Search

In heap bioleaching and waste-rock dumps, complex microbial communities exist in the flowing and interstitial liquid phases and mineral surface-associated biofilms, often embedded in extracellular polymeric substances (EPS). Microbial activity in the interstitial phase and mineral ore surface facilitates mineral degradation, resulting in either metal recovery or acidic, metal -bearing drainage from sulfidic waste-rock. Determining microbial presence and activity through microorganisms leaving the heap or dump has severe limitations. Hence, increasingly the ore-bed is sampled to quantify and characterise this. Here, methods for cell detachment and quantification, microbial activity measurement on the mineral surface and evaluation of EPS, quantitatively and biochemically, were refined and validated to assess microbial presence, using mineral coated beads in continuous flow-through columns. Number of wash steps required were assessed over increasing colonisation times over 30 days. Microbial cells colonising the mineral surface, pre- and post-washing were visualised by scanning electron microscopy (SEM) and their activity quantified by isothermal microcalorimetry (IMC). Using IMC, detachment and enumeration of detached cells, we demonstrated that 6–8 washes provided a reliable estimation of mineral-associated microorganisms, with less than 10% of cells or microbial activity associated with the surface following treatment. This allowed consolidated refinement of the protocol using traditional detachment method, SEM and IMC to provide correlative data. Extraction of EPS in a complete flow-through system is reported for the first time and the biochemical composition was similar to those reported under batch bioleaching conditions.

The quantitative effect of particle shape, porosity, wettability, particle size, and solution viscosity on the residence time distribution (RTD) profiles of non-reactive, steady-state, drip-irrigated ore beds characteristic of heap leaching systems is presented. Results were obtained using step-up tracer tests and allowed for the analysis of preferential flow behaviour within the systems. The key findings were as follows. Increased particle sphericity enhanced channelling in beds of smaller particles, but not for larger particle sizes. Higher particle wettability caused greater liquid dispersion during both initial wetting studies and at steady-state fluid flux. Higher porosity levels and the inclusion of fines in mixed sized beds resulted in longer average solute residence times, higher liquid hold-up, longer solution and tracer breakthrough times, and increases in drain-down moisture percentages. Increasing the irrigation fluid’s viscosity, reflective of the increase in ionic concentrations in leach solutions, reduced both the solution and tracer breakthrough times and increased dispersion with signs of more discontinuous or isolated fluid volumes at steady-state. These results highlighted the importance of the inclusion of fines in agglomerated beds to improve uniform wetting especially those with low to moderate particle porosities (

Highlights • Saccharides assessed as combined cryoprotectant, preservative and prebiotic. • Application is freeze dried topical probiotic of Lactobacillus plantarum. • Inulin was best as cryoprotectant, but did not protect cells over storage. • Best combined performance using sucrose with storage at 4 °C. • Room temperature storage only feasible with skimmed milk (positive control)., Probiotic formulations must contain the right strain(s) in sufficient numbers when administered to confer the desired health benefit. However, significant cell death can occur during freeze-drying and over storage. This study assesses various saccharides for their ability to protect Lactobacillus plantarum cells over freeze-drying and storage, as well as their potential to act as prebiotics. The cryoprotective potential of 10% (m/v) of skimmed milk, inulin, maltodextrin, and sucrose were investigated during freeze-drying. Storage was assessed over 12 weeks at 4 °C and room temperature. Improved cell survival over freeze drying was observed with all the saccharides. However, only maltodextrin and sucrose retained cell viability over storage at 4 °C. Overall, skimmed milk demonstrated the highest survival up to 91%. Despite good cryoprotectant performance, inulin provided the least protection over storage, with

