Solubility of Organophosphorus Metal Extractants in Supercritical Carbon Dioxide

The solubility ( S ) of several liquid organophosphorus compounds, tributyl phosphate (TBP), diisodecylphosphoric acid (DIDPA), di-(2-ethylhexyl)phosphoric acid (DEHPA), dihexyl-( N, N -diethylcarbamoyl)methylphosphonate (CMP), and octyl(phenyl)( N, N -diisobutylcarbamoyl)methylphosphine oxide (CMPO), in supercritical CO 2 was determined over temperature and pressure ranges of 30−90 °C and 7.5−25 MPa. The solubility of the compound increases with increasing density (ρ) of CO 2 fluid, showing a linear relationship: ln S = p ln ρ + q ( p and q are constants). The slope p measured at 60 °C is 21.8, 11.4, 13.1, 10.8, or 7.5, and the constant q, given solubility at ρ = 1 g/mL, is 26.3, 0.4, 2.7, 5.1, or −0.1 respectively for TBP, DIDPA, DEHPA, CMP, or CMPO. A homogeneous mixture of TBP or CMP with CO 2 is readily obtained even at relatively low pressure, where the density of CO 2 is relatively low. It is found that all the compounds examined are soluble enough in CO 2 to prepare an organophosphorus compound−CO 2 mixture which can be used in supercritical CO 2 fluid extraction of metal ions.