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Gravity Separation

time：2015-10-10 15:45:03  Click number：2368

Gravity separation is the separation of two or more minerals of different specific gravity by their relative movement in response to the force of gravity and one or more other forces (such as centrifugal forces, magnetic forces, buoyant forces), one of which is resistance to motion (drag force) by a viscous medium such as heavy media, water or, less commonly, air.

Gravity separation is one of the oldest technique in mineral processing but has seen a decline in its use since the introduction of methods like flotation, classification, magnetic separation and leaching. Gravity separation dates back to at least 3000 BC when Egyptians used the technique for separation of gold.

It is necessary to determine the suitability of a gravity concentration process before it is employed for concentration of an ore. The concentration criterion is commonly used for this purpose, designated  in the following equation (where  represents specific gravity):

• for CC > 2.5, suitable for separation of particles above 75 micron in size
• for 1.75 < CC < 2.5, suitable for separation of particles above 150 micron in size
• for 1.50 < CC < 1.75, suitable for separation of particles above 1.7 mm in size
• for 1.25 < CC < 1.50, suitable for separation of particles above 6.35 mm in size
• for CC < 1.25, not suitable for any size

Although concentration criteria is a useful rule of thumb when predicting amenability to gravity concentration, factors such as particle shape and relative concentration of heavy and light particles can have a dramatic impact on separation efficiency in practice.

There are several methods that make use of the weight or density differences of particles:

• Heavy media or dense media separation (these include the Sepro Condor DMS, baths, drums, larcodems, dyana whirlpool separators, and dense medium cyclones)
• Shaking tables
• Spiral separators
• Jig concentrators are continuous processing gravity concentration devices using a pulsating fluidized bed.(RMS-Ross Corp. Circular Jig Plants)
• Centrifugal bowl concentrators, such as the Knelson concentrator and Falcon Concentrator
• Multi gravity separators (Falcon Concentrator, Knelson, Mozley (Enhanced Gravity Separator), Salter Cyclones (Multi-Gravity Separator) and the Kelsey Jig)
• Sluices

These processes can be classified as either density separation or gravity (weight) separation.

In dense media separation a media is created with a density in between the density of the ore and gangue particles. When subjected to this media particles either float or sink depending on their density relative to the media. In this way the separation takes place purely on density differences and does not, in principle, relay on any other factors such as particle weight or shape. In practice, particle size and shape can have an impact on separation efficiency. Dense medium separation can be performed using a variety of mediums. These include, organic liquids, aqueous solutions or suspensions of very fine particles in water or air. The organic liquids are typically not used due to their toxicity, difficulties in handling and relative cost. Industrially, the most common dense media is a suspension of fine magnetite and/or ferrosilicon particles. An aqueous solution as a dense medium is used in coal processing in the form of a belknap wash and suspensions in air are used in water-deficient areas, like areas of China, where sand is used to separate coal from the gangue minerals.

Gravity separation is also called relative gravity separation as it separates particles due to their relative response to a driving force. This is controlled by factors such as particle weight, size and shape. These processes can also be classified into multi-G and single G processes. The difference is the magnitude of the driving force for the separation. Multi-G processes allow the separation of very fine particles to occur (in the range of 5 to 50 micron) by increasing the driving force of separation in order to increase the rate at which particles separate. In general, single G process are only capable of processing particles that are greater than approximately 80 micron in diameter.

Of the gravity separation processes, the spiral concentrators and circular jigs are two of the most economical due to their simplicity and use of space. They operate by flowing film separation and can either use washwater or be washwater-less. The washwater spirals separate particles more easily but can have issues with entrainment of gangue with the concentrate produced.