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Phosphorite, phosphate rock or rock phosphate is a non-detrital sedimentary rock which contains high amounts of phosphate bearing minerals. The phosphate content of phosphorite is at least 20% which is a large enrichment over the typical sedimentary rock content of less than 0.2%.[1] The phosphate is present as fluorapatite typically in cryptocrystalline masses (grain sizes < 1 μm) referred to as collophane.[1] The dark brown to black beds range from a few centimetres to several metres in thickness. The layers contain the same textures and structures as fine grained limestones and may represent diagenetic replacements of carbonate minerals by phosphates.[1]

Phosphorites are known from Proterozoic banded iron formations in Australia, but are more common from Paleozoic and Cenozoic sediments. The Permian Phosphoria Formation of the western United States represents some 15 million years of sedimentation reaches a thickness of 420 metres and covers an area of 350 000 km2.[1] Commercially mined phosporites occur in France, Belgium, Spain, Morocco, Tunisia and Algeria. In the United States phosphorites have been mined in Florida, Tennessee, Wyoming and Idaho.[2]

Production and use

Deposits which contain phosphate in quantity and concentration which are economic to mine as ore for their phosphate content are not particularly common. The two main sources for phosphate are guano, formed from bird droppings, and rocks containing concentrations of the calcium phosphate mineral, apatite.

Phosphate rock is mined, beneficiated, and either solubilized to produce wet-process phosphoric acid, or smelted to produce elemental phosphorus. Phosphoric acid is reacted with phosphate rock to produce the fertilizer triple superphosphate or with anhydrous ammonia to produce the ammonium phosphate fertilizers. Elemental phosphorus is the base for furnace-grade phosphoric acid, phosphorus pentasulfide, phosphorus pentoxide, and phosphorus trichloride. Approximately 90% of phosphate rock production is used for fertilizer and animal feed supplements and the balance for industrial chemicals.

For general use in the fertilizer industry, phosphate rock or its concentrates preferably have levels of 30% phosphorus pentoxide (P2O5), reasonable amounts of calcium carbonate (5%), and <4% combined iron and aluminium oxides. Worldwide, the resources of high-grade ore are declining, and the beneficiation of lower grade ores by washing, flotation and calcining is becoming more widespread.

In addition to phosphate fertilisers for agriculture, phosphorus from rock phosphate is also used in animal feed supplements, food preservatives, anti-corrosion agents, cosmetics, fungicides, ceramics, water treatment and metallurgy.[3 ]

In the world, the United States is the leading producer and exporter of phosphate fertilizers, accounting for about 37% of world P2O5 exports.[4] The world’s total economic demonstrated resource of rock phosphate is 18 Gt, which occurs principally as sedimentary marine phosphorites.[3 ]

China, the United States and Morocco are the world's largest miners of phosphate rock, each producing about a quarter of the total world production. Other countries with significant production include Brazil, Russia, Jordan and Tunisia. Historically, large amounts of phosphates were obtained from guano deposits on small islands such as Christmas Island and Nauru, but these sources are now largely depleted.

References

  1. ^ a b c d Blatt, Harvey and Robert J. Tracy, Petrology, Freeman, 1996, 2nd ed. pp. 345-349 ISBN 0-7167-2438-3
  2. ^ Klein, Cornelis and Cornelius S. Hurlbut, Jr., Manual of Mineralogy, Wiley, 1985, 20th ed., p. 360, ISBN 0-471-80580-7
  3. ^ a b Phosphate AIMR Report 2007
  4. ^ US Geological Survey Minerals Yearbook 2006 Rock Phosphate
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