Irrigation and Solution Management
When a new panel of concentrate-coated support rock is stacked on the pad, the solution distribution piping is extended and the panel is irrigated with a solution containing active, adapted bacteria, nutrients, and ferric iron. The solution 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 on-solution 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.
