Heap Leach Pads Research Articles

Bituminous geomembranes in mining applications in the Americas

Bituminous geomembranes (BGM) have been used over the last 20 years in mining applications in innovative ways, to reduce the hazards of mining in the environment. BGM is a reinforced geomembrane made of a geotextile from 250 gr/m² to 400 gr/m2 impregnated by elastomeric bitumen. This gives the geomembrane unique features that enables their use in demanding weather conditions of low temperatures (– 45°C or -49°F in Northern Canada or Europe) or high temperatures (+40°C in Brazil) due to its low thermal expansion coefficient that avoids the formation of wrinkles. This allows the installation and welding at high temperatures in the countries of Latin America. This paper will describe how several unique characteristics have been used for challenging and environmentally sensitive mining projects throughout the Americas, including treatment of solid waste spoil dumps (heap leach pads, closure, and encapsulation of mining facilities) of different types of ore: gold, diamonds, lithium, copper, silver, rare earth: niobium and in hydraulic applications for tailings or clear water dams and reservoirs.

Modelling the scale-up of the iodine-assisted chalcopyrite leaching in ferric sulfate media from reactors to columns and simulating heap operations

The iodine-assisted heap leaching of chalcopyrite involves the addition of iodide in ferric sulfate media to accelerate the dissolution of chalcopyrite at ambient temperature. The previous studies using reactor leaching tests identified the solution potential, the amount of iodide added, and temperature as the factors controlling the process performance. In the present study, the iodine-assisted chalcopyrite leaching process was scaled up from reactors to 1-m and 6-m columns to further understand how these parameters control the process performance at a larger scale. This is essential to optimize the process in industrial-scale heap leach pads. The HeapSim-2D model was calibrated with the 1-m and 6-m column leach tests together with the kinetic parameter values obtained in the previous reactor leaching tests. When the iodine-assisted chalcopyrite leaching was scaled up from reactors to 1-m and 6-m columns, the leaching became slower because the ore used in the column leaching tests had larger particle sizes with partially-liberated chalcopyrite particles. The calibrated model was then used to carry out the sensitivity analyses to assess the response of the process performance to key design and operating variables, including irrigation rate, ferric concentration, total iodide concentration, temperature, and heap height. The analyses show that the copper extraction could be optimized by adjusting these key design and operating parameters.

Pilot Scale Testing of Lignite Adsorption Capability and the Benefits for the Recovery of Rare Earth Elements from Dilute Leach Solutions

Naturally occurring organic materials containing humic acids show a strong affinity towards rare earth elements (REE) and other critical elements. Leaching experiments on lignite coal waste produced from construction sand production revealed that the contained REEs were associated with the organic matter. Furthermore, adsorption studies revealed that the lignite waste was capable of extracting REEs from a model solution and increased the REE content of the lignite waste by more than 100%. As such, this study aimed to utilize the lignite waste to adsorb REEs from pregnant leach solutions and acid mine drainage sources having low REE concentrations and subsequently leach the lignite material to produce pregnant leach solutions containing relatively high amounts of REEs, which benefits the performance and economic viability of downstream separation and purification processes. An integrated flowsheet was developed based on this concept and tested at a pilot scale. The pregnant leachate solution (PLS) was generated from a heap leach pad containing 2000 tons of Baker seam coarse refuse. The pilot scale circuit was comprised of aluminum precipitation, adsorption using the waste lignite, and rare earth-critical metal (RE-CM) precipitation stages in succession. The results indicated that the aluminum precipitation stage removed over 88% and 99% of the Al and Fe, respectively. The adsorption stage increased the REE content associated with the waste lignite from 457 ppm to 1065 ppm on a whole mass basis. Furthermore, the heavy REE (HREE) content in the feedstock increased by approximately 250%, which raised the percentage of HREE in the REE distribution by 19 absolute percentage points. In addition to the REEs, concentrations of other critical elements such as Mn, Ni, and Zn also improved by 75%, 37%, and 250%, respectively. Bench-scale tests revealed that increasing the solids concentration in the waste lignite and PLS mix from 1% to 20% by weight enhanced the adsorption efficiency from 32.0% to 99.5%, respectively. As such, a new flowsheet was proposed which provides significantly higher REE concentrations in the PLS that can be fed directly to solvent extraction and/or oxalic acid precipitation and, thereby, enhancing process efficiency and economics.

