In view of the rapid development and extension of the methods of
electro-plating with silver and gold, and of the large amount of spent
liquors containing silver or gold thus produced, it has long been
desirable to find methods by which these metals can be recovered from
the spent liquors. The processes hitherto adopted generally
necessitate the tedious and unpleasant evaporation of the cyanide
liquors, or else involve a series of chemical operations which are
somewhat difficult to carry out, so that actually the used-up baths
are sold to some firm which undertakes this recovery as a particular
branch of its business.
A process invented by Stockmuir and Fleischmann, and worked out by
them in the chemical laboratory of the Bavarian Industrial Museum, is,
however, exceedingly simple, and is employed in many establishments.
In order to remove silver from a potassium cyanide silver solution, it
is only necessary to allow a clean piece of plate zinc to remain in
the liquid for two days; even better results are obtained by the use
of iron conjointly with the zinc. In the first case, the silver often
adheres firmly to the zinc, while in the second it always separates
out as a powder. It is then only necessary to wash the precipitated
powder, which usually contains copper (since spent silver solutions
always contain copper), dry it, and then dissolve it in hot
concentrated sulphuric acid, water being added, and the dissolved
silver precipitated by strips of copper. The silver thus obtained is
perfectly pure. If the amount of copper present is only small, it can
usually be removed by fusing the precipitated powder with a little
niter and borax.