Kinetics of the dissolution of sand into alkaline solutions: application of a modified shrinking core model

International audienceThe batch dissolution kinetics of sand particles in concentrated alkaline sodium hydroxide solutions was investigated at high temperature and high pressure. The influence of the particle size, the operating temperature (150-220 degreesC) and the hydroxide ion molality on the kinetic rate were studied. Experimental data fit well into a developed model based on the shrinking core model approach in which a variable activation energy term was introduced. Kinetic analysis of the experimental results reveals that the associated energy and the rate-controlling mechanism are dependent upon both sample temperature as well as the extent of the reaction

Chemical and structural changes of raw phosphate during heat treatment

International audienceIn the wet phosphoric acid production using calcined phosphate particles, physical structure and chemical composition of used material have an impact on the reaction speed. In this study, porous properties of phosphate fraction size 125-200 mum after heating at different temperature between 240degreesC to 940degreesC were investigated. Fluorapatite, carbonate fluorapatite (Francolite), dolomite and calcite constitute the major minerals of the phosphate rock used. The results of TG/DTA, CRTA, IP, and ESEM observations were used to understand the chemical changes observed. The decline of the specific surface area (SSA) from 16.61 m(2)/g down to 0.81 m(2)/g was interpreted according to the thermal results. The increasing of the linear shrinkage % by sintering process is observed. This process begins near 240degreesC and reaches a maximum near 940degreesC. These results are in agreement with those in the literature. Some kinetic tests were undertaken at laboratory scale to elucidate the relationship between reactivity and porous properties of phosphate particles

Fractal nature of bentonite - water - NaCl gel systems evidenced by viscoelactic properties and model of gels

International audienceThe effect of ionic strength on the viscoelastic behaviour of bentonite dispersion has been investigated and quantified by the elastic modulus (G′) to insure the fractal nature of the bentonite-water-NaCl gel systems. Whatever the ionic strength of the medium, this rheological parameter followed a scaling law as a function of the bentonite volume fraction. Shih's fractal model (strong-link regime) was used to better fit the experimental data in coagulated and non-coagulated gels. A good agreement between the experimental results at different ionic strength and calculated data was obtained. The fractal dimension (D) varied with ionic strength and had a minimum value at maximum coagulation which is in agreement with the fractal nature of the bentonite gels