Aeration and Temperature Control
The sulfide oxidation reactions taking place in a GEOLEACH™ heap are exothermic, which raises the temperature of the ore. Heat is removed from the system mainly by the saturated air as it leaves the top of the heap. Heat is also lost from the system by cooling of the solution as it contacts the incoming, unsaturated air flowing into the heap, and from the solution ponds and as the solution is returned to the heap via the sprinklers.
The optimum temperature in the heap is 38-42°C for systems employing mesophilic bacteria, and 65-70°C for thermophilic microorganisms. Moderate microbes are used in climates where the atmospheric temperature is higher then mesophilic conditions. The aeration fans and air distribution piping are sized to maintain the desired temperature taking into account the amount of sulfide in the heap, the chemical and mineralogical composition, and the site ambient conditions. Fan sizing also takes into account the expected peak oxidation rates, which may be up to twice the average rate, and the least favorable ambient conditions, including high air temperatures and high relative humidity. Under conditions of lower reaction rates and lower ambient air temperature and humidity, the rate of heat removal required is reduced. The rate at which air is supplied to the heap is regulated to maintain temperature in the optimum range. This is accomplished by partially closing dampers in the main air ducts. Long-term changes in concentrate mineralogy or sulfur grade are also handled by adjusting aeration rates.
Heat balance calculations show that forced aeration in a GEOLEACH™ heap is several times more effective in removing heat than is the circulating solution. Both the aeration rate and the solution application rate are manipulated to control temperature in a GEOLEACH™ heap. The HotHeap™ control and operating system provides the necessary monitoring to balance heat loads.
The progress of biooxidation in a GEOLEACH™ heap can be effectively monitored through the analysis of solution samples, the measurement of temperature and the review of oxygen profiles. The GEOLEACH™ system when equipped with the HotHeap™ control system is robust and self-regulating.
In cold climates and at high altitudes plastic sheeting may be applied to the surface of the heap to help conserve heat.