It’s a common misconception that ores are mined directly from rock in a pure form. Instead, it’s far more common to powder all of the rock containing a valuable material and extract it in other ways. Pregnant, barren, and processing ponds are mainly used for the dissolution method of extracting ore from rock dust. This allows for the capture of lower grade and mixed ores that aren’t pure or consolidated enough to harvest with other methods. But this kind of hydrometallurgical technique, usually called heap leach mining, requires carefully designed processing ponds and ongoing management. Anything from heavy rainfall to dry and hot conditions can affect the speed and efficiency of the leaching technique. Pregnant and barren ponds require slightly different design considerations, despite having many similarities.
Heap Leaching Techniques
As the name suggest, heap leaching processing involves piling the particles of rock dust or sand into large heaps on the ground. The heaps are generally constructed on an artificial or natural slope to encourage the processing solution to drain at a specific rate. Some minerals and metals are broken down with an advanced bioleaching technique that relies on a combination of bacteria and other microorganisms added to the leaching pile with the irrigation drip. The microorganisms extract more ore from the heaps than alkaline or acidic solutions on their own. Traditional leaching accomplishes a similar rate of ore extraction with the use and reuse of strong solutions known as lixivants. The right choice of a lixivant solution maximizes the accumulation of the desired ore with minimal extraction of unwanted materials.
Processing Ponds
Once the solution seeps through the aggregate of the heap pile, it’s directed through a series of lined ditches or trenches to further processing ponds. These ponds include pregnant, barren, and holding ponds. In processing ponds containing pregnant solution in particular, equipment and further solutions are added to the mix to settle out the valuable ore and minerals. By the time the solution moves to a long-term holding pond or a reuse tank, ore has been harvested as much as possible.
Barren vs Pregnant
The solution exiting the leaching area first arrives in a processing pond for pregnant solution. When there’s still a majority of harvestable ore left in the liquid, it’s considered pregnant or loaded. Some systems only call for a single set of pregnant ponds to settle or skim out ore and minerals with minimal work. For more advanced processing needs, pregnant solution may travel through multiple ponds or tanks to harvest as much of the target ore or mineral as possible. Solutions are often held in these ponds for multiple days to give the system time to process out as much of the valuable compounds as feasible.
Once the solution has released as much of its ore as achievable, it’s considered barren. Yet, it’s most likely not anywhere near the water quality required for discharge due to the addition of alkaline or acidic solutions. It’s not feasible to store the solution indefinitely, but some barren ponds are designed to hold the liquid for weeks or even months at a time. This barren solution is usually reused to start the next round of leaching from the pile or heaps. If it’s not a good choice for reuse on site, it’s usually pumped out of the holding ponds or dried in an evaporation pond into a sludge or solid cake.
The Downhill Flow
Leaching systems are based around the free and constant force of gravity to drain the ore-rich solution into pregnant processing ponds. Site selection is crucial to find either a natural slope or a soil that’s conducive to extensive grading. Sloped ponds, ditches, and pads work together to reduce the need for pumps until the end of the system. Since there are usual multiple components to every barren and processing pond system, the total slope may add up to a substantial figure by the end of the run. Even a very gradual slope per foot results in a large dip from the beginning to the end.
Industries Relying on These Ponds
Barren and processing ponds are almost exclusively used in the mining and mineral processing industries. Precious metals like gold and silver are widely processed in this way, especially at mines that produce primarily low-grade ore that isn’t a good candidate for other methods. Other minerals and metals harvested by this method include copper, uranium, and nickel. Some barium and mercury processing methods also rely on similar processing ponds for carbon bonding and accumulation.
Understanding the types of processing ponds used for metal refinement will help you determine what’s right for your facility. BTL Liners can help no matter what kind of barren or pregnant storage ponds you need to build. Check in with our project managers for tips on matching a liner to your various processing ponds.
