M. Parker

Particle aggregation in colloids in high magnetic fields

Abstract The influence of high magnetic fields on the stability of colloidal dispersions of paramagnetic and diamagnetic particles has been investigated. The theory of Svoboda is extended to allow examination of the dynamics of particle aggregation in colloids. Particle trajectories are calculated leading to binary pair formation. Experimental results are presented on particle aggregation in high magnetic fields (flocculation). Here binary pairs seem to form and to be stable at enhanced rates of settling.

Limits of magnetic flocculation in colloidal dispersions

The limits of particle aggregation in high magnetic fields are described. These limits are determined, for any given colloid-field system, by electrochemical factors and by particle size. The latter is discussed in terms of thermal diffusion processes at the lower size limit and by the influence of differential gravitational settling in larger particle systems.

Kinetics of magnetic flocculation in colloidal dispersions

Abstract A description is given of the kinetics of particle agglomeration in colloidal dispersions in applied magnetic fields, a phenomenon referred to as magnetic flocculation. The influence of monosize particles and dissimilar particle sizes on the position and field value for secondary flocculation minima is re-examined. The concept of isochronal volume is introduced through which a relaxation time parameter as well as a flocculation frequency is described for the case of magnetically induced agglomeration. The flocculation half life (the relaxation time parameter) and flocculation frequency both have characteristics similar to parameters used in the literature to describe rapid coagulation in the absence of a magnetic field.

Study of particle behaviour in high field magnetic flocculation

A recently developed dynamic theory of magnetic field-induced particle-particle aggregation in suspensoids is further extended in terms of characteristic velocity coefficients in the manner of the inertialess theory of H.G.M.S. This is done by expressing the flocculation process in terms of familiar parameters such as magnetic velocity and capture radius. Hybrid flocculation of diamagnetic-paramagnetic pairs is described. Also demonstrated here is the importance of the suspensoid settling velocity following the occurrence of binary flocculation.

Application of magnetic flocculation in a continuous flow magnetic separator

A Continuous Flow Magnetic Separation (CFMS) technique is introduced, capable of processing slurries in a continuous mode. The separation process is based on creating differences in the magnetically induced settling rate of the individual particles. Results concerning separator performance are given for both two-component test slurries and slurries of naturally occurring mixtures under a variety of experimental conditions.

Contribution of interparticle forces to the buildup of ultrafine colloidal particles around a magnetized fiber

The influence of interparticle double‐layer and dipole‐dipole forces on the buildup of ultrafine colloidal particles around a magnetized collection center in a static colloid is considered. Expressions are given and developed, respectively, that describe the double layer and dipole‐dipole interparticle effects in a one‐dimensional particle number gradient. These expressions lend themselves to a description of the effects by a double‐layer diffusion and a dipole‐dipole flocculation coefficient. The size of these coefficients and of the normal thermal diffusion coefficient is compared over a wide range of colloidal particle radius, and ranges of radius are identified in which each coefficient can be dominant. Steady‐state solutions of the transport equation including these coefficients are computed and show that this inclusion can be justified, and is indeed necessary, for significant ranges of particle radius.

Flocculation of diamagnetic particles in high magnetic fields

The combined influence of high magnetic fields and concentration of a paramagnetic electrolyte on the stability of a suspension of diamagnetic quartz particles has been investigated. For the first time, formation of magnetically induced binary pairs of particles has been observed over a broad range of electrolyte concentration and is explained in terms of an extended Derjaguin–Landau–Verwey–Overbeek (DLVO)‐theory. Anomalous effects at low fields are shown to be caused by particle rotation due to an anisotropic magnetic susceptibility.

Sedimentation of colloids in high magnetic fields

Magnetic flocculation experiments are described for a number of mineral suspensions in high fields. Their sedimentation behaviour has been observed in situ by a novel optical monitoring technique. Anomalous negative enhancement of sedimentation rates in these suspensions has been observed at low fields, and this phenomenon is explained in terms of particle anisotropy. Results are shown of field-induced selective sedimentation in heterogeneous suspensions of fly ash. Finally, for the first time, binary magnetic flocculation is reported for a diamagnetic colloid.

Magnetosedimentation of particles in heterogeneous colloids

When binary mixtures of colloidal particles settle under gravity the structure of the sediment is a function of the relative dimensions and densities of the particles. All this is changed if the colloids are allowed to settle in an attractive vertical field gradient. Here, the particles acquire a contribution to the effective density which results in selective magnetosedimentation. A description is given of exploratory settling experiments of this type, using binary mixtures of paramagnetic materials. In certain of these experiments clear evidence of sediment bands is found.

AGGREGATION OF COLLOIDAL PARTICLES IN HIGH MAGNETIC FIELDS

A description is given of the behaviour of colloidal particles in high magnetic fields. This includes a theoretical model of the dynamics of particle aggregation in terms of London, van der Waals, double layer and magnetic dipolar faces together with an account of experimental investigations in the magnetic field-induced aggregation of various paramagnetic and diamagnetic colloidal dispersions. In the case of colloidal dispersions of diamagnetic quartz a linear relationship is observed between the value of the threshold magnetic field for onset of binary pair formation and the pH-value of the paramagnetic ionic solution carrying the particles.