INFLUENCE OF CALCINATION PROCESS ON STABILITY AND REACTIVITY OF CARBONATE FLUORAPATITE

Laboratory tests were conducted to investigate the effect of some calc ination parameters, such as temperature and time, on the stability and reactivity of chemically prepared carbonate-fluorapatite (francolite) . X-ray diffraction and infrared analyses showed that the synthetic ma terial was very similar in character to natural francolite. Leaching t he product with Silverman's reagent indicated that the prepared carbon ate-fluorapatite was free of excess carbonates. The unit-cell dimensio ns of the prepared apatite lay within the range shown by natural franc olite. The infrared analysis also indicated that the carbonate substit utes for phosphate and is, thus, B-type carbonate. Calcination tests e stablished that the carbonate-fluorapatite is thermally stable up to 5 00-degrees-C. Above this temperature it starts to lose its carbonate c ontent to form a more stable compound of fluorapatite of larger unit-c ell dimensions. At temperatures in the range 950-1050-degrees-C the fl uorapatite becomes very similar in structure to the natural, type fluo rapatite from Durango, Mexico. The reactivity of the prepared carbonat e-fluorapatite is very high and is unaffected by calcination at 500-de grees-C owing to the stability of apatite at this temperature. However , a dramatic reduction in reactivity occurs when the calcination tempe rature is raised to 650-degrees-C. It decreases further at higher temp eratures, when the crystal lattice of apatite loses its carbonate cont ent. This indicates that the carbonate present in the crystal lattice of a given apatite plays a very important role in determining its reac tivity.