Magnetic separators can be found in most mineral processing operations, especially those processing non-metallic minerals and magnetic ores. This article investigates the use of high intensity magnetic separators in the minerals sector with a focus on processing dry materials (in the -15mm, +45 micron size range).
Principles of Magnetic Separation
There are three magnetic properties of minerals. Each type enables separation using magnetic technology and are:
- Paramagnetic: Minerals that are only slightly affected by an applied magnetic field. They move towards concentration in lines of magnetic flux (e.g. hematite, ilmenite, and chromite).
- Ferromagnetism: Minerals capable of achieving a high degree of magnetic alignment (e.g. magnetite).
- Diamagnetism: A mineral (e.g. silica) that is very weakly repelled by the pole of a strong magnet. When a magnetic field is applied, a diamagnetic mineral will develop a magnetic moment through induction but in the opposite direction and is therefore repelled.
The magnetic separation of minerals is based on a three-way competition between:
- Magnetic forces;
- Gravitational or inertial forces;
- Inter-particle attractive and repulsive forces;
The combination of these forces determines the outcome of any given magnetic separation and is much affected by the nature of the feed such as size distribution, magnetic susceptibility and other physical and chemical characteristics.
The control variables to optimise separation efficiency can be summarised as follows:
- Volume of Particle (V) (function of feed particle size distribution).
- Magnetic Susceptibility of Particle (K) (a function of the mineral type and liberation).
Machine Operation Variables
- Magnetic Field Strength of Separator (H) (Machine Design Parameter)
- Magnetic Field Gradient of Separator (H/R) (Machine Design Parameter)
The magnetic force F(m) generated on a paramagnetic particle in a magnetic separator is given by:
F(m) = V.K.H.H/R
For a successful separation, the magnetic force F(m) must be able to move the paramagnetic mineral from its natural path immediately after leaving a separator, or lift it from a belt (overcoming gravity) in the case of a Disc type magnetic separator.
Magnetic Separator Designs
These principles are used in the design and application of 3 types of magnetic separator used for the dry processing of mineral deposits.
Permanent High Intensity Magnetic Roll Separator (MASTEROLL)
The Rare Earth MASTEROLL uses Neodymium Iron Boron permanent magnets (the most powerful permanent magnets available) built into a composite high-intensity magnetic head pulley. High separation efficiencies are generated by:
- The design of the magnetic roll assembly (using high grade neodymium magnets);
- The optimum pole spacing to generate high magnetic field strengths and magnetic field gradients;
- The optimum combination maximises the magnetic force exerted on a paramagnetic particle as it passes over the roll;
Fig 1: Principle of operation of MASTEROLL Magnetic Separator.
Non-magnetic material is discharged forward of the roll, in a natural trajectory. Any magnetic particles are influenced by the roll’s magnetic force and are discharged on the underside of the roll into a separate chute. Separation trajectories are set by adjusting the conveyor speed using an inverter control and adjusting the splitter chutes. The captured material can be captured in 3 collection areas or chutes:
- Middlings (weakly or paramagnetic minerals);
- Magnetics (ferromagnetic minerals);
Magnetic rolls are available in 75mm, 150mm and 200mm diameters, up to a width of 1 metre. Multiple configurations of rolls are available, giving the non-magnetic fraction a further pass for improved product purity or recovery. The MASTEROLL can process a wide size range of material ranging from 75 microns up to 15mm. Although, as with all physical separation processes, a tight control of the particle size range improves separation efficiency.