Chemical durability of bituminous geomembranes (BGMs) in heap leach pad applications

The degradation behaviour of a 4.8 mm thick elastomeric bituminous geomembrane (BGM) immersed in pH 0.5, 9.5, and 11.5 synthetic mining solutions is examined over 26 months at 22, 40, 55 and 70°C. The low pH solution simulates the leach solutions found in copper, nickel, and uranium heap leach pads while the two high pH solutions simulate the chemistry and pH found in gold and silver heap leaching facilities. The mechanical, rheological, and chemical properties are examined at different incubation times to assess the degradation in the BGM at different temperatures. It is shown that the degradation rates of all properties are faster in pH 11.5 and 9.5 than in pH 0.5. Additionally, the BGM started to exhibit degradation in its mechanical properties even with a slightly degraded bitumen coat in all the mining solutions at elevated temperatures. The time to nominal failure of the BGM is predicted at different field temperatures using Arrhenius modelling. Due to the relatively fast degradation in the mechanical properties of the BGM, especially at temperatures above 50°C, the tensile strains in the BGM in the field should be limited so it can meet the required liner design life of heap leaching applications.

Global mining footprint mapped from high-resolution satellite imagery

Mining is of major economic, environmental and societal consequence, yet knowledge and understanding of its global footprint is still limited. Here, we produce a global mining land use dataset via remote sensing analysis of high-resolution, publicly available satellite imagery. The dataset comprises 74,548 polygons, covering ~66,000 km2 of features like waste rock dumps, pits, water ponds, tailings dams, heap leach pads and processing/milling infrastructure. Our polygons finely contour the edges of mine features and do not include the space between them. This distinguishes our dataset from others that employ broader definitions of mining lands. Hence, despite our database being the largest to date by number of polygons, comparisons show relatively lower global land use. Our database is made freely available to support future studies of global mining impacts. A series of spatial analyses are also presented that highlight global mine distribution patterns and broader environmental risks.

Physical and mechanical performance of an HDPE geomembrane in 10 mining solutions with different pHs

The degradation in physical and mechanical properties of a 1.5 mm thick HDPE geomembrane immersed in seven different low pH and three high pH simulated mining solutions is examined at temperatures between 95 and 40 °C for 3 years. The solutions with pH between 0.5 and 13.5 encompass solutions found in copper, nickel, uranium, gold, and silver heap leach pads. The geomembrane did not exhibit any chemical degradation during the three years of incubation in all the low-pH solutions. However, in the solutions with pH 9.5, 11.5, and 13.5, some of the geomembrane’s physical and mechanical properties are shown to reach nominal failure at 95 and 85 °C. While the geomembrane examined shows superior performance in the acidic environments than in the basic solutions examined, its performance in such extremely basic environments is still better than in neutral reduced municipal solid waste leachate. Using Arrhenius’ modelling, the predicted times for the antioxidant depletion stage of the geomembrane examined in composite liner configuration range between 31 years in pH 13.5 to 51 years in pH 0.5 for pad liners at 50 °C, exceeding a typical leaching period of the ore of around 20 years in different heap leaching operations.

Heap leach pad mapping with drone-based hyperspectral remote sensing at the Safford Copper Mine, Arizona

Mapping the distribution of minerals, pipes, ponded lixiviant, and other surficial features on heap leach pads is a continuing challenge for hydrometallurgy operations, maintenance, and design. Recently, commercial drone technology has enabled remote sensing techniques to be deployed for some leach pad mapping and surveying applications, but applications to hydrometallurgical operations are still emerging. This project tested one potential mapping technology, drone-based hyperspectral remote sensing, for mapping leach pads, using the Safford mine as a test site.The results show that hyperspectral remote sensing can acquire information about mineral distribution, piping system, and lixiviant ponding. The main minerals detected on the leach pads are kaolinite, muscovite, and gypsum, although the detection of gypsum is probably due to the widespread sulfate precipitating from agglomerating acid and lixiviant. The path of the stacker loading rocks, lixiviant ponding, and drip lines can also be seen in the spectral images. Chlorite and biotite, long-term acid consumers, were not detected in the results, but are on the leach pads. Best practices for hyperspectral remote sensing in mapping leach pads include: building a reference library from ground-truth samples, using high spatial resolution for mapping piping systems and low spatial resolution for mapping minerals, scanning leach pads in sunshine, and scanning before leaching starts.

