Magnetic Separation

Magnetic separations take advantage of the magnetic properties of minerals. All minerals will have one of three magnetic properties: ferromagnetic, paramagnetic, and diamagnetic. Ferromagnetic minerals (i.e., magnetite and pyrrhotite) are magnetic and are easily separated from other minerals, since they will be attracted to the poles of a magnet. Paramagnetic and diamagnetic minerals are not magnetic, but differ in how they interact with magnetic fields. Paramagnetic minerals are weakly attracted whereas diamagnetic minerals are weakly repelled along lines of magnetic forces. Thus, if a mixture of paramagnetic and diamagnetic minerals is passed through a magnetic field; the paramagnetic minerals will be pulled into the field and the diamagnetic minerals will be repelled or separated from the field. Furthermore, paramagnetic minerals have different degrees of paramagnetism that can also be used to effect separations.

Magnetic fields of various intensities can be provided by permanent or electromagnets. Generally, magnetic separators are classified as low or high intensity and whether they work in wet or dry applications. In concurrent separators, slurry flows in the same direction as the rotation of the drum surrounding the magnets.

Separation and Concentration Techniques

The separation and concentration of the valuable mineral can take place after the ore is crushed, ground, and classified into the required particle size distribution. There a number of different techniques are employed in concentrating the valuable minerals. These techniques exploit differences in physical or chemical properties of the valuable and gangue minerals.

Basically, there are four kinds of separation and concentration techniques:

i. Sorting – based on appearance, colour, texture, optical properties and radioactivity

ii. Gravity and Dense-Medium Separation – Separation based on specific gravity of the valuable mineral relative to the gangue and the carrying medium such as water. In dense-medium separation, the a carrying medium is a mixture of water, magnetite, or ferrosilicon. The paramagnetic properties of the medium allow it to either remain in suspension at a predetermined slurry density or to be separated from water for cleaning and reuse.

iii. Magnetic Separation – separation based upon natural or induced differences in magnetic susceptibility of the minerals within the ore.

iv. Froth Flotation – separations based on the surface chemistry properties of a mineral. The natural or modified surface property of the mineral determines its ability to attach to an air bubble and float to the surface.

Physical Mineral Concentration Methods

Physical Mineral Concentration Methods :

l. Separation dependent on optical and radioactive properties of minerals, i.e. hand pickling, optical sorting, radioactive sorting, etc.

2. Separation dependent on specific gravity (density) difference of minerals, i.e. heavy-media separation, gravity concentration by use of tables, jigs, cones, etc.

3. Separation utilizing the different surface properties (i.e. surface chemistry) of the minerals, i.e. froth flotation, etc.

4. Separation dependent on magnetic properties of the minerals, i.e. low and high, dry and wet magnetic separation, etc.

5. Separation dependent on electrical conductivity properties of the minerals, i.e. electrostatic separation, etc.

So mineral processing is concerned mainly with the physical methods of separation of minerals.

Introduction of Magnetic Separation

Dry low-intensity magnetic separation is confined mainly to the concentration of coarse sands which are strongly magnetic, the process being known as cobbing, and often being carried out in drum separators. Below the 0.5 cm size range, dry separation tends to be replaced by wet methods, which produce much less dust loss and usually a cleaner product. Low-intensity wet separation is now widely used for purifying the magnetic medium in the dense medium separation process, as well as for the concentration of ferromagnetic sands.
Read more