Constraints of bioleaching in in-situ recovery applications

The potential role of microorganisms in the in-situ recovery (ISR) of technology metals, in particular from reduced ores through acidic leaching is not well understood, but attracts increasing interest worldwide. Based on electron balance criteria, upper limits for metal recovery in general and bioleaching effects in particular are deduced in this work. These criteria define restrictions valid for any metal (U, Cu, Zn, etc.), oxidant (ferric iron, O2), and type of leaching (pure chemical leaching and/or bioleaching). The indirect catalysis of leaching by microbial (re-)oxidation of Fe+2 to Fe+3 is a well-verified mechanism; however, for practical applications it is constrained by the in-situ availability of O2. The content of O2 in the injected leachant is limited by its low solubility as a function of temperature and pressure. The ex-situ bio-oxidation of Fe in an aerated bioreactor is considered as an alternative to the direct (and fast) chemical oxidation, e.g. by the more economic H2O2 injection into the leachant pipeline. Reactive transport modeling of in-situ leaching of sulfidic Cu ores demonstrates the role of key parameters for (bio)leaching productivity: pH, amount of oxidants, flow rate, and porosity. In reality, sulfides dissolve kinetically, as demonstrated. A pure thermodynamic approach, however, already provides a theoretical upper limit of both the (predominant) direct chemical leaching by Fe+3 and the possible effect of (indirect) bioleaching by re-oxidizing Fe+2 that is constrained by in-situ O2.