Designing bioleaching reactors: challenges and innovations
Résumé
Bioleaching is a hydrometallurgical technique which uses the metabolic activity of lithotrophic microorganisms for the extraction of metals from sulfide ores. These microorganisms draw their energy from the oxidation of iron and/or reduced inorganic sulfur compounds, producing sulfuric acid and ferric iron. The result is a highly corrosive " bioleaching " solution that dissolves the sulfide minerals by oxidation, releasing the metals to solution. In these reactions oxygen is used as an electron acceptor. Bioleaching is a proven technology already industrially applied. Its main advantages over other processes are to be cost-effective and to provide the same duty in a simpler way in terms of operability. Bioleaching is also attractive insofar as it presents few environmental hazards. However, up to now, it remains a niche application mainly because of the limitations and the unsatisfactory performances of the reactors. One reason to explain this situation is probably the lack of research work dedicated to process development and reactor design in comparison to the great effort devoted worldwide to the biology and physiology of microorganisms. Out of the papers devoted to bioleaching published in the last decade, less than 2% deal with basic process engineering issues (such as heat and mass transfer) and bioreactor design. One of the key factors of bioleaching performances in agitated tanks is mixing. It has to be efficient in order to reach a high level of oxygen transfer rate (OTR) to comply with oxygen uptake rates of about 1500 mg L −1 h −1 , and even distribution of the various components of the slurry throughout the tank. The main costs of bioleaching operations are the investment costs of the tanks and the impellers and the operating costs of gaseous mass transfer. This presentation will give an overview of the recent advances achieved to overcome some of the limitations of bioleaching processes with a particular emphasis on the development of an alternative bioleaching reactor based on the use of floating agitators to mix and to suspend solids in the solution as well as to inject gases in the pulp. This new concept enables to decrease the costs of bioleaching processes by operating (i) in lagoons or ponds instead of tanks, (ii) at higher solid loading (> 15% w/w) than in conventional stirred tank bioreactors (STR). In these conditions of high solid load, the demand for oxygen is significantly increased and air is replaced by oxygen to provide an adequate oxygen supply The experimental testwork performed from lab to pilot scales (2 L up to 2 m 3) has confirmed that this new device operated at solid load up to 30% leads to similar leaching performances (kinetics and metal yields) than those obtained in STR at solid load below 20%. The injection of oxygen instead of air improves OTR but must be managed carefully to avoid high dissolved oxygen (DO) level in the slurry: a decrease of the activity of the bioleach microorganisms was observed when the DO concentration was above 17 ppm, which might be attributed to an oxidative stress due to the formation of toxic reactive oxygen species [1]. [1] IMLAY et al., Annual Review of Biochemistry, 77, 755–776, 2008.