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Study of Biological Extraction Kinetics of Heavy Metals from Spent Catalyst by Thermophilic Bacteria

Gerayeli, Faezeh | 2011

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 42671 (06)
  4. University: Sharif University of Technology
  5. Department: Chemical and Petroleum Engineering
  6. Advisor(s): Yaghmaei, Soheyla; Mousavi, Mohammad
  7. Abstract:
  8. Every year, an enormous amount of catalysts are consumed in oil refineries during refining processes. After some regeneration cycles in order to recover their activity, they are not anymore effective and must be disposed. These spent catalysts are regarded as dangerous solid wastes as they significantly include some contaminations like vanadium, nickel, arsenic, sodium. From another point of view, the metals present in spent catalysts are highly valuable and are used extensively in steel industry and manufacture of special alloys. So, it sounds profitable to extract and recover these metals from such a cheap source instead of minerals and ores. Basically, there are two main processes, hydro- and pyro-metallurgical processes to recover metal values from spent catalysts and also a combination of them. Due to several drawbacks of these conventional techniques, biotechnological leaching processes (also called bioleaching) have been developed as a potential alternative. Bacteria can produce various metabolites which are capable of extracting metal values from waste materials.According to high ability of two mesophilic acidophilic starins, Acidithiobacillus ferrooxidans and Acidithiobacillus thiooxidans, they play a major role in practical bioleaching of various metals and are studied more than other strains. But there are a few investigations about bioleaching processes using thermophils. According to literature, thermophils may be more effective in metal extraction as they can tolerate extreme situations.Although the role of micro-organism in bioleaching processes is studied extensively, there is not a definite mechanism for different reactions. Moreover there is a lack of information about kinetics of such reactions which can help with the scaled-up operations.So, by using thermophilic acidophilic bacteria called Acidianus Brierleyi, the batch bioleaching kinetics of spent hydrocracking spent catalyst was investigated in this study. Four significant factors in bioleaching (pH, pulp density, Inoculation and elemental sulfur) were realized among 10 key factors using Plackett–Burman factorial design. Furthermore, a central composite design (CCD) was applied to examine the interactions of these 4 factors in order to find the optimal condition. Finally, the best condition for getting maximum efficiency in bioleaching process was determined as follow: pH (1.6), pulp density (0.6 g/L), inoculation (4%) and elemental sulfur (4 g/L). Second-order polynomial models were developed to find the impact of four variables on metal recovery. The predicted recoveries from these models were in good agreement with experimental results in optimal conditions that were %35.27 for aluminum, %83.32 for molybdenum and %68.74 for nickel. To better understand the mechanism of the leaching reaction, the rate limiting step of the leaching process was determined by applying the optimum values and it revealed that diffusion through product layer is the rate limiting step
  9. Keywords:
  10. Spent Catalyst ; Bioleaching ; Kinetics Study ; Thermophilic Bacteria

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