From colloidal dispersions to colloidal pastes through solid-liquid separation processes

Abstract

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. 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 osmotic 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. In these conditions, each particle is surrounded by an ionic layer, which repels other ionic layers. This results in a structure with strong short-range order. At high particle volume fractions, the overlap of ionic 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 aggregated particles, which are silica nanoparticles, dispersed in water and flocculated by multivalent cations. This produces large bushy aggregates, with fractal structures that are maintained through interparticle surface-surface bonds. As the paste is submitted to osmotic pressures, small relative displacements of the aggregated particles lead to structural collapse. The final structure is made of a dense skeleton 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 may be calculated through a numerical model that takes into account the noncentral interparticle forces. According to this model, the response of aggregated pastes to applied stress may be controlled through the manipulation of interparticle adhesion.

Equation of State of Colloidal Dispersions

“…For experiments that were performed through the dialysis of colloidal silica dispersions against a solution of PEG 35000 (Figures 2 and 4), this discrepancy could originate from an inadequate equation of state for PEG 35 kDa. For example, the molecular weight distribution of the PEG 35 kDa used by Chang 8 and Persello 36 may not be identical to that used by Bouchoux. 12,13 We note that experiments made on the same colloidal silicas using dextran T110 as an osmotic stress polymer yield pressures that are in very good agreement with the calculated ones (Figure 5).…”

“…The dispersions were made of silica particles of the same mean radius of 10 nm, but they were synthesized in separate experiments [8,38]. Also, the compression curves were obtained by different researchers, both using the osmotic stress technique.…”

In Situ Characterization by SAXS of Concentration Polarization Layers during Cross-Flow Ultrafiltration of Laponite Dispersions

“…Depending on the transmembrane pressure and cross-flow conditions, a transition from stable to unstable filtration performance has been identified and a limit has been proposed. , The limit separates a stable domain where reversible polarization layer is formed and an unstable domain where the accumulation of matter growth leads to an incessant decrease of the permeation flux with time. Until now, these phenomena have been investigated by theoretical − and modeling approaches, − but no real time observation of these phenomena at the pertinent length scales under cross-flow of the colloids has been completed during the filtration process. Some experimental investigations have probed the structural organization of deposits, − and some others have allowed following the structural organization of colloids near the membrane surface during frontal ultrafiltration by small-angle X-ray scattering (SAXS) and small-angle neutron scattering (SANS). − …”

How a Colloidal Paste Flows—Scaling Behaviors in Dispersions of Aggregated Particles under Mechanical Stress—

“…Such universality allows direct comparison between results of the numerical simulations and experimental data on compression of colloidal pastes. This was done successfully in recent works [19,20,21].…”