The kinetic of the reaction of sand with aqueous NaOH corresponding to the ratio Si02/Na20 = 2 was studied in a pressure vessel at 22OOC and 2.7 MPa. Since the kinetic curves could not be obtained directly from the experimental data, a new method is proposed to plot the entire kinetic graph from experimental data. An analytical expression of the type Q = A [I -exp(-Bt)] describes the system perfectly. The constants A and B were calculated for a silica sample having a narrow granulometric distribution (range i.e. 300-315 pn). The value ofA is found to be almost constant, between 0.95 to 0.99 and B ranges from 0.03 to 0.14 when [OH-] increases from 0.5 to 12.5 mol/L. The kinetic order with respect to O K is equal to 0.470 f 0.013 and the kinetic constant at 220°C is 3.933 x lod g/m2 .s. ~~On a etudie la cinetique de la reaction du sable avec du NaOH aqueux correspondant au rapport SiO,/N%O = 2 dans un reservoir sous pression a 22OOC et 2,7 Mpa. Etant donne que les courbes de cinetique n'ont pas pu &tre obtenues directement des donnees experimentales, une nouvelle methode est proposee pour tracer le diagramme cinetique au complet au moyen des donnees experimentales. Une expression analytique du type a = A Keywords: solid-fluid reaction, sand, sodium hydroxide, silica dissolution, sodium silicate.he study of non-catalytic reactions involving a fluid and T a solid is of a growing interest in chemistry (in applications such as leaching, silica descaling, metal corrosion). There are several difficulties when the measurements of the kinetics of the transfer of material from the solid to the dissolved phase are made. Some difficulties occur because the reaction products can be insoluble precipitates which may adhere to the reagent surface and induce reaction-blockage. Furthermore, differences of reactivity are also observed if the solid is covered with an amorphous layer such as silica, or an oxide layer, such as may occur for some metals.The solid can also be comprised of an aggregate of phases which have different physical and chemical properties, such as a phosphate ore where the apparent reactivity is the sum of the reactions of each phase. On the other hand, the rate of solid dissolution depends markedly on the solid morphology, as well as on its porosity. If the external form of the grains is irregular, the study of surface kinetics becomes more complicated. The presence of asperities, which have a better contact with the fluid, leads to a faster surface reaction.The great variety of cases which may occur explains the various forms of kinetic curves. Studying the weight loss or change in solution concentration during the dissolution of many solids, Barret (1973) found that the general form of the kinetic curves can be sigmoidic, parabolic, linear, logarithmic, exponential, etc.Sigmoidal dissolution curves show an induction period, which corresponds to a very slow starting of the reaction during an initial time, most commonly called "induction time". Such curves were observed by Cornell and Giovanoli (1 993) during t...

The 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 jC) 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.

The initial dissolution rate of sand into sodium hydroxide solution (12.5 mol/L) over a wide range of temperatures (1 50 to 220°C) and pressures (0.47 to 2.29 MPa) has been determined. A semi-empirical equation was found to fit the dissolution data obtained at different temperatures and at different particle sizes. The influence of temperature on the reaction extent led to the activation energy of the dissolution, E, = 74.41 kJ/mol. Six samples of sieved silica having narrow particle size distribution 38-40, 100-125, 212-200, 400-500 and 850-1000 pn were reacted with 12.5 mol/L sodium hydroxide solution at 220°C. The effect of particle size, i.e., surface area of the grains, on the initial rate of dissolution (denoted vo) has been described by an empirical equation. Evidence is given here showing that the general assumption of the simple proportionality between the rate of reaction and the exposed surface of solid is not always confirmed.On a determine la vitesse de dissolution initiale du sable dans une solution d'hydroxyde de sodium (12,5 mol/L) pour une vaste gamme de temperatures (de 150 a 220°C) et de pressions (de 0,47 a 2,29 Mpa). On a trouve qu'une equation semi-empirique calait les donnees de dissolution obtenues A diffkrentes temperatures et tailles de particules. L'influence de la temperature sur la conversion a permis d'etablir I'energie d'activation de la distillation, E, = 74,41 kJ/mol. On a fait reagir six echantillons de silice tamisee ayant des distributions de tailles des particules etroites, de 38-10, 100-145, 21 2-200,400-500 et 850-1000 pn, avec une solution d'hydroxyde de sodium de 12,5 mol/L a 220°C. L'effet de la taille des particules, soit la surface specifique des grains, sur la vitesse de dissolution (appelke vo), est decrit par une equation empirique. On a la preuve ici que l'hypothese generale de la proportionnalitk simple entre la vitesse de reaction et la surface de solide exposee ne se confirme pas toujours.