the
platinum was caught by the same methods (always excepting amalgamation
processes) as were used for the gold, and perhaps no special
precautions were taken either to catch it at all or to increase in any
way the quantity so obtained. The machines used were the ordinary ones
employed in gold washing, such as the "torn," tables supplied with
riffles or covered with burlap and cocoa matting, sluices with riffles
of different varieties, etc. The heavy concentrates obtained by these
machines were further treated, after having been run over amalgamating
plates or treated in various forms of amalgamators to remove the gold,
by hand panning.
The
concentrates thus obtained were still further enriched by removing with
a hand magnet any magnetite or other magnetic material, such as nails,
pick points, miscellaneous pieces of old iron, etc. In some cases the
miner went further and removed the greater part of the other heavy
materials, such as chromite, zircon, ilmenite, etc., by blowing in a
pan or horn spoon, thus obtaining a final product of quite pure
platinum metals.
Even
after all this trouble and with the small return which the miner
ordinarily gets for the material the process paid. Many managers of
placer mines have been convinced for a long time that it would pay them
handsomely if they could save the platinum in their gravels by some
inexpensive method.
Attempts
in this direction have generally taken the shape of specially designed
undercurrents, very similar to those used for catching fine gold.
These, so far as the catching of platinum is concerned, have not been
successful; but their installation has often been profitable by reason
of the increased saving of fine gold. The next attempt along this line
was to effect the largest possible saving of black sand in the sluices
and to pan by hand the entire quantity thus saved in large vats of
still water. With expert panners this method will undoubtedly save the
larger portion of the platinum in the sand, but it has the objection of
being very laborious and expensive.
The
experiments of the Geological Survey have conclusively shown that from
95 to 98 per cent of the precious metals, both gold and platinum,
contained in the sluice-box sands can be saved on concentrating tables
of the Pinder or AVilfley type, such as are used in everyday mill
practice; and that in most cases the concentrates thus obtained will
represent less than 1 per cent of the total weight of sand fed to the
table.
In
some cases, where the sand is especially heavy and the precious metals
extremely fine, such a high ratio of concentration can not be effected
without some loss. In many cases also it has proved good practice to
use only a small part of the material at first, and to rerun the
concentrates from this preliminary treatment in order to obtain a
second and final concentrate. These table tests are described in detail
in the report of the Geological Survey, and therefore will not be
discussed at greater length here.
The
concentrates from the tables are generally rich enough to ship directly
to some smelter; but it is not only possible but perfectly practicable
to clean them successfully at the concentrating plant. The best method
of doing this will, of course, vary greatly according to the character
of the concentrates. In general, good results may be obtained as
follows:
Treat
the material with a weak hand magnet or an electro-magnet (using an
extremely low amperage) to remove magnetite, bits of scrap iron, etc. The strength
of the magnetic field should be very slight to avoid picking up the
more magnetic portions of the platinum, and the material thus extracted
should be re-treated to avoid the possibility of mechanical inclusion
of fine particles of the precious metals. The magnetic material can
then generally be thrown away without loss.
The
residue from this treatment should then be passed under an
electro-magnet the field of which is strong enough to pick up monazite.
In this way it will be divided into a magnetic and a nonmagnetic
portion. The former will contain any chromite, ilmenite, garnet,
olivine, etc., together with most of the crude platinum.
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