A heap leach pad may seem simple to construction and similar in design to any other containment basin. While it does share the same excavation needs and berm designs as many other waste-holding storage-ponds and similar features, it is unique in needing an extensive drainage system at the base to collect the leachate that is the source of the ore. Only landfills share a similar design, so many heap leach pad owners have looked to the waste management industry for more data on designing better drainage systems. Today’s heap leach pads have advanced drainage systems despite their affordable costs, making it easy to maximize leachate collection without spending more than what’s expected. By putting together all of the following elements, it’s easy to design a system that captures as much valuable ore as possible with each cycle while reducing leaks and seepage to practically none.
Storage Ponds
Leachate requires further processing after it soaks through the heap and exits the pad. In almost all facilities, the most effective way to handle the volume of leachate is to direct it to a set of storage ponds built next to the heap pad. Some designs only use a single storage pond, but most of them use two. The first pond to hold the leachate when it has just emerged from the pad is known as the pregnant pond. It’s rich in both valuable ore and other contaminants that could be hazardous to the environment. The second pond is known as the barren pond and it’s only for holding wastewater that needs no further processing for ore. However, almost all water that goes into a barren pond will still need further treatment to improve to the point where it can be safely discharged into the environment. That’s why both pregnant and barren storage ponds for leachate need impermeable liners. The same liner materials that are used for protecting the heap leach pad from seepage are generally a good choice for these ponds as well. However, make sure the material offers the correct level of UV exposure resistance if you plan to build the storage ponds in a way that would expose the liner.
Double Liner for Leak Detection
Leaks are often permitted by state and federal regulations as long as they are limited in volume and only limited lixiviants are used. If a concentrated solution is necessary to extract a higher concentration of ore or to deal with a lack of processing and crushing, a double liner system may be necessary for leak control and detection. Double liners consist of two layers of impermeable geomembrane material such as RPE. The first layer is intended to stop seepage and leaks, but the second layer provides backup in case of damage. Collection pipes and sumps, or even sensors, are installed in between the two layers of liner. This allows for leaks to gather and set off the detection system before they can overwhelm the outer liner can cause a real leak. Not all heap leach pads need this level of leak protection and detection, but it is easy enough to achieve with the right materials.
Geotextile Covers for Top Protection
The next material to be applied to a heap pad, right after the impermeable geomembrane liner, is often a thick layer of large crushed rock or coarse gravel. This creates the ideal amount of porosity for leachate to rapidly flow down and collect in the perforated pipes buried in the gravel. Unfortunately, the material used for this collection layer can also create a lot of extra wear and tear for the liner. Punctures are particularly a problem when crushed rock is used as part of a collection system. However, there are materials available known as geotextile covers that work well to separate rough or sharp materials from liners without compromising the performance of either. Geotextiles can be woven or non-woven, but they’re generally permeable and not designed to block water. Yet, adding just a thin layer of another material over the geomembrane liner can go a long way in extending its lifespan and preventing leaks. If a system doesn’t require a double liner installation for leak detection and protection, a geotextile cover is an affordable alternative that still gives extra peace of mind.
Underlayment for Bottom Protection
While geotextiles may help control damage and abrasion from the top down on a pad liner, it doesn’t work as well below. Instead, a thicker material known as underlayment is commonly recommended as a cushion. Underlayment is similar to geotextiles in that it’s not impermeable or designed to be a liner. Instead, it’s a felt-like texture that helps cushion liners and prevents rocks and sharp edges from creating stress points. If an underlayment isn’t going to be used in the construction of a new heap pad, then a thick layer of compacted clay is usually required instead. Trying to simply excavate and grade raw soil won’t work because even small rocks and roots can damage the liner when there are millions of tons of material pressing down from above.
Perforated Drainpipes
Drainpipes are essential to every heap leaching pad design because it’s hard to slope the base enough to collect all the leachate with gravity alone. These perforated pipes still collect the leachate through the natural flow of seepage, but they help direct it to trenches and ponds rather than allowing it to pool at the base of the heap. As with the liner materials, these pipes must be able to withstand a lot of weight pressing down on them. It’s all too easy for the drainpipes to become crushed during loading and fail to collect any leachate at all. In addition to crushing damage, these pipes are also prone to filling with fine sediment from the ore. Only specially designed heap leach pad piping will work for this application due to the challenges of the installation.
Gravel or Soil Cover
While gravel or crushed stone is commonly used as the top layer of a heap leach pad for its porosity, soil can also be used. This soil is usually compacted to keep it in place and stabilize it, but only to a minimal level so it still allows leachate to percolate through and into the collection pipes. Without this layer of rock or soil the ore fines would simply press into the openings on the pipe and fill them with sediment. This means that the cover material used must resist this effect over time as well. That’s why gravel is generally selected as a more stable material than soil, which can always shift or compact with time to affect the drainage speed in the heap.
With the right materials, it’s easier than you might expect to create a complete drainage system for a heap leach pad. The combination of geotextiles, cover materials, and geomembranes ensures that leachate is directed where it’s needed and can’t escape anywhere else. Without enough layers of material in the pad design, compression tends to occur that shuts down collection temporarily or permanently. Find the materials you need to put together a reliable and productive heap leaching pad from BTL Liners.
