Comparative study of dewatering characteristics of metal precipitates generated during treatment of monometallic solutions

The sludge dewatering properties (settling, filtration and centrifugation) of metal precipitates generated during treatment of monometallic solutions (0.020 mol/L) have been evaluated in this research. The precipitation tests carried out on 15 different metals gave metal removal yields generally similar to those predicted by MINEQL+ software, with the exception that kinetic aspects should be considered during precipitation of metal sulphides and phosphates. Hydroxides precipitation at pH 10.0 was the most efficient technique for the removal of eight metallic ions (Al3+, Cd2+, Co2+, Fe2+, Fe3+, Mg2+, Mn2+, Ni2+), whereas phosphates precipitation (at pH 6.0 with an addition of 0.0133 mol PO43−/L) gave highest removal yields for Ba2+, Ca2+, Cr3+. Sulphides precipitation (at pH 7.0 and using, 0.020 mol S2−/L) has been found the most efficient technique only for Cu2+ and Sn2+ precipitation, whereas carbonates precipitation (at pH 8.0 and using 0.020 mol CO32−/L) gave better removal yield only for Pb2+. Results have also shown that metal phosphates have generally better dewatering characteristics (SVI, filtration capacity, SRF, sludge solids content) than metal carbonates, sulphides and hydroxides. In fact, considering only the sludge dewatering characteristics, phosphates precipitation appears the most appropriate technique for the precipitation of many metals (Al3+, Ba2+, Cd2+, Co2+, Cr3+, Fe2+, Fe3+, Ni2+ and Zn2+). Metal hydroxides formation constitutes the best option for Ca2+, Cu2+, Mg2+ and Sn2+ removal, whereas precipitation of metal carbonates is particularly interesting for treatment of Mn2+ and Pb2+ containing solutions.