16. Petkova V., V. Koleva, B. Kostova, S. Sarov, Structural and thermal transformations on high energy milling of natural apatite, J Therm Anal Calorim, ISSN 1388-6150, (2015), Volume 121, Issue 1, 217-225 IF=2.042

Abstract: In this work we are investigated the isomorphic substitution and thermal decomposition of sedimentary fluorine apatite (FAp) (with Ca/P ratio >1.67) from Tunisia after high-energy- milling (HEM) activation at different time from 10 to 600 min. The chemical composition of the material includes: 29.6 % Р₂О₅^total and 46.5% CaO (main components) and 3.5 % F; 0.55 % R₂O₃ (R = Al, Fe); 1.1 % SO₃; 1.9 % SiO₂ (a low content in a comparison with other natural apatites from North Africa or Asia); 0.35 % MgO; 0.05 % Cl; 6,6 % CO₂ as the impurities. HEM is a well-known approach for preparing various solid materials and for increasing their reactivity. The solid-state transformation of the initial and HEM activated apatite samples was examined by chemical analysis, BET, powder XRD, FTIR spectroscopy and thermal analysis. The structure of natural apatite allows isomorphic substitutions of carbonate, hydroxyl and metal ions by PO₄^3-, Ca²⁺ and F^-. The obtained powder XRD data indicate an increased defectiveness of the apatite structure in the course of the HEM. The solid state transformations of the initial and HEM activated apatite are examined by TG-DTA analyses. It is found that the thermal stability of the activated samples decreases as compared to the initial sample. This is related with the increased defectivity of the apatite structure during the high energy milling shown by the XRD data. The thermal analysis allows the differentiation of the structurally bonded A, B and A-B types carbonate ions from these originating from the calcite and dolomite admixtures.

The results obtained demonstrate that the mechanical distortion and the structural changes related with the migration of the carbonate ions from B type to A type channel positions are the main factors responsible for the enhanced solubility of the high energy activated FAp.