dition
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 a 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
