Bioleaching Characteristics Of Zinc Smelting Slag And Toxicity Reduction For Bioleached Residue

Zinc smelting slag contains a huge amount of metals including As, Cd, Cr, Cu, Pb and Zn, etc., which is a smelting waste solid with high potential pollution risk and low recycling efficiency. In this study, a zinc smelting slag was treated by bioleaching and EDTA-Na2leaching to recycle valuable metals and detoxify the slag. The biochemical and electrochemical effects between moderate thermophilic bacteria and slag as well as the feasibility of extraction of Pb in bioleached residue by EDTA-Na2were investigated. In addition, the environmental activity and potential toxicity of original slag, bioleached slag and sequential leached residue were also studied. Rsults were as followed:About81.4%bacteria were absorbed on slag paticle surface in conditions of pH of1.0, temperature of65℃, pulp density of2%and slag grain size of100mesh within30min. Bacterial adsorption during bioleaching was profoundly affected by pH, pulp density and grain size. Higher or lower pH had detrimental effect on bacterial adsorption efficiency. Higher pulp density and larger particle size are benifical to bacterial adsorption.In bioleaching system, Cu and Zn extraction could reach84.8%and81.2%, respectively, with the action of dominant bacterium. Cu and Zn extraction percentages increased with rising oxidation-reduction potential (ORP), however, ORP did not have any obvious correlations with values of Fe3+/Fe2+, indicating that mechanism in boleaching procedure were not only indirect interaction, but other biochemical reactions. Adsorbed bacteria had significant impact on extraction of metals in slag, especially Zn where extraction percentage increased from66.7%to84.5%. The electrochemical characteristics of zinc smelting slag electrode showed that the oxidation of Cu2S, FeS2and their intermediate products was found and corrosion potential and corrosion current were410mv and0.166mA/cm2in bioleaching system, repectively, higher than that of control system, indicating that the corrosion of zinc smelting slag was enhanced by bacteria in bioleaching by oxidation of sulfide.Pb in zinc smelting slag could be effectively extracted by EDTA-Na2as its concentration of0.1mol/L, temperature of65℃, rotate speed of120rpm and time of2h and Pb extraction reached71.6%. The extraction percentage of Pb in bioleached residue arrived to67.9%at the same conditons. The results of toxicity investigation of bioleached residue and sequential leached residue showed that the concentrations of As, Cd, Cr, Cu, Pb and Zn did not exceed the limits of USA EPA Regulations, method1311and pH of original slag, bioleached residue and sequential leached residue did not exceed the limits of GB5085.3-2007as well. Overall, the zinc smelting slag was totally detoxified through a two-step sequential leaching procedure combined bioleaching and EDTA-Na2leaching.