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Heavy media gravity separation involves using a mixture of fine media material, such as magnetite (SG 5.1) or ferrosilicon (SG 6.8), suspended in a slurry of water, to produce a media slurry with a specific gravity that will allow low density material(s) to float, and other high density material(s) to sink. T he SG of separation is the SG of the media slurry, and can range from 1.45 (for coal), to around 2.8 (for diamonds). The separation principal is very simple, if the "liquid" media has a SG of 2.5, every mineral with a SG greater than 2.5 will sink and those lighter than 2.5 will rise to the top and float.  Very early media separation used sand mixtures and water to separate coal from rocks such as shale and sandstone, however the sand was very difficult to recover and much was lost, so these early media vessels (called Chance Cones) lost favor quickly. Other areas where heavy media systems are used include iron ores, zinc, garnet, magnesite, lead and fluorspar. This system is limited by the maximum specific gravity of the media solution, since with too many solids the slurry becomes more plastic and less fluid. Static heavy media separation vessels comprise the majority of separations, and include the Wemco drum and cone vessels, the McNally lo flow vessel used in coal and a host of other vessels, which form a structure where separation between two materials of differing densities can take place. Generally speaking, the light density material must differ from the sink material density by a difference of 1 specific gravity unit. It trying to separate diamonds (SG 3.5) from Kimberlite, (SG 2.5), there is a difference of 1, so it would be appear to be possible using the heavy media process. And, in fact it does work, since most new diamond mines use heavy media vessels to separate the heavy fraction containing diamonds from the low density material, Kimberlite. Typically the size range of ore treated in heavy media separation ranges from 10 mesh to 8 inch, and sometimes down to 28 mesh in particle size. The particle size of the media is also important, since this affects the separation size. Generally a good grade of media of magnetite or ferrosilicon needs to be 90% to 100% -325 mesh. Most heavy media processes are similar, in that the pre screened feed is fed to the heavy media vessel, which has the required media specific gravity (separation SG), the light density material floats and is discharged from the float section of the vessel, while the higher SG material sinks and is discharged from the sink section of the vessel. The material is discharged on drain and rinse screens, which have the first few feet partitioned in the fines pan, to separate the material that drains through from the remainder of the screen. This is the undiluted media, since it is separated before the rinse spray bars wash the material of the media. The washed media needs to be re-concentrated and is sent back to the magnetic separator circuit for recovery of the media. From the magnetic separator, the media reports to the densifier or the media thickener, where it is pumped back to the heavy media separator to maintain the proper SG of the vessel and replace the media continuously lost with the discharge of the float and sink material. Generally, if a ore can be beneficiated in a heavy media system, it is cost effective, since most of the media is reused, and typical losses is only a pound of media per ton of ore processed. The cost of heavy media systems is more than that of tables of jigs, but is has a much finer degree of separation and can process a wide range of particle sizes. The US Bureau of Mines, before their demise, did a lot of research into the feasibility of grinding the magnetite to several microns in particle size and attempting to recover -100 mesh material in the heavy media cyclones (a dynamic heavy media system). They had mixed results, which looked promising, but I am not aware of any commercial plant using this process today, although there could be one somewhere that I am unaware of. The link below is from a heavy media pilot plant for the beneficiating of diamonds that I designed for a company that had some Kimberlite pipes in Africa, which were diamond bearing. They are in adobe acrobat reader format. Diamond Plant Heavy Media Flowsheet, heavy media separator Diamond plant heavy media flowsheet, media recovery Information provided by Charles Kubach, Mining and Mineral Processing Engineer |
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