THE INFLUENCE OF INTERPARTICLE SURFACE FORCES ON THE COAGULATION OF WEAKLY MAGNETIC MINERAL ULTRAFINES IN A MAGNETIC-FIELD

In this paper, it is shown that the coagulation of dispersions of weak ly magnetic mineral ultrafines (such as hematite and chromite) in an e xternal magnetic field can be described theoretically by invoking inte rparticle forces. Essentially, coagulation occurs when the short-range London-van der Waals interactions and the long-range magnetic forces outweigh the stabilizing electric double layer repulsion. From classic al colloid chemistry theory, we have calculated the various components of the potential energy for different-sized particles at a series of ionic strengths and magnetic field intensities. Principles governing t he stability of the suspensions were derived and the computations lead to the establishment of criteria which can be used to predict the sta bility of the suspensions of weakly magnetic oxide mineral ultrafines in a ''wet magnetic separation process''. Experimentally, the magnetic -field induced coagulation of ultrafines of natural hematite and chrom ite in aqueous suspensions at moderate ionic strength was investigated using a laboratory-scale electromagnetic solenoid. The experimental r esults relate the coagulation process (as determined by magnetosedimen tation analysis) to particle size, slurry pH and the external magnetic field. In the magnetic fields, maximum coagulation occurred near the pH of the point of zero charge (pH(PZC)) of the minerals (where the el ectrostatic double layer repulsion was reduced to a minimum) enabling the particles to enter the ''primary minimum'' energy sink. In contras t, in cases where the electrostatic repulsion was not suppressed, the long-range magnetic forces enabled coagulation to occur in the ''secon dary minimum''. This caused the formation of chains which appeared to be relatively stable at enhanced rates of settling. The experimental r esults could be interpreted from a theoretical analysis of the interpa rticle forces controlling the process.