REE11-2: Water purification by magnetic separation

Researcher: Almut Eisentraeger
Team Leader(s): Dr Ian Griffiths & Dr Dominic Vella
Collaborators: Prof. Gary Amy, KAUST
Dr Roel Dullens
Dr Scott Tsai, Ryerson University

Part 1 of project completed May 31, 2012

Background

To meet increasing demand for clean water, it is imperative that techniques are improved and new strategies are developed for the desalination of water and removal of heavy metals.

The use of magnetic separation is a popular new technology for the removal of heavy metals from water. In magnetic separation, a wire mesh is used to generate high magnetic field gradients to capture particles. However, the smaller the particle size, the weaker the magnetic force they experience and thus the harder it is to control and trap them. If the particles are too small, the magnetic attractive forces experienced in a field gradient will not be large enough to overcome the random (Brownian) motion of the particles, and so separation should not occur in principle.

However, magnetic separation experiments have yielded results that are better than expected with the ability to collect nano-sized particles [1]. It is believed that particle aggregation (Figure 1) plays a decisive role in the unexpected success of magnetic separation for these small particles.

Techniques and Challenges

To better understand this unexpected behaviour, researchers at the Oxford Centre for Collaborative Applied Mathematics (OCCAM) are developing mathematical models that describe the behaviour of magnetisable particle systems to provide insight and quantitative predictions about the system behaviour.

Upon the application of a magnetic field, magnetisable particles attract one another to form aggregates. A key challenge is to understand the most energetically favourable form of aggregate in a variety of circumstances.

The Future

The mathematical models developed in this project will provide a solid fundamental basis on which subsequent modelling and analysis of the behaviour of magnetic particle aggregates may be developed.

The results of such theories will provide crucial insight in guiding the design and operation of new magnetic separation technologies, leading to faster, more efficient methods for the production of clean water.

References

[1] Yavuz C.T., Mayo J.T., Yu W.W., Prakash A.: Low-field magnetic separation of monodisperse Fe3O4 nanocrystals, Science, 314:964-967, 2006 

[2] Tsai S.S., Griffiths I.M., Stone H.A.: Microfluidic immunomagnetic multitarget sorting - a model for controlling deflection of paramagnetic beads, Lab Chip, 11:2577-2582, 2011 

[3] Tsai S.S., Wexler J.S., Griffiths I.M., Stone H.A.: Separation of magnetic beads in a microfluidic device - modelling and experimentation, In Proceedings of the ASME 2011 International Mechanical Engineering Congress & Exposition IMECE2011, 2011