Dry Magnus separation q N. Fraunholcz, P.C. Rem * , P.A.C.M. Haeser Faculty of Mining and Petroleum Engineering, Delft University of Technology, Mijnbouwstraat 120, 2628 RX Delft, Netherlands Received 3 August 2001; accepted 15 October 2001 Abstract The Magnus effect is the lift force on a body moving through a fluid as a result of the rotation of that body. In theory, the Magnus effect can be used to recover small non-ferrous metal particles from dry bulk streams. The Magnus separation process would consist of passing a feed stream next to a fast-spinning magnet in order to create a selective rotation of the non-ferrous particles so as to deflect them away from the stream by the Magnus effect. The present work shows the experimental deflection for flat and granular particles with a size between 1 and 3 mm and gives a theoretical background of the separation. It is also shown that the measured deflections relate well to values from numerical simulations. Ó 2002 Elsevier Science Ltd. All rights reserved. Keywords: Mineral processing; Recycling; Modelling; Simulation 1. Introduction A simple experiment with a copper cylinder, a ramp made of cardboard and a small transparent vessel filled with water shows a force of surprising strength (see Fig. 1). While the cylinder rolls down the slope, its angular velocity grows proportional to its speed. As it then en- ters the water, its rotation creates a lift force that eventually reverses its horizontal motion. The Magnus effect has been known for a very long time (see Magnus, 1852 or Barkla and Auchterlonie, 1971), yet apart from manipulating the trajectory of the ball in sports like golf and baseball, few applications of the effect have been recognized (see Flettner, 1926 for a rare example of a naval application). It was recently discovered, however, that the effect can be used to sep- arate fine non-ferrous particles from a mixture, for ex- ample pieces of copper and aluminium from the bottom ashes of municipal waste incinerators (see Rem and Karstens, 2001). The principle of this kind of separation is shown in Fig. 2: rotating particles are deflected by the Magnus effect as they fall through the water. By selec- tively bringing the non-ferrous particles into rotation with the help of a spinning magnetic field, these particles are removed from the feed stream. The particle size for a wet Magnus separator is typically between 1 and 10 mm. Particles smaller than 1 mm have a relatively small ter- minal velocity which makes the process less attractive, while particles larger than 10 mm can easily be recovered by standard eddy current separators. In principle, Magnus separation can also be applied as a dry process. Since the terminal velocities of particles are higher in air than in water this could be an option for smaller particle sizes, typically from 0.2 mm up to 2 mm. Several issues need to be resolved, however, in order to createapracticalprocess.Itisnotclear,forexample,how the material should be fed into the field region and whe- ther the separation should be effected while the particles traverse the rotating magnetic field (as in the wet Magnus process) or afterwards. The present work compares the results of experiments with theoretical predictions in or- der to create a fundamental framework of theory and experiment for designing a dry Magnus process. 2. Theory The trajectory of a spinning particle falling in a fluid can be analysed to derive the forces of drag and lift, F D ¼ c D qU 2 A=2 ð1Þ and F L ¼ c L qU 2 A=2; ð2Þ respectively. The coefficients of lift and drag, c L and c D , then follow from the known values of the fluid density q, Minerals Engineering 15 (2002) 45–51 www.elsevier.com/locate/mine q Presented at MEES ’01, Falmouth, UK, June 2001. * Corresponding author. Tel.: +31-15278-3617; fax: +31-15278-2836. E-mail address: p.c.rem@ta.tudelft.nl (P.C. Rem). 0892-6875/02/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII:S0892-6875(01)00198-4