Recovery of Silver from Laboratory Wastes J. P. Rawat and S. lqbal M. Kanmmpuri Allgarh Muslim University, Aligarh-202001, India A simple and inexpensive procedure combining ion exclusion with ion exchange has been applied for the recovery of silver from silver chloride residues in laboratory wastes. The yield is 87-91% as solid silver nitrate. In laboratories many reagents and chemicals are discarded unknowingly in daily routine as wastes. Some of these substances may be of enormous importance if they can be converted in usable forms. Unwisely discarded silver in the form of silver chloride is one such example (1-5). The material is discarded after quantitative and qualitative analyses. The commercial realization of ion exchange technology to this end may be helpful in many ways. As AgCl is a sparingly soluble salt, it miaht be ~ossibletheoreticallv t o recover silver from it usin; a catibn exchanger by removing silver i m s and thus enabling silver chloride to dissociate further as required by the equilibrium AgCl (solid) t Ag+
Results of AgNOp Recovery Sample No.
AgNo. expected (g)
AgNol found (8)
Yield (%)
+ Cl-
which should shift toward the right-hand side when Ag+ is taken up by the cation exchanger. However, practically this is less feasible. Perman has tried to recover silver bv converting silver nitrate into a silver amine complex and then reducinc with ascorbic acid ( 6 ) .T o have asirnnlified nrocedure. we have tried the ion exchange combined kith ion exclusion in order to get a good yield of silver as silver nitrate. Procedure
The silver chloride residue obtained from undergraduate laboratories as waste was acidified with hydrochloric acid to ensure complete precipitation of silver as silver chloride. It was heated a t 100 OC until the residue appeared white. After cooling, the supernatant liquid was decanted, and silver chloride was filtered off on Whatmann No. 42 paper on a Buchner funnel. The silver chloride was transfered to a beaker, and 25% ammonia solution was added until dissolution was complete. In order to make this solution chloride-ion-free, it was passed over an anion exchange resin column in nitrate form (step 1 in figure). The effluent obtained was then passed over cation exchanger in Hfform, resulting in the breakage of the silver amine complex. The flow rate was adjusted to 0.5 mllmin. The silver and ammonium ions were eluted with 0.50 M nitric acid (step 2 in figure). The effluent so obtained was again passed over a cation exchange resin column in ammonium form. As a result of ion exclusion. the ammonium ions move faster, leaving behind Agt, which is exchanged on the column. Ammonium ions were collected as an effluent in a separate beaker. After passing all the solution of silver andammonium, the column was thoroughly washed with deionized water to remove silver and ammonium ions sticking t o the glass wall and the exchanger. The silver was then eluted with 0.50 M nitric acid (step 3 in figure). The effluent so obtained was evaporated until all the water was removed, leaving salt in the beaker; the salt was melted a t 280-300 "C until brown fumes ceased (7). I t was then cooled and treated with water and evaporated over a controlled-temperature bath. The flow diagram (figure) summarizes the procedure. Resuns The efficiency of this method was checked by recovering known amounts of silver nitrate. Fifty milliliters of a stan-
Flow chan of ion exchange procedure. step t
step 2
Ag(NH&N03 + RHf * RAg(NH&+ HN03 Ag(NH3):+ HN03 + AgN03 + NHdNOa RHi
+
step 3
dard solution of silver nitrate containing 0.70 g of silver nitrate was precipitated with a sodium chloride solution. The precipitate so obtained was treated in the same way as the waste collected following the recommended procedure. The silver nitrate obtained was weighed and also titrated after dissolving a part of it. The results are summarized in the table. These results show that the method is quite successful for recovering Ag as silver nitrate from laboratory wastes. Volume 63 Number 8 June 1986
537
Acknowledgment
The authors are thankful to M. S. Ahmad, Chairman of the Department of Chemistry at A.M.U., for providing research facilities. We extend our thanks to Balbir Singh and Masood Alam for their help during this work. Thanks are also due to the University Grants Commission for financial assistance.
538
Journal of Chemical Education
Literature Cited (1, W ~ I ~ ~ ~ . , O . L . . I C ~ P19m.m.347. ~.E~~C. (2) BuahK. J.:Diehl.H. J. Chom.Edue. 1979.55,-. (3) C1ske.G. U.S.BureualMiwaBull. 1976.4 (throughChrm.Abrfr.). (4) Kdak Pub. No. 7-10. Eaatman Kodak Co.. Raeheate~.NY.1972. (5) Bvrinasa Week 1914(2).BOF(tbr0whChom.Abatr.).
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