Modeling pyrite bioleaching in isothermal test columns with the HeapSim model

Abstract The HeapSim model developed to design heap leach processes was employed herein to evaluate unknown parameters and to identify the rate-controlling steps governing a simple leach system consisting of only pyrite under isothermal conditions. The temperature at which the column tests were performed encompassed the range of the mesophilic cells (15–40 °C), moderate thermophilic cells (30–55 °C), and extreme thermophilic cells (50–80 °C). The ore-, geometry-, and hydrology-related parameters characteristic of the column tests were known from previous experiments. This left only the biological parameters of iron- and sulfur-oxidizing cells and the oxygen gas–liquid mass transfer rate to be found by trial and error from simultaneous best fits of five important leach data sets: extent of sulfide oxidation, effluent solution potential, iron concentration, cell numbers, and sulfur grade. The challenge was to find a unique value of the oxygen mass transfer rate common to all temperatures. Good to excellent fits of the leach indicators were obtained, while the values of the parameters were largely within the ranges expected. The model revealed the rate-limiting step to shift from particle kinetics to oxygen gas–liquid mass transfer with increasing temperature, increasing proportion of fine pyrite grains, and higher pyrite head grades. Competition for oxygen between sulfur- and iron-oxidizing microorganisms lowered potentials and retarded pyrite oxidation.