Coal Preparation » General
The elimination of organic chemicals, used in float/sink testing from laboratories is a priority for the coal industry. A variety of replacements are under investigation including ferro-fluids, zirconium dioxide and caesium formate. This project initially investigated the application of magnetorheological (MR) fluids to float sink testing. MR fluids contain ferromagnetic particles which are too heavy for Brownian motion to keep in suspension. Modelling indicated that a ferrofluid layer of homogeneous density could be achieved by application of an alternating magnetic field at a certain frequency. The frequency will cause the magnetic domains to flip and hence hold them in a fixed position in space. This frequency is thought to be in the kilohertz region. Experiments were conducted; however, the apparatus was unable to generate the required frequency and field strength. Hence the theory could not be validated. It was also determined that to build a system to achieve the required magnetic field would be beyond the project budget.
Consequently, the project then focussed on completing the experimental work using DC magnetic fields for both MR and colloidal type ferrofluids. A colloidal type ferrofluid is a solution containing ferromagnetic nanoparticles that are suspended by Brownian motion and hence do not settle or clump at the pole face when subjected to a magnetic field. The result of the application of the magnetic field is a change in the apparent density due to a change in buoyancy. A series of experiments were performed using density tracers and ferrofluids subjected to varying magnetic field strengths. The experiments were to place density tracers in the ferrofluid and to increase the magnetic field strength acting on the ferrofluid. The results were that as the magnetic field strength was increased the tracers rose to the surface of the ferrofluid and were recovered. The experiments confirmed that density control was possible with the float/sink status of a tracer being able to be controlled through changes in the strength or the applied magnetic field.
Future work is required to determine the practicality of using ferrofluid to separate coal samples. The key issues to evaluate are cleaning of ferrofluid from the coal particles, viscosity, and costs and 'sharpness' of the float/sink cut point.