Continuous liquid–liquid extraction using modified subcritical water for the demetalisation of used industrial oils Original Research Article

Modified subcritical water is proposed as an extractant for the development of a liquid–liquid extraction method for the demetalisation of used industrial oils. The two immiscible liquid phases (the used oil and water modified with 4% (v/v) HNO3+0.1 M KCl) enter into contact in an approach designed by the authors, consisting of an extraction coil in stainless steel located into an electrically heated oven. After close contact between the two immiscible phases for proper mass transfer, the oil–water segments leave the oven through the restrictor, then are cooled by passage through a coil located in a bath at room temperature. Spontaneous separation of both phases takes place in the collection flask at the end of the cooling coil. The metals (Cu, V, Pb, Ni, Cd and Cr) extracted into the aqueous phase are determined by graphite furnace atomic absorption spectrometry. An in-depth study of variables affecting the demetalisation-extraction step as well as those concerning 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 the oil) for Cu, V, Pb, Cd, Cr and Ni, respectively. While the extraction time 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.