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.
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