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Heap leaching is an industrial mining process used to extract precious metals, copper, uranium, and other compounds from ore using a series of chemical reactions that absorb specific minerals and re-separate them after their division from other earth materials. Similar to in situ mining, heap leach mining differs in that it places ore on a liner, then adds the chemicals via drip systems to the ore, whereas in situ mining lacks these liners and pulls pregnant solution up to obtain the minerals. Heap leaching is widely used in modern large-scale mining operations as it produces the desired concentrates at a lower cost compared to conventional processing methods such as flotation, agitation, and vat leaching.Petersen, J., & Dixon, D. G. (2002). Thermophilic heap leaching of a chalcopyrite concentrate. Minerals engineering, 15(11), 777-785.
Additionally, dump leaching is an essential part of most copper mining operations and determines the quality grade of the produced material along with other factors
Due to the profitability that the dump leaching has on the mining process, i.e. it can contribute substantially to the Economic growth of the mining process, it is advantageous to include the results of the leaching operation in the economic overall project evaluation.Bouffard, Sylvie C., and David G. Dixon. "Investigative study into the hydrodynamics of heap leaching processes." Metallurgical and Materials Transactions B 32.5 (2001): 763-776.
The process has ancient origins; one of the classical methods for the manufacture of copperas (iron sulfate) was to heap up iron pyrite and collect the leachate from the heap, which was then boiled with iron to produce iron(II) sulfate.Industrial England in the Middle of the Eighteenth Century, Nature, Vol, 83, No. 2113, Thursday, April 28, 1910; page 267.
The essential questions to address during the process of the heap leaching are:Roman, Ronald J., Blair R. Benner, and George W. Becker. "Diffusion model for heap leaching and its application to scale-up." TRANS SOC MIN ENG, AIME 256.3 (1974): 247-252.
In recent years, the addition of an agglomeration drum has improved on the heap leaching process by allowing for a more efficient leach. The rotary drum agglomerator works by taking the crushed ore fines and agglomerating them into more uniform particles. This makes it much easier for the leaching solution to percolate through the pile, making its way through the channels between particles.
The addition of an agglomeration drum also has the added benefit of being able to pre-mix the leaching solution with the ore fines to achieve a more concentrated, homogeneous mixture and allow the leach to begin prior to the heap.
Although heap leach design has made significant progress over the last few years through the use of new materials and improved analytical tools, industrial experience shows that there are significant benefits from extending the design process beyond the liner and into the rock pile itself. Characterization of the physical and hydraulic (hydrodynamic) properties of ore-for-leach focuses on the direct measurement of the key properties of the ore, namely:
Theoretical and numerical analysis, and operational data show that these fundamental mechanisms are controlled by scale, dimensionality, and heterogeneity, all of which adversely affect the scalability of metallurgical and hydrodynamic properties from the lab to the field. The dismissal of these mechanisms can result in a number of practical and financial problems that will resonate throughout the life of the heap impacting the financial return of the operation. Through procedures that go beyond the commonly employed metallurgical testing and the integration of data gleaned through real time 3D monitoring, a more complete representative characterization of the physicochemical properties of the heap environment is obtained. This improved understanding results in a significantly higher degree of accuracy in terms of creating a truly representative sample of the environment within the heap.Guzmán-Guzmán, A., Y. Cáceres Hernández, O., Srivastava, R., & W. Jones, J. (2014). Integrated process control to enhance heap leach performance. Paper presented at the Second International Conference on Heap Leach Solutions, Lima, Peru.
By adhering to the characterization identified above, a more comprehensive view of heap leach environments can be realized, allowing the industry to move away from the de facto black-box approach to a physicochemically inclusive industrial reactor model.
The production of one gold ring through this method, can generate 20 tons of waste material.Gage B. & G. 2008. American Outrage. USA: Human Rights Watch.
During the extraction phase, the gold ions form complex ions with the cyanide:
Recuperation of the gold is readily achieved with a redox-reaction:
The most common methods to remove the gold from solution are either using activated carbon to selectively absorb it, or the Merrill-Crowe process where zinc powder is added to cause a precipitation of gold and zinc. The fine product can be either doré (gold-silver bars) or zinc-gold sludge that is then refined elsewhere.
In 2011 leaching, both heap leaching and in-situ leaching, produced 3.4 million metric tons of copper, 22 percent of world production.US Geological Survey, 2011 Minerals Yearbook - Copper, 2012. The largest copper heap leach operations are in Chile, Peru, and the southwestern United States.
Although heap leaching is a low cost-process, it normally has recovery rates of 60-70%. It is normally most profitable with low-grade ores. Higher-grade ores are usually put through more complex milling processes where higher recoveries justify the extra cost. The process chosen depends on the properties of the ore.
The final product is cathode copper.
Nickel recovery from the leach solutions is much more complex than for copper and requires various stages of iron and magnesium removal, and the process produces both leached ore residue ("ripios") and chemical precipitates from the recovery plant (principally iron oxide residues, magnesium sulfate and calcium sulfate) in roughly equal proportions. Thus, a unique feature of nickel heap leaching is the need for a tailings disposal area.
The final product can be nickel hydroxide precipitates (NHP) or mixed metal hydroxide precipitates (MHP), which are then subject to conventional smelting to produce metallic nickel.
The final product is yellowcake and requires significant further processing to produce fuel-grade feed.
The conventional pads simplest in design are used for mostly flat or gentle areas and hold thinner layers of crushed ore. Dump leach pads hold more ore and can usually handle a less flat terrain. Valley fills are pads situated at valley bottoms or levels that can hold everything falling into it. On/off pads involve putting significantly larger loads on the pads and removing and reloading it after every cycle.
Many of these mines which previously had digging depths of about 15 meters are digging deeper than ever before to mine materials, approximately 50 meters, sometimes more, which means that, in order to accommodate all of the ground being displaced, pads will have to hold higher weights from more crushed ore being contained in a smaller area (Lupo 2010).Lupo, John F. 2009 Liner system design for heap leach pads. Geotextiles and Geomembranes 28(12): 163-173 With that increase in build up comes in potential for decrease in yield or ore quality, as well as potential either weak spots in the lining or areas of increased pressure buildup. This build up still has the potential to lead to punctures in the liner. As of 2004 cushion fabrics, which could reduce potential punctures and their leaking, were still being debated due to their tendency to increase risks if too much weight on too large a surface was placed on the cushioning (Thiel and Smith 2004).Thiel, Richard, and Mark E. Smith 2004 State of the practice review of heap leach pad design issues. Geotextiles and Geomembranes 22(5): 555-568 In addition, some liners, depending on their composition, may react with salts in the soil as well as acid from the chemical leaching to affect the successfulness of the liner. This can be amplified over time.
One solution proposed to reclamation problems is the privatization of the land to be mined (Woody et al. 2011).
2009 Avoiding Mine-Community Conflict: From Dialogue to Shared Futures. In: Jacques Wiertz and Chris Moran, Proceedings of the First International Seminar on Environmental Issues in the Mining Industry. Enviromine 2009, Santiago, Chile, (x-x). 30 Sept- 2 Oct 2009Many have argued that by cycling mine power through local citizens, this disagreement can be alleviated, since both interest groups would have shared and equal voice and understanding in future goals. However, it is often difficult to match corporate mining interests with local social interests, and money is often a deciding factor in the successes of any disagreements. If communities are able to feel like they have a valid understanding and power in issues concerning their local environment and society, they are more likely to tolerate and encourage the positive benefits that come with mining, as well as more effectively promote alternative methods to heap leach mining using their intimate knowledge of the local geography (Franks 2009).
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