The Processing of Diamond-Bearing Ore
Once a mining operation yields ore, the diamonds must be sorted from the other materials. This process relies primarily on diamond's high density. An old but effective method is to use a washing pan, which forces heavy minerals like diamond to the bottom and waste to the top. Cones and cyclones use swirling heavy fluids mixed with crushed ore to achieve density separations. With 99 percent of the waste in the ore removed, further separations may use either a grease table or an x-ray separator. Final separation and sorting is done by eye.
Crushed ore is mixed
with a muddy
water suspension, called puddle, and all is stirred by angled rotating
blades in the circular washing pan. Heavier minerals settle to the
and are pushed toward an exit point, while lighter waste rises to the
and overflows as a separate stream of material.
A working grease table at
Cape Province, South Africa. The surface of diamond is highly unusual
that it resists being wetted by water but sticks readily to grease.
wet gravel washes across 3 inclined surfaces covered with beeswax and
Diamonds stick to the grease while wetted waste minerals flow past. The
operator routinely scrapes the material that adheres to the table into
a grease pot, using a trowel. The grease in the pot is melted and the
are removed in a strainer. More automated systems use a rotating grease
belt and scraper.
Cones (left) and cyclones (right) use heavy-media separation. Diamond-bearing concentrate is mixed with a fluid near the density of diamond. Separation occurs in cones and cyclones by swirling the mixture at low and high velocities respectively.In the cone, rotational mixing permits lighter minerals to float to the top and run out as overflow, while diamonds and dense minerals sink to the bottom and are sucked out with a compressed air siphon.
In the cyclone, fast rotation of the suspension drives heavy minerals to the conical wall, where they sink to the bottom and are extracted, while float waste minerals are sucked from the center of the vortex. Cyclones are about 99.999% efficient at concentrating diamonds and similarly dense minerals from the original ore. Adapted from Bruton (1978)
The x-ray separator system acts
thin stream of particles from the concentrate accelerated off a moving
belt into the air, where they encounter an intense beam of x-rays. Any
diamond fluoresces in the x-rays, activating a photomultiplier that
a jet of air, deflecting the diamonds (blue) into a collector bin.
from Bruton (1978)