Load Capacity of Perforated Reinforced Concrete Sewer Pipes

In cities, reinforced concrete pipes are often employed to construct storm sewer systems. Due to impervious base layers, stormwater runoff dominates rather than rainwater infiltration, leading to problems in aquatic ecosystems (e.g., degradation in urban watersheds, flood attenuation, and water quality). A thermoplastic perforated pipe can provide drainage channels in applications of solid waste landfills, mining heap leach pads, or irrigation systems. Inspired by these applications, perforation can be implemented in a reinforced concrete sewer to alter the moisture conditions in the ground under impervious base layers. Three-edge bearing tests and four-point flexural tests are conducted to investigate the structural integrity of perforated sewer pipes. A numerical model is also calibrated for use to optimize the layout design of drainage holes. It is found that perforation can result in strength degradation in concrete pipe, but the reduction in strength is not significant. A correlation between the ultimate load and opening ratio is established for the two testing conditions. The allowable opening ratio is determined as 0.2%, below which a perforated sewer can fulfill the requirements for both drainage and load resistance.

Regulations, risk management and experience about tailings dams in Chile

Chile is the biggest copper producer in the world, the production of fine copper in 2018 was approximately 5.5 Mton and it is estimated that in 2025, 7.2 Mton of copper will be produced. This production, added to copper grades of less than 1%, causes 99% of the material that is processed to become mine waste such as tailings storage, heap leach pads and rubble deposits. Tailings are classified as high risk environmental waste, and these tailings are also located in a country of high seismicity. Due to this, Chile has a regulatory framework that allowed the improvement of the standards in the design, construction and operations of tailings deposits based on the know-how about the behaviour of these structures. Currently, a risk management system is being implemented to identify the risks and take decisions and actions to ensure the physical and chemical stability of these different types of storage. This article describes the Chilean experience about the behaviour of tailings storage under high magnitude seismic events. A summary about the regulatory framework and the recently implemented risk management is also presented.

Shear strength of the geomembrane–subgrade interface in heap leaching applications

A series of large-scale direct shear tests was carried out to examine the effectiveness of smooth, textured and structured surface geomembranes (GMBs) with different soil subgrades for heap leach pad applications. Four different subgrades – namely, sand, two different coarse-grained underliners and a clayey soil representing the layers directly underlying the GMB liner in heap leach pads – were used to examine the shear strength of the GMB–subgrade interfaces at normal stresses between 50 and 1000 kPa. It was found that increasing the normal stresses can change the mechanisms contributing to the shear resistance at the interface. This resulted in a statistically insignificant increase in the interface friction of the GMB–granular soil interfaces when using GMBs with surface roughness relative to the interface friction of the smooth GMB. Furthermore, depending on the type of subgrade, establishing the shear envelopes over a wide range of normal stresses was found to overestimate or underestimate the shear strength at the field stresses even when linear regressions present the best fit for the data.

Water retention curves of a geosynthetic clay liner under non-uniform temperature-stress paths

This paper presents a novel suction-controlled chamber that permits the determination of the full water retention curves of geosynthetic clay liners (GCLs) under non-uniform temperature-stress paths. It investigates field conditions encountered in brine ponds (low confining stress settings) and heap leach pads (high confining stress settings) during construction and operation stages. Consequently, the analysis of the moisture dynamics in a GCL was defined under the wetting path (construction) and drying path (operation). High vertical stresses were found to facilitate a more rapid water uptake as capillarity is established faster than at low, confined stresses. In general, the drying curves increase the water desorption over the suction range investigated due to the low water viscosity caused by high temperatures. The wetting of the GCL at 20 °C and drying at 70 °C under either low, confined stress (2 kPa) or high confining stress (130 kPa) shows a reduction in the volumetric water contents. Furthermore, on the drying path, the coupled effect of elevated temperature and high confining stress accelerates water desorption leading possibly to potential desiccation.

