Heap leaching is one of several leaching methods. Heap leaching is, in contrast to many dump leaching operations, a pre-planned process where arrangements are made to optimize conditions for leaching. In brief, the mineral ore or concentrate is piled in a heap and lixiviant fluid is distributed over the surface to leach metal from the heap. More detailed:
- The soil on a slightly sloping ground is first compacted and then covered with an impermeable pad like an asphalt layer or a flexible plastic sheet.
- Crushed ore is stacked in big heaps on the pad. Fine particles are agglomerated to increase permeability.
- The heap is sprayed with leaching reagent.
- As the reagent percolates through the heap the wanted metals are solubilized.
- The leachate (metal containing solution) drained from the heap is collected in a pond and the solution is subsequently sent for metal recovery.
Heap leaching takes months rather than years as for dump leaching. Compared to stirred tank reactors, heap reactors form undesired gradients of pH and reagent levels. The reaction conditions in a heap varies from top to bottom, from core to surface and sometimes also locally in the heap. Still, the reaction conditions are less heterogeneous in a heap than in a dump or in situ leaching operations.
Important factors for a successful heap leaching operation are:
- Maintain a good permeability. Precipitates like gypsum, ferric hydroxides and clay minerals might cause clogging and flooding.
- A good water balance. Sprinklers or drippers buried in the heap are used to deliver the leaching solution over the heap.
- pH control
- A steady supply of leaching reagents. See for example aeration.
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Permeability is critical for the recovery rates from a heap (as well as from dump and in-situ operations). If the heap material is too compact, the leaching operation time increases as the reagent liquid will only slowly percolate through the heap. If permeability in the heap is uneven, the leaching solution might cause channeling leaving parts of the heap untreated. Therefore, fine particles are often agglomerated to ensure a good permeability.
Comments on heap bioleaching
- Thermofilms are sometimes used to cover the heap in order to reduce heat loss from exothermic reactions. Usually a high temperature will increase the leaching rate.
- The most common bioleaching process mainly used for bioleaching of copper and refractory gold-bearing ores.
- Although one can rely on the natural movement of microbes to eventually inoculate the heap, initial rates of bioleaching can be improved by effective heap inoculation, but this is difficult to achieve.
- BioHeap – A Bioleaching Process for Nickel Extraction. http://www.azom.com/details.asp?ArticleID=1601
- Heap leaching of sulfide concentrates is employed by a company called GeoBiotics
- Other examples of heap bioleaching ...