Heap leaching is an industrial mining process to extract precious metals, copper, uranium, and other compounds from ore via a series of chemical reactions that absorbs specific minerals and then re-separate them after their division from other earth materials. Comparable to in situ mining, heap leach mining differs in that it uses a liner to place amounts of ore on, then adds the chemicals via drip systems to the ore, whereas in situ mining lacks these pads and pulls pregnant solution up to obtain the minerals. This method is only slightly more friendly environmentally, however, and has still seen copious amounts of negative feedback from both environmentalists and health experts in the past twenty or more years. Since its original peak of popularity in the 1970s, the heap leach mining technique has been applied throughout the earth, but due to recent increases in negative environmental impact assessments has received more discussion regarding rehabilitation than perpetuation of these types of mines. Yet this method continues to be a profit-earning endeavor for many mining companies across the globe.
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 sulfate
The mined ore is usually crushed into small chunks and heaped on an impermeable plastic and/or clay lined leach pad where it can be irrigated with a leach solution to dissolve the valuable metals. While sprinklers are occasionally used for irrigation, more often operations use drip irrigation to minimize evaporation, provide more uniform distribution of the leach solution, and avoid damaging the exposed mineral. The solution then percolates through the heap and leaches both the target and other minerals. This process, called the "leach cycle," generally takes from one or two months for simple oxide ores (e.g., most gold ores) to two years (for nickel laterite ores). The leach solution containing the dissolved minerals is then collected, treated in a process plant to recover the target mineral and in some cases precipitate other minerals, and then recycled to the heap after reagent levels are adjusted. Ultimate recovery of the target mineral can range from 30% of contained (run-of-mine dump leaching sulfide copper ores) to over 90% for the easiest to leach ores (some oxide gold ores).
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 allowing the leach to begin prior to the heap. 
Precious metals 
The crushed ore is irrigated with a dilute alkaline cyanide solution. The solution containing the dissolved precious metals ("pregnant solution") continues percolating through the crushed ore until it reaches the liner at the bottom of the heap where it drains into a storage (pregnant solution) pond. After separating the precious metals from the pregnant solution, the dilute cyanide solution (now called "barren solution") is normally re-used in the heap-leach-process or occasionally sent to an industrial water treatment facility where the residual cyanide is treated and residual metals are removed. In very high rainfall areas, such as the tropics, in some cases there is surplus water that is then discharged to the environment, after treatment, posing possible water pollution if treatment is not properly carried out.
The production of one gold ring through this method, can generate 20 tons of waste material.
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.
Copper Ores 
The method is similar to the cyanide method, above, except sulfuric acid is used to dissolve copper from its ores. The acid is recycled from the solvent extraction circuit (see solvent extraction-electrowinning, SX/EW) and reused on the leach pad. A byproduct is iron(II) sulfate, jarosite, which is produced as a byproduct of leaching pyrite, and sometimes even the same sulfuric acid that is needed for the process. Both oxide and sulfide ores can be leached, though the leach cycles are much different and sulfide leaching requires a bacterial or "bio-leach" component. The largest copper heap leach operations are in Chile, Peru, and the southwestern United States.
Although the heap leaching is a low cost-process, it normally has recovery rates of 60-70%, although there are exceptions. 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 Ores 
The method is an acid heap leaching method like that of the copper method in that it utilises sulfuric acid instead of cyanide solution to dissolve the target minerals from crushed ore. The amount of sulfuric acid required is much higher than for copper ores (as high as 1,000 kg of acid per tonne of ore, but 500 kg is more common.) The method was originally patented by Australian miner BHP Billiton and is being commercialized by Cerro Matoso S.A. in Colombia (a wholly owned subsidiary of BHP Billiton), Vale in Brazil, and European Nickel PLC for the rock laterite deposits of Turkey, Talvivaara mine in Finland, Balkans, and the Philippines. There currently are no operating commercial scale nickel laterite heap leach operations, but there is a sulphide HL operating in Finland.
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.
Uranium Ores 
Similar to copper oxide heap leaching, also using dilute sulfuric acid. Rio Tinto is commercializing this technology in Namibia and Australia, the French nuclear power company Areva in Niger (two mines) and Namibia, and several other companies are studying its feasibility.
The final product is yellowcake and requires significant further processing to produce fuel-grade feed.
