Coal Preparation

Application of Magnetic Fluids for Sink-Float Analysis - Literature Review

Coal Preparation » General

Published: June 04Project Number: C13055

Get ReportAuthor: Kevin Galvin | University of Newcastle

It is now more likely than ever that the Australian Coal industry will need to find an alternative to the heavy liquids used to produce a sink-float analysis of coal. Indeed, the Australian Standard on sink-float analysis is likely to be phased out over the next five years. Thus there is a strong motivation to find an alternative method. This report provides a literature review on the use of magnetized ferrofluids for producing a sink-float analysis.

A ferrofluid is a colloidal dispersion of 10 nm magnetite particles in a liquid, stabilized by the use of surfactants. When magnetized, and subjected to a magnetic field gradient, the dispersion experiences a powerful downwards force, much like the weight force of a dense liquid. In other words, by adjusting the current supplied to an electromagnet it is possible to produce a fluid of virtually any apparent density.

This technology is not new, and hence is not protected by patents. However, DeBeers have developed significant know-how, in addition to commercial devices that could be used immediately for sink-float analysis in the coal industry. It would be necessary, however, to investigate the accuracy of the method in the low density range of coal and mineral matter, and hence confirm its suitability. A comprehensive experimental study is therefore recommended in order to examine these issues.

The preferred technology for exploiting magnetized ferrofluids is the Ferro Hydrostatic Separator (FHS). Two poles of an electromagnet are separated by a physical distance known as the air gap, with the separation higher at the top than at the bottom. This arrangement creates a field strength that is smaller at the top than at the bottom, and hence a field gradient. The fluid in between is also magnetized, and, at sufficient magnetization, can be held in place on its own without the need for a container. The system is also placed on a slight incline in order to generate a horizontal gradient and hence horizontal motion. Particles that are added either float or sink, and hence can be seen to "pop" out the front end, either at the top or near the base. In this way, the FHS can function as a continuous production unit.

After studying this topic, it is concluded that a kerosene based ferrofluid offers significant advantages over a water based ferrofluid. Firstly, a kerosense based fluid is more readily diluted since the surfactant adsorbs onto the particles and is not at an equilibrium level within the fluid. If a water based fluid was diluted, additional surfactant would need to be added to ensure the surfactant remained adsorbed on the particles and did not desorb. Further, a kerosene based ferrofluid is much less dense which means a higher field gradient can be applied to achieve the minimum density for the sink-float work. Thus, the accuracy of the apparent densities should be higher. Kerosene is much less viscous and hence the drainage of the sink and float fractions will be quicker, and with the higher volatility the particles will dry more quickly.

DeBeers would like to secure a buyer for their know-how which is considerable, or at least a licensing arrangement. Unfortunately the sink-float market is not large, and hence the purchase of their know-how is not highly attractive. The technology is not protected by patents, though DeBeers does have some related patents, and details necessary to produce their current range of FHS machines. DeBeers also has significant know-how in the area of kerosene ferrofluid manufacture.

DeBeers have indicated they are prepared to provide an FHS machine for a comprehensive trial for the coal industry. Some minor improvements would be introduced, including the use of solid-state pressure transducers to measure and in turn control the density. The sink-float analysis would be conducted in the batch mode using a basket and scoop arrangement.


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