Influence of non-wetting conditions on aggregation of divided solids in liquid phase

National audienceWe present here the main results of an experimental study of aggregation which was performed on two silica granular samples: one is naturally hydrophilic; the other has been made hydrophobic by superficial grafting of hydrophobic groups. Aggregation is studied in water-ethanol mixtures of variable compositions and followed by in situ turbidimetry at different stirring rates. The aggregation behaviors of the two samples are quite different. Aggregates of hydrophilic particles are small and fragile whereas aggregates of hydrophobic particles are large and particularly solid. Interpretation of these differences is presented; it is based on several elements: modeling of hydrophobic interaction and of aggregate morphology, modeling of the aggregate optical properties and determination of the aggregation and fragmentation rates. Essential modification introduced by non-wetting consists of the formation of gas bridges around contact points between particles in aggregates

Model of aggregation of solid particles in nonwetting liquid medium

International audienceProblem of modelling solid particles aggregation in non-wetting liquids is in close relation with the problem of interactions between hydrophobic particles in water. Hydrophobic aggregation is discussed from a thermodynamic point of view, then the main aspects of aggregation dynamics are envisaged in the particular conditions created by non-wetting. Consequences on aggregation and fragmentation kernels are studied. An experimental illustration is provided

Etude de l'agrégation de particules solides en milieu non mouillant. Interprétation et modélisation

The understanding of solid particles aggregation mechanisms in a non wetting medium is of great importance in many scientific fields. To study it, a model system is used ; it is composed of hydrophobic silica particles in a water – ethanol mixture. Since the aggregates are fragile, aggregation had to be studied in-situ. This was possible thanks to a "laboratory-made" turbidimetric sensor. To know the aggregates' final size and morphology , other methods have also been used and their respective interest or validity determined. The influence of various parameters on aggregation have been studied, such as stirring speed, wettability, presence or not of gas, bubble injection...The modelling of aggregation - fragmentation phenomena takes place in the framework of a population balance (validated by experimental particle sizing) and accounts for the hydrodynamics of the suspension as well as the physicochemical aspects of the non wetting medium. One of the most important points which comes out from this study is a reduction of fragmentation due to the presence of gas bridges between the solid particles in a non wetting liquid.La compréhension des mécanismes d'agrégation de particules solides en milieu non mouillant revêt une grande importance dans de nombreuses disciplines scientifiques. Pour réaliser cette étude, le système modèle retenu est constitué de particules de silice rendues hydrophobes plongées dans un mélange eau - éthanol. Etant donné le caractère fragile des agrégats, l'agrégation se devait d'être étudiée in-situ ce qui a pu être réalisé grâce a un capteur turbidimétrique développé au laboratoire. Pour connaître la taille finale et la morphologie des agrégats, d'autres méthodes ont également été utilisées ce qui a permis de confronter les résultats entre eux. L'influence de différents paramètres sur l'agrégation ont ensuite été étudiés telles que la vitesse d'agitation, la mouillabilité, la présence ou non d'un milieu désaéré, l'injection de bulles...La modélisation des phénomènes d'agrégation – fragmentation utilise l'approche des bilans de populations et prend en compte l'hydrodynamique de la suspension ainsi que les aspects physico-chimiques propres à la non mouillabilité. L'un des points les plus importants qui ressort de cette étude concerne la diminution de la fragmentation due à la présence de ponts gazeux entre les particules solides plongées dans un milieu liquide non mouillant

Experimental study of small aggregate settling

International audienceThe drag coefficient and hydrodynamic radius of particles are important parameters needed in crystallization science. Small aggregates of micrometric primary particles are mainly produced in stirred crystallizers. We present experimental results on the drag coefficient of macroscopic aggregates consisting of glass beads in the number range [2, 100]. The drag coefficient is calculated from settling measurements in glycerol in order to preserve the Stokesian nature of typical flow around particles in a crystallizer. We show that the hydrodynamic radius of these aggregates is almost the radius based on the average projected area over all orientations. This result is extended to larger and more porous aggregates

Etude de l'agrégation de particules solides en milieu non mouillant (Interprétation et modélisation)

ST ETIENNE-ENS des Mines (422182304) / SudocSudocFranceF

Influence of nonwetting on the aggregation dynamics of micronic solid particles in a turbulent medium

International audienceThe aim of this work was to determine and to interpret the influence of nonwetting on the aggregation dynamics of micronic solid particles in a turbulent medium. Two silica granular samples were studied: one was naturally hydrophilic; the other was made hydrophobic. Aggregation in an aqueous ethanol solution was followed by in situ turbidimetry. The influence of stirring rate and deaeration was determined. Aggregates of hydrophilic particles were small and fragile, whereas aggregates of hydrophobic particles were large and solid. Moreover, they differred greatly in optical properties. Within the proposed approach, different features of the aggregate morphology were identified: fractal dimension, maximum size, and gas content of the hydrophobic clusters. These elements are taken into account in the models of aggregation dynamics proposed here

<i>On-line</i> determination of aggregate size and morphology in suspensions

National audienceInformation about the aggregation state of fine particles is an important element for process control, product quality monitoring and fundamental understanding in many cases of industrial slurries. When aggregates are small or fragile objects, their withdrawal is difficult and off-line characterization may be a source of error. This work deals with the application of different in line methods to the characterization of silica aggregate size and morphology. These methods are based on turbidimetry. One of them consists of the analysis of the turbidity fluctuations and is operated on a commercial instrument. The other one uses the aggregate settling velocity which is determined by turbidimetry too, however with a home-made apparatus. This work gives us the opportunity to define morphological models for small aggregates and to calculate their drag coefficient. Thanks to these models, the aggregate morphological characteristics and the number of their constituting particles can be derived from the experimental results. Agreement between the different methods is examined and discussed