Heap Leach Pad Surface Moisture Monitoring Using Drone-Based Aerial Images and Convolutional Neural Networks: A Case Study at the El Gallo Mine, Mexico

An efficient metal recovery in heap leach operations relies on uniform distribution of leaching reagent solution over the heap leach pad surface. However, the current practices for heap leach pad (HLP) surface moisture monitoring often rely on manual inspection, which is labor-intensive, time-consuming, discontinuous, and intermittent. In order to complement the manual monitoring process and reduce the frequency of exposing technical manpower to the hazardous leaching reagent (e.g., dilute cyanide solution in gold leaching), this manuscript describes a case study of implementing an HLP surface moisture monitoring method based on drone-based aerial images and convolutional neural networks (CNNs). Field data collection was conducted on a gold HLP at the El Gallo mine, Mexico. A commercially available hexa-copter drone was equipped with one visible-light (RGB) camera and one thermal infrared sensor to acquire RGB and thermal images from the HLP surface. The collected data had high spatial and temporal resolutions. The high-quality aerial images were used to generate surface moisture maps of the HLP based on two CNN approaches. The generated maps provide direct visualization of the different moisture zones across the HLP surface, and such information can be used to detect potential operational issues related to distribution of reagent solution and to facilitate timely decision making in heap leach operations.

Mapping Surface Moisture of a Gold Heap Leach Pad at the El Gallo Mine Using a UAV and Thermal Imaging

An understanding of the spatial and temporal variations of surface moisture content over a heap leach pad (HLP) is essential for leaching production and to achieve a high ore recovery. The current practice of leach pad monitoring and data collection remains highly manual and labor-intensive, and exposes technical staff to hazardous material (i.e., cyanide solution). To address these challenges, we propose using unmanned aerial vehicles (UAVs) equipped with thermal imaging sensors to remotely obtain high temporal and spatial resolution image data for monitoring the surface moisture distribution over HLPs. A field study was conducted over a sprinkler-irrigated HLP at El Gallo gold mine in Sinaloa State, Mexico, and the acquired data were used to derive an empirical relationship between the surface moisture content and the remotely sensed surface temperature using linear regression. Moreover, the data were used to generate moisture distribution maps of the entire HLP surface. In situ samples were taken manually during the field experiments to measure the ground-truth material moisture at selected sampling locations. The results show a good agreement between the remote sensing method and the measured ground-truth samples.

The Effect of Hydro-Jex Operation on the Stability of Heap Leach Pads: a Case Study of a Heap Leach Operation in Central Mexico

Hydro-Jex® is a new enhanced method for heap leach treatment with significantly higher mineral recovery. In this case study at the Los Filos Mine in Mexico, the Hydro-Jex operation is evaluated for heap leach stability. This is pursued by numerical modeling of the heap leach pad stability and well monitoring of the phreatic surface before and after the Hydro-Jex operation. Experimental and numerical results both indicate an improvement in mineral processing under the influence of Hydro-Jex. Numerical results show an insignificant decrease and a significant increase in stability during and after Hydro-Jex injection application, respectively. The integrity of the liner is maintained in both cases. The zonal injection pressure, operationally below the overburden pressure, is found to be negligible compared with the dimensionality of the heap, and stability of the structure is not exposed to any significant risk, even up to 25% above overburden pressure. Furthermore, heap leach factor of safety increases after Hydro-Jex application. This is attributed to breakdown of the water solution build-up and thus a decrease in the phreatic surface depth below the top of the pad. It is suggested that injection wells be drilled, based on thorough geophysical data, in locations where overcompaction of heap material results in water solution build-up and pore pressure enhancement. Compared with traditional heap leaching, the Hydro-Jex technique not only expedites mineral processing by increasing chemical kinetics extraction but also increases the stability of the heap by unclogging drains.

Global-scale remote sensing of mine areas and analysis of factors explaining their extent

Mines are composed of features like open cut pits, water storage ponds, milling infrastructure, waste rock dumps, and tailings storage facilities that are often associated with impacts to surrounding areas. The size and location of mine features can be determined from satellite imagery, but to date a systematic analysis of these features across commodities and countries has not been conducted. We created detailed maps of 295 mines producing copper, gold, silver, platinum group elements, molybdenum, lead-zinc, nickel, uranium or diamonds, representing the dominant share of global production of these commodities. The mapping entailed the delineation and classification of 3,736 open pits, waste rock dumps, water ponds, tailings storage facilities, heap leach pads, milling infrastructure and other features, totalling ~3,633 km2. Collectively, our maps highlight that mine areas can be highly heterogeneous in composition and diverse in form, reflecting variations in underlying geology, commodities produced, topography and mining methods. Our study therefore emphasises that distinguishing between specific mine features in satellite imagery may foster more refined assessments of mine-related impacts. We also compiled detailed annual data on the operational characteristics of 129 mines to show via regression analysis that the sum area of a mine's features is mainly explained by its cumulative production volume (cross-validated R2 of 0.73). This suggests that the extent of future mine areas can be estimated with reasonable certainty based on expected total production volume. Our research may inform environmental impact assessments of new mining proposals, or provide land use data for life cycle analyses of mined products.

