GOLD BULLION.
There
are several patents covering the electrolytic refining of gold bullion,
detailed descriptions of some of which will be found in the references
listed. The process used by the United States Mint, described by
Durham,1 may be briefly summarized as follows: Anodes of
gold bullion 3-1/4 by 8-3/4 by 1/2 inch, which contain about 90 per
cent gold and 7 per cent silver, are treated in porcelain cells in an
acid electrolyte of gold trichloride. Cathodes of pure gold 4 inches
wide and 0.012 inch thick and weighing 4.5 ounces when new receive the
gold deposited. They are not removed from the tanks till the weight
reaches about 160 ounces. The cathodes, with their deposited gold, are
remelted, cast as anodes, and a second deposit of gold is made, raising
the fineness at least to 0-.9997. During this process silver chloride
is formed, which drops to the bottom of the cells and is collected.
From the silver chloride metallic silver is recovered by treatment
with metallic zinc and sulphuric acid, and this is refined to pure
silver by electric deposition on pure silver cathodes in an electrolyte
of silver nitrate. Copper and the platinum metals are dissolved in the
gold chloride electrolyte. In practice the platinum metals are usually
precipitated by adding ammonium chloride to the electrolyte when it
contains about 5 per cent platinum and 0.5 per cent palladium, because
if the proportion of these metals becomes much greater there is a
tendency for them to deposit with the gold at the cathode.2
In
time the electrolyte becomes spent when, together with all wash waters,
it is passed over scrap iron, which precipitates copper and any other
metals that may be carried by it. This impure cement copper is cast
into anodes 5 by 14 by f inch, and the copper is deposited on
copper-plated lead cathodes in an acid copper-sulphate electrolyte.
The
gold and silver and the platinum metals are insoluble in the
copper-sulphate electrolyte, and fall to the bottom of the lead-lined
wooden cells and are saved for further treatment for the extraction of
gold and platinum.
BLISTER COPPER.
The
aim of copper refining is to produce a fine copper and the saving of
gold, silver, platinum, and other metals is of secondary importance in
the process, though these metals recovered in many plants are of large
aggregate value. The processes known as the Elkington (multiple) and
Hayden (series) systems are in use. They differ in manipulation, but
depend on the same action. Anodes of blister copper are treated in an
acid electrolyte of copper sulphate. Fine copper is deposited on pure
copper cathodes. During the process nickel, cobalt, iron, manganese,
zinc, lead, and tin go into solution in the electrolyte, though some
nickel goes into the mud also if the copper contains much of that
metal. Lead, tin, gold, platinum, silver, tellu-rium, selenium, and
sulphur are precipitated as "anode mud" and saved for treatment.
i
Durham, E. C, Electrolytic refining at the United States Mint, San
Francisco, Cal.: Am. Inst. Min. Eng. Trans., vol. 42, pp. 874-901,
October, 1911.
2 Rose, T. K., The precious metals comprising gold, silver, and platinum, p. 195, New York, Van Nos-trand,1909.
