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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 132 kg "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.

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During start-up of heap bioleaching, low grade ores are typically treated with acid for agglomeration and to combat the acid neutralising capacity of the gangue minerals. This may stress the bioleaching inocula, particularly upon inoculation during ore agglomeration. Acid addition for agglomeration varies across operations, ore types and their neutralising capacity, with limited information published on recommended concentrations. The initial pH in the agglomeration mix is typically below pH 1.0 and may be as low as pH 0.5. This paper investigates the effect of acid stress in terms of initial acid concentration and exposure duration in submerged culture on mesophilic bacteria typically implicated in mineral sulphide bioleaching and critical for heap colonisation at start-up. Following acid stress, cultures were returned to standard operating conditions in batch stirred slurry reactors and their performance assessed in terms of mineral leach rates, ferrous oxidation and the rate of microbial growth. Increasing acid stress resulted in an increase in the lag period before onset of microbial growth and iron oxidation. Following adaptation, typical growth and ferrous iron oxidation rates were observed under low stress conditions while reduction in the rate and extent of microbial growth and ferrous iron oxidation persisted at extreme conditions. A reduction in yield (microbial cells produced per kg iron oxidised) was observed with increased acid concentration over comparative times. Microbial speciation analysis indicated a substantial decrease in the diversity of surviving bacterial species.

In heap bioleaching iron and/or sulfur oxidising microorganisms are used to facilitate the oxidation of base metal sulfides in ore, thereby liberating the metal ions (e.g. Cu2+) into the leach solution. The heap performance is consequently strongly influenced by the contacting of the leach solution and the ore particles. In this study two setups were used to examine irrigation from a single drip emitter, one of the most common methods of heap irrigation. The distribution of liquid, microbial colonisation and mineral recovery in a bioleach of a 132kg “ore slice” of agglomerated ore were monitored using sample ports positioned along the breadth and height of the box over a period in excess of 500 days. A specialist magnetic resonance imaging (MRI) method which is insensitive to the metal content of the ore was subsequently used to examine the effect of flow rate and particle size distribution on the liquid flow into a smaller bed. Overall 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 top corners of the samples. This had a significant impact on the local leaching efficiencies and microbial colonisation of the ore. The MRI studies revealed at steady state, the majority (~60%) of the liquid flowed into established large channels. There was minimal exchange with low liquid content regions (presumably stagnant liquid) despite their accounting for more than 16% of the total liquid hold-up. The effect of increasing the flow rate was to retard lateral liquid distribution while slightly increasing the liquid hold‑up in large channels in the region below the irrigation point. Hence poor lateral liquid distribution in drip irrigation was identified as a significant disadvantage of the method.