20 Years of Evapotranspiration Cover Performance of the Leach Pads at Richmond Hill Mine

The Richmond Hill Mine heap leach pads (HLP 1&2, and HLP 3) were reclaimed in 1996 using a three-layer evapotranspiration (ET) cover designed to limit percolation of precipitation through the facilities. HLP seepage outflow rates have been measured on an approximately weekly basis. Large seasonal and wet/dry outflow cycles have been observed. Several years after the cover system installation, increased HLP drainage rates were observed, which could be due to increased permeability of the compacted layer in response to pedogenic processes such as wetting/drying, freeze/thaw, and root development. The estimated net infiltration rate during the monitoring period is about 34% of precipitation. To evaluate the cover system efficiency, a monthly time-step, spreadsheet based water balance model was created. The model considers close inter-relation between precipitation, ET, and cover net infiltration on a monthly scale and continuous feedback of water movement in the soil-plant-atmosphere continuum. The model accurately reproduced the recorded seepage rates for the monitoring period after pedogenic processes reach a stable condition.

Interactions of mimic weathered pyrite surfaces (FeS2) with acidic culture media (0 K): An approach for (bio)leaching applications

In biohydrometallurgical processes, a mineral exhibits different oxidation phases due to the system's heterogeneity, especially in heap leach pads. Oxidation chemically modifies the mineral surface altering its interface, and thereby affecting the bacteria-mineral interaction, mineral reactivity and leaching velocity. Given that the mineral can be found in different oxidation states in heap bioleaching processes, three oxidation conditions of FeS2, in which iron and/or sulfur related compounds are formed on the mineral surface were assayed. This paper studies the interaction between the modified surface of a sulfide mineral, FeS2, and 0 K culture medium, typically used in biomining processes. Chemical and electrochemical changes on surfaces were characterized by subjecting them to weathering in an acidic culture medium (pH 1.8), without applying potential or current. The chemical species formed were identified by Raman spectroscopy. The modified pyrite surfaces showed significant interfacial transformations upon immersion in the culture medium, and the formation of passive chemical species, such as elemental sulfur, jarosite, phosphates and oxides, were identified. These interfacial modifications are correlated with changes in the open circuit potential (OCP) values during immersion of pyrite and surface modified pyrites in 0 K culture medium. Electrochemical characterization showed a decrease in mineral oxidation capacity, which directly affects the extent of leaching and possibly, of the interaction with other elements participating in the process, such as microorganisms. To study the interactions among bacteria and the pyrite mineral suffering different surface modifications, the attachment of the bioleaching bacterium Leptospirillum sp. was evaluated.

Quantitative mineralogy and geochemistry of pelletized sulfide-bearing gold concentrates in an alkaline heap leach

Sulfide-rich gold mill concentrates from the Cripple Creek & Victor Gold Mining Co. (CC&V) in Colorado, USA were pelletized with Portland cement at 20, 80 or 200 lbs./ton. The hand-formed ~1 in. diameter pellets were leached in a bench-scale heap leach system for 16 weeks in a 0.3 mM NaOH solution at pH 10.5, in a matrix of crushed ore or clear acrylic balls. Leached pellets were sampled every two weeks. The sampled pellets were encapsulated in epoxy, cut, polished, and examined using a field emission scanning electron microscope equipped with an automated mineralogy system. Leachate samples were collected after 2, 4, 6, 8 and 16 weeks and analyzed for major cations. Higher cement concentrations produced pellets with better macroscopic integrity, although microscopically and chemically detectable degradation occurred at all cement concentrations tested. Pellets with 200 lbs./ton cement retained their physical integrity macroscopically throughout the trial, whereas pellets with 20 and 80 lbs./ton cement were significantly fractured within 2 days of leaching. Mineralogical analyses by electron microscopy showed that physical degradation was localized where cement had been removed. The electron microscopy also revealed that all pellets had cement-leached rims by 8 weeks, and sulfide:oxide ratios decreased in most experiments. Leachate concentrations of Ca, Ba and Sr were initially high and decreased over the experiment, consistent with rapid dissolution of cement from the pellets in the first 4 weeks. These results show that pellets will break apart and dissolve on the heap leach pad at all of the tested binder concentrations; thus pelletizing CC&V mill concentrates is not likely to improve leach pad permeability or minimize acid generation. The combination of automated mineralogy, element mapping, and chemical analysis of leachates proved to be an effective means of monitoring leach behavior of fine-grained materials. The Ca element mapping was the most efficient method of monitoring cement binder degradation.

