In
order to save very fine and especially rusty particles of gold,
so-called "under-current sluices" are used; these are shallow wooden
tanks, 50 square yards and upwards in area, which are placed somewhat
below the main sluice, and communicate with it above and below, the
entry being protected by a grating, so that «nly the finer material is
admitted. These are paved with stone blocks or lined with mercury
riffles, so that from the greatly reduced velocity of flow, due to
sudden increase of surface, the finer particles of gold may collect. In
order to save finely-divided gold, amalgamated copper plates are
sometimes placed in a nearly level position, at a considerable
distance from the head of the sluice, the gold which is retained in it
being removed from time to time. Sluices are often made double, and
they are usually cleaned up—that is, the deposit rich in gold is
removed from them— once a week. The gold is then recovered by "
panning."
The
application of a jet of water to the removal of auriferous gravels by
the so-called hydraulic system of mining has already been noticed at
vol. iv., p. 70t. (a) This method has for the most part been
confined to the country of its invention.- California, and the western
territories of America, where the conditions favourable for its use are
more fully developed than elsewhere,— notably the presence of thick
banks of gravel that cannot be utilized by other methods, and abundance
of water, even though considerable work may be required at times to
make it available. The general conditions to be observed in such
workings may be briefly stated as follows:—(1) The whole of the
auriferous gravel, down to the " bed rock," must be removed—that is, no
selection of rich or poor parts is possible; (2) this must be
accomplished by the aid of water alone, or at times by water
supplemented by gunpowder; (3) the conglomerate must be mechanically
disintegrated without interrupting the whole system ; (4) the gold must
be saved without interrupting the continuous flow of water; and (5)
arrangements must be made for dis'posing of the vast masses of
improverished gravel.
The
general appearance of an hydraulic gold working is seen in fig. 3, the
water being brought from a ditch on the high ground, and through a line
of pipes to the distributing box, whence the branch pipes supplying the
three jets diverge. The stream issues through a nozzle resembling that
of a fire engine (fig 4), which is movable in a horizontal plane around
the vertical axis a, and in a vertical plane on the spheric I joint and centre b, so
that the direction of the jet may be varied through considerable angles
by simply moving a handle. The material of the bank, being loosened by
the cutting action of the water, crumples into holes, or "caves in,"
and the superincumbent mass, often with large trees and stones, falls
into the lower ground. The stream, laden with stones and gravel, passes
into the sluices, where the gold is recovered in the manner already
described. Under the most advantageous conditions the loss of gold may
be estimated at 15 or 20 per cent, the amount recovered representing a
value of about two shillings per ton of gravel treated. The loss of
mercury is about the same, from 5 to 6 cwt. being in constant use per
mile of sluice. About 1 cwt. is added daily in at least two charges.
The average half-yearly consumption is estimated at about one hundred
flasks of 74 lb. each, after allowing for the amount recovered in
clearing up and distillation of the amalgam. The latter operation is
performed at intervals of seven or fourteen days in the upper lengths
of the sluice, and half-yearly in the lower parts.
The
dressing or mechanical preparation of vein stuff containing gold is
generally similar to that of other ores, except that the precious metal
should be removed from the waste substances as quickly as possible,
even although