While most mining companies have shifted from a previously accepted sprinkler method to the percolation of slowly dripping choice chemicals (cyanide or sulfuric acid) closer to the actual ore bed (Krauth 1990), heap leach pads have not changed too much throughout the years. There are still four main categories of pads: conventional, dump leach, Valley Fills, and on/off pads (Thiel and Smith 2004). Typically, each pad only has a single, geomembrane liner for each pad, with a minimum thickness of 1.5mm (usually it is thicker).
The simplest in design, conventional pads 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 the use of putting significantly larger loads on the pads, and removing it 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). 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). 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.
Environmental Concerns 
Heap leach mining works well for high concentrations of less ores, as less Earth needs to be ground onto leach pads in order to extract the same amount of materials. While yield is usually approximately 60-70%, there are significant amounts of damage to the surface environment (Norgate and Haque 2012). Yet the amount of overall harm caused by heap leaching is often lower than more traditional techniques, reducing costs to the process (Norgate and Haque 2012). It also requires less energy consumption to use these methods, which many consider to be an environmental alternative.
In some cases, waste materials from this process are transported to a facility to be treated. However, after a month (or more) of setting in a mat, there is often still more wait time involved in recovering the chemicals—both pregnant and excess—thus allowing for additional chemicals to potentially leach out of the pad into the soil below (Franks et al 2011). This could possibly cause damage to the environment, which has a chance at contaminating surrounding bodies of water. All in all, it is only slightly more environmentally friendly than in-situ mining, which involves leaving chemicals directly into the soil before pulling pregnant chemicals up. Still, most soil in heap leaching is seldom replaced or additionally treated (Franks et al 2011). Evidence has also been found to show that there increased levels of erosion in mining sites with open chemicals, including these heap leach mines, which could be exacerbated through natural phenomena like storms and wind, or more serious occurrences like earthquakes (Franks et al 2011).
The threat to ecosystem composition and biodiversity has been commented on repeatedly in terms of these mines, and it is noted that, while they do have a higher yield, they also have tendencies to accumulate wear and tear from being outside, and pose a threat to the immediate environment by crushing and dumping dirt that would otherwise have been left untouched. As noted earlier, with the reduction of readily available Rare Earth Minerals, there has been increased in the amount of ore piled onto these pads, suggesting that there may come a time when the amount of ore dumped is not worth the amount of returned mineral collected. Therefore, alternatives need to be considered in the near future. Currently, depths are being mined faster than research can provide information about the effects of more ore on the system.
There is also very little study for long-term viability of liners, as this type of mining and the increased ore depths are still a relatively new field, especially given the changes in depths that have been put into practice. With the increase in weight, pressure, and chemicals put on this method of mining, as well as the already small level of knowledge regarding long term benefits, it is difficult to predict the extent of damage from previous leaks, as well as the durability of present day pads and mining sites.
Examples of Case Studies 
Rum Jungle Mine 
One of the oldest and most famous uranium mines in the world, the Rum Jungle Mine in Northern Australia was constructed in the 1950s, and is today experiencing extreme amounts of environmental degradation and acid rock drainage that are leading to further negative impacts on the surrounding river and ecosystems (Ferguson et al 2011). This mine includes three overburdened heaps, two flooded open cuts, and a backfilled open cut, as well as numerous former tailings and heap leach pads. These leach pads caused considerable contamination to soils as chemicals seeped through them. There was attempted rehabilitation in the 1980s, but there are still high evidences of environmental problems today (Mudd and Patterson 2010). These waste sites caused the local river to maintain water that is highly unsafe due to its acidity, high concentrations of target minerals, and other toxic chemicals, many of which are said to have originally leached out of (Ferguson et al 2011). While the highest concentrations have stayed near the buffer zones of the mine and in the East Finniss River, those that did make it into the Finniss River pose a serious and ongoing public threat to those living nearby who used the river daily (Ferguson et al 2011). Now, years later, it is still posing a serious environmental risk to those around the mine (Mudd and Patterson 2010).
Ranger Uranium Mine 
Ranger Uranium Mine in Northern Australia showed a significant increase in erodibility of lands when in contact with materials treated from chemical mining (Riley 1995). This could manifest itself in landslides and loss of habitat, as well as an increase in gravel composition that could cause other potential problems. If these lands can be planted with vegetation that can survive more acidic conditions, however likely that may be, they may be able to avoid eroding materials into other, less mine-contaminated ecosystem (Riley 1995). Given the likelihood of this, though, more rehabilitation measures are being sought after. This case study, however, shows the complexities of mining and the necessity to factor in more preventative measures when dealing with toxic chemicals in natural settings.
