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COVERING THE WORLD || PROTECTING THE EARTH

W W W. S O L M A X . C O M

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MINING HEAP LEACH PADS & TAILINGS DAMS

HEAD OFFICE | Varennes | Canada Gières | France Shanghai | China Santiago | Chile Pelabuhan Klang | Malaysia


MINING HEAP LEACH PADS & TAILINGS DAMS

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Telephone Canada | 450 929 1234 Telephone Malaysia | 03 3375 3600

TECNICAL NOTES || MINING HEAPLEACH PADS & TAILINGS DAMS

BACKGROUND

Mining is the removal of valuable raw materials or other geological materials from the earth, from an ore body, vein or seam. The term also includes the removal of soil. Any material that cannot be grown through agricultural processes, or created synthetically in a laboratory or factory, is usually mined. Mining in a wider sense includes taking away of any non-renewable resource. Mining of stone and metal has been done since pre-historic times. Modern mining processes involve the prospect for ore bodies, analysis of the profit possible of a proposed mine, removal of the desired resources and finally, retrieval of the land to prepare it for other uses once the mine is closed.

The nature of the mining process creates a potentially detrimental impact on the environment both during the mining operation and for years after the mine is closed. This impact has led to most of the world’s nations adopting systems to moderate the negative effects of mining operations.

Heap leaching is an industrial mining process to extract precious metals, copper, uranium, and other compounds from ore. The process has ancient origins; one of the classical methods for the manufacturing of iron sulfate was to heap up iron pyrite and collect the leachate from the heap, which was then boiled with iron to produce iron sulfate.

Tailings are the materials left over after the process of separating the valuable fraction from the uneconomic fraction of an ore. Tailings are distinct from overburden or waste rock, which are the materials overlying an ore or mineral body that is displaced during mining without being processed. Mine tailings are usually produced from the mill in slurry form (a mixture of fine mineral particles and water). Tailings represent an external cost of mining, particularly true of early mining operations which did not take adequate steps to make tailings areas environmentally safe after closure. Modern day mines, particularly in jurisdictions with well-developed mining regulations or operated by responsible mining companies, incorporate the rehabilitation and proper closure of tailings areas in the mining costs and activities. For example, the province of Quebec, Canada, requires not only submission of a closure plan before the start of mining activity, but also the deposit of a financial guarantee equal to 70% of the estimated rehabilitation costs. Tailings dams are often the most significant environmental liability for a mining project.

ENVIRONMENTAL IMPACTS IN HEAP LEACH MINING

Today, large mining companies must deal with major economic as well as ecological concerns. They must recover the maximum amount of ore through leaching, working large quantities of material with a large or small ore content at a reasonable cost. And they must do so while protecting soil and groundwater tables neighbouring their prospecting zones.




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REV. 01.12Telephone Canada | 450 929 1234 Telephone Malaysia | 03 3375 3600

To this end, they have little choice but to forego inappropriate operating methods in favour of a modern approach and technique for the construction of impervious heap leach pad systems, storage areas and mine tailing covers. The use of polyethylene or PVC geomembranes has made it possible to continue working mines in regions subjected to harsh weather conditions (intense cold in Siberia, the heat of South American deserts) or on complex and unstable soil (saline or sandy soil of the Chilean desert), while meeting owners’ economic and ecological imperatives.

The elements and compounds uncovered and liberated through mining and processing, which are not usually part of the ecological systems have the potential to alter the receiving environment to its detriment. Most mining and processing wastes contain minerals, such as sulphides, which are formed at higher temperatures and pressures at geological depth.

Disposal of mine tailings is one of the most important environmental issues for any mine during the project’s life. The sustainability challenge in the management of tailings and waste rock is to dispose of material, such that it is inert or, if not, stable and contained, to minimise water and energy inputs and the surface footprint of wastes and to move toward finding alternate uses.

Although, the tailings would be ideally made up of gangue materials (i.e. silica), to some degree, the sought-after mineral also appears in the tailings. Tailings also commonly contain unmineralised sulphides that can breakdown and release metals that can generate acidic conditions. In operations that recover lead, uranium and other toxic heavy metals, this represents a significant environmental hazard. In addition to the minerals themselves, some processing methods involve marine pollutants such as copper sulfate, xanthate or cyanide which will be present to some degree in the tailings. In some operations, components of the gangue may also be toxic, though it is rare for these materials to be present above trace levels. An example is thallium in sulfide ores.

In order to prevent the uncontrolled release of tailings material into the environment, mines usually have a disposal facility which quite often takes the form of a dam or pond. This is a convenient method of storage since tailings are often in the form of a slurry when they are discharged from the concentrator.

