Dissolution of Precious Metals in Supercritical Carbon Dioxide

Precious metals including copper, gold, and palladium can be dissolved in supercritical CO2 by oxidation with HNO3 and subsequent chelation with hexafluoroacetylacetone to form CO2-soluble metal (beta)-diketonate complexes. The oxidizing agent HNO3 is carried into the supercritical fluid phase by the CO2-soluble Lewis base tri-n-butyl phosphate (TBP) as a complex of the general form TBP(HNO3)x(H2O)y. For the dissolution of Pd metal, a Lewis acid-base complex of the form TBP(HNO3)1.0 (H2O)0.4 is required. Au and Cu can be oxidized with a lesser amount of HNO3 in the complex. This supercritical fluid dissolution technique provides a "dry" method for recovering precious metals from abandoned electronics and spent catalysts with minimum waste generation.