The grease table

The grease table is a remarkable invention, discovered by F. Kirsten in 1897. Kirsten was an employee at de Beers and when washing some ores containing diamond he accidentally spilt the contents of his wash-pan, which contained both diamond and crushed ore, over the well-greased axle hub of A cart-wheel. He noticed that the diamonds stuck to the grease, but not the ore. He thereupon designed a simple inclined table of corrugated iron and thickly coated this with grease. When diamond-containing crushed ore concentrates were mixed with water and then washed over the tables (which were shaken), the diamonds were found to be trapped on the grease, whilst everything else rolled off with the water stream. It soon transpired that the trapping action was extraordinarily efficient and almost all diamonds (to within less than 1 %) were trapped within the first 12 inches of grease.

This grease-table system is now used on a big scale, the tables being mechanically vibrated. Every hour the grease is scraped off (with diamond adhering) and packed in cans covered with gauze and then heated. The grease melts, is removed and re-used, the diamonds accumulating in the can.

The saving in labor, compared with the old hand-sorting methods of last century is enormous. The grease table acts because of a curious property of diamond surfaces. Neither diamond nor grease are wetted by Water, so the diamonds adhere to the grease. All other minerals are effectively coated (in the water) by a water layer and so slide off the sloping grease table, assisted by a jerking vibrating movement. The modern grease table is an aluminium sheet 4 feet wide, with steps on it, and with surface sloping down at 15 degrees to the horizontal. Petroleum jelly is the best grease medium. Used grease is melted down and a ¼-inch-thick layer first spread over the table. On this is usually spread a thin layer of new refined jelly.

We have described above the mining and extraction of diamonds later used for diamond jewelry, diamond wedding bands from the great South African deep pipes, those which are under the control of the de Beers Group. These mines constitute a most important source of gems and also, of course, produce a large quantity of industrial diamond. A great deal of capital is sunk into these mines. The de Beers Group has an annual production of diamond worth over £50 million. The Premier mine alone during the past 60 years has yielded nearly 200 million tons of blue ground, and clearly the excavating, hauling and processing of all this is a major engineering feat. It has produced perhaps 10 tons of diamond. Yet, in terms of total output, more diamonds are recovered from the great alluvial deposits, especially those of South West Africa, Congo, Orange River and Angola. Both the mining methods and the recovery techniques for alluvial diamonds are entirely different to those used in deep pipes, and in the next section we shall briefly review these.

These diamonds appear either in what are called marine terraces, or in river-beds. For our purpose we can classify both types as alluvial deposits. Secondary alluvial deposits are also mined, off-shore from the sea-bed itself, these being called 'marine' diamonds.

MARINE DIAMONDS

Much publicity of late has been devoted to the new marine method of extraction of diamond. This has not proved very profitable, however. In connection with marine terraces and with river-bed sources, it was evident that such sources did not automatically stop at coast-lines; there must be some diamondiferous-bearing gravel off-shore on the sea-bed itself. Such marine diamond 'fields' have been sought out and are being worked. Special ships have been designed and built. Huge suction pumps have been installed, and the gravel front the sea-bed is sucked up, sorted through and then discharged some distance away in order not to foul the region being worked.
There are great difficulties in the way. Only in calm weather can the great suction tubes be trailed on the sea bottom. The depth to be 'fished' is limited by the weight of the suction tube. The tubes are constantly jamming through being blocked by large boulders loosened in the operation. The results of the sea mining are disappointing in terms of the costs of the operations. Diamonds are certainly recovered, as predicted, but the volume acquired makes the operation work at a loss, In spite of this, the method is being pursued doggedly.