Processing ponds are far from an isolated system on their own. They’re a part of a larger leaching system for processing ores and minerals, and therefore, they only provide storage for certain steps of the process. However, process ponds are essential in most leaching and dissolution metallurgy techniques. Underestimating their design or skipping on important materials will only result in serious environmental issues for years to come. Since barren ponds are often used as the permanent storage space for residues and tailings left after the mine shuts down, they should be lined with the long-term in mind. Understanding how processing ponds fit into the larger heap leaching system is essential to designing them correctly.
Gravity Fed Pond Systems
Since this type of metallurgy is usually chosen specifically to reduce energy usage and save money, using extensive slurry pumps works against those benefits. The leachate must flow down through the leaching pile and off of the heap pad and then into the rest of the processing pond system. Since this system is designed to use as little energy as possible, designing it to rely on gravity makes sense. The concentration of the fine particles, dissolved minerals and metals in the slurry creates a challenging liquid to pump. Minimizing pumping not only saves money on energy costs but also on specialty slurry pumps required to move the thickened material. Yet, the site usually limits the options for gravity fed systems. Identifying areas with enough depth or existing slope to keep the solution channels angled will help ensure success with a gravity-fed system.
Importance of Crushing and Grading
Leachate, appropriate for processing in ponds, doesn’t always come from percolation through large pieces of raw rock. Most materials coming directly out of the mine need additional crushing and sorting to ensure they’ll release as much ore as possible. This process is known as agglomeration. While some heap leaching is done on raw run-of-mine materials that aren’t sorted or graded, most facilities have a number of crushing and sorting steps before the heap is formed. Ponds designed to hold leachate from finely crushed material will need extra depth, or dredging more often, to counter the larger amount of fines that will build up in the bottom. There’s little other processing or preparation required for most heap leaching processes, allowing leaching to begin as soon as the heap is completed.
Leaching
After the pad is covered with a compacted heap, it’s time to apply the solvent solutions with the use of various types of irrigation equipment. Percolation rates depend on the size of the crushed material, the amount of lixiviant chemical used, the ambient temperature of the area, and many other factors. Leaching periods are mostly controlled by the type of ore being extracted and the desired recovery rate. Some precious metals like gold only take between 10 and 30 days to reach full dissolution, while more resistant materials like nickel can take multiple years to complete a cycle. Even short-lived cycles are usually repeated over and over again. This means that any liners installed in the leachate collection system will need to last for decades without leaking or breaking down.
Concentration and Separation
Various methods are then used to concentrate the ore that’s suspended in the pregnant solution. Once it leaves the leaching pad, leachate is also known as a pregnant solution. Pregnant processing ponds must be able to withstand the chemicals used for the leaching process since concentrations will still be high during those stages. Multiple methods are available to separate the waste rock from the valuable ore, including settling through gravity, flocculation to make certain materials float, and even magnetic sorting for certain metals. Agitated separation methods often use tanks rather than open ponds to keep the material contained, but slower, low-energy, settling processes usually take place in ponds instead.
Overflow and Containment Planning
Care must be taken during the design of the entire leaching and processing system to plan for overflow. All the secure lining in the world can’t protect the surrounding environment if flooding and overflowing aren’t taken into account with each new processing pond. Everything from the dams holding back these impoundments, to the surrounding ditches and overflow areas, should be covered with the impermeable liner material. Setting up for secondary containment will ensure that historical flooding events won’t damage the system. For example, it’s generally fine to plan for barren ponds to flow to pregnant ponds when overfilled. This is because the barren water is cleaner than the pregnant solution and therefore unlikely to affect it negatively. In contrast, it’s unsafe to let water flow the other way in a flood since it could result in unexpected environmental exposure. Secondary containment often extends far into the drainage and catchment areas around the ponds, heap leaching pad, roads, and more.
Monitoring Wells and Leak Detection Systems
Finally, processing ponds are protected and maintained according to the monitoring wells and leak detection systems. Monitoring wells are small sumps drilled around the edges of the leaching pad, processing ponds, and other parts of the system. If they fill with liquid, it’s easy to check and determine that there’s a leak somewhere in the system. Leak detection is a more active approach. It relies on a double layer of impermeable liner at the base of the pond or pad. If liquid escapes past the primary liner layer, sensors above the secondary liner will alert the facility operators. This stops leaks before they truly escape the containment system. For the most precious metals, and materials with the greatest environmental risk, these double liner systems are well worth the extra cost and effort.
Processing ponds play a role in any leaching or dissolution system in metallurgy. Tanks are simply impractical to choose for holding all of the volume at a large mining site. With facilities producing thousands of tons of raw material to leach each year, open ponds are the most affordable way to create holding and processing space. Flexible geomembranes from BTL Liners make it easier than you might expect to build process ponds that last through multiple cycles of ore recovery.
