### Browsing by Subject "Heap leaching"

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- ItemOpen AccessComparison of non-reactive solute transport models for the evaluation of fluid flow in leaching beds(2023) Odidi, Michael Dumisane; Harrison, Susan; Fagan MarijkeHeap leaching is a hydrometallurgical process used for the extraction of minerals within complex and typically low-grade ores. An important factor in the mineral dissolution process is the contact efficiency between the irrigation fluid (lixiviant) and the targeted mineral, which is influenced by both the solid and fluid properties of the system. One of the principal challenges related to the contact efficiency is preferential flow, cited to result in low extraction rates and in extreme cases, heap failure. Preferential flow reveals itself on two scales in drip irrigated heaps, referred to as the bed and solution scale. The bed scale takes a macro view of the heap and deals with uneven wetting profiles characterized by the presence of wet and dry sections. Linked to this is capillary suction effects which play an important role in the establishment of fluid flow profiles within the heap. The solution scale focuses on preferential flow behaviour in the wetted sections of the heap characterized by variations in the residence times of fluid elements. Such variations produce fast flowing, slow flowing and stagnant solution pools. Therefore, ideal solution flow behaviour in a heap result in uniform wetting at the bed scale and plug flow behaviour with similar fluid residence times at the solution scale. Though bed scale preferential flow can be visually observed, diagnosing symptoms at the solution scale typically requires the generation, analysis, and modelling of residence time distribution (RTD) curves. The main objectives of this study were to firstly explore the effects that important material and fluid properties have on the steady state fluid flow profiles in drip irrigated beds characteristic of those used in laboratory scale column leaching studies and quantified using step tracer tests. This is based on the underlying principle that the movement of inert tracer molecules within an irrigated bed at steady state is identical to the solution flow path within the bed. The second objective was to test the ability of nine empirical and semi-empirical solute transport models to adequately fit the generated flow profiles or RTD curves. The third was to compare the magnitudes of the quantified model parameters to ascertain the level of solution scale preferential flow in the different beds and determine the adequate level of model complexity needed to describe their flow profiles which facilitates identifying the controlling variables within the system. Properties of the loading material that were identified as potentially most impactful with respect to heap operations were: porosity, wettability, particle shape and size distribution. Therefore, four different materials with unique inherent characteristics were selected for this study: glass beads (GB - spherical and non-porous), glass shards (GS - irregularly shaped and non-porous), greywacke (GW - irregularly shaped, porous, and highly wettable) and malachite ore (MO - irregularly shaped, highly porous, low wettability and non-uniform composition). In terms of fluid properties, current models have already established a correlation between the concentration of dissolved chemical species within a fluid and its viscosity. This was relevant due to the variety of lixiviant compositions used in previous heap hydrology studies and the fact that the composition also varies with time within a reactive heap. To study the effects of this parameter on the establishment of flow profiles, glycerol was used as a viscosity modifier to formulate solutions with viscosities ranging from 0.8 to 2.2 cP, representative of the range experienced in heap leaching systems due to varying SO4 2- concentrations. The packed beds were characterized using their bulk densities, void age, total liquid holdups, total bed saturations, 24-hour drain-down moisture percentages, solution and tracer breakthrough times. Beds containing both narrow and mixed particle size fractions were tested. The nine solute transport models used for RTD modelling included three compartmental model configurations (CM-1, CM-2, CM-3) and the tanks-in-series (TIS) model, all empirical in nature. The five semi-empirical models selected were the advection-dispersion (AD), piston exchange (PE), piston exchange-diffusion variant (PE-D) and piston dispersion and exchange (PDE) models. A novel model formulation called the piston dispersion and exchange-diffusion variant (PDE-D) model was also coded and tested, which incorporated both the longitudinal dispersion coefficient