Procedure for Recovering Elemental Silver from Silver Residues As the price of silver nitrate increases and chemistry department budgets tighten, the recovery of silver from silver residues of analytical hhoratory experiments becomes a necessity rather than a luxury. Although many methods have been proposed, most are complex%nd/or are limited by the anions present in the residue.a The following recovery procedure was selected, after msny experiments, as the most efficient and profitable method. I t is based upon the traditional ore refining method of forming the impurities into slow melting slag. Silver residue (a mixture of AgCI, Ag,(CNL, AgSCN, AgI, AgBr, and Ag,CrOd was dried in an oven a t llO°C until it lost its moist appearance. The oven employed here and the furnace used later in the procedure were hooded bemuse of the toxic reaction products formed from the chloride, thiocyanste, and cyanide ions present in the precipitate. This safety precaution is strongly recommended. If the residue is not predried, the mixture sputters when placed in the furnace. The dried residue was intimately mixed with an equivalent amount of potassium carbonate assuming the residue consists entirely of AgCl. Potassium carbonate, rather than sodium carhonate, was chosen because it forms a slag with a lower melting point. I t was determined that the mare intimate mixtures of K&08 and residue gave higher yields. When the ingredients were ground in a ball mill for two hours, we obtained 95% recovery from a. silver chloride sample, whereas s. ct~sualstrirring of the ingredients resulted in only s.60-70% yield. The mixture was placed in a clay crucible and baked in a furnace a t 1000°C. At this temperature the reduced silver melts, forming a puddle a t the bottom of the crucible. When the reaction is carried out a t lower temperatures, a. sponge-like silver is obtained that has impurities trapped in its porous structure. The crucible is filled no more than three-fourths full as some foaming occurs. The furnace floor was protected by a sheet of asbestors which had been preheated a t lOOO0C for 2 hr to remove combustibles. The furnace was hooded because COCL is formed. The reaction was considered complete when the temperature had returned to 1000'C. The crucible was removed, the molten slag was discarded, and the molten silver was poured into another clay crucible. The casting crucibles employed were clay combustion boats (size No. 6) marketed by Leco, Inc. These shaped the silver as small ingots. The inner surfaces of the boats had been previously coated with chalk to facilitate the release of the ingot. When theingot had cooled the chalk dust and slag remnants were removed with water. Silver nitrate may be obtained by dissolving the silver in nitric a c i d . V h e silver metal may he used as such or sold. Wesold most of ours to the BYU Art Department whichused it for classes in metal and jewelry design. We would like t o express our appreciation to the faculty of the Chemistry Department and especially Professor John Wing for technical assistance and the use of the muffle furnaces, and to the hundreds of chemistry students who diligently prepared the silver residues. National Science Foundation Trainee.
' WILLBANKS, 0. L., J. CHBM.EDUC.30,347 (1953). ' GARIN,D. L., AND HENDERSON, K. O.,J. CAEM.EDUC.,47,741 (1970). S. PAULSTEED M. HAYES'
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