Recycling Of P₂O₅ From White Phosphorus Fertilizer And Kinetic Model For Dissolution With Dilute Sulfuric Acid

White phosphorus fertilizer is produced in the first neutralization stage of two-section (or three-section) neutralization process for production of feed grade calcium hydrogen phosphate with wet-process phosphoric acid (WPA). White phosphorus fertilizer contains 20～35% P2O5, which is approximately equivalent to the content of P2O5 in phosphate ores, and a lot of impurities such as Fe, Al, F and heavy metals carried from WPA are enriched in white phosphorus fertilizer. Though the phosphorus in white phosphorus fertilizer occupies 15-30% of the total phosphorus from the WPA feed, its application is extremely limited by its high content of impurities. In a way, the existence of white phosphorus fertilizer has restricted the development of feed grade calcium hydrogen phosphate industry. Therefore it is of practical significance to find more effective processes for application of white phosphorus fertilizer.In this paper, with white phosphorus fertilizer from a feed-grade phosphate plant as raw material, experimental study has been conducted to investigate suitable methods to make proper use of the white phosphorus fertilizer as a phosphorus source. With chemical composition and microstructure considered, a technological process for manufacturing first grade mono-ammonium phosphate fertilizer using white phosphorus fertilizer has been developed. In this process, phosphoric acid is leached from white phosphorus fertilizer with dilute sulfuric acid, and the leached-out phosphoric acid is then mixed with WPA in proper proportion according to the relative content of impurities. After defluorization by precipitation, the leached phosphoric acid (in mixture with WPA) is neutralized with ammonia, monoam-monium phosphate thus produced can be used as a qualified first grade monoammonium phosphate fertilizer.The process for leaching P2O5 from white phosphorus fertilizer with dilute acid has been investigated. The key factors governing the leaching of white fertilizer such as temperature, leaching time, mole ratio of H2SO4/P2O5 and initial particle size of white phosphate fertilizer, have been experimentally studied. To get a leaching liquor of P2O5 with less impurities, leaching of white fertilizer should be conducted with less sulfuric acid, i.e. H2SO4/P2O5 (mol) = 0.8～1.0. With leaching conditions being properly selected, P2O5 can be recycled with a yield of over 70%, while only 40% of Fe, Al, and about 35% of the F are leached. Selectivity of leaching and the leaching yield of phosphorus have to be compromised by proper choice of appropriate leaching conditions.Finally, based on study of the leaching process, kinetics for leaching of P2O5 from white fertilizer at high yield of P2O5 were experimentally inverstigated and mathematical models have been proposed for proper design of industrial eaching processs.