The Australian government, who has recently had to deal with negative environmental effects from many historic mines, has taken now to requiring measures that require accounting environmental and social concerns. Alternative locations should be listed and analyzed in mining proposals, as well as rehabilitation plans, externalities (and possible solutions), groundwater and infrastructure changes, gained or lost opportunities, socio-economic impacts, and any risks, as well as any measures taken to reduce or eliminate said risks (Department of Natural Resources, Environment, the Arts and Sport 2009).
Fort Belknap, Montana 
Located on the Fort Belknap Indian Reservation, the Zortman-Landusky gold mine in Montana was one of many early heap leach mines that experienced problems with spills and contamination of surface and groundwater. Although the leaks happened in the 1980s, and the mine was eventually shut down in 1996, health problems on the reservation continue to be a problem, and, as not all of the mine was properly cleaned up, could potentially cause further damage to the people of Fort Belknap (Woody et al 2011). Zortman-Landusky eventually filed bankruptcy when the Bureau of Land Management stepped in to assist the lawsuit that was not heard by the residents of the reservation. Once the bankruptcy was filed, however, all health care and studies ceased, and compensation for the destruction of culturally significant mountain peaks to the local Assiniboine and Gros Ventre people was never achieved (Klauk 2012). Today, there are still abnormally high reports of health problems including thyroid problems, lead poisoning, chemical burns, and emphysema (especially in children) (Klauk 2012).
In a victory for anti-heap leach endeavors, in 2006, Idaho legislatures were failed to allow the Canadian-based Atlanta Gold Company to use cyanide-leaching on a mountain top to extract predicted gold placed at the headwaters of a main river, especially after the State of Montana had so many problems with water contamination in leach mining endeavors. However, Atlanta Gold was able to buy public lands, and is now attempting to clean up toxic levels of arsenic dripping from previous mining endeavor in order to proceed with less stringent permit processes for mining. Those chemical levels have affected local species, as well as the small group of citizens residing in Atlanta, Idaho.
Yet another mining endeavor that lead to the increased health risk of neighboring citizens includes the Coeur d’Alene Mining District in Coeur d’Alene, Idaho. Dozens of various mines in a close area started leaking contaminants to the surrounding streams, poisoning local biodiversity, including the Salmon populations, many species of which were already struggling or else a key source of nutrition for local populations (Woody et al 2011). These populations eventually experienced countless health problems until the Department of Health and Welfare stepped in to take measured to promote awareness and demand clean up measures, which ultimately cost the government $212 million (Woody et al 2011).
Additional Notes 
Legal Claims 
Mining laws have been lobbied against in the United States for the past few decades, and many are only beginning to hear the environmentalist argument. The US General Mining Law of 1872 previous gave liberal rights to miners in terms of establishing and exploring claims, yet did not require any sort of environmental rehabilitation aspect to its process (Woody et al 2011). Today, this is being disputed due to the number of environmental problems being found in heap leach sites, as well as the increase in scientific knowledge that could make mining more efficient and less costly as well. There has been much debate going into the levels of revision the U.S. General Mining Law of 1872, including whether rehabilitation measures should be added to a decrease in the amount of liberal rights given to mining companies. Australia has already addressed much of this with the increased amounts of impact and externality knowledge required in any mining proposal endeavor.
However, as seen with many case studies, a simple way around these measures is the privatization of the land to be mined (Woody et al 2011). In this case, environmental standards need to be made priority, as their effects spread beyond simple legal boundaries and into the ecosystems present at each mine, many of which are affecting poorer people less likely to speak up for their health and their environment (see Environmental Justice).
Cultural and Social Concerns 
With the rise of the environmentalist movement has also come an increased appreciation for social justice, and mining has showed similar trends lately. Societies located near potential mining sites are at increased risk to be subjected to injustices as their environment is affected by the changes made to mined lands—either public or private—that could eventually lead to problems in social structure, identity, and physical health (Franks 2009). Many 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 indeed 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). Through increased dialogue and environmental legislation, many corporations and citizens hope to bridge the gap between interests in order to obtain the rare natural resources that most people depend on in daily life.
See also 
- Gold cyanidation
- Gold extraction
- In-situ leach
- Mineral processing
- Environmental Justice
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