Gravity separation, or gravity concentration, is an important mineral processing technique that exploits the differences in density between various minerals in order to separate them for further processing. Gravity separation is typically used in the beneficiation of tin, tungsten, gold, iron, lead, zinc, and other metals.
Gravity separation is based upon density differences between the mineral of interest and the gangue waste material. A difference in density between the two will allow for the proper application of a force field, usually in the form of gravity, to effect a separation. Separation techniques are based on the densities of minerals and their relative motion in a force field. Gravity separation is typically used in the beneficiation of ores with complex mineral assemblages, where the presence of gangue or underlying rock makes classical chemical extraction stages difficult or uneconomic.
Gravity separation processes can be classified into two distinct categories: free-fall concentration and hindered-settling concentration. Free-fall concentration makes use of the law of gravity, in which two particles of different densities will naturally have different trajectories under the influence of gravity, and thus can be separated. On the other hand, hindered-settling processes use a dense medium in which the particles of interest are separated from the gangue surfaces.
In the concentrating process, a force is applied that accelerates the settling of the heavier particles. In free-fall concentration processes, gravity is the dominant force, while hindered-setting processes make use of other forces, such as buoyancy and drag.
Free-fall concentration processes consist of processes such as jigging, tabling, and spiraling. In jigging, the most important acceleration force applied is gravity. Jigging utilizes a pulsating gravitational field and creates a stratification in which the more dense particles sink to the bottom, leaving the light particles to be carried by a stream of water at the top. Tabling is similar to jigging except that it uses a horizontal table vibrated in the longitudinal direction at a low frequency and is often used for the separation of heavy minerals. Lastly, the spiral concentrator makes use of an inclined ramp in the form of a helix to achieve separation.
Hindered-settling is a more complex process in which the particles are separated in a dense medium, usually a suspension of liquid and solid phase of a specific density that serve as a barrier between two particle streams. An external force, such as gravity, is used to continuously accelerate the settling of the heavier particles in the medium. Common hindered-settling processes include dense-media separation (DMS) and the Reichert cone process.
DMS is used to separate minerals based on their density by subjecting them to a concentrated suspension of fine, low-density particles that create a barrier between two particle streams, allowing the separation of the denser minerals from the less dense minerals. The Reichert cone is an inclined, spinning cone with variable sluice size and other features that are designed to achieve a higher separation efficiency than DMS.
Gravity separation is an efficient mineral processing technique that is used in a wide range of applications, from processing coal and other minerals, to separating heavy minerals from aggregates. The technique is advantageous because it is inexpensive and does not require a great deal of energy. It is often used in the beneficiation of ores with complex mineral assemblages, where the presence of gangue minerals makes chemical extraction difficult or uneconomic.
