Phosphorus and Phosphates
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INTRODUCTION
Phosphates, compounds of the element phosphorous, are produced from relatively abundant supplies of phosphate rock. The major use of phosphate is to supply phosphorous, one of the three essential plant foods, nitrogen, phosphorus, and potassium. Phosphate rock extraction from its ore, and its subsequent conversion into fertilizer materials and industrial chemicals, is a relatively mature art. Single superphosphate, a mixture of monocalcium monohydrate and gypsum formed by the reaction of sulfuric acid with phosphate rock, has been used as a fertilizer since the mid-1800s. Phosphoric acid, derived by the treatment of phosphate rock with sulfuric acid so as to produce gypsum in a separable form, was manufactured in many locations by batch and countercurrent decantation methods in the 1920s. Phosphoric acid produced by the later process is called the "wet process acid" to distinguish it from "furnace phosphoric acid,"
which is produced by hydrating the phosphorous pentoxide formed by burning elemental phosphorous in air. The "wet" processes utilized for the bulk of today's phosphoric acid and fertilizer production trace their origins to the early 1930s. 1 Higher-purity industrial and food-grade phosphates, until recently, were most often derived from furnace processes. New plants recover purified phosphoric acid suitable for food-grade uses from relatively impure wet process acid, using solvent extraction technology. PHOSPHATE ROCK
*Jacobs Engineering.
Naturally occurring mineral products having sufficient phosphate content to be of commercial value are classified as phosphate rock. The grade or phosphate content of these products has been traditionally reported as percent C:1:3(P04) 2, which is referred to as bone phosphate of lime (BPL), tricalcium phosphate (TCP), or triphosphate of lime (TPL). Stoichiometric factors relating traditional rock analysis to other commonly used analytical
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Riegel Handbook of Industrial Chemistry, 1Oth Edition Edited by Kent. Kluwer Academic/Plenum Publishers, New York 2003
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PHOSPHORUS AND PHOSPHATES
_ . Direct Application Rock
grinding
_ . Defluorinated Rock (AFI)
calcination
_ . AI, Ca, Mg Phosphates
fusion
_ . White Phosphorous Phosphorous Compounds Thermal Acid & Derivatives
electric reduction
!Phosphate Rock
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t
I partial acidulation- H2S04
_.
Single Super Phosphate
partial acidulation- H 3P04
I _.
Triple Super Phosphate
full acidulation- H 2S04
~----------~~~~
_.
Wet Process Acid Merchant Acid & Super Acid Ammonium Phosphates NPK Fertilizers Pure Acid & Derivatives
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Fig. 10.1. Phosphate rock treatments and end products.
terms are 100.00 percent BPL = 45.77 percent P20 5 = 19.96 percent P. International trade of phosphate rock is based on dry metric tons, whereas U.S. domestic sales are in short tons, on a dry or as is moisture basis. Specifications for purchased rock may address grade, particle size, moisture content, and chemical impurities such as CaO, MgO, Si02 , Al2 0 3 , Fez03 , F, Cl, Na20, and KzO. The content of organic
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