Maybe of interest:HEAP LEACHING
Heap leaching was introduced in the 1970’s as a means to drastically reduce gold recovery costs. This process has literally made many mines by taking low grade geological resources and transforming them to the proven ore category. Ore grades as low as 0.01 oz Au per ton have been economically processed by heap leaching.
Heap leaching involves placing crushed or run of mine ore in a pile built upon an impervious liner. Cyanide solution is distributed across the top of the pile and the solution percolates down through the pile and leaches out the gold. The gold laden pregnant solution drains out from the bottom of the pile and is collected for gold recovery by either carbon adsorption or zinc precipitation. The barren solution is then recycled to the pile.
Heap leaching generally requires 60 to 90 days for processing ore that could be leached in 24 hours in a conventional agitated leach process. Gold recovery is typically 70% as compared with 90% in an agitated leach plant. Even with this inferior performance, the process has found wide favor, due to the vastly reduced processing costs compared with agitated leaching.
The cost advantage areas are largely as follows:
1. Comminution: Where as heap leaching is typically done on –3/4 inch rock, agitated leaching requires reduction to –200 mesh. This additional step is typically done with large grinding mills that consume roughly one horsepower per ton per day of capacity.
2. Solids liquid separation steps are not required for heap leaching.
3. Tailings disposal costs are quite high for a modern agitated leach plant. Large expensive liquid containment dams are required. By comparison, heap leach pads can generally be left in place after reclamation.
Disadvantages, in addition to lower recovery of heap leaching compared with agitated leaching, include:
1. The stacked ore must be porous enough to allow solution to trickle through it. There have been many recovery failures due to the inability to obtain solution flow. This is widely experienced when ores have a high clay content. This problem is often alleviated by agglomeration prior to heap stacking.
2. In areas of high rainfall, solution balance problems can arise, resulting in the need to treat and discharge process water.
3. In extremely cold areas, heap freezing can result in periods of low recovery. Operational procedure modifications such as subsurface solution application have reduced, but not eliminated, this concern.
4. Ice and snow melting can result in excessive accumulation of leach solutions. This concern can often be mitigated by use of diversion structures.
Quite frequently, mines will use agitated leaching for high grade ore and heap leaching for marginal grade ores that otherwise would be considered waste rock. A common recovery plant is often employed for both operations.
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