Acidic extractants (HA) are a type of extractant widely employed in solvent extraction. Such extractants have the peculiarity of releasing protons when extracting metal ions, making the extraction strongly dependent on the pH at which the system reaches equilibrium after extraction. Therefore, accurate adjustment of such pH is essential to selectively extract the target metal with satisfactory efficiency and purity [1].
Cyanex 272 (bis(2,4,4-trimethylpentyl) phosphinic acid) is widely used to extract cobalt (Co) from nickel (Ni)-bearing solutions as it ensures high selectivity for a broad range of Co/Ni ratios. To allow efficient separation, the pH of the system is typically adjusted to about 5 with a base (e.g., NaOH, NH3) [2]. The base can be added to the two phases during the mixing process to maintain a selected pH in all the extraction stages or, pre-neutralization of the solvent can be performed. Pre-neutralization, also known as saponification, involves contacting the solvent with the base before extraction to partially or completely convert the extractant from its acidic form (HA) to the corresponding salt (e.g., NaA). It is widely accepted in the literature that pre-neutralized extractants allow for easier control of the system pH. While Co extraction with saponified Cyanex 272 has been documented in various studies [3], [4], [5], the rationale behind the choice of the conditions (e.g., base concentration, temperature) at which the saponification is carried out is often not reported. Furthermore, determining the number of stages for counter-current operations with saponified organic phases is not always straightforward as the equilibrium pH might change among the stages.
In this work, the impact on saponification of NaOH concentration, the ratio of Cyanex 272 to diluent (Isopar L), and temperature are examined. The findings are then utilized to determine conditions (NaOH concentration and temperature) for the pre-neutralization of the solvent. To achieve this, pseudo-ternary phase diagrams for the system NaOH - Cyanex 272 - Isopar L for 2, 5 and 10 M NaOH were experimentally determined in a region of the diagram considered relevant for the application of the solvent in liquid-liquid extraction processes. A region with biphasic liquid-liquid characteristics, a monophasic microemulsion area, and a zone featuring a biphasic system made up of a diluent-rich phase and a diluent-depleted phase were identified. Based on the results, 10 M NaOH was selected for the pre-neutralization of the solvent at room temperature (21 ± 2°C).
An industrially pre-treated feed produced from Ni-rich NMC LIBs was used as feed for the solvent extraction experiments. End-of-life lithium-ion batteries (LIBs) are indeed one of the main secondary sources of Co, making the recovery of Co from this waste essential to secure a stable supply [6]. Initially, batch experiments were conducted to identify conditions suitable for Co extraction. It was found that 0.3 M Cyanex 272 in Isopar L granted satisfactory Co extraction at pH = 5.5 with a phase ratio of 1. A comparison between batch counter-current extraction simulations performed maintaining a fixed equilibrium pH of 5.5 throughout all the stages and using saponified Cyanex 272 is provided. Finally, some advantages and disadvantages associated with using saponified extractants in counter-current solvent extraction processes are discussed.