Gravity concentration was the dominant mineral processing method for about two thousand years. and it is only in the twentieth century that its importance has declined, with the development of such processes as flotation, magnetic separation and leaching. Gravity concentration is still the main method to treat most iron, tungsten, and tin ores, beach sands, and some rare metal ores such as tantalum and niobium. Gravity also plays an important role in the production of gold because of its high specific density(19.3) and natural occurrence as liberated species.
The gold concentrates produced by gravity equipment such as Knelson Concentrators, jigs and spirals are called gravity concentrates, and are normally pumped to a secured area, the gold room, for further upgrading. The tails and middlings of the gold room cleaner are also concentrates of a sort, as they generally contain liberated gold at a significantly greater concentration than in the ore itself.
Upgrading of the gold is achieved by rejecting gangue minerals and tramp metals (steel and copper) whilst retaining free gold particles. For gold-bearing sulphide ores, the main gangue minerals in gold gravity concentrates are pyrite and arsenopyrite, with small amounts of magnetite. For oxide ores, the gold concentrates may contain some heavy minerals, such as cassiterite, magnetite, ilmenite, rutile, zircon and monazite.
Tramp steel can be rejected by magnetic separation, and tramp copper, usually blasting wire, can be washed out of the final concentrates using nitric acid. Gangue minerals, however, are typically rejected using gravity methods. This proves particularly challenging for higher density minerals, such as sulphides and some oxides.