The effect of temperature on hydroxyoxime stability in the LIX 63 – Versatic 10 – tributyl phosphate synergistic solvent extraction system under synthetic nickel laterite conditions

The chemical stability of LIX® 63 hydroxyoxime (‘hydroxyoxime’) when in the presence of Versatic 10 and tributyl phosphate (TBP) has been assessed under synthetic nickel laterite extract conditions between 30 and 60 °C and strip conditions at 60 °C.In all extract systems, nominally at pH 5.5, syn to anti hydroxyoxime isomer inter-conversion was observed. The hydroxyoxime half life under extract conditions at 30 °C was almost two years. Increasing the operating temperature to 60 °C under otherwise identical conditions saw this decrease to 8 weeks. Hydroxyoxime degradation resulted in the generation of previously identified products 5,8-diethyl-6,7-dodecanedione (diketone) and 5,8-diethyldodecan-6-oxime-7-one (‘keto-oxime’) and the nett loss of 5,8-diethyl-7-hydroxy-6-dodecanone (acyloin), as expected. These products do not quantitatively account for all hydroxyoxime loss. The activation energy for hydroxyoxime decomposition under the chosen extract operating conditions was calculated to be 70 kJ/mol, with a pre-exponential factor of 9.0 × 109 per week.The hydroxyoxime half life when determined under strip conditions, using 10 g/l sulphuric acid, at 60 °C was 26 weeks, with degradation yielding diketone and acyloin in a near-stoichiometric manner. Analysis, in conjunction with studies performed previously, allowed the activation energy for hydroxyoxime decomposition under strip conditions to be determined as being 58 kJ/mol, with a pre-exponential factor of 3.0 × 107 per week.

► LIX 63 stability in the presence of Versatic 10 and TBP was assessed under synthetic nickel laterite conditions. ► Tests were performed between 30 and 60 °C under extract conditions at pH 5.5. ► Decreasing temperature increased the half life, reaching 105 weeks at 30 °C. ► Strip stage stability was also assessed at 60 °C. ► Activation energies of hydroxyoxime degradation under extract and strip conditions were determined.