Extraction behavior of nickel(II) in ammoniacal sulfate solution with sterically hindered β-diketone

Abstract

► Nickel extraction from ammoniacal solution is sensitive to pH and total ammonia concentration. ► The formation of nickel ammine species in the aqueous phase depresses nickel extraction. ► The hydrated nickel extracts in the organic phase can decrease nickel extraction. ► Ammonia can be co-extracted in the form of NiA 2 ·NH 3 in the organic phase. ► Extraction reaction of nickel is endothermic at pH 7.5 but exothermic at pH 8.5 and 9.5. The extraction behavior of nickel in ammoniacal sulfate solution using sterically hindered β-diketone (4-ethyl-1-phenyl-1,3-octadione, HA) was investigated systematically by consideration of various parameters, i.e. HA concentration, aqueous pH, total ammonia concentration, ionic strength and temperature. The results indicated that the nickel extraction is dependent not only on the pH, total ammonia concentration and temperature in the aqueous phase, but also on the nickel extract in the organic phase. It was found that the increase in the total ammonia concentration and pH depresses the nickel extraction in ammoniacal solutions. The overall extraction process is endothermic at pH 7.5 and exothermic at pH 8.5 and 9.5, respectively. This can be mainly attributed to the formation of nickel ammine species in the aqueous phase. Extraction of ammonia and water in the organic phase was studied in the presence and absence of nickel. And the structure of nickel extracts was examined by the UV–Vis and FT-IR spectroscopies. The results showed that ammonia molecule should be co-extracted by coordinating with nickel extracts (NiA 2 ) as the form of NiA 2 ⋅NH 3 . The water molecule is associated with β-diketone molecule by intermolecular hydrogen bonding in the absence of nickel, but extracted as the form of hydrated nickel extracts in the presence of nickel.