Recovery of silver from laboratory wastes

Abstract

We have tried various procedures for the recovery of silver from the wastes from experiments in the courses in quantitative analysis and have settled on cathodic deposition of silver from acyanide solution in a non-partition cell with stainless steel electrodes. All of the equipment needed can be found in most laboratories or can be purchased at little expense. The procedure is unusually good for instruction and projects at the upper class level. Of the older procedures we have used that of Willbanks ( 1 ) many times with success. It is based on converting the various insoluble silver salts collected to silver hydroxide by digesting with sodium hydroxide, collecting the silver hydroxide, decomposing the oxide to silver metal at 5OO0c, and finally melting the silver into a bar. The chemistry is simple and the procedure an enormous improvement over an earlier scheme in which the solid silver residues (chloride, etc.) are mixed with sodium carbonate and heated to 1 0 o 0 O to llOO°C. Our troubles with these procedures curiously came, on two occasions, from breakage of crucibles and consequent damage to the lining and heating elements of the electric muffle. Steed and Hayes (2) improved the older fusion procedure by using potassium carbonate and provided some useful hints on how to improve the yield and protect the furnace. Our feeling is that securing the large clay crucibles, ball-milling the silver residues with the carbonate (as in the Steed and Hayes procedure), and risking the furnace is too much trouble, even with silver at $0.16 per gram (1977). Of course, if the residues are silver and silver oxide only (from organic preparations), dissolution and digestion with nitric acid, evaporation, and crystallization of silver nitrate, as recommended by Garin and Henderson (3), is more straightforward. An older procedure for converting silver chloride to silver nitrate, Meyer (4, by digesting a suspension of the silver chloride in dilute hydrochloric acid with metallic zinc (powder or fine wire) and working up the metallic silver so formed we found simply too cumbersome and expensive; the digestion is time-consuming, and a second digestion of the metallic silver with hydrochloric acid is necessary to remove the excess zinc. Meyer was interested in the recovery of silver from alloys (silver solder, coinage) and recommended precipitation of the chloride to separate the silver from copper, cadmium, and other metals. We chose electrodeposition of the silver from a cyanide solution rather than from an ammonia solution primarily because we feared that silver-ammonia solutions bpa'reless handling might be slopped about, allowed to dry and produce a shock-sensitive explosive residue. Silver can, of course, be deposited very nicely from an ammonia solution (5,6,7), but silver iodide will not dissolve in a solution of ammonia. On the other hand, all silver salts, except for the sulfide, dissolve in solutions of the alkali cyanides, the electrodeposition can be made from a non-partition cell of simple construction, and excellent deposits of large amounts of silver can be obtained (as for example in the fabrication of silver crucibles for analytical work (8 ,9) ) . The chemistry involved is