Local governments and mine operators are increasingly aware of the ecological dangers and economic losses related to the failure of impervious systems. Geomembranes used to ensure the imperviousness of heap leach pads and tailings dams must therefore be produced according to the most stringent standards of the geosynthetics industry.

Mining operations are often carried out in regions that are not only hard to reach, but whose ecosystems are fragile. Solmax produces membranes adapted to specific conditions, taking into account the characteristics of a given natural environment as well as the needs of major mining companies.



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GEOSYNTHETIC SOLUTIONS IN MINING HEAP LEACH PADS

Out of the many types of hydraulic barriers available today, HDPE and PVC geomembranes definitely make up the prime alternatives for mining designers as they are first and foremost both highly impervious and chemically resistant to a very wide range of contaminants including typical heap leaching solvents. HDPE and PVC geomembranes are hence rot-proof and designed to withstand long term use. Furthermore, as opposed to natural soils which require specific water contents to achieve proper barrier status, HDPE geomembranes are inherently impervious regardless of their environment, unharmed by freeze-thaw cycles and extreme subzero temperatures. They also constitute great space savers from their essentially bi-dimensional structure, achieving even higher levels of imperviousness in less than a few millimeters rather than several meters thick of natural clay.

HDPE and PVC geomembranes offer great engineering properties from their high elasticity and overall excellent mechanical resistance under tensile, tear and puncture modes, making them especially reliable when used in seismic zones. And since they are manufactured under controlled conditions, all material properties of HDPE and PVC geomembranes, including waterproofing are also much more isotropic than natural soils. Moreover, they can be custom manufactured to meet specific friction requirements thereby insuring heap leaching pad stability. The use of HDPE and PVC geomembranes also offers a great added feature as every phase of their manufacturing and installation processes may be fully quantified and documented, enabling fast and accurate conformance verification to project technical requirements.

Heap leaching is a low-cost industrial mining process to extract both precious and non-precious metals such as gold, silver, nickel, copper, uranium and other compounds from lower grade ores. It involves spreading crushed ore over large HDPE or PVC geomembrane lined pads and spraying them with a leaching solvent such as cyanide or sulfuric acid which will dissolve the valuable minerals into a so-called pregnant solution within several weeks of percolation. Metal recovery is then performed through either absorption, precipitation, smelting or electro-winning methods.

GEOMEMBRANE BENEFITS

Geomembranes are impervious sheets of polymeric material primarily used to reduce the hydraulic conductivity of in situ soils. They represent a branch of engineering materials called geosynthetics which are specifically designed to enhance the building properties of natural soils which have been deemed deficient from an engineering point of view. Geomembranes are thus extensively used to provide additional imperviousness to natural soils in order to protect groundwater tables against contamination from industrial activities such as landfilling, mining, and more generally, hazardous material storage.

Geomembranes come in a variety of polymers of which polyethylene represents the most significant percentage because of its inherent engineering and endurance properties, including chemical resistance to a wide range of contaminants. In particular, ever since their inception in the early 80’s HDPE geomembranes have indeed become the material of choice of every industrialized nation when it comes to mining containment applications. Their rapid growth and industry wide acceptance through worldwide governmental endorsements and ensuing regulations have been acknowledged from their many advantages when compared to other more traditional waterproofing materials such as natural clayey soils. As opposed to yesteryear’s unlined facilities and associated dismal containment properties, modern engineered heap leach pads lined with HDPE or PVC geomembranes represent today the safest and most economical mining process method.




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REV. 01.12Telephone Canada | 450 929 1234 Telephone Malaysia | 03 3375 3600

For both stability & financial reasons, the most preferred conventional single-use or static pad liner system is the single composite soil/geomembrane liner system. Looking at the generic heap leach pad cross-section in Figure 1 from the bottom up e.g. according to the installation sequence, a slightly inclined (+/-1%) and rectangular subgrade layer is first prepared and fenced in by shallow earth berms. Where conditions prevent the preparation of a rock free subgrade surface, the placement of a natural material underliner of selected fill will be called for. An HDPE or PVC geomembrane is then installed over the whole earthen area followed by a matrix of slotted drainage pipes leading to a series of collector pipes or open geomembrane-lined swales. The preferred type of HDPE geomembrane is usually of the textured kind which offers superior surface friction in order to prevent lateral sliding of the heap piles which are expected to settle to up to 15% from consolidation of ore degradation, erosion and progressive stacking. PVC geomembranes can also provide a useful option with respect to this element due to their relatively high surface friction coefficients, even in a smooth configuration. PVC geomembranes are also available in surface roughened formats in order to further in enhance friction angle performance. In addition, since typical heap slope angles correspond to the ore’s natural angle of repose which can be fairly steep in conjunction with the saturated interface between the ore and the geomembrane, adequate friction must be supplied so as to prevent heap failure from flow slides as well. For that matter, puncture-protective geotextiles are seldom used between the geomembranes and the crushed ore since they weaken the shear plane supporting the heaps. A protective granular aggregate layer may then be installed on top of the geomembrane/pipe system should the ore not offer adequate permeability (in any case, granular material is usually compulsory around pipes in order to prevent crushing). Crushed ore is then usually delivered to the leach pads by either overland conveyors or off-road vehicles or stackers in 4 to 8 meter lifts. Solvent irrigation then follows with the installation of a sprinkler or drip irrigation system which is often preferred in order to minimize evaporation. Once the dissolving solution will have percolated through the heap for several weeks leaching out the sought after minerals, the now pregnant solution will be directed to the extraction process.

