The application of metallic zinc to recover gold from waste solutions is based in the fact that zinc is a good agent for the detection and precipitation of gold from solutions containing a mixture of several acids. Basically the zinc can be used with some advantages. If a plate of zinc is immersed in these solutions from coloring baths, it will be precipitate all the gold existing therein and the liquid will be clear without color tint. In this case, copper and gold are separated together. The solution containing gold, silver and copper is put into a vessel, which has a cross-bar to which is attached the zinc plate.
The solution containing the gold must be diluted with at least twice its volume of water, after the old color, rinsing and pot washings have been emptied into the collecting vessel. Large or small quantities of the liquid may be treated with equally good results if sufficiently diluted, for diluted colors and its auxiliaries does not act on the metallic zinc with more than necessary energy due the processes produce dissolution of zinc, precipitation of gold and formation of hydrogen. The idea is to expose the greatest amount of surface to the liquid as is required to reduce the gold without adding too much the size and weight of the zinc plate employed. If more zinc than necessary is used and the solution contains a high level of free acid, the zinc plate is dissolved and would be produced a fluffy sediment with gold, and some zinc compounds.
The metallic zinc yields good results in the form of a rectangular sheet and the gauge size may be about that used for making thick gutter spouting, which expose the necessary surface to the action of the liquid for the development of the greatest amount of hydrogen. It has been noted that the hydrogen that have left the vessel is not so effective in the precipitation of metals as that produced in the same vessel. By using metallic zinc into waste color solutions properly diluted, the free acid acts upon the zinc and though ever so slightly, it produces nascent hydrogen gas to be given off. One equivalent volume of hydrogen combined with one volume of chlorine forms hydrogen chloride and the excess of hydrogen goes to the surface of the liquid and escapes. And the remaining equivalent volumes of chlorine which assisted in holding the gold and copper in solution are extracted by the zinc as it is slowly dissolved. Gold and copper chlorides are decomposed and reduced to metallic state, and the zinc chloride is produced. The latter is a very soluble salts and remains in solution in the liquid above the gold, silver and copper precipitate.
In this case, it is important the physical condition of the zinc plate due to tarnished surfaces prevent the gold depositing upon them and the action of the acid liquid does the work of presenting fresh surfaces to the influence of the mixture. The larger is the volume of free acid in the liquid residues resulting from coloring gold, there is a more capacity of dissolving the zinc, and larger must be water added to save the unnecessary waste of zinc.