The entire hydrometallurgical process begins with a bulk collection of the raw materials containing the desired ore or mineral. While there are many options for organizing and storing this collection, many projects use the reliable heap leach pad method for extraction. Mounds of the mined dust is piled on top of an impermeable liner so leaching liquids are easily added and collected. Leach pads need proper lining just as much as the rest of the barren and processing pond system. Understanding how heap leach pads work will guide the construction of a safe and productive beginning to the project.
The First Step in Processing Ores
Leaching heaps or mounds are the first steps in hydrometallurgical processing. Even if a strong slope is desired to encourage good flow of leachate liquids, it’s necessary to use a gentle enough angle to prevent erosion and slides. Many leach pads are based around the same flexible geomembrane liners you’ll find in the processing ponds and connecting trenches. Without a liner or with a material prone to leaks, much of the liquid produced on the pad will simply escape into the soil below. Only a durable liner will ensure it reaches the trenches so it can flow onward to the processing ponds and harvesting equipment.
Supporting the Weight of the Pad
Once loaded with a large mound of rock dust, and saturated with a liquid solution, the load on the heap pad begins to dramatically add up. The ground below the pad needs to support thousands of pounds in total weight without shifting or collapsing under the load. This is also true for the processing and holding ponds used later in the process, but those ponds tend to spread the weight of the water over a larger area. The soil and rock below the heap leach pad needs testing and geoengineering to ensure it can support the proposed heap before any work is done for grading and preparation.
Since the raw material from the mine contains both the highest amount of useful ore and the greatest amounts of contaminants, it’s essential to plan for containment under and around the pad. Leaching heaps, especially those processed with bio-active ingredients, tend to heat up significantly in the sun each day. This causes a lot of stretching during the hottest part of the day and then shrinking as the heap cools. In order to keep the liner from tearing or moving in place, look for a liner material with a low thermal expansion rate.
In addition to the drainage ditches used to collect and direct leachate, most leach pads are surrounded by berms. These are raised dirt ridges primarily meant to redirect surface runoff that would otherwise dilute the solution running out of the heap. Berms can also keep overflow from escaping the pad and its drainage ditches during periods of heavy rainfall. A single processing project may call for both lined and unlined berms in order to completely control both incoming and outgoing liquid. Lined berms can hold back greater quantities of moisture without suffering from erosion or damage, but they don’t allow for water infiltration that may be desired to reduce runoff on the surface like unlined berms.
Renewing the Heap
The heap is constantly losing size and volume as leachate washes through the material and removes dissolved minerals and ores. Even if you’re only aiming to refine a single ore from your processing water, multiple minerals are likely to end up in the water as they react to the leaching solution. This means that fresh layers of rock dust are constantly added to the heap to continue the same level of production. Heavy equipment is constantly driving on and off of the leach pad, risking damage to the liners. Reinforced liners are the best option for areas where traffic is a concern since the design of the material offers greater resistance to both sudden tears and ongoing wear.
Supplying the Solution
Drainage ditches are usually humble constructions found on the side of the road, but they play a much bigger and more important role in the mining processing system. These ditches directly connect the main heap leaching pad or pads with the rest of the ponds and tanks required for processing and refining the ores. Without lining, these ditches would lose a significant amount of water to seepage through the soil. They’d also flow much more slowly, increasing losses to evaporation.
Other Types of Leaching for Ores
While this section mainly explores the process of heap leaching, it’s far from the only option for hydrometallurgical processing. Other common ways of using leaching solutions to dissolve and consolidate ore include:
- Dump leaching, a process very similar to heap techniques but without the pre-processing steps that increase productivity and speed
- Tank leaching, which relies on costly, size limited tanks rather than large open processing ponds
- Pressure leaching, a more complex procedure that keeps the heap under pressure inside a vessel to force extra solution out at a faster rate.
Regardless of how you plan to leach ores and minerals out of mined material, you’ll need drainage ditches and ponds to deal with the resulting liquids. Make sure you’re designing those ponds with the challenges of leachates in mind. Reuse of the solution is common, so it’s important to maintain proper dilution rates throughout the process by keeping water in and possibly out. Turn to a flexible liner from BTL Liners for ditches and ponds that won’t leak or lose water unexpectedly during the processing stages.