Continuous liquid-liquid extraction using modified subcritical water for the demetalisation of used industrial oils

Modified subcritical water is proposed as an extracrant for the development of a liquid-liquid extraction method for the demetalisation of used indust rial oils. The two immiscible liquid phases (the used oil and water modifie d with 4% (v/v) HNO3 + 0.1 M KCl) enter into contact in an approach designe d by the authors, consisting of an extraction coil in stainless steel locat ed into an electrically heated oven. After close contact between the two im miscible phases for proper mass transfer, the oil-water segments leave the oven through the restrictor, then are cooled by passage through a coil loca ted in a bath at room temperature. Spontaneous separation of both phases ta kes place in the collection flask at the end of the cooling coil. The metal s (Cu, V, Pb, Ni, Cd and Cr) extracted into the aqueous phase are determine d by graphite furnace atomic absorption spectrometry. An in-depth study of variables affecting the demetalisation-extraction step as well as those con cerning the hydrodynamic variables was performed. The method was validated using a Certified Reference Material (C.R.M. 1086a; Wear Metals in Oil, N.I .S.T., Gaithersburg, MA) and applied to five different used industrial oils . A residence time of 270 s provides oil samples purified to 75, 85, 95, 90 , 98 and 90% (with respect to the initial concentration of each metal in th e oil) for Cu, V, Pb, Cd, Cr and Ni, respectively. While the extraction tim e was 270 s, the complete method was achieved in about 40 min. The proposed approach proves to be cheap, quick and avoids handling the organic samples . (C) 2001 Elsevier Science B.V. All rights reserved.