Cobalt separation from nickel in sulfate aqueous solution by a new extractant: Di-decylphosphinic acid (DDPA)

The ability of the recently-synthesised phosphinic acid di-decylphosphinic acid (DDPA) to separate Co(II) from Ni(II) in Ni–Co sulfate aqueous solution relating to the leach liquors obtained from the treatment of Ni–Co hydroxide precipitation sludge has been assessed. Factors investigated under batch test conditions included equilibrium pH, temperature, metal concentration, neutralization level of DDPA and operating matrix, namely sulfate, chloride and nitrate. The optimum equilibrium pH for separation was found to be 5.6. In the temperature range of 16–42 °C, the increase in temperature was favorable for the separation. DDPA was suitable for Co/Ni separation in the solution with high Ni concentration and low Co concentration. Co extraction ratio increased with the increase of neutralization level of DDPA. The maximum of separation factor was obtained at a neutralization level of 60%. Chloride ion was favorable for the separation, whereas the nitrate ion had almost no effect on separation. Based on the batch test, the Co separation from Ni in sulfate aqueous solution (Co 4.14 g/L, Ni 42.02 g/L, pH 4.14) was carried out with 15% vol DDPA in 5-stages at equal phase ratio. Finally, extraction ratio of cobalt reached 99.8% and the extraction separation factor of Co/Ni was up to 9.5 × 103. Experimental results showed that DDPA was a promising extractant for Co/Ni separation in sulfate aqueous solution.

► DDPA has the similar extraction performance with Cyanex 272.
► DDPA has an advantage over EHEHPA in Co/Ni separation.
► The optimum equilibrium pH for Co/Ni separation is 5.6.
► The optimum neutralization level of DDPA is 60%.
► Increasing temperature is favorable for the separation.