Heap leaching is just one of many techniques leveraged today to make mining operations for precious metals as effective as possible. Some methods work together to maximize the total percentage of ore recovered, while others are used separately. Heap leaching works well with primary ore recovery methods like conventional milling, but it can’t be used in conjunction with other techniques like in-situ mining. Deciding if heap leaching is the right method for a particular mine and ore source requires comparing the various challenges and benefits of each technique.
Heap leaching is an older technology than conventional milling processes to separate ore out directly in the mine. In the standard mill, a majority of the ore can be separated by grinding and screening alone. Then a combination of cyanide, gravity, and agitation is used to quickly separate out most of the rest of the ore in pits drilled into the mine. However, these processes are expensive and can reduce the profit margin of the mine, making them ineffective for lower value ores like copper and iron sulfate. In this case, the slightly lower recovery percentage of the heap leach process is balanced out by the far lower operation cost. The pad process takes longer as well, but the resulting process is also less environmentally impactful than conventional milling if the right liners are used to contain the leachate. Conventional milling can be combined with heap leaching if high value ores are directed to a different treatment than low value fines. This reduces the costs to extract ore from less effective materials while still increasing total output.
An alternative mining technique, similar to heap leaching but incompatible with it, is in-situ mining. Instead of pulling ore-containing rock to the surface at all, wells are filled with water and lixiviant chemicals to break down the ores while they’re still underground. When the liquid is pumped to the surface, it’s easily condensed or processed to concentrate out the valuable ore. In-situ mining doesn’t actually save surface space though, because each well only produces a small amount of ore. So, the boreholes often stretch over thousands of acres of ground. In contrast, heap leach pads are limited in size to 40 acres each. This method is primarily only used for uranium and sometimes gold because it’s simply too expensive to be feasible for copper or other mid to low-value ores. It’s not as flexible as heap leaching either.
Vat leaching is a technique commonly substituted for heap leaching in conventional milling operations. While using an above-ground, metal or concrete vat creates a controlled environment that can speed up ore processing, the cost of construction often far outweighs the benefits of choosing this method. These vats also require agitation equipment or flocculation to properly circulate and separate the various compounds. Heap leaching only needs gravity and low-cost irrigation equipment to achieve the same results, albeit at slightly slower efficiency levels. Vats are far more limited in size than heap pads, making it easier to extend the processing volume of a mine with pads than vats.
Even within the heap leaching process, there are multiple options for the actual leaching process itself. Cyanide treatment to release gold ore is one of the most effective leaching procedures, but it also has a larger environmental footprint than other methods. Other options that can be effective with the heap process include:
- Bioleaching, the use of bacteria and other microbes to loosen the bonds holding ore to the rock
- Dump leaching, an alternative form of heap leaching where the ore isn’t crushed or screened first, and therefore, far less ore is recovered
- Acid leaching, which is affordable and effective for low-value ores
- Chlorination and peroxide use, which generate oxygen to split ore from the waste rock
- Alkali and ammonia leaching, which can work for a wide variety of ores when adjusted to the specific conditions of the fines.