Heap leaching of sulfide ore is one of only a few examples of large-scale solid-state bioprocess. The environment of a mineral heap leach is one of low pH, high dissolved metals and elevated temperatures. This makes it an interesting example of biology functioning in extreme environments. Commercial development of a solid-state process is more challenging because of the difficulty in controlling temperature, pH, toxic metals and oxygen levels within the reactor as compared to stirred liquid reactors. The benefit is that they are of much lower in capital and operating cost than tank submerged processes. For any biochemical reaction that is more efficient at higher temperatures the energy cost of heating a large volume of water is high. The lower water use, and therefore lower heat capacity, of a solid-state reactor make it an ideal choice for thermophilic processes. Archaea from hydrothermal environments have been shown to bioleach chalcopyrite 10 times faster at 60 degrees C than leaching by mesophiles at 40 degrees C. The heap must be constructed with enough sulfides exposed to oxidation so that the metabolic heat of the process can increase the temperature of these massive heaps of ore to accelerate the process. The challenge for bioleaching at high temperature is to maintain the correct environment for the microbes to do their work. This can be achieved by careful control of air and liquid flow within the heap.