Studies on the Mechanism of Bioleaching Pyrrhotite
|School||Central South University|
|Course||Mineral Processing Engineering|
|Keywords||pyrrhotite A.ferrooxidans L.ferriphilum A.caldus surface properties electrochemistry bioleaching|
With the development of economy, the mineral resources are reducing greatly. As a result, we ought to turn to use new methods to process mineral resources of low grade. Bioleaching technology now attracts more attention than ever before for its low cost, environmental acceptance and simplicity. Pyrrhotite is a common iron sulfide in nature and is usually associated with various useful components, such as nickel, cobalt, copper, zinc and other valuable metals. Therefore, the study of the oxidation dissolution of pyrrhotite in bioleaching process means a lot in comprehensive utilization of low grade sulfide minerals and the extraction of valuable metals.On the basis of property of pyrrhotite crystal and its leaching thermodynamics, the variation of surface properties of pyrrhotite after biological conditioning with Acidithiobacillus ferrooxidans, Leptospirillum ferriphilum and Acidithiobacillus caldus was evaluated by zeta-potential, adsorption and contact angle measurements. The effect of slurry concentration, the amount of bacteria, pH value, etc. on pyrrhotite leaching in the presence of A.ferrooxidans and L.ferriphilum were conducted with SEM; the electrochemistry behavior of pyrrhotite with and without bacteria were conducted by electrochemical analysis methods (cyclic voltammetry (CV), Electrostatic potential, polarization curves and electrochemical impedance spectroscopy) to disclose the oxidation mechanism of pyrrhotite in microbic leaching system.The leaching process of pyrrhotite was analysed by the figure of Eh-pH of FeS1.12-H2O.The best conditions of bioleaching of pyrrhotite are Eh=0.4-0.8V and pH=1-3. When the system Eh is between 0.36-0.8V, pH is in 2.28-4.0, the ferric ion may mainly exist in the form of precipitates.In acidic solution, the stable existing zone is blow 0.4V, which means that pyrrhotite would be oxidized and dissolved at very low potential. The characteristics of pyrrhotite crystal determines the oxidization dissolution rate, and the polymorphism of monoclinic system is likely to present unstable chemistry property and be easily oxidized by acid and other oxidants.The results of research on surface properties showed that all types of bacterium adhered to pyrrhotite surface rapidly, but the pyrrhotite adapted bacteria had the stronger adsorption ability. The pyrrhotite isoelectric point (IEP) shifts towards the cell isoelectric point after interacting with bacteria cells, indicating the adsorption of cells on the pyrrhotite surface, and the shift degree was different. After treatment by different bacteria, due to the formation of sulfur membrane or the ferric ion was relatively enriched on the pyrrhotite surface, the change of contact angle and surface hydrophobicity were not the same. The results indicate that the pyrrhotite dissolution may be dominated by the acid dissolution during the initial bioleaching, the energy source for the microorganism growth determines its function mechanism in the bioleaching system. The iron-oxidizing bacteria offer an indirect mechanism function (sulfide mineral was oxidized by dissolved Fe3+) and the sulfur-oxidizing bacteria offer a direct mechanism function (sulfide mineral was oxidized by O2 in bioleaching process).The electrochemical study of oxidation of pyrrhotite in the presence and absence of bacteria in acidic system showed that pyrrhotite was firstly oxidized to sulfur or iron (ⅲ) complex at the potential of 0.2V, which covered the electrode and made electrode passivated; as the scanning potential rising, S was oxidized to SO42-, making the passive film dissolved. The result of AC impedance further showed within the electrode potential region of 0.2-0.7V, the oxidation of pyrrhotite firstly formed passive film and then passive film dissolved. In the solution with bacteria, the mechanism of pyrrhotite oxidation didn’t changed, but the corrosion potential and corrosion current of pyrrhotite increased, which showed bacteria promoted the oxidation of pyrrhotite.The results of bioleaching experiments showed that bacteria can improved the leaching rate of pyrrhotite; at the 7th day of bioleaching experiment, the total-iron extraction reached 30.14%, much higher than that in sterile solution. The temperature of reaction system is the main factor for pyrrhotite leaching, and pyrrhotite can be easily oxidized. The system electric potential of leaching keeps above 0.3V and pyrrhotite was oxidized to Fe2+ and SO42-.The iron-oxidizing bacteria offer an indirect mechanism function. Increasing the mineral concentration can depress the dissolution of pyrrhotite; increasing inoculation amount and adjusting the pH value to initial number are beneficial to the growth of pyrrhotite and thus promoted the leaching of pyrrhotite.The dissertation was supported by "National Basic Research Program" (2010CB630903,2004CB619204) and "National Nature Science Foundation of China" (50621063).