| Ionic liquids have been widely used as extractants in recent years due to the advantages of satisfactory extraction percentage and selectivity.But ionic liquids have the disadvantages of high viscosity,water pollution and loss of extractant caused by ion exchange mechanism.Therefore,we designed and synthesized a neotype soft alkali extractant,1-hexyl-3-methylimidazole-2-thione(HMImT).The solubility of HMImT in aqueous phase was determined by high performance liquid chromatography(HPLC)and the value was 0.015%which was little and almost could be ignored.As many problems exist in the process of extraction,such as difficult extraction of Rh and Pt,poor separation effect of Pd,Pt,Rh and Ir,the loss of extractant,and environmental pollution.In this paper,HMImT was used to extract and separate Pd,Pt,Rh and Ir from HCl medium.The extraction mechanism is neutral complexing or ion association mechanism,which could avoid the loss of extractant.The main content of this paper contains three parts as follows:First,we investigated the liquid-liquid extraction of Rh by HMImT from HCl solution.Rh(Ⅲ)is usually a stable valence in HCl medium,however Rh(Ⅲ)exists in a variety of forms which are difficult to be extracted and result in complicated extraction mechanism.Rh(Ⅲ)was reduced to Rh(Ⅰ)when SnCl2 solid was added,which greatly improved extraction effect.The behavior of extracting Rh(Ⅰ)was evaluated.Extraction equilibrium was reached when time was 20 min and the complete reduction of Rh(Ⅲ)to Rh(Ⅰ)only need 3 min.HMImT is an efficient extractant and extraction percentage of Rh(Ⅰ)reached 99%when the dosage of HMImT is 6 times of Rh(Ⅰ).HMImT has wide application since extraction percentage of Rh(Ⅰ)is hardly influenced by HCl concentration.Then XPS spectra,FT-IR spectra and molar ratio method were used to analyze the extracted complexes and thestoichiometry of HMImT-Rh complexes was 2:1.UV-vis spectra,1H NMR spectra,titration method and quantum chemical calculations was used to confirm neutral complexing mechanism of Rh(Ⅰ).The extraction percentage of Rh(Ⅰ)is 99.6%while the extraction percentage of base metals(Fe(Ⅲ)、Mn(Ⅱ)、Al(Ⅲ)、Ce(Ⅲ)、Li(Ⅰ)、Mg(Ⅱ))was around 10%.Thus HMImT showed excellent extractability and high selectivity for rhodium(Ⅰ)extraction.Then we investigated the separation of some platinum metals.Though liquid-liquid extraction has advantages of simple operation,high extraction efficiency,but its disadvantages of solvent pollution or high price of ionic liquids as dispersant limit its application.Therefore liquid-liquid extraction was replaced by microextraction in this system.The greatest characteristic of this system is thatthe separation of four kinds of metals is achieved by sole HMImT.Pd(Ⅱ)was first separated by HMImT from mixed metal solution and the extraction percentages of Pd(Ⅱ),Rh(Ⅱ),Pt(Ⅳ)and Ir(Ⅳ)were 99.4%,0.9%,2.0%and 2.3%separately.So Pd(Ⅱ)was completely separated.Pt(Ⅳ)was secondly separated from mixed solution by adjusting HCl concentration to 9.0 M and the extraction percentage of Pt(Ⅳ),Rh(Ⅲ)and Ir(Ⅳ)were 99.8%,0.9%and 0.5%.Thirdly,we separated Rh(Ⅲ)and Ir(Ⅳ)according to our previous study of HMImT extracting Rh(Ⅰ)in which SnCl2 was added to aqueous phase,and the extraction percentage of Rh and Ir were 99.6%and 0.1%.After finishing separation,we investigated extraction capacity and purity.HMImT shows outstanding extraction capacity(Pd 0.54 mmol/g,Pt 0.14 mmol/g,Rh 0.82 mmol/g)and perfect purity(beyond 99%).We have known the extraction mechanism of Rh(Ⅰ)and Pd(Ⅱ)according to our previous work and our team work,so the emphasis of this section was investigation of Pt(Ⅳ).Based on Job’s method,FT-IR spectra,UV-vis spectra,1H NMR spectra and theoretical natural population analysis(NPA),the ion association mechanism of HMImT on Pt(Ⅳ)extraction was confirmed.By precipitating-calcining method,Pd(Ⅱ),Pt(Ⅳ)and Rh(Ⅲ)could be recycled in the form of Pd,Pt2O and Rh2O3.Finally,we improved the extraction method of Pt(Ⅳ).Separation of Pt(Ⅳ)mentioned above was under the condition of 9.0 M HCl from mixed solution which destroyed extractant and caused water pollution,thus we investigated new method to reduce extraction acidity.Pt(Ⅳ)was reduced to Pt(Ⅱ)when SnCl2 was added,which lowered extraction acidity to 0.8 M HCl and improved extraction percentage to 99.4%.Meanwhile the ionic association mechanism was transformed into neutral complex mechanism.To investigate the potential application of HMImT extracting platinum metals,extraction process was modeled by Langmuir,Freundlich and Dubinin-Radushkevich isotherms,furthermore the thermodynamic and kinetics models were also studied,which would have some guidance importance to the industry application. |
