SAMPLING

Monitoring of the extent of biooxidation, either directly or indirectly, permits optimization of the GEOCOAT® heap operations. The direct methods involve sampling of either the solution streams or the heap solids or both. Solids samples provide more information but also require more effort to collect. Solution samples are easily collected but may not provide all the desired information. When processing concentrates consisting mainly of pyrite or pyrrhotite, the predominant metal in solution is iron. However, measurement of the concentration of iron in solution does not provide a reliable measure of the extent of oxidation of these sulfide minerals because of the tendency of iron to precipitate in the heap. Precipitation occurs particularly at the higher temperatures (60-70ºC) associated with the use of thermophile microorganisms. In contrast, the oxidation of sulfides of copper, nickel, cobalt and zinc can be followed relatively easily by analysis of the solution for one or more of these metals, as they do not readily reprecipitate. Indirect methods of determining the extent of biooxidation include temperature profiles and time under biooxidation.

During the initial stages of operation of a new GEOCOAT® heap, sufficient data is collected to allow operation using indirect measurements and solution samples. Solids sampling would be conducted during the early life of the heap to establish a database of performance measurements and then used only as necessary for metallurgical investigations. Solids samples are obtained using a vacuum system to withdraw material from a 100mm PVC casing driven into the heap. The casing is advanced, one meter at a time, and the sample extracted using a vacuum hose. With each advance, the casing is extended by adding a onemeter length. Using this method, a complete suite of samples can be obtained at any time from various levels and locations in the heap.