Each heap leaching facility is unique in its design and the process it uses for ore recovery. This is because everything from the base mineral hosting the ore, to the availability of certain chemicals, affects the profitability of the operation. However, all of the ponds used for holding mine materials for processing or storage require impermeable liners to protect the environment. These liners are also responsible for keeping drinking water supplies safe and keeping workers from being exposed to unnecessary risk. With so much riding on a thin layer of polymer in the form of a geomembrane, it’s important to get the material selection right. Material can play more of a role in the success or failure of the liner than any other particular factor.
State and Federal Regulations
Start by investigating what federal and state regulations apply to the project. There are numerous EPA regulations relevant to mining and ore recovery operations, but most of them are less strict than the laws applied to other forms of hazardous waste. It’s state regulations that can vary the most and are often stricter than expected. The EPA allows for the use of nearly any impermeable liner material for the ponds and pad of a leaching operation, but state regulations are likely to be far stricter. Some states specifically state a polymer geomembrane must be used, while others even state a minimum thickness and permeability rating.
Choosing a Geomembrane Material
As mentioned above, the material is the single most important factor in liner selection. This is especially true for processing and pregnant ponds because of the chemicals mixed into the solution. Settling for the first pond liner material found could result in failure due to chemical weathering in as little as a few months. Considering the cost of draining the processing ponds or uncovering the pad to repair it, it’s best to choose a material that you can trust to last despite chemical exposure.
The best material for lining ore recovery systems, in general, is reinforced polyethylene (RPE). This material is widely used for industrial ponds and basins of various types for its combination of durability and chemical resistance. While polyethylene is not the only material available in a reinforced style, it is the most chemical resistant polymer that’s commonly reinforced. Reinforced liners are recommended for challenging industrial installations like processing ponds. The use of heavy equipment to retrieve collected ore or for dredging won’t cause damage to the material. BTL Liners uses RPE for all of our toughest liner materials and we recommended it for applications like processing ponds and leaching pads.
PVC, or polyvinyl chloride, is a widely used material for pond liners. However, that doesn’t make a good choice for industrial settings like a mine or ore recovery facility. It’s not a truly impermeable material, especially when installed on site with seams that are prone to leaking. PVC liners tend to only offer a 10% leakage rate at best, while RPE liners can offer a practically zero seepage rate. Why settle for leakage that can result in fines from the EPA when RPE liners are available for far more reliable performance? Finally, don’t forget that PVC is weakened by both sun and chemical exposure, especially over long periods of time.
High Density Polyethylene
HDPE, or high-density polyethylene, is the next best choice after RPE for lining processing ponds. It’s chemically resistant enough to last and has better durability than LDPE or PVC. However, it’s still not as easy to install or quite as long lasting as an RPE product. It’s also stiffer and harder to conform to shallow curves and banks that are erosion resistant. HDPE should only be used for lining processing ponds when RPE isn’t an option. Since BTL Liners ships our industrial grade RPE liners all over the world, it’s unlikely you’ll have to make that choice.
Low Density Polyethylene
Low density polyethylene, or LDPE for short, is a highly flexible material that lacks some of durability of both RPE and HDPE. Since durability is an essential feature for liners used in mining or ore recovery, it can’t be compromised for these installations. LDPE also tends to lack the chemical resistance that HDPE and RPE provide. Finally, it’s not quite as tough against rips and tears as RPE since it lacks the reinforcement of RPE. Stick to RPE and leave the LDPE for industrial settings that don’t involve harsh chemicals or abrasive tailings.
Why Not Concrete or Asphalt?
Since processing and tailings ponds are harsh environments, full of ways for liners to get damaged, it’s easy to assume that concrete or asphalt might be a better choice. While these materials are long-lasting and can help stabilize loose soil around the banks of a pond, they don’t take the place of impermeable lining materials. On their own, both concrete and asphalt allow water to slowly seep through them. Openings in the surfaces that are too small for the bare eye to see are still large enough to let moisture seep through over time. Since the leaching and recovery process can spread out over many months to years, there is plenty of opportunity for seepage to occur over time. These materials only work as liners when paired with impermeable geomembranes like RPE.
Issues with Natural Liners
Natural liner products sold for fish or recreational ponds are definitely not reliable enough for an industrial use like ore recovery. Even if the thick layers of Bentonite clay offer good seepage control when left perfectly undisturbed, the constant influx of fresh leachate is sure to disturb the material along the bottom. Even adding new layers of Bentonite every few months won’t be sufficient to keep seepage and leaks under control. Natural liners aren’t worth the cost or effort, although a thick layer of clay may be used underneath the impermeable geomembrane liner to create a smooth surface to support it properly.
Double Liner Systems
RPE is compatible with all the double liner systems used today for constant leak detection. There’s no need to choose between leak monitoring and liner durability when using products from BTL Liners. This includes double liners used as part of the multi-layered systems for leachate collection at the bottom of a heap pad. A constant leak detection system is more expensive to install than basic monitoring wells around the exterior, but it’s the only way to detect a leak before it escapes the actual containment system for the pond. Other leak detection methods all rely on finding the moisture after it’s already on the other side of the barrier and in the surrounding environment.
Monitoring wells are depressions placed around the edges of individual processing ponds or the general facility area. These wells naturally fill up with a small amount of water, but that water is easily pumped up for testing on a regular basis. If heavy metals, chemicals, or other contaminants are detected in these samples, it’s a clear sign there is a leak or similar issue with the processing pond. While monitoring wells are inexpensive and reliable, they can’t detect a leak before it affects the environment. Only a double liner system can stop leaks before they actually reach the soil and water around the pond. These features are usually considered sufficient for barren ponds and low risk ores, but materials like uranium may necessitate the use of a more complex leak control system.
Preparing the Ground for Lining Installation
The amount of effort put into ground preparation, grading, and rock removal before the liner goes in, largely determines how successful it is in the long run. Even one root or rock left behind can cause a small amount of damage that grows into a much larger problem. Yet, ground preparation may not be enough to deal with shifting soil or texture issues that cause wear and tear from below. Laying down a cushion-like layer underlayment material can prevent tears for the lifespan of the liner.
Liner selection for processing ponds in the mining industry might be challenging, but it’s a job worth the effort. Taking care to find the right liner material goes a long way in protecting the environment and reducing costly fines and fees. If you’re still wondering what to choose for a particular leaching pad or pond installation, chat with us here at BTL Liners. We have a liner for every wastewater holding situation, including processing ponds and tailings pits for long-term storage.