Diamond Mining

In the conventional sawing and drilling of a diamond, diamond particles continuously chip the stone at high speed. There are occasions when this method fails, such as when a hard 'knaat' is encountered, an average packet of rough diamonds contains about three to five per cent knaats. If the knaats are more than about one millimetre in size within the rough stone, they could be seen and so rejected. Smaller inclusions are dangerous, as they could damage the saw blades or the scaifes. Again, a hard inclusion might cause vibration of the thin saw blade and shatter it, unless the cutting is slowed down so much that a five carat knaat takes several weeks to cut. Laser drilling and sawing overcomes these problems. In addition, unlike the conventional saw or drill, there is no physical contact between the laser hardware and the stone. Consequently, oil residues or material of the saw blade do not contaminate the stone.

When the focused laser beam hits the stone at a very small spot and the beam energy is absorbed, the temperature at the target spot shoots up to more than 3000C. The heating is extremely fast and so is the cooling, beacuse of the pulsed nature of the laser light. Consequently, the rest of the stone is not heated by the laser beam. To increase the absorption of the laser light by the diamond, a spot of graphite is first apllied to the entrance point of the desired hole. As the beam hits the stone this graphite spot absorbs all the energy of the laser and evaporates. The diamond below the spot becomes graphite locally because of the high temperature and, in turn, provides an absorptive spot. The diamond is therefore cut by evaporation and not by abrasion. The hole produced by the beam is V shaped and as it gets deeper, it becomes wider. However, good design of the optics of the laser system can narroe the diameter of the hole to less than ten microns. The weight loss of the diamond by laser sawing is therefore much less than by conventional sawing, provided special care is taken. Twinned or grained crystals of diamond are cut in a few hours instead of the weeks required by conventional sawing.

Laser sawing is therefore superior to conventional methods, as the cuts can be made irrespective of crystallographic direction. Accuracies of twenty-five microns are routine, limited only by the movement of the holder and vibrations of the table. When a laser is employed for kerfing, the kerf can be as thin as 0.3 micron. Kerf depths of four-hundred micron are achieved in about twenty-five seconds.

The laser beam is produced by flashing light from xenon flash tubes on to a rod of laser material. The rod absorbs this energy and emits it as an exceedingly parallel and coherent beam. The laser material is a solid rod of ruby, Neodymium glass (Nd glass) or Neodymium aluminum garnet (NdYGA,) with accurately parallel and polished faces. Carbon dioxide gas in a quartz tube also acts as a laser material but the focusing is not as fine as with a solid rod. A serious problem in all high power lasers is the overheating of the laser material and the choice of the laser equipment depends on the use that is made of the system.

A typical laser drilling or cutting machine used for diamond processing would have a pulsed NdYAG emitting light of wavelength of 1060 nm, with a peak power one to twenty five kilowatt and a pulse width of 0.2 microseconds. The repetition rate of the pulses is ten to ten housand per second. Other factors affect laser drilling, of holes than the minimal requirements of power density. Not only are the diameter and the depth of the hole dependent on the wavelength of the laser light, but they are also determined by the mode of the laser output. The lowest order mode and the shortest wavelength give the sharpest focussed spot.

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