HEAT BALANCE

The sulfide oxidation reactions taking place in a GEOLEACH ™ heap are exothermic, producing heat 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. The extent to which heat is removed by this mechanism is dependent on the rate at which air is forced into the heap, and by the temperature and relative humidity of the ambient air. To a lesser extent, heat is also lost from the system by cooling of the solution as it contacts the incoming, unsaturated air flowing into the heap. Heat is also lost 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. 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. In practice both the aeration rate and the solution application rate can be manipulated to control temperature in a GEOLEACH ™ heap. The HotHeap™ control and operating philosophy provides the necessary monitoring system to balance heat loads. The HotHeap™ control and operating philosophy provides the necessary monitoring system to balance heat loads. See the figure below for a schematic of the HotHeap™ system.

Temperature Monitoring and Control

The progress of biooxidation in a GEOLEACH ™ heap can be effectively monitored through the analysis of solution samples, the measurement of temperature and oxygen profiles. The GEOLEACH ™ system when equipped with the HotHeap™ control system is robust and self-regulating. Microbial populations will generally adapt to the conditions prevailing, tending to push the system towards their optimum environment. This makes controlling and monitoring the process relatively simple.

A typical biooxidation rate profile consists of three stages with each stage requiring adjustments to the aeration and solution application rates. The first stage is characterized by a lag during which little biooxidation occurs while the bacteria are propagating and colonizing the heap. At this stage the aeration rate is minimized to allow heat to build up within the heap while still ensuring that the bacterial population has sufficient oxygen. During this stage, the solution application rate may be increased to deliver as much acid as possible to the heap to reduce solution pH as rapidly as possible. The second stage is one of rapid biooxidation, during which the heap begins to reach optimal temperatures. During this stage, the solution application rate may also be adjusted. The aeration rate is increased during this stage, usually to the maximum designed rate. During the third stage, biooxidation nears completion, temperatures begin to decline, and the air supply is reduced to near stoichiometric levels. The following figure shows a conceptual control cycle for the irrigation and aeration and the corresponding response of heap temperatures for a sulfide ore containing some acid soluble metal.

The level of instrumentation employed to monitor the heap is dependent on the variability of the ore, the pad configuration, panel sizes, degree of control required and on the amount of historical operating information available. The instrumentation involves the measurement of heap temperature, airflow and off-gas oxygen concentration coupled with air damper controls, irrigation timing and data acquisition through a PLC or SCADA.