Effect of sulfide and hydroxide on the removal of heavy metal ions from hydrometallurgical zinc effluent

Heavy metals removal from Specialized Industrial Park of Zinc of Zanjan effluent was tested using sulfide, hydroxide, and the combination precipitation. The initial effluent includes 200 ppm of zinc, 55 ppm of cadmium, 113 ppm of manganese, 0.85 ppm cobalt, 0.65 ppm nickel, 0.2 ppm iron, and 0.55 ppm lead. Sulfide precipitation using Na2S as a sulfide source is inefficient in the precipitation of manganese except in high concentrations of sulfide, while hydroxide precipitation using lime is not successful in cadmium removal. Combining these two methods in one and two steps is helpful in the precipitation and settling of all heavy metal ions. The optimum temperature, pH, sulfide concentration, and time were determined. The concentration of zinc, cadmium, manganese, cobalt, nickel, and iron were reduced to 0.1, 0.1, 0.14, 0.23, 0.19, and 0.16 ppm, while lead was not detected in the solution in optimum condition. The experiments were repeated for effluent with higher amounts of lead, iron, and nickel, and its success in efficient removing of these ions is also confirmed. Scanning electron microscope (SEM) images and X-ray diffraction (XRD) patterns were used to study morphologies and structures of precipitation. The concentration of Na2S plays a crucial role in removal efficiency. Excess sulfide causes the increment of ions such as manganese in the solution and decrements the efficiency discussed in this research. The lower use of lime, almost no excess sulfide ions in solution, fast kinetics, and precipitating of a wide range of metal ions are some advantages of this method.