Heap leaching has become a popular method of extracting gold, writes Nicolaas C. Steenkamp and Leon Louw.
Heap leaching is used to extract gold, copper, silver, uranium, and iodine. This method was first employed to extract copper at the Bluebird mine in the US in the early 1960s, and then by several gold mines in the western parts of the US a few years later. Since then, heap leaching has been adopted successfully by many mines across the world.
Heap leaching occurs after the mining and crushing of low-grade ores, usually in an opencast mine. High-grade ores and ores not amenable to cyanide leaching at coarse particle sizes require further processing to recover the gold values. These processing methods can include further grinding, concentration, pressure oxidation, and roasting, which is used to treat these ores to expose the gold particles prior to cyanidation.
The crushed ore undergoes an agglomeration stage, after which the agglomerated ore is deposited onto the heap leach pad. The heaped ore is irrigated with a lixiviant (a liquid medium) to dissolve the metals and generate the leachate. The lixiviant will depend on the target metal being extracted.
The pad is compacted and then lined with a high-density polyethylene membrane, which prevents toxic compounds and elements (such as cyanide and the leachate solution) from entering the groundwater system.
The leachate is collected in a pond or tank, and it is referred to as a pregnant or value-bearing solution. The solution is then processed to recover the metals. In gold operations, recovery is affected through carbon adsorption or the Merrill-Crowe process. The barren solution, together with additional lixiviant, is recycled back to the heap.
Heap leaching can take anything from a couple of months to several years. In the case of gold recovery, heap leaching generally requires 60 to 90 days to leach the ore, compared to the 24 hours required by a conventional agitated leach process. Gold recovery is also usually only 70% compared with 90% recovery in an agitated leach plant. Other metals, such as copper, use solvent extraction and electrowinning to extract the target metal from the solution.
According to Phil Bundo, process engineering director at Senet, mines need large reserves, a large resource, and significant real estate if they want to employ the heap leach method. “To build a big heap and accommodate all the associated equipment, a large space is required; and to fill that space, a mine needs to produce enough ore,” says Bundo. Bundo adds that climatic conditions like rain can negatively affect a heap leach operation, although this would not ordinarily be enough cause to discard heap leaching as a processing method. Resources and reserves play a much bigger role in determining whether heap leaching will be viable.
Heap leaching is not that popular in South Africa because it is more applicable to shallow opencast mining, and South African gold mines are mostly underground operations. The method is, however, being used on a large scale in the copper mines of South America and, according to Bundo, it will be used more and more on the African continent in future.
Weighing up the alternatives
According to Bundo, there are other recovery methods, besides heap leaching, to consider when mining gold. These include gravity concentration, carbon in pulp (CIP), and carbon in leach (CIL). “The methodology selected is a function of the mineralogy of the ore. If the gold is associated with oxides, for example, it can be amenable to heap leaching. The grade also plays a key role. Heap leaching is used for low-grade oxides, while high-grade ore (with or without oxides) is better suited to CIP or CIL methods,” says Bundo.
Bundo explains that while heap leaching is not as costly as CIL or CIP, the recovery achieved is also not as efficient. “CIP and CIL are costlier in terms of initial capital and operational costs, but they provide the benefit of high recovery,” says Bundo. In cases where the oxides are amenable to heap leaching and cannot economically justify the construction of a CIL or CIP plant, then the operations start off with heap leaching to generate sufficient capital to finance the CIP or CIL circuits when mining more refractory ore.
“It is also important to remember that if the gold is associated with sulphides or other minerals, heap leaching cannot be effectively employed as recoveries are generally low. In such cases, it is advisable to install a CIP or CIL system from the beginning,” says Bundo.
The higher capital and operational investment in methods such as CIP and CIL are due to the more aggressive techniques used to recover the gold. A CIP process will, for example, involve crushing, milling, and agitator tanks. Heap leaching requires less aggressive techniques: a chemical solution is simply sprayed on heap ore. As the solution percolates through the heaped ore, it dissolves the gold. The solution is then collected and treated further by adsorption.
The heap leach stack must be porous enough to allow the solution to drip or to drain through the stack. There are potential recovery failures due to the inability to obtain the optimal flow solution. High clay content is achieved by agglomeration prior to stacking the piles.
Further research is underway to both increase the recovery of metals and reduce the risks of the solutions used and generated during the process. Heap leaching has the potential of extending the life of mine or bringing mines under care-and-maintenance back into production by reprocessing the tailing or fine-residue dumps.
