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 0.5 l to which solutions of ZnSO4 (800–5800 mg l-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 3.2×10^-19 to 3.2×10^-4 mg l-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 (mu)m for S2levels dropping from 3.2×10^-4 to 3.2×10^-18 mg l-1. At 3.2×10^-11 mg l-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 mg l-1 while ZnS particles formed with a mean geometric diameter of about 10 (mu)m. Below 3.2×10^11 mg l-1 of S2- insufficient sulfide was added for complete zinc precipitation. At S2- levels higher than 3.2×10^-11 mg l-1 the effluent zinc concentration increased due to the formation of soluble zinc sulfide complexes as confirmed by chemical equilibrium model calculations.