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Rheology and stability of magnetite dense media Klein, Bernard

Abstract

The efficiency of the dense medium separation process is known to depend on the rheology and stability of the medium. In particular, the medium should exhibit a low viscosity and a high settling stability. Despite this knowledge, little information existed on these medium properties. The lack of information stems partially from the difficulties associated with measuring the rheological properties of unstable suspensions. In order to measure these properties, it was necessary to design a rheometer for settling suspensions. Once this was achieved, the rheology and stability of magnetite suspensions were characterized and the influences of various medium parameters on these properties were investigated. Settling experiments revealed that magnetite dense media exhibit bulk zone settling properties that are characterized by the presence of (from top to bottom): a supernatant, a transition zone, a constant density zone and a sediment. The constant density zone was found to have a solids content that was the same as that of the initial suspension. Test results indicated that the suspension mudline settled at approximately the same rate as the constant density zone and should therefore provide a good indication of the media stability. Based on knowledge of the settling properties of magnetite suspensions, a rheometer fixture was designed that could be used to measure the rheological properties of such suspensions. The fixture is an elongated double gap concentric cylinder cup and bob arrangement that positions the bob in the constant density zone of the settling suspension during measurements. Rheological measurements revealed that magnetite dense media exhibits yield shear thinning and thixotropic flow properties. The Casson flow curve model was found to fit the rheological flow curves for these suspensions better than other well known models. Investigations into the effects of various parameters on medium rheology and stability revealed that solids content, magnetite particle size and, in the presence of clays, pH are the most important suspension variables. Other parameters that significantly affect the suspension properties include magnetization, dispersing agents and the presence of clay and fine coal contaminants. Several of these parameters significantly affected the Casson yield stress, while only a few parameters affected the Casson viscosity, indicating that the yield stress is the most controllable rheological parameter. In addition, over the tested shear rate range, the yield stress term contributed much more to the apparent viscosity of the suspensions than the Casson viscosity term, and is therefore the most important rheological property. It was also found that the yield stress is inversely related to the mudline settling rate such that when the yield stress is high the settling rate is low and vice versa. Investigations into the effect of particle size distribution on the properties of magnetite dense media revealed that media properties can be improved by using bimodal particle size distributions. In particular, at low medium densities, where stability is of concern, the size ratio of the two particle fractions and the proportion of fine magnetite particles were found to have a large effect on the settling rate. At high medium densities, where the media can be excessively viscous, optimum size ratios and proportions of fine particles can be selected to reduce the suspension Casson yield stress.

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