Reduction of silver chloride by zinc powder is the appropriate process to recover silver. The reaction is fast since zinc is more electronegative than iron. The stoichiometric dosage of zinc should be added, preferable as a slurry and an excess must be avoided. At the end of the reaction, HCl is added to dissolve any possible surplus of zinc and to avoid the hydrolysis of ZnCl2. A pH of about 1.8-2.4 is sufficiently low for this purpose. Reduction of the particle size of silver chloride to 100 passing 2 mm tends to increase the overall reaction rate. Very fine material must be avoided since this may give rise to difficulties with decantation and agitation.
The tanks employed are rubber lined conical tanks with a more acutely angled second conical bottom section. The charge is usually agitated by rubber covered mechanical stirrers. Water, air and steam can be admitted through 1” diameter pipes by valves on a manifold at the bottom of each tank. Supernatant liquor is run off directly into a pan filter through a fixed decanter pipe in the side of the tank, leaving about the third part of tank capacity in the cone. A valve at the bottom of the tank allow for the release of the silver sponge into the pan filter. For example, if 400 kg of quenched chloride slurry is loaded to the tank, which is half filled with water, and both stirred and air agitated. Then, it will be required to add 5.5 kg of sodium chlorate and 22 L of 33% hydrochloric acid for the leaching operation. The tank is filled with water, heated to 55-65 oC, the agitation sopped, time allowing for settling and the liquor decanted. Seven to eight washes follow, each with 0.12-0.17 g/L HCl to produce an effluent free of copper and lead, as tested by ammonium hydroxide and potassium dichromate. The whole leach operation takes about three to five hours.
The reduction process is performed by adding the stoichiometric amount of zinc to the leach tank over a period of 25-35 minutes. It is important to mention that the reaction is fast and exothermic and some care is required. Then, stirring is continued for 45-60 minutes, when the completion of the reaction is indicated by the absence of AgCl. The latter is tested by adding a sample a little of ammonium hydroxide and then acidifying the solution with hydrochloric acid. After stirring, silver particles are allowed to settle and the supernatant liquor decanted. Approximately, a total of 9 to 11 washes at 40oC are needed to produce liquor free of zinc chloride, which can be tested with potassium dichromate. The silver sponge is run out of the tank, filtered and dried. A gas fired tilting furnace can be employed to melt the silver and cast anodes for the electrolytic plant. The product of this stage contents 99.8-99.9% Ag, 0.01-0.16% Cu, 8-45 ppm Pb, 9-45 ppm Zn and 45-95 ppm Au.