Economy in acid
being therefore imperative, the silver solution finally becomes much
concentrated, and it requires high heat and careful management to
finish the solution of the bullion. Bars containing more than about 10
per cent. of copper cannot be dissolved at all, owing to the
separation of copper sulphate insoluble in the small amount of free
acid finally remaining. The advantage gained by dissolving bullion
with abundance of free acid in the improved process is so evident that
it merely requires to be pointed out. For bullion containing 20 per
cent. of copper the author employs six parts of acid to one of
bullion; for baser metal still more acid, and so on, never losing more
than the stochiometrical percentage of acid and recovering the
remainder. In this description he, however, confines himself to the
treatment of ordinary silver ore with less than 10 per cent. of
copper.
In the diagram A A represent two refining pots, 4 ft. in diameter and
3 ft. in depth, each capable of dissolving at one operation as much as
400 pounds of bullion. The acid is stored in the cast iron reservoir,
B, which is placed on a level sufficiently high to charge into A by
gravitation, and is composed of fresh concentrated acid mixed with the
somewhat dilute acid regained from a previous operation.
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