The presence of surfactants in the lixiviant solution has the potential to improve solution penetration into small pores and cracks of large ore particles by decreasing surface tension, thereby enhancing access of lixiviant to internal mineral grains and associated metal recovery from low grade sulphide ores. The effect of surfactants on the growth and activity of microorganisms implicated in bioleaching must also be considered, because of the microorganisms’ principal role in the regeneration of leaching reagents and facilitating metal recovery from an ore. The effect of surfactant addition on ferrous ion oxidation and microbially-mediated leaching of chalcopyrite by a mixed thermophilic microbial culture was investigated using five non-ionic surfactants: Tween® 20, Tween® 80, Plurafac® LF 120, Plurafac® LF 600 and Lutensol® XL 90 at concentrations of 5, 10 and 20 mg L−1. The addition of 5 and 10 mg L−1 Tween® 20 showed no negative effect on microbial ferrous ion oxidation and chalcopyrite bioleaching. Further, its presence was observed to reduce the initial leaching lag time. Microbial ferrous ion oxidation was inhibited in the presence of 10 mg L−1 Plurafac® LF 120 while a partial inhibition was observed with 10 mg L−1 Lutensol XL 90. Plurafac® LF 600 at 5 and 10 mg L−1 and Tween® 80 at 10 mg L−1 inhibited microbial growth completely, and thereby leaching, hence precluding it from a role in bioleaching.

In heap (bio)leaching processes, a substantial fraction of the valuable mineral is positioned below the ore particle surface. X-ray micro-computed tomography (&mu, CT) was used to quantify the effect of ore type and structure and operating temperature on the leaching of this mineral, to investigate the rate-controlling factors. Mini-leaching columns containing agglomerated chalcopyrite, pyrite, and malachite ores were scanned by X-ray &mu, CT (13.40 &micro, m resolution) at select time intervals. The leaching of a relatively porous malachite ore was reaction-controlled, with no leaching penetration limitation into the ore particles. For two less porous ore types, the structure and higher porosity of the agglomerate rim and conditions that resulted in the degradation of the full ore matrix structure were found to be the determining variables of the leaching extent and time. In the case of a chalcopyrite ore, an enhancement of copper recovery and sulphide mineral dissolution with increasing temperature was attributable to the increased leaching penetration distance and crack development in addition to thermodynamically expected increased leaching rate. Increasing temperature did not affect the maximum penetration distance for the waste rock containing pyrite, with no crack development observed. Overall increases in iron recovery were due to accelerated leaching rates, though diffusion or mineral access limitations were evident at a higher temperature.

X-ray micro-Computed Tomography (X-ray μCT) allows for non-invasive 3D investigations of heap leaching processes. It permits study of mineral liberation, pore network structure in particles or beds, and bioleaching of non-surface mineral grains. A premise for bioleaching studies integrating X-ray μCT is that the metabolic activity of microorganisms is not affected by X-ray energy doses. This study therefore evaluates the compatibility of bioleaching microorganisms with X-ray energy exposure. The X-ray effect on a mixed thermophilic culture suspended in solution was assessed using imaging conditions of 35–150 kV, 200–280 μA and 1–2 hours. Microorganisms were unaffected between 35–90 kV and 200–280 μA while there was some detrimental effect at 120 and 150 kV. The X-ray influence on a mixed mesophilic culture colonising a mineral surface was investigated at 90 kV and 225 μA for a 1 h exposure time. The system was monitored before and after exposure through measurement of: redox potential of the effluent solution, microbial cell concentration, heat output from the colonised surface using isothermal microcalorimetry (IMC), and cell attachment on the ore surface using scanning electron microscopy (SEM). The redox and cell concentration measurements showed insignificant differences between exposed and unexposed samples. Equivalent heat generation and good colonization of the mineral surface was demonstrated, confirming continued microbial activity post exposure, hence recommending its use in support of bioleaching studies.

The large cost of the flotation reagents used in the first-of a two-stage desulphurization flotation process, developed for the treatment of South African waste coal ultrafine tailings, has motivated the investigation of Mycobacterium phlei as an alternative coal bioflotation reagent. Attachment experiments were used to confirm that the microbe attaches to South African coal selectively over pyrite or gangue material present in the coal. Subsequent small scale batch floats using M. phlei successfully showed that the bioflotation process can upgrade and desulphurize the coal tailings feed, and delivered approximately the same concentrate yields as the optimised chemical flotation conditions (37 – 39%). However, a projected financial feasibility analysis that assessed the incorporation of the M. phlei into the two-stage flotation process found that the bioflotation process was not profitable due to the very large flow rate of cells required (2×1016 cells/ton coal) and the associated additional equipment and growth media component costs.

