HEAP OPERATING TEMPERATURE

Heat Balance

The sulfide oxidation reactions taking place in a GEOCOAT® heap are exothermic, producing heat which raises the temperature of the support rock and concentrate. 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 concentrate in the heap (coating ratio), the concentrate 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 GEOCOAT® 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 GEOCOAT® heap.

Because of the open structure of the GEOCOAT® heap and the close spacing of the air distribution piping, heat is carried to all parts of the heap. Heat balance modeling has demonstrated that the surface of the heap can be maintained at or close to the design temperature, suggesting that bacterial activity will be maintained at high levels even at the top surface and on the side slopes of the heap. Provided that solution is applied to all heap surfaces, the rate and extent of bioleaching can be expected to be uniform throughout the heap.

Temperature Monitoring and Control

The progress of biooxidation in a GEOCOAT® heap can be effectively monitored through the analysis of solution samples and the measurement of temperature profiles. The GEOCOAT® system is robust and self-regulating. Bacterial 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 level of instrumentation employed to monitor heap temperatures is dependent on the amount of historical operating information available. During early stage operation it will be necessary to measure temperatures in the heap. Conventional thermocouples would be placed in vertical thermowells installed in the heap during stacking. Each thermowell would contain three thermocouples at different levels. Additionally, permanent thermocouples would be placed under the heap in the solution drainage layer. Once enough heap and solution temperature data have been collected, a temperature model can be constructed to allow the inference of the bulk heap temperature from the underlying solution temperatures. At this stage the use of the in-heap thermocouples would be discontinued and temperature monitoring would be limited to the solution streams.

After several process cycles, sufficient data should be available to establish the base operating parameters. Control measures will then be based on the stage reached in the biooxidation cycle, as measured by elapsed time since the start of solution application, rather than on continuous temperature or solution monitoring.