Induced structural changes within a natural apatite-calcite system via short-time mechanochemical activation

This work studies the transformations occurring in Tunisian phosphorite ore (TPh) under mechanochemical activation (MCA) by ball milling for 30 min with plain steel grinding bodies of different sizes - 3, 5, 10, and 20 mm, using a planetary ball mill. The raw ore is composed of carbonate-fluorapatite B-type and accompanying minerals – calcite, dolomite, and quartz (in minimal amounts). By varying the size of the grinding bodies, a mechanochemical effect similar to that of varying the activation duration is sought, but with reduced energy consumption due to a shortened activation period of up to 30 min. The mechanochemical activation leads to a decrease in the size of the particles, an accumulation of mechanical energy, and a decrease in the stability of the source minerals, which causes the occurrence of structural changes (induced defects) and solid-phase synthesis. The following induced changes were examined: revealing of a fresh reactive surface, structural changes in apatite and calcite, and solid-phase synthesis upon apatite-calcite interaction. All of them were registered upon activation with the four types of grinding bodies. The results of the studies demonstrate that the grinding bodies with the smallest sizes of 3 mm and 5 mm were the most effective when the aim is to obtain a fresh reactive surface and increase the chemical reactivity. The effect is illustrated by a rapid trend of increasing chemical activity from 21 % P 2 O 5 ass /P 2 O 5 total for the non-activated apatite, to 32 % for 3 mm and 34 % for 5 mm steel grinding bodies. When the aim is to obtain an energy-rich solid phase with pronounced defects in the structure, the time for mechanochemical treatment should be increased beyond 30 min. The samples were analyzed using chemical analysis, powder X-ray diffraction (PXRD), Wavelength Dispersive X-ray fluorescence (WD-XRF) analysis, Scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR). The obtained results have practical significance for the development of eco-agriculture - production of fertilizers, soil improvers, and activators. • Anionic isomorphism leads to the formation of A-type carbonate-fluorapatite. • Substitution of F − with OH − near the hexagonal axis in B-type apatite. • Solid-phase synthesis with formation of hydrogen ortho- and pyrophosphates.