In heap bioleaching, a process in which microorganisms are required for the regeneration of leach reagents and control of reaction products, inaccessibility of non-surface mineral grains is a key cause of low recovery and long extraction times. High resolution, non-destructive 3D X-ray micro-computed tomography (μCT) is an imaging technique that has been successfully demonstrated for the study of abiotic leaching of non-surface minerals. For this technique to be applied to biotic leaching, it is required that the iron and sulphur oxidizing abilities of the microorganisms are not affected by the irradiation experienced. In the current study, the feasibility of investigating biotic leaching by X-ray μCT is explored by examining the relative energies required to achieve the high image resolution needed for mineral grain mapping while avoiding microbial deactivation. A mixed mesophilic and moderately thermophilic culture in solution was used and exposed to various X-ray energy doses. Direct microscopic cell counting and redox potential were measured to quantify the microbial activity and growth. The results showed that exposure to X-ray does not affect microbial activity at 35-90 kV, 200-280 μA and a distance of 7.2 cm between energy source and sample, however, it has an influence at 120 and 150 kV. This indicates that while X-ray μCT does influence the microbial cultures, it can be used for bioleaching studies at lower energy doses.

Microbial association with and colonisation of mineral surfaces plays a key role in enhancing the extraction of metals from ores during heap bioleaching processes. On the other hand, if uncontrolled, the same association can also lead to the generation of acid rock drainage (ARD) effluents from mine waste. This study aims to measure microbial metabolic activity of a mixed mesophilic culture on the surfaces of pyrite-bearing waste rocks of different grades over time. The waste rocks are milled, size fractionated and coated onto glass beads, to provide a defined surface area. The metabolic activity on the mineral surface is measured with isothermal microcalorimetry (IMC) complemented with scanning electron microscopy (SEM) and analysis of solution chemistry to measure leach agents and metal release into the pregnant leach solution (PLS). The waste rock samples showed a similar degree of leaching when the solution chemistry was analysed, despite having different sulphide content. However, when metabolic activity of the micro-organisms on the mineral surface was measured, greater activity was seen with higher sulphide content. This data informs an ongoing study to establish a flow-through configuration of the biokinetic test for ARD prediction accounting for both leach solution and microbial-mineral interaction as well as differing kinetics of acid-neutralising and generating reactions to enable the refinement of the current batch method.

Microbial colonisation of mineral ore is critical in efficient mineral solubilisation for metal recovery in heap bioleaching processes. A better understanding of the processes and behaviour by which microorganisms attach to, and colonise, mineral surfaces may help to enhance bioleaching processes. The direct quantification of metabolic activity of mineral associated cells is difficult and has not been demonstrated adequately in ore beds typical of heap leaching. In this investigation, isothermal microcalorimetry (IMC) was used to measure metabolic activity of a mixed mesophilic culture colonising the mineral surface. IMC is non-destructive and measures continuous heat flow from chemical or biological processes. Our results showed an increase of heat output from the colonised surfaces of mineral concentrate in the biotic system with respect to the abiotic system, caused by oxidative reactions facilitated by the mineral-microbial biofilm. This confirmed that the attached microorganisms were metabolically active and facilitated ongoing mineral leaching through regeneration of lixiviants. The progression of mineral colonisation in a mini-column system was monitored using IMC, scanning electron microscopy and conventional wet chemistry measurements.

