A basic triboelectric series for heavy minerals from inductive electrostatic separation behaviour

Electrostatic separation is used in many heavy mineral sand operations for the separation of minerals, particularly where other separation means (density, size, magnetic susceptibility) are less effective due to close similarity in such properties, as is experienced with minerals such as rutile and zircon. Electrostatic separation is achieved by exploiting differences in mineral conductivity giving rise to differences in particle charging and discharging rates. By the appropriate design of equipment an efficient separation can be accomplished. A good review of the fundamentals relating to electrostatic separation is given by Manouchehri et al.1 There are three major mechanisms for particle charging in commercial electrostatic separators: corona or ion-bombardment charging, inductive charging, and triboelectric charging to a lesser extent. In practice, inductive separation techniques experience some triboelectric charging effects as the overall induced charge level is generally lower than that of corona charging; the frictional effects between particles and the separator surface are therefore more significant. Triboelectric charging is a complex process caused by the frictional contact between dissimilar materials with differing filled electron levels.1–2 In simple terms this results in a flow of electrons to equalize electron levels across the two materials while in contact. When the surfaces are separated the charge may be retained by the two materials, with the net result that one material retains a negative charge while the other a positive charge. The extent of charging is dependent on many factors, including the speed and pressure of the contact, ambient conditions, as well as surface contamination such as coatings. The effect of triboelectric charging during inductive electrostatic separation can be determined by changing the polarity of the induced field and comparing mineral recoveries under both polarity regimes. The objective of this work was to use this method to better understand the triboelectric behaviour of minerals in the Richards Bay Minerals (RBM) deposit, with the potential that this information could lead to improved electrostatic separations.