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Mineral Processing in the mining industry can be defined as the economic recovery of a valuable commodity from a mineral deposit. In the case of De Beers, diamond extraction is our core business.
De Beers recovers diamonds from both kimberlite (type of rock in which diamonds are found) and alluvial deposits. The extraction process can be broken down into four primary areas.
| COMMINUTION AND LIBERATION |
Comminution is essentially the breakage and fracture techniques used to reduce the size of the ore particles in the feed material and in the process to liberate the entrained diamonds. Over the past number of years De Beers has worked closely with comminution equipment manufacturers to develop processing equipment which is effective in the reduction of particle size without damaging the diamonds. In most diamond recovery plants, a re-crush stage is added, after the concentration stage. This allows for the liberation of the smaller diamonds after the larger ones have been removed.
The Concentration step, also referred to as Dense Medium Separation (DMS), in a diamond plant utilizes the relative density differential between the diamonds and the host rock to separate the diamonds into a small parcel of high density material. The feed material is mixed with a suspension of finely ground ferrosilicon slurry at a density of approximately 2.65g/cm3. The mixture is pumped or gravity fed into a cyclone (a dynamic separator which uses centrifugal force to effect separation) which produces a concentrate containing diamonds and high density material. The ferrosilicon is recovered on floats and sinks screens for re-use.
Concentrate from the DMS plant is then processed through the Recovery section. In this section, the specific properties of diamonds are used to recover the diamonds from the concentrated material. These properties include; magnetic susceptibility, X-ray luminescence and crystallographic laser fluorescence. The concentrate will either be dried or processed wet through a magnetic separation process. The non-magnetic fraction is subjected to an X-ray recovery process. Under X-ray radiation, the diamonds absorb energy which is given off in the form of light. This light is detected, enabling the diamonds to be ejected from the process stream. After X-ray recovery, the material will undergo a final round of diamond recovery and concentration. Single particle Laser Raman sorters are used to concentrate the diamonds to greater than 95% diamond by weight. The Raman sorter utilizes the unique molecular signature of the diamond to distinguish them from the luminescent gangue material. The final diamond concentrate is packed and weighed prior to being shipped to the final sorting, cleaning and valuation facility.
| PROCESS REJECT AND SLIMES HANDLING |
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Process reject can be defined as either slimes or coarse reject. Fine processed material is densified in thickeners in the process plant with the recovered water being re-used as process water. Flocculant is added to the feed slurry to assist with the settling of the fine material. The process reject fractions are generally disposed of together with the coarse reject being used to build the containment walls of the rejects disposal area. Thickener slimes from the process plant will be deposited within cells created in the containment area. All chemicals used in the mining operations are benign (ferrosilicon and flocculant) and do not emit toxic effluents into the environment.
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