About Early Methods of Diamond Mining

The diamond-bearing blue earth from the mines is automatically dumped into ore bins and thence conveyed in trucks drawn by endless wire rope and impelled by steam to the depositing floors on the receiving grounds, which are planed and rolled hard as if for use as tennis courts or brick drying floors. The De Beers mine floors are rectangular sections, six hundred yards long and two hundred yards wide, and extend for four miles; each floor holds about fifty thousand truck loads, a full load weighing about sixteen hundred pounds; spread out until about a foot in thickness, such a load covers about twenty-one square feet. In this great area of blue earth lie the invisible diamonds, for, although some of the rough diamonds may be as large as walnuts, persons walking over the blue earth have almost never seen one. Weathering disintegrates the breccia or blue earth, which process is carried and hastened by wheeled harrows drawn by steam traction engines. Rain accelerates this weathering process and drought retards it. The blue ground from Kimberley mine becomes well pulverised in six months, with the favourable condition of a heavy summer rainfall, while the De Beers earth under similar conditions requires a year's time. About five per cent. of the De Beers mine blue ground is intractable; this, in large pieces, is removed to be reduced by crushers and rolls in the method commonly used for mineral ores. When thoroughly disintegrated the blue ground is hauled to the washing machines to enter the first stage of concentration. Automatic feeders supply the washing machines and the wet mixture from them goes through chutes into a revolving cylinder perforated with holes one and one quarter inches in diameter; lumps too large to pass through these outlets emerge from the ends of the cylinders by way of pan conveyor to crushing rolls. The pulverised ground which passes through the perforations is fed into shallow circular pans, where the contents are swept around by revolving arms, tipped with wedge-shaped teeth, on a vertical shaft, which forces the diamonds and other heavy minerals to the outer side of the pan, while the thin mud is discharged near the centre through an outlet into which it is guided by an inner rim. The concentrates go from this process into trucks with locked covers in which they are conveyed to the pulsator, where they are sifted into five sizes, ranging from one sixth to five eights of an inch diameter, and passed into a combination of jigs or pulsators with stationary bottoms covered with screens with square meshes a little coarser than the perforated plates of the cylinders that size the concentrate for the jigs. Upon the jig screens, a layer of leaden bullets for the finer sizes and of iron bullets for the coarser sizes is spread, forming a bed that prevents the deposit from passing through the screen too rapidly. The heaviest part of the deposit, with the diamonds, passes through the screens into pointed boxes from which the deposit is drawn off and taken to the sorting tables. The refuse goes to the tailing heap.

But one per cent. of the total amount of blue ground washed goes to the pulsator, and fifty-eight per cent. of this flows over the jigs as waste. Numerous experiments were unsuccessfully made to effect the separation of the diamonds from the worthless concentrates in a less tedious and expensive way than sorting them by hand, when a De Beers employee, Fred Kirsten, suggested coating a shaking or percussion table with grease; and this resulted in the notable discovery that diamonds only, of all the blue ground minerals, adhered to grease, while all else would flow off with water as tailings. The improved shaking tables now used at the South African mines are corrugated, and while a first table fails to detain one third of the diamonds a second table recovers these, almost to the last diamond; so that this invention is practically as certain in its accomplishments as the human eye and hand, while proving a great economy in its operation. It has been demonstrated also that these greased shaking tables will hold other precious stones of high specific gravity and hardness. The diamonds which are heavily coated with grease, of about the consistency of axle grease, by their experience with this process, are cleaned by boiling them in a solution of caustic soda. The quantity of deposit (diamonds) which reaches the sorting tables equals but one cubic foot in 192 cubic feet.