DEVELOPMENT OF A LABORATORY-SCALE LEACHING PLANT FOR METAL EXTRACTIONFROM FLY-ASH BY THIOBACILLUS STRAINS

Semicontinuous biohydrometallurgical processing of fly ash from munici pal waste incineration was performed in a laboratory-scale leaching pl ant (LSLP) by using a mixed culture of Thiobacillus thiooxidans and Th iobacillus ferrooxidans. The LSLP consisted of three serially connecte d reaction vessels, reservoirs for a fly ash suspension and a bacteria l stock culture, and a vacuum filter unit, The LSLP was operated with an ash concentration of 50 g liter(-1), and the mean residence time wa s 6 days (2 days in each reaction vessel), The leaching efficiencies ( expressed as percentages of the amounts applied) obtained for the econ omically most interesting metal, Zn, were up to 81%, and the leaching efficiencies for Al were up to 52%, Highly toxic Cd was completely sol ubilized (100%), and the leaching efficiencies for Cu, Ni, and Cr were 89, 64, and 12%, respectively, The role of T. ferrooxidans in metal m obilization was examined in a series of shake flask experiments, The r elease of copper present in the fly ash as chalcocite (Cu2S) or cuprit e (Cu2O) was dependent on the metabolic activity of T. ferrooxidans, w hereas other metals, such as Al, Cd, Cr, Ni, and Zn, were solubilized by biotically formed sulfuric acid. Chemical leaching with 5 N H2SO4 r esulted in significantly increased solubilization only for Zn, The LSL P developed in this study is a promising first step toward a pilot pla nt with a high capacity to detoxify fly ash for reuse for construction purposes and economical recovery of valuable metals.