Palladium and platinum binding on an imidazol containing resin
Introduction
The development of catalytic systems (for the oil industry, fine chemistry, automotive catalysts ...) has led an increasing demand of industry for precious metals (such as those of the platinum group, referred to as PGMs). For some of these PGMs demand is not balanced by supply, due to limited resources. For these reasons, a series of metallurgical processes has been investigated during recent decades for recovering precious and strategic metals from low-grade ores and more specifically from industrial waste (spent catalysts, electronic devices, etc.) (Barakat and Mahmoud, 2004, Brooks, 1991). Particular attention has been paid to pyro- and hydro-metallurgical processes after waste grinding (Brooks, 1991, Pinheiro et al., 2004, Shams et al., 2004). Preliminary physical separation, such as gravimetric or magnetic separation, and so on, enables the relative proportion of these valuable metals to be increased and justifies the treatment of these secondary sources by conventional processes. In hydro-metallurgical processes the ground material is subjected to strong acidic leaching (using for example HCl solutions or aqua reggia) resulting in acidic mixtures containing a wide diversity of metals including PGMs and also base metals (BMs) (Angelidis and Skouraki, 1996, Gaita and Al-Bazi, 1995, Jafarifar et al., 2005, Kononova et al., 1998). Solvent extraction (Alguacil et al., 1997, Chavan and Dhadke, 2003, Hung et al., 2007, Kakoi et al., 1994, Lokhande et al., 1998, Rane and Venugopal, 2006, Regel-Rosocka et al., 2007, Sarkar and Dhadke, 2000) and impregnated resins/liquid membranes (Dakshinamoorthy and Venugopal, 2005, Guibal and Vincent, 2006, Kakoi et al., 1996, Kolev et al., 2000, Mimura et al., 2002, Rovira et al., 1999, Uheida et al., 2004, Zuo and Muhammed, 1995b) are powerful techniques for the recovery and separation of metals in concentrated solutions but its efficiency decreases for the treatment of less concentrated solutions. Sorbents, including ion-exchange resins and chelating resins, may be an alternative to solvent extraction processes (Antico et al., 1994, Guibal et al., 2001, Iglesias et al., 1999, Juang and Chen, 1997, Ruiz et al., 2000, Serarols et al., 2001, Warshawsky et al., 2002). A wide range of materials have been investigated for the recovery of PGMs from dilute solutions using biosorbents (Chassary et al., 2005, Fujiwara et al., 2007, Guibal et al., 2001, Inoue et al., 1995, Kim and Nakano, 2005, Parajuli et al., 2006, Ruiz et al., 2000, Wang et al., 2005) or synthetic resins (Beauvais and Alexandratos, 1998, Bogacheva, 2002, Guino and Hii, 2005, Hubicki and Leszczynska, 2005, Hubicki and Wojcik, 2004, Iglesias et al., 1999, Jermakowicz-Bartkowiak et al., 1999, Jermakowicz-Bartkowiak et al., 2005, Kaledkowski and Trochimczuk, 2006). Several resins for gold recovery have been developed at the Wrocław University of Technology (Jermakowicz-Bartkowiak et al., 1999, Kaledkowski and Trochimczuk, 2006, Kolarz et al., 1998, Kolarz et al., 1999, Kolarz et al., 2002, Trochimczuk, 2001, Trochimczuk, 2002, Trochimczuk et al., 1999).
Several steps should be taken into account for the optimal design of a sorption process: (a) equilibrium parameters (sorption isotherms) in function of the composition of the solution (pH, presence of competitor metals, counter ions, etc.); (b) kinetic data (influence of diffusion mechanisms on the control of equilibrium time); (c) concentration factors (including the desorption of the metals in a selected elution medium); and (d) recycling (sorption/desorption cycles). A fifth criterion is especially important in the case of PGM recovery from waste catalysts: the separation of PGMs from BMs is an important issue, together with the possible separation of PGMs from multi-component solutions. This scheme will be followed in the present study for the recovery of Pd and Pt from synthetic solutions using an imidazol-bearing resin. Imidazol groups are known to be very reactive for metal ions and more specifically for precious and noble metals (Kim et al., 2002, Lee and Tavlarides, 2002, Li et al., 2000, Liu et al., 2002, Venkatesan et al., 2007). The influence of pH (and HCl concentration) and the presence of chloride ions will be investigated; the recovery of Pd and Pt from multi-component solutions (Pd–Pt, and PGMs–BMs) will be also carried out. In the study of uptake kinetics, particular attention will be paid to diffusion mechanisms (influence of resin drying). Finally, the desorption of Pd and Pt from loaded resins will be tested, together with the recycling of the sorbents.
Section snippets
Preparation of imidazol resin
The synthesis of this imidazol-based resin has previously been described by Trochimczuk (1998). Fig. 1 describes the synthesis route. A copolymer of vinylbenzyl chloride and divinylbenzene, having the nominal cross-linking degree 2% and a chlorine content of 5.20 mmol Cl/g, was reacted with the sodium derivative of diethyl malonate. Diethyl malonate (2-times molar excess with respect to the chlorine content of the polymer) in dry THF was reacted with an equimolar amount of sodium hydride. When
Influence of HCl concentration
Pd and Pt are generally leached from exhausted catalysts and waste materials in very acidic solutions (mainly HCl solutions). The influence of acid concentration is a key parameter in the design of Pd/Pt sorption systems for metal recovery from acidic leachates. Fig. 2 shows the evolution of sorption capacity with increasing concentrations of HCl (from 0.1 to 5 M) for both Pd and Pt uptake. As expected, increasing the acidity resulted in a decrease in sorption capacities. Increasing the
Conclusion
The imidazol-bearing resin proved very efficient for Pd and Pt recovery from dilute acidic solutions. Sorption capacities reached up to 1 mmol metal g− 1 in HCl solutions as concentrated as 2 M (for highly substituted resin, containing 9.4% N). As expected, the sorption capacity depended on the substitution degree of the resin: comparison of two batches showed that reducing the nitrogen content to 80% of its previous value considerably reduced the sorption capacity. The resin has comparable
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