Stability of the Versatic 10-decyl-4-pyridinecarboxylate system under continuous SX conditions: chemical analysis and phase disengagement studies

The chemical stability of n-decyl-4-pyridinecarboxylate (decyl-4PC) in a Versatic 10/decyl-4PC/aliphatic diluent system under conditions expected in a typical plant solvent extraction (SX) circuit has been assessed. This was achieved by monitoring phase disengagement properties and chemical composition of each organic solution with time when tested under the following conditions: continuous batch mixing over 12 weeks at 40 degreesC and pH 3.0 (long-term stability); continuous batch mixing over 20 days at 60 degreesC and pH 3.0 (high-temperature stability); and continuous extraction-stripping operation (approx. 600 cycles; pH 5.8 and 3.0) over 16 days at. 40 degreesC (operating stability). Nickel, cobalt, zinc and copper were present in all aqueous solutions. Samples taken throughout each trial were analysed by gas chromatography (GC) and high-performance liquid chromatography (HPLC) for time-related compositional changes. GC analysis revealed the time-related appearance and increase of n-decanol in the continuous mixing trials, attributable to hydrolysis of decyl-4PC. A loss of approximately 1.0% of the total amount of decyl-4PC per annum by hydrolysis was calculated to occur with continuous mixing at 40 degreesC and pH 3.0, increasing to about 5.9% per annum at 60 degreesC and pH 3.0. A smaller loss of decyl-4PC by hydrolysis would be expected in an operating SX plant where only a fraction of the organic is undergoing mixing at any given time. No other products arising from ongoing degradation of decyl-4PC were detected by GC or HPLC in any of the trials. No adverse changes in phase disengagement were noted in the decyl-4PC systems as a function of extended mixing time and mixing temperature. It is concluded that commercial application of this system to separate nickel and cobalt from calcium, magnesium and manganese would not be compromised by the instability of this synergistic mixture. (C) 2004 Elsevier B.V All rights reserved.