The selective removal of Zn with Na2S from a mixture of Zn and Ni was studied in a continuously stirred tank reactor. At pH 5 and pS 18 the selectivity was improved from 61% to 99% by reducing the supersaturation at the dosing points by means of the reduction of the influent concentrations. The particle size distribution (PSD) of the precipitates increased as the supersaturation decreased up to a mode of about 22 mu m at pH 5. PSD values on the 100-200 mu m range were obtained when the sulfide was dosed through a polytetrafluoroethylene (Teflon) membrane at low supersaturation. Zn precipitated as sphalerite, whereas Ni mainly formed amorphous particles with a pH dependent stoichiometry. (C) 2010 Elsevier B.V. All rights reserved.