Separation of nickel and calcium by solvent extraction using mixtures of carboxylic acids and alkylpyridines

The solvent extraction of some base metals by mixtures containing a carboxylic acid and an alkylpyridine synergist was studied, with particular reference to the separation of nickel from calcium in sulphate media. The dependence of the synergistic shift in the pH(50) value (Delta pH(50)) and the Ni-Ca separation (pH(50)(Ca)-pH(50)(Ni)) on the identities of the carboxylic acid, the alkylpyridine and the organic diluent was examined. The effect of extractant concentration in the organic phase and the nature of the anion present in the aqueous phase (nitrate, chloride and sulphate) was also investigated. Large synergistic shifts were found for nickel, whereas antagonistic shifts were found for calcium, resulting in marked enhancement of the Ni-Ca separation, e.g. from 1.04 pH units for Versatic 10 acid (0.50 M in an aliphatic diluent) to 3.48 pH units for its mixture with 4-(5-nonyl)pyridine (0.50 M). The selectivity series for the extraction of divalent metals from sulphate media by the above-mentioned mixture lies in the order (pH(50) values in parentheses): Cu (3.16) > Ni (4.73) > Zn (4.94) > Co (5.41) > Fe (5.65) > Mn (6.45) > Ca (7.96) > Mg (8.43). This series suggests that in the recovery of nickel from laterite leach liquors, the mixed extractant systems would allow the early rejection of manganese, ferrous iron, calcium and magnesium to the raffinate, whilst the more valuable impurity metals (copper, cobalt and zinc) would report to the nickel strip liquor, from which they could be subsequently recovered. In batch countercurrent experiments, a simulated leach liquor containing (in g L-1): Ni 5, Mg 5, Mn 2, Co 0.5, Ca 0.5, Cu 0.1 and Zn 0.1 was contacted with the mixed extractant in three stages at unit organic-to-aqueous phase ratio and an equilibrium pH of 5.8. The recovery of nickel was 99.9% and that of cobalt was 98.8%, with co-extractions of manganese, calcium and magnesium of only 4.2%, 1.3% and 0.04%, respectively. Stripping of nickel with 0.25 M sulphuric acid was essentially complete (greater than or equal to 98%) in a single stage at an organic-to-aqueous phase ratio of 3.33, uptake of acid by the alkylpyridine component of the mixed extractant being avoided by operating the stripping stage in the equilibrium pH range of 3-4. The nickel-containing skip liquor can be purified by removal of the co-extracted cobalt, copper and zinc using a commercial organophosphinic acid extractant. (C) 2000 Elsevier Science B.V. All rights reserved.