Solvent Extraction Separation Of Platinum, Palladium And Rhodium And Its Application In The Recovery Of Platinum Metals From Secondary Resources

The platinum metals-containing wastes are one of important and valuable secondary resources. Recovery of platinum metals from secondary resources is significant on sustainable development and recycling economic. Solvent extraction technique has been widely used in the recovery of platinum metals from secondary resources due to its strong extraction and separation ability. The studies of this technique focus on the selection of novel extractants, research of extraction performance of platinum metals, mastery of extraction mechanism of platinum metals and development of high efficient extraction technique are emphases for the application of solvent extraction technique in the field of recovering platinum metals from secondary resources.In order to know the extraction mechanism of platinum metals from molecule structure and develop the high efficient extraction technique, two kinds of extractants, namely iso-amyl benzothiazolyl sulfoxid (ABSO) and N-butyl isooctylamide (BiOA), were synthesised.ABSO was used in the extraction of palladium( II) from hydrochloric acid media. The effects of extractant concentration, HC1 concentration, temperature, H⁺ and Cl^-concentration on the percentage extraction of Pd( II) with ABSO had been studied. The percentage extraction of Pd(II) achieved 99.72% from 0.1 molL^-1 HC1 with 0.5 mol·L^-1 ABSO as an extractant. Pd(II) was recovered using 1 mol·L^-1 ammonium solution as the stripping agent. The distribution coefficient of Pd(II) was not affected by the H⁺ concentration, but decreased dramatically with the increase of Cl" concentration. The stoichiometry of the extracted species had been found as PdCl₂·2ABSO with the molar ratio of palladium( II) to extractant 1:2.The X-ray crystal structure of the extracted species which was not reported previously was obtained, which explained the extraction mechanism of ABSO extracting Pd( II) from molecule structure. The extraction mechanism was determined to be neutral coordination. The crystal structure reveals the PdCl₂(ABSO)₂ complex as a whole is in complete symmetry with palladium atom at the symmetry center of the quare-planar complex, and ABSO coordinates with Pd(II) via thiazole N atom. This result indicates that sulfoxides containing thiazolyl substituent are quite different from general dialkyl sulfoxides which are usually coordinated with Pd( II) via O or S atom.The extraction of Pt(IV) from HC1 media with N-butyl isooctylamide was studied.The results suggested that the percentage extraction of Pt(IV) achieves 99.5% by using 2 mol-L"1 BiOA-5%TBP in octane to extract Pt(IV) in 5 mol-L"1 HC1 media. This is an exothermic reaction with an enthalpy of -24.18 kJ-mol"1. Water is a high efficient stripping agent to recovery of Pt(IV). The stoichiometry of the extracted species had been found as (BiOAH+)2-PtCl62" by using spectral analysis and slope analysis method. The extraction mechanism could be determined to be an ion-pair formation with the protonated BiOA.The direct extraction of Rh(III) from chloride solution with commonly used extractants is known not to be feasible due to the formation of mixed aquo-chloro complexes. The interaction of Rh(III)-Cl complexes with Sn(II) in aqueous chloride solution to form new Rh( I )-Sn( II) complexes can be used to achieve quantitative Rh extraction. BiOA was used in the extraction of Rh from hydrochloric acid media after pre-treatment of the Rh-containing chloride solution with stannous chloride. The Rh aquo-chloro complexes was completely convert to hydrophobic [Rh(SnCl3)5]4" when the mole ratio of Sn:Rh is 12 : 1. The formation of hydrophobic [Rh(SnCl3)s]4" largely increases the extractability of Rh with BiOA.A complete extraction of Rh in 2 mol-L'1 HC1 media has been achieved with 1.5 mol-L"1 BiOA-0.5 mol-L'1 TBP. The system of extraction of [Rh(SnCl3)5]4 with BiOA-TBP is a synergistic extraction. The ratio of BiOA and TBP in this synergistic extraction system is 3:1. The Rh loaded in organic phase is completely stripped with a mixture of 4 mol-L"1 HC1 and 0.03 mol-L"1 NaC103 as a stripping agent. The oxidation of Sn(II) to Sn(IV) in air, however, caused the breakdown of Rh( I )-Sn( II) complexes. The activation power will lose if the time of storage is too long. The stoichiometry of the extracted species had been found as (BiOA-H+)3(TBP-H+)-[Rh(SnCl3)5]4^. The extraction mechanism could be determined to be an ion-pair associated synergistic extraction with the protonated BiOA and TBP.Based on the results of extraction of Pd( II) and Pt(IV) with synthesis sulfoxide MSO as a extractant by our group, MSO was used in the extraction of Rh from hydrochloric acid media after pre-treatment of the Rh-containing chloride solution with stannous chloride. The extraction of activated Rh with MSO is a rapid and exothermic reaction. The percentage extraction of Rh achieves 99.7% from 2 mol-L"1 HC1 media with 1 mol-L"1 MSO in kerosene. The rhodium accumulated in the 1 mol-L"1 MSO can reach 13.6 g-L"1 or above. The percentage stripping of Rh achieves 90.5% with a mixture of 5 mol-L"1 HC1 and 0.03 mol-L"1 NaC103 as the stripping agent. The stoichiometry of the extracted species had been found as (MSOH3O+)4[Rh(SnCl3)5]4".And the extraction mechanism could be determined to be an ion-pair formation with the protonated MSO.Separation of palladium, platinum and rhodium from spent catalysts leaching solution with MSO and BiOA was studied. The separation process include two-stage cross-current extraction of Pd(II) with MSO in low acidity, two-stage cross-current extraction of Pt(IV) with BiOA in high acidity and two-stage cross-current extraction of activated Rh with BiOA. The total percentage extraction of Pd(II) achieved 99.9%, and the percentage stripping of Pd(II) achieved 99.9%. The total percentage extraction of Pt(IV) achieved 99.9% and the percentage stripping of the Pt(IV) was nearly 100%. The percentage extraction and stripping of Rh both achieved nearly 100%. The ratio of Pt to Pd was 1.5 X 10"4 in the stripping palladium solution. The ratio of Pd to Pt was 1.4 X 10"5 in the stripping platinum solution. The ratio of Pt to Rh was 3.4 X 10"4 in the stripping rhodium solution. These results indicate the process is feasible to separate Pd, Pt and Rh from the leaching solution of spent catalysts.