G. Alan Turner

The effect of vibrations on the flow properties of a saturated paste of iron ore and water

Abstract Many stiff pastes, made up of finely ground solid material with enough liquid to saturate the voids, have both a yield stress and a viscosity. However, in some cases both these rheological attributes can be reduced in value by the application of vibrations. This fact, already widely made use of in the construction industry, is of interest also for practical applications in the processing industries as well as in fundamental investigations of the physical structure and behaviour of such two- (or perhaps three-) phase systems. This paper reports the magnitudes of the reduction both of the yield stress and of the viscosity of a paste of iron ore and water, of a fixed composition in the capillary state. A squeeze-flow apparatus was used to make these measurements. It was modified to allow the specimen of paste to be vibrated. The iron ore was a Knob Lake flotation concentrate, ground to 62% through 325 mesh. The water content, constrained to a narrow range by the physics of this two-phase system, was 16% w/w (wet basis). The vibration frequency was in the range 10 to 30 Hz, whilst the peak-to-peak amplitude lay between 0.254 to 1.27 mm. The results showed that the yield stress decreased from 20 kPa in the unvibrated state to almost zero when vibrated at 10 Hz and with an amplitude of 0.254 mm. Again, the viscosity could be reduced markedly by the vibrations. Furthermore, the viscosity of the vibrated paste (i.e., with zero yield stress) could be correlated with the shear rate by a power-law equation. Because of the complexity of the system and the uncertainty of the influence of all possible variables, no attempt was made at this stage to set up any correlations. However, by inspection, the viscosity of this particular sample was fitted to a function of amplitude ( A ) and frequency ( f ), namely A 2 f 0.5 . No theoretical justification is put forward.

The setting of a dense bed of fine powders in air Part II. A simple model of the settling process

Abstract In the sedimentation of fine particles it is widely recognized that there is flocculation and that the settling rate is governed by that. By using a simple model it is possible to deduce an order-of-magnitude estimate of the size of the agglomerates from the sedimentation rate at the surface of a settling bed of powder in air. For some of the materials used, namely fine clay particles, the results seem plausible. On the other hand, the results for glass beads indicate that in this case the assumptions were open to doubt; the one most suspect was that the agglomerate could be assumed to be impermeable. All beds were very dense with a sharp upper interface, hence the model dealt with dense beds of fine powders which were assumed to form spherical agglomerates of relatively large diameter, settling in close proximity. The variables used had to take account, at least in an empirical fashion, of the effect of flow of air around each agglomerate due to (a) their proximity, (b) air displaced upwards by solids settling in a closed vessel. Thus the quantities of significance were (a) the diameter of the agglomerate, (b) the separation between agglomerates; these were combined into variables used in the equations, namely the hydraulic diameter and the density of the suspension.