The silver chloride is then reduced in lead-lined tanks by means of granulated zinc and water acidulated with sulphuric acid. Thirty-three pounds of commercial granulated zinc are stated to be enough to reduce 100 lbs. of silver from the chloride. The reactions involved are as follows:
- 2AgCl + Zn = ZnCl2 + 2Ag
- Zn + H2SO4 = ZnSO4 + H2
- H2 + 2AgCl = 2Ag + 2HCl
- 2HCl + Zn = ZnCl2 + H2
These reactions explain the fact that, while zinc slowly reduces silver chloride in the presence of water only, the action is quickened by the addition of free acid, by which the zinc is attacked and hydrogen evolved. Nascent hydrogen is a powerful reducing agent, and decomposes silver chloride much more rapidly than zinc does, hydrochloric acid being formed and rendered available for the production of more hydrogen. The result of this is that the action, which is at first slow, becomes more and more rapid as hydrochloric acid accumulates in the solution. The sulphuric acid is only needed to start the reaction, but, of course, the more that is added, the more quickly will the operation proceed, and if much is added, the chemical action is more violent at first than afterwards, the amount of free acid present in this case falling off. At the San Francisco Mint 1 lb of acid of 60° B. is added for every 2 lbs. of silver to be reduced. Hydrogen is evolved copiously and is carried off by a hood and flue. Energetic stirring with wooden paddles is desirable to prevent the formation of lumps of chloride, protected by a layer of silver powder. The white chloride of silver gradually turns black as the silver is reduced. To test whether the reaction is complete, some of the silver is taken out, washed well with ammonia, filtered, and the clear solution acidified with nitric acid. A white precipitate signifies that undecomposed silver chloride is still present in the vat.
When the reduction is complete, the vats are allowed to settle, the solution drawn off, and a little sulphuric acid added to dissolve any residue of zinc that may be present. The dark grey pulverulent silver is then washed by decantation, after which it is removed to a wooden filter, and sweetened by washing with boiling water, and finally pressed, dried, and melted into bars, which are about 998 fine. The zinc and sulphuric acid used in this process are lost, and a considerable quantity of undecomposed nitric acid is also run to waste, being contained in the solution from which the silver chloride is precipitated.
The cost of refining and parting by the nitric acid process at the United States Mints at Philadelphia and New York was formerly somewhat less than 2 cents per oz. of the parting alloy, and in San Francisco nearly 3 cents. The cost for dore silver was considerably lower. In Europe, the cost was less than in the United States. The present costs are not published.