock phosphate source, not only the availability but also aspects regardinglogistics, design of the processing plant, type and amounts of other components (Ca, Fe, Al, C,SiO 2 , etc.), and various local factors are taken into account. These are mainly:• P 2 O 5 content determining logistic cost• CaO/P 2 O 5 ratio determining amount of acid and by-products (gypsum, calcium carbonate)• physical quality of the rock (dust formation through handling)• presence of organic carbon interfering with the dissolution, interfering with the process,producing, e.g. excessive amounts of NO x and odour nuisance• presence of further components like F, Fe, Al, interfering with the process, producing, e.g.excessive amounts of NO x or fluoride emissions or producing thixotropic slurries (due to Feand Al compounds) that cannot be easily handled; scaling of equipment, in particular of heatexchangers and off-gas lines, can be promoted by such components; filterability maybecome impaired; for certain grades the presence of limited amounts of such componentscan be advantageous• presence of other components like Si, As, Cd, that may be undesirable for various reasons.Also, certain trace elements serve as micronutrients and their presence is desired• the ability to process any rock phosphate (or combinations) cannot be assessed from theanalyses only: tests on plant scale are required and in all cases, the long time experience isnecessary in order to combine and optimise process design, process operating parametersand rock quality.Achieved environmental benefitsProper choice of rock phosphate for a given design of plant will:• minimise the acid required• optimise the type and amounts of co-products• minimise emissions into the environment.Cross-media effectsNone believed to be likely.Large Volume Inorganic Chemicals – Ammonia, Acids and Fertilisers 241