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MINING IN CALIFORNIA AND NEVADA.
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ore,
after passing through the screens, is passed over a series of
copper-plate (electro) ripples into four grinding pans. Before I
proceed any further 1 am compelled to notice what the American mine
superintendents term their "pan system," which is the successor of the old arastras, then Chilian mills, Berdan's basins, until eventually the present "pans "
were perfected to such a degree of usefulness as to give not only every
satisfaction but likewise prove their admirable utility with both gold
and silver ores ; their general adoption is therefore, in California,
the outcome of well-tested work against many other appliances invented
for similar purposes, but which could only be successful with this "pan system." To
resume, these grinding pans are of cast iron, 5 feet 6 inches in
diameter, and 1 foot 9 inches high. Their central spindle is set in
motion by a bevel gear running underneath each pan, and a vertical
spindle sets in motion the " mutter" a kind of inverted cone
that can be raised or be depressed, as required, by the screw thread
cut into the central spindle. This muller communicates the required
circular motion to a double layer of triangular, bevelled, and
perforated plates of chilled cast iron, so that the sand is subjected
to a severe grinding and thorough mixing for such time as is requisite
at this stage. The pans have a false bottom, furnished with grooves and
recesses for the mercury, so that any gold ground out of this already
very fine sand is readily amalgamated. Of course, the speed to be given
to the muller depends on the character of the ore, but a higher speed
is generally preferred, because by it the golden particles are freed
from films coating same and burnished for immediate action by the
mercury. This "pulp," as it is termed, is regularly removed and placed into the " settlers," which
are also pans worked the same way, but instead of grinding plates four
equi-distant arms extend from the muller, from which arms depend a
series of iron teeth and plates, by means of which this pulp is stirred
with a graduated and copious supply of water, so that all particles of
amalgam and mercury fall to the bottom of their own gravity, and are
then further dealt with. At certain times, which suggest themselves by
practice only, the so enriched strata of the pulp is removed into the
last set of pans, termed the " concentrators; " these are
double the size of the last, viz., 10 feet diameter by 3 feet' 4 inches
high, and they are made of wooden staves at the sides only, the bottom
being cast iron. In these concentrators the weekly results of the
treatment are collected in the shape of amalgam, to be retorted
monthly; besides that, the pyrites are principally obtained also from
these concentrators. The above process, so much more complicated than
ours, includes therefore the following features, viz.:—Crushing through
gratings, 1,400 holes per square inch; electro-plated copper plates and
shallow ripples with mercury; grinding the fine sand still finer;
stirring the pulp to deposit amalgam ; concentration of amalgam, and
finally collection of it and of the pyrites.
Elaborate
as the above process may appear to us on Bendigo, that of the Idaho
Company, in the same district, is still more so, and I submit that, by
describing these two, all practical purposes are served, because at the
other mills inspected, either one or the other, or part of one and part
of another, had been adopted by the superintendents in charge.
The
Idaho Company is the most successful proprietary mining auriferous
quartz on the Pacific slopes. Their profits, at the rate of 7-1/2 per
cent, per month on their capital, have been regularly paid for the last
hundred and twenty odd times (July 1877), and the mine has been opened
so as to provide similar work for at least eight to twelve years
longer. Their machinery for treating their quartz is on a very
comprehensive scale, which
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