The use of PVC geomembranes in mining operations is quite common. Technically, they provide considerable puncture resistance through the deformability along with a reduced expansion coefficient and a high degree of flexibility that allows them to conform to the adjacent soil. Due to their high elasticity facilitating factory fabrication into large panels, their ease of installation reduces the time needed for this operation. Therefore significantly enhancing the economic benefit of this type of membrane system with respect to the installation and operational cost models.

In similar fashion, HDPE and in some cases PVC geomembranes are also the material of choice for multiple-use Dynamic Heap Leach Pads, Valley Fill Heap Leach Pads, Dump Heap Leach Pads and so-called Super High Heap Leach Pads, whereby elevated loading conditions will usually call for more robust gauges. HDPE geomembranes are also used for other customary heap leach pad applications such as for secondary containment structure (accidental spill prevention), pregnant solution and waste water storage reservoirs. HDPE and PVC geomembranes are used as well for inter-lift liners in order to the reduce consumption of extracting solutions, and rain liners so as to prevent dilution of extracting solutions.

FIG

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Typical Heap Leach Pad Cross-Section



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GEOSYNTHETIC SOLUTIONS IN TAILING DAMS

A wide range of industrial products and processes require metals and minerals obtained from mining, extraction and transformation activities requiring large quantities of rocks to be crushed and pulverized. As a fine grind is often necessary to release those metals and minerals, the mining industry produces enormous quantities of fine rock particles, in sizes ranging from sand-sized down to as low as a few microns particularly associated for instance with phosphogypsum and bauxite residues (red mud). These fine-grained wastes are known as “tailings.” As a matter of fact, by far the larger proportion of ore mined in most industry sectors ultimately becomes tailings that must be disposed of.

Several methods have been used for tailings disposal, but modern environmentally-conscientious techniques typically call for the disposal of dry or thickened tailings in vast engineered impoundments. Of all the different impoundment layouts, valley impoundments (or retention dams) are the most common since it is generally economically advantageous to use natural depressions to contain tailings. Whereby natural topographic relief is unsuited, raised embankment enclosure configurations such as ring dikes are then appropriate. As opposed to ring dikes whereby tailings are physically contained on all sides, valley impoundments have the frequent particularity of only having three actively retaining sides consisting of the downhill retention dam itself and adjoining two natural side rises, while the uphill section does not necessitate closure on account of the likely steep incline (≥10%). Whether used in a single or double-lined scheme, HDPE geomembranes constitute a prime choice when it comes to insuring the imperviousness of those disposal facilities. The HDPE geomembranes are critical in preventing downstream seepage across retaining embankments and guarding against potential cataclysmic rupture, as well as the downward percolation of contaminants leading to pollution of the underlying strata.

SOLMAX EXPERIENCE AND QUALITY

As a world leader, Solmax HDPE, LLDPE, and PVC geomembranes are manufactured to meet the industry’s most stringent specifications. Made from first quality polymeric resins and manufactured on the most advanced plastic processing equipment, Solmax geomembranes are internationally recognized and considered as being of the highest guaranteed quality, having been utilized on all five continents for the protection of the groundwater tables in well over 60 countries throughout the world.

In particular, Solmax is building a strong worldwide presence in the use of geomembranes in the mining industry and has provided geomembranes for some of the largest mining projects in the world. Some examples of Solmax’s presence in this industry include the Minera Dolores gold mine in Chihuahua, Mexico, the Minera Escondida copper mine (largest copper producing mine in the world) and Codelco Norte copper mines in Chile, the COMARSA and CIDELSA mining projects in Peru, and closer to home, the Faro Mine Complex (Grum Sulphide Cell Cover) in Yukon, Canada. Solmax plans to capitalize on the booming global mining industry by strengthening current relationships with the major players in the industry while continuously meeting and surpassing geomembrane specifications and requirements.

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