Reclamation of Gold It has been the experience of ourselves and others' thrtt pmcedures utilizing q u a regiaPfar recovering gold from reaction residues are not satisfactory when Lewis bases such as organophosphines and pyridine are present. Large quantities of magnesium bromide and iodide from Grignard alkylations also interfere. Electroohemicd methods are well suited for homogeneous solutions, but not for semi-did residues in which metallic gold forms on long standing. After experimenting with several chemical processes, the method outlined below was developed. It should be useful in orgmometallic and coordination chemistry laboratories where Lewis base adducts of gold are encountered. The recovered gold is particulate and may be used without further powdering. The concentrated residues are diluted with water, adjusted to pH 9-10 with potassium hydroxide, and heated to near dryness in a beaker. The process is repeated until the odor of anlines and phosphines is no longer detectable over the solution. Concentrated nitric acid is used to acidify the mixture which is then added in small batches to a large porcelain evspporating dish containing hot concentrated sulfuric acid. Care must be exercised as spattering and foaming may occur. Iodine and bromine will be liberated if present; a. thick tar-like foam of organic matter may float to the surface and be digested slowly. These oxidation reactions should be complete before additional residues are added to the sulfuric acid. Traces of metallic gold may form on the surface of the mixture and sides of the vessel during the additions. Concentrated sulfuric acid from a. polyethylene wash bottle should be used to rinse the sides occasionally. After all the residues have been added, heating is increased to concentrate the mixture. Dense white fumes of sulfur trioxide ?re evolved, and metallic gold forms on the evaporating dish. When the solid matter on the bottom is visible, sulfuric acid should be added and the concentration procedure repeated. The resulting mixture is cooled (but not to the point of solidifying), transferred to s beaker containing a largeexcess of water, andmechanically stirred for at least an hour. Vigorous stirring will help break spongy clumps of metal into smaller particles. The liquid together with suspendedmaterid is decantedleaving behind the gold metal which rapidly settles to the bottom. (Note the similarity to panning for gold!) Several washings may be necessary to remove all the suspended matter. The gold is washed with 3M hydrochloric acid to remove some soluble material and leach out any reactive metal contaminants. Final washings with water and methanol in a. fine sintered glass funnel give a product of 99+% purity. The washings may be combined and evaporated to begin a, new batchof gold residues. Additional purification can he effectedby rossting a t 350-400°C. (At higher tempecrtures sintering of the gold occurs.) Necessary ssfety precautions are (a) use 8. good hood to carry off the noxious fumes, (b) use of gloves and shields to protect the hands and fece from splattering acid, (c) carefulwatching of the reaction as unexpected foaming may occur. In five days, we obtained 132 g of gold from 4 1 of residues which had accumulated for several years. The current market value8iis $305.
' STYNES,D. V., Univemity of British Columbirt, personal communication. BRAUER, G., ed, "Handbookof Preparative Inorganic Chemistry," Academic Press, New York, Val. 2, 1965, p. 1054. 365/oe. for 200 mesh gold, D. F. Goldsmith, Evanston, Ill. C. FRANKS a w I11 R. STUART TOBIAS PWRDUE UNIVERSITY LAFAYETTE, 1 ~ ~ 1 ~ ~ ~ 4 7 9 0 7
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Journal of Chemical Education
