Development of tertiary thioamide derivatives to recover palladium(II) from simulated complex chloride solutions

• Two tertiary thioamides, MPHTA and MCHTA in toluene, recover Pd(II) from HCl media.
• Compounds stay effective upon sequential SX cycles with modified commercial diluents.
• Extractant:Pd(II) molar ratios of 1.7 are reached in toluene, at saturation.
• Selectivity profiles are encouraging, since only Fe(III) affected Pd(II) extraction.
• Thioamides did not noticeably extract any of the metals tested, except Pd(II).

Two thioamide derivatives, N-methyl-N-phenyl-octanthioamide (MPHTA) and N-methyl-N-cyclohexyl-octanthioamide (MCHTA), were synthesized from their amide analogues, adequately characterized, and systematically evaluated as extractants for palladium(II) from chloride media. In sequence of previous research, which revealed the adequacy of both compounds, when dissolved in toluene, to efficiently recover Pd(II) from a wide range of differently concentrated hydrochloric acid solutions, the Pd(II) solvent extraction performance of both MPHTA and MCHTA dissolved in commercial diluents is evaluated in this work. Additionally, results obtained on successive extraction–stripping cycles, involving thiourea as stripping agent, confirm the robustness and stability of these Pd(II) solvent extraction systems, even when commercial diluents are employed. Equilibrium extraction isotherms, obtained for both compounds in toluene, show a high Pd(II) loading capacity. Selectivity data has been collected for complex 4.0 M and 6.0 M HCl mixtures containing two, three, four or five metal ions — palladium(II), platinum(IV), rhodium(III), aluminum(III), and iron(III). Fe(III) is the only metal that affects Pd(II) extraction of both MPHTA and MCHTA, as Pd(II) distribution ratios are significantly reduced; on the other hand, the other metals are practically not extracted. These selectivity results are encouraging from a practical point of view, since both extractants may prove suitable to efficiently and selectively recover Pd(II) from hydrometallurgical solutions coming from the leaching of secondary raw material sources.

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