A detailed study of the microwave pyrolysis of the Moroccan (Youssoufia) rock phosphate

The microwave pyrolysis of rock phosphate is studied. The results obtained by various analytical techniques are presented and compared in order to better understand the mechanisms involved in the microwave heating and decomposition of the rock phosphate. The pyrolysis curves showed that only drying of the sample takes place for microwave powers less than 400 W as a result of the relaxation losses of water molecules. For powers near 700 W, the attained temperatures allowed the decomposition of both organic and mineral matters. The products of this decomposition, presenting conduction and/or relaxation losses, are responsible for further heating the samples. Dielectric constant measurements performed on the phosphate with different relative humidity levels suggested that the origin of the heating process is due to the dielectric relaxation losses of water molecules initially contained within the material. The oil yield is on the order of that obtained by conventional pyrolysis. The percentage of asphaltenes produced under microwave irradiation was found to be half of that produced by conventional heating, while the oils obtained by microwave pyrolysis are more paraffinic as shown by gas chromatography as well as IR and NMR spectroscopies. Structural and micro-structural studies performed on the pyrolysis residues obtained by both techniques revealed that they were well crystallized in the case of microwave heating as a result of the higher temperatures encountered in this case. (c) 2005 Elsevier B.V. All rights reserved.