“…However, many metal combinations exist that are difficult to separate from one another. A variety of extractants have been designed for the separation of critical metals such as rare earth metals, − precious metals, , and transition metals such as nickel (Ni 2+ ), cobalt (Co 2+ ), and so on. The selective extraction of Ni 2+ and Co 2+ from impurity metals such as zinc, magnesium, and calcium has been examined. − However, difficulties still remain for their separation from manganese (Mn 2+ ), which often coexists in large amounts in natural and secondary resources. , Commercial organophosphorous extractants, such as 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester (PC-88A), di-(2-ethylhexyl) phosphoric acid (D2EHPA), and bis(2,4,4-trimethylpentyl) phosphinic acid (Cyanex 272), , are typically used in hydrometallurgy; however, the separation of Ni 2+ and Co 2+ from Mn 2+ using these extractants is difficult because they are selective for Mn 2+ .…”