as well as a diffusional flux mass transfer mechanism. The CM-2, AD and TIS were mono porosity models assuming all solution volumes within the beds were actively flowing which limited their ability to account for solution scale preferential flow. The CM-1, CM-3, PE, PE-D, PDE, PDE-D models were dual porous, accounting for the presence of either dead or stagnant solution volumes. The model parameters used to account for preferential flow in the RTD profiles included: the fraction of dead to total solution volume, dynamic to total saturation fraction, number of TIS, ratio of parallel continuously stirred tank volumes, longitudinal dispersion coefficient, overall mass transfer coefficient and maximum diffusional pore length. The cumulative RTD responses for the bed systems composed of narrow size fractions were noticeably impacted by particle size. These systems displayed symptoms of increased solution channelling behaviour at steady state, based on their relatively short tracer breakthrough times, as the average particle size was increased from ∼1 to 15 mm. The incorporation of semi-empirical models which could account for stagnant volumes. The main comparative modelling results across all systems studied showed that the PDE and PDE-D models were the top performers, based on a model fit analysis. This was due to their dual porous nature and relatively higher levels of complexity. The mono porosity models (CM-2, TIS and AD) performed the worst due to their inability to account for isolated and immobile liquid volumes. However, when 10 mm during agglomeration will aid in increasing the fraction of mobile (actively flowing) liquid within the heap due to the increased presence of macro voids. High levels of particle porosities (>2.5 m2/g) will also aid in this aspect. This is proposed to be due to greater void network connectivity with an increase in porosity facilitating better mass transfer. These insights were obtained through the analysis of experimentally generated data and model simulations. They have provided a better understanding of the movement of fluid molecules within drip irrigated beds, which is essential for improved leaching performance. Building on this, the next step is to consider the effects of scale up and reactive systems on both empirical and simulated data.
- ItemRestrictedCompetitive Bioleaching of Pyrite and Chalcopyrite(Elsevier, 2006) Petersen, Jochen; Dixon, David GAn experimental study was conducted to investigate the heap bioleaching of a copper-gold concentrate using the Geocoat™ technique. Small-scale columns were operated at selected temperatures using bacterial consortia suited to these temperatures. In all experiments secondary copper sulfides would leach more rapidly than pyrite. Chalcopyrite, however, appeared to leach only in the presence of extremely thermophile microbes and selectivity towards chalcopyrite decreased with leach time. Analysis of the leach tailings showed that, surprisingly, the overall rate of bio-oxidation did not significantly increase with temperature. Further, analysis of ferric demand for mineral oxidation vs. ferric supply by microbial oxidation suggests that chalcopyrite leaching is promoted by extreme thermophiles due to a favourable interplay between reaction kinetics, solution potential and chalcopyrite ‘passivation’ phenomena, whereas pyrite leaching is favoured at lower temperatures. This analysis also explains why selectivity towards chalcopyrite decreases with time. The results are of some significance for the development of a high temperature whole ore chalcopyrite heap bioleach process.
- ItemRestrictedDetermination of oxygen gas-liquid mass transfer rates in heap bioleach reactors(Elsevier, 2010) Petersen, JA detailed experimental study is described which was conducted to determine the rate of oxygen gas–liquid mass transfer within the packed bed of heap bioleach reactors at different temperatures (22–68 °C), using the Na2SO3 method. The raw data was analysed using a simplified film mass transfer model, making corrections for oxygen solubility in concentrated solution and for increased water vapour partial pressure at elevated temperatures. The results compared favourably against two independent experimental leach studies, indicating kLa values between 33 and 46 h−1. The value varied with the particle size distribution of the packing, with kLa assuming larger values for those packings that had a higher fines contents. While kLa increases with temperature, the solubility of oxygen decreases simultaneously, resulting in net mass transfer rates being relatively unaffected by temperature in the range studied. This indicates that thermophile heap bioleach reactors are likely to operate under gas–liquid mass transfer limitations, especially at high altitude.
