Since heap leaching can help pull more valuable ore from tailings and other waste products left behind by other methods, it’s a great way to reduce the environmental impact of conventional mining. However, leachate still poses a risk to the environment if it’s allowed to seep or leak out of the pad. That’s what makes it essential to choose the right geomembrane for lining the pad rather than settling for a clay product or compacted soil alone. Only a flexible geomembrane can conform to every curve and bump of a real-world site, making it the best choice for any size of heap leach pad. Here’s what you need to know about liner selection for leach pads for all types of ores and processing methods.
The Four Types of Heap Leach Pads
Heap leach pads all share similar construction materials consisting of a compacted soil or clay base topped with an impermeable geomembrane liner. Yet, there are still four major designs that make up the majority of the pads in use today. The options are:
1.Conventional pads, built on flat ground or gentle slopes only. These pads can only hold thinner layers and lower total heights, but they can be used for long periods of time if the right liner is selected.
2.Dump leach pads, which can hold raw and uncrushed materials for leaching. These pads need thick liners but can be built on sloped terrain and handle heavier loads than conventional designs.
3.Valley fill pads, designed to take advantage of natural or excavated low points to hold a large volume without risk of slipping or spilling. These pads often feature interlift designs to help maximize leachate capture at each level of ore material added.
4.On/Off pads, the largest and heaviest pads that can support the biggest loads of ore fines. As the name suggests, each load of ore fines or tailings must be removed at the end of leaching cycle before new material is added. Other pads tend to be topped with new material over time until the operation shuts down, but the loads supported by On/Off pads are too heavy to continue covering.
Lengthy Leach Cycles
Depending on the ore being extracted, leach cycles can range anywhere from just a few weeks to multiple years. The longest time frames can stretch out to a decade for uranium heap leach pads, although the last few years of the cycle tend to produce only low concentrations of the ore. This means that liners selected for the pads must be able to withstand harsh conditions and heavy weight for multiple years without tearing, sliding, or developing leaks. Lengthy leach cycles are common when testing of the leachate reveals there’s a higher concentration of ore still being produced later than expected. Choosing a liner with a warranty that’s far longer than the expected lifespan of the pad can pay off when it allows for an extended recovery window.
Why Clay Isn’t Enough
Clay liners were some of the first materials used to secure heap leach pads and separate them from the native soil. Unfortunately, compacted or even modified clay is not impermeable enough to protect the environment and groundwater under a pad. Adding polymers and woven fabrics to the clay liner still doesn’t address its permeability, which allows leachate to seep out over time and travel into the surrounding soil. While clay is still required as a shaping and sealing component for most pad designs, it shouldn’t be treated as the primary liner. Only an impermeable geomembrane can truly secure the bottom of a heap leach pad from leaking and seepage through years of heavy use.
Selecting a Material
Material selection is essential for geomembranes installed in heap leach pads. It’s especially important for valley and On/Off pad designs because they receive some of the heaviest loads and most wear and tear out of all types. Yet, even a conventional heap leach pad designs a durable liner that won’t leak or seep with age. Since any number of caustic or abrasive materials may be added to the irrigation water to encourage ore extraction, the geomembrane must be able to handle constant exposure to a wide range of chemicals without reacting. There are only a few geomembranes commonly used for this kind of industrial lining purpose, and one among them clearly stands out as the best choice for heap leach pads.
Reinforced Polyethylene
For its combination of UV resistance, chemical resistance, tear and puncture resistance, and overall durability, reinforced polyethylene (RPE) stands out as the best choice for lining heap leach pads. It’s affordable, widely available, and easy to install thanks to the reinforced design that encourages it to lay flat with fewer wrinkles and creases. Wrinkling and creasing are two major problems in heap leach pad design because of the sheer amount of weight pressing down on the material. Even the toughest materials quickly crack and develop leaks if they’re allowed to fold and overlap during installation. RPE is easy to stretch smoothly over the carefully shaped pad so the weight is perfectly distributed for fewer stress points. It’s also able to withstand exposure to various lixiviants and other chemicals mixed in with the leachate, even for long periods of time. Most heap pads call for buried installation under a layer of coarse fill material to contain the collection system, but RPE liners can also handle the exposed installation in the storage ponds or other features that call for it. It’s also warrantied to last for decades and available in custom fabricated sizes to reduce the number of seams in the system as much as possible.
Alternatives to Avoid
Of course, RPE is far from the only material used to manufacture geomembranes for lining industrial basins like mining pads. There are many alternatives on the market that often claim to be durable or flexible enough to withstand this particular application. However, all of them have significant drawbacks that interfere with their long-term use. It’s clear that RPE is the best choice when it comes to designing durable heap leach pads.
- Polyvinyl chloride (PVC): One of the most widely used geomembrane liners, but it suffers from durability issues when compared to RPE. PVC is also less durable against UV exposure and isn’t compatible with some of the lixiviant chemicals used for ore processing. It’s relatively easy to install but lacks the lay flat ability of RPE that helps reduce wrinkling and creasing. It’s usually only available in limited sizes requiring seems sealing on site to cover even the smallest heap pad designs.
- High density polyethylene (HDPE): While this material is based on polyethylene like RPE, it’s not quite the same. First, the higher density makes it stiffer and therefore more likely to crack if folded or creased under the weight of the heap. It’s also a little harder to install for that reason. HDPE tends to have good UV and chemical resistance, but it’s not necessarily a good choice for cold climates since it can crack under pressure when frozen.
- Low density polyethylene (LDPE): The lower density of this polyethylene material results in easy installation and good conformity to the site, but it also weakens the overall strength of the material. Even when reinforced, LDPE isn’t quite as strong as RPE. This makes it a poor choice for dump leach pads, in particular, where the lack of crushing leaves material sharp and abrasive. It does have good chemical resistance, but it can still react with salts from the soil in some areas and experience a reduced lifespan. In general, it doesn’t offer as much durability or tear resistance as RPE.
RPE liners are a great choice for protecting heap leach pads and ensuring they’re as effective and productive as possible. If you’re looking for RPE liners to complete a pad project or renovate older pads before they’re put back into use, look no further than BTL Liners. Our heavy-duty industrial liner products work well for both the heap leach pad and the leachate storage ponds.
