From colloidal dispersions to colloidal pastes through solid/liquid separation processes
Résumé
Solid-liquid separation is an operation that starts with a dispersion of solid particles in a liquid, and removes some of the liquid from the particles, producing a concentrated solid paste and a clean liquid phase. From a conceptual point of view, it is similar to thermodynamic processes where pressure is applied to a system in order to reduce its volume. In dispersions, the resistance to this compression depends on interactions between the dispersed particles. The first part of this work deals with dispersions of repelling particles, which are either silica nanoparticles or synthetic clay platelets, dispersed in aqueous solutions at high pH and low ionic strength. In these conditions, each particle is surrounded by an ionic double layer, which repels other double layers. This results in a structure with strong short-range order. At high particle volume fractions, the overlap of double layers generates large osmotic pressures; these pressures may be calculated, through the cell model, as the cost of reducing the volume of each cell. The variation of osmotic pressure with volume fraction is the equation of state of the dispersion. The second part of this work deals with dispersions of particles that attract each other. The particles are silica nanoparticles, dispersed in water and flocculated by addition of multivalent cations. This produces large bushy aggregates, with fractal structures that are maintained through interparticle surface-surface bonds. As the paste is submitted to increasing osmotic pressures, most bonds are retained, but reordering processes at the scale of 2-20 diameters cause the structure to collapse. The final structure is made of dense grains immersed in a nearly homogeneous matrix of aggregated particles. The variation of osmotic resistance with volume fraction is the compression law of the paste; it has been calculated through a numerical model that consists of spheres connected by springs, which bind to their surfaces. The response of such networks to osmotic pressure follows some scaling laws, which depend only on the nature of the springs (elastic vs. dissipative). According to these predictions, the response of aggregated pastes to applied stress may be controlled through the manipulation of interparticle adhesion.
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