The separation of nickel and cobalt in a mixed sulfide precipitate
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C o 2+ -q-
H2S + CuS
CuS -]- 02 -]- xNH3 - - Cu" x N H 3 2+ + 5042-
[21 [3]
(The terms x and y are the average ligand numbers of copper and nickel ammines, respectively. The hydrogen sulfide generated in Reaction [2] could be used to precipitate more of the mixed cobalt/nickel sulfide.) All reagents were of analytical reagent quality, except sodium sulfide nonahydrate, which could not be purchased as such because of its deliquescent nature. According to the manufacturers specification, it was free of heavy metal impurity and had a purity of better than 95 pct. The mixed cobalt/nickel sulfide was prepared by dissolving 70 g (0.25 mole) each of cobalt sulfate heptahydrate and nickel sulfate heptahydrate in two liters of water and slowly precipitating the mixed sulfide at 60 ~ with a solution of 134 g of sodium sulfide nonahydrate in 500 ml (a 10 pct excess). The sodium sulfide solution was added slowly over 10 minutes and the mixture stirred during the precipitation. At the completion of the precipitation, the mixed sulfide settled, the supernatant was decanted, and the residue was washed and centrifuged at room temperature three times with deionized water. R.I. HOLLIDAY, Senior Lecturer, is with the Department of Chemistry, University of Western Australia, Nedlands 6009, Australia. Manuscript submitted August 24, 1990.
METALLURGICAL TRANSACTIONS B
The lixiviant to extract nickel from the mixed sulfide was 0.5 M with respect to copper II ion for all leaches and 6.0 M with respect to total ammonia for the copper II ammine leaches. The volume of lixiviant for each run is given in Table I. The mixed sulfide and lixiviant were heated at 90 ~ (363 K) for 1 hour in a closed container and then centrifuged and the residue washed and centrifuged three times. All supernatants were mixed and diluted so that the final concentration of metal ions was suitable for atomic absorption analysis. Effectively, the dilution was equivalent to making 250 liters of solution from the supematants, although, in actuality, the total volume of solution made was much less than this. To remove cobalt from the copper/cobalt sulfide residue, a 100 pct excess of 2 M sulfuric acid was added to it and the whole heated to about 80 ~ on a hot plate. It was then centrifuged and washed three times, as before. All of the supernatants were mixed and diluted to a fixed volume for analysis. The final residue was continually boiled with an excess of concentrated nitric acid, aliquots being added until the whole was dissolved. It was again diluted for analysis. All three final solutions (labeled A, B, and C, respectively in Tables I and II) were analyzed for nickel, cobalt, and copper by atomic absorption spectrometry. Theoretically, if there is an excess of a mixed cobalt/ sulfide precipitate reacting with an aqueous copper II solution, all of the copper should be precipitated as sulfide and the ratio of aN~ 2+ /aco 2+ (hereafter called the selectivity - 2+ 2+ ratio) should equal Ksp(Nis)/KspfCoS),w h e r e ahi and aco are the activities of the appropriat
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