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Diamonds off the conveyor belt

Fraunhofer-Gesellschaft Zur Forderung Der Angewandten Forschung E.V. : 19 December, 2006  (Technical Article)
Precious stones are hard to see when they are extracted from the mine: They are hidden among masses of bedrock. A new sorting plant retrieves the precious diamonds and even detects the particularly valuable pure white and green gems.
The ancient Greeks called them the tears of the gods, the Romans saw them as fragments of fallen stars. In no other material is the dispersion of visible light as beautiful as in a diamond. It is precisely this strong ability to refract light that is now being put to use by researchers at the Fraunhofer Institute for Information and Data Processing IITB: A new optical sorting method identifies the coveted gems in the midst of the extracted kimberlite bedrock and separates them out at lightning speed.

At the core of the diamond sorting plant is a high-resolution linescan camera, which, unlike a conventional camera, produces a continuous image rather than sequences of images. The camera faces the fragments of broken rock, which are thrown from a conveyor belt into an intercepting pit. The stones are illuminated from a certain angle as they fall. If the beams encounter a diamond, the light is deflected towards the camera. This registers the flashes of light and transmits a signal showing their exact position to a computer. The computer, in turn, is connected with 200 air jets with valves that can be separately opened and closed. “The computer has 60 milliseconds in which to decide whether or not to actuate a jet in order to blow out a diamond”, states project manager Günter Struck of the Visual Inspection Systems business unit at the IITB. In collaboration with OptoSort, a company based near Itzehoe, the project group has developed the illumination technique and adapted the rapid-response image evaluation process to suit the task in hand. The new sorting plant also uses a special conveyer belt that runs at a constant speed: “We need to be certain that the diamond identified by the camera will arrive at the appropriate air jet at a particular point in time”, says Struck.

The novel diamond extraction method has been in operation in two mining regions in South Africa since the beginning of 2006. The plants sort several tons of rock per hour and identify diamonds as small as 0.6 millimeters in diameter. The new technology is thus faster and more efficient than the traditional X-ray method of diamond sorting. It detects all diamonds except rough black ones. “Instead, though, it retrieves the extremely precious pure white diamonds and the even rarer green gemstones that the X-ray method fails to find”, declares Struck.
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