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Effect of
temperature on the attack of fluorapatite by a phosphoric
acid solution
Khamaïs
Brahim, Kaïs Antar, Ismaïl Khattech* and Mohamed
Jemal
Faculty of Science, Chemistry Department, Applied
Thermodynamics Laboratory, 2092 Tunis El Manar – Tunisia.
*Corresponding author. E-mail:
khattech@fst.rnu.tn.
Tel: +21698208884; Fax: +216 71885 008.
Accepted 27 June, 2007 |
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The attack of fluorapatite (Fap) by phosphoric acid solution
between 25 and 65°C was followed by microcalorimetry. The
molar enthalpy decreases until a temperature close to 45°C,
and then increases. This behaviour was attributed to a
change of mechanism. Iteration of the results enables us to
propose a two or one step mechanism, depending on the
temperature range. Kinetic and thermochemical parameters
corresponding to each step were also deduced. For
temperature lower than 45°C, the activation energy, Ea,
equals about 20 kJ mol-1, suggesting a diffusion
phenomenon control, whereas for higher temperature the
attack seems to be controlled by a chemical process (Ea
= 101 kJ mol-1).
Key words: Kinetics, microcalorimetry, fluorapatite,
phosphoric acid, temperature, dissolution. |
