16 THE DIAMOND MINES OF SOUTH AFRICA
An
endless wire rope haulage carries all the refractory ground to the
mill, where it is put through a series of crushing machinery. The
first or "comet" crushers reduce the ground so that the largest pieces
will pass through a two-inch ring. From these crushers the ground
passes through revolving screens which separate the finely crushed
from the coarse pieces. The fine size is conveyed to the washing pan,
and the coarser ground passes from the end of the screen to revolving
picking tables, where diamonds of the larger size may be seen and
removed without risk of crushing by further pulverization. From the
picking tables the ground is scraped automatically into two sets of
rolls, and the pulverized product screened again and graded into three
sizes. The finest size, passing a half-inch screen, goes to the washing
pans, and the two coarser sizes to jigs. Large diamonds which have been
separated from their envelope of blue are retained in the jig. The
ground still holding the smaller diamonds passes out of the end of the
jig and then through a series of rolls, screens, and jigs until the
finished product is drawn from the bottom jigs into locked trucks
running on tramways to the pulsator for further concentration and
sorting.
From
beginning to end of this process the crushed ground is carried by
water, and the plant requires a flow of 400,000 gallons an hour. After
leaving the last jig the water is separated from the fine ground by a
revolving screen and the tailings are taken away in trucks to the
tailing heap. Within the past three years the ordinary rotary pans have
supplanted the jigs, and are found to be more economical.
The
coarse ground, which passes out of the end of the revolving cylinders
of the washing plants, is called " lumps." As the lumps leave the end
of the cylinders they fall upon a conveyor and are taken to the end of
the washing machines, where they are reduced by a similar, though
smaller, crushing plant, with the exception that pans only are used for
saving the diamonds.
Thus
the screened and sized product from the washing pans and the crushing
machines reaches the final stage of concentration in the Pulsator.
This is a combination of jigs with station-
