Ismail Khattech

Thermochemistry of phosphate products. Part I: Standard enthalpy of formation of tristrontium phosphate and strontium chlorapatite

Abstract Tristrontium phosphate Sr 3 (PO 4 ) 2 (triSr) and strontium chlorapatite Sr 3 (PO 4 ) 6 Cl 2 (ClapSr) have been prepared and characterized by X-ray diffraction, IR spectroscopy and chemical analysis. Using an isoperibol calorimeter the heat of solution of these products in a 9% weight nitric acid solution were determined. They are (−110.1 ± 1.5) and (−283.6 ± 2.4) kJ mol −1 for triSr and ClapSr, respectively. Combining these results with others found by dissolving the products and entities involved in thermochemical cycles, allows to determine the standard heat of formation of these Sr-phosphates. These quantities are: Δ f H °(triSr, 298 K) = −4110 kJ mol −1 and Δ f H °(ClapSr, 298 K) = −13233 kJ mol −1 . The results are discussed on the basis of the differences in bond enthalpies.

Attack of Tunisian Phosphate Ore by a Mixture of Sulfuric and Phosphoric Acid: Thermochemical Study by Means of Differential Reaction Calorimetry

A calorimetric, Differential Reaction Calorimetry (DRC), study of the thermochemical of the attack of a phosphate ore from Gafsa region (Tunisia) by a mixture of sulfuric and phosphoric acid is undertaken at different solid-liquid ratio and different temperature. The plot of the quantity of heat measured by integrating the raw signal as a function of the dissolved mass in the same volume solution at 25℃ presents three straight segments attributed at the formation of the hemihydrate (CaSO4·1/2H2O:HH), the dihydrate (CaSO4·2H2O:DH) or a mixture of these two products checked by X-ray diffraction, infrared spectroscopy and thermal analysis (Differential Scanning Calorimeter: DSC). The attack by the acid mixture was performed at higher temperature and showed in addition the formation of another form of calcium sulfate:anhydrous (CaSO4:AH). Moreover, the variation of mass enthalpy versus temperature presents a break at the T = 45℃. According to our results, it seems that the effect of the temperature on the sulfo-phosphoric attack reaction on the natural phosphate (NP) generates a change of mechanism at around 45℃.