Finding diamonds in the rough

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A sorting plant is using optics to sort diamonds from the bedrock they were mined from, and can even detect the particularly valuable pure white and green gems.

A sorting plant is using optics to separate diamonds from the bedrock they were mined from, and can even detect the particularly valuable pure white and green gems.

A diamond’s sparkle is down to its strong light refraction which disperses visible light. This attribute is being put to used by researchers at the Fraunhofer Institute for Information and Data Processing (IITB). A new optical sorting method identifies the gems among 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 is connected to 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’, said 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’, said Struck.

The diamond extraction method has been in operation in two mining regions in South Africa since the beginning of 2006. The plants sort several tonnes of rock per hour and identify diamonds as small as 0.6mm in diameter, making the new technology 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’, said Struck.