Abstract
Heap bioleaching performance is dependent on the contacting of the leach solution with the ore bed, hence on the system hydrodynamics. In this study two experimental setups were used to examine hydrodynamics associated with irrigation from a single drip emitter, one of the most common methods of heap irrigation. A specialist magnetic resonance imaging (MRI) method which is insensitive to the metal content of the ore was used to examine the liquid flow into an ore bed in the immediate vicinity of an irrigation point. The distribution of liquid in, microbial colonisation of and mineral recovery from a bioleach of a large scale 132kg “ore slice” were subsequently monitored using sample ports positioned along the breadth and height of the reactor. In both systems the lateral movement of the liquid increased with bed depth, though preferential flow was evident. The majority of the liquid flow was in the region directly below the irrigation point and almost no liquid exchange occurred in the areas of lowest liquid content at the upper corners of the bed in which fluid exchange was driven by capillary action. The MRI studies revealed that the liquid distribution was unchanging following an initial settling of the ore bed and that, at steady state, the majority (~60%) of the liquid flowed directly into established large channels. The limited lateral movement of the liquid had a significant impact on the local leaching efficiencies and microbial colonisation of the ore with cell concentrations in the regions of lowest liquid content lying below the detection limit. Hence poor lateral liquid distribution with drip irrigation, and the associated impact on colonisation was identified as a significant disadvantage of this irrigation approach. Further, the need to optimise fluid exchange throughout the ore bed was identified as key for optimisation of leaching performance.
Highlights
Heap bioleaching systems are unsaturated ore beds, typically stacked using agglomerated ore that are irrigated with lixiviant from the top of the bed
In both the magnetic resonance imaging (MRI) experiments and in the larger “ore-slice” box reactor system, the liquid distribution was observed to increase laterally with depth. This is because the high moisture regions were wetted by liquid channels that flowed under gravity, whereas the upper regions of the ore bed were wetted through capillary movement of liquid
The MRI data showed that slumping of the ore during an initial irrigation cycle permanently altered the liquid flow path which subsequently remained stable
Summary
Heap bioleaching systems are unsaturated ore beds, typically stacked using agglomerated ore that are irrigated with lixiviant from the top of the bed. The result of the leaching reactions is that the metal ions (e.g. Cu2+) are liberated into the leach solution. They are subsequently transported out of the heap and recovered from the effluent pregnant leach solution (PLS) which exits at the base of the heap. The heap hydrology plays two key roles as both reaction and transport medium. These determine the degree of recovery that is possible in an ore bed because contact is required between the leach solution and the exposed mineral surfaces on the ore particles for the leaching reactions to occur (Rossi, 1990)
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