The feasibility of extending the aqueous biphase extraction process to mineral sulfides was investigated with pyrite (FeS2) as a model metal sulfide in polyethylene glycol (PEG)-based biphase systems. The phase diagram for the PEG-2000/Na2CO3/H2O system was determined; with increase in the PEG molecular weight, the binodal curve shifted in the direction of lower polymer and salt concentrations. The partitioning behavior of pyrite in PEG/Na2CO3/H2O and PEG/Na2SO4/H2O systems was investigated and the effects of conditioning time, pH, and PEG molecular weight were also studied. Pyrite particles distributed to the interface initially; but with increasing conditioning time, the particles transferred preferentially to the bottom salt-rich aqueous phase. Pyrite partition was also pH-dependent. When the pH was above 9, pyrite transferred into the salt-rich phase; between pH 5 and 9, it preferred the interface; and below pH 5, pyrite went into the top polymer-rich phase. The above trends an rationalized in terms of the effects of conditioning time and pH on the surface oxidization of pyrite and how this, in turn, changes the relative concentrations of hydrophilic and hydrophobic sites on the solid surface. (C) 2000 Elsevier Science B.V. All rights reserved.