1st experiment 1.5706 grms.
2d " 1.5694 "
------
Mean 1.5700 "
The presence of silver could not be qualitatively ascertained in the
residual liquor.
Although sheet zinc, or zinc and iron sheets, serve so well for the
precipitation of silver, they cannot be employed for the recovery of
gold. The latter separates out in such a case very incompletely and as
a firmly adhering lustrous film in the zinc. On the other hand, finely
divided zinc, the so-called zinc dust, is an excellent substance to
employ for precipitating gold quantitatively and in the form of powder
from spent cyanide liquors. When zinc dust is added to a spent gold
bath and the liquid periodically stirred or shaken, all the gold is
precipitated in two or three days. The amount of zinc to be added
naturally depends on the quantity of gold present. Freshly prepared
gold baths for gilding in the cold contain on the average 3.5 grms.
gold per liter, while those used for the hot process contain 10.75
grms. To precipitate all the gold in the original bath, 1.74 grms. or
0.37-0.5 grms. zinc dust would be necessary, and, of course, a much
smaller quantity would be sufficient for the spent liquors. Since the
precipitation takes place more rapidly when an excess of zinc dust is
present, it is generally advisable to add ¼ or at the most ½ kilo, of
zinc dust to every 100 liters of solution.
The precipitated gold, which contains zinc dust and usually silver and
copper, is washed, freed from zinc by hydrochloric acid, and then from
silver and copper by nitric acid and thus obtained pure.
A spent bath treated in this way gave the following amounts of gold
per liter:
1st experiment 0.2626
2d " 0.2634
Mean 0.2630 grms.
The presence of gold in the residual cyanide solution could not be
qualitatively detected. The potassium cyanide of the solutions
obtained by this process should be converted into ferrocyanide by
heating with ferrous sulphate and milk of lime, since this substance
is not poisonous and can therefore be got rid of without danger. It
would, however, be more economical and, considering the large amount
of cyanide present, more profitable to work it up into Prussian blue.
* * * * *