© 2017 Elsevier LtdThis study presents an investigation, using 3D X-ray micro computed tomography (μCT), into the effect of sulfide mineral position within an ore particle on leaching efficiency. Three sections of an unsaturated mini-leaching column that had been packed with agglomerated low-grade, low-porosity chalcopyrite ore and leached with an acidified ferric iron solution were imaged at different stages of a 102 day experiment. Image analysis was used to quantify changes in the mineral content and the influence on this of the mineral distance from the ore particle surface, local voidage and radial position within the column. The main factor affecting the mineral recovery was identified to be proximity of the mineral to the ore particle surface, with recovery decreasing with increasing distance from the ore surface. A maximum leaching penetration was observed to exist at 2 mm from the surface, beyond which no recovery was achieved. Higher recoveries at the column wall indicated that preferential flow in this higher voidage had an additional, albeit smaller, impact on leaching efficiency.

Residence time distribution experiments were performed using chemical and microbial tracers to elucidate solution transport mechanisms within low grade, agglomerated ore beds. Multiple, fixed bed, mini-column reactors were packed with representative grab samples of low grade ore. The tracers were introduced to the ore bed in three ways: as a pulse added with the irrigant, included during agglomeration of the ore, and introduced by submerging the ore bed in tracer solution for a fixed time period. These methods of tracer introduction allowed for the characterisation of flow interchange in packed bed systems, under unsteady state conditions. Dispersion and diffusion of solution through the ore beds were characterised based on the resulting residence time distribution curves. Distinct stagnant and flowing regions within the agglomerated ore bed were observed. Flow characteristics provided information regarding solution exchange between the fast flowing and largely stagnant liquid phases. Increased retention times of both the solute and micro-organisms were noted when the tracer was agglomerated onto the ore surface. This was followed by an increase in solute transport rate from the ore surface driven by concentration gradient.

Liquid irrigation is one of the key process control parameters following the construction of an ore leaching heap. This study uses 3D magnetic resonance imaging (MRI) to examine non-invasively the effect of liquid flowrate changes on heap hydrology when drip irrigation is used. Experimental results from a vertical column show that the increase in flowrate causes an increase in the number of rivulets in the ore bed. The new rivulets were found to be thicker, and their development caused an increase in liquid–solid contacting area which is considered advantageous for metal ion recovery. Experiments performed on larger samples showed that the effects of flowrate changes were limited to the region directly below the drip emitter because the increase in flowrate caused an increase in macro-pore flow and not capillary retention of liquid. Therefore the increase in flowrate was not found to perturb liquid distribution patterns in a way that would be substantially advantageous to heap leaching recoveries.

Raw algal lipids (RALs) and their derivatives (fatty acid methyl esters, FAMEs) were investigated as biocollectors for the recovery of coal from ash-rich fine coal waste by froth flotation. Testing was done on fine coal discards from two South African sites&mdash, a high ash (50%), high sulphur (5.7%) sample and a lower ash (26%), low sulphur (0.91%) sample. The yield and recovery of combustibles on using biocollectors was similar to, or better than, with oleic acid, a polar collector demonstrated as a good collector in previous research. The RALs and FAMEs reduced the ash and sulphur content of the combustible fraction of the high ash-high sulphur sample to between 23&ndash, 28% and 1.9&ndash, 3.3%, respectively. The chemical modification of RALs to FAMEs provided potential for improving the selectivity of the recovery of combustibles. It also resulted in a change of the optimum operating pH, with RALs yielding better results at the natural pH of the sample (pH 2.7), while FAMEs performed best at pH 7.

The distribution of the metal-bearing mineral grains within a particulate ore prepared for leaching, and the impact of this spatial heterogeneity on overall extraction efficiency is of key importance to a mining industry that must continuously target ever-reducing grades and more complex ore bodies. If accessibility and recovery of the target minerals is to be improved, a more detailed understanding of the behaviour of the system must be developed. We present an in situ analysis using X-ray computed tomography to quantify the rates of volume reduction of sulfide mineral grains in low grade agglomerated copper bearing ores during a miniature laboratory scale column leaching experiment. The data shows the scale of the heterogeneity in the leaching behaviour, with an overall reduction of sulphide mineral grains of 50%, but that this value masks significant mm3 to cm3 scale variability in reduction. On the scale of individual ore fragments, leaching efficiency ranged from 22% to 99%. We use novel quantitative methods to determine the volume fraction of the sulfide that is accessible to the leachate solution.