Optimization Of Parameters And Microbial Community Characteristics In Bioleaching System For The Pb/Zn Smelting Slag

Pb/Zn smelting slag contains large amount of metal elements, such as As, Cd, Cu, Zn, Ag, In, Ga and Pb, Developing a high-effectively and environmental-friendly process for Pb/Zn smelting slag is important both for reusing metal resources and the environment sustainability. In this study, the Pb/Zn smelting slag was collected from a Pb/Zn smelter in Hunan Province of China. The characteristics of the Pb/Zn smelting slag were studied through XRD, SEM-EDX analysis and three-stage BCR sequential extraction procedure. Flask experiments were conducted to optimize the bioleaching parameters in bioleaching system, including temperature, pH, pulp density and particle size, and bacteria community was studied through a culture-independent restriction fragment length polymorphism (RFLP) of 16S rRNA genes.The results show that the valuable metals in the slag mainly are Zn, Cu, Pb, Ag, In, Ga, and the concentration of these elements are 28378, 13115, 5740, 96.6, 96, 916 mg·kg^-1, respectively. The main toxic elements in the slag are Pb, Cd and As, and concentration of Cd and As are 29.6 and 4587 mg·kg^-1, respectively. The fraction of Cd, Cu, Pb and Zn in the slag mainly exists in residual form and that of As mainly exist in oxidizable form. Though the percentage of acid-extractable As, Cd, Cu, Pb and Zn are significantly less than those of oxidizable and residual froms, the total concentrations of extractable As, Cu, Pb and Zn in the slag are 2620,4602,356 and 9365 mg·kg^-1, respectively, which are relatively high.The results from the flask experiments show that the bioleaching rates of As, Cu, Zn, Ag and In in the slag are mainly affected by the pH and pulp density. Under optimized condition (temperature 65℃, pH 1.25-1.5, 10% pulp density, particle size less than 0.83 mm and a leaching time of 4 d), the bioleaching rates of As, Cd, Cu, Zn, Ag, In and Ga in the Pb/Zn smelting slag are 86%-91%, 80%-85%, 90%-93%, 85%-86%, 75%-80%, 90%-94% and 85%-90%, respectively. For Pb, however, only about 3%-4% was extracted from the smelting slag.The cultured indigenous moderate-thermophilic bacteria in the bioleaching system are mainly composed of Brevibacterium and Cocci. The community characteristic of the indigenous moderate-thermophilic bacteria in this bioleaching system through a culture-independent restriction fragment length polymorphism (RFLP) of 16S rRNA genes revealed that the indigenous moderate-thermophilic bacteria in the bioleaching system were mainly affiliated with Bacillus spp. (33.33 %), Sporosarcina spp. (20.99 %) and Pseudomonas spp. (16.05 %).The concentration for leaching toxicity of Cd and Pb in the bioleached residues are always less than the Identification Standard for Hazardous Wastes (GB5085.3-1996) in the whole bioleaching process; that of Cu and Zn are less than the standards after bioleaching of 2 days and concentration for leaching toxicity of As can meet the standards after bioleaching of 6 days.