Coupled with growing economies, efforts to move toward a low-carbon future are demanding more copper than ever before, with global usage of refined copper more than tripling over the last half century.1 And while production has largely kept pace, technological advancements and diminishing ore grades have resulted in a transition of beneficiation techniques that promises to help maximize this critical resource.
Thanks to low capital and operating expenses, heap leaching is opening up new opportunities in exploiting low-grade ore bodies and tailings. Where only around 3 percent of copper and gold was extracted through heap leaching decades ago, more than 30 percent of copper and gold are now produced courtesy of this hydrometallurgical technique.2
Unfortunately, one challenge can stand in the way of a viable heap leaching operation: poor heap permeability. However, agglomeration, a particle size enlargement process, has shown to significantly improve heap permeability and metal recovery, while offering some additional benefits along the way.
Copper heap leaching with agglomeration
Heap leaching relies on a leaching agent moving through a bed of heaped ore to extract the metallic components: Crushed ore is stacked on a leaching pad and irrigated with the leaching agent. The leaching agent chemically reacts with the metal in the ore to dissolve the copper into the solution as it percolates through the heap. The impregnated solution can then be collected at the bottom of the heap so that the copper can be recovered.
Ores with a high presence of fines and/or clayey particles result in poor heap permeability as the fines clog spaces between larger particles, creating pockets where leaching is inhibited or altogether prevented. The agglomeration of ore fines and clay, however, proves to greatly improve heap permeability when carried out properly by reducing these fines and transforming them into uniform pellets, which allow for noninhibited permeability.
In the case of heap leaching, agglomeration relies on a binder (typically cement) and tumbling motion to cause coalescence, or the building of fines into larger particles. Agglomeration drums (sometimes referred to as ore drums or agglomerators) are the equipment of choice for agglomerating ore fines in preparation of heap leaching.
Why agglomeration drums?
Ranging in size between 3 feet (ft) and 15 ft (1 to 4.6 meters) in diameter, agglomeration drums are known for their high throughput (typically around 500 pounds per hour [lb/hr] – 3,500+ tons per hour [TPH]) and heavy-duty design and construction — both essential qualities for meeting the demands of the mining industry. When designed and maintained properly, agglomeration drums offer long-term, reliable processing, despite the rigorous demands of the job.
Depending on the manufacturer, these rotary drums can also be highly customizable. Since ore characteristics vary from one source to the next, the ability to optimize the drum around the unique source of ore provides a superior processing solution. Customizations might include the materials of construction, various liners to deter buildup and alteration of bed turning/flight designs, among other design options.
Figure 1. Agglomeration creates uniformity in the particles, leading to a more loosely packed bed, which promotes more pathways between particles in the ore bed through which the leachate can flow more freely.
How agglomeration drums work
Ore fines, leachate and binder are continuously fed into the rotating drum at predetermined rates to promote optimal agglomeration.
The binder causes the fines to become tacky, picking up other fines as they tumble in the drum. Combined with the rolling action, this creates an effect similar to that of rolling a snowball. The drum is specifically sized and designed to accommodate the retention time necessary for the agglomerates to form to the desired size. The drum is set on a slight angle to allow gravity to assist in moving material through the drum.
How agglomeration improves heap leaching
A few key ways in which agglomeration results in improved copper (or other metal) recovery in the leaching process are improved heap permeability, an early start to the leaching process, increased particle penetration and improved particle strength control.
Greater heap permeability
Particle size and shape are two important parameters in optimizing heap permeability. Without agglomeration, fines clog spaces between larger ore particles in the ore bed, preventing the leachate from flowing through and ultimately reducing the amount of recovered ore. A similar issue can exist when a wide range in particle size distribution results in segregation of particles.
Agglomeration is used to create particles that are more uniform in both size and shape. This uniformity creates a more loosely packed bed, which promotes channels or pathways between particles in the ore bed through which the leachate can flow more freely. This is demonstrated in Figure 1.
An early start to leaching
In addition to creating a more permeable heap, agglomeration offers another advantage: a jump start to the leaching process.
By incorporating the leaching agent into the agglomeration stage, the ore fines and leaching agent are homogeneously mixed. This provides an early start to the leaching process and helps to ensure a uniform dispersion of the leaching agent throughout the ore fines.
Uniform dispersion allows every particle to be exposed to the leachate early in the process, improving efficiency of the recovery.
Figure 2. Agglomeration creates void spaces between the smaller particles that make up the larger agglomerate, which results in a porous agglomerate similar to a pumice stone.
Improved particle penetration
In addition to promoting improved leachate flow through the material bed, agglomeration can also be used to increase penetration of each individual particle. This is done through controlling the bulk density of the particles; when carried out with expertise, agglomeration creates void spaces between the smaller particles that make up the larger agglomerate, similar to the channels in the heap. This results in a porous agglomerate similar to a pumice stone, where surface area, and therefore access to the metal components, is maximized, allowing the leachate to saturate the entirety of each particle to collect all the metal components contained within (see Figure 2).
Particle strength control
Particle strength can also be controlled through agglomeration. Particle strength is important for two reasons: First, it maintains pellet integrity, preventing pellets from breaking down into fines and clogging the heap; second, it allows pellets at the bottom of the heap to support the weight of the heap without breaking down, allowing ore to be stacked higher on the heap.
Agglomerate quality
For these reasons, agglomeration can greatly improve ore recovery rates in a heap leaching operation. As has been demonstrated, however, the quality of the agglomerates is critical to maximizing recovery.
Many factors influence agglomerate characteristics including drum rpms, ore moisture content, feed rates and retention time.
It is important to note that all ores are different and will respond differently to agglomeration. For this reason, testing is often needed to define the necessary process parameters to produce agglomerates with the desired specifications.
It is also important to recognize that while agglomeration has the potential to maximize recovery rates, other factors will play a part in this endeavor as well.
Conclusion
The heap leaching process opens up a wealth of opportunities in exploiting low-grade ore sources, however, without optimal heap permeability, recoveries will not reach full potential. When carried out properly, agglomeration through the use of an agglomeration drum can offer a greatly improved leaching process through improved heap permeability, an early start to the leaching process, increased particle penetration and greater particle strength. Ultimately, this aids in the effort to recover as much ore as possible in a time when resources — particularly copper — are being put under increasing pressure.
References
- The World Copper Factbook 2017. International Copper Study Group, 2018, www.icsg.org.
- "Heap Leaching." NAUE GmbH & Co. KG, www.naue.com/applications/mining/heap-leaching.
Shane Le Capitaine has more than 23 years of experience as a process sales engineer at FEECO International, where he specializes in custom rotary drums. He has worked with many of the mining industry’s top firms, providing feasibility testing, process development, custom equipment and aftermarket services. In partnership with FEECO, Le Capitaine has engineered many of the agglomeration drums serving the mining industry today, including copper ore drums for the world’s largest and most environmentally advanced copper mine.
