Dia Met’s diamond pipe may be one of several — (Part three

Kimberlite pipes are uncommon but they rarely occur singly. If one is discovered, there will almost certainly be others in the vicinity.

The pipes form patternless clusters seemingly devoid of any geometric control. A similar chaotic arrangement would result if a handful of pennies were thrown into a gusting wind from a 20-storey building.

The pipes are roughly circular in plan view, 2-200 hectares in area and carrot-shaped in section. They pinch out at depth and the minable portion will depend on how much of the upper pipe has been eroded.

Some pipes are minable for only a few hundred metres. The Orapa pipe in Botswana, on the other hand, is estimated to be technically minable to a depth of 2,000 metres.

Initial mining by open pit is universal. Once the pit reaches its economic limit, underground mining takes over. High productivity block caving or sub-level caving methods have been used exclusively in South Africa since the mid-1950s.

In terms of the weight of valuable mineral in the ore, diamond-bearing kimberlite is the lowest-grade rock mined anywhere in the world. An ore containing 20 carats per 100 tonnes is equivalent to only four parts per 100 million, yet diamond recoveries exceeding 90% are common.

By comparison, 0.15 oz.-per-ton gold ore is 128 times more concentrated at a gold content of 5.1 parts per million.

Without a diamond’s high specific gravity (3.52), its property of fluorescing in X-rays and affinity for grease, today’s high recoveries would be impossible. (“Fluorescence” means “a phosphorescent glowing.”)

Underscoring the efficiency of modern methods, De Beers is currently re-treating old dump material at Kimberley and recovering 18 carats per 100 tonnes.

Rough diamonds are one millimetre in size and upwards. Consequently, there is no need for milling the ore. Crushing is usually to about 25 mm, which is sufficient to expose and/or liberate the stones.

If the mine produces large diamonds from time to time, the ore is crushed in a series of stages. Large stones are identified by optical scanning and recovered before the stream of rock reaches the next crusher.

After crushing, most of the waste rock is scalped by gravity processing (heavy-media). The resulting concentrate of diamonds and other heavy minerals is further upgraded by an X-ray separator.

Other processes such as electrostatic or magnetic separation may then follow depending on the nature of the new concentrate. Finally, the diamonds are picked out by hand.

X-ray separation is not fully effective on diamonds less than 1.5 mm in size. In such cases, the time-honored grease table is used. The ore pulp is delivered to a moving belt covered with a thick layer of specially formulated grease. (Vaseline was used at one time.) The diamonds stick to the grease, and the waste flows to discharge.

The question uppermost in the minds of many interested parties is, do synthetic diamonds pose a threat to natural, gem-quality diamonds? No, say the experts.

Synthetic diamonds have been manufactured since 1954 and the demand for them is insatiable. But they are exclusively for industrial use.

Clear, synthetic stones weighing up to a carat have been made experimentally. Pressure of 725,000 lb. per square inch and 1,200C are required and a single, 1-carat stone will take a week to manufacture. The cost is astronomic.

Manufactured industrial diamonds are dark-colored, contain metallic inclusions and are relatively inexpensive. They range from 0.75 mm to dust sizes. A 0.

1-mm grit is in heavy demand for bonded, grinding wheels.

Larger stones, such as those needed for cutting tools and the crowns of drilling bits, are natural, mined stones.