- ItemRestrictedLarge particle effects in chemical/biochemical heap leach processes - A review(Elsevier, 2011) Yousef, Ghorbani; Becker, Megan; Mainza, Aubrey; Franzidis, Jean-Paul; Petersen, JochenThe application of heap leach technology to recovery of economically important metals, notably copper, gold, silver, and uranium, is wide-spread in the mining industry. Unique to heap leaching is the relatively coarse particle size, typically 12–25 mm top size for crushed and agglomerated ores and larger for run-of-mine dump leaching operations. Leaching from such large particles is commonly assumed to follow shrinking core type behaviour, although little evidence for the validity of this assumption exists. This review investigates the current state of knowledge with respect to the understanding of the characteristics and mineralogy of large particles and how these influence leaching in a heap context and the tools to characterize these. This includes the study of ore and particle properties, visualization techniques for ore characterization, the connection between comminution and leaching behaviour, as well as particle models within heap leach modelling. We contend that the economics of heap leaching are strongly governed by the trade-off between the slow rate and limited extent of leaching from large particles and the cost of crushing finer. A sound understanding of the underlying large particle effects will therefore greatly inform future technology choices in the area of heap leaching.
- ItemRestrictedLarge particle effects in chemical/biochemical heap leach processes – A review(Elsevier, 2011) Ghorbani, Yousef; Becker, Megan; Mainza, Aubrey; Franzidis, Jean-Paul; Petersen, JochenThe application of heap leach technology to recovery of economically important metals, notably copper, gold, silver, and uranium, is wide-spread in the mining industry. Unique to heap leaching is the relatively coarse particle size, typically 12–25 mm top size for crushed and agglomerated ores and larger for run-ofmine dump leaching operations. Leaching from such large particles is commonly assumed to follow shrinking core type behaviour, although little evidence for the validity of this assumption exists. This review investigates the current state of knowledge with respect to the understanding of the characteristics and mineralogy of large particles and how these influence leaching in a heap context and the tools to characterize these. This includes the study of ore and particle properties, visualization techniques for ore characterization, the connection between comminution and leaching behaviour, as well as particle models within heap leach modelling. We contend that the economics of heap leaching are strongly governed by the trade-off between the slow rate and limited extent of leaching from large particles and the cost of crushing finer. A sound understanding of the underlying large particle effects will therefore greatly inform future technology choices in the area of heap leaching.
- ItemRestrictedLeaching of a zinc ore and concentrate using the Geocoat™ technology(Elsevier, 2011) Soleimani, Mohammad; Petersen, Jochen; Roostaazad, Reza; Hosseini, Soheil; Mohammad, Mousavi; Najafi, Alireza; Vasiri, Akhtarolmolouk KazemiIn this study, the Geocoat™ technology was used for the extraction of zinc from a mineral concentrate obtained from the Kooshk mine (Yazd, Iran) by a culture dominated by the mesophilic bacterium Acidithiobacillusferrooxidans in a packed column bioreactor. A low grade sphalerite ore was used as support for the concentrate coating. During the 100 days of leaching pH, Fe3+, Fetotal, microbial population density and zinc extraction were measured. The final zinc extraction from concentrate and low grade support was 97% and 78%, respectively, and it was found that leaching from the support does not proceed significantly before leaching from the coating is completed.
- ItemRestrictedModelling of bioleach processes: connection between science and engineering(Elsevier, 2010) Petersen, JThis paper gives a brief introduction to the modelling of bioleach processes, developed from a careful analysis of the fundamental process steps at the gas–liquid, biological and mineral interfaces, and how these interact in a given reactor environment (tanks and heaps). The insights gained from such modelling work can guide both engineers in the optimisation of processes and scientist in directing their research at areas not yet well understood. From this perspective, some future directions of the bioleaching field are discussed.
- ItemRestrictedModelling zinc heap bioleaching(Elsevier, 2007) Petersen, J; Dixon, D GA comprehensive modelling study of the HydroZinc™ heap bioleach process, using the HeapSim modelling tool, is described. The model was calibrated on the basis of a small number of column leach experiments and compared against pilot heap test results. The model calibration thus confirmed, a detailed sensitivity study was conducted in order to establish the key parameters that determine the overall rate of Zn extraction. In the present case these were found to be oxygen gas–liquid mass transfer, various factors affecting the delivery of acid into the heap, and factors affecting the temperature distribution within the heap. A set of improved design parameters are proposed that would almost double the zinc conversion rate measured in the pilot plant – from 83% in 740 days to 78% in 383 days – and increase zinc production rate from 1.77 to 4.35 kg/m2/day. However, this improvement must be evaluated in the context of various implications for the downstream process.