Gravity concentration can be defined as the process by which particles of different sizes, shapes, and densities are separated from each other by the force of gravity or the centrifugal force. Gravity concentration, one of the oldest mineral beneficiation processes, has evolved over the decades and continues to play an important role in modern mineral processing operations especially for ores that are difficult to beneficiate using froth flotation; such as tin, tungsten, niobium/tantalum, iron, industrial minerals and oxidized coal.
Types of gravity separators
Since the dense medium cyclone (DMC) was first patented in the 1940s it has become the process of choice in coal preparation, and is also widely used for upgrading iron ore and in the pre-concentration of diamonds and metalliferous and industrial minerals.
The mineral jig has been widely used in recovering valuable heavy minerals such as gold, platinum, tin, tungsten and lower density minerals, gemstones such as diamond and sapphire from alluvial or placer deposits. Jigs have also been widely used in hardrock mill applications for recovering of coarse heavy minerals liberated in open or closed grinding circuits. Base metals, iron, manganese ores and barite have also been recovered using jigs.
The shaking table is one of the most versatile gravity concentration devices and is found in most types of gravity separation plants. However, its major use is in the coal industry, where especially enlarged units are available, and in the recovery of metal oxides-tin, tungsten, tantalum, and chrome.
Spiral concentrators are simple low energy-consuming devices that separate minerals mainly on the basis of density or by shape. It is developed for concentration of low-grade ores and industrial minerals in slurry form. Spirals are widely used in mineral processing as a method for pre-concentration and have proven to be metallurgically efficient and cost-effective. Spiral concentrators are used for a number of mineral processing applications. They’re extensively used to process heavy mineral sand deposits, including monazite, zircon, ilmenite and rutile deposits. More recently, spiral concentrators have been widely used to recover fine coal as well as gold, iron ore and other minerals.
The Gold Centrifugal Concentrator is a brilliant gravity separation equipment widely used in the mining industry, especially for the placer gold, fine grain rock gold, and other precious minerals. Centrifugal separators are a promising way to increase the extraction of fine particles of precious metals from loose ores and tailings. Due to the high speed of rotation of the centrifugal separator cone, it is possible to separate particles of small sizes.
Advantages and disadvantages of gravity concentration
Gravity concentration, where applicable and effective, has the lowest installed and operating costs when compared with other beneficiation technologies. It also tends to have the lowest environmental impact as gravity concentration does not require the use of chemicals and reagents.
One of the main problems of gravity concentration processes has been its limitation in treating particles in relatively fine size range. In the fine size ranges the fluid and viscous forces become dominant relative to the gravity and this in turn affects the separation efficiency.
Applications of gravity concentration
Gravity concentration, or density-based separation methods, declined in importance in the first half of the twentieth century due to the development of froth flotation, which allowed for the selective treatment of low-grade complex ores. They remain, however, the main concentrating methods for iron and tungsten ores and are used extensively for treating tin ores, coal, gold, beach sands, and many industrial minerals. Gravity concentration can remain an attractive option only if it can be implemented with low capital and operating costs. It finds diverse applications in the treatment of coal, beach sands, iron, gold, diamonds platinum, baryte, fluorspar, tin, tungsten ores etc.
Dense-medium separators have proven to be the most efficient processes for removing the undesirable gangue material from run-of-mine coal. Coal dense medium separation is a popular beneficiation process used for the upgrading of coal ore into power station and metallurgical coal. Many coal preparation plants use a heavy medium cyclone to re-jigger rough cleaned coal or broken medium coal, so as to improve the production of fine coal, reduce equipment wear and production costs.
Gravity concentration is a process to concentrate the mineral of interest using the difference of specific gravity of gold and gangue minerals. The specific gravity of gold is 19.5 and the specific gravity of quartz (the common gangue mineral associated with gold) is 2.65 (i.e., gravity concentration works because gold is heavy, and quartz is light). Gravity causes gold to settle in water faster than silica and other gangue minerals. Gravity concentration spirals are optional in gold processing, used for carbon degritting.
Platinum group metals (PGMs), together with gold and silver, are classified as noble or precious metals because of their high corrosion and oxidation resistance. The PGM industry faces challenges with increasing chrome content in the feed. Gravity concentration solutions are used to separate chrome from a stream that has already been classified from the main platinum streams by a desliming screen.
Gravity concentration is widely used in iron ore processing, especially weak magnetic type. Other applications can be found in the pre-concentration of hematite by spirals, the concentration of beach sands and weak magnetic minerals by shaking table for finer concentrate, the effective recovery on fine grade iron by centrifugal concentrator ore, etc. Following a range of screening processes, large assemblies of spiral concentrators are located downstream of a cyclone cluster and are used in classifying iron ore fines from waste.
Chromite ore is the main source of chromium metal, chemical and refractory. During the beneficiation of chromite ore, about 50% (by weight) of the total feed is discarded into tailings, which consist of huge quantity of the chromite values. Chromite values from ultrafine tailings of chrome ore beneficiation plant can be recovered by right combination of process parameters of spiral concentrator.
Mineral sands are old beach, river or dune sands that contain concentrations of the important minerals, rutile, ilmenite, zircon and monazite. These 'heavy' minerals have a relative density of between 4 and 5.5 g/cm3 and are much heavier than common sand minerals such as quartz which has a density of around 2.65 g/cm3. Garnet, magnetite, sapphire, diamond and staurolite are also mined from some mineral sand deposits. Spiral concentrators play an important role in the typical mineral sand processing plant.