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174 E.J. Beckman / J. of Supercritical Fluids 28 (2004) 121–191 containing an extractant (one of a variety of amines, phosphates or oximes) to draw the copper selectively into the organic phase (usually a high flash point alkane mixture). The copper is then back-extracted into water, from where it is electrochemically re- duced (electrowinning) to pure (99.99%+) copper. The solvent extraction step is, from a process per- spective, somewhat simple, consisting of a series of mixer-settler tanks that are open to the environment. Previous work has shown that one can extract cop- per into carbon dioxide; further it is likely that one could synthesize a highly CO2-soluble analog to one of the currently used commercial extractants for cop- per. Hence, one could construct a CO2-based analog to the current solvent extraction process. However, it is not likely that the cost of such a step would justify the move away from the currently used organic solvents. At present, the solvent extraction/back extraction steps contribute ≈10–20% of the $0.2/lb processing cost of copper using SX-EW, assuming that >90% of the ex- tractant is recovered after each use [277]. Indeed, per- haps a far better target for green processing applied to copper refining would involve either conversion of the remaining traditional smelters over to SX-EW [278] or finding ways in which to lower the energy demand of the ore excavating/crushing/grinding process or the electrowinning step [279]. A further complication is that most copper refining is performed in either South America or Africa, where the regulatory and/or so- cietal driving force for adopting green chemical pro- cessing is substantially less than in either Europe or the US. Platinum group metals, either those derived from ore or during the recycling of catalytic converters or electronics components, are also refined using solvent extraction [280]. Here, the metal is extracted using strong acid (usually HCl), then purified by extraction into organic solvent using an auxiliary, where selec- tivity is achieved via both the design of the auxiliary and subsequent aqueous washing steps to remove un- wanted trace metals. The extraction is multi-step, in order to sequentially remove the gold, platinum, pal- ladium and other PGMs. The metals are then reduced either chemically or electrochemically and recovered. The opportunities for the use of carbon dioxide to re- place organic solvents in such processes mirror those in copper refining; here, however, the value of the metal is five orders of magnitude greater. Further, it has been shown that one can design CO2-soluble analogs to those compounds used to extract PGMs into organic solvents [281]. However, just as the value of PGMs makes the use of CO2 more viable, so too does it pro- mote the development of competing technologies. For example, IBC (Utah) has developed solid metal ab- sorbents comprised of macrocycles tethered to poly- meric resins [282]. These resins have been shown to selectively bind PGMs of various types, where the metals are recovered by back extraction following pro- cessing. If CO2 is to be competitive in this arena, the ligands must be selective, should be as inexpensive as possible and/or one must be able to recover them fol- lowing binding and release of the metal. Both the lig- ands and their metal complexes must be highly soluble at low pressures (preferably CO2’s vapor pressure) as throughputs in this application will be very high. As in the case of coffee decaffeination, it would be highly preferable to reduce and/or capture the metals without depressurization of the CO2. Given Watkin’s research, it may be possible, for example, to reduce the met- als using added hydrogen. Unlike in the case of con- ventional organic solvents, adding hydrogen to CO2 produces neither safety nor mass transport problems. There are two features of this process that weigh in favor of CO2: (a) the metal concentration is relatively low, meaning that employing a high ligand:metal ra- tio still allows for dilute ligand concentrations; and (b) aqueous flow rates can be higher than the point that causes breakthrough problems for solid sorbents. Hence, there may be opportunities for use of CO2 in this industry. Another application of potential interest is in the upgrading of so-called vacuum resid (or vacuum resid- ual) in petroleum refineries [283]. Vacuum resid refers to low vapor pressure (hence relatively high molecu- lar weight) fractions of the initial petroleum stream. In addition to hydrocarbons, this fraction contains a substantial quantity (over 1000 ppm) of a wide spec- trum of metals (owing to the concentration effects of numerous upstream unit operations). Included in this mix of metal contaminants are considerable amounts of vanadium and nickel, metals that can de-activate the catalysts employed to crack petroleum into use- able (salable) materials. Further, both the nickel and vanadium are complexed by porphyrin type materials present in the vacuum resid. If these metals could be easily and economically extracted, more of the initialPDF Image | Supercritical and near-critical CO2 in green chemical synthesis and processing
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