Agglomerating the ore
Part of the preparation stage is known as agglomeration, in which an agglomeration drum is used. Although heap leaching is efficient on its own, its efficiency is greatly improved by adding an agglomeration drum. Agglomeration drums are also called ore drums, agglomerators, and heap leaching drums. Agglomeration is based on a rotary drum design that tumbles ore fines, in the presence of the leachate, through its interior to promote uniformity and to mix the leachate and fines. The agglomeration step happens after the ore is crushed, but before it is heaped.
Using an agglomeration drum to agglomerate the ore fines will ensure that the crushed ore particles are more uniform, making it easier for the leaching solution to travel through the channels between the particles to help maximise recovery (increased percolation). In addition, adding the agglomeration drum allows the leaching solution to mix with the ore fines. As the ore fines are agglomerated, the solution is sprayed throughout the drum and mixed thoroughly with the ore fines, which ultimately makes the process more efficient.
“The agglomeration drum is the start of the heap leach stacking process, which ends at the leaching pad,” says Theo Winterbach, mechanical and materials handling manager at Senet. From the agglomeration drum, the material is transferred via an intricate conveyor system to the heap leach pad. “The heap leach pad covers a large area and the product has to reach the entire pad. The system thus requires multiple conveyors to transfer the material to the furthest point of the pad,” says Winterbach. The agglomerate should be transferred as smoothly as possible, as it is important to keep it intact. One benefit of having multiple and mobile equipment is that a company will not have to lay out all the capital in the beginning. Once the team has decided where the pad will be located, they can acquire only the necessary equipment until production starts ramping up. “The added advantage is that the mobile equipment allows for a certain amount of flexibility; therefore, the entire system can be moved to ensure that the conveyors feed the stacker at the end of the chain,” says Winterbach. A typical heap is between eight and 10 metres high. The normal flow of material is from the agglomeration drum to the grasshoppers and then to the linear index conveyor, which delivers it to the stacker machine.
Stacking the heap
The linear index and stacker machine need to move backwards as the stockpile is stacked in preparation for the leaching. At times (depending on the size of the heap leach pad), the entire set of machines has to be fully retracted and repositioned to a new position on the heap leach pad before the process is repeated. When the entire system needs to be adjusted, front-end loaders or other applicable equipment can be used to move the equipment, as most of the equipment, including the stacker, is wheel or track driven. A combination of wheels and tracks is used to minimise the bearing pressure on the pad.
The stacker can slew and move backwards and forwards in a linear fashion.
Heap leach pads require special preparation in construction. According to Bundo, a pad should slope by one or two degrees so that the solution can gravitate to the pond, and there should be sufficient aggregates underlying the surface. Moreover, the high-density polyurethane (HDPE) liners that cover the surface are critical to prevent the chemical solution from seeping into the groundwater. Special collection pipes are installed underneath the pad to direct the solution to the relevant ponds.
Pros and cons
For Breton Scott, managing director of Bowline Professional Services, the most important advantage of heap leaching is that it lowers the capital and operating expenses relative to other traditional methods like flotation, agitation, and vat leaching, especially where low-grade ores and tailings are present. It also has a potentially rapid payback period.
“Heap leaching further eliminates some environmental concerns and restraints. The main benefit, in terms of the environmental impact, is that it requires less energy and water,” says Scott. Moreover, the method has uncomplicated design and equipment requirements, and the construction phase is a lot faster than other treatment methods.
Although Scott says that the heap leach method is not seriously affected by climate, he mentions that a lower efficiency has been noted at low temperatures. “High rainfall areas may also dilute the solution, requiring additional monitoring,” he says.
The risks associated with heap leaching are mainly related to environmental concerns, should the pad construction process not be done correctly from the design stage. Potential issues with the regional water balance are highlighted as a risk, along with the possible exposure of the solutions used to the surrounding areas. Heap leaching does, however, have a much lower potential of acid mine drainage. The costs associated with pollution control and closure efforts are one of the main continual expenses in such operations.
“The drilling of water-monitoring boreholes and regular testing of the groundwater by an accredited water-quality laboratory would be required if the heap leach method is used,” says water laboratory analyst, Ben Steyn. “Tests would generally include pH, dissolved solids, and heavy metals.”
“The biggest question a mine needs to ask itself is whether it has an ore body that is amenable to heap leaching. Senet prefers to get involved in a project from the test work phase, which enables us to prove that heap leaching, as a processing method, will work for the project. We not only consider heap leaching, but also all the other options available. It is always a trade-off between the capital investment and recovery,” Bundo concludes.