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excess of 'base metals', such as copper and iron. With the growing emphasis on economically feasible recycling and recovery processes, there are increasing demands for highly selective separation techniques and reagents for the separation and recovery of precious metals from low quality ores and secondary sources. High selectivity can be provided by macrocyclic extractants. SELECTIVE MACROCYCLIC EXTRACTANTS FOR GOLD A vast array of experimental data available demonstrates that the most effective Au(III) sorbents and extractants from HCl solutions should be found among the compounds containing atoms of ether and oxide oxygen as reaction centres (15). On going from linear polyethers to their macrocyclic analogues, crown ethers have been shown to be the most promising class of extractants for selective gold recovery from chloride solutions. The size of the macrocyclic cavity essentially influences the gold extraction. Derivatives of 18- crown-6 were used by Yakshin and coworkers (15) in the extraction of trace quantities of gold from HC1, while Fang and Fu (16) used benzo-15-crown-5 in the presence of potassium chloride to extract Au(III). Trom et a1 (17) studied the solvent extraction and transfer through bulk liquid membranes of gold and silver cyanide complexes using dicyclohexano-18-crown-6. The same group investigated the solvent extraction and transport through a supported liquid membrane of metal cyanide complex salts of gold (I) and silver (I) by macrocyclic extractant carriers (18). Bradshaw et a1 have developed silica gel bonded thiamacrocycles which have shown high selectivity for Au (III) (19). GOLD EXTRACTION USING SUPERCRITICAL FLUIDS Reports of gold extraction by SFE are few and far between but what is in the literature is interesting. Wai et a1 have reported the extraction of Au (III) from cellulose paper using bistriazolo-crowns in supercritical CO2 (20). In the presence of 5% methanol as a modifier and the addition of microlitre quantities of water, up to 80% of the spiked gold was extracted. In the absence of the modifier and spiked water, the extraction efficiency was insignificant. Otu recently reported the desorption of gold from activated carbon using supercritical carbon dioxide (21). Ion pair solvation of sodium dicyanoaurate by tributyl phosphate facilitates the charge neutralization necessary for the elution of the ionic Au(CN)z- by the non-polar supercritical carbon dioxide. The objective of this research into supercritical fluid extraction is to avoid the use of organic modifiers and to maximize the extraction selectivity for gold. The key here is the matching of the gold coordination chemistry with a SF-C02 soluble ligand, such as through the careful choice of donor atoms in a macrocyclic cavity. In the work reported in this paper, the use of new fluorinated macrocyclic reagents (Figure 2), based on the aptly named calixarene 'molecular baskets' is described. The SFE of Au(III) from spiked cellulose-based filter paper is chosen as a convenient approach to test the efficiencies of the new reagents. 54 <:00' ColdBulletin 1999,32(2) RRR R Figure 2 Chemicalstructuresofthefluorinatedcalixarenes studiedfortheSFEofgold Cl: R = -(CHZ}sS(CHz}z(CFz};£F3; R'= -CHz(C=O)NHOH C2: R = -(CHz}3S(CHz}z(CFz}7CF3; R'= -CHz(C=S}N(CzHsJz C3: R = -(CHZ}sS(CHz}z(CFz}7CF3; R ' = -CHz(C=O)N(CHzCH;:SH)(CHCH3CHz C H ) C4: R = tert-butyl-; R'= CHz(C=O)O(CHz}zNH- (C=S)NH(CHz}3S(CHz}z(CFz}7CF3 EXPERIMENTAL Materials A 1000 ppm Spectrosol standard solution of Au(III) as AuC14- was obtained from Merck (Germany). Hydroxylammonium chloride (NH20H.HC1) was also bought from Merck. All CO2 gas cylinders were fitted with dip tubes and bought from Irish Oxygen (Cork, Ireland). All extracted samples after SFE were collected in either methyl iso-butyl ketone (MIBK) or DMSO (both purchased from Merck) as indicated.PDF Image | Carrying Gold in Supercritical C02
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