Control of the sulfide (S2-) concentration for optimal zinc removal by sulfide precipitation in a continuously stirred tank reactor.

Precipitation of Zn2+ with S2- was studied at room temperature in a continuously stirred tank reactor of 0.5l to which solutions of ZnSO4 (800-5800 mgl(-1) Zn2+) and Na2S were supplied. The pH was controlled at 6.5 and S2- concentration in the reactor was controlled at set point values ranging from 3.2x10(-19) to 3.2x10(-4) mgl(-1), making use of an ion-selective S2- electrode. In steady state, the mean particle size of the ZnS precipitate decreased linearly from 22 to 1 microm for S2- levels dropping from 3.2x10(-4) to 3.2x10(-18) mgl(-1). At 3.2x10(-11) mgl(-1) of S2-, the supplies of ZnSO4 and Na2S solutions were stoichiometric for ZnS precipitation. At this S2- level, removal of dissolved zinc was optimal with effluent zinc concentration <0.03 mgl(-1) while ZnS particles formed with a mean geometric diameter of about 10 microm. Below 3.2x10(-11) mgl(-1) of S2- insufficient sulfide was added for complete zinc precipitation. At S2- levels higher than 3.2x10(-11) mgl(-1) the effluent zinc concentration increased due to the formation of soluble zinc sulfide complexes as confirmed by chemical equilibrium model calculations.