Mechanochemical transformation of apatite to phosphoric slow-release fertilizer and soluble phosphate

This paper introduces a mechanochemical process for the clean production of phosphoric slow-release fertilizer and soluble phosphate powders directly from apatite (Ca-5(PO4)(3)(OH, F)) by just milling with sulfuric acid (H2SO4, 98 wt%) and ammonium sulfate ((NH4)(2)SO4) at room temperature. It was found that, at certain molar ratios of the additives, water soluble ammonium hydro-phosphate was simply produced from apatite and the ratio of the watersoluble one to the insoluble part (soluble in 2% citrate acid taken as slow release fertilizer) could be regulated by changing the added molar ratio of sulfuric acid. The less the (NH4)(2)SO4H2SO4 addition was, the higher the proportion of obtained phosphoric slow-release fertilizer to the soluble phosphates would be. 25% addition of the required stoichiometric ratio of (NH4)(2)SO4-H2SO4 for a complete transformation, for example, gave the product with 50% slow release composition as well as 30% soluble phosphate. The obtained fine solid powders after milling operation may serve directly as fertilizers. Besides its simplicity and easy regulation in product compositions, one significant advantage of the proposed mechanochemical process is the no emission of gaseous and aqueous wastes from the current production line, offering a truly environment-friendly approach. (C) 2017 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.