Conception of an integrated flowsheet for rare earth elements recovery from coal coarse refuse

The majority of rare earth elements (REEs) existing in the feed to coal preparation plants report to the coarse refuse streams which are transported for permanent storage in contained piles. In this study, an integrated flowsheet was developed based on laboratory test data which combines physical separation, pyrite bio-oxidization, heap leaching, selective precipitation and solvent extraction processes. The test data was obtained from (1) characterization of a number of natural leachate and solid samples collected from different preparation plants which process coals from a number of coal seams and (2) laboratory acid leaching and selective precipitation tests results. The highly-valued critical REEs (i.e., Y, Nd, Eu, Tb and Dy) were selectively leached from the refuse samples in the natural environment due to the acid generated by pyrite oxidization. The leachate samples were evaporated to remove water and obtain residual solids (i.e., dissolved solids in the leachates). The total REE content in the dissolved solids from a given leachate sample was 380 ppm, which was higher than the REE content of the coarse refuse material that generated the leachate (322 ppm). Acid leaching tests recovered as much as 80% of the total REEs from the coarse refuse samples using a 1.2 M sulfuric acid solution. Afterwards, the pH of the leachate was increased in a step-wise fashion which resulted in the production of precipitates containing 0.3–1.1% total REEs. A significant amount of contaminants, such as Fe, Al, and Ca, were eliminated in the sequential precipitation process, which allowed further upgrading using oxalic acid precipitation and/or solvent extraction. In the proposed flowsheet, the coarse refuse is arranged in heap leach pads and the acid needed for REE leaching is primarily produced from pyrite bio-oxidization, which enhances the selectivity of REE recovery and significantly reduces the cost. The successful application of the flowsheet would result in significant benefits to both the coal and rare earth industries.

Problems and solutions for GCLs used in waste containment facilities: temperature concerns and polymer treatment related to GCLs used in waste containment areas

In this study, first, various geosynthetic clay liner (GCL) application areas were briefly defined and afterwards, the details of GCL usage in mine applications were given. GCLs are barrier materials that are preferred in mining facilities such as heap leach pads, tailing dams or waste landfills. In these applications, GCLs might be in contact with aggressive leachates that could deteriorate the hydraulic properties of the GCLs. The hydraulic capability of the GCLs is decreased by an increase in hydraulic conductivity or a decrease in swell index of the bentonite component of the GCL. Due to the biodegradation of the organic substances in a waste containment area, heat is generated and the temperature of the leachates increases. To investigate the effect of temperature, triaxial permeability and free swell tests were conducted on the GCLs that were permeated with 0.5 M MgCl2 solution and deionized water. The temperature of the permeation fluid was chosen as 20 and 40 °C. Furthermore, a cationic polymer having 1 and 2% amounts of mass was added to the bentonite component of the GCL to enhance the hydraulic capability of the GCL. Test results indicated that temperature increase in 0.5 M MgCl2 solution from 20–40 °C caused both an increase in the swell index and the permittivity of the GCLs. The increase in permittivity could be attributed to the lower viscosity of the fluid at higher temperatures. Finally, adding a cationic polymer up to the amount of 2% by mass to the bentonite resulted in almost two orders of magnitude decrease in the permittivity of the GCLs that were permeated with 0.5 M MgCl2 solution. In conclusion, cationic polymer-treated GCLs might be used effectively in waste containment facilities by both decreasing the permittivity and increasing the swell index of the GCLs.