Sedimentation and Compaction of Calcium Carbonate Aggregating Suspensions : Scaling Analysis of the Equilibrium

Abstract

Les suspensions agrégées se divisent généralement en un surna geant transparent et un sédiment gélifié volumineux. La suspension demeure homogène (gel stable sous l'effet de la gravité) seulement quand la fraction volumique des particules dépasse une valeur don née La présente étude porte sur le calcul de $**. Un modèle simplifié décrivant l'équilibre du gel a été mis au point et sa validité est vérifiée en comparant de ses prédictions avec des mesures sys tématiques du volume du sédiment effectuées sur des suspensions de carbonate de calcium. Si nous appliquons ce modèle à la prédic tion de nous démontrons que n'est pas une caractéristique du système physicochimique mais qu'il dépend de la hauteur de l'échantillon, de son rapport d'aspect et des frottements entre le gel et la paroi. Pour les grands (respectivement petits) rapports d'as pect, nous montrons que Φ** est une fonction en loi de puissance de la largeur (respectivement la hauteur) de l'échantillon compor tant un exposant 1 / ( k -1) qui est relié à la dépendance du seuil d'écoulement avec la fraction volumique de la suspension. SEDIMENTATION AND COMPACTION OF CALCIUM CARBONATE AGGREGATING SUSPENSIONS: SCALING ANALYSIS OF THE EQUILIBRIUMAggregating suspensions generally separate into a clear super natant and a voluminous gelled sediment. The suspension remains homogeneous (Le. a gel stable under gravity forms) only when the volume fraction of particles exceeds a given value Φ**. The pre sent study is devoted to the determination of Φ**. A simplified model describing the gel equilibrium has been developed and its validity is checked comparing its predictions to systematic mea surements of the sediment volume performed on calcium carbon ate suspensions. Applying this model to the prediction of Φ **, we show that Φ ** is not a characteristic of the physico-chemical sys tem but depends on the height of the sample, its aspect ratio and friction between the gel and the side wall. For large (respectively small) aspect ratios, we find that Φ ** is a power law function of the width (respectively the height) of the sample involving an exponent (1) Bat SEDIMENTACIÓN Y COMPACTACIÓN DE SUSPENSIONES DE AGREGADOS DE CALCIO : ANÂLISIS DE ESCALA DES EQUILIBRIOLas suspensions agregadas se dividen, por lo general, en una fracción transparante que sobrenada y un sedimento geleficado voluminoso. La suspension quedarse homogénea (gel estable sobre el efecto de la gravedad) unicamente cuando la fracción de volumen de particuias sobrepasa un valor determinado Φ**. El presente estudio presenta un al càlculo de Φ**. Se ha desarrollado un modelo simplificado que describe el estado de equilibrio del gel, cuya validez esta verificado mediante una comparación de sus predicciones con mediciones sistemàticas del volumen de sedimento debajo de suspensiones de carbonato de calcio. Si se aplica este modelo para la predicción de Φ **, se demuestra que Φ** no constituye una caracterlstica del sistema fisicoquimico pero que, en cambio depende de la altura de la muestra, de su relación de ...

Electrical conductivity measurements and the nonhomogeneous settling behavior of aqueous suspensions of barite

“…Both formation of channels and particle agglomeration phenomena enhance the settling velocity of the solids and may also explain the nonhomogeneous behavior of Region II. Particle agglomeration and channeling have indeed been reported in the literature as a recurrent sedimentation phenomenon in Newtonian fluids, in situations where particles are dispersed in colloidal suspensions 23,24 or tend to form floccules 19,41–43 …”

“…in the literature as a recurrent sedimentation phenomenon in Newtonian fluids, in situations where particles are dispersed in colloidal suspensions 23,24 or tend to form floccules. 19,[41][42][43]…”

“…However, the scientific literature has also reported particle agglomeration as a recurrent settling phenomenon in Newtonian fluids where particles are dispersed in colloidal suspensions. 23,24 Thus, on account of its microscopic structure, the settling dynamics of such a colloidal system strongly differs from the one observed in usual macroscopic suspensions constituted by non-Brownian particles. 24 Based on the discussions above, the sedimentation of aqueous suspensions is still a nonconsolidated theme, with much more to be gained in understanding.…”

“…As a convergence point, those studies have shown that settling is homogeneous in macroscopic suspensions of non‐Brownian particles. However, the scientific literature has also reported particle agglomeration as a recurrent settling phenomenon in Newtonian fluids where particles are dispersed in colloidal suspensions 23,24 . Thus, on account of its microscopic structure, the settling dynamics of such a colloidal system strongly differs from the one observed in usual macroscopic suspensions constituted by non‐Brownian particles 24 …”

Electrophoretic Deposition of Silicon Carbide

“…Theoretically, the volume fraction of a uniform sediment should be ⌽ ‫ס‬ 0.63 for a random close packing of equal-sized, unaggregated spheres. 31,32 The difference between the theoretical value and the maximum in Table I (⌽ ‫ס‬ 0.43) is attributed to the shape (platelets) and to some residual aggregation of the particles. Disaggregation of SiC by the aluminum salt also is apparent in the settling behavior of the suspension.…”