Selective recovery of critical metals from Base metal Refinery secondary leach liquor through Liquid-Liquid extraction

Process streams from Base Metal Refineries, after high pressure nickel reduction, still contain significant levels of critical metals (cobalt and nickel). These secondary leach liquor streams have not been given much attention, yet they pose an environmental concern due to the incomplete separation of the two metal ions. This study investigates and recommends novel conditions for the treatment of such unique process streams to selectively recover the valuable cobalt and nickel using bis(2,4,4-trimethylpentyl)phosphinic acid (Cyanex 272). The secondary leach liquor was an industrial solution from a Base Metal Refinery with 404.49 mg/L cobalt and 1535.6 mg/L nickel in ammonium sulphate (pH 7.2). The investigation was conducted through the determination of optimum extraction conditions, optimum stripping conditions, the extraction mechanism and process thermodynamic parameters. The Design Expert software was used to design the experiments and to optimise the process variables. The factors considered were; Cyanex 272 concentration (5 – 25 v/v%), Extraction Temperature (20 – 70 °C) and Organic to Aqueous ratios (1:4 – 5:2). The responses were Cobalt Extraction Percentage and Separation Factor. Selective cobalt extraction was significant within 3 – 5 v/v% Cyanex 272, 40 – 60 °C extraction temperature and 0.5O/A loading. The optimum conditions were 5 v/v% Cyanex 272, 60 °C extraction temperature and 0.5O/A loading. The extraction was endothermic within the range (20 – 40 °C) with a standard enthalpy change of 47.722 kJ/mol. The optimum Cyanex 272 concentration and O/A loading proved to provide the stoichiometric requirement for selective cobalt extraction. The maximum separation factor and percentage cobalt extracted were 169.78 and 88.85 % respectively. A second stage batch extraction resulted in the complete isolation of nickel in the raffinate. Selective stripping of the loaded organic phase resulted in a complete isolation of cobalt. By utilizing the new conditions recommended in this study, we were able to isolate cobalt and nickel aqueous streams completely. Fourier Transform Infrared Spectroscopy (FTIR) and Ultraviolet–Visible Spectroscopy (UV–VIS) analysis confirmed the formation of the tetrahedral cobalt-Cyanex 272 complex, the absence of the nickel-Cyanex complex and possible regeneration of the extractant. The authors recommend further studies into the design and implementation of the extraction process described in this paper using the data provided herein as a basis.