Heap leaching is a unique technique, but it’s based on some fairly basic concepts. Even the most complex multi-stage leaching operation, with specially formulated alkaline and biological agents, will still follow the same principles. By understanding how the pad is filled and run, it’s easy to understand why care is needed during the design and construction of these features. Only a well-graded and carefully lined pad can support the heavy weight of both crushed rock and millions of gallons of water combined. Of course, variations on the basic technique of heap leaching also affects how the pad must be designed to compensate for extra challenges like soaring heights and increasing weights.
Sourcing the Mined Ore
Unlike some commonly compared methods, heap leaching is not a mining method on its own. It’s generally used as a processing method for mined ore from hard rock operations that use drilling, blasting, or other methods to generate crushed material. Leaching is most effective when the ore fines have a uniform and small particle size. This means most mined ore fines and tailings need additional processing before going to the leach pad.
Building Pads
Fully lined and stabilized leaching pads require at least a few weeks for excavation and liner application, while some complex designs with multiple layers of extraction can take months to build. The pads must be completed before ore fines are brought to the site unless there are temporary storage options available like silos or tanks.
Crushing, Comminution, and Agglomeration
Uneven particle size, in the ore fines used for leaching, leads to flow issues that reduce total ore or mineral recovery. This means that investment in crushing equipment for comminution and agglomeration can pay off when recovery rates rise, especially for precious metals like gold. Rotating drums are most commonly used for this process since they’re easily added to other conveyor-based systems used for moving materials from trucks to the heap leach pads. Some facilities even introduce the concentrated lixiviant chemicals used to maximize ore recovery at this stage, ensuring an even distribution throughout the material.
Heaping the Material and Building the Lifts
Once the hard rock ore fines are on-site and crushed into a uniform size, the material can be heaped on the constructed pad. Leach pads rarely see just a single layer of material heaped on top of the liner and collection system. Most heaps are built in multiple layers known as lifts. Each lift is slightly smaller than the one below it, creating a step-sided pyramid shape that is stable and effective for irrigation. Liquids flowing from the very top layer can pass through hundreds of feet of crushed ore, resulting in ore-rich concentrate exiting from the heap. Some of today’s largest heap leaching facilities have topped their pads with 500 feet or more of material. Liners and collection pipes may also be inserted between the lifts for the greatest amount of environmental protection and ore capture.
Irrigating with Lixiviants
Some heap leach operations run on water alone, but this is relatively rare today. Most facilities use at least one lixiviant, a chemical or other liquid compound used to help extract the ores from the rock. These chemicals are chosen for their ability to dissolve or break down just the desired ores with minimal effect on other materials. This minimizes the amount of secondary processing needed to turn the leach concentrate into purified metals and minerals. Many heap leaching irrigation liquids combine multiple lixiviants to achieve the highest possible amount of ore recovery from each pad. Lixiviants are also widely used for related mining and processing methods like in-situ mining.
Irrigation, for heap leaching, often relies on modified forms of the same equipment used for watering field crops. Sprinklers and drip irrigation both remain the most common ways of applying the liquid leaching medium to the heap. Some heaps may be covered to keep rainwater out, but most are open because the addition of rainwater only accelerates the process rather than interrupting it. Open heap leaching is primarily done in drier climates, but pads built in wet climates are more likely to need covers and runoff management systems.
Collecting and Concentrating the Leachate
Once the water and lixiviants trickle through the multiple layers of mounded ore fines, it finally reaches the liner layer at the bottom. This part of the pad should contain a base layer of larger crushed rock or other material that is highly porous. Perforated collection pipes are set into it that slope across the pad and towards the collection sump or basin. Most pads produce such a high volume of leachate at the peak of operation that it is immediately channeled to an open storage pond for further treatment and processing. Other facilities use pumps to drain the sumps routinely into tanks or other storage containers. Storage ponds offer better concentration effects even before processing begins due to the natural effects of evaporation. The collection process can take anywhere from one month to multiple years depending on the size of the heap, the desired recovery rate, the durability of the materials used to construct the pad, and the particular ore or mineral being extracted. This is why leach pads must be built to last, even if they’ll only be used once.
Long-Term Storage
After the leachate is drained away and all the valuable ore is concentrated or processed out of it, there is still usually a large volume of waste left behind in need of storage. Permanent storage for solid waste like fines and tailings is usually handled either by digging a lined pit on site or by transporting the material to a separate solid waste landfill. If concentrated wastewater products are also left over after the ore is processed out, they’re either injected in empty wells underground or treated until it’s clean enough for discharge.
Effective and profitable operation of a heap leach pad starts with a leak-proof and seepage-controlled environment. Modern heap leach pads rely on flexible geomembrane liners, especially if complete control of the leachate is required to protect the surrounding environment. Go beyond just the state and federal regulations for pad design by choosing a geomembrane liner from BTL Liners. Our flexible liner products are reliable and strong enough for the biggest leaching heaps, even during extended periods of use.
