SOLUTION MANAGEMENT

Once sufficient new support rock and concentrate have been stacked on the pad, the solution distribution piping is extended and the newly-stacked area placed under irrigation. Biooxidation is rapidly initiated as the new area is populated with active, adapted bacteria introduced with the solution and recycled with the stripped support rock. Solution is applied at a rate of 10-20 L/h.m2 by “Wobbler™” sprinklers, and percolates through the heap to the drain layer where it is collected in perforated piping. The drainage pipes conduct the solution to a collection ditch running along one of the long sides of the heap. The ditch discharges into the off-solution pond which overflows to the on-solution pond. Solution is pumped from the onsolution pond back to the distribution piping and sprinklers on the active areas of the heap.

To maintain acidity and dissolved metal contents of the circulating solution at target levels, a portion of the solution is continuously bled from the circuit. In the processing of refractory gold ores where the solution contains no values, a pump in the off-solution pond delivers the bleed stream to the effluent treatment system where limestone and lime are added to neutralize acidity and precipitate iron, arsenic and other metals.

In the GEOCOAT® treatment of base metal concentrates where the valuable constituents are solubilized, the bleed stream is the pregnant leach solution (PLS) which feeds the downstream recovery processes. In the case of copper concentrates, for example, downstream processing would typically include solvent extraction and electrowinning. The flow rate of the bleed stream or PLS is adjusted to control the concentration of the valuable metal at the optimum value for downstream processing. Because the GEOCOAT® heap drains almost completely in a few hours and retains only a very small solution inventory, recirculation of relatively high-grade solution to the heap is feasible. Residual metal values are easily and rapidly recovered from the heap by rinsing with fresh water after bioleaching is complete.

During periods of heavy rainfall, it may be necessary to reduce or eliminate the addition of fresh make-up water to the system and to maintain the PLS grade by reducing the solution bleed rate. In extreme cases, low-grade solution may be diverted to an emergency pond and later returned to the on-solution pond as make-up. A detailed water balance, taking into account the site climatic conditions, must be performed for each GEOCOAT® project and the results used to size operating ponds and emergency storage ponds.