Table of Contents
The basic condition controlling the choice of a dump or heap leaching process versus a fine grinding-agitation leaching process is the grade and tonnage of the ore body. Low grade ore and small tonnages are best treated by a low-cost method such as dump or heap leaching.
The permeability of the material in a dump or heap undergoing a percolation leach is a factor independent of either the grade or reserve tonnage of the ore but such permeability is the primary and critical factor in any successful leaching operation. Without permeability in the leach bed, no percolation, no dissolution, no extraction, and no success are possible.
Permeability
A permeable heap is normally achieved by taking into account two parameters:
- Heap building/construction
- Agglomeration/pelletizing
Even though heap building is a critical factor for good permeability, it won’t be discussed here. It has been described in full detail by many authors. Instead, the analysis will concentrate on the second parameter.
As mentioned above, we emphasize the difference between the concepts of agglomeration and pelletizing. We refer to agglomeration when small particles are being attached to larger particles i.e., the ore presents a complete size distribution. On the other hand, pelletizing is a process where all particles are fine (less than 20 mesh) as with mill tailings.
The higher the amount of clays, the softer the ores, and/or the finer the size distribution, the higher the number of transfer points required i.e., the more complex the system will be.
Solution Delivery
A good solution delivery system should:
- Provide uniform barren solution distribution to the entire surface of the heap.
- Minimize solution losses by direct evaporation and/or entrainment by high winds.
- Minimize temperature drop of the barren solution going onto the heap. This is particularly important in cold weather where freezing temperatures can produce serious operational problems.
- Be easy and inexpensive to maintain.
The designer of a proper solution delivery system should then consider irrigation systems and flowrate control.
A uniform solution distribution will be possible only when the solution is uniformly fed into the different sprinklers at the same pressure. For a large heap with a high number of sprinklers, a suggested practice is to split out the total irrigation area into several equal individual sections and provide every section with flowmeter and pressure regulator. The solution will then be of fairly equal pressure as fed into the different sprinklers.
Solution Recovery
Pregnant and wash solutions draining from heaps flow by gravity in plastic or asphalt lined ditches and steel or plastic pipes to either a sediment pond or to the pregnant pond, depending on the clarity of the leach effluent.
Sediment ponds have proven to be very useful as clarifiers particularly before a zinc dust precipitation unit where the clarity of the solution is a critical parameter.
In summary, three critical areas in the planning of a heap leaching operation are permeability, solution delivery, and solution recovery. Other factors such as existence of a known ore reserve are equally critical to the operation but we have chosen to limit our presentation to these three often-neglected factors.
