At the
end of this time the amalgam is run out through the stop-cock at bottom of
the vat, is washed, and is put into hydraulic presses, by means of which
the mercury is squeezed out, leaving behind a thick, pulpy mass, composed
mainly of silver, and locally termed a "pina," from its resembling in
shape the cone of a pine tree. These pinas are then carefully weighed and
put into a subliming furnace, Figs. 5 and 6, in order to drive off the
rest of the mercury, the silver being subsequently run into bars. About
four ounces of mercury are lost for every pound of silver made.
The actual quantities of mercury to be added in the grinding vat, and the
times of its addition, are based entirely on practical experience of the
process. With ore assaying 150 oz. to 175 oz. of silver to the ton, 75
lb. of mercury are put in at the commencement, another 75 lb. at intervals
during the middle of the process, and finally another lot of 75 lb.
shortly before the termination. When treating "pacos," or earthy chlorides
of silver, assaying only 20 oz. to 30 oz. of silver to the ton, 36 lb. of
mercury is added to 21/2 tons of ore at three different stages of the
process as just described.
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