Processing Diamond 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 bottom and are pushed toward an exit point, while lighter waste rises to the top and overflows as a separate stream of material.

A working grease table at Longlands, Cape Province, South Africa. The surface of diamond is highly unusual in that it resists being wetted by water but sticks readily to grease. Here, wet gravel washes across 3 inclined surfaces covered with beeswax and paraffin. 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 diamonds are removed in a strainer. More automated systems use a rotating grease belt and scraper.

This diagram shows how 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 on a 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 triggers a jet of air, deflecting the diamonds (red) into a collector bin. Adapted from Bruton (1978)