at
the same speed; then the pulps are "thinned" down by means of jets of
pure water, as necessary, and the upper plugs are opened to convey away
the waste for future treatment; the lower plugs are not taken out until
the amalgam is clear of all sand and slimes. This amalgam, of which
there are large quantities, owing to the richness of the ore, is then
conveyed in trucks upon a tramway to the " strainer."
The
preliminary action taken with this amalgam consists, however, in its
being placed in bags of common navy canvas, in order to permit the free
mercury to dribble through of its own weight; another bag of canvas, of
a conical form, is then used for this dried amalgam, and this bag is
placed into the strainer, which is made of strong cast iron, and
resembles a very large bucket, smaller at the bottom, which latter is
besides pierced with a number of small holes; the lid is then screwed
down on the close packing, and, through a pipe leading from a main,
water is turned on under great pressure, by means of which all the rest
of the mercury is squeezed through the canvas into a vessel placed
below. The strainer is furnished, about half way up the two sides, with
two spindles and a handle, by means of which it can be tilted over
easily, thereby permitting the amalgam to be removed for the retorts,
which have already been described,
(5.) MODES OF TREATMENT.
The
treatment at the Comstock, it will be admitted, is very simple and
direct, though in some mills other means are added in order to assist
in the reduction of the ores. To recapitulate: crushing through No. 40
gauge screens, sands falling into tanks, 7 feet square by 30 inches in
depth ; for grinding, amalgamation, and concentration into Horn's pans,
5 feet diameter by a depth of 32 inches, muller rotating fourteen times
per minute for three hours, then each charge of two tons is sapplied
with 400 lbs. of mercury, ending with thorough mixing for two more
hours ; then plugs are opened successively for settler pan, 10 feet
diameter by 3 feet depth, muller a like speed, until amalgam is left
pure by jets of water removing impurities.
Where
the ore is not working well in the pans, solutions of sulphates of
copper and iron are added occasionally, and superheated steam is also
forced through pipes in the bottom of the pans with very good results ;
the consumption of bluestone at the Comstock has reached 4,000 tons per
annum.
Section V.—Tailings.
These
residues, accruing in enormous quantities from the mills at the
mines—some mills crush and treat from 400 to 500 tons of ore per diem—
are treated in a skilful manner in specific tailings mills located on
the Carson River, in the Gold Canon, at Silver City, and Gold Hill, and
as a matter of fact most of these mills receive tailings which have
repeatedly been treated in mills situated higher up the valleys. As far
as I could ascertain, these tailings are primarily conserved in large
dams, where they gradually settle, and after having remained therein
for some months or longer in order to induce spontaneous decomposition
in the open air, they are then ground in pans with additional solutions
of rocksalt, bluestone, and interjected superheated steam. Inasmuch as
they are by this time ground very fine, and are therefore more like
slimes than sands, electro-copper plates and blanket strakes complete
the process of intercepting the auriferous silver they are charged
with. The blanket strakes have a novel appliance
