Effect of high-energy milling and thermal treatment on the solid-phase reactions in apatite–ammonium sulphate system

Abstract

The phosphorous fertilizers are a product of natural sedimentary phosphorite ores. Using this raw material to produce phosphoric acid and classic phosphorous fertilizers has generated well-known ecological problems. A new and perspective way to use the same materials is creating a new type of time-delayed fertilizers applying high-energy milling (HEM) activation method. The impact of the mechanical forces over the solids is mostly revealed through the changes of the quantities being related to the energetic stability and reactivity of the solid phase. The aim of this work is to report the results from the investigation on the chemical and thermal reactions in composites of natural apatite , which are HEM activated for different times and thermally treated, (from Tunisia) and ammonium sulphate. The Tunisian phosphorite belongs to the ‘basic’ apatites having a Ca/P ratio of 1.70–1.77 and is characterized by a complex mineral composition with major component carbonate-fluorapatite. The used ammonium sulphate—(NH4)2SO4 is obtained as a by-product from cleaning industrial waste gases, using e-beam technology. The composites of Tunisian phosphorite ores and ammonium sulphate, mixed in a mass ratio 1:1, were HEM activated during 10 min to 50 h with 20 mm Fe-milling bodies and temperature treated up to 1,100 °C. As a result, the chemical properties of the treated composites changed. Proofs were found for (i) formation of new phases during HEM activation such as NH4Ca(PO3)3 (NH4)2CaH4(P2O7)2, (NH4)2Ca3(P2O7)2.6H2O, CaH2P2O7 and α-Ca2P2O7; and (ii) decreasing of temperature intervals of phase changes in comparison to untreated composite.