PLATINUM ASSAYING.
As
is well known, the textbook methods for the assay of platinum have not
proved effective. Many people have been fooled by the results of
assayers, most of whom, presumably following standard methods, are not
sufficiently familiar with the difficulties of platinum analysis to
recognize the errors that might arise in their work. The difficulties
of platinum analysis are well known, and statements have been
repeatedly published emphasizing the necessity of employing only
assayers who are recognized as competent to undertake this class of
work. The Bureau of Mines has recently issued a comprehensive reviewls
of the various methods that have been used and gives in detail a method
which can be considered standard. As this method has been shown to be
absolutely reliable, when followed by a competent assayer, for the
detection of platinum metals in all classes of samples, it is believed
that it should be accepted. It is suggested that any one desiring
assays for platinum should insist upon the assayer using this method.
If this is done and check assays are required later, the results can be
compared with safety.
The
importance to all those interested in platinum of the method published
by the Bureau of Mines seems to justify its repetition here, though the
whole report should be in the hands of each assayer who may be called
upon to assay ores for platinum.
SELECTED METHOD FOR THE COMMERCIAL ESTIMATION OP PLATINUM IN ORES.
In
the investigation of methods for the detection and estimation of
platinum in vein material, the method following has been selected as
satisfactory for commercial use. It contains nothing new, but has been
Compiled from various sources and is similar to one used by L. W,
Dunham, Irvington, N. J.14 The manipulation has been given
in detail and notes added to explain the various steps in the process.
Although a separation of the platinum metals is not given, the rough
group separations are included to aid in determining the value of an
ore.
The bead obtained by usual fire-assay methods may contain gold, silver, and the platinum metals and is parted with HN03.
The silver and most of the platinum and palladium go into solution. The
residue may contain gold, iridium, rhodium, and some ruthenium and
osmium, as well as platinum. The gold and. the rest of the platinum are
dissolved from the residue of other platinum metals with dilute aqua
regia and filtered off, leaving iridium, rhodium, part of the
ruthenium, and a small amount of osmium, which may be ignited and
weighed. The HN03 solution may contain silver, platinum, and
palladium. The silver is removed, as the chloride and the platinum and
palladium are precipitated from the solution (which has been made
slightly alkaline with Na2C03), by boiling with
formic acid, then filtered, ignited, and weighed. To separate the
palladium from the platinum the mixed metals are treated with warm
dilute HN03, which dissolves the palladium and leaves the
platinum. The palladium is found by difference, or may be precipitated
with formic acid, after evaporating to dryness with HC1, taking up with
water, and neutralizing with Na2C03. The aqua
regia solution is evaporated to dryness twice with HC1, taken up with
water, and, after the gold is removed with oxalic acid, the platinum
and palladium are recovered as in the preceding process. A qualitative
test is applied to a solution of the platinum precipitate.
The
ore in all cases is ground to pass a 100-mesh screen and when
substances difficult to fuse, such as chromite and zircon, are present,
the sample should be made to pass a 150-mesh screen.
When
the quantity of platinum is small, it is advisable to run several
samples of an assay ton each, combine the lead buttons, and scorify to
about 30 grams.
An
assay ton of the carefully sampled ore is thoroughly mixed with a
suitable flux. As iron has a greater affinity for iridium than has
lead, the old iron-nail method of treating sulphide ores should not be
used.
>3 Davis, C W., The detection and estimation of platinum in ores: U. S. Bur. Mines Tech. Paper 270. H Private communication, 1920.
