SEPARATION OF PLATINUM GROUP METALS.
It
is not known what processes are used by the refiners of platinum in
this country, as the methods employed are trade secrets and jealously
guarded by each refiner. Crude platinum and the muds from the gold and
copper refineries can be refined by the Wollaston 1 process, though they usually go through a preliminary refining to remove lead, copper, and other base impurities.
The
material to be refined is treated several hours in aqua regia, which
dissolves most of the metals of the platinum group. The resulting
solution is evaporated nearly to dryness to remove excess acid, diluted
with water, and siphoned from the insoluble residue. This solution is
treated with ammonium chloride, which precipitates ammonium
chloropiatinate, from which spongy platinum is obtained by heating.
This spongy platinum, containing about 2 per cent of iridium, is
broken, sieved, and the fines made into paste with water and then
compressed. This cake is strongly heated and hammered while red hot to
make what is known as "hard metal." Pure platinum can be obtained by
repeated refining of hard metal, the same method being used and care
being taken to have an excess of acid in the solution, as this tends to
prevent the precipitation of iridium.
The
liquor is not entirely free from platinum even after most of this metal
has been extracted, and it contains the other metals of the platinum
group. Iridium chloride is precipitated from this solution by
evaporation to a density the exact degree of which depends on various
conditions and can not be definitely stated.
Two
other methods of treating crude platinum, one for the production of
pure platinum and the other for extraction of the platinum-group
metals, quoted by Schnabel,2 are given in full, as published by him:
G.
Matthey produces pure platinum from the commercial crude metal by
melting it with six times its amount of lead, granulating this alloy,
and treating it with dilute hydrochloric acid, which disolves iron,
lead, palladium, and rhodium, while platinum remains behind with
iridium and small quantities of lead, rhodium, and other platinoid
metals. This residue is boiled with aqua regia, when platinum and lead
dissolve, and iridium remains behind. The lead is precipitated by
sulphuric acid. The liquid is filtered from lead sulphate, and treated
with excess of ammonium chloride and common salt to precipitate
platinum in the usual way. If rhodium is present in the solution, the
precipitate is rose-color instead of pure yellow. It is ignited with
bisulphate of potash, which forms rhodium-potassium sulphate, while the
platinum separates as metal. The double salt is dissolved by boiling
the whole with water.
Wyott
has brought forward a process for extracting the platinoid metals from
residues and mother liquors. Platinum, palladium, and rhodium are
dissolved out of the ores by aqua regia. The first is precipitated by
ammonium chloride. The liquid is filtered off, neutralized by soda, and
palladium cyanide (PdCy2) precipitated from it by mercuric
cyanide. Rhodium remains in the solution. The residue after treatment
with aqua regia is to be heated in a stream of air, whereby osmium is
converted into tetroxide, which volatilizes, and rhodium oxide is
deposited in the hotter parts of the exit tube. The residue, after this
heating, is mixed with salt and heated in a stream of chlorine.
Sodium-iridium chloride is formed, which is dissolved by boiling water.
i Rose, T. K.,The precious metals comprising gold, silver, and platinum, pp. 267-699, New York, Van Nostrand. 1909. 2 Schnabel, Carl, Handbook of metallurgy, vol. 2, p. 623, New York, Macmillan & Co., 1898.
